feat: list_dir tool (#4817)

Add a tool to list_dir. It is useful because we can mark it as
non-mutating and so use it in parallel

codex-rs/core/src/model_family.rs

@@ -120,6 +120,7 @@ pub fn find_family_for_model(mut slug: &str) -> Option<ModelFamily> {
             base_instructions: GPT_5_CODEX_INSTRUCTIONS.to_string(),
             experimental_supported_tools: vec![
                 "read_file".to_string(),
+                "list_dir".to_string(),
                 "test_sync_tool".to_string()
             ],
             supports_parallel_tool_calls: true,
@@ -133,7 +134,7 @@ pub fn find_family_for_model(mut slug: &str) -> Option<ModelFamily> {
             reasoning_summary_format: ReasoningSummaryFormat::Experimental,
             base_instructions: GPT_5_CODEX_INSTRUCTIONS.to_string(),
             apply_patch_tool_type: Some(ApplyPatchToolType::Freeform),
-            experimental_supported_tools: vec!["read_file".to_string()],
+            experimental_supported_tools: vec!["read_file".to_string(), "list_dir".to_string()],
             supports_parallel_tool_calls: true,
         )
 

codex-rs/core/src/tools/handlers/list_dir.rs

@@ -0,0 +1,476 @@
+use std::collections::VecDeque;
+use std::ffi::OsStr;
+use std::fs::FileType;
+use std::path::Path;
+use std::path::PathBuf;
+
+use async_trait::async_trait;
+use codex_utils_string::take_bytes_at_char_boundary;
+use serde::Deserialize;
+use tokio::fs;
+
+use crate::function_tool::FunctionCallError;
+use crate::tools::context::ToolInvocation;
+use crate::tools::context::ToolOutput;
+use crate::tools::context::ToolPayload;
+use crate::tools::registry::ToolHandler;
+use crate::tools::registry::ToolKind;
+
+pub struct ListDirHandler;
+
+const MAX_ENTRY_LENGTH: usize = 500;
+const INDENTATION_SPACES: usize = 2;
+
+fn default_offset() -> usize {
+    1
+}
+
+fn default_limit() -> usize {
+    25
+}
+
+fn default_depth() -> usize {
+    2
+}
+
+#[derive(Deserialize)]
+struct ListDirArgs {
+    dir_path: String,
+    #[serde(default = "default_offset")]
+    offset: usize,
+    #[serde(default = "default_limit")]
+    limit: usize,
+    #[serde(default = "default_depth")]
+    depth: usize,
+}
+
+#[async_trait]
+impl ToolHandler for ListDirHandler {
+    fn kind(&self) -> ToolKind {
+        ToolKind::Function
+    }
+
+    async fn handle(&self, invocation: ToolInvocation) -> Result<ToolOutput, FunctionCallError> {
+        let ToolInvocation { payload, .. } = invocation;
+
+        let arguments = match payload {
+            ToolPayload::Function { arguments } => arguments,
+            _ => {
+                return Err(FunctionCallError::RespondToModel(
+                    "list_dir handler received unsupported payload".to_string(),
+                ));
+            }
+        };
+
+        let args: ListDirArgs = serde_json::from_str(&arguments).map_err(|err| {
+            FunctionCallError::RespondToModel(format!(
+                "failed to parse function arguments: {err:?}"
+            ))
+        })?;
+
+        let ListDirArgs {
+            dir_path,
+            offset,
+            limit,
+            depth,
+        } = args;
+
+        if offset == 0 {
+            return Err(FunctionCallError::RespondToModel(
+                "offset must be a 1-indexed entry number".to_string(),
+            ));
+        }
+
+        if limit == 0 {
+            return Err(FunctionCallError::RespondToModel(
+                "limit must be greater than zero".to_string(),
+            ));
+        }
+
+        if depth == 0 {
+            return Err(FunctionCallError::RespondToModel(
+                "depth must be greater than zero".to_string(),
+            ));
+        }
+
+        let path = PathBuf::from(&dir_path);
+        if !path.is_absolute() {
+            return Err(FunctionCallError::RespondToModel(
+                "dir_path must be an absolute path".to_string(),
+            ));
+        }
+
+        let entries = list_dir_slice(&path, offset, limit, depth).await?;
