core: add a sandbox-backed fs helper

codex-rs/Cargo.lock

@@ -1557,6 +1557,7 @@ version = "0.0.0"
 dependencies = [
  "anyhow",
  "codex-apply-patch",
+ "codex-fs-ops",
  "codex-linux-sandbox",
  "codex-shell-escalation",
  "codex-utils-home-dir",
@@ -1846,6 +1847,7 @@ dependencies = [
  "codex-exec-server",
  "codex-execpolicy",
  "codex-file-search",
+ "codex-fs-ops",
  "codex-git",
  "codex-hooks",
  "codex-keyring-store",
@@ -2087,6 +2089,17 @@ dependencies = [
  "tokio",
 ]
 
+[[package]]
+name = "codex-fs-ops"
+version = "0.0.0"
+dependencies = [
+ "anyhow",
+ "pretty_assertions",
+ "serde",
+ "serde_json",
+ "tempfile",
+]
+
 [[package]]
 name = "codex-git"
 version = "0.0.0"

codex-rs/Cargo.toml

@@ -11,6 +11,7 @@ members = [
     "apply-patch",
     "arg0",
     "feedback",
+    "fs-ops",
     "codex-backend-openapi-models",
     "cloud-requirements",
     "cloud-tasks",
@@ -109,6 +110,7 @@ codex-exec-server = { path = "exec-server" }
 codex-execpolicy = { path = "execpolicy" }
 codex-experimental-api-macros = { path = "codex-experimental-api-macros" }
 codex-feedback = { path = "feedback" }
+codex-fs-ops = { path = "fs-ops" }
 codex-file-search = { path = "file-search" }
 codex-git = { path = "utils/git" }
 codex-hooks = { path = "hooks" }

codex-rs/arg0/Cargo.toml

@@ -14,6 +14,7 @@ workspace = true
 [dependencies]
 anyhow = { workspace = true }
 codex-apply-patch = { workspace = true }
+codex-fs-ops = { workspace = true }
 codex-linux-sandbox = { workspace = true }
 codex-shell-escalation = { workspace = true }
 codex-utils-home-dir = { workspace = true }

codex-rs/arg0/src/lib.rs

@@ -4,6 +4,7 @@ use std::path::Path;
 use std::path::PathBuf;
 
 use codex_apply_patch::CODEX_CORE_APPLY_PATCH_ARG1;
+use codex_fs_ops::CODEX_CORE_FS_OPS_ARG1;
 use codex_utils_home_dir::find_codex_home;
 #[cfg(unix)]
 use std::os::unix::fs::symlink;
@@ -105,6 +106,17 @@ pub fn arg0_dispatch() -> Option<Arg0PathEntryGuard> {
         };
         std::process::exit(exit_code);
     }
+    if argv1 == CODEX_CORE_FS_OPS_ARG1 {
+        let mut stdin = std::io::stdin();
+        let mut stdout = std::io::stdout();
+        let mut stderr = std::io::stderr();
+        let exit_code =
+            match codex_fs_ops::run_from_args(args, &mut stdin, &mut stdout, &mut stderr) {
+                Ok(()) => 0,
+                Err(_) => 1,
+            };
+        std::process::exit(exit_code);
+    }
 
     // This modifies the environment, which is not thread-safe, so do this
     // before creating any threads/the Tokio runtime.

codex-rs/core/Cargo.toml

@@ -38,6 +38,7 @@ codex-exec-server = { workspace = true }
 codex-shell-command = { workspace = true }
 codex-skills = { workspace = true }
 codex-execpolicy = { workspace = true }
+codex-fs-ops = { workspace = true }
 codex-file-search = { workspace = true }
 codex-git = { workspace = true }
 codex-hooks = { workspace = true }

codex-rs/core/src/exec.rs

@@ -332,6 +332,58 @@ pub(crate) async fn execute_exec_request(
     finalize_exec_result(raw_output_result, sandbox, duration)
 }
 
