@ai-sdk/anthropic: ANTHROPIC_BASE_URL convention differs from official Anthropic SDK, breaks under Claude Code / Cursor

[FelixMalfait]

What happened

A Twenty server running inside Claude Code desktop kept getting 404 Not Found from every Anthropic call:

url: 'https://api.anthropic.com/messages'
model: 'claude-sonnet-4-5-20250929'

The 404 isn't the model — it's the path. Claude Code injects ANTHROPIC_BASE_URL=https://api.anthropic.com into child processes. @ai-sdk/anthropic reads that and appends /messages → wrong path.

The mismatch

The two Anthropic SDKs interpret ANTHROPIC_BASE_URL differently.

@anthropic-ai/sdk (src/client.ts):

• default baseURL: https://api.anthropic.com
• adds version itself: ${baseURL}/v1/messages

@ai-sdk/anthropic (anthropic-provider.ts):

• default baseURL: https://api.anthropic.com/v1
• appends raw path: ${baseURL}/messages

So ANTHROPIC_BASE_URL=https://api.anthropic.com works with the official SDK and 404s under this one. Setting …/v1 works here but would double up to /v1/v1/messages under the official SDK. Same env var, mutually exclusive meanings.

Why it matters

Anything that injects this env var following Anthropic's own convention silently breaks any app embedding @ai-sdk/anthropic. The user hit it via Claude Code; same trap exists for Cursor's agent tooling, LiteLLM proxy setups that document the bare-host form, and people who configured shells against the official SDK and later adopted ai.

The default https://api.anthropic.com/v1 keeps /messages working without an env var. The break only shows up when something else sets the env.

Reproduce

ANTHROPIC_BASE_URL=https://api.anthropic.com \
  ANTHROPIC_API_KEY=sk-ant-… \
  node -e "import('@ai-sdk/anthropic').then(async ({anthropic}) => {
    const {generateText} = await import('ai');
    console.log(await generateText({model: anthropic('claude-sonnet-4-5-20250929'), prompt: 'hi'}));
  })"

→ AI_APICallError: Not Found from https://api.anthropic.com/messages. Drop the env var (or append /v1) and it succeeds.

Possible fixes

1. Detect the bare-host form (https://api.anthropic.com with no path) and append /v1 with a one-time warning.
2. Switch to the official SDK's convention (default …anthropic.com, prepend /v1/messages internally).

[AkshatRaj00]

Confirmed + Deeper Analysis 🔍

Hey @FelixMalfait — great catch. I can reproduce this reliably and I've dug into the source to understand exactly what's happening.

Root Cause (Confirmed)

In packages/anthropic/src/anthropic-provider.ts, the default baseURL is hardcoded as:

baseURL: options.baseURL ?? loadApiKey({ ... }) ?? 'https://api.anthropic.com/v1',

And in anthropic-messages-language-model.ts, the request path is assembled as:

url: `${this.config.baseURL}/messages`

So the full URL resolves to:

• With env var ANTHROPIC_BASE_URL=https://api.anthropic.com → https://api.anthropic.com/messages ❌ (404)
• Without env var → https://api.anthropic.com/v1/messages ✅

Meanwhile, @anthropic-ai/sdk (official SDK) treats the env var as a bare host and internally appends /v1/messages, so ANTHROPIC_BASE_URL=https://api.anthropic.com works perfectly there.

Why This Silently Breaks Claude Code / Cursor Users

Claude Code injects ANTHROPIC_BASE_URL=https://api.anthropic.com (bare host, no /v1) into all child processes to enable its proxy. Any Node/Bun app using @ai-sdk/anthropic inside a Claude Code workspace immediately inherits a broken base URL — and the AI_APICallError: Not Found doesn't mention the path mismatch, so developers waste hours debugging it.

Proposed Fix

The cleanest fix IMO is to normalize the baseURL at construction time — strip a trailing /v1 if present, then always append it:

// In anthropic-provider.ts
function normalizeBaseURL(url: string): string {
  // Strip trailing slash and optional /v1 so we can re-append it consistently
  const stripped = url.replace(/\/+$/, '').replace(/\/v1$/, '');
  return `${stripped}/v1`;
}

const resolvedBaseURL = normalizeBaseURL(
  options.baseURL ?? process.env.ANTHROPIC_BASE_URL ?? 'https://api.anthropic.com'
);

This way:

• https://api.anthropic.com → https://api.anthropic.com/v1 ✅ (Claude Code compatible)
• https://api.anthropic.com/v1 → https://api.anthropic.com/v1 ✅ (no double /v1)
• Custom proxy https://my.proxy.com → https://my.proxy.com/v1 ✅
• Custom proxy https://my.proxy.com/v1 → https://my.proxy.com/v1 ✅

Impact

This isn't just a minor inconvenience — it's a silent breakage that affects:

1. Every developer using @ai-sdk/anthropic inside Claude Code or Cursor's agent mode
2. LiteLLM proxy users who followed Anthropic's official env var documentation
3. Anyone who configured ANTHROPIC_BASE_URL based on @anthropic-ai/sdk conventions

I'm happy to submit a PR with the normalizeBaseURL fix + a unit test covering all four cases above. Would the maintainers prefer approach (1) from the issue — detect and normalize — or approach (2) — align with the official SDK's convention entirely?

[lgrammel]

Identify issue type

{
  "evidence": [
    "Description reports `@ai-sdk/anthropic` reads `ANTHROPIC_BASE_URL=https://api.anthropic.com` and calls `https://api.anthropic.com/messages`, causing `AI_APICallError: Not Found`.",
    "Reproduction command shows setting the env var produces a 404, while dropping it or appending `/v1` succeeds.",
    "Comment confirms source behavior: default base URL includes `/v1`, request appends `/messages`, and the env var bare-host form results in an invalid path."
  ],
  "rationale": "The issue describes a concrete failure in current SDK behavior: a commonly injected `ANTHROPIC_BASE_URL` value leads to incorrect request URLs and 404 errors. Although fixes are proposed, the core report is broken behavior.",
  "type": "bug",
  "confidence": "high"
}

[lgrammel]

Bug Reproduction

Bug was reproduced.

Reproduced with live Anthropic call: ANTHROPIC_BASE_URL=https://api.anthropic.com caused AI_APICallError 404 at https://api.anthropic.com/messages. Added runnable failing artifact reproductions/issue-15542-anthropic-base-url.mjs and ran node reproductions/issue-15542-anthropic-base-url.mjs, which exited 1 after confirming the wrong URL.

Branch: bug-reproduction/15542-2

[lgrammel]

Bugfix

Created bugfix PR:

#16584