chore: add codex debug app-server tooling (#10367)

codex debug app-server <user message> forwards the message through
codex-app-server-test-client’s send_message_v2 library entry point,
using std::env::current_exe() to resolve the codex binary.

for how it looks like, see:

```
celia@com-92114 codex-rs % cargo build -p codex-cli && target/debug/codex debug app-server --help                       
    Finished `dev` profile [unoptimized + debuginfo] target(s) in 0.34s
Tooling: helps debug the app server

Usage: codex debug app-server [OPTIONS] <COMMAND>

Commands:
  send-message-v2  
  help             Print this message or the help of the given subcommand(s)
````
and
```
celia@com-92114 codex-rs % cargo build -p codex-cli && target/debug/codex debug app-server send-message-v2 "hello world"
   Compiling codex-cli v0.0.0 (/Users/celia/code/codex/codex-rs/cli)
    Finished `dev` profile [unoptimized + debuginfo] target(s) in 1.38s
> {
>   "method": "initialize",
>   "id": "f8ba9f60-3a49-4ea9-81d6-4ab6853e3954",
>   "params": {
>     "clientInfo": {
>       "name": "codex-toy-app-server",
>       "title": "Codex Toy App Server",
>       "version": "0.0.0"
>     },
>     "capabilities": {
>       "experimentalApi": true
>     }
>   }
> }
< {
<   "id": "f8ba9f60-3a49-4ea9-81d6-4ab6853e3954",
<   "result": {
<     "userAgent": "codex-toy-app-server/0.0.0 (Mac OS 26.2.0; arm64) vscode/2.4.27 (codex-toy-app-server; 0.0.0)"
<   }
< }
< initialize response: InitializeResponse { user_agent: "codex-toy-app-server/0.0.0 (Mac OS 26.2.0; arm64) vscode/2.4.27 (codex-toy-app-server; 0.0.0)" }
> {
>   "method": "thread/start",
>   "id": "203f1630-beee-4e60-b17b-9eff16b1638b",
>   "params": {
>     "model": null,
>     "modelProvider": null,
>     "cwd": null,
>     "approvalPolicy": null,
>     "sandbox": null,
>     "config": null,
>     "baseInstructions": null,
>     "developerInstructions": null,
>     "personality": null,
>     "ephemeral": null,
>     "dynamicTools": null,
>     "mockExperimentalField": null,
>     "experimentalRawEvents": false
>   }
> }
...
```

codex-rs/Cargo.lock

@@ -1379,6 +1379,7 @@ dependencies = [
  "clap_complete",
  "codex-app-server",
  "codex-app-server-protocol",
+ "codex-app-server-test-client",
  "codex-arg0",
  "codex-chatgpt",
  "codex-cloud-tasks",

codex-rs/Cargo.toml

@@ -70,6 +70,7 @@ codex-ansi-escape = { path = "ansi-escape" }
 codex-api = { path = "codex-api" }
 codex-app-server = { path = "app-server" }
 codex-app-server-protocol = { path = "app-server-protocol" }
+codex-app-server-test-client = { path = "app-server-test-client" }
 codex-apply-patch = { path = "apply-patch" }
 codex-arg0 = { path = "arg0" }
 codex-async-utils = { path = "async-utils" }

codex-rs/app-server-test-client/BUILD.bazel

@@ -1,6 +1,6 @@
 load("//:defs.bzl", "codex_rust_crate")
 
 codex_rust_crate(
-    name = "codex-app-server-test-client",
+    name = "app-server-test-client",
     crate_name = "codex_app_server_test_client",
 )

codex-rs/cli/Cargo.toml

@@ -21,6 +21,7 @@ clap = { workspace = true, features = ["derive"] }
 clap_complete = { workspace = true }
 codex-app-server = { workspace = true }
 codex-app-server-protocol = { workspace = true }
+codex-app-server-test-client = { workspace = true }
 codex-arg0 = { workspace = true }
 codex-chatgpt = { workspace = true }
 codex-cloud-tasks = { path = "../cloud-tasks" }

codex-rs/cli/src/main.rs

@@ -110,9 +110,11 @@ enum Subcommand {
     Completion(CompletionCommand),
 
     /// Run commands within a Codex-provided sandbox.
-    #[clap(visible_alias = "debug")]
     Sandbox(SandboxArgs),
 
+    /// Debugging tools.
+    Debug(DebugCommand),
+
     /// Execpolicy tooling.
     #[clap(hide = true)]
     Execpolicy(ExecpolicyCommand),
@@ -150,6 +152,36 @@ struct CompletionCommand {
     shell: Shell,
 }
 
+#[derive(Debug, Parser)]
+struct DebugCommand {
+    #[command(subcommand)]
+    subcommand: DebugSubcommand,
+}
+
+#[derive(Debug, clap::Subcommand)]
+enum DebugSubcommand {
+    /// Tooling: helps debug the app server.
+    AppServer(DebugAppServerCommand),
+}
+
+#[derive(Debug, Parser)]
+struct DebugAppServerCommand {
+    #[command(subcommand)]
+    subcommand: DebugAppServerSubcommand,
+}
+
+#[derive(Debug, clap::Subcommand)]
+enum DebugAppServerSubcommand {
+    // Send message to app server V2.
+    SendMessageV2(DebugAppServerSendMessageV2Command),
+}
+
+#[derive(Debug, Parser)]
+struct DebugAppServerSendMessageV2Command {
+    #[arg(value_name = "USER_MESSAGE", required = true)]
+    user_message: String,
+}
+
 #[derive(Debug, Parser)]
 struct ResumeCommand {
     /// Conversation/session id (UUID) or thread name. UUIDs take precedence if it parses.
@@ -425,6 +457,15 @@ fn run_execpolicycheck(cmd: ExecPolicyCheckCommand) -> anyhow::Result<()> {
     cmd.run()
 }
 
+fn run_debug_app_server_command(cmd: DebugAppServerCommand) -> anyhow::Result<()> {
+    match cmd.subcommand {
+        DebugAppServerSubcommand::SendMessageV2(cmd) => {
+            let codex_bin = std::env::current_exe()?;
+            codex_app_server_test_client::send_message_v2(&codex_bin, &[], cmd.user_message, &None)
+        }
+    }
+}
+
 #[derive(Debug, Default, Parser, Clone)]
 struct FeatureToggles {
     /// Enable a feature (repeatable). Equivalent to `-c features.<name>=true`.
@@ -693,6 +734,11 @@ async fn cli_main(codex_linux_sandbox_exe: Option<PathBuf>) -> anyhow::Result<()
                 .await?;
             }
         },
+        Some(Subcommand::Debug(DebugCommand { subcommand })) => match subcommand {
+            DebugSubcommand::AppServer(cmd) => {
+                run_debug_app_server_command(cmd)?;
+            }
+        },
         Some(Subcommand::Execpolicy(ExecpolicyCommand { sub })) => match sub {
             ExecpolicySubcommand::Check(cmd) => run_execpolicycheck(cmd)?,
         },