+pub(crate) async fn execute_exec_request_output_bytes(
+    exec_request: ExecRequest,
+    sandbox_policy: &SandboxPolicy,
+    stdout_stream: Option<StdoutStream>,
+    after_spawn: Option<Box<dyn FnOnce() + Send>>,
+) -> Result<ExecToolCallOutputBytes> {
+    let ExecRequest {
+        command,
+        cwd,
+        env,
+        network,
+        expiration,
+        sandbox,
+        windows_sandbox_level,
+        windows_sandbox_private_desktop,
+        sandbox_permissions,
+        sandbox_policy: _sandbox_policy_from_env,
+        file_system_sandbox_policy,
+        network_sandbox_policy,
+        justification,
+        arg0,
+    } = exec_request;
+    let _ = _sandbox_policy_from_env;
+
+    let params = ExecParams {
+        command,
+        cwd,
+        expiration,
+        env,
+        network: network.clone(),
+        sandbox_permissions,
+        windows_sandbox_level,
+        windows_sandbox_private_desktop,
+        justification,
+        arg0,
+    };
+
+    let start = Instant::now();
+    let raw_output_result = exec(
+        params,
+        sandbox,
+        sandbox_policy,
+        &file_system_sandbox_policy,
+        network_sandbox_policy,
+        stdout_stream,
+        after_spawn,
+    )
+    .await;
+    let duration = start.elapsed();
+    finalize_exec_result_bytes(raw_output_result, sandbox, duration)
+}
+
 #[cfg(target_os = "windows")]
 fn extract_create_process_as_user_error_code(err: &str) -> Option<String> {
     let marker = "CreateProcessAsUserW failed: ";
@@ -574,6 +626,64 @@ fn finalize_exec_result(
     }
 }
 
+fn finalize_exec_result_bytes(
+    raw_output_result: std::result::Result<RawExecToolCallOutput, CodexErr>,
+    sandbox_type: SandboxType,
+    duration: Duration,
+) -> Result<ExecToolCallOutputBytes> {
+    match raw_output_result {
+        Ok(raw_output) => {
+            #[allow(unused_mut)]
+            let mut timed_out = raw_output.timed_out;
+
+            #[cfg(target_family = "unix")]
+            {
+                if let Some(signal) = raw_output.exit_status.signal() {
+                    if signal == TIMEOUT_CODE {
+                        timed_out = true;
+                    } else {
+                        return Err(CodexErr::Sandbox(SandboxErr::Signal(signal)));
+                    }
+                }
+            }
+
+            let mut exit_code = raw_output.exit_status.code().unwrap_or(-1);
+            if timed_out {
+                exit_code = EXEC_TIMEOUT_EXIT_CODE;
+            }
+
+            let exec_output = ExecToolCallOutputBytes {
+                exit_code,
+                stdout: raw_output.stdout,
+                stderr: raw_output.stderr,
+                aggregated_output: raw_output.aggregated_output,
+                duration,
+                timed_out,
+            };
+
+            if timed_out {
+                return Err(CodexErr::Sandbox(SandboxErr::Timeout {
+                    output: Box::new(exec_output.to_utf8_lossy_output()),
+                }));
+            }
+
+            let string_output = exec_output.to_utf8_lossy_output();
+            if is_likely_sandbox_denied(sandbox_type, &string_output) {
+                return Err(CodexErr::Sandbox(SandboxErr::Denied {
+                    output: Box::new(string_output),
+                    network_policy_decision: None,
+                }));
+            }
+
+            Ok(exec_output)
+        }
+        Err(err) => {
+            tracing::error!("exec error: {err}");
+            Err(err)
+        }
+    }
+}
+
 pub(crate) mod errors {
     use super::CodexErr;
     use crate::sandboxing::SandboxTransformError;
@@ -741,6 +851,16 @@ pub struct ExecToolCallOutput {
     pub timed_out: bool,
 }
 
+#[derive(Clone, Debug)]
+pub(crate) struct ExecToolCallOutputBytes {
+    pub exit_code: i32,
+    pub stdout: StreamOutput<Vec<u8>>,
+    pub stderr: StreamOutput<Vec<u8>>,
+    pub aggregated_output: StreamOutput<Vec<u8>>,
+    pub duration: Duration,
+    pub timed_out: bool,
+}
+
 impl Default for ExecToolCallOutput {
     fn default() -> Self {
         Self {
@@ -754,6 +874,19 @@ impl Default for ExecToolCallOutput {
     }
 }
 
