fix(agent): close unclosed JSON delimiters in LIFO order when repairing truncated tool_call arguments

[Frowtek]

What does this PR do?

_repair_tool_call_arguments salvages malformed tool-call JSON from
local/quantized models. When closing unclosed structures it appended all
missing } first, then all missing ] — a fixed order that is wrong for
nested payloads. {"items": [1, 2 became {"items": [1, 2}] (invalid)
instead of {"items": [1, 2]}, every later repair pass then failed, and the
whole argument payload was silently dropped to {}. The tool then ran with
empty arguments.

The fix walks the text tracking string state (so braces/brackets inside
string values are ignored) and closes open delimiters in LIFO nesting order,
so nested truncations recover correctly. The flat-object case keeps working.

Related Issue

N/A

Type of Change

• 🐛 Bug fix (non-breaking change that fixes an issue)

Changes Made

• agent/message_sanitization.py — replace the count-and-append delimiter
  close in _repair_tool_call_arguments with a string-aware, stack-based
  LIFO close. Trailing-comma strip and the excess-closer pass are unchanged.
• tests/run_agent/test_repair_tool_call_arguments.py — turn the misleading
  test_unclosed_bracket_and_brace (asserted only "valid JSON", so it passed
  on the dropped {}) into a real value assertion, plus 5 regression tests.

How to Test

pytest tests/run_agent/test_repair_tool_call_arguments.py -q
pytest tests/run_agent/test_streaming_tool_call_repair.py -q

Results:

• test_repair_tool_call_arguments.py — 26 passed
• test_streaming_tool_call_repair.py (uses the same repair path) — 12 passed

Before the fix, {"items": [1, 2 → {} (args lost); after, it recovers to
{"items": [1, 2]}.

Checklist

• I've read the Contributing Guide
• My commit messages follow Conventional Commits
• I searched for existing PRs to make sure this isn't a duplicate
• My PR contains only changes related to this fix
• All tests pass
• I've added tests for my changes
• I've updated relevant documentation — N/A (internal helper)

[liuhao1024]

Positive verification — LIFO JSON delimiter repair is correct and well-tested.

Reviewed the full diff (agent/message_sanitization.py + tests).

The replacement of the naive count-and-append approach ('}' * n then ']' * m) with a proper stack-based LIFO closer is correct. The key improvements:

• String-awareness: The in_string / escaped state machine correctly ignores braces/brackets inside string values, so {"msg": "a { b" doesn't count the { as an open delimiter.
• LIFO ordering: reversed(stack) produces closers in nesting order — {"k": [1, 2 → ]} (valid) instead of }] (invalid).
• No regression on flat cases: Simple {"a": 1, "b": 2 still closes correctly.

Test coverage is thorough: unclosed bracket+brace, array of strings, nested write_file args, object inside array, flat object, and brace-in-string-value. All 6 new test cases exercise distinct edge paths.

No issues found.

[tonydwb]

Code Review Summary

Verdict: Approved

Correctness

• Critical bug fix: the old count-based approach closed delimiters in wrong order (} then ]), which would corrupt nested JSON like {"k": [1, 2 -> invalid {"k": [1, 2]}.
• The new LIFO stack-based approach correctly closes in reverse order of opening.
• Properly handles string literals (ignores braces/brackets inside quoted strings).

Code Quality

• Clean implementation using a stack with proper string/escape state tracking.
• Comments clearly explain the edge case being addressed.

Testing

• Excellent test coverage with 6 new test cases covering:
  • Unclosed bracket and brace (the original bug)
  • Unclosed array of strings
  • Unclosed nested write_file args
  • Object inside array closes LIFO
  • Flat object regression guard
  • Brace inside string value not treated as delimiter

Looks Good

• Important bug fix with comprehensive test coverage. The fix is minimal and focused.

---

Reviewed by Hermes Agent

[teknium1]

Thanks for the focused investigation and regression coverage. The nested-delimiter case is real in the existing legacy repair path, but this PR takes a repair direction we do not accept.

• Automated hermes-sweeper review: agent/message_sanitization.py:227-233 currently contains the legacy delimiter-repair logic, and this PR adds a new stack-based reconstruction pass for malformed/truncated model output.
• This matches the standing model-output-repair policy: Hermes does not add new content-reconstruction passes for malformed tool-call JSON, including delimiter/boundary reconstruction.
• Please keep model output valid upstream or focus a follow-up on a verified wire-transport defect rather than reconstructing malformed model arguments.

---

Closed as not-planned per standing maintainer policy (model-output-repair). This is a design-direction decision, not a code-quality judgment — see the Contribution Rubric in AGENTS.md for what the project is looking for. If you believe this policy was misapplied to your change, comment here and a maintainer will take a look.