Summary

• Pass 3 of _prune_old_tool_results sliced the raw JSON of function.arguments mid-string and appended the literal text ...[truncated], producing invalid JSON
• Strict providers (observed on MiniMax) 400 this payload as non-retryable with invalid function arguments json string
• The broken call survives in the session history, so every subsequent turn re-sends the same malformed payload and locks the session in a re-send loop
• Fix: parse the arguments, shrink long string leaves inside the parsed structure, re-serialise. Non-string leaves (paths, ints, lists) pass through. Non-JSON arguments pass through unchanged

Before / after

Original (800 chars):

{"path": "~/.hermes/skills/shopping/browser-setup-notes.md",
 "content": "# Shopping Browser Setup Notes\n\n## Overview\n...[~700 more chars]"}

Old output (invalid JSON — caused the 400):

{"path": "~/.hermes/skills/shopping/browser-setup-notes.md", "content": "# Shopping Browser Setup Note...[truncated]

New output (valid JSON, same shrunk quality):

{"path": "~/.hermes/skills/shopping/browser-setup-notes.md",
 "content": "# Shopping Browser Setup Notes\n\n## Overview\n...[truncated]"}

Reproducer

Exact scenario that hit this in the wild:

1. Agent session writes a ~730-byte markdown file via write_file
2. Session grows, compressor kicks in
3. Pass 3 truncates the arguments JSON past its opening quote and never closes it
4. Next turn sends history including that call → MiniMax returns 400 invalid function arguments json string, tool_call_id: call_function_l22bfzt6fe2y_1 (2013)
5. Error is non-retryable and history is immutable — loop

Test plan

• pytest tests/agent/test_context_compressor.py -v — 48 passed (40 existing + 8 new TestTruncateToolCallArgsJson)
• New end-to-end test reproduces the failure payload shape through _prune_old_tool_results Pass 3 and asserts the output parses as JSON
• Unicode test asserts ensure_ascii=False so CJK content (which triggered the reproducer — markdown about 非德满) stays intact rather than bloating with \uXXXX escapes
• Non-JSON arguments pass through untouched

Notes

• Threshold unchanged: arguments >500 chars get shrunk; each long string leaf is capped at 200 chars + ...[truncated] marker
• Backwards compatible with any existing downstream consumers that parse function.arguments as JSON — only strengthens the contract

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