diff --git a/src/flightdeck/server/routes/actions.py b/src/flightdeck/server/routes/actions.py
index ad807a3..bc3123e 100644
--- a/src/flightdeck/server/routes/actions.py
+++ b/src/flightdeck/server/routes/actions.py
@@ -3,7 +3,7 @@
 from fastapi import APIRouter, HTTPException, Request
 from pydantic import BaseModel, Field
 
-from flightdeck.operations import OperationError, compute_diff, promote_release, rollback_release
+from flightdeck.operations import ActionOutcome, OperationError, compute_diff, promote_release, rollback_release
 from flightdeck.server.routes.common import ensure_app_state
 
 router = APIRouter()
@@ -32,6 +32,29 @@ def _raise_bad_request(exc: OperationError) -> HTTPException:
     return HTTPException(status_code=400, detail=str(exc))
 
 
+def _action_body(outcome: ActionOutcome) -> dict[str, object]:
+    return {
+        "action_id": outcome.action_id,
+        "action": outcome.action,
+        "release_id": outcome.release_id,
+        "agent_id": outcome.agent_id,
+        "environment": outcome.environment,
+        "baseline_release_id": outcome.baseline_release_id,
+        "promoted_pointer_changed": outcome.promoted_pointer_changed,
+        "policy": outcome.policy.model_dump(mode="json"),
+    }
+
+
+def _raise_policy_blocked(action: str, outcome: ActionOutcome) -> HTTPException:
+    return HTTPException(
+        status_code=409,
+        detail={
+            "message": f"{action.capitalize()} blocked by policy.",
+            "outcome": _action_body(outcome),
+        },
+    )
+
+
 def _require_mutation_access(request: Request) -> None:
     ensure_app_state(request)
     expected_token: str | None = request.app.state.local_api_token
@@ -123,16 +146,10 @@ def post_promote(request: Request, req: ActionRequest) -> dict[str, object]:
     except OperationError as exc:
         raise _raise_bad_request(exc) from exc
 
-    return {
-        "action_id": outcome.action_id,
-        "action": outcome.action,
-        "release_id": outcome.release_id,
-        "agent_id": outcome.agent_id,
-        "environment": outcome.environment,
-        "baseline_release_id": outcome.baseline_release_id,
-        "promoted_pointer_changed": outcome.promoted_pointer_changed,
-        "policy": outcome.policy.model_dump(mode="json"),
-    }
+    if not outcome.policy.passed:
+        raise _raise_policy_blocked("promotion", outcome)
+
+    return _action_body(outcome)
 
 
 @router.post("/v1/rollback")
@@ -152,13 +169,7 @@ def post_rollback(request: Request, req: ActionRequest) -> dict[str, object]:
     except OperationError as exc:
         raise _raise_bad_request(exc) from exc
 
-    return {
-        "action_id": outcome.action_id,
-        "action": outcome.action,
-        "release_id": outcome.release_id,
-        "agent_id": outcome.agent_id,
-        "environment": outcome.environment,
-        "baseline_release_id": outcome.baseline_release_id,
-        "promoted_pointer_changed": outcome.promoted_pointer_changed,
-        "policy": outcome.policy.model_dump(mode="json"),
-    }
+    if not outcome.policy.passed:
+        raise _raise_policy_blocked("rollback", outcome)
+
+    return _action_body(outcome)
diff --git a/tests/test_sdk_client.py b/tests/test_sdk_client.py
index 2a0a5f8..f9045f3 100644
--- a/tests/test_sdk_client.py
+++ b/tests/test_sdk_client.py
@@ -101,6 +101,37 @@ def handler(request: httpx.Request) -> httpx.Response:
         assert client.list_releases() == {"releases": []}
 
 
+def test_flightdeck_client_promote_raises_on_policy_block() -> None:
+    def handler(request: httpx.Request) -> httpx.Response:
+        assert request.method == "POST"
+        assert request.url.path == "/v1/promote"
+        return httpx.Response(
+            409,
+            json={
+                "detail": {
+                    "message": "Promotion blocked by policy.",
+                    "outcome": {
+                        "promoted_pointer_changed": False,
+                        "policy": {"passed": False, "reasons": ["candidate cost exceeds max"]},
+                    },
+                }
+            },
+        )
+
+    transport = httpx.MockTransport(handler)
+    with httpx.Client(transport=transport, base_url="http://flightdeck.test") as http:
+        client = FlightdeckClient("http://flightdeck.test", client=http)
+        with pytest.raises(httpx.HTTPStatusError) as excinfo:
+            client.post_promote(
+                release_id="rel_blocked",
+                environment="local",
+                window="7d",
+                reason="policy check",
+            )
+
+    assert excinfo.value.response.status_code == 409
+
+
 def test_flightdeck_client_ingest_batch_chunks_payload() -> None:
     seen_lengths: list[int] = []
 
diff --git a/tests/test_server_actions.py b/tests/test_server_actions.py
index 87c74ba..25e9a17 100644
--- a/tests/test_server_actions.py
+++ b/tests/test_server_actions.py
@@ -160,6 +160,40 @@ def test_http_promote_requires_reason(tmp_path: Path) -> None:
     assert res.status_code == 422
 
 
+def test_http_promote_policy_block_returns_conflict_and_keeps_audit(tmp_path: Path) -> None:
+    ws = tmp_path / "ws_policy_block"
+    runner, baseline_id, candidate_id = _seed_workspace(ws)
+    with _cwd(ws):
+        policy = write_policy(ws, max_cost_per_run_usd=0.0001)
+        assert runner.invoke(cli, ["policy", "set", str(policy)]).exit_code == 0
+
+        with TestClient(create_app()) as client:
+            res = client.post(
+                "/v1/promote",
+                json={
+                    "release_id": candidate_id,
+                    "environment": "local",
+                    "window": "7d",
+                    "reason": "too expensive",
+                    "actor": "http-test",
+                },
+            )
+
+        assert res.status_code == 409
+        detail = res.json()["detail"]
+        assert detail["message"] == "Promotion blocked by policy."
+        outcome = detail["outcome"]
+        assert outcome["promoted_pointer_changed"] is False
+        assert outcome["policy"]["passed"] is False
+
+        storage = Storage(load_config().db_path)
+        storage.migrate()
+        assert storage.get_promoted_release_id("agent_support", "local") == baseline_id
+        last = storage.list_release_actions(agent_id="agent_support", environment="local")[0]
+        assert last.release_id == candidate_id
+        assert last.policy_result.passed is False
+
+
 def test_ui_root_serves_vite_index(tmp_path: Path, monkeypatch) -> None:
     monkeypatch.chdir(tmp_path)
     assert CliRunner().invoke(cli, ["init"]).exit_code == 0