diff --git a/src/plyngent/tools/file/edit_lineno.py b/src/plyngent/tools/file/edit_lineno.py
index d8f25a7..f366316 100644
--- a/src/plyngent/tools/file/edit_lineno.py
+++ b/src/plyngent/tools/file/edit_lineno.py
@@ -77,6 +77,14 @@ def _append_after(target: Path, path: str, text: str, lines: list[str], new_cont
     return f"appended content after line {n} in {path}"
 
 
+def _excerpt(text: str, limit: int = 40) -> str:
+    """Collapse newlines and truncate for a one-line message excerpt."""
+    one = text.replace("\r\n", " ").replace("\n", " ")
+    if len(one) > limit:
+        return one[:limit] + "…"
+    return one
+
+
 def _replace_range(
     target: Path,
     path: str,
@@ -92,7 +100,11 @@ def _replace_range(
     new_lines = lines[: start_line - 1] + replacement + lines[end:]
     _ = target.write_text("".join(new_lines), encoding="utf-8")
     removed = end - start_line + 1
-    return f"replaced lines {start_line}-{end} ({removed} lines) with {len(replacement)} lines in {path}"
+    old_first = _excerpt(lines[start_line - 1].rstrip("\r\n"))
+    return (
+        f"replaced lines {start_line}-{end} ({removed} lines; first: {old_first!r}) "
+        f"with {len(replacement)} lines in {path}"
+    )
 
 
 @tool(tags=ToolTag.LOCAL | ToolTag.INSTANCE_STATE | ToolTag.YOLO)
diff --git a/src/plyngent/tools/file/edit_replace.py b/src/plyngent/tools/file/edit_replace.py
index c90e2c0..a791571 100644
--- a/src/plyngent/tools/file/edit_replace.py
+++ b/src/plyngent/tools/file/edit_replace.py
@@ -18,15 +18,31 @@ def _count_non_overlapping(text: str, needle: str) -> int:
         start = index + len(needle)
 
 
-def _success_message(path: str, *, replaced: int, found: int) -> str:
+def _excerpt(text: str, limit: int = 40) -> str:
+    """Collapse newlines and truncate for a one-line message excerpt."""
+    one = text.replace("\r\n", " ").replace("\n", " ")
+    if len(one) > limit:
+        return one[:limit] + "…"
+    return one
+
+
+def _success_message(
+    path: str,
+    *,
+    replaced: int,
+    found: int,
+    old_string: str,
+    new_string: str,
+) -> str:
     remaining = found - replaced
     unit = "occurrence" if replaced == 1 else "occurrences"
+    change = f" ('{_excerpt(old_string)}' → '{_excerpt(new_string)}')"
     if remaining == 0:
         if found == 1:
-            return f"replaced 1 occurrence in {path}"
-        return f"replaced {replaced} {unit} in {path} (all {found} matches)"
+            return f"replaced 1 occurrence in {path}{change}"
+        return f"replaced {replaced} {unit} in {path}{change} (all {found} matches)"
     return (
-        f"replaced {replaced} {unit} in {path} "
+        f"replaced {replaced} {unit} in {path}{change} "
         f"({replaced} of {found} matches; {remaining} remain — "
         f"raise max_replaces or use a more specific old_string)"
     )
@@ -56,4 +72,10 @@ async def edit_replace(path: str, old_string: str, new_string: str, max_replaces
     _ = target.write_text(updated, encoding="utf-8")
     # The write changes the file's mtime, so edit_lineno's freshness check
     # rejects stale line numbers until the model re-reads with_lineno.
-    return _success_message(path, replaced=n, found=found)
+    return _success_message(
+        path,
+        replaced=n,
+        found=found,
+        old_string=old_string,
+        new_string=new_string,
+    )
diff --git a/tests/test_tools/test_edit_lineno.py b/tests/test_tools/test_edit_lineno.py
index e83f2b0..0c55c22 100644
--- a/tests/test_tools/test_edit_lineno.py
+++ b/tests/test_tools/test_edit_lineno.py
@@ -19,6 +19,7 @@ def test_edit_lineno_replace_middle(workspace: object) -> None:
     _ = _read_lineno("a.txt")
     out = call_sync(edit_lineno, "a.txt", 2, 3, "TWO\nTHREE\n")
     assert "replaced lines 2-3" in out
+    assert "first: 'two'" in out  # old-content excerpt for self-correction
     assert call_sync(read_file, "a.txt") == "L1-4\none\nTWO\nTHREE\nfour\n"
 
 
diff --git a/tests/test_tools/test_file.py b/tests/test_tools/test_file.py
index 8258eae..17726d2 100644
--- a/tests/test_tools/test_file.py
+++ b/tests/test_tools/test_file.py
@@ -74,6 +74,7 @@ def test_edit_replace_first_only(workspace: object) -> None:
     assert call_sync(read_file, "t.txt") == "L1-1\nbb aa"
     assert "1 of 2" in result or "1 of 2 matches" in result
     assert "remain" in result
+    assert "'aa' → 'bb'" in result  # old→new excerpt for self-correction
 
 
 def test_edit_replace_max_replaces(workspace: object) -> None: