diff --git a/cmd/server/main.go b/cmd/server/main.go
index 20675ed..f7a8108 100644
--- a/cmd/server/main.go
+++ b/cmd/server/main.go
@@ -246,6 +246,10 @@ func main() {
 			// SSoT. Drives Verfahrensablauf + Mode B result-view conditional
 			// rendering and per-rule selection state (`rule:<uuid>` keys).
 			ScenarioFlags: services.NewScenarioFlagsService(pool, projectSvc),
+			// t-paliad-340 / m/paliad#153 B0 (mig 157) — Litigation Builder.
+			// CRUD over the new normalised scenarios + scenario_proceedings
+			// + scenario_events + scenario_shares tables.
+			ScenarioBuilder: services.NewScenarioBuilderService(pool),
 		}
 
 		// t-paliad-246 Slice A — Backup Mode runner. Wired only when
diff --git a/internal/db/migrations/157_scenario_builder_foundation.down.sql b/internal/db/migrations/157_scenario_builder_foundation.down.sql
new file mode 100644
index 0000000..c016de2
--- /dev/null
+++ b/internal/db/migrations/157_scenario_builder_foundation.down.sql
@@ -0,0 +1,94 @@
+-- 157_scenario_builder_foundation — down
+--
+-- Rolls back mig 157 in reverse order. Down files are reference material
+-- (not auto-applied); operator recovery path is:
+--
+--   psql ... < 157_scenario_builder_foundation.down.sql
+--   DELETE FROM paliad.applied_migrations WHERE version = 157;
+--
+-- This restores the legacy paliad.scenarios shape from mig 145 — the
+-- builder columns and the three sibling tables are dropped wholesale.
+-- Any builder data in the dropped tables is lost (the tables CASCADE to
+-- their children, and DROP TABLE doesn't keep a backup).
+
+BEGIN;
+
+SELECT set_config(
+    'paliad.audit_reason',
+    'mig 157 rollback: tear down Scenario builder foundation (t-paliad-340)',
+    true
+);
+
+-- 8. updated_at triggers
+DROP TRIGGER IF EXISTS scenario_events_touch_updated_at_trg       ON paliad.scenario_events;
+DROP TRIGGER IF EXISTS scenario_proceedings_touch_updated_at_trg  ON paliad.scenario_proceedings;
+
+-- 7. RLS — drop new policies + restore legacy four
+DROP POLICY IF EXISTS scenario_shares_mutate        ON paliad.scenario_shares;
+DROP POLICY IF EXISTS scenario_shares_select        ON paliad.scenario_shares;
+DROP POLICY IF EXISTS scenario_events_mutate        ON paliad.scenario_events;
+DROP POLICY IF EXISTS scenario_events_select        ON paliad.scenario_events;
+DROP POLICY IF EXISTS scenario_proceedings_mutate   ON paliad.scenario_proceedings;
+DROP POLICY IF EXISTS scenario_proceedings_select   ON paliad.scenario_proceedings;
+DROP POLICY IF EXISTS scenarios_owner_mutate        ON paliad.scenarios;
+DROP POLICY IF EXISTS scenarios_select              ON paliad.scenarios;
+
+-- Restore the four mig-145 policies verbatim.
+CREATE POLICY scenarios_project_select ON paliad.scenarios
+    FOR SELECT
+    USING (project_id IS NOT NULL AND paliad.can_see_project(project_id));
+
+CREATE POLICY scenarios_project_mutate ON paliad.scenarios
+    FOR ALL
+    USING (project_id IS NOT NULL AND paliad.can_see_project(project_id))
+    WITH CHECK (project_id IS NOT NULL AND paliad.can_see_project(project_id));
+
+CREATE POLICY scenarios_abstract_select ON paliad.scenarios
+    FOR SELECT
+    USING (project_id IS NULL AND created_by = auth.uid());
+
+CREATE POLICY scenarios_abstract_mutate ON paliad.scenarios
+    FOR ALL
+    USING (project_id IS NULL AND created_by = auth.uid())
+    WITH CHECK (project_id IS NULL AND created_by = auth.uid());
+
+-- 6. helper function
+DROP FUNCTION IF EXISTS paliad.can_see_scenario(uuid);
+
+-- 5. paliad.projects.origin_scenario_id
+DROP INDEX IF EXISTS paliad.projects_origin_scenario_idx;
+ALTER TABLE paliad.projects DROP COLUMN IF EXISTS origin_scenario_id;
+
+-- 4. paliad.scenario_shares
+DROP TABLE IF EXISTS paliad.scenario_shares;
+
+-- 3. paliad.scenario_events
+DROP TABLE IF EXISTS paliad.scenario_events;
+
+-- 2. paliad.scenario_proceedings
+DROP TABLE IF EXISTS paliad.scenario_proceedings;
+
+-- 1. paliad.scenarios — restore mig-145 shape
+DROP INDEX IF EXISTS paliad.scenarios_updated_idx;
+DROP INDEX IF EXISTS paliad.scenarios_owner_status_idx;
+
+-- Restore the unique constraint mig 145 had.
+ALTER TABLE paliad.scenarios
+    ADD CONSTRAINT scenarios_unique_per_scope
+    UNIQUE NULLS NOT DISTINCT (project_id, created_by, name);
+
+-- spec was NOT NULL in mig 145. Restore that — but only after backfilling
+-- any NULL specs the builder might have created (none in legacy paths;
+-- only builder rows have NULL spec, and those are dropped together with
+-- the builder schema if a real rollback is needed).
+UPDATE paliad.scenarios SET spec = '{}'::jsonb WHERE spec IS NULL;
+ALTER TABLE paliad.scenarios ALTER COLUMN spec SET NOT NULL;
+
+ALTER TABLE paliad.scenarios DROP COLUMN IF EXISTS notes;
+ALTER TABLE paliad.scenarios DROP COLUMN IF EXISTS stichtag;
+ALTER TABLE paliad.scenarios DROP COLUMN IF EXISTS promoted_project_id;
+ALTER TABLE paliad.scenarios DROP COLUMN IF EXISTS origin_project_id;
+ALTER TABLE paliad.scenarios DROP COLUMN IF EXISTS status;
+ALTER TABLE paliad.scenarios DROP COLUMN IF EXISTS owner_id;
+
+COMMIT;
diff --git a/internal/db/migrations/157_scenario_builder_foundation.up.sql b/internal/db/migrations/157_scenario_builder_foundation.up.sql
new file mode 100644
index 0000000..635c30d
--- /dev/null
+++ b/internal/db/migrations/157_scenario_builder_foundation.up.sql
@@ -0,0 +1,500 @@
+-- 157_scenario_builder_foundation — t-paliad-340 / m/paliad#153 B0
+--
+-- Schema foundation for the Litigation Builder (PRD
+-- docs/plans/prd-procedures-litigation-planner-2026-05-27.md §5.1 + §5.2).
+-- Phase B0 of the 7-slice train described in PRD §7.1. DB-only — no UI
+-- depends on these tables yet; B1 wires the builder shell on top.
+--
+-- What this migration adds:
+--
+--   1. Six new columns on paliad.scenarios for the builder shape:
+--        owner_id, status, origin_project_id, promoted_project_id,
+--        stichtag, notes.
+--      Two relaxations on existing columns:
+--        - spec NOT NULL → NULL (the builder normalises spec contents
+--          into scenario_proceedings / scenario_events; new rows skip
+--          spec entirely. Legacy callers from mig 145 still provide it
+--          explicitly, so they keep inserting valid rows.)
+--        - DROP CONSTRAINT scenarios_unique_per_scope (the builder
+--          allows multiple "Unbenanntes Szenario" + multiple scratch
+--          scenarios per user — uniqueness on (project_id, created_by,
+--          name) blocks that. The legacy service treated the constraint
+--          as UX collision avoidance, not correctness.)
+--
+--   2. Three new tables for the normalised builder shape:
+--        - paliad.scenario_proceedings (one row per proceeding in a
+--          scenario; multi-proceeding constellations + spawned children)
+--        - paliad.scenario_events       (one row per event card on the
+--          canvas; planned / filed / skipped state + actual_date + notes
+--          + per-card optional horizon)
+--        - paliad.scenario_shares       (read-only team shares; owner is
+--          the sole editor)
+--
+--   3. One new column on paliad.projects:
+--        - origin_scenario_id — audit trail for promote-to-project
+--          (B5; the column lands now so the FK is in place when the
+--          wizard arrives).
+--
+--   4. New helper function paliad.can_see_scenario(_scenario_id) that
+--      mirrors paliad.can_see_project's STABLE SECURITY DEFINER shape.
+--      Visibility logic:
+--        - global_admin sees everything,
+--        - owner_id = auth.uid() (builder-owned scenarios),
+--        - scenario_shares.shared_with_user_id = auth.uid()
+--          (read-only shared scenarios),
+--        - legacy project-scoped scenarios (owner_id IS NULL AND
+--          project_id IS NOT NULL) follow can_see_project(project_id),
+--        - legacy abstract scenarios (owner_id IS NULL AND project_id
+--          IS NULL) follow created_by = auth.uid().
+--
+--   5. Replacement RLS policies on paliad.scenarios that fold builder
+--      visibility together with the legacy shape. The legacy
+--      project_* / abstract_* policies are dropped (they covered only
+--      legacy paths) and rewritten as a single pair of policies that
+--      treats owner_id, scenario_shares, and the legacy paths uniformly.
+--
+--      Builder-only RLS for the three new tables: read = scenario
+--      visibility; write = scenario owner (or legacy editor) only.
+--
+-- PRD §5.1 deviations called out for the reader:
+--
+--   - PRD specs `proceeding_type_id uuid REFERENCES paliad.proceeding_types(id)`.
+--     The live column is `integer` (see paliad.proceeding_types.id);
+--     scenario_proceedings.proceeding_type_id is integer here to match
+--     the real FK target. PRD authors did not check the column type;
+--     this migration uses the truth on disk.
+--
+--   - PRD references `auth.users(id)` for owner_id and share columns;
+--     the established paliad convention (see paliad.projects.created_by,
+--     paliad.scenarios.created_by) uses `paliad.users(id)`. Same UUIDs
+--     either way (paliad.users.id == auth.users.id), but the FK targets
+--     paliad.users to stay consistent with project tables.
+--
+-- Audit-first: all DDL ran clean against a BEGIN/ROLLBACK probe on the
+-- live DB before this file was committed. paliad.scenarios has 0 rows
+-- (verified pre-mig), so the column additions and constraint relaxations
+-- have no data impact.
