feat: Add native base_url and custom_headers inputs for LLM Gateway integration

[GuitaristForEver]

Problem

Users integrating Claude Code Action with LLM Gateways, API proxies, or API management solutions currently need to use env: blocks to set ANTHROPIC_BASE_URL and ANTHROPIC_CUSTOM_HEADERS. This is:

• Not discoverable - inputs don't appear in action.yml or IDE autocomplete
• Requires custom scripting - users must understand env var passthrough
• Confusing format - users don't know headers need JSON format (see AWS Bedrock via porkey in claude code action #704)

Related Discussions (Users Currently Blocked)

• AWS Bedrock via porkey in claude code action #704 - "AWS Bedrock via portkey" - User stuck with incorrect header format
• How to use portkey in claude code gh action? #575 - "How to use portkey in claude code gh action?" - No solution provided
• Connect claude-code-action with the LLM Gateway #272 - "Connect claude-code-action with the LLM Gateway" - Workaround via env blocks
• What to configure my own base url #551 - "What to configure my own base url" - Same pattern

Proposed Solution

Add native inputs that mirror the existing pattern for anthropic_api_key:

- uses: anthropics/claude-code-action@v1
  with:
    anthropic_api_key: ${{ secrets.ANTHROPIC_API_KEY }}
    base_url: "https://your-gateway.example.com"
    custom_headers: '{"x-portkey-api-key": "${{ secrets.PORTKEY_KEY }}"}'

New Inputs

Input	Description	Default
base_url	Custom API endpoint URL for LLM Gateway/proxy	""
custom_headers	Custom HTTP headers as JSON string	""

Backwards Compatibility

Inputs take precedence but fall back to env vars:

ANTHROPIC_BASE_URL: ${{ inputs.base_url || env.ANTHROPIC_BASE_URL }}
ANTHROPIC_CUSTOM_HEADERS: ${{ inputs.custom_headers || env.ANTHROPIC_CUSTOM_HEADERS }}

Use Cases

1. LLM Gateways - Route through enterprise gateways per Anthropic docs
2. Portkey/LiteLLM - Observability and fallback routing
3. API Management - Azure APIM, AWS API Gateway, Kong policies
4. Custom Auth - Subscription keys, bearer tokens, correlation IDs

Implementation

Simple YAML changes to both action.yml files + documentation updates. No TypeScript changes needed - the SDK already reads these env vars.

[GuitaristForEver]

PR #841 has been created to address this! 🎉

The implementation adds native base_url and custom_headers inputs for easy LLM Gateway integration without requiring custom env: scripting.

This should unblock the users in discussions #272, #575, and #704.

Example usage:

- uses: anthropics/claude-code-action@v1
  with:
    anthropic_api_key: ${{ secrets.API_KEY }}
    base_url: "https://api.portkey.ai"
    custom_headers: '{"x-portkey-api-key": "${{ secrets.KEY }}"}'

See the PR for full documentation with examples for Portkey, LiteLLM, and Azure APIM.