+        let mut output = Vec::with_capacity(entries.len() + 1);
+        output.push(format!("Absolute path: {}", path.display()));
+        output.extend(entries);
+        Ok(ToolOutput::Function {
+            content: output.join("\n"),
+            success: Some(true),
+        })
+    }
+}
+
+async fn list_dir_slice(
+    path: &Path,
+    offset: usize,
+    limit: usize,
+    depth: usize,
+) -> Result<Vec<String>, FunctionCallError> {
+    let mut entries = Vec::new();
+    collect_entries(path, Path::new(""), depth, &mut entries).await?;
+
+    if entries.is_empty() {
+        return Ok(Vec::new());
+    }
+
+    let start_index = offset - 1;
+    if start_index >= entries.len() {
+        return Err(FunctionCallError::RespondToModel(
+            "offset exceeds directory entry count".to_string(),
+        ));
+    }
+
+    let remaining_entries = entries.len() - start_index;
+    let capped_limit = limit.min(remaining_entries);
+    let end_index = start_index + capped_limit;
+    let mut selected_entries = entries[start_index..end_index].to_vec();
+    selected_entries.sort_unstable_by(|a, b| a.name.cmp(&b.name));
+    let mut formatted = Vec::with_capacity(selected_entries.len());
+
+    for entry in &selected_entries {
+        formatted.push(format_entry_line(entry));
+    }
+
+    if end_index < entries.len() {
+        formatted.push(format!("More than {capped_limit} entries found"));
+    }
+
+    Ok(formatted)
+}
+
+async fn collect_entries(
+    dir_path: &Path,
+    relative_prefix: &Path,
+    depth: usize,
+    entries: &mut Vec<DirEntry>,
+) -> Result<(), FunctionCallError> {
+    let mut queue = VecDeque::new();
+    queue.push_back((dir_path.to_path_buf(), relative_prefix.to_path_buf(), depth));
+
+    while let Some((current_dir, prefix, remaining_depth)) = queue.pop_front() {
+        let mut read_dir = fs::read_dir(&current_dir).await.map_err(|err| {
+            FunctionCallError::RespondToModel(format!("failed to read directory: {err}"))
+        })?;
+
+        let mut dir_entries = Vec::new();
+
+        while let Some(entry) = read_dir.next_entry().await.map_err(|err| {
+            FunctionCallError::RespondToModel(format!("failed to read directory: {err}"))
+        })? {
+            let file_type = entry.file_type().await.map_err(|err| {
+                FunctionCallError::RespondToModel(format!("failed to inspect entry: {err}"))
+            })?;
+
+            let file_name = entry.file_name();
+            let relative_path = if prefix.as_os_str().is_empty() {
+                PathBuf::from(&file_name)
+            } else {
+                prefix.join(&file_name)
+            };
+
+            let display_name = format_entry_component(&file_name);
+            let display_depth = prefix.components().count();
+            let sort_key = format_entry_name(&relative_path);
+            let kind = DirEntryKind::from(&file_type);
+            dir_entries.push((
+                entry.path(),
+                relative_path,
+                kind,
+                DirEntry {
+                    name: sort_key,
+                    display_name,
+                    depth: display_depth,
+                    kind,
+                },
+            ));
+        }
+
+        dir_entries.sort_unstable_by(|a, b| a.3.name.cmp(&b.3.name));
+
+        for (entry_path, relative_path, kind, dir_entry) in dir_entries {
+            if kind == DirEntryKind::Directory && remaining_depth > 1 {
+                queue.push_back((entry_path, relative_path, remaining_depth - 1));
+            }
+            entries.push(dir_entry);
+        }
+    }
+
+    Ok(())
+}
+
+fn format_entry_name(path: &Path) -> String {
+    let normalized = path.to_string_lossy().replace("\\", "/");
+    if normalized.len() > MAX_ENTRY_LENGTH {
+        take_bytes_at_char_boundary(&normalized, MAX_ENTRY_LENGTH).to_string()
+    } else {
+        normalized
+    }
+}
+
+fn format_entry_component(name: &OsStr) -> String {
