bug: apply_diff mishandles :start_line: markers in REPLACE section

[KJ7LNW]

The apply_diff tool incorrectly handles cases where :start_line: or :end_line: markers are present within the REPLACE section of a diff block. This problem is particularly prevalent with Gemini 2.5, which frequently places these markers incorrectly.

Current Behavior

Currently, if a diff includes :start_line: markers in the REPLACE section (after the ======= separator), they are treated as literal text to be inserted into the file rather than being rejected as improperly formatted diff content:

<<<<<<< SEARCH
:start_line:996
-------
}))

...
{task ? (
=======
:start_line:996    <<<<<< PROBLEM - This should be rejected
-------
}))

return (
<div className={isHidden ? "hidden" : "fixed top-0 left-0 right-0 bottom-0 flex flex-col overflow-hidden"}>
{showAnnouncement && <Announcement hideAnnouncement={hideAnnouncement} />}
{task ? (
>>>>>>> REPLACE

Expected Behavior

The apply_diff tool should validate that metadata markers like :start_line: and :end_line: only appear in their designated positions before the ------- separator in the SEARCH section, and should produce an error if these markers are found in the REPLACE section.

Issue Location

This validation should be added to the validateMarkerSequencing function in src/core/diff/strategies/multi-search-replace.ts.

Additional Context

This issue is a frequent problem when using Gemini 2.5, which has a tendency to place these metadata markers in the REPLACE section. When this happens, the markers are inserted as literal text rather than being reported as formatting errors, causing incorrect file modifications.

[gurgelff]

As a temporary workaround, the user can add to "Custom instructions for all modes" settings something along the lines of:
When you are applying diff, only use :start_line: marker in the SEARCH block, but don't use it in REPLACE block.

[dosubot]

Hi @KJ7LNW! I'm Dosu and I’m helping the Roo-Code team.

Based on the available documentation, the apply_diff tool uses markers like :start_line: and :end_line: as part of its diff format, with the expectation that these markers are used as hints for locating the correct lines to modify. The documentation specifies that the :start_line: marker is mandatory within each SEARCH block, and the :end_line: marker is optional but supported by the parser. These markers are intended to appear before the ------- separator in the SEARCH section, and not in the REPLACE section. The tool requires an exact match for the SEARCH block content, including these markers, and treats any content in the REPLACE section as literal text to be inserted into the file, which is the root of the issue you described.
See documentation

There is no documentation or release note indicating that the apply_diff tool currently validates or rejects misplaced markers in the REPLACE section. The documentation does not mention the validateMarkerSequencing function, nor does it provide details about its implementation or any recent changes related to this function or to marker validation in the REPLACE section.
See release notes

No existing issues, tickets, or discussions about this specific problem were found in the documentation or release notes.

In summary:

• The tool currently treats :start_line: or :end_line: markers in the REPLACE section as literal text, which can lead to incorrect file modifications if these markers are misplaced.
• There is no documented validation to reject these markers in the REPLACE section, and no documentation about the validateMarkerSequencing function or recent updates addressing this issue.
• The expected validation behavior you described does not appear to be implemented or documented at this time.

If you need guidance on how to implement this validation or want to discuss possible solutions, let me know!

To reply, just mention my name @dosu.

---

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[KJ7LNW]

As a temporary workaround, the user can add to "Custom instructions for all modes" settings something along the lines of: When you are applying diff, only use :start_line: marker in the SEARCH block, but don't use it in REPLACE block.

Good idea . Hopefully #4015 will land soon, too.

[bhaktatejas922]

Could use a fast apply model? :)