docs(prompt): enhance apply_diff tool instructions to improve Gemini model success rate

.changeset/improve-apply-diff-prompt.md

@@ -0,0 +1,5 @@
+---
+"zoo-code": patch
+---
+
+Enhance the `apply_diff` tool description and parameter instructions to enforce strict `:start_line:` formatting and copy-paste exact matching requirements, improving success rates for Gemini Flash and other smaller/faster models.

plans/improve-apply-diff-prompt.pr.md

@@ -0,0 +1,35 @@
+### Related GitHub Issue
+
+Closes: #611
+
+### Description
+
+This PR enhances the documentation and instructions for the `apply_diff` tool to improve its success rate, particularly for Gemini models. It addresses issues with malformed `:start_line:` syntax and imprecise diff matching by enforcing stricter, clearer requirements in the tool's system instructions.
+
+Note: While the issue notes that adding these guidelines to `AGENTS.md` resolved the issue locally, we have integrated them directly into the core prompts (`src/core/prompts/tools/native-tools/apply_diff.ts`) and have purposely omitted modifying `AGENTS.md` to adhere to repository PR hygiene guidelines.
+
+### Test Procedure
+
+- Verify existing `apply_diff` behavior with `apps/vscode-e2e/src/suite/tools/apply-diff.test.ts`.
+- Prompt instructions updated in `src/core/prompts/tools/native-tools/apply_diff.ts`.
+
+### Pre-Submission Checklist
+
+- [x] **Issue Linked**: This PR is linked to an approved GitHub Issue.
+- [x] **Scope**: My changes are focused on the linked issue (one major feature/fix per PR).
+- [x] **Self-Review**: I have performed a thorough self-review of my code.
+- [x] **Testing**: Existing tests cover the functionality; no new functionality requiring tests was added.
+- [x] **Documentation Impact**: I have considered if my changes require documentation updates.
+- [x] **Contribution Guidelines**: I have read and agree to the [Contributor Guidelines](/CONTRIBUTING.md).
+
+### Documentation Updates
+
+- [ ] No documentation updates are required. (Updates are purely internal prompt instruction refinements).
+
+### Additional Notes
+
+None.
+
+### Get in Touch
+
+Zoo AI Assistant

src/core/prompts/tools/native-tools/apply_diff.ts

@@ -9,7 +9,12 @@ const DIFF_PARAMETER_DESCRIPTION = `A string containing one or more search/repla
 [exact content to find]
 =======
 [new content to replace with]
->>>>>>> REPLACE`
+>>>>>>> REPLACE
+
+CRITICAL:
+- The ':start_line:[line_number]' header is strictly required. It must follow the exact syntax ':start_line:[integer]' (for example: ':start_line:220'). Do not write headers with shorthand forms like ':220' or variations like ':start_line=220'.
+- Copy the exact lines from the source file for a 100% string match including all whitespace, indentation, and newlines.
+- Ensure the separator '-------' is on its own line immediately following ':start_line:[line_number]' with a newline.`
 
 export const apply_diff = {
 	type: "function",