+    let normalized = name.to_string_lossy();
+    if normalized.len() > MAX_ENTRY_LENGTH {
+        take_bytes_at_char_boundary(&normalized, MAX_ENTRY_LENGTH).to_string()
+    } else {
+        normalized.to_string()
+    }
+}
+
+fn format_entry_line(entry: &DirEntry) -> String {
+    let indent = " ".repeat(entry.depth * INDENTATION_SPACES);
+    let mut name = entry.display_name.clone();
+    match entry.kind {
+        DirEntryKind::Directory => name.push('/'),
+        DirEntryKind::Symlink => name.push('@'),
+        DirEntryKind::Other => name.push('?'),
+        DirEntryKind::File => {}
+    }
+    format!("{indent}{name}")
+}
+
+#[derive(Clone)]
+struct DirEntry {
+    name: String,
+    display_name: String,
+    depth: usize,
+    kind: DirEntryKind,
+}
+
+#[derive(Clone, Copy, PartialEq, Eq)]
+enum DirEntryKind {
+    Directory,
+    File,
+    Symlink,
+    Other,
+}
+
+impl From<&FileType> for DirEntryKind {
+    fn from(file_type: &FileType) -> Self {
+        if file_type.is_symlink() {
+            DirEntryKind::Symlink
+        } else if file_type.is_dir() {
+            DirEntryKind::Directory
+        } else if file_type.is_file() {
+            DirEntryKind::File
+        } else {
+            DirEntryKind::Other
+        }
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+    use tempfile::tempdir;
+
+    #[tokio::test]
+    async fn lists_directory_entries() {
+        let temp = tempdir().expect("create tempdir");
+        let dir_path = temp.path();
+
+        let sub_dir = dir_path.join("nested");
+        tokio::fs::create_dir(&sub_dir)
+            .await
+            .expect("create sub dir");
+
+        let deeper_dir = sub_dir.join("deeper");
+        tokio::fs::create_dir(&deeper_dir)
+            .await
+            .expect("create deeper dir");
+
+        tokio::fs::write(dir_path.join("entry.txt"), b"content")
+            .await
+            .expect("write file");
+        tokio::fs::write(sub_dir.join("child.txt"), b"child")
+            .await
+            .expect("write child");
+        tokio::fs::write(deeper_dir.join("grandchild.txt"), b"grandchild")
+            .await
+            .expect("write grandchild");
+
+        #[cfg(unix)]
+        {
+            use std::os::unix::fs::symlink;
+            let link_path = dir_path.join("link");
+            symlink(dir_path.join("entry.txt"), &link_path).expect("create symlink");
+        }
+
+        let entries = list_dir_slice(dir_path, 1, 20, 3)
+            .await
+            .expect("list directory");
+
+        #[cfg(unix)]
+        let expected = vec![
+            "entry.txt".to_string(),
+            "link@".to_string(),
+            "nested/".to_string(),
+            "  child.txt".to_string(),
+            "  deeper/".to_string(),
+            "    grandchild.txt".to_string(),
+        ];
+
+        #[cfg(not(unix))]
+        let expected = vec![
+            "entry.txt".to_string(),
+            "nested/".to_string(),
+            "  child.txt".to_string(),
+            "  deeper/".to_string(),
+            "    grandchild.txt".to_string(),
+        ];
+
+        assert_eq!(entries, expected);
+    }
+
+    #[tokio::test]
+    async fn errors_when_offset_exceeds_entries() {
+        let temp = tempdir().expect("create tempdir");
+        let dir_path = temp.path();
+        tokio::fs::create_dir(dir_path.join("nested"))
+            .await
+            .expect("create sub dir");
+
+        let err = list_dir_slice(dir_path, 10, 1, 2)
+            .await
+            .expect_err("offset exceeds entries");
+        assert_eq!(
+            err,
+            FunctionCallError::RespondToModel("offset exceeds directory entry count".to_string())
+        );
+    }
+
+    #[tokio::test]
+    async fn respects_depth_parameter() {
+        let temp = tempdir().expect("create tempdir");
+        let dir_path = temp.path();
+        let nested = dir_path.join("nested");
+        let deeper = nested.join("deeper");