+
+BEGIN;
+
+SELECT set_config(
+    'paliad.audit_reason',
+    'mig 157: Scenario builder foundation (t-paliad-340 / m/paliad#153 B0)',
+    true
+);
+
+-- ----------------------------------------------------------------
+-- 1. paliad.scenarios — additive columns + constraint relaxations
+-- ----------------------------------------------------------------
+
+ALTER TABLE paliad.scenarios
+    ADD COLUMN owner_id            uuid NULL REFERENCES paliad.users(id) ON DELETE CASCADE,
+    ADD COLUMN status              text NOT NULL DEFAULT 'active'
+        CHECK (status IN ('active','archived','promoted')),
+    ADD COLUMN origin_project_id   uuid NULL REFERENCES paliad.projects(id) ON DELETE SET NULL,
+    ADD COLUMN promoted_project_id uuid NULL REFERENCES paliad.projects(id) ON DELETE SET NULL,
+    ADD COLUMN stichtag            date NULL,
+    ADD COLUMN notes               text NULL;
+
+ALTER TABLE paliad.scenarios ALTER COLUMN spec DROP NOT NULL;
+ALTER TABLE paliad.scenarios DROP CONSTRAINT IF EXISTS scenarios_unique_per_scope;
+
+CREATE INDEX scenarios_owner_status_idx
+    ON paliad.scenarios(owner_id, status)
+    WHERE owner_id IS NOT NULL;
+
+CREATE INDEX scenarios_updated_idx
+    ON paliad.scenarios(owner_id, updated_at DESC)
+    WHERE owner_id IS NOT NULL;
+
+COMMENT ON COLUMN paliad.scenarios.owner_id IS
+    'Litigation Builder owner (PRD §5.1). NULL = legacy composition-spec '
+    'scenario from m/paliad#124 Slice D (mig 145). Builder rows MUST have '
+    'owner_id set; the application enforces it via ScenarioBuilderService.';
+
+COMMENT ON COLUMN paliad.scenarios.status IS
+    'Lifecycle: active (default; user-editable) / archived (soft-deleted, '
+    'still visible in side panel) / promoted (converted to project via '
+    'B5 wizard; read-only). Legacy mig-145 rows default to active.';
+
+COMMENT ON COLUMN paliad.scenarios.origin_project_id IS
+    'Set when the scenario was exported from an existing project '
+    '("Im Builder öffnen" — Akte mode, PRD §2.3).';
+
+COMMENT ON COLUMN paliad.scenarios.promoted_project_id IS
+    'Set after the scenario was promoted to a real project via the 3-step '
+    'wizard (PRD §5.4). Together with paliad.projects.origin_scenario_id, '
+    'forms the bidirectional audit link.';
+
+COMMENT ON COLUMN paliad.scenarios.stichtag IS
+    'Scenario-level default Stichtag; per-proceeding overrides in '
+    'paliad.scenario_proceedings.stichtag take precedence.';
+
+-- ----------------------------------------------------------------
+-- 2. paliad.scenario_proceedings — one proceeding per scenario row
+-- ----------------------------------------------------------------
+
+CREATE TABLE paliad.scenario_proceedings (
+    id                              uuid PRIMARY KEY DEFAULT gen_random_uuid(),
+    scenario_id                     uuid NOT NULL
+        REFERENCES paliad.scenarios(id) ON DELETE CASCADE,
+    proceeding_type_id              integer NOT NULL
+        REFERENCES paliad.proceeding_types(id),
+    primary_party                   text NULL
+        CHECK (primary_party IN ('claimant','defendant')),
+    scenario_flags                  jsonb NOT NULL DEFAULT '{}'::jsonb
+        CHECK (jsonb_typeof(scenario_flags) = 'object'),
+    parent_scenario_proceeding_id   uuid NULL
+        REFERENCES paliad.scenario_proceedings(id) ON DELETE CASCADE,
+    spawn_anchor_event_id           uuid NULL
+        REFERENCES paliad.sequencing_rules(id),
+    ordinal                         int NOT NULL DEFAULT 0,
+    stichtag                        date NULL,
+    detailgrad                      text NOT NULL DEFAULT 'selected'
+        CHECK (detailgrad IN ('selected','all_options')),
+    appeal_target                   text NULL,
+    collapsed                       boolean NOT NULL DEFAULT false,
+    created_at                      timestamptz NOT NULL DEFAULT now(),
+    updated_at                      timestamptz NOT NULL DEFAULT now()
+);
+
+CREATE INDEX scenario_proceedings_scenario_idx
+    ON paliad.scenario_proceedings(scenario_id, ordinal);
+
+CREATE INDEX scenario_proceedings_parent_idx
+    ON paliad.scenario_proceedings(parent_scenario_proceeding_id)
+    WHERE parent_scenario_proceeding_id IS NOT NULL;
+
+COMMENT ON TABLE paliad.scenario_proceedings IS
+    'One proceeding inside a Litigation Builder scenario. Multiple rows '
+    'per scenario for multi-proceeding constellations. '
+    'parent_scenario_proceeding_id self-refs for spawned children '
+    '(e.g. upc.ccr.cfi spawned by with_ccr on upc.inf.cfi). '
+    'PRD §5.1, m/paliad#153 B0.';
+
+COMMENT ON COLUMN paliad.scenario_proceedings.primary_party IS
+    'Per-proceeding perspective ("our side"). NULL = no perspective '
+    'picked yet (both party columns render with natural labels). '
+    'Per-proceeding so multi-jurisdiction constellations can flip side '
+    'independently (PRD §3.3).';
+
+COMMENT ON COLUMN paliad.scenario_proceedings.scenario_flags IS
+    'Per-proceeding flags (e.g. {"with_ccr": true, "with_amend": false}). '
+    'Mirrors paliad.projects.scenario_flags shape but lives per-proceeding-'
+    'per-scenario. Validated by the application against '
+    'paliad.scenario_flag_catalog at write time.';
+
+COMMENT ON COLUMN paliad.scenario_proceedings.spawn_anchor_event_id IS
+    'Which sequencing_rule of the parent proceeding caused this spawn. '
+    'NULL for root proceedings. Used by the UI to place the spawned child '
+    'triplet directly below the parent at the spawn node (PRD §3.6).';
+
+COMMENT ON COLUMN paliad.scenario_proceedings.ordinal IS
+    'Stack order on canvas (top to bottom). Siblings under the same '
+    'parent (or top-level) are ordered by ordinal asc, then created_at.';
+
+COMMENT ON COLUMN paliad.scenario_proceedings.detailgrad IS
+    'Per-proceeding optional-detail toggle: selected (only explicitly '
+    'chosen optionals + mandatories) or all_options (every optional '
+    'sequencing_rule surfaces). Matches today''s Verfahrensablauf pattern.';
+
+-- ----------------------------------------------------------------
+-- 3. paliad.scenario_events — one event card on the canvas
+-- ----------------------------------------------------------------
+
+CREATE TABLE paliad.scenario_events (
+    id                     uuid PRIMARY KEY DEFAULT gen_random_uuid(),
+    scenario_proceeding_id uuid NOT NULL
+        REFERENCES paliad.scenario_proceedings(id) ON DELETE CASCADE,
+    sequencing_rule_id     uuid NULL
+        REFERENCES paliad.sequencing_rules(id),
+    procedural_event_id    uuid NULL
+        REFERENCES paliad.procedural_events(id),
+    custom_label           text NULL,
+    state                  text NOT NULL DEFAULT 'planned'
+        CHECK (state IN ('planned','filed','skipped')),
+    actual_date            date NULL,
+    skip_reason            text NULL,
+    notes                  text NULL,
+    horizon_optional       int NOT NULL DEFAULT 0
+        CHECK (horizon_optional >= 0),
+    created_at             timestamptz NOT NULL DEFAULT now(),
+    updated_at             timestamptz NOT NULL DEFAULT now(),
+    CONSTRAINT scenario_events_one_anchor CHECK (
+        (sequencing_rule_id  IS NOT NULL)::int +
+        (procedural_event_id IS NOT NULL)::int +
+        (custom_label        IS NOT NULL)::int >= 1
+    )
+);
+
+CREATE INDEX scenario_events_proceeding_idx
+    ON paliad.scenario_events(scenario_proceeding_id);
+
+-- A single proceeding can't carry two cards for the same sequencing rule
+-- (each rule maps to one card). Free-form / procedural_event-only cards
+-- skip this uniqueness — multiple custom cards per proceeding are OK.
+CREATE UNIQUE INDEX scenario_events_rule_uniq_idx
+    ON paliad.scenario_events(scenario_proceeding_id, sequencing_rule_id)
+    WHERE sequencing_rule_id IS NOT NULL;
+
+COMMENT ON TABLE paliad.scenario_events IS
+    'One event card on the Litigation Builder canvas. Captures state '
+    '(planned/filed/skipped), actual_date, notes, skip_reason, and the '
+    'per-card optional-horizon setting. At least one of '
+    '(sequencing_rule_id, procedural_event_id, custom_label) must be '
+    'set — sequencing-rule-backed cards are the common case; free-form '
+    'cards exist for events the catalog doesn''t cover yet. '
+    'PRD §3.4 / §5.1.';
+
+COMMENT ON COLUMN paliad.scenario_events.state IS
+    '3-state machine: planned (default, future event with computed date) '
+    '/ filed (past event, actual_date set) / skipped (user chose not to '
+    'file; optional skip_reason). No "overdue" enum — that''s derived '
+    '(date < today AND state=planned), not stored. PRD Q10 / §3.4.';
+
+COMMENT ON COLUMN paliad.scenario_events.actual_date IS
+    'Set when state=filed (real-world filing date) OR when state=planned '
+    'and the user overrode the computed date (court-set events, manual '
+    'tweaks). NULL when the computed date is canonical.';
+
+COMMENT ON COLUMN paliad.scenario_events.horizon_optional IS
+    'Per-card "show N more optional follow-ups" affordance. Default 0 '
+    '(hidden). PRD Q4 / §3.4.';
+
+-- ----------------------------------------------------------------
+-- 4. paliad.scenario_shares — read-only team shares
+-- ----------------------------------------------------------------
+
+CREATE TABLE paliad.scenario_shares (
+    id                  uuid PRIMARY KEY DEFAULT gen_random_uuid(),
+    scenario_id         uuid NOT NULL
+        REFERENCES paliad.scenarios(id) ON DELETE CASCADE,
+    shared_with_user_id uuid NOT NULL
+        REFERENCES paliad.users(id) ON DELETE CASCADE,
+    created_at          timestamptz NOT NULL DEFAULT now(),
+    created_by          uuid NOT NULL REFERENCES paliad.users(id),
+    UNIQUE (scenario_id, shared_with_user_id)
+);
+
+CREATE INDEX scenario_shares_user_idx
+    ON paliad.scenario_shares(shared_with_user_id);
+
+COMMENT ON TABLE paliad.scenario_shares IS
+    'Read-only team shares for Litigation Builder scenarios. Owner '
+    '(paliad.scenarios.owner_id) is the sole editor; rows here grant '
+    'view-only access to other paliad users. PRD Q12 / §5.1.';
+
+-- ----------------------------------------------------------------
+-- 5. paliad.projects.origin_scenario_id — promote-to-project trail
+-- ----------------------------------------------------------------
+
+ALTER TABLE paliad.projects
+    ADD COLUMN origin_scenario_id uuid NULL
+        REFERENCES paliad.scenarios(id) ON DELETE SET NULL;
+
+CREATE INDEX projects_origin_scenario_idx
+    ON paliad.projects(origin_scenario_id)
+    WHERE origin_scenario_id IS NOT NULL;
+
+COMMENT ON COLUMN paliad.projects.origin_scenario_id IS
+    'FK to the scenario this project was promoted from (B5 wizard). '
+    'NULL = project was created directly, not via Builder. Together with '
+    'paliad.scenarios.promoted_project_id, forms the bidirectional audit '
+    'link. PRD §5.2.';
+
+-- ----------------------------------------------------------------
+-- 6. paliad.can_see_scenario — visibility helper
+-- ----------------------------------------------------------------
+
+CREATE OR REPLACE FUNCTION paliad.can_see_scenario(_scenario_id uuid)
+    RETURNS boolean
+    LANGUAGE sql STABLE SECURITY DEFINER
+    SET search_path TO 'paliad', 'public'
+AS $func$
+    SELECT EXISTS (
+        SELECT 1 FROM paliad.users u
+         WHERE u.id = auth.uid() AND u.global_role = 'global_admin'
+    )
+    OR EXISTS (
+        SELECT 1 FROM paliad.scenarios s
+         WHERE s.id = _scenario_id AND s.owner_id = auth.uid()
+    )
+    OR EXISTS (
+        SELECT 1 FROM paliad.scenario_shares sh
+         WHERE sh.scenario_id = _scenario_id
+           AND sh.shared_with_user_id = auth.uid()
+    )
+    -- Legacy project-scoped scenarios (mig 145) — visible via project
+    -- team membership.
+    OR EXISTS (
+        SELECT 1 FROM paliad.scenarios s
+         WHERE s.id = _scenario_id
+           AND s.owner_id IS NULL
+           AND s.project_id IS NOT NULL
+           AND paliad.can_see_project(s.project_id)
+    )
+    -- Legacy abstract scenarios (mig 145) — owner-only via created_by.
+    OR EXISTS (
+        SELECT 1 FROM paliad.scenarios s
+         WHERE s.id = _scenario_id
+           AND s.owner_id IS NULL
+           AND s.project_id IS NULL
+           AND s.created_by = auth.uid()
+    );
+$func$;
+
+COMMENT ON FUNCTION paliad.can_see_scenario(uuid) IS
+    'Returns true if the caller (auth.uid()) can see the given scenario. '
+    'Mirrors paliad.can_see_project. Covers builder-owned scenarios '
+    '(owner_id), read-only shares (scenario_shares), and the two legacy '
+    'paths from mig 145 (project-scoped via can_see_project, abstract '
+    'via created_by). Used by RLS on all four scenario_* tables.';
+
+-- ----------------------------------------------------------------
+-- 7. RLS — replace legacy scenarios policies + new tables
+-- ----------------------------------------------------------------
+
+-- Replace mig-145's four policies with a single pair that handles
+-- builder + legacy shapes together.
+DROP POLICY IF EXISTS scenarios_project_select   ON paliad.scenarios;
+DROP POLICY IF EXISTS scenarios_project_mutate   ON paliad.scenarios;
+DROP POLICY IF EXISTS scenarios_abstract_select  ON paliad.scenarios;
+DROP POLICY IF EXISTS scenarios_abstract_mutate  ON paliad.scenarios;
+
+CREATE POLICY scenarios_select ON paliad.scenarios
+    FOR SELECT USING (paliad.can_see_scenario(id));
+
+-- Write rule: builder owner, legacy project team member (if no owner),
+-- or legacy abstract creator (if no owner + no project). Shares are
+-- read-only — they don't grant mutate.
+CREATE POLICY scenarios_owner_mutate ON paliad.scenarios
+    FOR ALL
+    USING (
+        owner_id = auth.uid()
+        OR (owner_id IS NULL AND project_id IS NOT NULL AND paliad.can_see_project(project_id))
+        OR (owner_id IS NULL AND project_id IS NULL AND created_by = auth.uid())
+    )
+    WITH CHECK (
+        owner_id = auth.uid()
+        OR (owner_id IS NULL AND project_id IS NOT NULL AND paliad.can_see_project(project_id))
+        OR (owner_id IS NULL AND project_id IS NULL AND created_by = auth.uid())
+    );
+
+-- scenario_proceedings — visibility piggybacks on the parent scenario.
+ALTER TABLE paliad.scenario_proceedings ENABLE ROW LEVEL SECURITY;
+
+CREATE POLICY scenario_proceedings_select ON paliad.scenario_proceedings
+    FOR SELECT USING (paliad.can_see_scenario(scenario_id));
+
+CREATE POLICY scenario_proceedings_mutate ON paliad.scenario_proceedings
+    FOR ALL
+    USING (EXISTS (
+        SELECT 1 FROM paliad.scenarios s
+         WHERE s.id = scenario_id
+           AND (s.owner_id = auth.uid()
+             OR (s.owner_id IS NULL AND s.project_id IS NOT NULL AND paliad.can_see_project(s.project_id))
+             OR (s.owner_id IS NULL AND s.project_id IS NULL AND s.created_by = auth.uid()))
+    ))
+    WITH CHECK (EXISTS (
+        SELECT 1 FROM paliad.scenarios s
+         WHERE s.id = scenario_id
+           AND (s.owner_id = auth.uid()
+             OR (s.owner_id IS NULL AND s.project_id IS NOT NULL AND paliad.can_see_project(s.project_id))
+             OR (s.owner_id IS NULL AND s.project_id IS NULL AND s.created_by = auth.uid()))
+    ));
+
+-- scenario_events — visibility piggybacks on the parent scenario via
+-- the proceeding row.
+ALTER TABLE paliad.scenario_events ENABLE ROW LEVEL SECURITY;
+
+CREATE POLICY scenario_events_select ON paliad.scenario_events
+    FOR SELECT
+    USING (EXISTS (
+        SELECT 1 FROM paliad.scenario_proceedings sp
+         WHERE sp.id = scenario_proceeding_id
+           AND paliad.can_see_scenario(sp.scenario_id)
+    ));
+
+CREATE POLICY scenario_events_mutate ON paliad.scenario_events
+    FOR ALL
+    USING (EXISTS (
+        SELECT 1 FROM paliad.scenario_proceedings sp
+         JOIN paliad.scenarios s ON s.id = sp.scenario_id
+         WHERE sp.id = scenario_proceeding_id
+           AND (s.owner_id = auth.uid()
+             OR (s.owner_id IS NULL AND s.project_id IS NOT NULL AND paliad.can_see_project(s.project_id))
+             OR (s.owner_id IS NULL AND s.project_id IS NULL AND s.created_by = auth.uid()))
+    ))
+    WITH CHECK (EXISTS (
+        SELECT 1 FROM paliad.scenario_proceedings sp
+         JOIN paliad.scenarios s ON s.id = sp.scenario_id
+         WHERE sp.id = scenario_proceeding_id
+           AND (s.owner_id = auth.uid()
+             OR (s.owner_id IS NULL AND s.project_id IS NOT NULL AND paliad.can_see_project(s.project_id))
+             OR (s.owner_id IS NULL AND s.project_id IS NULL AND s.created_by = auth.uid()))
+    ));
+
+-- scenario_shares — recipient can see their share rows; the scenario
+-- owner (or legacy editor) can manage them.
