diff --git a/codex-rs/core/tests/suite/mod.rs b/codex-rs/core/tests/suite/mod.rs
index 5b131e0493..3b67b335d1 100644
--- a/codex-rs/core/tests/suite/mod.rs
+++ b/codex-rs/core/tests/suite/mod.rs
@@ -13,6 +13,7 @@ mod json_result;
 mod live_cli;
 mod model_overrides;
 mod otel;
+mod parallel_read_only;
 mod prompt_caching;
 mod read_file;
 mod review;
diff --git a/codex-rs/core/tests/suite/parallel_read_only.rs b/codex-rs/core/tests/suite/parallel_read_only.rs
new file mode 100644
index 0000000000..c2e22d1719
--- /dev/null
+++ b/codex-rs/core/tests/suite/parallel_read_only.rs
@@ -0,0 +1,305 @@
+#![cfg(not(target_os = "windows"))]
+
+use std::ffi::CString;
+use std::os::unix::ffi::OsStrExt;
+use std::path::Path;
+use std::path::PathBuf;
+use std::sync::Arc;
+use std::time::Duration;
+use std::time::Instant;
+
+use anyhow::Context;
+use codex_core::protocol::AskForApproval;
+use codex_core::protocol::EventMsg;
+use codex_core::protocol::InputItem;
+use codex_core::protocol::Op;
+use codex_core::protocol::SandboxPolicy;
+use codex_protocol::config_types::ReasoningSummary;
+use core_test_support::responses;
+use core_test_support::responses::ev_apply_patch_function_call;
+use core_test_support::responses::ev_assistant_message;
+use core_test_support::responses::ev_completed;
+use core_test_support::responses::ev_function_call;
+use core_test_support::responses::sse;
+use core_test_support::responses::start_mock_server;
+use core_test_support::skip_if_no_network;
+use core_test_support::test_codex::TestCodex;
+use core_test_support::test_codex::test_codex;
+use tokio::fs::OpenOptions;
+use tokio::io::AsyncWriteExt;
+use tokio::sync::Mutex;
+use tokio::sync::oneshot;
+use wiremock::matchers::any;
+
+#[tokio::test(flavor = "multi_thread", worker_threads = 2)]
+async fn read_only_tools_execute_before_apply_patch() -> anyhow::Result<()> {
+    // Bail out early if the sandbox does not allow network traffic, because the
+    // mocked Codex server still communicates over HTTP.
+    skip_if_no_network!(Ok(()));
+
+    // Stand up a mock Codex backend that will stream tool calls and responses.
+    let server = start_mock_server().await;
+
+    let mut builder = test_codex().with_config(|config| {
+        config.enable_parallel_read_only_tools = true;
+        config.include_apply_patch_tool = true;
+    });
+    let TestCodex {
+        codex,
+        cwd,
+        session_configured,
+        ..
+    } = builder.build(&server).await?;
+
+    // Create two FIFOs that the mocked read-only tools will try to read from in
+    // order to simulate long running, blocking I/O.
+    let fifo_one = cwd.path().join("parallel_fifo_one");
+    let fifo_two = cwd.path().join("parallel_fifo_two");
+    create_fifo(&fifo_one)?;
+    create_fifo(&fifo_two)?;
+
+    let read_call_one = "read-file-1";
+    let read_call_two = "read-file-2";
+    let patch_call = "apply-patch";
+
+    let read_args_one = serde_json::json!({
+        "file_path": fifo_one.to_string_lossy(),
+        "offset": 1,
+        "limit": 1,
+    })
+    .to_string();
+    let read_args_two = serde_json::json!({
+        "file_path": fifo_two.to_string_lossy(),
+        "offset": 1,
+        "limit": 1,
+    })
+    .to_string();
+
+    let patch_path = "parallel_patch_output.txt";
+    let patch_content = format!(
+        "*** Begin Patch\n*** Add File: {patch_path}\n+parallel apply_patch executed\n*** End Patch"
+    );
+
+    // Queue the first SSE response that drives the session: fire two read-only
+    // tool calls, then schedule the apply-patch call.
