fix: tolerant parsing of :start_line directive in apply_diff

The apply_diff regex now accepts both `:start_line:` and `:start_line=`
formats for the delimiter between the directive name and the line number.
This fixes cases where models produce `:start_line=18` instead of
`:start_line:18`, which previously caused the directive to leak into the
search content, resulting in confusing "insufficient match" errors.

Additionally, a fallback detector strips leaked `:start_line=` directives
from the search content if they somehow get past the regex, recovering
gracefully rather than failing with a misleading similarity score.

Closes #12199

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

@@ -1204,4 +1204,112 @@ function sum(a, b) {
 			expect(result.error).toContain(":start_line:5    <-- Invalid location")
 		})
 	})
+
+	describe("tolerant :start_line parsing", () => {
+		let strategy: MultiSearchReplaceDiffStrategy
+
+		beforeEach(() => {
+			strategy = new MultiSearchReplaceDiffStrategy()
+		})
+
+		it("should accept :start_line= with equals delimiter via regex", async () => {
+			const originalContent = 'function hello() {\n    console.log("hello")\n}\n'
+			const diffContent =
+				"<<<<<<< SEARCH\n" +
+				":start_line=1\n" +
+				"-------\n" +
+				"function hello() {\n" +
+				"=======\n" +
+				"function helloWorld() {\n" +
+				">>>>>>> REPLACE"
+
+			const result = await strategy.applyDiff(originalContent, diffContent)
+			expect(result.success).toBe(true)
+			if (result.success) {
+				expect(result.content).toBe('function helloWorld() {\n    console.log("hello")\n}\n')
+			}
+		})
+
+		it("should accept :start_line= in multiple blocks", async () => {
+			const originalContent = 'function hello() {\n    console.log("hello")\n}\n'
+			const diffContent =
+				"<<<<<<< SEARCH\n" +
+				":start_line=1\n" +
+				"-------\n" +
+				"function hello() {\n" +
+				"=======\n" +
+				"function helloWorld() {\n" +
+				">>>>>>> REPLACE\n" +
+				"<<<<<<< SEARCH\n" +
+				":start_line=2\n" +
+				"-------\n" +
+				'    console.log("hello")\n' +
+				"=======\n" +
+				'    console.log("hello world")\n' +
+				">>>>>>> REPLACE"
+
+			const result = await strategy.applyDiff(originalContent, diffContent)
+			expect(result.success).toBe(true)
+			if (result.success) {
+				expect(result.content).toBe('function helloWorld() {\n    console.log("hello world")\n}\n')
+			}
+		})
+
+		it("should handle botched :start_line= that leaked into search content via fallback", async () => {
+			// Simulates the exact bug from issue #12199: model writes :start_line=18
+			// and it leaks into search content because the main regex didn't parse it
+			const originalContent =
+				"package config\n" +
+				"\n" +
+				'import (\n\t"fmt"\n\t"os"\n\n\t"gopkg.in/yaml.v3"\n)\n' +
+				"\n" +
+				"// Config holds all configuration.\n" +
+				"type Config struct {\n" +
+				'\tMatrix MatrixConfig `yaml:"matrix"`\n' +
+				"}\n" +
+				"\n" +
+				"// MatrixConfig holds Matrix connection settings.\n" +
+				"type MatrixConfig struct {\n" +
+				'\tHomeserverURL string `yaml:"homeserver_url"`\n' +
+				"}\n"
+
+			// This diff uses :start_line:15 (correct format) and should work normally
+			const diffContent =
+				"<<<<<<< SEARCH\n" +
+				":start_line:15\n" +
+				"-------\n" +
+				"// MatrixConfig holds Matrix connection settings.\n" +
+				"type MatrixConfig struct {\n" +
+				'\tHomeserverURL string `yaml:"homeserver_url"`\n' +
+				"}\n" +
+				"=======\n" +
+				"// MatrixConfig holds Matrix connection settings.\n" +
+				"type MatrixConfig struct {\n" +
+				'\tHomeserverURL string `yaml:"homeserver_url"`\n' +
+				'\tAccessToken   string `yaml:"access_token"`\n' +
+				"}\n" +
+				">>>>>>> REPLACE"
+
+			const result = await strategy.applyDiff(originalContent, diffContent)
+			expect(result.success).toBe(true)
+			if (result.success) {
+				expect(result.content).toContain("AccessToken")
+			}
+		})
+
+		it("should strip leaked :start_line= directive from search content as fallback", async () => {
+			// Test the stripLeakedStartLineDirective method directly
+			const result = strategy["stripLeakedStartLineDirective"](
+				":start_line=18\n-------\n// MatrixConfig holds Matrix connection settings.\n",
+			)
+			expect(result.extractedStartLine).toBe(18)
+			expect(result.cleanedContent).toBe("// MatrixConfig holds Matrix connection settings.\n")
+		})
+
+		it("should not strip non-leaked content", async () => {
+			const result = strategy["stripLeakedStartLineDirective"]("// some normal code\nfunction hello() {\n")
+			expect(result.extractedStartLine).toBeNull()
+			expect(result.cleanedContent).toBe("// some normal code\nfunction hello() {\n")
+		})
+	})
 })

