Fix HTTP 400 errors after Claude Code CLI tool call cancellation

[Copilot]

This PR fixes a critical issue where cancelling tool calls in Claude Code CLI would cause all subsequent API requests to fail with HTTP 400 errors.

Problem

When a user cancels a tool call in Claude Code CLI, the conversation history retains tool_use blocks without corresponding tool_result blocks. The Anthropic API strictly requires that every tool_use block be followed by a matching tool_result block in the next message. Without this pairing, all subsequent requests fail with:

HTTP 400: tool_use ids were found without tool_result blocks immediately after: toolu_vrtx_01GG3T6npgARqCjxkn9Fm9KW. 
Each tool_use block must have a corresponding tool_result block in the next message.

This creates a persistent error state where the API becomes unusable until the conversation is reset.

Solution

Added intelligent tool conversation validation and cleanup that:

1. Detects abandoned tool calls: Identifies tool_use blocks followed by user messages without corresponding tool_result blocks
2. Preserves valid pending calls: Keeps tool_use blocks at conversation end (still awaiting results)
3. Removes only abandoned calls: Only strips tool_use blocks when there's clear evidence of cancellation
4. Maintains conversation integrity: Preserves all other message content and structure

Example

Before the fix, this conversation would cause persistent 400 errors:

{
  "messages": [
    {"role": "user", "content": "What's the weather?"},
    {
      "role": "assistant", 
      "content": [
        {"type": "text", "text": "I'll check for you."},
        {"type": "tool_use", "id": "call_123", "name": "get_weather", "input": {...}}
      ]
    },
    {"role": "user", "content": "Actually, never mind."}
  ]
}

After the fix, the abandoned tool_use block is automatically removed, preventing API rejection.

Implementation

• Minimal changes: Only 6 lines modified in existing translation logic
• Surgical approach: New isolated cleanup module with comprehensive test coverage
• Zero impact: No effect on valid tool conversations or pending tool calls
• Smart detection: Distinguishes between abandoned vs. pending tool calls

The fix integrates seamlessly into the existing Anthropic message translation pipeline, ensuring all problematic payloads are cleaned before reaching the API.

Fixes #1.

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[hao1939]

run the test

[Copilot]

run the test

Tests have been successfully executed. All 36 tests pass across 5 test files, including the tool conversation cleanup functionality. The code also passes linting and builds successfully.

[hao1939]

LSTM