fix: parse nested bare JSON tool calls

[alectimison-maker]

Summary

• replace the flat bare-JSON tool-call regex with a string-aware balanced-object scanner
• recover nested argument objects and multiple bare calls in model text
• keep the Chrome and Firefox parser modules byte-identical and add regressions

Motivation

Local and OpenAI-compatible model backends can emit a tool call in ordinary message content
instead of the structured tool_calls field. The existing bare-JSON fallback matched only
objects without nested braces, so a valid call such as a click with nested metadata was
ignored and could surface as final assistant text instead of executing.

Design

The fallback now scans the already bounded model text for balanced JSON objects while respecting
quoted strings and escapes. Each candidate still has to parse as JSON and pass the existing tool
allowlist. Wrapped JSON, XML, call:name{} handling, the 10,000-character cap, and the OpenAI-style
fallback output shape are unchanged.

The scanner is used only when the earlier wrapped/XML parsers did not find a call. A greedy regex
was rejected because it can merge adjacent calls, and parsing the entire response was rejected
because models commonly include prose around tool-call JSON.

Testing

• targeted nested-object, escaped-string, multiple-call, allowlist, and Chrome/Firefox parity checks — passed
• node test/security/injection-corpus.mjs — 60/60 passed
• node --check src/chrome/src/agent/tool-call-parser.js — passed
• node --check src/firefox/src/agent/tool-call-parser.js — passed
• node test/run.js — 1,449 passed; one pre-existing repository-version assertion fails because package.json is 26.0.10 while the newest CHANGELOG.md entry is 26.0.0
• git diff --check — passed

Compatibility and risks

No public API or provider behavior changes for already supported formats. The parser remains
bounded and allowlisted. The main residual risk is accepting a valid allowlisted JSON object
embedded in explanatory model text; that is the intended purpose of this fallback and matches
the previous flat-object behavior.

Scope

This does not add new tool-call syntaxes, change provider normalization, or relax parser limits.

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[Copilot]

🟡 Changes recommended

An unmatched opening brace before a valid tool call prevents the scanner from recovering that call.

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Pull request overview

Updates fallback parsing to support nested bare-JSON tool calls while preserving browser parity.

Changes:

• Adds a string-aware balanced-object scanner.
• Adds nested and multiple-call regression tests.
• Keeps Chrome and Firefox implementations identical.

File summaries

File	Description
src/chrome/src/agent/tool-call-parser.js	Adds balanced JSON extraction.
src/firefox/src/agent/tool-call-parser.js	Mirrors Chrome parser changes.
test/run.js	Tests nested arguments and call ordering.

Review details

• Files reviewed: 3/3 changed files
• Comments generated: 2
• Review effort level: Balanced

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