fix: repair malformed JSON in model-written tool arguments

[elkaix]

Summary

Model-written tool arguments sometimes fail strict JSON.parse and kill the tool call at preflight. Two failure classes were observed in real Dynamic Workflow launches:

• Invalid escape sequences — markdown-style escapes such as \* inside JSON string values (Bad escaped character at position 3458).
• Unescaped inner quotes — a literal " inside a string value that terminates it early (Expected ',' or ']' after array element in JSON at position 2832).

Both now repair instead of failing the call. The repair runs only as a fallback after JSON.parse rejects the input, and the original parse error is still reported when the repaired text also fails to parse.

Approach

A single scanner pass rewrites invalid escapes to a literal backslash plus character, and rewrites an in-string " to \" unless the next non-whitespace character is structural (, : } ]) or end of input. A content quote immediately followed by a structural character stays ambiguous and still closes the string; that limit is documented in the function comment.

Test plan

• packages/agent-core/test/loop/tool-call.e2e.test.ts — 45 tests, including a reproduction of the observed items-array failure, mixed-class repair, whitespace before the terminator, and preservation of the original error on still-broken input.
• Full suite green locally.

Summary by CodeRabbit

• Bug Fixes

  • Improved handling of malformed tool arguments, including invalid escape sequences and unescaped quotation marks.
  • Preserves valid escapes while repairing recoverable formatting issues.
  • Provides clearer errors when arguments remain structurally invalid.

• Tests

  • Added coverage for malformed escapes, embedded quotes, combined errors, valid escapes, and invalid JSON structures.

[coderabbitai]

No actionable comments were generated in the recent review. 🎉

ℹ️ Recent review info

⚙️ Run configuration

Configuration used: Path: .coderabbit.yaml

Review profile: CHILL

Plan: Pro Plus

Run ID: adaddcf9-74da-46f7-8762-a81708575d16

📥 Commits

Reviewing files that changed from the base of the PR and between 83663b6 and 8ef6d30.

📒 Files selected for processing (1)

• packages/agent-core/test/loop/tool-call.e2e.test.ts

🚧 Files skipped from review as they are similar to previous changes (1)

• packages/agent-core/test/loop/tool-call.e2e.test.ts

---

📝 Walkthrough

Walkthrough

parseToolCallArguments now repairs selected malformed JSON string escapes and quotes before retrying parsing. Tests cover repaired arguments, preserved valid escapes, and structurally invalid input.

Changes

Tool argument repair

Layer / File(s)	Summary
Parser repair implementation packages/agent-core/src/loop/tool-call.ts, .changeset/tool-args-escape-repair.md	The parser repairs invalid string escapes and ambiguous interior quotes, then retries JSON parsing. The changeset documents the patch.
Parser repair validation packages/agent-core/test/loop/tool-call.e2e.test.ts	Tests cover malformed escapes, unescaped quotes, valid escapes, and invalid syntax that remains unrepaired.

Estimated code review effort: 3 (Moderate) | ~20 minutes

Possibly related PRs

• Pythoughts-labs/pythinker-code#23: Handles malformed StrReplaceFile tool arguments in a different implementation.

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title uses the fix: conventional-commit prefix, imperative mood, and 58 characters, which is within the 72-character limit.
Description check	✅ Passed	The description clearly explains the problem, implementation approach, limitations, and test plan, although it does not use the repository template headings.
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check.
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.

---

_{Comment @coderabbitai help to get the list of available commands.}

[pkg-pr-new]

pnpm dlx https://pkg.pr.new/@pythoughts/pythinker-code@8ef6d30

npx https://pkg.pr.new/@pythoughts/pythinker-code@8ef6d30

commit: 8ef6d30

[coderabbitai]

Actionable comments posted: 1

🤖 Prompt for all review comments with AI agents

Verify each finding against current code. Fix only still-valid issues, skip the
rest with a brief reason, keep changes minimal, and validate.

Inline comments:
In `@packages/agent-core/test/loop/tool-call.e2e.test.ts`:
- Around line 179-183: Update the test case for parseToolCallArguments to use
input containing both an invalid in-string escape and a separate structural
error, such as {"a":"bad \\*","b":[}, so repairInvalidStringEscapes is attempted
but reparsing still fails. Keep the assertion focused on returning the original
parseErrorMessage for the unmodified input.

🪄 Autofix

Fix all unresolved CodeRabbit comments on this PR:

• Push a commit to this branch (recommended)
• Create a new PR with the fixes

---

ℹ️ Review info

⚙️ Run configuration

Configuration used: Path: .coderabbit.yaml

Review profile: CHILL

Plan: Pro Plus

Run ID: 2871e31a-253c-4d0b-abd1-d831ae342719

📥 Commits

Reviewing files that changed from the base of the PR and between 065bf2e and 83663b6.

📒 Files selected for processing (3)

• .changeset/tool-args-escape-repair.md
• packages/agent-core/src/loop/tool-call.ts
• packages/agent-core/test/loop/tool-call.e2e.test.ts