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/gateway/server-methods/sessions.ts

@@ -128,6 +128,19 @@ 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,
+    contextTokens: undefined,
+    systemPromptReport: undefined,
+  };
+}
+
 function archiveSessionTranscriptsForSession(params: {
   sessionId: string | undefined;
   storePath: string;
@@ -507,9 +520,10 @@ export const sessionsHandlers: GatewayRequestHandlers = {
     const next = await updateSessionStore(storePath, (store) => {
       const { primaryKey } = migrateAndPruneSessionStoreKey({ cfg, key, store });
       const entry = store[primaryKey];
+      const resetEntry = stripRuntimeModelState(entry);
       const parsed = parseAgentSessionKey(primaryKey);
       const sessionAgentId = normalizeAgentId(parsed?.agentId ?? resolveDefaultAgentId(cfg));
-      const resolvedModel = resolveSessionModelRef(cfg, entry, sessionAgentId);
+      const resolvedModel = resolveSessionModelRef(cfg, resetEntry, sessionAgentId);
       oldSessionId = entry?.sessionId;
       oldSessionFile = entry?.sessionFile;
       const now = Date.now();
@@ -524,7 +538,7 @@ export const sessionsHandlers: GatewayRequestHandlers = {
         responseUsage: entry?.responseUsage,
         model: resolvedModel.model,
         modelProvider: resolvedModel.provider,
-        contextTokens: entry?.contextTokens,
+        contextTokens: resetEntry?.contextTokens,
         sendPolicy: entry?.sendPolicy,
         label: entry?.label,
         origin: snapshotSessionOrigin(entry),

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

@@ -591,6 +591,43 @@ 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",
+          contextTokens: 123456,
+        },
+      },
+    });
+
+    const { ws } = await openClient();
+    const reset = await rpcReq<{
+      ok: true;
+      key: string;
+      entry: { sessionId: string; modelProvider?: string; model?: string; contextTokens?: number };
+    }>(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");
+    expect(reset.payload?.entry.contextTokens).toBeUndefined();
+
+    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 }] };