+impl ExecToolCallOutputBytes {
+    fn to_utf8_lossy_output(&self) -> ExecToolCallOutput {
+        ExecToolCallOutput {
+            exit_code: self.exit_code,
+            stdout: self.stdout.from_utf8_lossy(),
+            stderr: self.stderr.from_utf8_lossy(),
+            aggregated_output: self.aggregated_output.from_utf8_lossy(),
+            duration: self.duration,
+            timed_out: self.timed_out,
+        }
+    }
+}
+
 #[cfg_attr(not(target_os = "windows"), allow(unused_variables))]
 async fn exec(
     params: ExecParams,

codex-rs/core/src/lib.rs

@@ -114,6 +114,7 @@ pub mod default_client;
 pub mod project_doc;
 mod rollout;
 pub(crate) mod safety;
+mod sandboxed_fs;
 pub mod seatbelt;
 pub mod shell;
 pub mod shell_snapshot;

codex-rs/core/src/sandboxed_fs.rs

@@ -0,0 +1,197 @@
+use crate::codex::Session;
+use crate::codex::TurnContext;
+use crate::error::CodexErr;
+use crate::error::SandboxErr;
+use crate::exec::ExecExpiration;
+use crate::exec::ExecToolCallOutputBytes;
+use crate::sandboxing::CommandSpec;
+use crate::sandboxing::SandboxPermissions;
+use crate::sandboxing::execute_env_output_bytes;
+use crate::sandboxing::merge_permission_profiles;
+use crate::tools::sandboxing::SandboxAttempt;
+use crate::tools::sandboxing::SandboxablePreference;
+use codex_fs_ops::CODEX_CORE_FS_OPS_ARG1;
+use codex_fs_ops::FsError;
+use codex_fs_ops::FsErrorKind;
+use codex_protocol::models::PermissionProfile;
+use std::collections::HashMap;
+use std::path::Path;
+use std::path::PathBuf;
+use std::sync::Arc;
+use std::time::Duration;
+
+const SANDBOXED_FS_TIMEOUT: Duration = Duration::from_secs(30);
+
+pub(crate) async fn read_file(
+    session: &Arc<Session>,
+    turn: &Arc<TurnContext>,
+    path: &Path,
+) -> Result<Vec<u8>, SandboxedFsError> {
+    let output = run_request(session, turn, path).await?;
+    Ok(output.stdout.text)
+}
+
+async fn run_request(
+    session: &Arc<Session>,
+    turn: &Arc<TurnContext>,
+    path: &Path,
+) -> Result<ExecToolCallOutputBytes, SandboxedFsError> {
+    let exe = std::env::current_exe().map_err(|error| SandboxedFsError::ResolveExe {
+        message: error.to_string(),
+    })?;
+    let additional_permissions = effective_granted_permissions(session).await;
+    let sandbox_manager = crate::sandboxing::SandboxManager::new();
+    let attempt = SandboxAttempt {
+        sandbox: sandbox_manager.select_initial(
+            &turn.file_system_sandbox_policy,
+            turn.network_sandbox_policy,
+            SandboxablePreference::Auto,
+            turn.windows_sandbox_level,
+            /*has_managed_network_requirements*/ false,
+        ),
+        policy: &turn.sandbox_policy,
+        file_system_policy: &turn.file_system_sandbox_policy,
+        network_policy: turn.network_sandbox_policy,
+        enforce_managed_network: false,
+        manager: &sandbox_manager,
+        sandbox_cwd: &turn.cwd,
+        codex_linux_sandbox_exe: turn.codex_linux_sandbox_exe.as_ref(),
+        use_legacy_landlock: turn.features.use_legacy_landlock(),
+        windows_sandbox_level: turn.windows_sandbox_level,
+        windows_sandbox_private_desktop: turn.config.permissions.windows_sandbox_private_desktop,
+    };
+    let exec_request = attempt
+        .env_for(
+            CommandSpec {
+                program: exe.to_string_lossy().to_string(),
+                args: vec![
+                    CODEX_CORE_FS_OPS_ARG1.to_string(),
+                    "read".to_string(),
+                    path.to_string_lossy().to_string(),
+                ],
+                cwd: turn.cwd.clone(),
+                env: HashMap::new(),
+                expiration: ExecExpiration::Timeout(SANDBOXED_FS_TIMEOUT),
+                sandbox_permissions: SandboxPermissions::UseDefault,
