Allow ACP providers without model lists

packages/server/src/server/agent/providers/acp-agent.test.ts

@@ -731,6 +731,31 @@ describe("ACPAgentSession Zed parity", () => {
     expect(setSessionConfigOption).not.toHaveBeenCalled();
   });
 
+  test("does not fail session start when configured model cannot be applied by ACP", async () => {
+    const logger = createTestLogger();
+    const childLogger = { trace: vi.fn(), warn: vi.fn() };
+    vi.spyOn(logger, "child").mockReturnValue(asInternals<typeof logger>(childLogger));
+    const session = createSessionWithConfig(
+      { provider: "deepseek-tui", model: "deepseek/v4" },
+      logger,
+    );
+    const { internals, setSessionConfigOption, unstableSetSessionModel } =
+      prepareConfiguredOverrideSession(session, {
+        currentModel: null,
+        availableModels: null,
+        configOptions: [],
+        connection: { unstable_setSessionModel: undefined },
+      });
+
+    await expect(internals.applyConfiguredOverrides()).resolves.toBeUndefined();
+    expect(unstableSetSessionModel).not.toHaveBeenCalled();
+    expect(setSessionConfigOption).not.toHaveBeenCalled();
+    expect(childLogger.warn).toHaveBeenCalledWith(
+      { value: "deepseek/v4" },
+      "deepseek-tui does not expose ACP model selection; using provider default model",
+    );
+  });
+
   test("routes config_option_update and refreshes derived mode, model, and thinking state", async () => {
     const session = createSession();
     const internals = asInternals<ACPSessionInternals>(session);

packages/server/src/server/agent/providers/acp-agent.ts

@@ -1376,7 +1376,7 @@ export class ACPAgentSession implements AgentSession, ACPClient {
       }
 
       if (typeof this.connection.unstable_setSessionModel !== "function") {
-        throw new Error(`${this.provider} does not expose ACP model selection`);
+        throw new Error(this.modelSelectionUnavailableMessage());
       }
 
       try {
@@ -1398,7 +1398,7 @@ export class ACPAgentSession implements AgentSession, ACPClient {
 
     const modelOption = selection.configOption;
     if (!modelOption) {
-      throw new Error(`${this.provider} does not expose ACP model selection`);
+      throw new Error(this.modelSelectionUnavailableMessage());
     }
     if (!selection.configChoice) {
       this.warnInvalidSelection(
@@ -1904,7 +1904,17 @@ export class ACPAgentSession implements AgentSession, ACPClient {
         availableModels: this.availableModels,
         configOptions: this.configOptions,
       });
-      await this.setModelWithSelection({ modelId: configuredModelId, selection });
+      try {
+        await this.setModelWithSelection({ modelId: configuredModelId, selection });
+      } catch (error) {
+        if (!this.isModelSelectionUnavailableError(error)) {
+          throw error;
+        }
+        this.logger.warn(
+          { value: configuredModelId },
+          `${this.provider} does not expose ACP model selection; using provider default model`,
+        );
+      }
     }
     if (this.config.thinkingOptionId && this.config.thinkingOptionId !== this.thinkingOptionId) {
       await this.setThinkingOption(this.config.thinkingOptionId);
@@ -1915,6 +1925,14 @@ export class ACPAgentSession implements AgentSession, ACPClient {
     this.logger.warn({ value }, message);
   }
 
+  private modelSelectionUnavailableMessage(): string {
+    return `${this.provider} does not expose ACP model selection`;
+  }
+
+  private isModelSelectionUnavailableError(error: unknown): boolean {
+    return error instanceof Error && error.message === this.modelSelectionUnavailableMessage();
+  }
+
   private translateSessionUpdate(update: SessionUpdate): AgentStreamEvent[] {
     switch (update.sessionUpdate) {
       case "user_message_chunk": {