Merge pull request #3000 from mpeter/fix/anthropic-small-model-base-url

fix(small-model): route anthropic provider through configured baseURL

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`, Gemini 2.x uses `thinkingBudget: 0`, and all other
     models omit `thinkingConfig` entirely.