+                additional_permissions,
+                justification: None,
+            },
+            /*network*/ None,
+        )
+        .map_err(|error| SandboxedFsError::ProcessFailed {
+            path: path.to_path_buf(),
+            exit_code: -1,
+            message: error.to_string(),
+        })?;
+
+    let output = execute_env_output_bytes(exec_request, /*stdout_stream*/ None)
+        .await
+        .map_err(|error| map_exec_error(path, error))?;
+    if output.exit_code != 0 {
+        return Err(parse_helper_failure(
+            path,
+            output.exit_code,
+            &output.stderr.text,
+            &output.stdout.text,
+        ));
+    }
+
+    Ok(output)
+}
+
+async fn effective_granted_permissions(session: &Session) -> Option<PermissionProfile> {
+    let granted_session_permissions = session.granted_session_permissions().await;
+    let granted_turn_permissions = session.granted_turn_permissions().await;
+    merge_permission_profiles(
+        granted_session_permissions.as_ref(),
+        granted_turn_permissions.as_ref(),
+    )
+}
+
+fn map_exec_error(path: &Path, error: CodexErr) -> SandboxedFsError {
+    match error {
+        CodexErr::Sandbox(SandboxErr::Timeout { .. }) => SandboxedFsError::TimedOut {
+            path: path.to_path_buf(),
+        },
+        _ => SandboxedFsError::ProcessFailed {
+            path: path.to_path_buf(),
+            exit_code: -1,
+            message: error.to_string(),
+        },
+    }
+}
+
+fn parse_helper_failure(
+    path: &Path,
+    exit_code: i32,
+    stderr: &[u8],
+    stdout: &[u8],
+) -> SandboxedFsError {
+    if let Ok(error) = serde_json::from_slice::<FsError>(stderr) {
+        return SandboxedFsError::Operation {
+            path: path.to_path_buf(),
+            error,
+        };
+    }
+
+    let stderr = String::from_utf8_lossy(stderr);
+    let stdout = String::from_utf8_lossy(stdout);
+    let message = if !stderr.trim().is_empty() {
+        stderr.trim().to_string()
+    } else if !stdout.trim().is_empty() {
+        stdout.trim().to_string()
+    } else {
+        "no error details emitted".to_string()
+    };
+
+    SandboxedFsError::ProcessFailed {
+        path: path.to_path_buf(),
+        exit_code,
+        message,
+    }
+}
+
+#[derive(Debug, thiserror::Error)]
+pub(crate) enum SandboxedFsError {
+    #[error("failed to determine codex executable: {message}")]
+    ResolveExe { message: String },
+    #[error("sandboxed fs helper timed out while accessing `{path}`")]
+    TimedOut { path: PathBuf },
+    #[error("sandboxed fs helper exited with code {exit_code} while accessing `{path}`: {message}")]
+    ProcessFailed {
+        path: PathBuf,
+        exit_code: i32,
+        message: String,
+    },
+    #[error("sandboxed fs helper could not access `{path}`: {error}")]
+    Operation { path: PathBuf, error: FsError },
+}
+
+impl SandboxedFsError {
+    pub(crate) fn operation_error_kind(&self) -> Option<&FsErrorKind> {
+        match self {
+            Self::Operation { error, .. } => Some(&error.kind),
+            _ => None,
+        }
+    }
+
+    pub(crate) fn operation_error_message(&self) -> Option<&str> {
+        match self {
+            Self::Operation { error, .. } => Some(error.message.as_str()),
+            _ => None,
+        }
+    }
+
+    #[allow(dead_code)]
+    pub(crate) fn to_io_error(&self) -> std::io::Error {
+        match self {
+            Self::Operation { error, .. } => error.to_io_error(),
+            Self::TimedOut { .. } => {
+                std::io::Error::new(std::io::ErrorKind::TimedOut, self.to_string())
+            }
+            Self::ResolveExe { .. } | Self::ProcessFailed { .. } => {
+                std::io::Error::other(self.to_string())
+            }
+        }
+    }
+}