+    let first_response = sse(vec![
+        serde_json::json!({
+            "type": "response.created",
+            "response": {"id": "resp-parallel"}
+        }),
+        ev_function_call(read_call_one, "read_file", &read_args_one),
+        ev_function_call(read_call_two, "read_file", &read_args_two),
+        ev_apply_patch_function_call(patch_call, &patch_content),
+        ev_completed("resp-parallel"),
+    ]);
+    responses::mount_sse_once_match(&server, any(), first_response).await;
+
+    // Queue a follow-up response so the session can complete once all tools run.
+    let second_response = sse(vec![
+        ev_assistant_message("msg-1", "all done"),
+        ev_completed("resp-final"),
+    ]);
+    responses::mount_sse_once_match(&server, any(), second_response).await;
+
+    // Start timers that resolve when each FIFO gets a writer, helping us measure
+    // when the corresponding read-only tool begins execution.
+    let start = Instant::now();
+    let wait_one = tokio::spawn(wait_for_writer(fifo_one.clone(), start));
+    let wait_two = tokio::spawn(wait_for_writer(fifo_two.clone(), start));
+
+    // Capture all Codex events so we can verify tool ordering once execution finishes.
+    let events = Arc::new(Mutex::new(Vec::new()));
+    let (done_tx, done_rx) = oneshot::channel();
+    let events_task = events.clone();
+    let codex_for_events = codex.clone();
+    tokio::spawn(async move {
+        loop {
+            let event = codex_for_events.next_event().await.expect("event");
+            let msg = event.msg;
+            let is_done = matches!(msg, EventMsg::TaskComplete(_));
+            {
+                let mut log = events_task.lock().await;
+                log.push(msg);
+            }
+            if is_done {
+                break;
+            }
+        }
+        let _ = done_tx.send(());
+    });
+
+    let session_model = session_configured.model.clone();
+
+    // Trigger the user turn that causes Codex to invoke the two read-only tools
+    // and subsequently the apply-patch tool.
+    codex
+        .submit(Op::UserTurn {
+            items: vec![InputItem::Text {
+                text: "please process the tools in parallel".into(),
+            }],
+            final_output_json_schema: None,
+            cwd: cwd.path().to_path_buf(),
+            approval_policy: AskForApproval::Never,
+            sandbox_policy: SandboxPolicy::DangerFullAccess,
+            model: session_model,
+            effort: None,
+            summary: ReasoningSummary::Auto,
+        })
+        .await?;
+
+    let wait_timeout = Duration::from_secs(5);
+
+    let (mut writer_one, elapsed_one) = tokio::time::timeout(wait_timeout, async {
+        wait_one.await.expect("wait fifo one task panicked")
+    })
+    .await
+    .expect("timeout waiting for first read-only tool")?;
+
+    let (mut writer_two, elapsed_two) = tokio::time::timeout(wait_timeout, async {
+        wait_two.await.expect("wait fifo two task panicked")
+    })
+    .await
+    .expect("timeout waiting for second read-only tool")?;
+
+    // Ensure the two read-only tools started within 200ms of each other so that
+    // they can be considered parallel.
+    let delta = if elapsed_one > elapsed_two {
+        elapsed_one - elapsed_two
+    } else {
+        elapsed_two - elapsed_one
+    };
+    assert!(
+        delta < Duration::from_millis(200),
+        "expected read-only tools to start in parallel (delta {delta:?})"
+    );
+
+    writer_one.write_all(b"fifo one line\n").await?;
+    writer_one.shutdown().await?;
+    drop(writer_one);
+
+    tokio::time::sleep(Duration::from_millis(100)).await;
+
+    {
+        // Confirm that apply_patch has not started while the second read-only
+        // tool is still blocked waiting for input.
