claude-fake-api

claude-fake-api

Stop burning API credits while you're testing your AI SaaS locally.

claude-fake-api is a Claude Code plugin that runs a local HTTP server on your machine that speaks the Anthropic Messages API. When your app calls it, the request gets forwarded — via the Claude Agent SDK — to your logged-in Claude Code session. That means the call is billed against your Claude Max / Pro subscription, not your API key.

Flip one env var. Keep coding.

What problem does it solve?

You're building a SaaS that uses Claude. Every time you test a PDF extractor, a chat feature, or a prompt change locally, you burn API credits. For exploratory work, that's annoying. For hot reload + iterate loops, it's painful.

With claude-fake-api running, your dev environment points at http://localhost:8787 instead of https://api.anthropic.com. The exact same SDK code works — it just goes through your Claude subscription.

This is for local development only. Do not point production traffic at this. It's a convenience tool, not an API replacement.

Scope (v1)

• ✅ POST /v1/messages — text-only messages, streaming + non-streaming
• ✅ Official anthropic Python + Node SDKs (set ANTHROPIC_BASE_URL)
• ✅ Per-request model passthrough (Opus / Sonnet / Haiku)
• ❌ Tool use (tool_use / tool_result blocks)
• ❌ Vision (image blocks)
• ❌ Files, batches, or any other endpoint

Unsupported requests return a clear 400 explaining what's missing.

Install

As a Claude Code plugin (recommended):

/plugin marketplace add dishishshawn/claude-fake-api
/plugin install claude-fake-api@claude-fake-api

The first time you run /fake-api start, the plugin will auto-install its npm dependencies (one-time, ~5 seconds).

Or clone and run standalone (no Claude Code plugin system):

git clone https://github.com/dishishshawn/claude-fake-api.git
cd claude-fake-api
npm install
node src/cli.mjs start

Use

Inside Claude Code:

/fake-api start

Claude Code will print something like:

claude-fake-api started on port 8787 (pid 12345).

Set these in the terminal where your app runs:
  export ANTHROPIC_BASE_URL=http://localhost:8787
  export ANTHROPIC_API_KEY=sk-fake-any-value

Paste those exports into the terminal where your SaaS backend runs. Then do your normal thing — client.messages.create(...) will now route through your Claude subscription.

When you're done:

/fake-api stop

Or check what's running:

/fake-api status

How it works

┌─────────────────────┐   POST /v1/messages     ┌──────────────────────┐
│  Your SaaS backend  │ ─────────────────────▶  │  claude-fake-api     │
│  (anthropic SDK)    │                         │  localhost:8787      │
└─────────────────────┘                         └──────────┬───────────┘
                                                           │
                                           translate req → │ query({...})
                                                           ▼
                                                ┌──────────────────────┐
                                                │  Claude Agent SDK    │
                                                │  (your Claude Code   │
                                                │   OAuth credentials) │
                                                └──────────┬───────────┘
                                                           │
                                                           ▼
                                                 Billed to Max/Pro sub

The proxy exposes the Anthropic Messages wire format. Under the hood it calls query() from @anthropic-ai/claude-agent-sdk, which reuses the OAuth token your claude CLI is already logged in with.

Limits & caveats

• One request at a time per host Claude session. If your app hammers the proxy in parallel, requests queue.
• Token counts are estimates (char-length / 4). Don't rely on usage for billing math.
• No tool use / images in v1. PRs welcome.
• Stop reasons are always end_turn. The underlying SDK doesn't surface max_tokens / stop_sequence in a way we can forward cleanly yet.
• Dev only. Seriously. Don't ship this anywhere.

License

MIT © dishishshawn