fix(small-model): don't double up /v1 in Anthropic baseURL override

Match @ai-sdk/anthropic exactly: baseURL is the full API prefix, so
/messages is appended as-is instead of unconditionally inserting /v1,
which broke configs where the baseURL already ends in /v1 (the common
form OpenCode itself passes to @ai-sdk/anthropic).

Addresses review feedback from @btriapitsyn on PR #3000.

packages/web/server/lib/small-model/DOCUMENTATION.md

@@ -103,7 +103,10 @@ other runtime API.
     `https://chatgpt.com/backend-api/codex/responses` with
     `ChatGPT-Account-Id`; expired tokens are refreshed against
     `auth.openai.com` (single-flight) and written back to `auth.json`.
-  - **Anthropic** (`type: api`): `/v1/messages` with `x-api-key`.
+  - **Anthropic** (`type: api`): `/messages` with `x-api-key`, against
+    `provider.anthropic.options.baseURL` when configured (used as-is, matching
+    `@ai-sdk/anthropic` — no `/v1` is inserted) or `https://api.anthropic.com/v1`
+    otherwise.
   - **Google** (`type: api`): `generateContent` with `x-goog-api-key`; Gemini 3
     uses `thinkingLevel` while older Flash models use `thinkingBudget: 0`.
   - Everything else: OpenAI-compatible `/chat/completions` against the

packages/web/server/lib/small-model/call.js

@@ -341,7 +341,10 @@ const callMessages = async ({ url, headers, modelID, prompt, system, maxOutputTo
 };
 
 const callAnthropic = async ({ apiKey, baseURL, modelID, prompt, system, maxOutputTokens, responseSchema, timeoutMs, signal }) => callMessages({
-  url: `${(baseURL || 'https://api.anthropic.com').replace(/\/+$/, '')}/v1/messages`,
+  // Matches @ai-sdk/anthropic: baseURL is the full API prefix (commonly
+  // already ending in /v1), so it gets /messages appended as-is rather than
+  // having /v1/messages appended, which would double up a configured /v1.
+  url: `${(baseURL || 'https://api.anthropic.com/v1').replace(/\/+$/, '')}/messages`,
   headers: {
     'x-api-key': apiKey,
     'anthropic-version': '2023-06-01',

packages/web/server/lib/small-model/call.test.js

@@ -325,7 +325,7 @@ describe('callSmallModel — custom provider config', () => {
 
     it('respects provider.anthropic.options.baseURL over the hardcoded Anthropic endpoint', async () => {
       readConfig.mockReturnValue({
-        provider: { anthropic: { options: { baseURL: 'http://127.0.0.1:3456' } } },
+        provider: { anthropic: { options: { baseURL: 'http://127.0.0.1:3456/v1' } } },
       });
       fetchMock.mockResolvedValue(anthropicOk('ok'));
 
@@ -344,6 +344,24 @@ describe('callSmallModel — custom provider config', () => {
       expect(init.headers['x-api-key']).toBe('dummy');
     });
 
+    it('uses a bare-host baseURL as-is without inserting /v1, matching @ai-sdk/anthropic', async () => {
+      readConfig.mockReturnValue({
+        provider: { anthropic: { options: { baseURL: 'http://127.0.0.1:3456' } } },
+      });
+      fetchMock.mockResolvedValue(anthropicOk('ok'));
+
+      await callSmallModel({
+        auth: { anthropic: { type: 'api', key: 'dummy' } },
+        catalog: {},
+        workingDirectory: '/proj',
+        providerID: 'anthropic',
+        modelID: 'claude-haiku-4-5',
+        prompt: 'hi',
+      });
+
+      expect(lastCall(fetchMock).url).toBe('http://127.0.0.1:3456/messages');
+    });
+
     it('falls back to https://api.anthropic.com when no anthropic baseURL override is configured', async () => {
       readConfig.mockReturnValue({});
       fetchMock.mockResolvedValue(anthropicOk('ok'));