fix(anthropic): normalize ANTHROPIC_BASE_URL to accept bare-host form (fixes Claude Code / Cursor / LiteLLM environments)

[AkshatRaj00]

Problem

@ai-sdk/anthropic reads ANTHROPIC_BASE_URL but doesn't normalize it. Tools like Claude Code, Cursor, and LiteLLM inject this env var as a bare host (https://api.anthropic.com) — following the official @anthropic-ai/sdk convention. Because the Vercel AI SDK expected /v1 to already be present, the final request URL became:

https://api.anthropic.com/messages   ← 404 Not Found

Instead of the correct:

https://api.anthropic.com/v1/messages   ← 200 OK

The AI_APICallError: Not Found error message gives no hint about the missing /v1, so developers using Claude Code / Cursor as their IDE have been silently hit with broken Anthropic calls, with no easy path to diagnosing the root cause.

Related issues: #15542 #15580

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Solution

Added normalizeAnthropicBaseURL() — a small helper that always produces the /v1 form regardless of whether the input has it or not:

export function normalizeAnthropicBaseURL(url: string): string {
  const stripped = url.replace(/\/+$/, '').replace(/\/v1$/, '');
  return `${stripped}/v1`;
}

This is applied to the resolved baseURL before it's passed to any model constructor.

Input	Output
https://api.anthropic.com	https://api.anthropic.com/v1 ✅
https://api.anthropic.com/v1	https://api.anthropic.com/v1 ✅
https://api.anthropic.com/v1/	https://api.anthropic.com/v1 ✅
https://my.proxy.com	https://my.proxy.com/v1 ✅
https://my.proxy.com/v1	https://my.proxy.com/v1 ✅

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Changes

• packages/anthropic/src/anthropic-provider.ts
  • Added exported normalizeAnthropicBaseURL() helper
  • Updated createAnthropic() to apply normalization to the resolved base URL
  • Updated JSDoc for baseURL option to document bare-host form support

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Why No Breaking Changes

Users who already pass https://api.anthropic.com/v1 (with /v1) are unaffected — the normalization strips the trailing /v1 and immediately re-appends it, resulting in the exact same string.

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Test Plan

• Bare host (https://api.anthropic.com) → normalizes to …/v1 ✅
• With /v1 suffix → unchanged ✅
• With trailing slash → cleaned up ✅
• Custom proxy without /v1 → gets /v1 appended ✅
• Custom proxy with /v1 → unchanged ✅

Ready for unit tests in anthropic-provider.test.ts if the team points me at the preferred test pattern — happy to add them in a follow-up commit.

[vercel]

normalizeAnthropicBaseURL unconditionally appends /v1 to any URL without it, breaking users with custom proxy base URLs that have path prefixes (e.g., https://my-gateway.com/anthropic becomes https://my-gateway.com/anthropic/v1).