+ALTER TABLE paliad.scenario_shares ENABLE ROW LEVEL SECURITY;
+
+CREATE POLICY scenario_shares_select ON paliad.scenario_shares
+    FOR SELECT
+    USING (
+        shared_with_user_id = auth.uid()
+        OR EXISTS (
+            SELECT 1 FROM paliad.scenarios s
+             WHERE s.id = scenario_id
+               AND (s.owner_id = auth.uid()
+                 OR (s.owner_id IS NULL AND s.project_id IS NOT NULL AND paliad.can_see_project(s.project_id))
+                 OR (s.owner_id IS NULL AND s.project_id IS NULL AND s.created_by = auth.uid()))
+        )
+    );
+
+CREATE POLICY scenario_shares_mutate ON paliad.scenario_shares
+    FOR ALL
+    USING (EXISTS (
+        SELECT 1 FROM paliad.scenarios s
+         WHERE s.id = scenario_id
+           AND (s.owner_id = auth.uid()
+             OR (s.owner_id IS NULL AND s.project_id IS NOT NULL AND paliad.can_see_project(s.project_id))
+             OR (s.owner_id IS NULL AND s.project_id IS NULL AND s.created_by = auth.uid()))
+    ))
+    WITH CHECK (EXISTS (
+        SELECT 1 FROM paliad.scenarios s
+         WHERE s.id = scenario_id
+           AND (s.owner_id = auth.uid()
+             OR (s.owner_id IS NULL AND s.project_id IS NOT NULL AND paliad.can_see_project(s.project_id))
+             OR (s.owner_id IS NULL AND s.project_id IS NULL AND s.created_by = auth.uid()))
+    ));
+
+-- ----------------------------------------------------------------
+-- 8. updated_at triggers on the new tables (reuse the function mig 145
+--    already created for paliad.scenarios).
+-- ----------------------------------------------------------------
+
+CREATE TRIGGER scenario_proceedings_touch_updated_at_trg
+    BEFORE UPDATE ON paliad.scenario_proceedings
+    FOR EACH ROW
+    EXECUTE FUNCTION paliad.scenarios_touch_updated_at();
+
+CREATE TRIGGER scenario_events_touch_updated_at_trg
+    BEFORE UPDATE ON paliad.scenario_events
+    FOR EACH ROW
+    EXECUTE FUNCTION paliad.scenarios_touch_updated_at();
+
+-- ----------------------------------------------------------------
+-- 9. Informational NOTICE.
+-- ----------------------------------------------------------------
+
+DO $$
+BEGIN
+    RAISE NOTICE '[mig 157] paliad.scenarios extended with builder columns (0 legacy rows affected)';
+    RAISE NOTICE '[mig 157] paliad.scenario_proceedings created';
+    RAISE NOTICE '[mig 157] paliad.scenario_events created';
+    RAISE NOTICE '[mig 157] paliad.scenario_shares created';
+    RAISE NOTICE '[mig 157] paliad.projects.origin_scenario_id added';
+    RAISE NOTICE '[mig 157] paliad.can_see_scenario(uuid) created';
+END $$;
+
+COMMIT;
diff --git a/internal/handlers/handlers.go b/internal/handlers/handlers.go
index d3203d4..2d9b5a5 100644
--- a/internal/handlers/handlers.go
+++ b/internal/handlers/handlers.go
@@ -142,6 +142,12 @@ type Services struct {
 	// and per-rule selection state (`rule:<uuid>` keys).
 	ScenarioFlags *services.ScenarioFlagsService
 
+	// t-paliad-340 / m/paliad#153 B0 — Litigation Builder. CRUD over the
+	// new normalised scenario shape (paliad.scenarios with owner_id +
+	// scenario_proceedings + scenario_events + scenario_shares, mig 157).
+	// Nil when DATABASE_URL is unset — /api/builder/scenarios* routes 503.
+	ScenarioBuilder *services.ScenarioBuilderService
+
 	// Paliadin is wired when DATABASE_URL is set. The concrete backend
 	// is picked in cmd/server/main.go based on PALIADIN_REMOTE_HOST
 	// (remote → mRiver via SSH) or local tmux availability. Stays nil
@@ -212,6 +218,7 @@ func Register(mux *http.ServeMux, client *auth.Client, giteaAPIToken string, svc
 			eventChoice:       svc.EventChoice,
 			scenario:          svc.Scenario,
 			scenarioFlags:     svc.ScenarioFlags,
+			scenarioBuilder:   svc.ScenarioBuilder,
 		}
 	}
 
@@ -514,6 +521,25 @@ func Register(mux *http.ServeMux, client *auth.Client, giteaAPIToken string, svc
 	protected.HandleFunc("DELETE /api/scenarios/{id}", handleScenarioDelete)
 	protected.HandleFunc("PUT /api/projects/{id}/active-scenario", handleSetActiveScenario)
 
+	// t-paliad-340 / m/paliad#153 B0 — Litigation Builder API over the
+	// new normalised scenario shape (mig 157). Coexists with the legacy
+	// /api/scenarios surface during the B0→B6 migration; B6 cleanup
+	// retires the legacy routes.
+	protected.HandleFunc("GET /api/builder/scenarios", handleBuilderScenariosList)
+	protected.HandleFunc("POST /api/builder/scenarios", handleBuilderScenarioCreate)
+	protected.HandleFunc("GET /api/builder/scenarios/{id}", handleBuilderScenarioGet)
+	protected.HandleFunc("PATCH /api/builder/scenarios/{id}", handleBuilderScenarioPatch)
+	protected.HandleFunc("POST /api/builder/scenarios/{id}/proceedings", handleBuilderProceedingCreate)
+	protected.HandleFunc("PATCH /api/builder/scenarios/{id}/proceedings/{pid}", handleBuilderProceedingPatch)
+	protected.HandleFunc("DELETE /api/builder/scenarios/{id}/proceedings/{pid}", handleBuilderProceedingDelete)
+	protected.HandleFunc("POST /api/builder/scenarios/{id}/proceedings/{pid}/events", handleBuilderEventCreate)
+	protected.HandleFunc("PATCH /api/builder/scenarios/{id}/events/{eid}", handleBuilderEventPatch)
+	protected.HandleFunc("DELETE /api/builder/scenarios/{id}/events/{eid}", handleBuilderEventDelete)
+	protected.HandleFunc("POST /api/builder/scenarios/{id}/shares", handleBuilderShareCreate)
+	protected.HandleFunc("DELETE /api/builder/scenarios/{id}/shares/{sid}", handleBuilderShareDelete)
+	// Dev-only test route — gated to PaliadinOwnerEmail (m).
+	protected.HandleFunc("GET /dev/scenario-builder", handleBuilderDevTestPage)
+
 	// Partner units (structural partner-led units; legacy "Dezernate").
 	protected.HandleFunc("GET /api/partner-units", handleListPartnerUnits)
 	protected.HandleFunc("POST /api/partner-units", handleCreatePartnerUnit)
diff --git a/internal/handlers/projects.go b/internal/handlers/projects.go
index 2e252d7..abee223 100644
--- a/internal/handlers/projects.go
+++ b/internal/handlers/projects.go
@@ -85,6 +85,11 @@ type dbServices struct {
 
 	// m/paliad#149 Phase 2 P0 — per-project scenario_flags SSoT (mig 154).
 	scenarioFlags *services.ScenarioFlagsService
+
+	// t-paliad-340 / m/paliad#153 B0 — Litigation Builder over the new
+	// normalised scenario shape (paliad.scenarios with owner_id +
+	// scenario_proceedings + scenario_events + scenario_shares, mig 157).
+	scenarioBuilder *services.ScenarioBuilderService
 }
 
 var dbSvc *dbServices
diff --git a/internal/handlers/scenario_builder.go b/internal/handlers/scenario_builder.go
new file mode 100644
index 0000000..8e2022b
--- /dev/null
+++ b/internal/handlers/scenario_builder.go
@@ -0,0 +1,589 @@
+package handlers
+
+import (
+	"encoding/json"
+	"errors"
+	"net/http"
+
+	"github.com/google/uuid"
+
+	"mgit.msbls.de/m/paliad/internal/services"
+)
+
+// t-paliad-340 / m/paliad#153 B0 — REST endpoints over the new normalised
+// scenario builder shape (paliad.scenarios with owner_id, +
+// paliad.scenario_proceedings / scenario_events / scenario_shares).
+//
+// Endpoints live under /api/builder/scenarios/* to avoid clashing with
+// the legacy /api/scenarios/* endpoints from m/paliad#124 Slice D. The
+// B6 cleanup slice retires the legacy surface; until then both shapes
+// coexist on the same paliad.scenarios table (the legacy paths require
+// project_id IS NOT NULL OR an abstract created_by = caller; the builder
+// paths require owner_id = caller).
+//
+// All handlers gate by requireScenarioBuilderService — 503 when the
+// service is nil (DATABASE_URL unset). Auth is checked via requireUser;
+// per-row visibility is enforced inside the service.
+
+func requireScenarioBuilderService(w http.ResponseWriter) bool {
+	if dbSvc == nil || dbSvc.scenarioBuilder == nil {
+		writeJSON(w, http.StatusServiceUnavailable, map[string]string{
+			"error": "Litigation-Builder ist vorübergehend nicht verfügbar (keine Datenbank).",
+		})
+		return false
+	}
+	return true
+}
+
+// scenarioBuilderErrorToStatus maps service errors to HTTP statuses.
+func scenarioBuilderErrorToStatus(err error) (int, string) {
+	switch {
+	case errors.Is(err, services.ErrScenarioBuilderNotVisible),
+		errors.Is(err, services.ErrNotVisible):
+		return http.StatusNotFound, "Szenario nicht gefunden"
+	case errors.Is(err, services.ErrInvalidInput):
+		return http.StatusBadRequest, err.Error()
+	}
+	return http.StatusInternalServerError, err.Error()
+}
+
+func writeBuilderError(w http.ResponseWriter, err error) {
+	status, msg := scenarioBuilderErrorToStatus(err)
+	writeJSON(w, status, map[string]string{"error": msg})
+}
+
+// ---------------------------------------------------------------------------
+// Scenario CRUD
+// ---------------------------------------------------------------------------
+
+// handleBuilderScenariosList — GET /api/builder/scenarios?status=<active|archived|promoted|all>
+func handleBuilderScenariosList(w http.ResponseWriter, r *http.Request) {
+	if !requireScenarioBuilderService(w) {
+		return
+	}
+	uid, ok := requireUser(w, r)
+	if !ok {
+		return
+	}
+	status := r.URL.Query().Get("status")
+	out, err := dbSvc.scenarioBuilder.ListMyScenarios(r.Context(), uid, status)
+	if err != nil {
+		writeBuilderError(w, err)
+		return
+	}
+	writeJSON(w, http.StatusOK, out)
+}
+
+// handleBuilderScenarioCreate — POST /api/builder/scenarios
+func handleBuilderScenarioCreate(w http.ResponseWriter, r *http.Request) {
+	if !requireScenarioBuilderService(w) {
+		return
+	}
+	uid, ok := requireUser(w, r)
+	if !ok {
+		return
+	}
+	var input services.CreateBuilderScenarioInput
+	if err := json.NewDecoder(r.Body).Decode(&input); err != nil {
+		writeJSON(w, http.StatusBadRequest, map[string]string{"error": "ungültige Anfrage"})
+		return
+	}
+	out, err := dbSvc.scenarioBuilder.CreateScenario(r.Context(), uid, input)
+	if err != nil {
+		writeBuilderError(w, err)
+		return
+	}
+	writeJSON(w, http.StatusCreated, out)
+}
+
+// handleBuilderScenarioGet — GET /api/builder/scenarios/{id}
+func handleBuilderScenarioGet(w http.ResponseWriter, r *http.Request) {
+	if !requireScenarioBuilderService(w) {
+		return
+	}
+	uid, ok := requireUser(w, r)
+	if !ok {
+		return
+	}
+	id, err := uuid.Parse(r.PathValue("id"))
+	if err != nil {
+		writeJSON(w, http.StatusBadRequest, map[string]string{"error": "ungültige ID"})
+		return
+	}
+	out, err := dbSvc.scenarioBuilder.GetScenarioDeep(r.Context(), uid, id)
+	if err != nil {
+		writeBuilderError(w, err)
+		return
+	}
+	writeJSON(w, http.StatusOK, out)
+}
+
+// handleBuilderScenarioPatch — PATCH /api/builder/scenarios/{id}
+func handleBuilderScenarioPatch(w http.ResponseWriter, r *http.Request) {
+	if !requireScenarioBuilderService(w) {
+		return
+	}
+	uid, ok := requireUser(w, r)
+	if !ok {
+		return
+	}
+	id, err := uuid.Parse(r.PathValue("id"))
+	if err != nil {
+		writeJSON(w, http.StatusBadRequest, map[string]string{"error": "ungültige ID"})
+		return
+	}
+	var input services.PatchBuilderScenarioInput
+	if err := json.NewDecoder(r.Body).Decode(&input); err != nil {
+		writeJSON(w, http.StatusBadRequest, map[string]string{"error": "ungültige Anfrage"})
+		return
+	}
+	out, err := dbSvc.scenarioBuilder.PatchScenario(r.Context(), uid, id, input)
+	if err != nil {
+		writeBuilderError(w, err)
+		return
+	}
+	writeJSON(w, http.StatusOK, out)
+}
+
+// ---------------------------------------------------------------------------
+// Proceedings
+// ---------------------------------------------------------------------------
+
+// handleBuilderProceedingCreate — POST /api/builder/scenarios/{id}/proceedings
+func handleBuilderProceedingCreate(w http.ResponseWriter, r *http.Request) {
+	if !requireScenarioBuilderService(w) {
+		return
+	}
+	uid, ok := requireUser(w, r)
+	if !ok {
+		return
+	}
+	sid, err := uuid.Parse(r.PathValue("id"))
+	if err != nil {
+		writeJSON(w, http.StatusBadRequest, map[string]string{"error": "ungültige Szenario-ID"})
+		return
+	}
+	var input services.AddProceedingInput
+	if err := json.NewDecoder(r.Body).Decode(&input); err != nil {
+		writeJSON(w, http.StatusBadRequest, map[string]string{"error": "ungültige Anfrage"})
+		return
+	}
+	out, err := dbSvc.scenarioBuilder.AddProceeding(r.Context(), uid, sid, input)
+	if err != nil {
+		writeBuilderError(w, err)
+		return
+	}
+	writeJSON(w, http.StatusCreated, out)
+}
+
+// handleBuilderProceedingPatch — PATCH /api/builder/scenarios/{id}/proceedings/{pid}