codex-rs/core/src/sandboxing/mod.rs

@@ -10,9 +10,11 @@ pub(crate) mod macos_permissions;
 
 use crate::exec::ExecExpiration;
 use crate::exec::ExecToolCallOutput;
+use crate::exec::ExecToolCallOutputBytes;
 use crate::exec::SandboxType;
 use crate::exec::StdoutStream;
 use crate::exec::execute_exec_request;
+use crate::exec::execute_exec_request_output_bytes;
 use crate::landlock::allow_network_for_proxy;
 use crate::landlock::create_linux_sandbox_command_args_for_policies;
 use crate::protocol::SandboxPolicy;
@@ -738,6 +740,20 @@ pub async fn execute_env(
     .await
 }
 
+pub(crate) async fn execute_env_output_bytes(
+    exec_request: ExecRequest,
+    stdout_stream: Option<StdoutStream>,
+) -> crate::error::Result<ExecToolCallOutputBytes> {
+    let effective_policy = exec_request.sandbox_policy.clone();
+    execute_exec_request_output_bytes(
+        exec_request,
+        &effective_policy,
+        stdout_stream,
+        /*after_spawn*/ None,
+    )
+    .await
+}
+
 pub async fn execute_exec_request_with_after_spawn(
     exec_request: ExecRequest,
     stdout_stream: Option<StdoutStream>,

codex-rs/fs-ops/BUILD.bazel

@@ -0,0 +1,6 @@
+load("//:defs.bzl", "codex_rust_crate")
+
+codex_rust_crate(
+    name = "fs-ops",
+    crate_name = "codex_fs_ops",
+)

codex-rs/fs-ops/Cargo.toml

@@ -0,0 +1,21 @@
+[package]
+name = "codex-fs-ops"
+version.workspace = true
+edition.workspace = true
+license.workspace = true
+
+[lib]
+name = "codex_fs_ops"
+path = "src/lib.rs"
+
+[lints]
+workspace = true
+
+[dependencies]
+anyhow = { workspace = true }
+serde = { workspace = true, features = ["derive"] }
+serde_json = { workspace = true }
+
+[dev-dependencies]
+pretty_assertions = { workspace = true }
+tempfile = { workspace = true }

codex-rs/fs-ops/src/command.rs

@@ -0,0 +1,38 @@
+use std::ffi::OsString;
+use std::path::PathBuf;
+
+#[derive(Debug, Clone, PartialEq, Eq)]
+pub enum FsCommand {
+    ReadFile { path: PathBuf },
+}
+
+pub fn parse_command_from_args(
+    mut args: impl Iterator<Item = OsString>,
+) -> Result<FsCommand, String> {
+    let Some(operation) = args.next() else {
+        return Err("missing operation".to_string());
+    };
+    let Some(operation) = operation.to_str() else {
+        return Err("operation must be valid UTF-8".to_string());
+    };
+    let Some(path) = args.next() else {
+        return Err(format!("missing path for operation `{operation}`"));
+    };
+    if args.next().is_some() {
+        return Err(format!(
+            "unexpected extra arguments for operation `{operation}`"
+        ));
+    }
+
+    let path = PathBuf::from(path);
+    match operation {
+        "read" => Ok(FsCommand::ReadFile { path }),
+        _ => Err(format!(
+            "unsupported filesystem operation `{operation}`; expected `read`"
+        )),
+    }
+}
+
+#[cfg(test)]
+#[path = "command_tests.rs"]
+mod tests;

