Preserve ACP catalogs when explicit model discovery fails

packages/provider-bridge-acp/src/bridge/bridge.test.ts

@@ -1210,11 +1210,49 @@ describe("acp bridge", () => {
     });
   });
 
-  it("falls back to the synthetic model when the list command prints no models", async () => {
-    const emptyId = sendModelList({ modelLines: "no model lines here" });
-    expect((await waitForResponse(emptyId)).result).toMatchObject({
-      models: [{ id: "acp-default", isDefault: true }],
+  it.each([
+    ["exits unsuccessfully", "process.exit(2)"],
+    ["prints no models", "process.stdout.write('no model lines here')"],
+    ["times out", "setInterval(() => {}, 1000)"],
+  ])(
+    "fails model/list when the configured command %s without a cached catalog",
+    async (_failure, script) => {
+      const failingId = sendModelList({
+        agent: { command: process.execPath, args: ["-e", script] },
+        modelListArgs: ["--"],
+      });
+      const response = await waitFor(
+        () => findResponse(failingId),
+        `response ${failingId}`,
+        40_000,
+      );
+      expect(response.error?.message).toBe(
+        "ACP model list command failed to provide a model catalog.",
+      );
+      expect(response.result).toBeUndefined();
+    },
+    45_000,
+  );
+
+  it("keeps the last good CLI catalog when the same command later fails", async () => {
+    const scriptPath = join(workspaceDir, "model-list.cjs");
+    writeFileSync(
+      scriptPath,
+      "process.stdout.write('cached-model - Cached Model\\n')",
+    );
+    const launch = {
+      agent: { command: process.execPath, args: [scriptPath] },
+      modelListArgs: ["--"],
+    };
+    const first = await waitForResponse(sendModelList(launch));
+    expect(first.result).toMatchObject({
+      models: [{ id: "cached-model", displayName: "Cached Model" }],
     });
+
+    writeFileSync(scriptPath, "process.exit(2)");
+    const second = await waitForResponse(sendModelList(launch));
+    expect(second.error).toBeUndefined();
+    expect(second.result).toEqual(first.result);
   });
 
   it("keeps CLI reasoning on the resolved model variant instead of ACP config", async () => {

packages/provider-bridge-acp/src/bridge/bridge.ts

@@ -2642,6 +2642,11 @@ async function handleModelList(
     );
     return;
   }
+  if (params.listCommand) {
+    throw new Error(
+      "ACP model list command failed to provide a model catalog.",
+    );
+  }
   const sessionDiscoveredModels =
     params.listCommand === undefined && params.agent
       ? await loadSessionDiscoveredModels(