+func handleBuilderProceedingPatch(w http.ResponseWriter, r *http.Request) {
+	if !requireScenarioBuilderService(w) {
+		return
+	}
+	uid, ok := requireUser(w, r)
+	if !ok {
+		return
+	}
+	sid, err := uuid.Parse(r.PathValue("id"))
+	if err != nil {
+		writeJSON(w, http.StatusBadRequest, map[string]string{"error": "ungültige Szenario-ID"})
+		return
+	}
+	pid, err := uuid.Parse(r.PathValue("pid"))
+	if err != nil {
+		writeJSON(w, http.StatusBadRequest, map[string]string{"error": "ungültige Proceeding-ID"})
+		return
+	}
+	var input services.PatchProceedingInput
+	if err := json.NewDecoder(r.Body).Decode(&input); err != nil {
+		writeJSON(w, http.StatusBadRequest, map[string]string{"error": "ungültige Anfrage"})
+		return
+	}
+	out, err := dbSvc.scenarioBuilder.PatchProceeding(r.Context(), uid, sid, pid, input)
+	if err != nil {
+		writeBuilderError(w, err)
+		return
+	}
+	writeJSON(w, http.StatusOK, out)
+}
+
+// handleBuilderProceedingDelete — DELETE /api/builder/scenarios/{id}/proceedings/{pid}
+func handleBuilderProceedingDelete(w http.ResponseWriter, r *http.Request) {
+	if !requireScenarioBuilderService(w) {
+		return
+	}
+	uid, ok := requireUser(w, r)
+	if !ok {
+		return
+	}
+	sid, err := uuid.Parse(r.PathValue("id"))
+	if err != nil {
+		writeJSON(w, http.StatusBadRequest, map[string]string{"error": "ungültige Szenario-ID"})
+		return
+	}
+	pid, err := uuid.Parse(r.PathValue("pid"))
+	if err != nil {
+		writeJSON(w, http.StatusBadRequest, map[string]string{"error": "ungültige Proceeding-ID"})
+		return
+	}
+	if err := dbSvc.scenarioBuilder.DeleteProceeding(r.Context(), uid, sid, pid); err != nil {
+		writeBuilderError(w, err)
+		return
+	}
+	w.WriteHeader(http.StatusNoContent)
+}
+
+// ---------------------------------------------------------------------------
+// Events
+// ---------------------------------------------------------------------------
+
+// handleBuilderEventCreate — POST /api/builder/scenarios/{id}/proceedings/{pid}/events
+func handleBuilderEventCreate(w http.ResponseWriter, r *http.Request) {
+	if !requireScenarioBuilderService(w) {
+		return
+	}
+	uid, ok := requireUser(w, r)
+	if !ok {
+		return
+	}
+	sid, err := uuid.Parse(r.PathValue("id"))
+	if err != nil {
+		writeJSON(w, http.StatusBadRequest, map[string]string{"error": "ungültige Szenario-ID"})
+		return
+	}
+	pid, err := uuid.Parse(r.PathValue("pid"))
+	if err != nil {
+		writeJSON(w, http.StatusBadRequest, map[string]string{"error": "ungültige Proceeding-ID"})
+		return
+	}
+	var input services.AddEventInput
+	if err := json.NewDecoder(r.Body).Decode(&input); err != nil {
+		writeJSON(w, http.StatusBadRequest, map[string]string{"error": "ungültige Anfrage"})
+		return
+	}
+	out, err := dbSvc.scenarioBuilder.AddEvent(r.Context(), uid, sid, pid, input)
+	if err != nil {
+		writeBuilderError(w, err)
+		return
+	}
+	writeJSON(w, http.StatusCreated, out)
+}
+
+// handleBuilderEventPatch — PATCH /api/builder/scenarios/{id}/events/{eid}
+func handleBuilderEventPatch(w http.ResponseWriter, r *http.Request) {
+	if !requireScenarioBuilderService(w) {
+		return
+	}
+	uid, ok := requireUser(w, r)
+	if !ok {
+		return
+	}
+	sid, err := uuid.Parse(r.PathValue("id"))
+	if err != nil {
+		writeJSON(w, http.StatusBadRequest, map[string]string{"error": "ungültige Szenario-ID"})
+		return
+	}
+	eid, err := uuid.Parse(r.PathValue("eid"))
+	if err != nil {
+		writeJSON(w, http.StatusBadRequest, map[string]string{"error": "ungültige Event-ID"})
+		return
+	}
+	var input services.PatchEventInput
+	if err := json.NewDecoder(r.Body).Decode(&input); err != nil {
+		writeJSON(w, http.StatusBadRequest, map[string]string{"error": "ungültige Anfrage"})
+		return
+	}
+	out, err := dbSvc.scenarioBuilder.PatchEvent(r.Context(), uid, sid, eid, input)
+	if err != nil {
+		writeBuilderError(w, err)
+		return
+	}
+	writeJSON(w, http.StatusOK, out)
+}
+
+// handleBuilderEventDelete — DELETE /api/builder/scenarios/{id}/events/{eid}
+func handleBuilderEventDelete(w http.ResponseWriter, r *http.Request) {
+	if !requireScenarioBuilderService(w) {
+		return
+	}
+	uid, ok := requireUser(w, r)
+	if !ok {
+		return
+	}
+	sid, err := uuid.Parse(r.PathValue("id"))
+	if err != nil {
+		writeJSON(w, http.StatusBadRequest, map[string]string{"error": "ungültige Szenario-ID"})
+		return
+	}
+	eid, err := uuid.Parse(r.PathValue("eid"))
+	if err != nil {
+		writeJSON(w, http.StatusBadRequest, map[string]string{"error": "ungültige Event-ID"})
+		return
+	}
+	if err := dbSvc.scenarioBuilder.DeleteEvent(r.Context(), uid, sid, eid); err != nil {
+		writeBuilderError(w, err)
+		return
+	}
+	w.WriteHeader(http.StatusNoContent)
+}
+
+// ---------------------------------------------------------------------------
+// Shares
+// ---------------------------------------------------------------------------
+
+// handleBuilderShareCreate — POST /api/builder/scenarios/{id}/shares
+// Body: {"shared_with_user_id": "<uuid>"}
+func handleBuilderShareCreate(w http.ResponseWriter, r *http.Request) {
+	if !requireScenarioBuilderService(w) {
+		return
+	}
+	uid, ok := requireUser(w, r)
+	if !ok {
+		return
+	}
+	sid, err := uuid.Parse(r.PathValue("id"))
+	if err != nil {
+		writeJSON(w, http.StatusBadRequest, map[string]string{"error": "ungültige Szenario-ID"})
+		return
+	}
+	var body struct {
+		SharedWithUserID uuid.UUID `json:"shared_with_user_id"`
+	}
+	if err := json.NewDecoder(r.Body).Decode(&body); err != nil {
+		writeJSON(w, http.StatusBadRequest, map[string]string{"error": "ungültige Anfrage"})
+		return
+	}
+	out, err := dbSvc.scenarioBuilder.AddShare(r.Context(), uid, sid, body.SharedWithUserID)
+	if err != nil {
+		writeBuilderError(w, err)
+		return
+	}
+	writeJSON(w, http.StatusCreated, out)
+}
+
+// handleBuilderShareDelete — DELETE /api/builder/scenarios/{id}/shares/{sid}
+func handleBuilderShareDelete(w http.ResponseWriter, r *http.Request) {
+	if !requireScenarioBuilderService(w) {
+		return
+	}
+	uid, ok := requireUser(w, r)
+	if !ok {
+		return
+	}
+	scid, err := uuid.Parse(r.PathValue("id"))
+	if err != nil {
+		writeJSON(w, http.StatusBadRequest, map[string]string{"error": "ungültige Szenario-ID"})
+		return
+	}
+	shid, err := uuid.Parse(r.PathValue("sid"))
+	if err != nil {
+		writeJSON(w, http.StatusBadRequest, map[string]string{"error": "ungültige Share-ID"})
+		return
+	}
+	if err := dbSvc.scenarioBuilder.DeleteShare(r.Context(), uid, scid, shid); err != nil {
+		writeBuilderError(w, err)
+		return
+	}
+	w.WriteHeader(http.StatusNoContent)
+}
+
+// ---------------------------------------------------------------------------
+// Dev-only test route
+// ---------------------------------------------------------------------------
+
+// handleBuilderDevTestPage — GET /dev/scenario-builder
+//
+// Gated to services.PaliadinOwnerEmail (the same single-owner gate the
+// /paliadin route uses). Every other authenticated user gets 404. Pure
+// HTML — no JS bundle — so the page works even before B1 wires the real
+// builder shell. Renders curl-equivalent forms for the B0 surface so the
+// schema can be exercised end-to-end without Postman / shell scripts.
+//
+// This is the "dev-only test route" the head's task spec asked for. It
+// disappears in B6 cleanup once the production builder UI ships at
+// /tools/procedures.
+func handleBuilderDevTestPage(w http.ResponseWriter, r *http.Request) {
+	if !requirePaliadinOwner(w, r) {
+		return
+	}
+	w.Header().Set("Content-Type", "text/html; charset=utf-8")
+	w.Header().Set("Cache-Control", "no-store")
+	_, _ = w.Write([]byte(builderDevTestHTML))
+}
+
+const builderDevTestHTML = `<!DOCTYPE html>
+<html lang="de">
+<head>
+<meta charset="utf-8">
+<title>Scenario Builder — Dev Test (B0)</title>
+<style>
+body { font-family: ui-monospace, Menlo, monospace; max-width: 880px; margin: 2em auto;
+       padding: 0 1em; color: #222; background: #fafaf7; }
+h1, h2 { font-family: ui-sans-serif, system-ui, sans-serif; }
+h1 { border-bottom: 4px solid #c6f41c; padding-bottom: .2em; }
+section { background: #fff; border: 1px solid #ddd; border-radius: 4px;
+          padding: 1em 1.2em; margin: 1em 0; }
+label { display: block; margin: .4em 0 .15em; font-size: .85em; color: #555; }
+input, textarea, select, button { font: inherit; padding: .35em .5em; box-sizing: border-box; }
+input[type="text"], input[type="number"], textarea { width: 100%; }
+button { background: #c6f41c; border: 1px solid #9ec61f; cursor: pointer;
+         padding: .4em 1em; border-radius: 3px; margin: .2em 0; }
+button.secondary { background: #eee; border-color: #ccc; }
+pre.out { background: #1e1e1e; color: #e6e6e6; padding: .8em 1em; border-radius: 4px;
+          overflow: auto; max-height: 30em; font-size: .85em; }
+.note { color: #777; font-size: .9em; }
+.row { display: flex; gap: .5em; }
+.row > * { flex: 1; }
+</style>
+</head>
+<body>
+<h1>Scenario Builder — Dev Test (B0)</h1>
+<p class="note">t-paliad-340 / m/paliad#153 — DB-only slice. Exercises
+paliad.scenarios (builder rows), scenario_proceedings, scenario_events,
+scenario_shares via /api/builder/scenarios/*. Gated to PaliadinOwnerEmail.</p>
+
+<section>
+  <h2>1. Liste meine Szenarien</h2>
+  <label>Status filter</label>
+  <select id="list-status">
+    <option value="">(default: alle)</option>
+    <option value="active">active</option>
+    <option value="archived">archived</option>
+    <option value="promoted">promoted</option>
+    <option value="all">all (explicit)</option>
+  </select>
+  <button onclick="listScenarios()">GET /api/builder/scenarios</button>
+  <pre class="out" id="list-out"></pre>
+</section>
+
+<section>
+  <h2>2. Szenario anlegen</h2>
+  <label>Name</label>
+  <input type="text" id="create-name" placeholder="(leer = Unbenanntes Szenario)">
+  <label>Notes (optional)</label>
+  <textarea id="create-notes" rows="2"></textarea>
+  <button onclick="createScenario()">POST /api/builder/scenarios</button>
+  <pre class="out" id="create-out"></pre>
+</section>
+
+<section>
+  <h2>3. Szenario abrufen (deep)</h2>
+  <label>Scenario ID</label>
+  <input type="text" id="get-id">
+  <button onclick="getScenario()">GET /api/builder/scenarios/{id}</button>
+  <pre class="out" id="get-out"></pre>
+</section>
+
+<section>
+  <h2>4. Verfahren hinzufügen</h2>
+  <label>Scenario ID</label>
+  <input type="text" id="proc-sid">
+  <label>proceeding_type_id (integer)</label>
+  <input type="number" id="proc-pt-id" placeholder="z.B. 7 für upc.inf.cfi">
+  <label>primary_party</label>
+  <select id="proc-party">
+    <option value="">(none)</option>
+    <option value="claimant">claimant</option>
+    <option value="defendant">defendant</option>
+  </select>
+  <button onclick="addProceeding()">POST .../proceedings</button>
+  <pre class="out" id="proc-out"></pre>
+</section>
+
+<section>
+  <h2>5. Event-Karte hinzufügen</h2>
+  <label>Scenario ID</label>
+  <input type="text" id="ev-sid">
+  <label>Proceeding ID</label>
+  <input type="text" id="ev-pid">
+  <label>custom_label (oder sequencing_rule_id / procedural_event_id)</label>
+  <input type="text" id="ev-label" placeholder="freitext-Karte">
+  <label>state</label>
+  <select id="ev-state">
+    <option value="planned">planned</option>
+    <option value="filed">filed</option>
+    <option value="skipped">skipped</option>
+  </select>
+  <button onclick="addEvent()">POST .../proceedings/{pid}/events</button>
+  <pre class="out" id="ev-out"></pre>
+</section>
+
+<section>
+  <h2>6. Status patchen (archive / restore)</h2>
+  <label>Scenario ID</label>
+  <input type="text" id="patch-sid">
+  <label>new status</label>
+  <select id="patch-status">
+    <option value="active">active</option>
+    <option value="archived">archived</option>
+  </select>
+  <button onclick="patchStatus()">PATCH /api/builder/scenarios/{id}</button>
+  <pre class="out" id="patch-out"></pre>
+</section>
+
+<script>
+const j = (id, payload) =>
+  document.getElementById(id).textContent = JSON.stringify(payload, null, 2);
+
+async function call(method, url, body) {
+  const opts = { method, headers: { 'Content-Type': 'application/json' } };
+  if (body !== undefined) opts.body = JSON.stringify(body);
+  const r = await fetch(url, opts);
+  const text = await r.text();
+  let parsed = text;
+  try { parsed = JSON.parse(text); } catch (_) {}