+        tokio::fs::create_dir(&nested).await.expect("create nested");
+        tokio::fs::create_dir(&deeper).await.expect("create deeper");
+        tokio::fs::write(dir_path.join("root.txt"), b"root")
+            .await
+            .expect("write root");
+        tokio::fs::write(nested.join("child.txt"), b"child")
+            .await
+            .expect("write nested");
+        tokio::fs::write(deeper.join("grandchild.txt"), b"deep")
+            .await
+            .expect("write deeper");
+
+        let entries_depth_one = list_dir_slice(dir_path, 1, 10, 1)
+            .await
+            .expect("list depth 1");
+        assert_eq!(
+            entries_depth_one,
+            vec!["nested/".to_string(), "root.txt".to_string(),]
+        );
+
+        let entries_depth_two = list_dir_slice(dir_path, 1, 20, 2)
+            .await
+            .expect("list depth 2");
+        assert_eq!(
+            entries_depth_two,
+            vec![
+                "nested/".to_string(),
+                "  child.txt".to_string(),
+                "  deeper/".to_string(),
+                "root.txt".to_string(),
+            ]
+        );
+
+        let entries_depth_three = list_dir_slice(dir_path, 1, 30, 3)
+            .await
+            .expect("list depth 3");
+        assert_eq!(
+            entries_depth_three,
+            vec![
+                "nested/".to_string(),
+                "  child.txt".to_string(),
+                "  deeper/".to_string(),
+                "    grandchild.txt".to_string(),
+                "root.txt".to_string(),
+            ]
+        );
+    }
+
+    #[tokio::test]
+    async fn handles_large_limit_without_overflow() {
+        let temp = tempdir().expect("create tempdir");
+        let dir_path = temp.path();
+        tokio::fs::write(dir_path.join("alpha.txt"), b"alpha")
+            .await
+            .expect("write alpha");
+        tokio::fs::write(dir_path.join("beta.txt"), b"beta")
+            .await
+            .expect("write beta");
+        tokio::fs::write(dir_path.join("gamma.txt"), b"gamma")
+            .await
+            .expect("write gamma");
+
+        let entries = list_dir_slice(dir_path, 2, usize::MAX, 1)
+            .await
+            .expect("list without overflow");
+        assert_eq!(
+            entries,
+            vec!["beta.txt".to_string(), "gamma.txt".to_string(),]
+        );
+    }
+
+    #[tokio::test]
+    async fn indicates_truncated_results() {
+        let temp = tempdir().expect("create tempdir");
+        let dir_path = temp.path();
+
+        for idx in 0..40 {
+            let file = dir_path.join(format!("file_{idx:02}.txt"));
+            tokio::fs::write(file, b"content")
+                .await
+                .expect("write file");
+        }
+
+        let entries = list_dir_slice(dir_path, 1, 25, 1)
+            .await
+            .expect("list directory");
+        assert_eq!(entries.len(), 26);
+        assert_eq!(
+            entries.last(),
+            Some(&"More than 25 entries found".to_string())
+        );
+    }
+
+    #[tokio::test]
+    async fn bfs_truncation() -> anyhow::Result<()> {
+        let temp = tempdir()?;
+        let dir_path = temp.path();
+        let nested = dir_path.join("nested");
+        let deeper = nested.join("deeper");
+        tokio::fs::create_dir(&nested).await?;
+        tokio::fs::create_dir(&deeper).await?;
+        tokio::fs::write(dir_path.join("root.txt"), b"root").await?;
+        tokio::fs::write(nested.join("child.txt"), b"child").await?;
+        tokio::fs::write(deeper.join("grandchild.txt"), b"deep").await?;
+
+        let entries_depth_three = list_dir_slice(dir_path, 1, 3, 3).await?;
+        assert_eq!(
+            entries_depth_three,
+            vec![
+                "nested/".to_string(),
+                "  child.txt".to_string(),
+                "root.txt".to_string(),
+                "More than 3 entries found".to_string()
+            ]
+        );
+
+        Ok(())
+    }
+}

