fix(gateway): handle brackets in JSON strings

[steebchen]

Summary

• Fixed mightBeCompleteJson heuristic to skip brackets/braces inside JSON string values
• Added 17 unit tests including the production failure case

The heuristic was incorrectly counting brackets inside strings (e.g., "delta":"\"]\"") as unbalanced, causing valid JSON to be marked as incomplete. This led to SSE events being concatenated together during streaming, resulting in JSON parse errors.

Test plan

• Unit tests pass (pnpm test:unit)
• Build succeeds (pnpm build)
• Format check passes (pnpm format)

🤖 Generated with Claude Code

Summary by CodeRabbit

• Bug Fixes

  • Improved JSON validation to correctly identify complete and incomplete JSON while properly handling strings containing special characters and escaped quotes.

• Tests

  • Added comprehensive unit test suite covering multiple scenarios: simple objects and arrays, incomplete JSON, empty and non-JSON strings, strings with special characters, unclosed strings, nested structures, arrays of objects, and complex edge cases.

[coderabbitai]

Walkthrough

Updates the mightBeCompleteJson utility function to properly handle JSON strings by tracking state and skipping escaped characters, replacing a naive brace/bracket counting approach. Comprehensive unit tests are added to validate behavior across various edge cases.

Changes

Cohort / File(s)	Summary
JSON Completion Validation apps/gateway/src/chat/tools/might-be-complete-json.ts, apps/gateway/src/chat/tools/might-be-complete-json.spec.ts	Implementation refactored to use string-aware traversal with inString state tracking and escaped quote handling; comprehensive test suite added covering simple structures, incomplete JSON, unclosed strings, nested objects/arrays, and edge cases with brackets/braces inside string literals.

Estimated code review effort

🎯 3 (Moderate) | ⏱️ ~20 minutes

Possibly related PRs

• perf(gateway): optimize CPU-intensive code paths #1530 — Modifies SSE parsing logic that relies on the mightBeCompleteJson heuristic being updated in this PR
• refactor(chat): extract pure utility functions #1551 — Directly introduces or previously modified the mightBeCompleteJson function that is refined here

Suggested labels

auto-merge

🚥 Pre-merge checks | ✅ 3

✅ Passed checks (3 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title clearly and specifically summarizes the main change: fixing a bug where brackets inside JSON strings are incorrectly counted, causing valid JSON to be marked incomplete.
Docstring Coverage	✅ Passed	Docstring coverage is 100.00% which is sufficient. The required threshold is 80.00%.

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✨ Finishing touches

• 📝 Generate docstrings

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Post copyable unit tests in a comment
• Commit unit tests in branch investigate-attachment

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[Copilot]

Pull request overview

This PR tightens the mightBeCompleteJson heuristic used in SSE parsing so that it no longer misclassifies valid JSON as incomplete when brackets or braces appear inside string values, and adds targeted unit tests around this behavior.

Changes:

• Updated mightBeCompleteJson to scan the JSON string character-by-character, tracking whether the cursor is inside a string and ignoring brackets/braces there, while handling escaped characters.
• Added a dedicated might-be-complete-json.spec.ts test suite with cases for simple structures, unbalanced input, escaped characters, and the real-world production failure scenario.

Reviewed changes

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

File	Description
apps/gateway/src/chat/tools/might-be-complete-json.ts	Refines the JSON completeness heuristic to skip bracket/brace counting inside string literals and to treat unterminated strings as incomplete.
apps/gateway/src/chat/tools/might-be-complete-json.spec.ts	Introduces comprehensive unit tests validating the heuristic across normal, edge, and production-derived JSON examples.

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