diff --git a/crates/app/cli/src/main.rs b/crates/app/cli/src/main.rs
index 8989ed8..50a2241 100644
--- a/crates/app/cli/src/main.rs
+++ b/crates/app/cli/src/main.rs
@@ -8,9 +8,11 @@ use std::io::{self, Write};
 
 #[derive(clap::Subcommand, Debug)]
 enum Cmd {
-    Read {path: String},
-    Glob {pattern: String},
-    Grep {root: String, pattern: String},
+    Read { path: String },
+    Glob { pattern: String },
+    Grep { root: String, pattern: String },
+    Write { path: String, content: String },
+    Edit { path: String, old_string: String, new_string: String },
 }
 
 #[derive(Parser, Debug)]
@@ -105,6 +107,42 @@ async fn main() -> Result<()> {
                     }
                 }
             }
+            Cmd::Write { path, content } => {
+                // Check permission
+                match perms.check(Tool::Write, None) {
+                    PermissionDecision::Allow => {
+                        tools_fs::write_file(&path, &content)?;
+                        println!("File written: {}", path);
+                        return Ok(());
+                    }
+                    PermissionDecision::Ask => {
+                        return Err(eyre!(
+                            "Permission denied: Write operation requires approval. Use --mode acceptEdits or --mode code to allow."
+                        ));
+                    }
+                    PermissionDecision::Deny => {
+                        return Err(eyre!("Permission denied: Write operation is blocked."));
+                    }
+                }
+            }
+            Cmd::Edit { path, old_string, new_string } => {
+                // Check permission
+                match perms.check(Tool::Edit, None) {
+                    PermissionDecision::Allow => {
+                        tools_fs::edit_file(&path, &old_string, &new_string)?;
+                        println!("File edited: {}", path);
+                        return Ok(());
+                    }
+                    PermissionDecision::Ask => {
+                        return Err(eyre!(
+                            "Permission denied: Edit operation requires approval. Use --mode acceptEdits or --mode code to allow."
+                        ));
+                    }
+                    PermissionDecision::Deny => {
+                        return Err(eyre!("Permission denied: Edit operation is blocked."));
+                    }
+                }
+            }
         }
     }
 
