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claude-code - 💡(How to fix) Fix [Bug] Opus 4.7 with extended thinking intermittently emits unparseable tool_use blocks

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Root Cause

Findings from my local session logs (~/.claude/projects/.../*.jsonl), 5/24-5/26:

  • ~20 "malformed and could not be parsed" retry prompts across 6 sessions; one session (83889c9d) hit it 8 times.
  • The failing assistant turns have stop_reason="tool_use" but NO parseable tool_use block — only a thinking block is present.
  • NOT caused by size: failing turns were only 164-1093 output tokens.
  • NOT caused by special chars: a plain 119-char single-line SQL (SELECT ... FROM information_schema.columns ...) triggered it.
  • Affects multiple tools: Read, Bash, mcp execute_sql.
  • Common factor: every failing turn is Opus 4.7 with an extended-thinking block.

Code Example

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Bug Description

Title: Opus 4.7 + extended thinking intermittently emits unparseable tool_use blocks (small calls too)

Body: Frequent "The model's tool call could not be parsed (retry also failed)" hard-stops. CLI 2.1.150 (latest). Model: claude-opus-4-7.

Findings from my local session logs (~/.claude/projects/.../*.jsonl), 5/24-5/26:

  • ~20 "malformed and could not be parsed" retry prompts across 6 sessions; one session (83889c9d) hit it 8 times.
  • The failing assistant turns have stop_reason="tool_use" but NO parseable tool_use block — only a thinking block is present.
  • NOT caused by size: failing turns were only 164-1093 output tokens.
  • NOT caused by special chars: a plain 119-char single-line SQL (SELECT ... FROM information_schema.columns ...) triggered it.
  • Affects multiple tools: Read, Bash, mcp execute_sql.
  • Common factor: every failing turn is Opus 4.7 with an extended-thinking block.

Looks like a tool_use serialization defect when extended thinking is active. Auto-retry sometimes recovers; "retry also failed" is when the retry repeats it.

Environment Info

  • Platform: darwin
  • Terminal: ghostty
  • Version: 2.1.150
  • Feedback ID: 59d64386-04ba-4c37-ace9-f2c3a0830f55

Errors

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