fix: validate separator newline in apply_diff to catch malformed ------- markers

[roomote-v0]

Related GitHub Issue

Closes: #12210

Description

This PR attempts to address Issue #12210. Feedback and guidance are welcome.

When the LLM omits the newline after the ------- separator (e.g. -------import { useTranslate }), the parser silently absorbs the separator into the search content, producing a confusing "No sufficiently similar match found" error. This causes smaller models like Qwen 3.6 to loop without recovering.

Implementation: Added detection in validateMarkerSequencing (in src/core/diff/strategies/multi-search-replace.ts) for lines starting with ------- that have non-whitespace content appended. When detected, a clear, actionable error message is returned telling the model to place the separator on its own line with search content starting on the next line.

Test Procedure

• Added 3 new tests in src/core/diff/strategies/__tests__/multi-search-replace.spec.ts:
  1. Detects malformed separator with content on the same line (the exact scenario from the issue with :start_line: present)
  2. Detects malformed separator without :start_line:
  3. Confirms properly formatted separator on its own line still works correctly
• All 61 tests pass: cd src && npx vitest run core/diff/strategies/__tests__/multi-search-replace.spec.ts

Pre-Submission Checklist

• Issue Linked: This PR is linked to an approved GitHub Issue.
• Scope: Changes are focused on the linked issue.
• Self-Review: I have performed a thorough self-review of my code.
• Testing: New tests have been added to cover the changes.
• Documentation Impact: No documentation updates required.
• Contribution Guidelines: I have read and agree to the Contributor Guidelines.

Documentation Updates

• No documentation updates are required.

Additional Notes

The fix is minimal and targeted -- it only adds a check in the existing AFTER_SEARCH state of the marker validation state machine. The regex /^-------\S/ catches lines where ------- is immediately followed by a non-whitespace character (i.e. missing newline). The error message includes the trailing content to help the model understand what went wrong.

Interactively review PR in Roo Code Cloud

[rkfg]

@roomote found another failure case: "diff": "<<<<<<< SEARCH\n:start_line:70\n-------\tapi.POST( Here the fix doesn't work because the regexp assumes \S (non-space characters) after the separator at the same line. In fact there could be spaces, tabs, and other characters. I think any characters after the separator except \n should be considered an error.

[roomote-v0]

Fixaroo   See task

Fixed the regex from /^-------\S/ to /^-------.+/ so whitespace characters (tabs, spaces) after the ------- separator are also detected as malformed. Added a test for the tab-prefixed content case. All 62 tests pass, lint and type checks clean.

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