anthropic-proxy

A local HTTP proxy that sits between Claude Code and the Anthropic API. It rewrites the system prompt before forwarding — letting you swap in custom personalities, tone, or style instructions.

Usage

go run . -port 8080

Then start Claude Code pointing at the proxy:

ANTHROPIC_BASE_URL=http://localhost:8080 claude

System prompt rewriting

Create a prompts/ directory to modify the system prompt before it reaches Anthropic.

Full replacement

Replace an entire system prompt block by index:

prompts/system-2-replace.txt

The file contents become the new text for that block.

Find and replace

Create prompts/replacements.yaml:

- block: 2
  find: |
    # Tone and style
     - Your responses should be short and concise.
  replace: |
    # Custom personality
    You are a friendly pirate. Respond in pirate speak.

- block: 3
  find: "some text"
  replace: "new text"
  disabled: true  # skip this rule

Full replacement files take precedence over find-and-replace rules for the same block. Unmatched find rules log a warning.

Block indices

The Claude Code system prompt is an array of text blocks. Typically:

Index	Content
0	Billing metadata
1	Identity intro
2	Main instructions, tone, and style
3	Memory and environment

Blocks 0 and 1 are generally left unchanged. Blocks 2 and 3 are where personality and style live.

Request logging

Opt-in with the -log flag:

go run . -port 8080 -log

Each API call creates files in requests/ named {timestamp}-{request-id}-{model}-{part}.json:

• tools.json — tool definitions
• messages.json — conversation messages
• system.json — system prompt
• usage.json — token usage from the response