diff --git a/src/activity_core/seed.py b/src/activity_core/seed.py
new file mode 100644
index 0000000..a8ffb22
--- /dev/null
+++ b/src/activity_core/seed.py
@@ -0,0 +1,88 @@
+"""Dev seed script — inserts fixture data for local development.
+
+Usage:
+    ACTCORE_DB_URL=postgresql+asyncpg://actcore:actcore@localhost:5433/actcore \
+        python -m activity_core.seed
+
+Safe to re-run: skips rows that already exist (matched by name).
+Never run against production.
+"""
+
+import asyncio
+import os
+import sys
+import uuid
+
+from sqlalchemy import select
+from sqlalchemy.ext.asyncio import AsyncSession, async_sessionmaker
+
+from activity_core.db import make_engine
+from activity_core.orm import ActivityDefinition  # noqa: F401 — registers metadata
+
+
+SEED_DEFINITIONS = [
+    {
+        "id": uuid.UUID("00000000-0000-0000-0000-000000000001"),
+        "name": "example-heartbeat",
+        "enabled": True,
+        "trigger_type": "cron",
+        "trigger_config": {
+            "trigger_type": "cron",
+            "cron_expression": "* * * * *",  # every minute
+            "timezone": "UTC",
+            "jitter_seconds": 0,
+            "misfire_policy": "skip",
+        },
+        "context_sources": [
+            {
+                "name": "stub",
+                "type": "static",
+                "config": {"value": {"ping": "pong"}},
+            }
+        ],
+        "task_templates": [
+            {
+                "task_type": "log_message",
+                "condition": None,
+                "params_template": {"message": "heartbeat from {context.stub.ping}"},
+            }
+        ],
+        "dedupe_key_strategy": "skip",
+        "version": 1,
+    },
+]
+
+
+async def seed(db_url: str | None = None) -> None:
+    engine = make_engine(db_url)
+    Session = async_sessionmaker(engine, expire_on_commit=False)
+
+    async with Session() as session:
+        async with session.begin():
+            for defn in SEED_DEFINITIONS:
+                existing = await session.scalar(
+                    select(ActivityDefinition).where(
+                        ActivityDefinition.name == defn["name"]
+                    )
+                )
+                if existing:
+                    print(f"  skip  {defn['name']!r} (already exists, id={existing.id})")
+                    continue
+
+                session.add(ActivityDefinition(**defn))
+                print(f"  insert {defn['name']!r} (id={defn['id']})")
+
+    await engine.dispose()
+    print("done.")
+
+
+if __name__ == "__main__":
+    url = os.environ.get("ACTCORE_DB_URL")
+    if not url:
+        print(
+            "error: set ACTCORE_DB_URL before running this script\n"
+            "  e.g. ACTCORE_DB_URL=postgresql+asyncpg://actcore:actcore@localhost:5433/actcore",
+            file=sys.stderr,
+        )
+        sys.exit(1)
+    asyncio.run(seed(url))
diff --git a/workplans/custodian-WP-0001-temporal-backbone.md b/workplans/custodian-WP-0001-temporal-backbone.md
index 3f91e77..3e893bd 100644
--- a/workplans/custodian-WP-0001-temporal-backbone.md
+++ b/workplans/custodian-WP-0001-temporal-backbone.md
@@ -52,7 +52,7 @@ tasks:
     state_hub_task_id: 491a6903-8189-43bb-958f-4d16abc84f8e
   - id: T12
     title: Seed one example ActivityDefinition
-    status: todo
+    status: done
     state_hub_task_id: f24662ff-4a26-48bd-b997-57e7586c7f11
   - id: T13
     title: Scaffold Python worker project