codex-rs/fs-ops/src/command_tests.rs

@@ -0,0 +1,16 @@
+use super::FsCommand;
+use super::parse_command_from_args;
+use pretty_assertions::assert_eq;
+
+#[test]
+fn parse_read_command() {
+    let command = parse_command_from_args(["read", "/tmp/example.png"].into_iter().map(Into::into))
+        .expect("command should parse");
+
+    assert_eq!(
+        command,
+        FsCommand::ReadFile {
+            path: "/tmp/example.png".into(),
+        }
+    );
+}

codex-rs/fs-ops/src/constants.rs

@@ -0,0 +1,3 @@
+/// Special argv[1] flag used when the Codex executable self-invokes to run the
+/// internal sandbox-backed filesystem helper path.
+pub const CODEX_CORE_FS_OPS_ARG1: &str = "--codex-run-as-fs-ops";

codex-rs/fs-ops/src/error.rs

@@ -0,0 +1,70 @@
+use serde::Deserialize;
+use serde::Serialize;
+use std::io::ErrorKind;
+
+#[derive(Debug, Clone, PartialEq, Eq, Serialize, Deserialize)]
+#[serde(rename_all = "snake_case")]
+pub enum FsErrorKind {
+    NotFound,
+    PermissionDenied,
+    IsADirectory,
+    InvalidData,
+    Other,
+}
+
+impl From<ErrorKind> for FsErrorKind {
+    fn from(value: ErrorKind) -> Self {
+        match value {
+            ErrorKind::NotFound => Self::NotFound,
+            ErrorKind::PermissionDenied => Self::PermissionDenied,
+            ErrorKind::IsADirectory => Self::IsADirectory,
+            ErrorKind::InvalidData => Self::InvalidData,
+            _ => Self::Other,
+        }
+    }
+}
+
+impl FsErrorKind {
+    pub fn to_io_error_kind(&self) -> ErrorKind {
+        match self {
+            Self::NotFound => ErrorKind::NotFound,
+            Self::PermissionDenied => ErrorKind::PermissionDenied,
+            Self::IsADirectory => ErrorKind::IsADirectory,
+            Self::InvalidData => ErrorKind::InvalidData,
+            Self::Other => ErrorKind::Other,
+        }
+    }
+}
+
+#[derive(Debug, Clone, PartialEq, Eq, Serialize, Deserialize)]
+pub struct FsError {
+    pub kind: FsErrorKind,
+    pub message: String,
+    pub raw_os_error: Option<i32>,
+}
+
+impl std::fmt::Display for FsError {
+    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
+        f.write_str(&self.message)
+    }
+}
+
+impl FsError {
+    pub fn to_io_error(&self) -> std::io::Error {
+        if let Some(raw_os_error) = self.raw_os_error {
+            std::io::Error::from_raw_os_error(raw_os_error)
+        } else {
+            std::io::Error::new(self.kind.to_io_error_kind(), self.message.clone())
+        }
+    }
+}
+
+impl From<std::io::Error> for FsError {
+    fn from(error: std::io::Error) -> Self {
+        Self {
+            kind: error.kind().into(),
+            message: error.to_string(),
+            raw_os_error: error.raw_os_error(),
+        }
+    }
+}

codex-rs/fs-ops/src/lib.rs

@@ -0,0 +1,13 @@
+mod command;
+mod constants;
+mod error;
+mod runner;
+
+pub use command::FsCommand;
+pub use command::parse_command_from_args;
+pub use constants::CODEX_CORE_FS_OPS_ARG1;
+pub use error::FsError;
+pub use error::FsErrorKind;
+pub use runner::execute;
+pub use runner::run_from_args;
+pub use runner::write_error;