diff --git a/crates/app/cli/tests/permissions.rs b/crates/app/cli/tests/permissions.rs
index 01a8f1e..2c33bcb 100644
--- a/crates/app/cli/tests/permissions.rs
+++ b/crates/app/cli/tests/permissions.rs
@@ -54,3 +54,97 @@ fn mode_override_via_cli_flag() {
         .arg(file.to_str().unwrap());
     cmd.assert().success().stdout("content\n");
 }
+
+#[test]
+fn plan_mode_blocks_write_operations() {
+    let dir = tempdir().unwrap();
+    let file = dir.path().join("new.txt");
+
+    // Write operation should be blocked in plan mode (default)
+    let mut cmd = Command::new(assert_cmd::cargo::cargo_bin!("owlen"));
+    cmd.arg("write").arg(file.to_str().unwrap()).arg("content");
+    cmd.assert().failure();
+}
+
+#[test]
+fn plan_mode_blocks_edit_operations() {
+    let dir = tempdir().unwrap();
+    let file = dir.path().join("test.txt");
+    fs::write(&file, "old content").unwrap();
+
+    // Edit operation should be blocked in plan mode (default)
+    let mut cmd = Command::new(assert_cmd::cargo::cargo_bin!("owlen"));
+    cmd.arg("edit")
+        .arg(file.to_str().unwrap())
+        .arg("old")
+        .arg("new");
+    cmd.assert().failure();
+}
+
+#[test]
+fn accept_edits_mode_allows_write() {
+    let dir = tempdir().unwrap();
+    let file = dir.path().join("new.txt");
+
+    // Write operation should work in acceptEdits mode
+    let mut cmd = Command::new(assert_cmd::cargo::cargo_bin!("owlen"));
+    cmd.arg("--mode")
+        .arg("acceptEdits")
+        .arg("write")
+        .arg(file.to_str().unwrap())
+        .arg("new content");
+    cmd.assert().success();
+
+    // Verify file was written
+    assert_eq!(fs::read_to_string(&file).unwrap(), "new content");
+}
+
+#[test]
+fn accept_edits_mode_allows_edit() {
+    let dir = tempdir().unwrap();
+    let file = dir.path().join("test.txt");
+    fs::write(&file, "line 1\nline 2\nline 3").unwrap();
+
+    // Edit operation should work in acceptEdits mode
+    let mut cmd = Command::new(assert_cmd::cargo::cargo_bin!("owlen"));
+    cmd.arg("--mode")
+        .arg("acceptEdits")
+        .arg("edit")
+        .arg(file.to_str().unwrap())
+        .arg("line 2")
+        .arg("modified line");
+    cmd.assert().success();
+
+    // Verify file was edited
+    assert_eq!(
+        fs::read_to_string(&file).unwrap(),
+        "line 1\nmodified line\nline 3"
+    );
+}
+
+#[test]
+fn code_mode_allows_all_operations() {
+    let dir = tempdir().unwrap();
+    let file = dir.path().join("test.txt");
+
+    // Write in code mode
+    let mut cmd = Command::new(assert_cmd::cargo::cargo_bin!("owlen"));
+    cmd.arg("--mode")
+        .arg("code")
+        .arg("write")
+        .arg(file.to_str().unwrap())
+        .arg("initial content");
+    cmd.assert().success();
+
+    // Edit in code mode
+    let mut cmd = Command::new(assert_cmd::cargo::cargo_bin!("owlen"));
+    cmd.arg("--mode")
+        .arg("code")
+        .arg("edit")
+        .arg(file.to_str().unwrap())
+        .arg("initial")
+        .arg("modified");
+    cmd.assert().success();
+
+    assert_eq!(fs::read_to_string(&file).unwrap(), "modified content");
+}
diff --git a/crates/tools/fs/Cargo.toml b/crates/tools/fs/Cargo.toml
index 69d5b2e..ad37b97 100644
--- a/crates/tools/fs/Cargo.toml
+++ b/crates/tools/fs/Cargo.toml
@@ -12,4 +12,7 @@ globset = "0.4"
 grep-regex = "0.1"
 grep-searcher = "0.1"
 color-eyre = "0.6"
+similar = "2.7"
+
+[dev-dependencies]
 tempfile = "3.23.0"
\ No newline at end of file
diff --git a/crates/tools/fs/src/lib.rs b/crates/tools/fs/src/lib.rs
index 768f7ce..63ee410 100644
--- a/crates/tools/fs/src/lib.rs
+++ b/crates/tools/fs/src/lib.rs
@@ -1,13 +1,62 @@
-use color_eyre::eyre::Result;
+use color_eyre::eyre::{Result, eyre};
 use ignore::WalkBuilder;
 use grep_regex::RegexMatcher;
 use grep_searcher::{sinks::UTF8, SearcherBuilder};
 use globset::Glob;
+use std::path::Path;
 
 pub fn read_file(path: &str) -> Result<String> {
     Ok(std::fs::read_to_string(path)?)
 }
 
+pub fn write_file(path: &str, content: &str) -> Result<()> {
+    // Create parent directories if they don't exist
+    if let Some(parent) = Path::new(path).parent() {
+        std::fs::create_dir_all(parent)?;
+    }
+    std::fs::write(path, content)?;
+    Ok(())
+}
+
+pub fn edit_file(path: &str, old_string: &str, new_string: &str) -> Result<()> {
+    // Read the current file content
+    let content = std::fs::read_to_string(path)?;
+
+    // Find all occurrences of old_string
+    let matches: Vec<_> = content.match_indices(old_string).collect();
+
+    match matches.len() {
+        0 => Err(eyre!("String to replace not found in file: '{}'", old_string)),
+        1 => {
+            // Exactly one match - safe to replace
+            let new_content = content.replace(old_string, new_string);
+
+            // Create a backup before modifying
+            let backup_path = format!("{}.backup", path);
+            std::fs::write(&backup_path, &content)?;
+
+            // Write the new content
+            match std::fs::write(path, new_content) {
+                Ok(_) => {
+                    // Success - remove backup
+                    let _ = std::fs::remove_file(&backup_path);
+                    Ok(())
+                }
+                Err(e) => {
+                    // Failed to write - restore from backup
+                    let _ = std::fs::rename(&backup_path, path);
+                    Err(e.into())
+                }
+            }
+        }
+        n => Err(eyre!(
+            "Ambiguous replacement: found {} occurrences of '{}' in file. Please make the old_string unique.",
+            n,
+            old_string
+        )),
+    }
+}
+
 pub fn glob_list(pattern: &str) -> Result<Vec<String>> {
     let glob = Glob::new(pattern)?.compile_matcher();
 
