edit tool: empty oldString silently overwrites file + permission prompt shows no diff preview #30841

Description

Two compounding bugs led to silent data loss of a user's ~/.zshrc:

1. The edit tool accepts empty string "" as oldString, which replaces the entire file content
2. The permission prompt for external directory edits shows only the file path — no diff or content preview — so the user approved the destructive edit without knowing what would happen
   Environment

• OpenCode version: 1.15.13
• OS: macOS (darwin)
• Installed via: opencode.ai/install
  Steps to Reproduce
  Scenario: Agent edits a file outside the project directory

1. Agent calls the edit tool with:
   {
   "filePath": "/Users/user/.zshrc",
   "oldString": "",
   "newString": "# opencode test\n"
   }
2. Permission prompt appears (because ~/.zshrc is outside the working directory)
3. Prompt shows:
   △Permission required
   ←Access external directory ~
   Patterns

• /Users/user/*
  No diff, no content preview, no indication of what will change.

4. User clicks "approve" — the entire file content is overwritten, only # opencode test remains
5. Original file content (~5KB of shell configuration) is permanently lost
   Why the user approved
   The user had no way to know what the edit would do. The prompt said "edit a file under ~" but didn't show:

• That oldString was empty (a red flag)
• What the actual change would be
• The diff or before/after content
  Expected Behavior

1. edit with empty oldString should be rejected or clearly labeled as "overwrite entire file" / "prepend to file". An empty match string is almost certainly a bug or misuse, not intentional.
2. Permission prompt should show the diff — at minimum the oldString → newString change, ideally a unified diff preview. Approving file modifications blind is a security and data safety issue.
   Actual Behavior
3. Empty oldString is silently accepted and matches everywhere, causing full content replacement
4. Permission prompt only shows the file path pattern, no content information
   Impact
   Data loss. User's ~/.zshrc (4947 bytes of shell configuration accumulated over time) was destroyed in a single approved edit. The agent then attempted a "revert" which made things worse — the file became empty.
   This is not an edge case. Any agent using oldString: "" on a user's config files, dotfiles, or important documents will cause silent corruption with no way for the user to intervene.
   Suggested Fixes
5. Reject edit calls where oldString is empty or whitespace-only
6. Add diff preview to the permission prompt for edit operations (show oldString → newString, or a unified diff)
7. For external directory edits, consider requiring a more explicit confirmation when the operation would replace large amounts of content

Plugins

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OpenCode version

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Steps to reproduce

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Screenshot and/or share link

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Operating System

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Terminal

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