fix(tools): make the empty/whitespace old_string rejection actionable

Port from cline/cline#13970: models that send patch calls with an empty
old_string got back 'old_string cannot be empty' — an error that names the
problem but not the recovery, so the next call was byte-identical and the
run burned turns until loop detection killed it (upstream repro: Kimi K3
looping on old_text: null).

The rejection now states the recovery: set old_string to the exact text the
replacement should replace, read the file first if unsure, use write_file
for new files/full rewrites, and do not re-send the call unchanged. The
whitespace-only rejection gets the same treatment. No behavior change for
valid calls.

tests/tools/test_fuzzy_match.py

@@ -21,9 +21,13 @@ class TestExactMatch:
         assert new == content  # untouched
 
     def test_empty_old_string_rejected(self):
+        """The rejection must carry a recovery path — a bare "cannot be empty"
+        leaves models re-sending the identical call until the loop detector
+        kills the run (cline/cline#13970)."""
         new, count, _, err = fuzzy_find_and_replace("abc", "", "x")
         assert count == 0
         assert err is not None
+        assert "read the file" in err and "write_file" in err
 
     def test_identical_strings(self):
         new, count, _, err = fuzzy_find_and_replace("abc", "abc", "abc")

tools/fuzzy_match.py

@@ -342,11 +342,21 @@ def fuzzy_find_and_replace(content: str, old_string: str, new_string: str,
     ``(content, 0, None, error)``.
     """
     if not old_string:
-        return content, 0, None, "old_string cannot be empty"
+        # Actionable recovery text: a terse "cannot be empty" leaves the model
+        # re-sending the identical call until the loop detector kills the run
+        # (upstream report: cline/cline#13970 — Kimi K3 looped on old_text: null).
+        return content, 0, None, (
+            "old_string is empty — nothing to match. Set old_string to the exact "
+            "existing text the replacement should replace (read the file first if "
+            "unsure). To create a new file or fully rewrite one, use write_file "
+            "instead. Do not re-send this call unchanged.")
     if not old_string.strip():
         # Whitespace-only anchors match trivially and mass-replace or
         # ambiguity-error; never meaningful.
-        return content, 0, None, "old_string is only whitespace — provide non-blank text to match"
+        return content, 0, None, (
+            "old_string is only whitespace — provide non-blank text to match. Set it "
+            "to the exact existing text the replacement should replace (read the file "
+            "first if unsure). Do not re-send this call unchanged.")
     if old_string == new_string:
         return content, 0, None, IDENTICAL_STRINGS_ERROR