fix: enhance null edit detection for Gemini models

- Add detailed error messages when search and replace content are identical
- Provide clear guidance on common causes (model hallucination, copy/paste errors)
- Add comprehensive tests for null edit detection
- Help AI models (especially Gemini) avoid producing phantom edits

Fixes #7360

src/core/diff/strategies/__tests__/null-edit-detection.spec.ts

@@ -0,0 +1,134 @@
+import { MultiSearchReplaceDiffStrategy } from "../multi-search-replace"
+
+describe("null edit detection", () => {
+	let strategy: MultiSearchReplaceDiffStrategy
+
+	beforeEach(() => {
+		strategy = new MultiSearchReplaceDiffStrategy()
+	})
+
+	it("should detect and reject null edits (identical search and replace)", async () => {
+		const originalContent = 'function hello() {\n    console.log("hello")\n}\n'
+		const diffContent = [
+			"<<<<<<< SEARCH",
+			"function hello() {",
+			'    console.log("hello")',
+			"}",
+			"=======",
+			"function hello() {",
+			'    console.log("hello")',
+			"}",
+			">>>>>>> REPLACE",
+		].join("\n")
+
+		const result = await strategy.applyDiff(originalContent, diffContent)
+		expect(result.success).toBe(false)
+		if (!result.success && result.failParts && result.failParts.length > 0) {
+			const firstError = result.failParts[0]
+			if (!firstError.success && firstError.error) {
+				expect(firstError.error).toContain("NULL EDIT DETECTED")
+				expect(firstError.error).toContain("Search and replace content are identical")
+				expect(firstError.error).toContain("This is a common issue with AI models (especially Gemini)")
+				expect(firstError.error).toContain("Model hallucination")
+			}
+		}
+	})
+
+	it("should detect null edits in multi-block diffs", async () => {
+		const originalContent =
+			'function hello() {\n    console.log("hello")\n}\nfunction world() {\n    return "world"\n}'
+		const diffContent = [
+			"<<<<<<< SEARCH",
+			"function hello() {",
+			'    console.log("hello")',
+			"}",
+			"=======",
+			"function hello() {",
+			'    console.log("hello world")',
+			"}",
+			">>>>>>> REPLACE",
+			"",
+			"<<<<<<< SEARCH",
+			"function world() {",
+			'    return "world"',
+			"}",
+			"=======",
+			"function world() {",
+			'    return "world"',
+			"}",
+			">>>>>>> REPLACE",
+		].join("\n")
+
+		const result = await strategy.applyDiff(originalContent, diffContent)
+		// Should partially succeed but report the null edit
+		expect(result.success).toBe(true)
+		if (result.success) {
+			expect(result.failParts).toBeDefined()
+			if (result.failParts && result.failParts[0] && !result.failParts[0].success) {
+				expect(result.failParts[0].error).toContain("NULL EDIT DETECTED")
+			}
+		}
+	})
+
+	it("should detect null edits with line numbers", async () => {
+		const originalContent = "function test() {\n    return true;\n}\n"
+		const diffContent = [
+			"<<<<<<< SEARCH",
+			":start_line:1",
+			"-------",
+			"function test() {",
+			"    return true;",
+			"}",
+			"=======",
+			"function test() {",
+			"    return true;",
+			"}",
+			">>>>>>> REPLACE",
+		].join("\n")
+
+		const result = await strategy.applyDiff(originalContent, diffContent)
+		expect(result.success).toBe(false)
+		if (!result.success && result.failParts && result.failParts.length > 0) {
+			const firstError = result.failParts[0]
+			if (!firstError.success && firstError.error) {
+				expect(firstError.error).toContain("NULL EDIT DETECTED")
+				expect(firstError.error).toContain("phantom")
+				expect(firstError.error).toContain("Ensure the REPLACE block contains the actual modified content")
+			}
+		}
+	})
+
+	it("should not trigger null edit detection for legitimate empty replacements (deletions)", async () => {
+		const originalContent = "function test() {\n    // Remove this comment\n    return true;\n}\n"
+		const diffContent = ["<<<<<<< SEARCH", "    // Remove this comment", "=======", ">>>>>>> REPLACE"].join("\n")
+
+		const result = await strategy.applyDiff(originalContent, diffContent)
+		expect(result.success).toBe(true)
+		// Should not contain null edit error
+		if (!result.success && result.error) {
+			expect(result.error).not.toContain("NULL EDIT DETECTED")
+		}
+	})
+
+	it("should not trigger for actual changes even if similar", async () => {
+		const originalContent = "function test() {\n    return true;\n}\n"
+		const diffContent = [
+			"<<<<<<< SEARCH",
+			"function test() {",
+			"    return true;",
+			"}",
+			"=======",
+			"function test() {",
+			"    return false;",
+			"}",
+			">>>>>>> REPLACE",
+		].join("\n")
+
+		const result = await strategy.applyDiff(originalContent, diffContent)
+		expect(result.success).toBe(true)
+		// Should not contain null edit error
+		if (!result.success && result.error) {
+			expect(result.error).not.toContain("NULL EDIT DETECTED")
+		}
+	})
+})

src/core/diff/strategies/multi-file-search-replace.ts

@@ -515,10 +515,17 @@ Each file requires its own path, start_line, and diff elements.
 				diffResults.push({
 					success: false,
 					error:
-						`Search and replace content are identical - no changes would be made\n\n` +
+						`NULL EDIT DETECTED: Search and replace content are identical - no changes would be made\n\n` +
+						`This is a common issue with AI models (especially Gemini) producing "phantom" edits.\n\n` +
 						`Debug Info:\n` +
-						`- Search and replace must be different to make changes\n` +
-						`- Use read_file to verify the content you want to change`,
+						`- The SEARCH block and REPLACE block contain exactly the same content\n` +
+						`- This means the edit would have zero effect on the file\n` +
+						`- To fix: Ensure the REPLACE block contains the actual modified content\n` +
+						`- Use read_file to verify the current file content before making changes\n\n` +
+						`Common causes:\n` +
+						`1. Model hallucination - thinking it made changes when it didn't\n` +
+						`2. Incorrect copy/paste of content between blocks\n` +
+						`3. Missing the actual modification in the REPLACE block`,
 				})
 				continue
 			}

src/core/diff/strategies/multi-search-replace.ts

@@ -432,10 +432,17 @@ Only use a single line of '=======' between search and replacement content, beca
 				diffResults.push({
 					success: false,
 					error:
-						`Search and replace content are identical - no changes would be made\n\n` +
+						`NULL EDIT DETECTED: Search and replace content are identical - no changes would be made\n\n` +
+						`This is a common issue with AI models (especially Gemini) producing "phantom" edits.\n\n` +
 						`Debug Info:\n` +
-						`- Search and replace must be different to make changes\n` +
-						`- Use read_file to verify the content you want to change`,
+						`- The SEARCH block and REPLACE block contain exactly the same content\n` +
+						`- This means the edit would have zero effect on the file\n` +
+						`- To fix: Ensure the REPLACE block contains the actual modified content\n` +
+						`- Use read_file to verify the current file content before making changes\n\n` +
+						`Common causes:\n` +
+						`1. Model hallucination - thinking it made changes when it didn't\n` +
+						`2. Incorrect copy/paste of content between blocks\n` +
+						`3. Missing the actual modification in the REPLACE block`,
 				})
 				continue
 			}