fix(agent): close unclosed JSON tool-call args in LIFO order

[swissly]

Summary

_repair_tool_call_arguments in agent/message_sanitization.py closed unclosed JSON structures by counting openers: it appended all } then all ]. For nested input like {"a": [1,2 this produced {"a": [1,2}] — invalid JSON — which then fell through to the {} last resort, silently discarding the model's arguments.

Root cause

# BEFORE — append all closers of one kind, then the other
open_curly = fixed.count('{') - fixed.count('}')
open_bracket = fixed.count('[') - fixed.count(']')
if open_curly > 0:  fixed += '}' * open_curly
if open_bracket > 0: fixed += ']' * open_bracket

Counting is order-insensitive. With {"a": [1,2 it appends } first, producing {"a": [1,2}], then json.loads fails → UNREPAIRABLE → {}.

Fix

Track the opening stack and append closers in LIFO order (last opened, first closed), so nested structures repair to valid JSON:

# AFTER
stack = []
for ch in fixed:
    if ch == "{": stack.append("}")
    elif ch == "[": stack.append("]")
    elif ch in ("}", "]"):
        if stack and stack[-1] == ch: stack.pop()
for closer in reversed(stack):
    fixed += closer

• {"a": [1,2 → {"a": [1,2]} (was {})
• {"a": [1, {"b": 2 → {"a": [1, {"b": 2}]}
• Balanced JSON is untouched (verified).

Tests

Added 5 regression tests to tests/run_agent/test_repair_tool_call_arguments.py (single brace, single bracket, nested LIFO, nested object-in-array, balanced passthrough). Verified the LIFO regression test fails on the buggy code and passes with the fix.

Validation

• pytest tests/run_agent/test_repair_tool_call_arguments.py → 8 passed
• No behavioral change to the repair pipeline's other stages (trailing commas, control-char escape, excess-closer removal, {} last resort).

[Copilot]

Pull request overview

This PR fixes a correctness bug in Hermes’ tool-call argument sanitization pipeline: when repairing truncated JSON, the previous “count openers then append closers” approach could generate invalid JSON for nested structures, causing _repair_tool_call_arguments to fall back to {} and silently discard the model’s intended tool arguments.

Changes:

• Update _repair_tool_call_arguments to close unclosed {/[ structures using a delimiter stack and append missing closers in LIFO order.
• Add regression tests covering single-delimiter truncation, nested truncation (the LIFO bug), and balanced JSON passthrough.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
agent/message_sanitization.py	Fixes JSON truncation repair by closing missing delimiters in LIFO order to preserve nested validity.
tests/run_agent/test_repair_tool_call_arguments.py	Adds regression tests to prevent reintroducing invalid closer ordering and to validate nested repair behavior.

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[isak-ialogics]

The LIFO stack still treats {/[ characters inside JSON strings as structural delimiters. On this head, _repair_tool_call_arguments('{"a":"[","b":[1,2', "t") still returns {} instead of {"a":"[","b":[1,2]} because the literal [ adds a spurious stack entry. Suggested next action: make the delimiter scan string/escape-aware (track quoted-string state and backslash escapes), and add this case as a regression so nested truncation with delimiter-valued arguments is preserved rather than discarded.

[swissly]

Thanks for the sharp catch — confirmed and fixed. The string-blind stack treated {/[ inside quoted string values as structural delimiters, so '{"a":"[","b":[1,2' pushed a spurious ] for the literal [, left the string unterminated, and fell back to {}.

Fix (commit 4a9d49f883): the delimiter scan now tracks quoted-string state and backslash escapes — {/[ inside a string are literal characters, not structure. Closing is still LIFO.

Regression tests added (4):

• bracket-in-string value ('{"a":"[","b":[1,2' → {"a":"[","b":[1,2]})
• brace-in-string value
• backslash-escaped quote inside a string (does not close it)
• delimiters after a closed string

Verified both core cases fail on the string-blind code and pass with the fix; full file 12/12 green.

[swissly]

Ping @teknium1 — this PR is mergeable/clean and ready for review (all review threads addressed, CI green). Open since 2026-08-03.