Error 400 with Tool Use: Number of toolResult blocks exceeds toolUse blocks

[BillWang-dev]

When attempting to use the Tool Use (Function Calling) feature via anthropic-proxy (with the backend model anthropic/claude-3.5-haiku routed through OpenRouter, where the error message indicates the actual provider is Amazon Bedrock), the API returns a 400 error. The error message states that the number of toolResult blocks in the request's messages exceeds the number of toolUse blocks from the model's previous turn.

Error Message:

  ⎿  Invalid tool parameters
  ⎿  API Error: 400 {"error":"{\"error\":{\"message\":\"Provider returned 
     error\",\"code\":400,\"metadata\":{\"raw\":\"{\\\"message\\\":\\\"The number of 
     toolResult blocks at messages.4.content exceeds the number of toolUse blocks of 
     previous turn.\\\"}\",\"provider_name\":\"Amazon 
     Bedrock\"}},\"user_id\":\"user_2m17fqmm2tL3UEVCpAlvfXvsXx5\"}"}

[xiangz19]

@BillWang-dev , I had similar issues, and I let cursor with GPT-4.1 to fix them for a while, and finally I gave up. Since I mainly uses claude modals on openrouter for testing. It should be easier than non-claude modals, so I asked GPT-4.1 to write a proxy from scratch. It works, I almost open sourced the code, then I found a repo https://github.com/1rgs/claude-code-proxy, which uses litellm, since it has 700+ stars, you can give it a try.
Actually claude code documention has instructions on working with litellm, you can install litellm proxy and configure it manually.

[BillWang-dev]

@xiangz19 thanks!

[xiangz19]

@BillWang-dev , I had similar issues, and I let cursor with GPT-4.1 to fix them for a while, and finally I gave up. Since I mainly uses claude modals on openrouter for testing. It should be easier than non-claude modals, so I asked GPT-4.1 to write a proxy from scratch. It works, I almost open sourced the code, then I found a repo https://github.com/1rgs/claude-code-proxy, which uses litellm, since it has 700+ stars, you can give it a try. Actually claude code documention has instructions on working with litellm, you can install litellm proxy and configure it manually.

sorry, it seems https://github.com/1rgs/claude-code-proxy doesn't support openrouter directly. (there is an open pull request adding openrouter support)

Anyway, you can install and configure litellm to use openrouter model.

[xiangz19]

@BillWang-dev , I just noticed https://github.com/1rgs/claude-code-proxy now supports openrouter too.

[St1ma]

@xiangz19 Did you succeed with claude-code-proxy?
It seems like they support OpenAI models, not OpenRouter directly

[xiangz19]

@xiangz19 Did you succeed with claude-code-proxy? It seems like they support OpenAI models, not OpenRouter directly

I haven't try it. I don't see openrouter mentioned now, it's only in some pull request.