codex-rs/core/src/tools/handlers/mod.rs

@@ -1,5 +1,6 @@
 pub mod apply_patch;
 mod exec_stream;
+mod list_dir;
 mod mcp;
 mod plan;
 mod read_file;
@@ -12,6 +13,7 @@ pub use plan::PLAN_TOOL;
 
 pub use apply_patch::ApplyPatchHandler;
 pub use exec_stream::ExecStreamHandler;
+pub use list_dir::ListDirHandler;
 pub use mcp::McpHandler;
 pub use plan::PlanHandler;
 pub use read_file::ReadFileHandler;

codex-rs/core/src/tools/spec.rs

@@ -356,6 +356,51 @@ fn create_read_file_tool() -> ToolSpec {
         },
     })
 }
+
+fn create_list_dir_tool() -> ToolSpec {
+    let mut properties = BTreeMap::new();
+    properties.insert(
+        "dir_path".to_string(),
+        JsonSchema::String {
+            description: Some("Absolute path to the directory to list.".to_string()),
+        },
+    );
+    properties.insert(
+        "offset".to_string(),
+        JsonSchema::Number {
+            description: Some(
+                "The entry number to start listing from. Must be 1 or greater.".to_string(),
+            ),
+        },
+    );
+    properties.insert(
+        "limit".to_string(),
+        JsonSchema::Number {
+            description: Some("The maximum number of entries to return.".to_string()),
+        },
+    );
+    properties.insert(
+        "depth".to_string(),
+        JsonSchema::Number {
+            description: Some(
+                "The maximum directory depth to traverse. Must be 1 or greater.".to_string(),
+            ),
+        },
+    );
+
+    ToolSpec::Function(ResponsesApiTool {
+        name: "list_dir".to_string(),
+        description:
+            "Lists entries in a local directory with 1-indexed entry numbers and simple type labels."
+                .to_string(),
+        strict: false,
+        parameters: JsonSchema::Object {
+            properties,
+            required: Some(vec!["dir_path".to_string()]),
+            additional_properties: Some(false.into()),
+        },
+    })
+}
 /// TODO(dylan): deprecate once we get rid of json tool
 #[derive(Serialize, Deserialize)]
 pub(crate) struct ApplyPatchToolArgs {
@@ -565,6 +610,7 @@ pub(crate) fn build_specs(
     use crate::exec_command::create_write_stdin_tool_for_responses_api;
     use crate::tools::handlers::ApplyPatchHandler;
     use crate::tools::handlers::ExecStreamHandler;
+    use crate::tools::handlers::ListDirHandler;
     use crate::tools::handlers::McpHandler;
     use crate::tools::handlers::PlanHandler;
     use crate::tools::handlers::ReadFileHandler;
@@ -640,6 +686,16 @@ pub(crate) fn build_specs(
         builder.register_handler("read_file", read_file_handler);
     }
 
+    if config
+        .experimental_supported_tools
+        .iter()
+        .any(|tool| tool == "list_dir")
+    {
+        let list_dir_handler = Arc::new(ListDirHandler);
+        builder.push_spec_with_parallel_support(create_list_dir_tool(), true);
+        builder.register_handler("list_dir", list_dir_handler);
+    }
+
     if config
         .experimental_supported_tools
         .iter()
@@ -786,6 +842,7 @@ mod tests {
 
         assert!(!find_tool(&tools, "unified_exec").supports_parallel_tool_calls);
         assert!(find_tool(&tools, "read_file").supports_parallel_tool_calls);
+        assert!(find_tool(&tools, "list_dir").supports_parallel_tool_calls);
     }
 
     #[test]
@@ -813,6 +870,7 @@ mod tests {
                 .iter()
                 .any(|tool| tool_name(&tool.spec) == "read_file")
         );
+        assert!(tools.iter().any(|tool| tool_name(&tool.spec) == "list_dir"));
     }
 
     #[test]