+        let log = events.lock().await;
+        assert!(
+            !log.iter()
+                .any(|msg| matches!(msg, EventMsg::PatchApplyBegin(_))),
+            "apply_patch began before the second read-only tool completed"
+        );
+    }
+
+    writer_two.write_all(b"fifo two line\n").await?;
+    writer_two.shutdown().await?;
+    drop(writer_two);
+
+    // Wait for the event collector to observe task completion so we can inspect
+    // the final event log.
+    done_rx.await.expect("event collector finished");
+
+    let events_log = events.lock().await;
+    let patch_begin_index = events_log
+        .iter()
+        .position(|msg| match msg {
+            EventMsg::PatchApplyBegin(begin) => begin.call_id == patch_call,
+            _ => false,
+        })
+        .expect("expected PatchApplyBegin event");
+    let patch_end_index = events_log
+        .iter()
+        .position(|msg| match msg {
+            EventMsg::PatchApplyEnd(end) => end.call_id == patch_call,
+            _ => false,
+        })
+        .expect("expected PatchApplyEnd event");
+    assert!(
+        patch_begin_index < patch_end_index,
+        "PatchApplyEnd occurred before PatchApplyBegin"
+    );
+
+    // Record whether apply_patch succeeded so the assertions below can verify
+    // either the patched file or the reported stderr output.
+    let patch_end_success = events_log.iter().find_map(|msg| match msg {
+        EventMsg::PatchApplyEnd(end) if end.call_id == patch_call => {
+            Some((end.success, end.stderr.clone()))
+        }
+        _ => None,
+    });
+    let (patch_success, patch_stderr) = patch_end_success.expect("expected PatchApplyEnd details");
+    drop(events_log);
+
+    if patch_success {
+        let patched_file = cwd.path().join(patch_path);
+        let patched_contents = std::fs::read_to_string(&patched_file)?;
+        assert!(
+            patched_contents.contains("parallel apply_patch executed"),
+            "unexpected patch contents: {patched_contents:?}"
+        );
+    } else {
+        assert!(
+            patch_stderr.contains("codex-run-as-apply-patch"),
+            "unexpected apply_patch stderr: {patch_stderr:?}"
+        );
+    }
+
+    // Check that the mock server observed outputs from every tool invocation.
+    let requests = server.received_requests().await.expect("recorded requests");
+    assert!(
+        !requests.is_empty(),
+        "expected at least one request recorded"
+    );
+
+    let mut seen_outputs = std::collections::HashSet::new();
+    for request in requests {
+        let body = request
+            .body_json::<serde_json::Value>()
+            .expect("request json");
+        if let Some(items) = body.get("input").and_then(|v| v.as_array()) {
+            for item in items {
+                if item.get("type").and_then(|v| v.as_str()) == Some("function_call_output") {
+                    if let Some(call_id) = item.get("call_id").and_then(|v| v.as_str()) {
+                        seen_outputs.insert(call_id.to_string());
+                    }
+                }
+            }
+        }
+    }
+
+    assert!(
+        seen_outputs.contains(read_call_one),
+        "missing read-only tool output for {read_call_one}"
+    );
+    assert!(
+        seen_outputs.contains(read_call_two),
+        "missing read-only tool output for {read_call_two}"
+    );
+    assert!(
+        seen_outputs.contains(patch_call),
+        "missing apply_patch tool output"
+    );
+
+    Ok(())
+}
+
+fn create_fifo(path: &Path) -> anyhow::Result<()> {
+    let c_path =
+        CString::new(path.as_os_str().as_bytes()).context("fifo path contained null byte")?;
+    let res = unsafe { libc::mkfifo(c_path.as_ptr(), 0o600) };
+    if res != 0 {
+        return Err(std::io::Error::last_os_error().into());
+    }
+    Ok(())
+}
+
+async fn wait_for_writer(
+    path: PathBuf,
+    origin: Instant,
+) -> anyhow::Result<(tokio::fs::File, Duration)> {
+    let file = OpenOptions::new()
+        .write(true)
+        .open(&path)
+        .await
+        .with_context(|| format!("open fifo {:?} for writing", path))?;
+    Ok((file, origin.elapsed()))
+}