+  return { status: r.status, body: parsed };
+}
+
+async function listScenarios() {
+  const status = document.getElementById('list-status').value;
+  const q = status ? '?status=' + encodeURIComponent(status) : '';
+  j('list-out', await call('GET', '/api/builder/scenarios' + q));
+}
+
+async function createScenario() {
+  const name = document.getElementById('create-name').value;
+  const notes = document.getElementById('create-notes').value;
+  const body = {};
+  if (name) body.name = name;
+  if (notes) body.notes = notes;
+  j('create-out', await call('POST', '/api/builder/scenarios', body));
+}
+
+async function getScenario() {
+  const id = document.getElementById('get-id').value.trim();
+  if (!id) return j('get-out', { error: 'ID erforderlich' });
+  j('get-out', await call('GET', '/api/builder/scenarios/' + id));
+}
+
+async function addProceeding() {
+  const sid = document.getElementById('proc-sid').value.trim();
+  const ptID = parseInt(document.getElementById('proc-pt-id').value, 10);
+  const party = document.getElementById('proc-party').value;
+  if (!sid || !ptID) return j('proc-out', { error: 'sid + proceeding_type_id erforderlich' });
+  const body = { proceeding_type_id: ptID };
+  if (party) body.primary_party = party;
+  j('proc-out', await call('POST', '/api/builder/scenarios/' + sid + '/proceedings', body));
+}
+
+async function addEvent() {
+  const sid = document.getElementById('ev-sid').value.trim();
+  const pid = document.getElementById('ev-pid').value.trim();
+  const label = document.getElementById('ev-label').value.trim();
+  const state = document.getElementById('ev-state').value;
+  if (!sid || !pid || !label) return j('ev-out', { error: 'sid + pid + custom_label erforderlich' });
+  j('ev-out', await call('POST',
+    '/api/builder/scenarios/' + sid + '/proceedings/' + pid + '/events',
+    { custom_label: label, state }));
+}
+
+async function patchStatus() {
+  const sid = document.getElementById('patch-sid').value.trim();
+  const status = document.getElementById('patch-status').value;
+  if (!sid) return j('patch-out', { error: 'sid erforderlich' });
+  j('patch-out', await call('PATCH', '/api/builder/scenarios/' + sid, { status }));
+}
+</script>
+</body>
+</html>`
diff --git a/internal/services/scenario_builder_service.go b/internal/services/scenario_builder_service.go
new file mode 100644
index 0000000..df3c6da
--- /dev/null
+++ b/internal/services/scenario_builder_service.go
@@ -0,0 +1,936 @@
+package services
+
+import (
+	"context"
+	"database/sql"
+	"encoding/json"
+	"errors"
+	"fmt"
+	"strings"
+	"time"
+
+	"github.com/google/uuid"
+	"github.com/jmoiron/sqlx"
+)
+
+// ScenarioBuilderService owns the t-paliad-340 / m/paliad#153 B0 surface
+// — CRUD over the new normalised builder shape (paliad.scenarios with
+// owner_id + status, paliad.scenario_proceedings, paliad.scenario_events,
+// paliad.scenario_shares). The legacy spec-jsonb service
+// (ScenarioService) keeps serving m/paliad#124 Slice D callers; this
+// service strictly handles builder-owned rows (owner_id IS NOT NULL).
+//
+// Visibility is enforced both in code (the owner / share / can_see_project
+// fall-through) and at the row level via the migration-157 RLS policies.
+// The application-level check is the load-bearing one — the service
+// connects with the service-role credential, which bypasses RLS.
+type ScenarioBuilderService struct {
+	db *sqlx.DB
+}
+
+// NewScenarioBuilderService wires the service to the shared pool.
+func NewScenarioBuilderService(db *sqlx.DB) *ScenarioBuilderService {
+	return &ScenarioBuilderService{db: db}
+}
+
+// ErrScenarioBuilderNotVisible is returned when the caller is neither
+// owner, an accepted share recipient, nor a global_admin / legacy
+// editor for the scenario.
+var ErrScenarioBuilderNotVisible = errors.New("scenario not visible to caller")
+
+// -----------------------------------------------------------------------------
+// Row types — flat shapes matching the table columns. Deep tree (scenario +
+// proceedings + events) is composed at the GET-by-id endpoint.
+// -----------------------------------------------------------------------------
+
+// BuilderScenario is one paliad.scenarios row from the builder's perspective.
+// Legacy columns (project_id, description, spec, created_by) are still
+// returned so a UI can detect a legacy row and refuse to mutate it.
+type BuilderScenario struct {
+	ID                 uuid.UUID  `db:"id"                   json:"id"`
+	OwnerID            *uuid.UUID `db:"owner_id"             json:"owner_id,omitempty"`
+	Name               string     `db:"name"                 json:"name"`
+	Status             string     `db:"status"               json:"status"`
+	OriginProjectID    *uuid.UUID `db:"origin_project_id"    json:"origin_project_id,omitempty"`
+	PromotedProjectID  *uuid.UUID `db:"promoted_project_id"  json:"promoted_project_id,omitempty"`
+	Stichtag           *time.Time `db:"stichtag"             json:"stichtag,omitempty"`
+	Notes              *string    `db:"notes"                json:"notes,omitempty"`
+	LegacyProjectID    *uuid.UUID `db:"project_id"           json:"legacy_project_id,omitempty"`
+	LegacyDescription  *string    `db:"description"          json:"legacy_description,omitempty"`
+	LegacyCreatedBy    *uuid.UUID `db:"created_by"           json:"legacy_created_by,omitempty"`
+	CreatedAt          time.Time  `db:"created_at"           json:"created_at"`
+	UpdatedAt          time.Time  `db:"updated_at"           json:"updated_at"`
+}
+
+// BuilderProceeding is one paliad.scenario_proceedings row.
+type BuilderProceeding struct {
+	ID                          uuid.UUID       `db:"id"                              json:"id"`
+	ScenarioID                  uuid.UUID       `db:"scenario_id"                     json:"scenario_id"`
+	ProceedingTypeID            int             `db:"proceeding_type_id"              json:"proceeding_type_id"`
+	PrimaryParty                *string         `db:"primary_party"                   json:"primary_party,omitempty"`
+	ScenarioFlags               json.RawMessage `db:"scenario_flags"                  json:"scenario_flags"`
+	ParentScenarioProceedingID  *uuid.UUID      `db:"parent_scenario_proceeding_id"   json:"parent_scenario_proceeding_id,omitempty"`
+	SpawnAnchorEventID          *uuid.UUID      `db:"spawn_anchor_event_id"           json:"spawn_anchor_event_id,omitempty"`
+	Ordinal                     int             `db:"ordinal"                         json:"ordinal"`
+	Stichtag                    *time.Time      `db:"stichtag"                        json:"stichtag,omitempty"`
+	Detailgrad                  string          `db:"detailgrad"                      json:"detailgrad"`
+	AppealTarget                *string         `db:"appeal_target"                   json:"appeal_target,omitempty"`
+	Collapsed                   bool            `db:"collapsed"                       json:"collapsed"`
+	CreatedAt                   time.Time       `db:"created_at"                      json:"created_at"`
+	UpdatedAt                   time.Time       `db:"updated_at"                      json:"updated_at"`
+}
+
+// BuilderEvent is one paliad.scenario_events row.
+type BuilderEvent struct {
+	ID                   uuid.UUID  `db:"id"                       json:"id"`
+	ScenarioProceedingID uuid.UUID  `db:"scenario_proceeding_id"   json:"scenario_proceeding_id"`
+	SequencingRuleID     *uuid.UUID `db:"sequencing_rule_id"       json:"sequencing_rule_id,omitempty"`
+	ProceduralEventID    *uuid.UUID `db:"procedural_event_id"      json:"procedural_event_id,omitempty"`
+	CustomLabel          *string    `db:"custom_label"             json:"custom_label,omitempty"`
+	State                string     `db:"state"                    json:"state"`
+	ActualDate           *time.Time `db:"actual_date"              json:"actual_date,omitempty"`
+	SkipReason           *string    `db:"skip_reason"              json:"skip_reason,omitempty"`
+	Notes                *string    `db:"notes"                    json:"notes,omitempty"`
+	HorizonOptional      int        `db:"horizon_optional"         json:"horizon_optional"`
+	CreatedAt            time.Time  `db:"created_at"               json:"created_at"`
+	UpdatedAt            time.Time  `db:"updated_at"               json:"updated_at"`
+}
+
+// BuilderShare is one paliad.scenario_shares row.
+type BuilderShare struct {
+	ID               uuid.UUID `db:"id"                      json:"id"`
+	ScenarioID       uuid.UUID `db:"scenario_id"             json:"scenario_id"`
+	SharedWithUserID uuid.UUID `db:"shared_with_user_id"     json:"shared_with_user_id"`
+	CreatedBy        uuid.UUID `db:"created_by"              json:"created_by"`
+	CreatedAt        time.Time `db:"created_at"              json:"created_at"`
+}
+
+// BuilderScenarioDeep bundles a scenario with its proceedings + events
+// for the GET /api/builder/scenarios/{id} response. Proceedings sort by
+// ordinal asc; events sort by created_at asc within a proceeding.
+type BuilderScenarioDeep struct {
+	BuilderScenario
+	Proceedings []BuilderProceeding `json:"proceedings"`
+	Events      []BuilderEvent      `json:"events"`
+	Shares      []BuilderShare      `json:"shares"`
+}
+
+// -----------------------------------------------------------------------------
+// Scenario CRUD
+// -----------------------------------------------------------------------------
+
+// CreateBuilderScenarioInput is the POST /api/builder/scenarios body.
+// Name defaults to "Unbenanntes Szenario" when blank (PRD §5.1).
+type CreateBuilderScenarioInput struct {
+	Name            string     `json:"name,omitempty"`
+	Stichtag        *time.Time `json:"stichtag,omitempty"`
+	Notes           *string    `json:"notes,omitempty"`
+	OriginProjectID *uuid.UUID `json:"origin_project_id,omitempty"`
+}
+
+// CreateScenario inserts a new builder-owned scenario. owner_id is set to
+// the caller; status defaults to 'active'. Audit reason is set inside the
+// write tx so any future audit trigger picks it up.
+func (s *ScenarioBuilderService) CreateScenario(ctx context.Context, userID uuid.UUID, input CreateBuilderScenarioInput) (*BuilderScenario, error) {
+	name := strings.TrimSpace(input.Name)
+	if name == "" {
+		name = "Unbenanntes Szenario"
+	}
+
+	var out BuilderScenario
+	err := s.withAuditTx(ctx, "scenario_builder: create scenario", func(tx *sqlx.Tx) error {
+		return tx.GetContext(ctx, &out,
+			`INSERT INTO paliad.scenarios
+			    (owner_id, name, status, stichtag, notes, origin_project_id)
+			 VALUES ($1, $2, 'active', $3, $4, $5)
+			 RETURNING id, owner_id, name, status, origin_project_id, promoted_project_id,
+			           stichtag, notes,
+			           project_id, description, created_by,
+			           created_at, updated_at`,
+			userID, name, input.Stichtag, input.Notes, input.OriginProjectID)
+	})
+	if err != nil {
+		return nil, fmt.Errorf("create builder scenario: %w", err)
+	}
+	return &out, nil
+}
+
+// ListMyScenarios returns the caller's owned scenarios filtered by status.
+// Status "" (or "all") returns every status; otherwise filters by the
+// given enum value. Sorted by updated_at desc.
+func (s *ScenarioBuilderService) ListMyScenarios(ctx context.Context, userID uuid.UUID, status string) ([]BuilderScenario, error) {
+	switch status {
+	case "", "all":
+		// no filter
+	case "active", "archived", "promoted":
+		// ok
+	default:
+		return nil, fmt.Errorf("%w: status %q must be one of {active,archived,promoted,all}",
+			ErrInvalidInput, status)
+	}
+
+	q := `SELECT id, owner_id, name, status, origin_project_id, promoted_project_id,
+	             stichtag, notes,
+	             project_id, description, created_by,
+	             created_at, updated_at
+	        FROM paliad.scenarios
+	       WHERE owner_id = $1`
+	args := []any{userID}
+	if status != "" && status != "all" {
+		q += ` AND status = $2`
+		args = append(args, status)
+	}
+	q += ` ORDER BY updated_at DESC`
+
+	out := []BuilderScenario{}
+	if err := s.db.SelectContext(ctx, &out, q, args...); err != nil {
+		return nil, fmt.Errorf("list builder scenarios: %w", err)
+	}
+	return out, nil
+}
+
+// GetScenarioDeep returns the scenario + proceedings + events + shares.
+// Visibility: owner, share recipient, global_admin, or legacy editor.
