apply_patch: parallel calls on the same file silently drop one change #53512

Description

When a model sends two apply_patch calls for the same file in one response, both calls can read the original file. Both report success, but the last write overwrites the first change.

I reproduced this with GPT-6.1 Sol and "permission": { "edit": "ask" }: both approval prompts showed the original file, I approved both, and only the second change was kept.

The edit tool already serializes same-file edits with a per-file lock (#23483), but apply_patch does not. Related but different: #31776 and PR #47956 concern a file changed outside OpenCode during approval.

Plugins

None

OpenCode version

1.18.34

Steps to reproduce

1. Create settings.txt with:

   top = 0
   middle = keep
   bottom = 0
   

2. Create opencode.json with:

   {
     "$schema": "https://opencode.ai/config.json",
     "permission": {
       "edit": "ask"
     }
   }

3. Start OpenCode 1.18.34 and select GPT-6.1 Sol.

4. Send this prompt:

   In settings.txt, change top = 0 to top = 1 and bottom = 0 to bottom = 2. Use two separate apply_patch calls, sent in parallel in the same response.
   

5. Confirm that both apply_patch calls appear in the same assistant response and that both approval requests show the original file.

6. Approve both requests.

Expected result

top = 1
middle = keep
bottom = 2

Actual result

Both calls report success, but the final file contains:

top = 0
middle = keep
bottom = 2

Screenshot and/or share link

No response

Operating System

macOS 26.3

Terminal

Terminal.app