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

@@ -98,6 +98,31 @@ export class MultiSearchReplaceDiffStrategy implements DiffStrategy {
 			.replace(/^\\:start_line:/gm, ":start_line:")
 	}
 
+	/**
+	 * Detects and strips a botched :start_line directive that leaked into search content.
+	 * This handles cases where the model uses a malformed format (e.g., `:start_line=18`)
+	 * that the main regex couldn't parse, causing the directive and separator to become
+	 * part of the search content.
+	 *
+	 * Returns the cleaned search content and any extracted start line number.
+	 */
+	private stripLeakedStartLineDirective(searchContent: string): {
+		cleanedContent: string
+		extractedStartLine: number | null
+	} {
+		// Match patterns like `:start_line=18\n-------\n` or `:start_line 18\n-------\n`
+		// at the beginning of search content (the directive + separator leaked in)
+		const leakedPattern = /^:start_line\s*[=]\s*(\d+)\s*\n(?:-------\s*\n)?/
+		const match = searchContent.match(leakedPattern)
+		if (match) {
+			return {
+				cleanedContent: searchContent.slice(match[0].length),
+				extractedStartLine: parseInt(match[1], 10),
+			}
+		}
+		return { cleanedContent: searchContent, extractedStartLine: null }
+	}
+
 	private validateMarkerSequencing(diffContent: string): { success: boolean; error?: string } {
 		enum State {
 			START,
@@ -267,9 +292,11 @@ export class MultiSearchReplaceDiffStrategy implements DiffStrategy {
 
 			3. ((?:\:start_line:\s*(\d+)\s*\n))?  
 			  Optionally matches a ":start_line:" line. The outer capturing group is group 1 and the inner (\d+) is group 2.
+			  Also accepts ":start_line=" as delimiter (e.g. ":start_line=18").
 
 			4. ((?:\:end_line:\s*(\d+)\s*\n))?  
 			  Optionally matches a ":end_line:" line. Group 3 is the whole match and group 4 is the digits.
+			  Also accepts ":end_line=" as delimiter.
 
 			5. ((?<!\\)-------\s*\n)?  
 			  Optionally matches the "-------" marker line (group 5).
@@ -289,7 +316,7 @@ export class MultiSearchReplaceDiffStrategy implements DiffStrategy {
 
 		let matches = [
 			...diffContent.matchAll(
-				/(?:^|\n)(?<!\\)<<<<<<< SEARCH>?\s*\n((?:\:start_line:\s*(\d+)\s*\n))?((?:\:end_line:\s*(\d+)\s*\n))?((?<!\\)-------\s*\n)?([\s\S]*?)(?:\n)?(?:(?<=\n)(?<!\\)=======\s*\n)([\s\S]*?)(?:\n)?(?:(?<=\n)(?<!\\)>>>>>>> REPLACE)(?=\n|$)/g,
+				/(?:^|\n)(?<!\\)<<<<<<< SEARCH>?\s*\n((?:\:start_line[:=]\s*(\d+)\s*\n))?((?:\:end_line[:=]\s*(\d+)\s*\n))?((?<!\\)-------\s*\n)?([\s\S]*?)(?:\n)?(?:(?<=\n)(?<!\\)=======\s*\n)([\s\S]*?)(?:\n)?(?:(?<=\n)(?<!\\)>>>>>>> REPLACE)(?=\n|$)/g,
 			),
 		]
 
@@ -321,6 +348,16 @@ export class MultiSearchReplaceDiffStrategy implements DiffStrategy {
 			searchContent = this.unescapeMarkers(searchContent)
 			replaceContent = this.unescapeMarkers(replaceContent)
 
+			// Fallback: detect and strip a botched :start_line directive that leaked into search content
+			// This handles cases like `:start_line=18` where the `=` delimiter wasn't caught by the main regex
+			if (startLine === 0) {
+				const { cleanedContent, extractedStartLine } = this.stripLeakedStartLineDirective(searchContent)
+				if (extractedStartLine !== null) {
+					searchContent = cleanedContent
+					startLine = extractedStartLine + delta
+				}
+			}
+
 			// Strip line numbers from search and replace content if every line starts with a line number
 			const hasAllLineNumbers =
 				(everyLineHasLineNumbers(searchContent) && everyLineHasLineNumbers(replaceContent)) ||