fix: sanitize malformed tool call arguments to prevent vLLM 400 errors

[thanhnnict]

Problem

When LLMs generate malformed JSON in tool_calls.function.arguments (e.g., missing commas, truncated due to max_tokens), Continue stores this in conversation history and sends it back to the server on subsequent turns. vLLM validates JSON in tool_calls arguments during request preprocessing and rejects malformed input with a 400 error, breaking the conversation.

Solution

Validate JSON before sending tool_calls arguments back to server. If invalid, wrap in a valid JSON envelope ({"_raw": original}) to prevent server rejection while preserving the original content.

Behavior

Input arguments	Valid JSON?	Output
{"path": "/tmp/x"}	✅	Pass-through unchanged
{"path": "/tmp/x" "content": "hi"}	❌	{"_raw": "..."}
"" (empty)	—	"{}" (fallback)
null / undefined	—	"{}" (fallback)

Root Causes of Malformed JSON

1. max_tokens truncation — cuts off mid-JSON
2. Reasoning token leak — thinking tokens leak into arguments field
3. Model hallucination — LLM occasionally produces invalid syntax
4. Encoding issues — special characters not properly escaped

Trade-offs

• Performance: JSON.parse() on every tool_call argument — negligible cost (microseconds)
• Semantics: Wrapped {"_raw": "..."} may confuse model, but strictly better than 400 error
• No data loss: Original malformed string preserved in _raw field

Related

• [Bug]: vllm-project/vllm#43995

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