hermes - 💡(How to fix) Fix [BUG] Context compaction truncates tool_call.function.arguments mid-string, produces invalid JSON, causes 400 errors with strict providers (e.g. MiniMax) [2 comments, 2 participants]

Summary

agent/context_compressor.py truncates tool_calls[*].function.arguments by raw string slicing (args[:200] + "...[truncated]"), which breaks JSON validity. Providers that strictly validate arguments as JSON (confirmed: MiniMax api.minimaxi.com) return HTTP 400 on subsequent requests, persistently poisoning the session because the truncated tool_call stays in history and gets replayed every turn.

Reproduction

1. Main model: MiniMax-M2.7 (any strict-JSON provider would trigger this)
2. Trigger a tool_call with long arguments (e.g. skill_manage with multi-KB markdown content field)
3. Let conversation grow until context compaction kicks in (compression.threshold default 0.5)
4. Post-compaction, observe tool_calls[0].function.arguments truncated to {"action": "create", "content": "# ...[truncated] — JSON string value with unterminated quote
5. Next API call returns:

   HTTP 400 invalid function arguments json string,
   tool_call_id: call_function_XXX (2013)

6. Every subsequent turn re-sends this broken history → persistent 400 → session must be archived to recover.

Root cause

agent/context_compressor.py around line 461 (Pass 3 of _prune_old_tool_results):

if len(args) > 500:
    tc = {**tc, "function": {**tc["function"],
                             "arguments": args[:200] + "...[truncated]"}}

Raw string chop doesn't preserve JSON validity. If truncation falls inside a string value, the value's closing quote (plus any nested structure) is lost. The concatenated "...[truncated]" suffix is literal text, not valid JSON continuation.

Proposed fix

Replace the string slice with a JSON-valid stub that preserves the fingerprint of the original call:

if len(args) > 500:
    try:
        parsed = json.loads(args)
        keys = list(parsed.keys()) if isinstance(parsed, dict) else []
        stub = json.dumps({
            "_truncated": True,
            "_original_length": len(args),
            "_keys": keys,
        }, ensure_ascii=False)
    except (json.JSONDecodeError, TypeError):
        stub = json.dumps({
            "_truncated": True,
            "_original_length": len(args),
            "_preview": args[:150],
        }, ensure_ascii=False)
    tc = {**tc, "function": {**tc["function"], "arguments": stub}}
    modified = True

Related community reports (all closed as not planned)

• kilocode #3967
• kilocode #3995
• kilocode #4350

All three report the same MiniMax (2013) error pattern. None diagnosed that the bug is in the client-side compaction module, not in the API layer.

Impact

• MiniMax (confirmed) — any session that triggers compaction with large tool_call arguments becomes permanently broken
• Likely affects other strict-JSON providers
• Permissive providers (OpenAI, Anthropic) may accept the truncated arguments silently, leading to subtle mis-behaviors in replayed history

Environment

• Hermes Agent v0.10.0 (v2026.4.16)
• Python 3.11.15
• MiniMax-M2.7 via api.minimaxi.com/v1 (OpenAI-compatible chat_completions)
• Ubuntu (Tencent Cloud Shanghai)