Fix stale runtime model reuse on session reset

src/auto-reply/reply/agent-runner.runreplyagent.e2e.test.ts

@@ -1255,6 +1255,76 @@ describe("runReplyAgent typing (heartbeat)", () => {
     });
   });
 
+  it("clears stale runtime model fields when resetSession retries after compaction failure", async () => {
+    await withTempStateDir(async (stateDir) => {
+      const sessionId = "session-stale-model";
+      const storePath = path.join(stateDir, "sessions", "sessions.json");
+      const transcriptPath = sessions.resolveSessionTranscriptPath(sessionId);
+      const sessionEntry: SessionEntry = {
+        sessionId,
+        updatedAt: Date.now(),
+        sessionFile: transcriptPath,
+        modelProvider: "qwencode",
+        model: "qwen3.5-plus-2026-02-15",
+        systemPromptReport: {
+          source: "run",
+          generatedAt: Date.now(),
+          sessionId,
+          sessionKey: "main",
+          provider: "qwencode",
+          model: "qwen3.5-plus-2026-02-15",
+          workspaceDir: stateDir,
+          bootstrapMaxChars: 1000,
+          bootstrapTotalMaxChars: 2000,
+          systemPrompt: {
+            chars: 10,
+            projectContextChars: 5,
+            nonProjectContextChars: 5,
+          },
+          injectedWorkspaceFiles: [],
+          skills: {
+            promptChars: 0,
+            entries: [],
+          },
+          tools: {
+            listChars: 0,
+            schemaChars: 0,
+            entries: [],
+          },
+        },
+      };
+      const sessionStore = { main: sessionEntry };
+
+      await fs.mkdir(path.dirname(storePath), { recursive: true });
+      await fs.writeFile(storePath, JSON.stringify(sessionStore), "utf-8");
+      await fs.mkdir(path.dirname(transcriptPath), { recursive: true });
+      await fs.writeFile(transcriptPath, "ok", "utf-8");
+
+      state.runEmbeddedPiAgentMock.mockImplementationOnce(async () => {
+        throw new Error(
+          'Context overflow: Summarization failed: 400 {"message":"prompt is too long"}',
+        );
+      });
+
+      const { run } = createMinimalRun({
+        sessionEntry,
+        sessionStore,
+        sessionKey: "main",
+        storePath,
+      });
+      await run();
+
+      expect(sessionStore.main.modelProvider).toBeUndefined();
+      expect(sessionStore.main.model).toBeUndefined();
+      expect(sessionStore.main.systemPromptReport).toBeUndefined();
+
+      const persisted = JSON.parse(await fs.readFile(storePath, "utf-8"));
+      expect(persisted.main.modelProvider).toBeUndefined();
+      expect(persisted.main.model).toBeUndefined();
+      expect(persisted.main.systemPromptReport).toBeUndefined();
+    });
+  });
+
   it("surfaces overflow fallback when embedded run returns empty payloads", async () => {
     state.runEmbeddedPiAgentMock.mockImplementationOnce(async () => ({
       payloads: [],

src/auto-reply/reply/agent-runner.ts

@@ -278,6 +278,9 @@ export async function runReplyAgent(params: {
       updatedAt: Date.now(),
       systemSent: false,
       abortedLastRun: false,
+      modelProvider: undefined,
+      model: undefined,
+      systemPromptReport: undefined,
       fallbackNoticeSelectedModel: undefined,
       fallbackNoticeActiveModel: undefined,
       fallbackNoticeReason: undefined,

src/gateway/server-methods/sessions.ts

@@ -128,6 +128,18 @@ function migrateAndPruneSessionStoreKey(params: {
   return { target, primaryKey, entry: params.store[primaryKey] };
 }
 
+function stripRuntimeModelState(entry?: SessionEntry): SessionEntry | undefined {
+  if (!entry) {
+    return entry;
+  }
+  return {
+    ...entry,
+    model: undefined,
+    modelProvider: undefined,
+    systemPromptReport: undefined,
+  };
+}
+
 function archiveSessionTranscriptsForSession(params: {
   sessionId: string | undefined;
   storePath: string;
@@ -509,7 +521,11 @@ export const sessionsHandlers: GatewayRequestHandlers = {
       const entry = store[primaryKey];
       const parsed = parseAgentSessionKey(primaryKey);
       const sessionAgentId = normalizeAgentId(parsed?.agentId ?? resolveDefaultAgentId(cfg));
-      const resolvedModel = resolveSessionModelRef(cfg, entry, sessionAgentId);
+      const resolvedModel = resolveSessionModelRef(
+        cfg,
+        stripRuntimeModelState(entry),
+        sessionAgentId,
+      );
       oldSessionId = entry?.sessionId;
       oldSessionFile = entry?.sessionFile;
       const now = Date.now();

src/gateway/server.sessions.gateway-server-sessions-a.test.ts

@@ -591,6 +591,41 @@ describe("gateway server sessions", () => {
     ws.close();
   });
 
+  test("sessions.reset recomputes model from defaults instead of stale runtime model", async () => {
+    await createSessionStoreDir();
+    testState.agentConfig = {
+      model: {
+        primary: "openai/gpt-test-a",
+      },
+    };
+
+    await writeSessionStore({
+      entries: {
+        main: {
+          sessionId: "sess-stale-model",
+          updatedAt: Date.now(),
+          modelProvider: "qwencode",
+          model: "qwen3.5-plus-2026-02-15",
+        },
+      },
+    });
+
+    const { ws } = await openClient();
+    const reset = await rpcReq<{
+      ok: true;
+      key: string;
+      entry: { sessionId: string; modelProvider?: string; model?: string };
+    }>(ws, "sessions.reset", { key: "main" });
+
+    expect(reset.ok).toBe(true);
+    expect(reset.payload?.key).toBe("agent:main:main");
+    expect(reset.payload?.entry.sessionId).not.toBe("sess-stale-model");
+    expect(reset.payload?.entry.modelProvider).toBe("openai");
+    expect(reset.payload?.entry.model).toBe("gpt-test-a");
+
+    ws.close();
+  });
+
   test("sessions.preview resolves legacy mixed-case main alias with custom mainKey", async () => {
     const { dir, storePath } = await createSessionStoreDir();
     testState.agentsConfig = { list: [{ id: "ops", default: true }] };