feat: establish unified administration and model context baseline

backend/test/routes-sessions.test.ts

@@ -27,10 +27,9 @@ function operationalWorkspace(id = "default") {
   } as const;
 }
 
-function sessionCatalog(defaultSession = "zai/glm-5.2", available = [defaultSession]) {
+function sessionCatalog(defaultInteraction = "zai/glm-5.2", available = [defaultInteraction]) {
   return {
-    defaultSession,
-    defaultMetadataGeneration: null,
+    defaultInteraction,
     embedding: { id: "ollama/qwen3-embedding:0.6b", dimensions: 1024 },
     sessionModels: () => [],
     metadataModels: () => [],
@@ -909,7 +908,7 @@ test("session lifecycle locates a B session when installation default is A", asy
 
   active = undefined;
   expect((await app.inject({ method: "POST", url: "/sessions/session-b/resume" })).json())
-    .toEqual({ id: "session-b", alreadyActive: false });
+    .toEqual({ id: "session-b", alreadyActive: false, workspaceId: "b-workspace" });
   expect(calls).toContain(`new:${bPath}`);
   expect(calls).toContain(`list:${bPath}`);
   expect(calls).toContain(`show:${bPath}`);
@@ -1179,6 +1178,29 @@ test("POST /sessions/:id/resume uses the manifest's retained workspace revision"
   );
 });
 
+test.each([undefined, { provider: "local", model: "qwen" }])("resume uses the global model/default, not the historical manifest (%j)", async (payload) => {
+  const configure = vi.fn(async () => {});
+  const manifest = { status: "open", archived: false, provider: "retired", model: "old-model" };
+  const runtime = { bridge: { onClientEvent: () => {}, emitClientEvent: () => {} } };
+  let current: any;
+  const app = buildApp(loadConfig({ THT_HARNESS_DIR: "../harness" }), {
+    mgr: {
+      get: () => current,
+      createFor: () => { current = runtime; return runtime; },
+      configure, start: () => {},
+    } as any,
+    thtRunner: { sessionShow: async () => manifest, reopenSession: async () => {} } as any,
+    readiness: { ensure: async () => ({ ok: true }) } as any,
+    runtimeModelCatalog: sessionCatalog("zai/glm-5.2", ["zai/glm-5.2", "local/qwen"]),
+    getSettings: () => ({ workspace: "psd", thinking: "medium" }) as any,
+  });
+  const response = await app.inject({ method: "POST", url: "/sessions/model-resume/resume", ...(payload ? { payload } : {}) });
+  expect(response.statusCode).toBe(200);
+  await new Promise((resolve) => setImmediate(resolve));
+  expect(configure).toHaveBeenCalledWith(expect.anything(), expect.objectContaining(payload ?? { provider: "zai", model: "glm-5.2" }));
+  expect(manifest).toMatchObject({ provider: "retired", model: "old-model" });
+});
+
 test("POST /sessions/:id/resume returns a sanitized error when its retained revision is unavailable", async () => {
   const rawFailure = "cannot read /data/workspace-registry/snapshots/secret-revision";
   const app = buildApp(loadConfig({ THT_HARNESS_DIR: "../harness" }), {
@@ -2691,7 +2713,7 @@ test("POST /sessions proceeds when ollamaEnsure succeeds", async () => {
   expect(ensureWs).toContain(`/snapshots/${"e".repeat(40)}/psd.yaml`);
 });
 
-test("POST /sessions falls back from a stale requested model to the catalog default", async () => {
+test("POST /sessions rejects a stale requested model without silently using the default", async () => {
   let created = 0;
   let persisted: any;
   const runtime = { bridge: { onClientEvent: () => {} } };
@@ -2721,13 +2743,10 @@ test("POST /sessions falls back from a stale requested model to the catalog defa
     payload: { question: "q", provider: "deepseek", model: "deepseek-v4-pro" },
   });
 
-  expect(res.statusCode).toBe(200);
-  expect(res.json()).toEqual({
-    id: "fallback",
-    warning: "Configured model deepseek/deepseek-v4-pro is unavailable; using zai/glm-5.2.",
-  });
-  expect(persisted).toMatchObject({ provider: "zai", model: "glm-5.2" });
-  expect(created).toBe(1);
+  expect(res.statusCode).toBe(503);
+  expect(res.json()).toMatchObject({ code: "model_unavailable" });
+  expect(persisted).toBeUndefined();
+  expect(created).toBe(0);
 });
 
 test("POST /sessions marks a persisted session failed when runtime construction throws", async () => {