old_string was not found in the file. Copy the exact text (i

Error message

old_string was not found in the file. Copy the exact text (including whitespace) from the file

What it means

applySandboxEdit searches the file content with strings.Count(content, oldString) and throws this error when the count is 0, i.e. the exact old_string (including whitespace) does not appear in the file. The edit requires an exact substring match; fuzzy or normalized matching is not performed.

Source

Thrown at internal/agent/tools/sandbox_edit.go:285

}

// Cleanup releases any resources.
func (t *EditSandboxFileTool) Cleanup(ctx context.Context) error {
	return nil
}

// applySandboxEdit performs an exact string replacement. replaceAll=false
// requires a unique match.
func applySandboxEdit(content, oldString, newString string, replaceAll bool) (string, int, error) {
	if oldString == "" {
		return "", 0, fmt.Errorf("old_string is required; copy the exact text to change, including whitespace")
	}
	if oldString == newString {
		return "", 0, fmt.Errorf("old_string and new_string are identical; no change would be made")
	}
	n := strings.Count(content, oldString)
	if n == 0 {
		return "", 0, fmt.Errorf("old_string was not found in the file. Copy the exact text (including whitespace) from the file")
	}
	if n > 1 && !replaceAll {
		return "", 0, fmt.Errorf(
			"old_string matched %d times. Include more surrounding context so it is unique, or set replace_all=true",
			n,
		)
	}
	if replaceAll {
		return strings.ReplaceAll(content, oldString, newString), n, nil
	}
	return strings.Replace(content, oldString, newString, 1), 1, nil
}

View on GitHub (pinned to 988cbb0330)

Solutions

Re-read the file and copy old_string exactly as it appears, preserving indentation, tabs, and line endings.
Shorten old_string to a small, definitely-unique snippet you can verify verbatim in the file.
Check for line-ending or trailing-whitespace mismatches (CRLF vs LF, trailing spaces).
Retry after a failed edit — the file may have been modified by a previous operation in the same run.

Example fix

// before: snippet from stale copy
oldString := "func Foo(x int) error {\n    return nil\n}"
// after: exact text re-read from the file (matching tabs/indentation)
oldString := "func Foo(x int) error {\n\treturn nil\n}"

Defensive patterns

Strategy: validation

Validate before calling

if !strings.Contains(fileContent, oldString) {
    return fmt.Errorf("old_string %q not present in file; re-read the file", oldString)
}

Try / catch

newContent, n, err := applySandboxEdit(content, oldString, newString, replaceAll)
if err != nil && strings.Contains(err.Error(), "was not found") {
    content, err = reReadFile(path) // refresh stale content and retry once
}

Prevention

Always copy old_string from the latest file read, never from memory or docs.
Normalize line endings (LF vs CRLF) and preserve exact indentation/tabs.
Prefer short, verifiable snippets over large copied blocks.

When it happens

Trigger: Calling Execute with an old_string that differs from the file text in whitespace, indentation, line endings (CRLF vs LF), unicode characters, or was taken from an outdated version of the file.

Common situations: The file changed (or was reformatted) after the snippet was copied; copying code from docs/diffs that doesn't match actual indentation; CRLF on Windows; tab/space mismatches; agent hallucinating the surrounding context.

Related errors

AI-assisted analysis of Tencent/WeKnora@988cbb0330 (2026-09-02). Data as JSON: /api/errors/fadd7fd2d46aad51. Report an issue: GitHub.