Fix stale runtime model reuse on session reset (#41173)

Merged via squash.

Prepared head SHA: d8a04a466a
Co-authored-by: PonyX-lab <266766228+PonyX-lab@users.noreply.github.com>
Co-authored-by: jalehman <550978+jalehman@users.noreply.github.com>
Reviewed-by: @jalehman

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

@@ -1255,6 +1255,79 @@ 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",
+        contextTokens: 123456,
+        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.contextTokens).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.contextTokens).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 +278,10 @@ export async function runReplyAgent(params: {
       updatedAt: Date.now(),
       systemSent: false,
       abortedLastRun: false,
+      modelProvider: undefined,
+      model: undefined,
+      contextTokens: undefined,
+      systemPromptReport: undefined,
       fallbackNoticeSelectedModel: undefined,
       fallbackNoticeActiveModel: undefined,
       fallbackNoticeReason: undefined,