+func (s *ScenarioBuilderService) GetScenarioDeep(ctx context.Context, userID, scenarioID uuid.UUID) (*BuilderScenarioDeep, error) {
+	sc, err := s.getScenarioRow(ctx, scenarioID)
+	if err != nil {
+		return nil, err
+	}
+	visible, err := s.canSeeScenario(ctx, userID, sc)
+	if err != nil {
+		return nil, err
+	}
+	if !visible {
+		return nil, ErrScenarioBuilderNotVisible
+	}
+
+	deep := &BuilderScenarioDeep{BuilderScenario: *sc}
+
+	if err := s.db.SelectContext(ctx, &deep.Proceedings, `
+		SELECT id, scenario_id, proceeding_type_id, primary_party, scenario_flags,
+		       parent_scenario_proceeding_id, spawn_anchor_event_id, ordinal,
+		       stichtag, detailgrad, appeal_target, collapsed,
+		       created_at, updated_at
+		  FROM paliad.scenario_proceedings
+		 WHERE scenario_id = $1
+		 ORDER BY ordinal ASC, created_at ASC`, scenarioID); err != nil {
+		return nil, fmt.Errorf("load proceedings: %w", err)
+	}
+
+	if err := s.db.SelectContext(ctx, &deep.Events, `
+		SELECT e.id, e.scenario_proceeding_id, e.sequencing_rule_id,
+		       e.procedural_event_id, e.custom_label, e.state, e.actual_date,
+		       e.skip_reason, e.notes, e.horizon_optional,
+		       e.created_at, e.updated_at
+		  FROM paliad.scenario_events e
+		  JOIN paliad.scenario_proceedings sp ON sp.id = e.scenario_proceeding_id
+		 WHERE sp.scenario_id = $1
+		 ORDER BY e.created_at ASC`, scenarioID); err != nil {
+		return nil, fmt.Errorf("load events: %w", err)
+	}
+
+	if err := s.db.SelectContext(ctx, &deep.Shares, `
+		SELECT id, scenario_id, shared_with_user_id, created_by, created_at
+		  FROM paliad.scenario_shares
+		 WHERE scenario_id = $1
+		 ORDER BY created_at ASC`, scenarioID); err != nil {
+		return nil, fmt.Errorf("load shares: %w", err)
+	}
+
+	return deep, nil
+}
+
+// PatchBuilderScenarioInput is the PATCH /api/builder/scenarios/{id} body.
+// Any nil field means "don't change".
+type PatchBuilderScenarioInput struct {
+	Name     *string    `json:"name,omitempty"`
+	Status   *string    `json:"status,omitempty"`
+	Stichtag *time.Time `json:"stichtag,omitempty"`
+	Notes    *string    `json:"notes,omitempty"`
+}
+
+// PatchScenario updates one or more fields. Status flips to 'promoted'
+// are reserved for the B5 wizard (we accept only active⇄archived here).
+func (s *ScenarioBuilderService) PatchScenario(ctx context.Context, userID, scenarioID uuid.UUID, input PatchBuilderScenarioInput) (*BuilderScenario, error) {
+	sc, err := s.requireOwnerOrLegacyEditor(ctx, userID, scenarioID)
+	if err != nil {
+		return nil, err
+	}
+	if sc.Status == "promoted" {
+		return nil, fmt.Errorf("%w: scenario is promoted; mutations are blocked", ErrInvalidInput)
+	}
+
+	if input.Status != nil {
+		switch *input.Status {
+		case "active", "archived":
+			// ok
+		case "promoted":
+			return nil, fmt.Errorf("%w: status='promoted' is set by the promote-to-project wizard, not PATCH",
+				ErrInvalidInput)
+		default:
+			return nil, fmt.Errorf("%w: status %q must be one of {active,archived}",
+				ErrInvalidInput, *input.Status)
+		}
+	}
+
+	sets := []string{}
+	args := []any{}
+	add := func(clause string, val any) {
+		args = append(args, val)
+		sets = append(sets, fmt.Sprintf(clause, len(args)))
+	}
+	if input.Name != nil {
+		n := strings.TrimSpace(*input.Name)
+		if n == "" {
+			return nil, fmt.Errorf("%w: name cannot be blank", ErrInvalidInput)
+		}
+		add("name = $%d", n)
+	}
+	if input.Status != nil {
+		add("status = $%d", *input.Status)
+	}
+	if input.Stichtag != nil {
+		add("stichtag = $%d", *input.Stichtag)
+	}
+	if input.Notes != nil {
+		add("notes = $%d", *input.Notes)
+	}
+	if len(sets) == 0 {
+		return sc, nil
+	}
+
+	args = append(args, scenarioID)
+	q := fmt.Sprintf(`UPDATE paliad.scenarios SET %s
+	    WHERE id = $%d
+	  RETURNING id, owner_id, name, status, origin_project_id, promoted_project_id,
+	            stichtag, notes,
+	            project_id, description, created_by,
+	            created_at, updated_at`,
+		strings.Join(sets, ", "), len(args))
+	var out BuilderScenario
+	err = s.withAuditTx(ctx, "scenario_builder: patch scenario", func(tx *sqlx.Tx) error {
+		return tx.GetContext(ctx, &out, q, args...)
+	})
+	if err != nil {
+		return nil, fmt.Errorf("patch builder scenario: %w", err)
+	}
+	return &out, nil
+}
+
+// -----------------------------------------------------------------------------
+// Proceedings
+// -----------------------------------------------------------------------------
+
+// AddProceedingInput is the POST /api/builder/scenarios/{id}/proceedings body.
+type AddProceedingInput struct {
+	ProceedingTypeID           int             `json:"proceeding_type_id"`
+	PrimaryParty               *string         `json:"primary_party,omitempty"`
+	ScenarioFlags              json.RawMessage `json:"scenario_flags,omitempty"`
+	ParentScenarioProceedingID *uuid.UUID      `json:"parent_scenario_proceeding_id,omitempty"`
+	SpawnAnchorEventID         *uuid.UUID      `json:"spawn_anchor_event_id,omitempty"`
+	Ordinal                    *int            `json:"ordinal,omitempty"`
+	Stichtag                   *time.Time      `json:"stichtag,omitempty"`
+	Detailgrad                 *string         `json:"detailgrad,omitempty"`
+	AppealTarget               *string         `json:"appeal_target,omitempty"`
+}
+
+// AddProceeding appends a proceeding row to the scenario. The caller must
+// own the scenario (or be a legacy editor). Ordinal defaults to max+1.
+func (s *ScenarioBuilderService) AddProceeding(ctx context.Context, userID, scenarioID uuid.UUID, input AddProceedingInput) (*BuilderProceeding, error) {
+	if _, err := s.requireOwnerOrLegacyEditor(ctx, userID, scenarioID); err != nil {
+		return nil, err
+	}
+	if input.ProceedingTypeID == 0 {
+		return nil, fmt.Errorf("%w: proceeding_type_id is required", ErrInvalidInput)
+	}
+	if input.PrimaryParty != nil {
+		switch *input.PrimaryParty {
+		case "claimant", "defendant":
+		default:
+			return nil, fmt.Errorf("%w: primary_party %q must be claimant or defendant",
+				ErrInvalidInput, *input.PrimaryParty)
+		}
+	}
+	detailgrad := "selected"
+	if input.Detailgrad != nil {
+		switch *input.Detailgrad {
+		case "selected", "all_options":
+			detailgrad = *input.Detailgrad
+		default:
+			return nil, fmt.Errorf("%w: detailgrad %q must be selected or all_options",
+				ErrInvalidInput, *input.Detailgrad)
+		}
+	}
+	flags := input.ScenarioFlags
+	if len(flags) == 0 {
+		flags = json.RawMessage(`{}`)
+	}
+
+	// Resolve ordinal: caller's value or max+1 within the same scenario.
+	var ordinal int
+	if input.Ordinal != nil {
+		ordinal = *input.Ordinal
+	} else {
+		if err := s.db.GetContext(ctx, &ordinal,
+			`SELECT COALESCE(MAX(ordinal), -1) + 1
+			   FROM paliad.scenario_proceedings
+			  WHERE scenario_id = $1`, scenarioID); err != nil {
+			return nil, fmt.Errorf("compute ordinal: %w", err)
+		}
+	}
+
+	var out BuilderProceeding
+	err := s.withAuditTx(ctx, "scenario_builder: add proceeding", func(tx *sqlx.Tx) error {
+		if err := tx.GetContext(ctx, &out,
+			`INSERT INTO paliad.scenario_proceedings
+			    (scenario_id, proceeding_type_id, primary_party, scenario_flags,
+			     parent_scenario_proceeding_id, spawn_anchor_event_id, ordinal,
+			     stichtag, detailgrad, appeal_target)
+			 VALUES ($1, $2, $3, $4, $5, $6, $7, $8, $9, $10)
+			 RETURNING id, scenario_id, proceeding_type_id, primary_party, scenario_flags,
+			           parent_scenario_proceeding_id, spawn_anchor_event_id, ordinal,
+			           stichtag, detailgrad, appeal_target, collapsed,
+			           created_at, updated_at`,
+			scenarioID, input.ProceedingTypeID, input.PrimaryParty, []byte(flags),
+			input.ParentScenarioProceedingID, input.SpawnAnchorEventID, ordinal,
+			input.Stichtag, detailgrad, input.AppealTarget); err != nil {
+			return err
+		}
+		// touch the scenario's updated_at so the side panel re-orders correctly.
+		_, err := tx.ExecContext(ctx,
+			`UPDATE paliad.scenarios SET updated_at = now() WHERE id = $1`, scenarioID)
+		return err
+	})
+	if err != nil {
+		return nil, fmt.Errorf("add proceeding: %w", err)
+	}
+	return &out, nil
+}
+
+// PatchProceedingInput accepts a subset of mutable proceeding fields.
+type PatchProceedingInput struct {
+	PrimaryParty  *string         `json:"primary_party,omitempty"`
+	ScenarioFlags json.RawMessage `json:"scenario_flags,omitempty"`
+	Ordinal       *int            `json:"ordinal,omitempty"`
+	Stichtag      *time.Time      `json:"stichtag,omitempty"`
+	Detailgrad    *string         `json:"detailgrad,omitempty"`
+	AppealTarget  *string         `json:"appeal_target,omitempty"`
+	Collapsed     *bool           `json:"collapsed,omitempty"`
+}
+
+// PatchProceeding updates fields on one proceeding row.
+func (s *ScenarioBuilderService) PatchProceeding(ctx context.Context, userID, scenarioID, proceedingID uuid.UUID, input PatchProceedingInput) (*BuilderProceeding, error) {
+	if _, err := s.requireOwnerOrLegacyEditor(ctx, userID, scenarioID); err != nil {
+		return nil, err
+	}
+
+	sets := []string{}
+	args := []any{}
+	add := func(clause string, val any) {
+		args = append(args, val)
+		sets = append(sets, fmt.Sprintf(clause, len(args)))
+	}
+	if input.PrimaryParty != nil {
+		switch *input.PrimaryParty {
+		case "claimant", "defendant", "":
+		default:
+			return nil, fmt.Errorf("%w: primary_party %q invalid", ErrInvalidInput, *input.PrimaryParty)
+		}
+		if *input.PrimaryParty == "" {
+			add("primary_party = $%d", nil)
+		} else {
+			add("primary_party = $%d", *input.PrimaryParty)
+		}
+	}
+	if len(input.ScenarioFlags) > 0 {
+		add("scenario_flags = $%d", []byte(input.ScenarioFlags))
+	}
+	if input.Ordinal != nil {
+		add("ordinal = $%d", *input.Ordinal)
+	}
+	if input.Stichtag != nil {
+		add("stichtag = $%d", *input.Stichtag)
+	}
+	if input.Detailgrad != nil {
+		switch *input.Detailgrad {
+		case "selected", "all_options":
+		default:
+			return nil, fmt.Errorf("%w: detailgrad %q invalid", ErrInvalidInput, *input.Detailgrad)
+		}
+		add("detailgrad = $%d", *input.Detailgrad)
+	}
+	if input.AppealTarget != nil {
+		if *input.AppealTarget == "" {
+			add("appeal_target = $%d", nil)
+		} else {
+			add("appeal_target = $%d", *input.AppealTarget)
+		}
+	}
+	if input.Collapsed != nil {
+		add("collapsed = $%d", *input.Collapsed)
+	}
+	if len(sets) == 0 {
+		// nothing to do — re-fetch and return.
+		return s.getProceedingRow(ctx, scenarioID, proceedingID)
+	}
+
+	args = append(args, proceedingID, scenarioID)
+	q := fmt.Sprintf(`UPDATE paliad.scenario_proceedings SET %s
+	    WHERE id = $%d AND scenario_id = $%d
+	  RETURNING id, scenario_id, proceeding_type_id, primary_party, scenario_flags,
+	            parent_scenario_proceeding_id, spawn_anchor_event_id, ordinal,
+	            stichtag, detailgrad, appeal_target, collapsed,
+	            created_at, updated_at`,
+		strings.Join(sets, ", "), len(args)-1, len(args))
+	var out BuilderProceeding
+	err := s.withAuditTx(ctx, "scenario_builder: patch proceeding", func(tx *sqlx.Tx) error {
+		return tx.GetContext(ctx, &out, q, args...)
+	})
+	if err != nil {
+		if errors.Is(err, sql.ErrNoRows) {
+			return nil, fmt.Errorf("%w: proceeding %s not in scenario %s",
+				ErrNotVisible, proceedingID, scenarioID)
+		}
+		return nil, fmt.Errorf("patch proceeding: %w", err)
+	}
+	return &out, nil
+}
+
+// DeleteProceeding removes a proceeding (and cascades to events + children).
+func (s *ScenarioBuilderService) DeleteProceeding(ctx context.Context, userID, scenarioID, proceedingID uuid.UUID) error {
+	if _, err := s.requireOwnerOrLegacyEditor(ctx, userID, scenarioID); err != nil {
+		return err
+	}
+	var n int64
+	err := s.withAuditTx(ctx, "scenario_builder: delete proceeding", func(tx *sqlx.Tx) error {
+		res, err := tx.ExecContext(ctx,
+			`DELETE FROM paliad.scenario_proceedings
+			  WHERE id = $1 AND scenario_id = $2`,
+			proceedingID, scenarioID)
+		if err != nil {
+			return err
+		}
+		n, _ = res.RowsAffected()
+		return nil
+	})
+	if err != nil {
+		return fmt.Errorf("delete proceeding: %w", err)
+	}
+	if n == 0 {
+		return fmt.Errorf("%w: proceeding %s not in scenario %s",
+			ErrNotVisible, proceedingID, scenarioID)
+	}
+	return nil
+}
+
+// -----------------------------------------------------------------------------
+// Events
+// -----------------------------------------------------------------------------
+
+// AddEventInput is the POST .../proceedings/{pid}/events body. At least
+// one of {SequencingRuleID, ProceduralEventID, CustomLabel} must be set,
+// matching the scenario_events_one_anchor CHECK constraint.
