diff --git a/core/edit/searchAndReplace/executeFindAndReplace.vitest.ts b/core/edit/searchAndReplace/executeFindAndReplace.vitest.ts
index de9f62cc079..e4c7473fa8f 100644
--- a/core/edit/searchAndReplace/executeFindAndReplace.vitest.ts
+++ b/core/edit/searchAndReplace/executeFindAndReplace.vitest.ts
@@ -326,5 +326,20 @@ describe("executeFindAndReplace", () => {
 
       expect(result).toBe("new content");
     });
+
+    it("should not corrupt the file when an earlier character changes length on lowercase", () => {
+      // "İ" (U+0130) lowercases to "i" + combining dot (2 UTF-16 units). The
+      // case-insensitive match must report indices into the original content,
+      // otherwise the replacement lands at the wrong offset and corrupts the file.
+      const content = "// İ marker comment\nconst value = 1;";
+      const result = executeFindAndReplace(
+        content,
+        "CONST VALUE = 1;",
+        "const value = 2;",
+        false,
+      );
+
+      expect(result).toBe("// İ marker comment\nconst value = 2;");
+    });
   });
 });
diff --git a/core/edit/searchAndReplace/findSearchMatch.ts b/core/edit/searchAndReplace/findSearchMatch.ts
index dd826985d92..5156715d83e 100644
--- a/core/edit/searchAndReplace/findSearchMatch.ts
+++ b/core/edit/searchAndReplace/findSearchMatch.ts
@@ -61,20 +61,50 @@ function trimmedMatch(
 
 /**
  * Case-insensitive matching strategy
+ *
+ * The match must be located in terms of the ORIGINAL file content. Searching
+ * the lowercased file directly is unsafe because `String.prototype.toLowerCase`
+ * is not guaranteed to be length-preserving (e.g. "İ" lowercases to "i̇", two
+ * UTF-16 units). A single such character before or within the match would shift
+ * every subsequent index, producing a misaligned slice and a corrupted edit.
+ *
+ * Instead we scan the original content and, for each candidate start position,
+ * lowercase only the slice that lines up with the lowercased search string. The
+ * matched region in the original may be longer or shorter than the search, so
+ * the end index is derived from that slice rather than from `searchContent.length`.
  */
 function caseInsensitiveMatch(
   fileContent: string,
   searchContent: string,
 ): BasicMatchResult | null {
-  const lowerFileContent = fileContent.toLowerCase();
   const lowerSearchContent = searchContent.toLowerCase();
-  const index = lowerFileContent.indexOf(lowerSearchContent);
-  if (index !== -1) {
-    return {
-      startIndex: index,
-      endIndex: index + searchContent.length,
-    };
+  if (lowerSearchContent.length === 0) {
+    return null;
   }
+
+  for (let startIndex = 0; startIndex < fileContent.length; startIndex++) {
+    // Grow the candidate slice until its lowercased form is at least as long as
+    // the search, then check for equality. This keeps indices anchored to the
+    // original string even when lowercasing changes length.
+    for (
+      let endIndex = startIndex + 1;
+      endIndex <= fileContent.length;
+      endIndex++
+    ) {
+      const lowerCandidate = fileContent
+        .slice(startIndex, endIndex)
+        .toLowerCase();
+      if (lowerCandidate.length < lowerSearchContent.length) {
+        continue;
+      }
+      if (lowerCandidate === lowerSearchContent) {
+        return { startIndex, endIndex };
+      }
+      // Once the candidate is long enough but doesn't match, advance the start.
+      break;
+    }
+  }
+
   return null;
 }
 
diff --git a/core/edit/searchAndReplace/findSearchMatch.vitest.ts b/core/edit/searchAndReplace/findSearchMatch.vitest.ts
index c2489d93e9c..8a84cec782e 100644
--- a/core/edit/searchAndReplace/findSearchMatch.vitest.ts
+++ b/core/edit/searchAndReplace/findSearchMatch.vitest.ts
@@ -179,6 +179,36 @@ describe("findSearchMatch", () => {
         strategyName: "caseInsensitiveMatch",
       });
     });
+
+    it("should return original-cased indices when an earlier character changes length on lowercase", () => {
+      // "İ" (U+0130) lowercases to "i" + combining dot (2 UTF-16 units), so an
+      // index found in the lowercased file would be shifted relative to the original.
+      const fileContent = "İ test";
+      const result = findSearchMatch(fileContent, "TEST");
+
+      expect(result).toEqual({
+        startIndex: 2,
+        endIndex: 6,
+        strategyName: "caseInsensitiveMatch",
+      });
+      expect(fileContent.slice(result!.startIndex, result!.endIndex)).toBe(
+        "test",
+      );
+    });
+
+    it("should return original-cased indices when the match itself changes length on lowercase", () => {
+      // The matched region in the file is a single code unit ("İ") that lowercases
+      // to two units, so endIndex must come from the original slice, not the search length.
+      const fileContent = "x İ y";
+      const result = findSearchMatch(fileContent, "i̇");
+
+      expect(result).toEqual({
+        startIndex: 2,
+        endIndex: 3,
+        strategyName: "caseInsensitiveMatch",
+      });
+      expect(fileContent.slice(result!.startIndex, result!.endIndex)).toBe("İ");
+    });
   });
 
   describe("Whitespace ignored strategy fallback", () => {