vercel · chadhietala · Aug 21, 2026
diff --git a/.changeset/pin-durable-session-turns.md b/.changeset/pin-durable-session-turns.md
@@ -0,0 +1,5 @@
+---
+"eve": patch
+---
+
+Keep turns for parked sessions on the deployment that started their durable driver, preventing newer deployments from writing incompatible session state.
diff --git a/packages/eve/src/execution/dispatch-turn-step.ts b/packages/eve/src/execution/dispatch-turn-step.ts
@@ -3,7 +3,8 @@ import {
   type TurnWorkflowDispatchInput,
 } from "#execution/durable-session-migrations/turn-workflow.js";
 import { buildTurnAttributes, readRootSessionId } from "#execution/eve-workflow-attributes.js";
-import { startWorkflowPreferLatest, turnWorkflowReference } from "#execution/workflow-runtime.js";
+import { turnWorkflowReference } from "#execution/workflow-runtime.js";
+import { start } from "#internal/workflow/runtime.js";
 import { normalizeEveAttributes } from "#runtime/attributes/normalize.js";
 
 /** Starts a per-turn child workflow for the current driver session. */
@@ -12,20 +13,16 @@ export async function dispatchTurnStep(
 ): Promise<{ readonly runId: string }> {
   "use step";
 
-  const run = await startWorkflowPreferLatest(
-    turnWorkflowReference,
-    [createTurnWorkflowInput(input)],
-    {
-      allowReservedAttributes: true,
-      attributes: normalizeEveAttributes(
-        buildTurnAttributes({
-          parentSessionId: input.sessionState.sessionId,
-          requestId: input.delivery.kind === "deliver" ? input.delivery.requestId : undefined,
-          rootSessionId: readRootSessionId(input.serializedContext) ?? input.sessionState.sessionId,
-        }),
-      ),
-    },
-  );
+  const run = await start(turnWorkflowReference, [createTurnWorkflowInput(input)], {
+    allowReservedAttributes: true,
+    attributes: normalizeEveAttributes(
+      buildTurnAttributes({
+        parentSessionId: input.sessionState.sessionId,
+        requestId: input.delivery.kind === "deliver" ? input.delivery.requestId : undefined,
+        rootSessionId: readRootSessionId(input.serializedContext) ?? input.sessionState.sessionId,
+      }),
+    ),
+  });
 
   return { runId: run.runId };
 }
diff --git a/packages/eve/src/execution/durable-session-store.ts b/packages/eve/src/execution/durable-session-store.ts
@@ -7,13 +7,12 @@
  * `"eve.session"` stream remains as a fallback for old in-flight
  * sessions that only carry a small state handle.
  *
- * The driver workflow run is pinned to the deployment that called
- * `start()`; child turn workflows run on latest. Both
- * {@link DurableSessionState} and {@link DurableSessionSnapshot} carry
- * a `version` so a pinned driver can ferry shapes written by newer
- * steps. Adding optional fields is forward-compatible (devalue
- * preserves unknown POJO fields); shape-breaking changes bump
- * `version` and add a migrator.
+ * The driver workflow run and its child turn workflows stay pinned to
+ * the same deployment. Both {@link DurableSessionState} and
+ * {@link DurableSessionSnapshot} carry a `version` for schema
+ * evolution within that deployment. Adding optional fields is
+ * forward-compatible (devalue preserves unknown POJO fields);
+ * shape-breaking changes bump `version` and add a migrator.
  */
 import type { ModelMessage } from "ai";
 

diff --git a/packages/eve/src/execution/next-driver-action.ts b/packages/eve/src/execution/next-driver-action.ts
@@ -1,7 +1,7 @@
 /**
  * Closed-contract dispatch surface between session-mutating step
- * bodies (latest deployment) and the durable driver workflow (pinned
- * to whichever deployment called `start()`).
+ * bodies and the durable driver workflow. Both run on the deployment
+ * that started the session.
  *
  * The driver matches on `kind` and follows a fixed playbook per arm.
  * Adding a new arm is breaking (pinned drivers can't dispatch an