diff --git a/crates/tools/fs/tests/fs_tools.rs b/crates/tools/fs/tests/fs_tools.rs
index e4a37ed..e42862a 100644
--- a/crates/tools/fs/tests/fs_tools.rs
+++ b/crates/tools/fs/tests/fs_tools.rs
@@ -1,4 +1,4 @@
-use tools_fs::{read_file, glob_list, grep};
+use tools_fs::{read_file, glob_list, grep, write_file, edit_file};
 use std::fs;
 use tempfile::tempdir;
 
@@ -26,4 +26,79 @@ fn grep_finds_lines() {
 
     let hits = grep(root.to_str().unwrap(), "hello").unwrap();
     assert!(hits.iter().any(|(_p, _ln, text)| text.contains("hello")));
+}
+
+#[test]
+fn write_file_creates_new_file() {
+    let dir = tempdir().unwrap();
+    let file_path = dir.path().join("new.txt");
+
+    write_file(file_path.to_str().unwrap(), "new content").unwrap();
+
+    assert_eq!(read_file(file_path.to_str().unwrap()).unwrap(), "new content");
+}
+
+#[test]
+fn write_file_overwrites_existing() {
+    let dir = tempdir().unwrap();
+    let file_path = dir.path().join("existing.txt");
+    fs::write(&file_path, "old content").unwrap();
+
+    write_file(file_path.to_str().unwrap(), "new content").unwrap();
+
+    assert_eq!(read_file(file_path.to_str().unwrap()).unwrap(), "new content");
+}
+
+#[test]
+fn edit_file_replaces_exact_match() {
+    let dir = tempdir().unwrap();
+    let file_path = dir.path().join("test.txt");
+    let original = "line 1\nline 2\nline 3\n";
+    fs::write(&file_path, original).unwrap();
+
+    edit_file(file_path.to_str().unwrap(), "line 2", "modified line 2").unwrap();
+
+    let result = read_file(file_path.to_str().unwrap()).unwrap();
+    assert_eq!(result, "line 1\nmodified line 2\nline 3\n");
+}
+
+#[test]
+fn edit_file_replaces_multiline() {
+    let dir = tempdir().unwrap();
+    let file_path = dir.path().join("test.txt");
+    let original = "line 1\nline 2\nline 3\nline 4\n";
+    fs::write(&file_path, original).unwrap();
+
+    edit_file(file_path.to_str().unwrap(), "line 2\nline 3", "new content").unwrap();
+
+    let result = read_file(file_path.to_str().unwrap()).unwrap();
+    assert_eq!(result, "line 1\nnew content\nline 4\n");
+}
+
+#[test]
+fn edit_file_fails_on_ambiguous_match() {
+    let dir = tempdir().unwrap();
+    let file_path = dir.path().join("test.txt");
+    let original = "duplicate\nsome text\nduplicate\n";
+    fs::write(&file_path, original).unwrap();
+
+    let result = edit_file(file_path.to_str().unwrap(), "duplicate", "changed");
+
+    assert!(result.is_err());
+    let err_msg = result.unwrap_err().to_string();
+    assert!(err_msg.contains("Ambiguous") || err_msg.contains("multiple") || err_msg.contains("occurrences"));
+}
+
+#[test]
+fn edit_file_fails_on_no_match() {
+    let dir = tempdir().unwrap();
+    let file_path = dir.path().join("test.txt");
+    let original = "line 1\nline 2\n";
+    fs::write(&file_path, original).unwrap();
+
+    let result = edit_file(file_path.to_str().unwrap(), "nonexistent", "changed");
+
+    assert!(result.is_err());
+    let err_msg = result.unwrap_err().to_string();
+    assert!(err_msg.contains("not found") || err_msg.contains("String to replace"));
 }
\ No newline at end of file