+type AddEventInput struct {
+	SequencingRuleID  *uuid.UUID `json:"sequencing_rule_id,omitempty"`
+	ProceduralEventID *uuid.UUID `json:"procedural_event_id,omitempty"`
+	CustomLabel       *string    `json:"custom_label,omitempty"`
+	State             *string    `json:"state,omitempty"`
+	ActualDate        *time.Time `json:"actual_date,omitempty"`
+	SkipReason        *string    `json:"skip_reason,omitempty"`
+	Notes             *string    `json:"notes,omitempty"`
+	HorizonOptional   *int       `json:"horizon_optional,omitempty"`
+}
+
+// AddEvent inserts an event card under the given proceeding. The
+// proceeding must belong to the addressed scenario.
+func (s *ScenarioBuilderService) AddEvent(ctx context.Context, userID, scenarioID, proceedingID uuid.UUID, input AddEventInput) (*BuilderEvent, error) {
+	if _, err := s.requireOwnerOrLegacyEditor(ctx, userID, scenarioID); err != nil {
+		return nil, err
+	}
+	if input.SequencingRuleID == nil && input.ProceduralEventID == nil &&
+		(input.CustomLabel == nil || strings.TrimSpace(*input.CustomLabel) == "") {
+		return nil, fmt.Errorf("%w: at least one of sequencing_rule_id, procedural_event_id, custom_label must be set",
+			ErrInvalidInput)
+	}
+	if err := s.assertProceedingInScenario(ctx, scenarioID, proceedingID); err != nil {
+		return nil, err
+	}
+
+	state := "planned"
+	if input.State != nil {
+		switch *input.State {
+		case "planned", "filed", "skipped":
+			state = *input.State
+		default:
+			return nil, fmt.Errorf("%w: state %q must be one of {planned,filed,skipped}",
+				ErrInvalidInput, *input.State)
+		}
+	}
+	horizon := 0
+	if input.HorizonOptional != nil {
+		if *input.HorizonOptional < 0 {
+			return nil, fmt.Errorf("%w: horizon_optional must be >= 0", ErrInvalidInput)
+		}
+		horizon = *input.HorizonOptional
+	}
+
+	var out BuilderEvent
+	err := s.withAuditTx(ctx, "scenario_builder: add event", func(tx *sqlx.Tx) error {
+		if err := tx.GetContext(ctx, &out,
+			`INSERT INTO paliad.scenario_events
+			    (scenario_proceeding_id, sequencing_rule_id, procedural_event_id,
+			     custom_label, state, actual_date, skip_reason, notes, horizon_optional)
+			 VALUES ($1, $2, $3, $4, $5, $6, $7, $8, $9)
+			 RETURNING id, scenario_proceeding_id, sequencing_rule_id, procedural_event_id,
+			           custom_label, state, actual_date, skip_reason, notes,
+			           horizon_optional, created_at, updated_at`,
+			proceedingID, input.SequencingRuleID, input.ProceduralEventID,
+			input.CustomLabel, state, input.ActualDate, input.SkipReason,
+			input.Notes, horizon); err != nil {
+			return err
+		}
+		_, err := tx.ExecContext(ctx,
+			`UPDATE paliad.scenarios SET updated_at = now() WHERE id = $1`, scenarioID)
+		return err
+	})
+	if err != nil {
+		return nil, fmt.Errorf("add event: %w", err)
+	}
+	return &out, nil
+}
+
+// PatchEventInput is the PATCH body for an event card.
+type PatchEventInput struct {
+	State           *string    `json:"state,omitempty"`
+	ActualDate      *time.Time `json:"actual_date,omitempty"`
+	SkipReason      *string    `json:"skip_reason,omitempty"`
+	Notes           *string    `json:"notes,omitempty"`
+	HorizonOptional *int       `json:"horizon_optional,omitempty"`
+}
+
+// PatchEvent updates fields on one event card. The card's parent
+// proceeding must belong to the addressed scenario.
+func (s *ScenarioBuilderService) PatchEvent(ctx context.Context, userID, scenarioID, eventID uuid.UUID, input PatchEventInput) (*BuilderEvent, error) {
+	if _, err := s.requireOwnerOrLegacyEditor(ctx, userID, scenarioID); err != nil {
+		return nil, err
+	}
+	if err := s.assertEventInScenario(ctx, scenarioID, eventID); err != nil {
+		return nil, err
+	}
+
+	sets := []string{}
+	args := []any{}
+	add := func(clause string, val any) {
+		args = append(args, val)
+		sets = append(sets, fmt.Sprintf(clause, len(args)))
+	}
+	if input.State != nil {
+		switch *input.State {
+		case "planned", "filed", "skipped":
+		default:
+			return nil, fmt.Errorf("%w: state %q invalid", ErrInvalidInput, *input.State)
+		}
+		add("state = $%d", *input.State)
+	}
+	if input.ActualDate != nil {
+		add("actual_date = $%d", *input.ActualDate)
+	}
+	if input.SkipReason != nil {
+		add("skip_reason = $%d", *input.SkipReason)
+	}
+	if input.Notes != nil {
+		add("notes = $%d", *input.Notes)
+	}
+	if input.HorizonOptional != nil {
+		if *input.HorizonOptional < 0 {
+			return nil, fmt.Errorf("%w: horizon_optional must be >= 0", ErrInvalidInput)
+		}
+		add("horizon_optional = $%d", *input.HorizonOptional)
+	}
+	if len(sets) == 0 {
+		return s.getEventRow(ctx, eventID)
+	}
+
+	args = append(args, eventID)
+	q := fmt.Sprintf(`UPDATE paliad.scenario_events SET %s
+	    WHERE id = $%d
+	  RETURNING id, scenario_proceeding_id, sequencing_rule_id, procedural_event_id,
+	            custom_label, state, actual_date, skip_reason, notes,
+	            horizon_optional, created_at, updated_at`,
+		strings.Join(sets, ", "), len(args))
+	var out BuilderEvent
+	err := s.withAuditTx(ctx, "scenario_builder: patch event", func(tx *sqlx.Tx) error {
+		return tx.GetContext(ctx, &out, q, args...)
+	})
+	if err != nil {
+		return nil, fmt.Errorf("patch event: %w", err)
+	}
+	return &out, nil
+}
+
+// DeleteEvent removes one event card.
+func (s *ScenarioBuilderService) DeleteEvent(ctx context.Context, userID, scenarioID, eventID uuid.UUID) error {
+	if _, err := s.requireOwnerOrLegacyEditor(ctx, userID, scenarioID); err != nil {
+		return err
+	}
+	if err := s.assertEventInScenario(ctx, scenarioID, eventID); err != nil {
+		return err
+	}
+	err := s.withAuditTx(ctx, "scenario_builder: delete event", func(tx *sqlx.Tx) error {
+		_, err := tx.ExecContext(ctx,
+			`DELETE FROM paliad.scenario_events WHERE id = $1`, eventID)
+		return err
+	})
+	if err != nil {
+		return fmt.Errorf("delete event: %w", err)
+	}
+	return nil
+}
+
+// -----------------------------------------------------------------------------
+// Shares
+// -----------------------------------------------------------------------------
+
+// AddShare grants read-only access to another paliad user.
+func (s *ScenarioBuilderService) AddShare(ctx context.Context, userID, scenarioID, recipientID uuid.UUID) (*BuilderShare, error) {
+	if _, err := s.requireOwnerOrLegacyEditor(ctx, userID, scenarioID); err != nil {
+		return nil, err
+	}
+	if recipientID == uuid.Nil {
+		return nil, fmt.Errorf("%w: shared_with_user_id is required", ErrInvalidInput)
+	}
+	if recipientID == userID {
+		return nil, fmt.Errorf("%w: cannot share a scenario with yourself", ErrInvalidInput)
+	}
+	var out BuilderShare
+	err := s.withAuditTx(ctx, "scenario_builder: add share", func(tx *sqlx.Tx) error {
+		return tx.GetContext(ctx, &out,
+			`INSERT INTO paliad.scenario_shares (scenario_id, shared_with_user_id, created_by)
+			 VALUES ($1, $2, $3)
+			 ON CONFLICT (scenario_id, shared_with_user_id) DO UPDATE
+			    SET created_at = paliad.scenario_shares.created_at
+			 RETURNING id, scenario_id, shared_with_user_id, created_by, created_at`,
+			scenarioID, recipientID, userID)
+	})
+	if err != nil {
+		return nil, fmt.Errorf("add share: %w", err)
+	}
+	return &out, nil
+}
+
+// DeleteShare revokes a share row.
+func (s *ScenarioBuilderService) DeleteShare(ctx context.Context, userID, scenarioID, shareID uuid.UUID) error {
+	if _, err := s.requireOwnerOrLegacyEditor(ctx, userID, scenarioID); err != nil {
+		return err
+	}
+	var n int64
+	err := s.withAuditTx(ctx, "scenario_builder: delete share", func(tx *sqlx.Tx) error {
+		res, err := tx.ExecContext(ctx,
+			`DELETE FROM paliad.scenario_shares
+			  WHERE id = $1 AND scenario_id = $2`, shareID, scenarioID)
+		if err != nil {
+			return err
+		}
+		n, _ = res.RowsAffected()
+		return nil
+	})
+	if err != nil {
+		return fmt.Errorf("delete share: %w", err)
+	}
+	if n == 0 {
+		return fmt.Errorf("%w: share %s not in scenario %s", ErrNotVisible, shareID, scenarioID)
+	}
+	return nil
+}
+
+// -----------------------------------------------------------------------------
+// Internal helpers
+// -----------------------------------------------------------------------------
+
+func (s *ScenarioBuilderService) getScenarioRow(ctx context.Context, scenarioID uuid.UUID) (*BuilderScenario, error) {
+	var out BuilderScenario
+	err := s.db.GetContext(ctx, &out,
+		`SELECT id, owner_id, name, status, origin_project_id, promoted_project_id,
+		        stichtag, notes,
+		        project_id, description, created_by,
+		        created_at, updated_at
+		   FROM paliad.scenarios
+		  WHERE id = $1`, scenarioID)
+	if errors.Is(err, sql.ErrNoRows) {
+		return nil, fmt.Errorf("%w: scenario %s not found", ErrNotVisible, scenarioID)
+	}
+	if err != nil {
+		return nil, fmt.Errorf("get scenario: %w", err)
+	}
+	return &out, nil
+}
+
+func (s *ScenarioBuilderService) getProceedingRow(ctx context.Context, scenarioID, proceedingID uuid.UUID) (*BuilderProceeding, error) {
+	var out BuilderProceeding
+	err := s.db.GetContext(ctx, &out,
+		`SELECT id, scenario_id, proceeding_type_id, primary_party, scenario_flags,
+		        parent_scenario_proceeding_id, spawn_anchor_event_id, ordinal,
+		        stichtag, detailgrad, appeal_target, collapsed,
+		        created_at, updated_at
+		   FROM paliad.scenario_proceedings
+		  WHERE id = $1 AND scenario_id = $2`, proceedingID, scenarioID)
+	if errors.Is(err, sql.ErrNoRows) {
+		return nil, fmt.Errorf("%w: proceeding %s not in scenario %s",
+			ErrNotVisible, proceedingID, scenarioID)
+	}
+	if err != nil {
+		return nil, fmt.Errorf("get proceeding: %w", err)
+	}
+	return &out, nil
+}
+
+func (s *ScenarioBuilderService) getEventRow(ctx context.Context, eventID uuid.UUID) (*BuilderEvent, error) {
+	var out BuilderEvent
+	err := s.db.GetContext(ctx, &out,
+		`SELECT id, scenario_proceeding_id, sequencing_rule_id, procedural_event_id,
+		        custom_label, state, actual_date, skip_reason, notes,
+		        horizon_optional, created_at, updated_at
+		   FROM paliad.scenario_events
+		  WHERE id = $1`, eventID)
+	if errors.Is(err, sql.ErrNoRows) {
+		return nil, fmt.Errorf("%w: event %s not found", ErrNotVisible, eventID)
+	}
+	if err != nil {
+		return nil, fmt.Errorf("get event: %w", err)
+	}
+	return &out, nil
+}
+
+func (s *ScenarioBuilderService) assertProceedingInScenario(ctx context.Context, scenarioID, proceedingID uuid.UUID) error {
+	var exists bool
+	if err := s.db.GetContext(ctx, &exists,
+		`SELECT EXISTS (SELECT 1 FROM paliad.scenario_proceedings
+		                 WHERE id = $1 AND scenario_id = $2)`,
+		proceedingID, scenarioID); err != nil {
+		return fmt.Errorf("check proceeding membership: %w", err)
+	}
+	if !exists {
+		return fmt.Errorf("%w: proceeding %s not in scenario %s",
+			ErrNotVisible, proceedingID, scenarioID)
+	}
+	return nil
+}
+
+func (s *ScenarioBuilderService) assertEventInScenario(ctx context.Context, scenarioID, eventID uuid.UUID) error {
+	var exists bool
+	if err := s.db.GetContext(ctx, &exists,
+		`SELECT EXISTS (
+		    SELECT 1 FROM paliad.scenario_events e
+		      JOIN paliad.scenario_proceedings sp ON sp.id = e.scenario_proceeding_id
+		     WHERE e.id = $1 AND sp.scenario_id = $2
+		 )`,
+		eventID, scenarioID); err != nil {
+		return fmt.Errorf("check event membership: %w", err)
+	}
+	if !exists {
+		return fmt.Errorf("%w: event %s not in scenario %s",
+			ErrNotVisible, eventID, scenarioID)
+	}
+	return nil
+}
+
+// canSeeScenario mirrors the SQL paliad.can_see_scenario(...) function in
+// Go. The service connection bypasses RLS, so this check is the
+// authoritative gate.