diff --git a/packages/eve/src/execution/workflow-runtime.ts b/packages/eve/src/execution/workflow-runtime.ts
@@ -101,9 +101,8 @@ export const workflowEntryReference = {
 /**
  * Stable workflow reference used by the driver to dispatch per-turn
  * child workflow runs. The id omits the package version stamp so
- * `start(turnWorkflowReference, args, { deploymentId: "latest" })`
- * routes to the latest deployment's turn workflow even when the eve
- * version differs from the caller's deployment.
+ * explicitly version-routed callers can find the same workflow on a
+ * newer deployment even when the eve version differs.
  */
 export const turnWorkflowReference = {
   workflowId: `workflow//${STABLE_ID_BASE}//${TURN_WORKFLOW_NAME}`,

diff --git a/packages/eve/src/execution/workflow-steps.test.ts b/packages/eve/src/execution/workflow-steps.test.ts
@@ -51,11 +51,7 @@ import { emitTerminalSessionFailureStep } from "#execution/terminal-session-fail
 import { resolveEffectiveOutputSchema } from "#execution/effective-output-schema.js";
 import { turnStep } from "#execution/workflow-steps.js";
 import { routeProxiedDeliverStep } from "#execution/proxied-deliver-step.js";
-import {
-  LATEST_DEPLOYMENT_UNSUPPORTED_MESSAGE,
-  turnWorkflowReference,
-  workflowEntryReference,
-} from "#execution/workflow-runtime.js";
+import { turnWorkflowReference, workflowEntryReference } from "#execution/workflow-runtime.js";
 
 vi.mock("./durable-session-store.js", async (importOriginal) => {
   const actual = await importOriginal<typeof import("./durable-session-store.js")>();
@@ -630,7 +626,7 @@ describe("dispatchTurnStep", () => {
     };
   }
 
-  it("starts turn workflows on the latest deployment in Vercel production", async () => {
+  it("keeps a parked session's turn workflow on its driver deployment in Vercel production", async () => {
     vi.stubEnv("VERCEL_ENV", "production");
     const input = createTurnInput();
     startMock.mockResolvedValue({ runId: "turn-run" });
@@ -648,12 +644,11 @@ describe("dispatchTurnStep", () => {
           "$eve.root": "sess-test",
           "$eve.type": "turn",
         },
-        deploymentId: "latest",
       },
     );
   });
 
-  it("starts turn workflows on the latest promoted generation in local development", async () => {
+  it("keeps a parked session's turn workflow on its driver generation in local development", async () => {
     vi.stubEnv("EVE_DEV", "1");
     const input = createTurnInput();
     startMock.mockResolvedValue({ runId: "turn-run" });
@@ -663,7 +658,7 @@ describe("dispatchTurnStep", () => {
     expect(startMock).toHaveBeenCalledWith(
       turnWorkflowReference,
       [createTurnWorkflowInput(input)],
-      expect.objectContaining({ deploymentId: "latest" }),
+      expect.not.objectContaining({ deploymentId: "latest" }),
     );
   });
 
@@ -689,37 +684,6 @@ describe("dispatchTurnStep", () => {
       },
     );
   });
-
-  it("falls back to the current deployment when latest is unsupported", async () => {
-    vi.stubEnv("VERCEL_ENV", "production");
-    const input = createTurnInput();
-    startMock
-      .mockRejectedValueOnce(new Error(LATEST_DEPLOYMENT_UNSUPPORTED_MESSAGE))
-      .mockResolvedValueOnce({ runId: "turn-run" });
-
-    await expect(dispatchTurnStep(input)).resolves.toEqual({ runId: "turn-run" });
-
-    const wireInput = createTurnWorkflowInput(input);
-    expect(startMock).toHaveBeenNthCalledWith(1, turnWorkflowReference, [wireInput], {
-      allowReservedAttributes: true,
-      attributes: {
-        "$eve.channel_request_id": "req_turn",
-        "$eve.parent": "sess-test",
-        "$eve.root": "sess-test",
-        "$eve.type": "turn",
-      },
-      deploymentId: "latest",
-    });
-    expect(startMock).toHaveBeenNthCalledWith(2, turnWorkflowReference, [wireInput], {
-      allowReservedAttributes: true,
-      attributes: {
-        "$eve.channel_request_id": "req_turn",
-        "$eve.parent": "sess-test",
-        "$eve.root": "sess-test",
-        "$eve.type": "turn",
-      },
-    });
-  });
 });
 
 describe("dispatchRuntimeActionsStep", () => {