feat: add support for ANTHROPIC_CUSTOM_HEADERS environment variable

[ibetitsmike]

Overview

This PR adds support for the ANTHROPIC_CUSTOM_HEADERS environment variable, allowing users to inject custom HTTP headers into all Anthropic API requests.

Changes

• Configuration: Added CustomHeaders field to AnthropicConfig
• Parsing: Implemented parseCustomHeaders() to parse the environment variable
  • Format: Name: Value with multiple headers separated by newlines
  • Handles edge cases: URLs with colons, whitespace, empty lines, malformed headers
• Injection: Modified newAnthropicClient() to inject headers using option.WithHeader()
• Tests: Added 15 comprehensive test cases
  • TestParseCustomHeaders: 12 test cases for parsing logic
  • TestCustomHeadersIntegration: 3 test cases for API integration
• Compatibility: Fixed test helpers for new config structure

Usage Example

# Single header
export ANTHROPIC_CUSTOM_HEADERS="X-Custom-Header: my-value"

# Multiple headers
export ANTHROPIC_CUSTOM_HEADERS="X-Header-1: value1
X-Header-2: value2"

# Headers with complex values (URLs, etc.)
export ANTHROPIC_CUSTOM_HEADERS="X-Callback-URL: https://example.com:8080/webhook"

Testing

✅ All tests pass (1.486s)
✅ Race detection passes (2.463s)
✅ No linting issues introduced
✅ Code formatted with gofumpt

Files Modified

• config.go - Added CustomHeaders field
• provider_anthropic.go - Header parsing and initialization
• intercept_anthropic_messages_base.go - Header injection
• anthropic_internal_test.go - Comprehensive tests
• bridge_integration_test.go - Test helper fixes

Total: 285 lines added across 5 files

[dannykopping]

Can you augment this to allow custom headers to be given generically, and then we inject them regardless of the provider?

[matifali]

Can you augment this to allow custom headers to be given generically, and then we inject them regardless of the provider?

Can there be a case where the provided headers don't apply to both OpenAI and Anthropic? Just make sure we handle both of them independently and gracefully (if applicable)

[dannykopping]

Can you augment this to allow custom headers to be given generically, and then we inject them regardless of the provider?

Can there be a case where the provided headers don't apply to both OpenAI and Anthropic? Just make sure we handle both of them independently and gracefully (if applicable)

We have provider-specific config, and they both embed the ProviderConfig type: https://github.com/coder/aibridge/blob/main/config.go

So yes, one could configure it on a per-provider basis.