Keep warm sessions across project revision transitions (#17461)

doc/DEVELOPING.md

@@ -1776,6 +1776,11 @@ preparation. Both phases keep their original ordering in `heartbeat.ts`; they
 acquire no runtime resources. Public-path characterization tests live in
 `server/src/__tests__/heartbeat-session-preparation.test.ts`.
 
+Session compatibility uses effective workspace policy and settings. Generic issue
+and project row timestamps do not reset a session. This includes the first
+project-owned directory binding. Changes to effective execution configuration
+still require a fresh session.
+
 Run-log formatting is in `server/src/services/heartbeat/run-log.ts`. It bounds
 stored event payloads, redacts and shortens log chunks, and caps stdout/stderr
 excerpts. `run-output.ts` owns per-attempt log handles, current-user redaction,

server/src/__tests__/heartbeat-workspace-session.test.ts

@@ -2427,6 +2427,57 @@ describe("effective run session config freshness", () => {
     });
   });
 
+  it.each([null, "2026-06-01T00:00:00.000Z"])("does not reset for project row timestamps (%s) but still resets for project policy", async (revision) => {
+    const base = await buildSessionConfigMetadata({
+      workspaceConfig: {
+        requestedMode: "agent_default",
+        effectiveMode: "agent_default",
+        projectConfigRevisionAt: revision,
+        projectPolicy: null,
+      },
+    });
+    const projectTouched = await buildSessionConfigMetadata({
+      workspaceConfig: {
+        requestedMode: "agent_default",
+        effectiveMode: "agent_default",
+        projectConfigRevisionAt: "2026-06-01T00:05:00.000Z",
+        projectPolicy: null,
+      },
+    });
+    const policyChanged = await buildSessionConfigMetadata({
+      workspaceConfig: {
+        requestedMode: "agent_default",
+        effectiveMode: "agent_default",
+        projectConfigRevisionAt: "2026-06-01T00:05:00.000Z",
+        projectPolicy: { enabled: true, defaultMode: "isolated_workspace" },
+      },
+    });
+
+    expect(
+      resolveTaskSessionConfigFreshness({
+        hasTaskSession: true,
+        configuredModel: "gpt-5.4-mini",
+        taskSessionParams: sessionParamsWithConfigMetadata(base),
+        configMetadata: projectTouched,
+      }),
+    ).toMatchObject({
+      reset: false,
+      changedCategories: [],
+      reasons: [],
+    });
+    expect(
+      resolveTaskSessionConfigFreshness({
+        hasTaskSession: true,
+        configuredModel: "gpt-5.4-mini",
+        taskSessionParams: sessionParamsWithConfigMetadata(base),
+        configMetadata: policyChanged,
+      }),
+    ).toMatchObject({
+      reset: true,
+      changedCategories: ["workspaceConfig"],
+    });
+  });
+
   it("does not reset when a reusable execution workspace becomes realized", async () => {
     const base = await buildSessionConfigMetadata({
       workspaceConfig: {

server/src/services/heartbeat/workspaces.ts

@@ -2800,6 +2800,7 @@ function buildSessionConfigCategoryValues(input: {
   // the timestamp here makes every comment invalidate an otherwise reusable
   // task session.
   delete workspaceConfig.issueConfigRevisionAt;
+  delete workspaceConfig.projectConfigRevisionAt;
   // This row is runtime state, not requested configuration. It is absent
   // before the first reusable run is realized and present on the next turn;
   // fingerprinting that transition would rotate the native session exactly