File Operations

file_read

Read the contents of a file. Supports reading specific line ranges.

Parameters

Parameter	Type	Required	Description
path	string	Yes	Path to the file (relative to working directory)
offset	integer	No	Line number to start reading from (0-based)
limit	integer	No	Maximum number of lines to read

Example

Reading a file with a line range:

{
  "path": "src/main.rs",
  "offset": 10,
  "limit": 20
}

This reads lines 11-30 of src/main.rs.

---

file_write

Create or overwrite a file. Automatically creates parent directories if they do not exist.

Parameters

Parameter	Type	Required	Description
path	string	Yes	Path to the file
content	string	Yes	Content to write

Example

{
  "path": "src/utils.rs",
  "content": "pub fn add(a: i32, b: i32) -> i32 {\n    a + b\n}\n"
}

Caution

file_write overwrites the entire file. For targeted edits, use file_edit or apply_patch instead.

---

file_edit

Edit a file by replacing an exact string match. The old_string must match exactly once in the file.

Parameters

Parameter	Type	Required	Description
path	string	Yes	Path to the file
old_string	string	Yes	Exact text to find (must be unique in the file)
new_string	string	Yes	Replacement text

Example

{
  "path": "src/main.rs",
  "old_string": "fn main() {\n    println!(\"Hello\");\n}",
  "new_string": "fn main() {\n    println!(\"Hello, world!\");\n}"
}

If old_string matches multiple locations or is not found, the edit fails with an error.

---

apply_patch

Apply a unified diff patch. Supports Claude Code-style patch format with fuzzy matching for context lines.

Patch Format

*** Begin Patch
*** Add File: path/to/new_file.rs
+line 1
+line 2
*** Update File: path/to/existing.rs
 context line
-old line
+new line
 context line
*** Delete File: path/to/old_file.rs
*** End Patch

Patch Sections

Section	Description
*** Add File	Create a new file with the specified content
*** Update File	Modify an existing file using diff hunks
*** Delete File	Remove a file

Fuzzy Matching

When applying updates, the patch tool uses fuzzy matching for context lines. If the exact context is not found, it searches within a 3-line window to find the best match. This makes patches more resilient to minor whitespace or formatting differences.

Example

*** Begin Patch
*** Update File: src/lib.rs
 use std::io;

-pub fn read_input() -> String {
+pub fn read_input() -> Result<String, io::Error> {
     let mut input = String::new();
-    std::io::stdin().read_line(&mut input).unwrap();
-    input
+    std::io::stdin().read_line(&mut input)?;
+    Ok(input)
 }
*** End Patch

---

Path Sandboxing

All file tools use resolve_path() to enforce path sandboxing:

• Paths are canonicalized, resolving symlinks and .. components.
• Any path that escapes the working directory is rejected.
• TOCTOU hardening: The implementation avoids checking path.exists() before canonicalize() to prevent symlink race conditions. Pre-write verification (verify_not_escaping_symlink()) runs before file creation, and post-write re-verification (verify_no_symlink_escape()) runs after writes to guard against symlink attacks.