diff --git a/codex-rs/Cargo.lock b/codex-rs/Cargo.lock
index f3903d6e..653c3e4e 100644
--- a/codex-rs/Cargo.lock
+++ b/codex-rs/Cargo.lock
@@ -610,6 +610,7 @@ name = "codex-arg0"
 version = "0.0.0"
 dependencies = [
  "anyhow",
+ "codex-apply-patch",
  "codex-core",
  "codex-linux-sandbox",
  "dotenvy",
@@ -718,14 +719,17 @@ name = "codex-exec"
 version = "0.0.0"
 dependencies = [
  "anyhow",
+ "assert_cmd",
  "chrono",
  "clap",
  "codex-arg0",
  "codex-common",
  "codex-core",
  "owo-colors",
+ "predicates",
  "serde_json",
  "shlex",
+ "tempfile",
  "tokio",
  "tracing",
  "tracing-subscriber",
diff --git a/codex-rs/arg0/Cargo.toml b/codex-rs/arg0/Cargo.toml
index 9ad18967..7c55ac0d 100644
--- a/codex-rs/arg0/Cargo.toml
+++ b/codex-rs/arg0/Cargo.toml
@@ -12,6 +12,7 @@ workspace = true
 
 [dependencies]
 anyhow = "1"
+codex-apply-patch = { path = "../apply-patch" }
 codex-core = { path = "../core" }
 codex-linux-sandbox = { path = "../linux-sandbox" }
 dotenvy = "0.15.7"
diff --git a/codex-rs/arg0/src/lib.rs b/codex-rs/arg0/src/lib.rs
index 624583b8..d7109176 100644
--- a/codex-rs/arg0/src/lib.rs
+++ b/codex-rs/arg0/src/lib.rs
@@ -30,7 +30,8 @@ where
     Fut: Future<Output = anyhow::Result<()>>,
 {
     // Determine if we were invoked via the special alias.
-    let argv0 = std::env::args_os().next().unwrap_or_default();
+    let mut args = std::env::args_os();
+    let argv0 = args.next().unwrap_or_default();
     let exe_name = Path::new(&argv0)
         .file_name()
         .and_then(|s| s.to_str())
@@ -41,6 +42,26 @@ where
         codex_linux_sandbox::run_main();
     }
 
+    let argv1 = args.next().unwrap_or_default();
+    if argv1 == "--codex-run-as-apply-patch" {
+        let patch_arg = args.next().and_then(|s| s.to_str().map(|s| s.to_owned()));
+        let exit_code = match patch_arg {
+            Some(patch_arg) => {
+                let mut stdout = std::io::stdout();
+                let mut stderr = std::io::stderr();
+                match codex_apply_patch::apply_patch(&patch_arg, &mut stdout, &mut stderr) {
+                    Ok(()) => 0,
+                    Err(_) => 1,
+                }
+            }
+            None => {
+                eprintln!("Error: --codex-run-as-apply-patch requires a UTF-8 PATCH argument.");
+                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.
     load_dotenv();
diff --git a/codex-rs/core/README.md b/codex-rs/core/README.md
index 9b3e59c8..9a4c255a 100644
--- a/codex-rs/core/README.md
+++ b/codex-rs/core/README.md
@@ -2,9 +2,18 @@
 
 This crate implements the business logic for Codex. It is designed to be used by the various Codex UIs written in Rust.
 
-Though for non-Rust UIs, we are also working to define a _protocol_ for talking to Codex. See:
+## Dependencies
 
-- [Specification](../docs/protocol_v1.md)
-- [Rust types](./src/protocol.rs)
+Note that `codex-core` makes some assumptions about certain helper utilities being available in the environment. Currently, this
 
-You can use the `proto` subcommand using the executable in the [`cli` crate](../cli) to speak the protocol using newline-delimited-JSON over stdin/stdout.
+### macOS
+
+Expects `/usr/bin/sandbox-exec` to be present.
+
+### Linux
+
+Expects the binary containing `codex-core` to run the equivalent of `codex debug landlock` when `arg0` is `codex-linux-sandbox`. See the `codex-arg0` crate for details.
+
+### All Platforms
+
+Expects the binary containing `codex-core` to simulate the virtual `apply_patch` CLI when `arg1` is `--codex-run-as-apply-patch`. See the `codex-arg0` crate for details.
diff --git a/codex-rs/exec/Cargo.toml b/codex-rs/exec/Cargo.toml
index c9d94deb..ced771f2 100644
--- a/codex-rs/exec/Cargo.toml
+++ b/codex-rs/exec/Cargo.toml
@@ -37,3 +37,8 @@ tokio = { version = "1", features = [
 ] }
 tracing = { version = "0.1.41", features = ["log"] }
 tracing-subscriber = { version = "0.3.19", features = ["env-filter"] }
+
+[dev-dependencies]
+assert_cmd = "2"
+predicates = "3"
+tempfile = "3.13.0"
diff --git a/codex-rs/exec/tests/apply_patch.rs b/codex-rs/exec/tests/apply_patch.rs
new file mode 100644
index 00000000..69ac1b8c
--- /dev/null
+++ b/codex-rs/exec/tests/apply_patch.rs
@@ -0,0 +1,38 @@
+use anyhow::Context;
+use assert_cmd::prelude::*;
+use std::fs;
+use std::process::Command;
+use tempfile::tempdir;
+
+/// While we may add an `apply-patch` subcommand to the `codex` CLI multitool
+/// at some point, we must ensure that the smaller `codex-exec` CLI can still
+/// emulate the `apply_patch` CLI.
+#[test]
+fn test_standalone_exec_cli_can_use_apply_patch() -> anyhow::Result<()> {
+    let tmp = tempdir()?;
+    let relative_path = "source.txt";
+    let absolute_path = tmp.path().join(relative_path);
+    fs::write(&absolute_path, "original content\n")?;
+
+    Command::cargo_bin("codex-exec")
+        .context("should find binary for codex-exec")?
+        .arg("--codex-run-as-apply-patch")
+        .arg(
+            r#"*** Begin Patch
+*** Update File: source.txt
+@@
+-original content
++modified by apply_patch
+*** End Patch"#,
+        )
+        .current_dir(tmp.path())
+        .assert()
+        .success()
+        .stdout("Success. Updated the following files:\nM source.txt\n")
+        .stderr(predicates::str::is_empty());
+    assert_eq!(
+        fs::read_to_string(absolute_path)?,
+        "modified by apply_patch\n"
+    );
+    Ok(())
+}