fix: preserve native Vertex endpoint defaults (#1104)

## Summary
- keep `ANTHROPIC_VERTEX_BASE_URL` unset when `providers/set` selects
Vertex's native default endpoint
- retain explicit `ANTHROPIC_VERTEX_BASE_URL` only for custom Vertex
gateways/endpoints
- add a regression test for the default Vertex endpoint env mapping

Closes #1087

## Test Plan
- `npm run test:run -- src/tests/providers.test.ts` — passed (21 tests)
- `npm run check` — passed (`eslint src --ext .ts` + `prettier --check
.`)
- `npm run build` — passed (`tsc`)
- `git diff --check origin/main..HEAD` — passed

Note: `claude -p` delegation was attempted for candidate discovery but
this cron shell's Claude Code auth is not logged in (`Not logged in ·
Please run /login`), so the implementation/validation above was done
directly with local tests.

Signed-off-by: Emre K <110906681+kocaemre@users.noreply.github.com>

src/acp-agent.ts

@@ -1300,6 +1300,7 @@ const AUTH_REQUIRED_CODE = RequestError.authRequired().code;
 const SUPPORTED_PROTOCOLS: LlmProtocol[] = ["anthropic", "bedrock", "vertex"];
 const PROVIDER_ID = "main";
 const DEFAULT_ANTHROPIC_BASE_URL = "https://api.anthropic.com";
+const DEFAULT_VERTEX_BASE_URL = "https://aiplatform.googleapis.com";
 
 /**
  * Vertex needs project + region that the standard `providers/set` payload
@@ -2691,7 +2692,7 @@ export class ClaudeAcpAgent {
     if (process.env.CLAUDE_CODE_USE_VERTEX) {
       return {
         apiType: "vertex",
-        baseUrl: process.env.ANTHROPIC_VERTEX_BASE_URL ?? "https://aiplatform.googleapis.com",
+        baseUrl: process.env.ANTHROPIC_VERTEX_BASE_URL ?? DEFAULT_VERTEX_BASE_URL,
         headers: {},
       };
     }
@@ -9133,10 +9134,16 @@ function createEnvForProvider(config: ProviderConfig | null): Record<string, str
   if (config.apiType === "vertex") {
     // `config.vertex` is guaranteed present for vertex by `unstable_setProvider`
     // validation; fall back to empty strings defensively.
+    // Leaving ANTHROPIC_VERTEX_BASE_URL empty for the default endpoint lets
+    // Claude Code use its native Vertex endpoint/model resolution. Supplying
+    // even the default host marks the session as a custom Vertex deployment and
+    // can hide otherwise-valid models.
     return {
       ...resetRouting,
       CLAUDE_CODE_USE_VERTEX: "1",
-      ANTHROPIC_VERTEX_BASE_URL: config.baseUrl,
+      ...(config.baseUrl !== DEFAULT_VERTEX_BASE_URL
+        ? { ANTHROPIC_VERTEX_BASE_URL: config.baseUrl }
+        : {}),
       ANTHROPIC_VERTEX_PROJECT_ID: config.vertex?.projectId ?? "",
       CLOUD_ML_REGION: config.vertex?.region ?? "",
       ANTHROPIC_CUSTOM_HEADERS: customHeaders,

src/tests/providers.test.ts

@@ -429,6 +429,32 @@ describe("providers", () => {
     );
   });
 
+  it("leaves the native Vertex base URL unset for the default endpoint", async () => {
+    const [agent, mockQuery] = await createAgentMock();
+    await agent.initialize({ protocolVersion: 1, clientCapabilities: {} });
+    await agent.unstable_setProvider({
+      providerId: "main",
+      apiType: "vertex",
+      baseUrl: "https://aiplatform.googleapis.com",
+      _meta: { claudeCode: { vertex: { projectId: "my-project", region: "global" } } },
+    });
+
+    await agent.newSession({ cwd: process.cwd(), mcpServers: [] });
+
+    expect(mockQuery).toHaveBeenCalledWith(
+      expect.objectContaining({
+        options: expect.objectContaining({
+          env: expect.objectContaining({
+            CLAUDE_CODE_USE_VERTEX: "1",
+            ANTHROPIC_VERTEX_BASE_URL: "",
+            ANTHROPIC_VERTEX_PROJECT_ID: "my-project",
+            CLOUD_ML_REGION: "global",
+          }),
+        }),
+      }),
+    );
+  });
+
   it("rejects vertex set without _meta project/region", async () => {
     const [agent] = await createAgentMock();
     await expect(