fix: reduce redundant context in apply_diff error messages

When apply_diff fails to find a match, the error message previously included:
- The search content that was just sent (redundant since model already has it)
- A large snippet of the original file (noisy and context-consuming)

This change streamlines the error output to only show the "Best Match Found"
section, which provides the most actionable information for debugging while
conserving valuable context window space.

The best match section shows exactly what was found in the file that was
closest to the search pattern, enabling targeted corrections without
re-reading the entire file.

Fixes: #5795
Signed-off-by: Eric Wheeler <[email protected]>

Author: Eric Wheeler

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

@@ -614,20 +614,6 @@ Each file requires its own path, start_line, and diff elements.
 					searchLines = aggressiveSearchLines
 					replaceLines = replaceContent ? replaceContent.split(/\r?\n/) : []
 				} else {
-					// No match found with either method
-					const originalContentSection =
-						startLine !== undefined && endLine !== undefined
-							? `\n\nOriginal Content:\n${addLineNumbers(
-									resultLines
-										.slice(
-											Math.max(0, startLine - 1 - this.bufferLines),
-											Math.min(resultLines.length, endLine + this.bufferLines),
-										)
-										.join("\n"),
-									Math.max(1, startLine - this.bufferLines),
-								)}`
-							: `\n\nOriginal Content:\n${addLineNumbers(resultLines.join("\n"))}`
-
 					const bestMatchSection = bestMatchContent
 						? `\n\nBest Match Found:\n${addLineNumbers(bestMatchContent, matchIndex + 1)}`
 						: `\n\nBest Match Found:\n(no match)`
@@ -644,7 +630,7 @@ Each file requires its own path, start_line, and diff elements.
 							bestMatchScore * 100,
 						)}%\n- Required Threshold: ${Math.floor(this.fuzzyThreshold * 100)}%\n- Search Range: ${
 							startLine ? `starting at line ${startLine}` : "start to end"
-						}\n- Tried both standard and aggressive line number stripping\n- Tip: Use the read_file tool to get the latest content of the file before attempting to use the apply_diff tool again, as the file content may have changed\n\nSearch Content:\n${searchChunk}${bestMatchSection}${originalContentSection}`,
+						}\n- Tried both standard and aggressive line number stripping\n- Tip: Use the read_file tool to get the latest content of the file before attempting to use the apply_diff tool again, as the file content may have changed\n\nSearch Content:\n${bestMatchSection}`,
 					})
 					continue
 				}

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

@@ -521,19 +521,6 @@ Only use a single line of '=======' between search and replacement content, beca
 					replaceLines = replaceContent ? replaceContent.split(/\r?\n/) : []
 				} else {
 					// No match found with either method
-					const originalContentSection =
-						startLine !== undefined && endLine !== undefined
-							? `\n\nOriginal Content:\n${addLineNumbers(
-									resultLines
-										.slice(
-											Math.max(0, startLine - 1 - this.bufferLines),
-											Math.min(resultLines.length, endLine + this.bufferLines),
-										)
-										.join("\n"),
-									Math.max(1, startLine - this.bufferLines),
-								)}`
-							: `\n\nOriginal Content:\n${addLineNumbers(resultLines.join("\n"))}`
-
 					const bestMatchSection = bestMatchContent
 						? `\n\nBest Match Found:\n${addLineNumbers(bestMatchContent, matchIndex + 1)}`
 						: `\n\nBest Match Found:\n(no match)`
@@ -542,7 +529,7 @@ Only use a single line of '=======' between search and replacement content, beca
 
 					diffResults.push({
 						success: false,
-						error: `No sufficiently similar match found${lineRange} (${Math.floor(bestMatchScore * 100)}% similar, needs ${Math.floor(this.fuzzyThreshold * 100)}%)\n\nDebug Info:\n- Similarity Score: ${Math.floor(bestMatchScore * 100)}%\n- Required Threshold: ${Math.floor(this.fuzzyThreshold * 100)}%\n- Search Range: ${startLine ? `starting at line ${startLine}` : "start to end"}\n- Tried both standard and aggressive line number stripping\n- Tip: Use the read_file tool to get the latest content of the file before attempting to use the apply_diff tool again, as the file content may have changed\n\nSearch Content:\n${searchChunk}${bestMatchSection}${originalContentSection}`,
+						error: `No sufficiently similar match found${lineRange} (${Math.floor(bestMatchScore * 100)}% similar, needs ${Math.floor(this.fuzzyThreshold * 100)}%)\n\nDebug Info:\n- Similarity Score: ${Math.floor(bestMatchScore * 100)}%\n- Required Threshold: ${Math.floor(this.fuzzyThreshold * 100)}%\n- Search Range: ${startLine ? `starting at line ${startLine}` : "start to end"}\n- Tried both standard and aggressive line number stripping\n- Tip: Use the read_file tool to get the latest content of the file before attempting to use the apply_diff tool again, as the file content may have changed\n\nSearch Content:\n${bestMatchSection}`,
 					})
 					continue
 				}