From 52ffb98453c0fc449760c978eb0cfe152bac32cc Mon Sep 17 00:00:00 2001
From: 32134178csa <321dsadsaca@protonmail.com>
Date: Fri, 27 Mar 2026 21:46:37 -0700
Subject: [PATCH] fix(filesystem): improve edit_file error message with
 nearest-match diagnostics

When edit_file fails to find a match, the error now shows the closest
near-match location with line numbers, matching percentage, and a
line-by-line diff highlighting which lines diverge. This helps agents
and users quickly identify whitespace or content mismatches.

Fixes #2034
---
 src/filesystem/__tests__/lib.test.ts |  6 ++---
 src/filesystem/lib.ts                | 38 +++++++++++++++++++++++++++-
 2 files changed, 40 insertions(+), 4 deletions(-)

diff --git a/src/filesystem/__tests__/lib.test.ts b/src/filesystem/__tests__/lib.test.ts
index f7e585af22..efb47ebeac 100644
--- a/src/filesystem/__tests__/lib.test.ts
+++ b/src/filesystem/__tests__/lib.test.ts
@@ -494,13 +494,13 @@ describe('Lib Functions', () => {
         );
       });
 
-      it('throws error for non-matching edits', async () => {
+      it('throws error for non-matching edits with diagnostic info', async () => {
         const edits = [
           { oldText: 'nonexistent line', newText: 'replacement' }
         ];
-        
+
         await expect(applyFileEdits('/test/file.txt', edits, false))
-          .rejects.toThrow('Could not find exact match for edit');
+          .rejects.toThrow('Could not find match for edit');
       });
 
       it('handles complex multi-line edits with indentation', async () => {
diff --git a/src/filesystem/lib.ts b/src/filesystem/lib.ts
index 17e4654cd5..aa07b389f5 100644
--- a/src/filesystem/lib.ts
+++ b/src/filesystem/lib.ts
@@ -248,7 +248,43 @@ export async function applyFileEdits(
     }
 
     if (!matchFound) {
-      throw new Error(`Could not find exact match for edit:\n${edit.oldText}`);
+      // Find the closest near-match to help diagnose the mismatch
+      const oldLines = normalizedOld.split('\n');
+      const contentLines = modifiedContent.split('\n');
+      let bestScore = 0;
+      let bestLineStart = 0;
+
+      for (let i = 0; i <= contentLines.length - oldLines.length; i++) {
+        const potentialMatch = contentLines.slice(i, i + oldLines.length);
+        let score = 0;
+        for (let j = 0; j < oldLines.length; j++) {
+          if (oldLines[j].trim() === potentialMatch[j].trim()) {
+            score++;
+          }
+        }
+        if (score > bestScore) {
+          bestScore = score;
+          bestLineStart = i;
+        }
+      }
+
+      let diagnostic = `Could not find match for edit in ${filePath}`;
+      if (bestScore > 0) {
+        const matchPct = Math.round((bestScore / oldLines.length) * 100);
+        diagnostic += `\nClosest match (${matchPct}% of lines) at lines ${bestLineStart + 1}-${bestLineStart + oldLines.length}:`;
+        const nearMatchLines = contentLines.slice(bestLineStart, bestLineStart + oldLines.length);
+        for (let j = 0; j < oldLines.length; j++) {
+          const marker = oldLines[j].trim() === nearMatchLines[j].trim() ? ' ' : '!';
+          diagnostic += `\n ${marker} line ${bestLineStart + 1 + j}: ${JSON.stringify(nearMatchLines[j])}`;
+          if (marker === '!') {
+            diagnostic += `\n     expected: ${JSON.stringify(oldLines[j])}`;
+          }
+        }
+      } else {
+        diagnostic += `\nNo similar content found in the file (${contentLines.length} lines).`;
+      }
+      diagnostic += `\n\nEdit oldText:\n${edit.oldText}`;
+      throw new Error(diagnostic);
     }
   }