harden persisted run resume contract

src/wf_api/__init__.py

@@ -39,6 +39,7 @@ from .operation_context import (
 
 from .runtime_dependencies import RuntimeDependencies, resolve_runtime_dependencies
 from .stores import WorkflowStores, file_workflow_stores
+from .durable_context import durable_workflow_api, require_workflow_stores
 
 __all__ = [
     "DEFAULT_CALL_STEP_ID",
@@ -79,4 +80,6 @@ __all__ = [
     "resolve_runtime_dependencies",
     "WorkflowStores",
     "file_workflow_stores",
+    "durable_workflow_api",
+    "require_workflow_stores",
 ]

src/wf_api/durable_context.py

@@ -0,0 +1,41 @@
+from __future__ import annotations
+
+from .operation_context import WorkflowOperationContext
+from .service import WorkflowApi
+from .stores import WorkflowStores
+
+
+def require_workflow_stores(context: WorkflowOperationContext) -> WorkflowStores:
+    """Return required stores or fail before constructing durable frontends.
+
+    `WorkflowOperationContext` keeps stores optional for compatibility tests and
+    lightweight MCP surfaces. Durable API surfaces need all stores up front so a
+    run cannot start without somewhere to persist artifacts, drafts, and stopped
+    execution state.
+    """
+    missing = []
+    if context.artifact_store is None:
+        missing.append("artifact_store")
+    if context.draft_workspace_store is None:
+        missing.append("draft_workspace_store")
+    if context.run_store is None:
+        missing.append("run_store")
+    if missing:
+        raise ValueError("durable workflow API requires stores: " + ", ".join(missing))
+    assert context.artifact_store is not None
+    assert context.draft_workspace_store is not None
+    assert context.run_store is not None
+    return WorkflowStores(
+        artifact_store=context.artifact_store,
+        draft_workspace_store=context.draft_workspace_store,
+        run_store=context.run_store,
+    )
+
+
+def durable_workflow_api(context: WorkflowOperationContext) -> WorkflowApi:
+    """Construct a WorkflowApi only after durable store dependencies exist."""
+    require_workflow_stores(context)
+    return WorkflowApi(context)
+
+
+__all__ = ["durable_workflow_api", "require_workflow_stores"]