Custom baseURL configuration returns "Route /api/messages not found" error

[nicehiro]

Question

Hi, thanks for your great work!

I'm trying to use opencode with Claude via a custom API endpoint. The same configuration works fine with Claude Code, but opencode returns the error: Not Found: Route /api/messages not found.

Environment Variables (working with Claude Code)

export ANTHROPIC_BASE_URL="my_url"
export ANTHROPIC_AUTH_TOKEN="my_key"

opencode Configuration

~/.local/share/opencode/auth.json:

{
  "anthropic": {
    "type": "api",
    "key": "my_key"
  }
}

~/.config/opencode/opencode.json:

{
  "$schema": "https://opencode.ai/config.json",
  "provider": {
    "anthropic": {
      "options": {
        "baseURL": "my_url"
      }
    }
  }
}

Error logs:

ERROR 2025-12-06T09:45:46 +56ms service=session.prompt error={"error":{"name":"AI_APICallError","url":"my_url","requestBodyValues":{"model":"claude-opus-4-5-20251101","max_tokens":32000, xxx
ERROR 2025-12-06T09:45:46 +0ms service=session.processor error=Not Found stack="AI_APICallError: Not Found\n    at <anonymous> (../../node_modules/.bun/@ai-sdk+provider-utils@3.0.18+d6123d32214422cb/node_modules/@ai-sdk/provider-utils/dist/index.mjs:950:18)\n    at processTicksAndRejections (native:7:39)" process

[github-actions]

This issue might be a duplicate of existing issues. Please check:

• max_tokens defaults to 32000 when using a custom provider #1735: max_tokens defaults to 32000 when using a custom provider - related to custom provider configuration issues
• Getting an issue integrating a custom provider: AI_TypeValidationError, failed type validation #1298: Getting an issue integrating a custom provider: AI_TypeValidationError - related to custom provider integration problems
• LiteLLM error: Anthropic doesn't support tool calling without tools= param specified #2915: LiteLLM error: Anthropic doesn't support tool calling without tools= param specified - related to Anthropic-specific configuration with custom providers
• ProviderInitError with Anthropic #1635: ProviderInitError with Anthropic - related to Anthropic API configuration issues

Feel free to ignore if none of these address your specific case.

[nicehiro]

Found a workaround.

Appending /v1 to the base URL (i.e., my_url/v1) resolves the issue.

I'm leaving this open as it may warrant either:

• A bug fix to align with how the official Anthropic SDK handles
• Documentation clarifying that baseURL should include /v1