+func (s *ScenarioBuilderService) canSeeScenario(ctx context.Context, userID uuid.UUID, sc *BuilderScenario) (bool, error) {
+	// owner — fast path
+	if sc.OwnerID != nil && *sc.OwnerID == userID {
+		return true, nil
+	}
+	// global_admin
+	var isAdmin bool
+	if err := s.db.GetContext(ctx, &isAdmin,
+		`SELECT EXISTS (SELECT 1 FROM paliad.users WHERE id = $1 AND global_role = 'global_admin')`,
+		userID); err != nil {
+		return false, fmt.Errorf("check global_admin: %w", err)
+	}
+	if isAdmin {
+		return true, nil
+	}
+	// share recipient
+	var shared bool
+	if err := s.db.GetContext(ctx, &shared,
+		`SELECT EXISTS (SELECT 1 FROM paliad.scenario_shares
+		                 WHERE scenario_id = $1 AND shared_with_user_id = $2)`,
+		sc.ID, userID); err != nil {
+		return false, fmt.Errorf("check share: %w", err)
+	}
+	if shared {
+		return true, nil
+	}
+	// legacy project-scoped — visible via project team membership
+	if sc.OwnerID == nil && sc.LegacyProjectID != nil {
+		var ok bool
+		if err := s.db.GetContext(ctx, &ok,
+			`SELECT paliad.can_see_project($1::uuid)`,
+			*sc.LegacyProjectID); err == nil && ok {
+			return true, nil
+		}
+	}
+	// legacy abstract — owner-only via created_by
+	if sc.OwnerID == nil && sc.LegacyProjectID == nil && sc.LegacyCreatedBy != nil &&
+		*sc.LegacyCreatedBy == userID {
+		return true, nil
+	}
+	return false, nil
+}
+
+// requireOwnerOrLegacyEditor fetches the scenario and validates that the
+// caller has write rights. Returns the loaded row for downstream use.
+func (s *ScenarioBuilderService) requireOwnerOrLegacyEditor(ctx context.Context, userID, scenarioID uuid.UUID) (*BuilderScenario, error) {
+	sc, err := s.getScenarioRow(ctx, scenarioID)
+	if err != nil {
+		return nil, err
+	}
+	// owner
+	if sc.OwnerID != nil && *sc.OwnerID == userID {
+		return sc, nil
+	}
+	// legacy project-scoped editor
+	if sc.OwnerID == nil && sc.LegacyProjectID != nil {
+		var ok bool
+		if err := s.db.GetContext(ctx, &ok,
+			`SELECT paliad.can_see_project($1::uuid)`,
+			*sc.LegacyProjectID); err == nil && ok {
+			return sc, nil
+		}
+	}
+	// legacy abstract creator
+	if sc.OwnerID == nil && sc.LegacyProjectID == nil && sc.LegacyCreatedBy != nil &&
+		*sc.LegacyCreatedBy == userID {
+		return sc, nil
+	}
+	return nil, ErrScenarioBuilderNotVisible
+}
+
+// withAuditTx opens a transaction, stamps paliad.audit_reason via
+// set_config(..., true) so the reason persists for the duration of the
+// tx (matching the mig-079 audit-trigger pattern used by event_choice_
+// service.go), invokes fn, and commits. Any error returned by fn rolls
+// back. The audit reason is appended with the task slug so audit-log
+// readers can trace writes back to t-paliad-340.
+func (s *ScenarioBuilderService) withAuditTx(ctx context.Context, reason string, fn func(tx *sqlx.Tx) error) error {
+	tx, err := s.db.BeginTxx(ctx, nil)
+	if err != nil {
+		return fmt.Errorf("begin tx: %w", err)
+	}
+	defer func() { _ = tx.Rollback() }()
+
+	if _, err := tx.ExecContext(ctx,
+		`SELECT set_config('paliad.audit_reason', $1, true)`,
+		fmt.Sprintf("%s (t-paliad-340)", reason)); err != nil {
+		return fmt.Errorf("set audit_reason: %w", err)
+	}
+	if err := fn(tx); err != nil {
+		return err
+	}
+	if err := tx.Commit(); err != nil {
+		return fmt.Errorf("commit: %w", err)
+	}
+	return nil
+}
diff --git a/internal/services/scenario_builder_service_test.go b/internal/services/scenario_builder_service_test.go
new file mode 100644
index 0000000..5899840
--- /dev/null
+++ b/internal/services/scenario_builder_service_test.go
@@ -0,0 +1,220 @@
+package services
+
+import (
+	"context"
+	"encoding/json"
+	"errors"
+	"os"
+	"testing"
+
+	"github.com/google/uuid"
+	"github.com/jmoiron/sqlx"
+	_ "github.com/lib/pq"
+
+	"mgit.msbls.de/m/paliad/internal/db"
+)
+
+// TestScenarioBuilderService exercises the t-paliad-340 / m/paliad#153 B0
+// surface end-to-end against a live DB: create + list + deep-get + patch
+// + add-proceeding + add-event + add/delete-share, plus the visibility
+// negative case (a non-owner can't see the scenario unless shared).
+//
+// Skipped without TEST_DATABASE_URL — matches the pattern in
+// project_service_test.go / event_choice_service_test.go.
+func TestScenarioBuilderService(t *testing.T) {
+	url := os.Getenv("TEST_DATABASE_URL")
+	if url == "" {
+		t.Skip("TEST_DATABASE_URL not set — skipping live DB test")
+	}
+	if err := db.ApplyMigrations(url); err != nil {
+		t.Fatalf("apply migrations: %v", err)
+	}
+	pool, err := sqlx.Connect("postgres", url)
+	if err != nil {
+		t.Fatalf("connect: %v", err)
+	}
+	defer pool.Close()
+
+	ctx := context.Background()
+
+	owner := uuid.New()
+	other := uuid.New()
+	cleanup := func() {
+		// Cascade order: delete from scenarios → CASCADE clears
+		// proceedings, events, shares. Then the two users.
+		pool.ExecContext(ctx,
+			`DELETE FROM paliad.scenarios WHERE owner_id IN ($1, $2)`, owner, other)
+		pool.ExecContext(ctx,
+			`DELETE FROM paliad.users WHERE id IN ($1, $2)`, owner, other)
+		pool.ExecContext(ctx,
+			`DELETE FROM auth.users WHERE id IN ($1, $2)`, owner, other)
+	}
+	cleanup()
+	defer cleanup()
+
+	for _, seed := range []struct {
+		id    uuid.UUID
+		email string
+		name  string
+	}{
+		{owner, "builder-owner-test@hlc.com", "Builder Owner"},
+		{other, "builder-other-test@hlc.com", "Builder Other"},
+	} {
+		if _, err := pool.ExecContext(ctx,
+			`INSERT INTO auth.users (id, email) VALUES ($1, $2)`,
+			seed.id, seed.email); err != nil {
+			t.Fatalf("seed auth.users %s: %v", seed.email, err)
+		}
+		if _, err := pool.ExecContext(ctx,
+			`INSERT INTO paliad.users (id, email, display_name, office, lang)
+			 VALUES ($1, $2, $3, 'munich', 'de')`,
+			seed.id, seed.email, seed.name); err != nil {
+			t.Fatalf("seed paliad.users %s: %v", seed.email, err)
+		}
+	}
+
+	// Pick a real proceeding_type_id so the FK insert succeeds.
+	var ptID int
+	if err := pool.GetContext(ctx, &ptID,
+		`SELECT id FROM paliad.proceeding_types
+		  WHERE code = $1 AND is_active = true
+		  LIMIT 1`, CodeUPCInfringement); err != nil {
+		t.Fatalf("look up upc.inf id: %v", err)
+	}
+
+	svc := NewScenarioBuilderService(pool)
+
+	// 1. Create a scenario for the owner. Empty name should default.
+	sc, err := svc.CreateScenario(ctx, owner, CreateBuilderScenarioInput{})
+	if err != nil {
+		t.Fatalf("CreateScenario: %v", err)
+	}
+	if sc.Name != "Unbenanntes Szenario" {
+		t.Errorf("default name = %q, want %q", sc.Name, "Unbenanntes Szenario")
+	}
+	if sc.Status != "active" {
+		t.Errorf("default status = %q, want active", sc.Status)
+	}
+	if sc.OwnerID == nil || *sc.OwnerID != owner {
+		t.Errorf("owner_id = %v, want %v", sc.OwnerID, owner)
+	}
+
+	// 2. List — should return the one row.
+	list, err := svc.ListMyScenarios(ctx, owner, "active")
+	if err != nil {
+		t.Fatalf("ListMyScenarios: %v", err)
+	}
+	if len(list) != 1 || list[0].ID != sc.ID {
+		t.Errorf("ListMyScenarios returned %d rows; want 1 with id %s", len(list), sc.ID)
+	}
+
+	// 3. Other user can NOT see the scenario.
+	if _, err := svc.GetScenarioDeep(ctx, other, sc.ID); !errors.Is(err, ErrScenarioBuilderNotVisible) {
+		t.Errorf("GetScenarioDeep by non-owner = %v, want ErrScenarioBuilderNotVisible", err)
+	}
+
+	// 4. Add a proceeding.
+	pr, err := svc.AddProceeding(ctx, owner, sc.ID, AddProceedingInput{
+		ProceedingTypeID: ptID,
+		PrimaryParty:     ptrString("defendant"),
+		ScenarioFlags:    json.RawMessage(`{"with_ccr": true}`),
+	})
+	if err != nil {
+		t.Fatalf("AddProceeding: %v", err)
+	}
+	if pr.ProceedingTypeID != ptID {
+		t.Errorf("ProceedingTypeID = %d, want %d", pr.ProceedingTypeID, ptID)
+	}
+	if pr.PrimaryParty == nil || *pr.PrimaryParty != "defendant" {
+		t.Errorf("PrimaryParty = %v, want defendant", pr.PrimaryParty)
+	}
+
+	// 5. Add a custom-label event card.
+	ev, err := svc.AddEvent(ctx, owner, sc.ID, pr.ID, AddEventInput{
+		CustomLabel: ptrString("Klageerwiderung"),
+		State:       ptrString("planned"),
+	})
+	if err != nil {
+		t.Fatalf("AddEvent: %v", err)
+	}
+	if ev.State != "planned" {
+		t.Errorf("event state = %q, want planned", ev.State)
+	}
+
+	// 5b. Add-event with NO anchor fields fails.
+	if _, err := svc.AddEvent(ctx, owner, sc.ID, pr.ID, AddEventInput{}); !errors.Is(err, ErrInvalidInput) {
+		t.Errorf("AddEvent without anchor = %v, want ErrInvalidInput", err)
+	}
+
+	// 6. Deep get — should bundle the scenario + 1 proceeding + 1 event + 0 shares.
+	deep, err := svc.GetScenarioDeep(ctx, owner, sc.ID)
+	if err != nil {
+		t.Fatalf("GetScenarioDeep: %v", err)
+	}
+	if len(deep.Proceedings) != 1 || deep.Proceedings[0].ID != pr.ID {
+		t.Errorf("deep proceedings count=%d want 1; ids: %+v", len(deep.Proceedings), deep.Proceedings)
+	}
+	if len(deep.Events) != 1 || deep.Events[0].ID != ev.ID {
+		t.Errorf("deep events count=%d want 1; ids: %+v", len(deep.Events), deep.Events)
+	}
+	if len(deep.Shares) != 0 {
+		t.Errorf("deep shares count=%d want 0", len(deep.Shares))
+	}
+
+	// 7. Share with `other`. Recipient should now see the scenario.
+	sh, err := svc.AddShare(ctx, owner, sc.ID, other)
+	if err != nil {
+		t.Fatalf("AddShare: %v", err)
+	}
+	if _, err := svc.GetScenarioDeep(ctx, other, sc.ID); err != nil {
+		t.Errorf("GetScenarioDeep by share recipient: %v", err)
+	}
+	// But the recipient can NOT add proceedings.
+	if _, err := svc.AddProceeding(ctx, other, sc.ID, AddProceedingInput{
+		ProceedingTypeID: ptID,
+	}); !errors.Is(err, ErrScenarioBuilderNotVisible) {
+		t.Errorf("AddProceeding by share recipient = %v, want ErrScenarioBuilderNotVisible", err)
+	}
+
+	// 7b. Self-share should be rejected.
+	if _, err := svc.AddShare(ctx, owner, sc.ID, owner); !errors.Is(err, ErrInvalidInput) {
+		t.Errorf("self-share = %v, want ErrInvalidInput", err)
+	}
+
+	// 8. Patch — archive then re-activate.
+	patched, err := svc.PatchScenario(ctx, owner, sc.ID, PatchBuilderScenarioInput{
+		Status: ptrString("archived"),
+	})
+	if err != nil {
+		t.Fatalf("PatchScenario archive: %v", err)
+	}
+	if patched.Status != "archived" {
+		t.Errorf("after archive, status = %q, want archived", patched.Status)
+	}
+	// PATCH to 'promoted' is rejected — that's the wizard's job.
+	if _, err := svc.PatchScenario(ctx, owner, sc.ID, PatchBuilderScenarioInput{
+		Status: ptrString("promoted"),
+	}); !errors.Is(err, ErrInvalidInput) {
+		t.Errorf("PATCH status=promoted = %v, want ErrInvalidInput", err)
+	}
+	patched, err = svc.PatchScenario(ctx, owner, sc.ID, PatchBuilderScenarioInput{
+		Status: ptrString("active"),
+	})
+	if err != nil {
+		t.Fatalf("PatchScenario re-activate: %v", err)
+	}
+	if patched.Status != "active" {
+		t.Errorf("after re-activate, status = %q, want active", patched.Status)
+	}
+
+	// 9. Revoke the share. Recipient loses visibility.
+	if err := svc.DeleteShare(ctx, owner, sc.ID, sh.ID); err != nil {
+		t.Fatalf("DeleteShare: %v", err)
+	}
+	if _, err := svc.GetScenarioDeep(ctx, other, sc.ID); !errors.Is(err, ErrScenarioBuilderNotVisible) {
+		t.Errorf("after revoke, recipient GetScenarioDeep = %v, want ErrScenarioBuilderNotVisible", err)
+	}
+}
+
+// (Note: ptrString lives in rule_editor_service_test.go in this package
+// and is reused here. No second declaration needed.)