codex-rs/fs-ops/src/runner.rs

@@ -0,0 +1,55 @@
+use crate::FsCommand;
+use crate::FsError;
+use crate::parse_command_from_args;
+use anyhow::Context;
+use anyhow::Result;
+use std::ffi::OsString;
+use std::io::Read;
+use std::io::Write;
+
+pub fn run_from_args(
+    args: impl Iterator<Item = OsString>,
+    stdin: &mut impl Read,
+    stdout: &mut impl Write,
+    stderr: &mut impl Write,
+) -> Result<()> {
+    let command = match parse_command_from_args(args) {
+        Ok(command) => command,
+        Err(error) => {
+            writeln!(stderr, "{error}").context("failed to write fs helper usage error")?;
+            anyhow::bail!("{error}");
+        }
+    };
+
+    if let Err(error) = execute(command, stdin, stdout) {
+        write_error(stderr, &error)?;
+        anyhow::bail!("{error}");
+    }
+
+    Ok(())
+}
+
+pub fn execute(
+    command: FsCommand,
+    _stdin: &mut impl Read,
+    stdout: &mut impl Write,
+) -> Result<(), FsError> {
+    match command {
+        FsCommand::ReadFile { path } => {
+            let mut file = std::fs::File::open(path).map_err(FsError::from)?;
+            std::io::copy(&mut file, stdout)
+                .map(|_| ())
+                .map_err(FsError::from)
+        }
+    }
+}
+
+pub fn write_error(stderr: &mut impl Write, error: &FsError) -> Result<()> {
+    serde_json::to_writer(&mut *stderr, error).context("failed to serialize fs error")?;
+    writeln!(stderr).context("failed to terminate fs error with newline")?;
+    Ok(())
+}
+
+#[cfg(test)]
+#[path = "runner_tests.rs"]
+mod tests;

codex-rs/fs-ops/src/runner_tests.rs

@@ -0,0 +1,76 @@
+use super::execute;
+use crate::FsCommand;
+use crate::FsErrorKind;
+use crate::run_from_args;
+use pretty_assertions::assert_eq;
+use tempfile::tempdir;
+
+#[test]
+fn run_from_args_streams_file_bytes_to_stdout() {
+    let tempdir = tempdir().expect("tempdir");
+    let path = tempdir.path().join("image.bin");
+    let expected = b"hello\x00world".to_vec();
+    std::fs::write(&path, &expected).expect("write test file");
+
+    let mut stdout = Vec::new();
+    let mut stderr = Vec::new();
+    let mut stdin = std::io::empty();
+    run_from_args(
+        ["read", path.to_str().expect("utf-8 test path")]
+            .into_iter()
+            .map(Into::into),
+        &mut stdin,
+        &mut stdout,
+        &mut stderr,
+    )
+    .expect("read should succeed");
+
+    assert_eq!(stdout, expected);
+    assert_eq!(stderr, Vec::<u8>::new());
+}
+
+#[test]
+fn read_reports_directory_error() {
+    let tempdir = tempdir().expect("tempdir");
+    let mut stdout = Vec::new();
+    let mut stdin = std::io::empty();
+
+    let error = execute(
+        FsCommand::ReadFile {
+            path: tempdir.path().to_path_buf(),
+        },
+        &mut stdin,
+        &mut stdout,
+    )
+    .expect_err("reading a directory should fail");
+
+    #[cfg(target_os = "windows")]
+    assert_eq!(error.kind, FsErrorKind::PermissionDenied);
+    #[cfg(not(target_os = "windows"))]
+    assert_eq!(error.kind, FsErrorKind::IsADirectory);
+}
+
+#[test]
+fn run_from_args_serializes_errors_to_stderr() {
+    let tempdir = tempdir().expect("tempdir");
+    let missing = tempdir.path().join("missing.txt");
+    let mut stdout = Vec::new();
+    let mut stderr = Vec::new();
+    let mut stdin = std::io::empty();
+
+    let result = run_from_args(
+        ["read", missing.to_str().expect("utf-8 test path")]
+            .into_iter()
+            .map(Into::into),
+        &mut stdin,
+        &mut stdout,
+        &mut stderr,
+    );
+
+    assert!(result.is_err(), "missing file should fail");
+    assert_eq!(stdout, Vec::<u8>::new());
+
+    let error: crate::FsError = serde_json::from_slice(&stderr).expect("structured fs error");
+    assert_eq!(error.kind, FsErrorKind::NotFound);
+    assert!(error.raw_os_error.is_some());
+}