enable-resume (#3537)

Adding the ability to resume conversations.
we have one verb `resume`. 

Behavior:

`tui`:
`codex resume`: opens session picker
`codex resume --last`: continue last message
`codex resume <session id>`: continue conversation with `session id`

`exec`:
`codex resume --last`: continue last conversation
`codex resume <session id>`: continue conversation with `session id`

Implementation:
- I added a function to find the path in `~/.codex/sessions/` with a
`UUID`. This is helpful in resuming with session id.
- Added the above mentioned flags
- Added lots of testing

codex-rs/exec/src/cli.rs

@@ -6,6 +6,10 @@ use std::path::PathBuf;
 #[derive(Parser, Debug)]
 #[command(version)]
 pub struct Cli {
+    /// Action to perform. If omitted, runs a new non-interactive session.
+    #[command(subcommand)]
+    pub command: Option<Command>,
+
     /// Optional image(s) to attach to the initial prompt.
     #[arg(long = "image", short = 'i', value_name = "FILE", value_delimiter = ',', num_args = 1..)]
     pub images: Vec<PathBuf>,
@@ -69,6 +73,28 @@ pub struct Cli {
     pub prompt: Option<String>,
 }
 
+#[derive(Debug, clap::Subcommand)]
+pub enum Command {
+    /// Resume a previous session by id or pick the most recent with --last.
+    Resume(ResumeArgs),
+}
+
+#[derive(Parser, Debug)]
+pub struct ResumeArgs {
+    /// Conversation/session id (UUID). When provided, resumes this session.
+    /// If omitted, use --last to pick the most recent recorded session.
+    #[arg(value_name = "SESSION_ID")]
+    pub session_id: Option<String>,
+
+    /// Resume the most recent recorded session (newest) without specifying an id.
+    #[arg(long = "last", default_value_t = false, conflicts_with = "session_id")]
+    pub last: bool,
+
+    /// Prompt to send after resuming the session. If `-` is used, read from stdin.
+    #[arg(value_name = "PROMPT")]
+    pub prompt: Option<String>,
+}
+
 #[derive(Debug, Clone, Copy, Default, PartialEq, Eq, ValueEnum)]
 #[value(rename_all = "kebab-case")]
 pub enum Color {

codex-rs/exec/src/lib.rs

@@ -30,11 +30,14 @@ use tracing::error;
 use tracing::info;
 use tracing_subscriber::EnvFilter;
 
+use crate::cli::Command as ExecCommand;
 use crate::event_processor::CodexStatus;
 use crate::event_processor::EventProcessor;
+use codex_core::find_conversation_path_by_id_str;
 
 pub async fn run_main(cli: Cli, codex_linux_sandbox_exe: Option<PathBuf>) -> anyhow::Result<()> {
     let Cli {
+        command,
         images,
         model: model_cli_arg,
         oss,
@@ -51,8 +54,15 @@ pub async fn run_main(cli: Cli, codex_linux_sandbox_exe: Option<PathBuf>) -> any
         config_overrides,
     } = cli;
 
-    // Determine the prompt based on CLI arg and/or stdin.
-    let prompt = match prompt {
+    // Determine the prompt source (parent or subcommand) and read from stdin if needed.
+    let prompt_arg = match &command {
+        // Allow prompt before the subcommand by falling back to the parent-level prompt
+        // when the Resume subcommand did not provide its own prompt.
+        Some(ExecCommand::Resume(args)) => args.prompt.clone().or(prompt),
+        None => prompt,
+    };
+
+    let prompt = match prompt_arg {
         Some(p) if p != "-" => p,
         // Either `-` was passed or no positional arg.
         maybe_dash => {
@@ -190,11 +200,29 @@ pub async fn run_main(cli: Cli, codex_linux_sandbox_exe: Option<PathBuf>) -> any
 
     let conversation_manager =
         ConversationManager::new(AuthManager::shared(config.codex_home.clone()));
+
+    // Handle resume subcommand by resolving a rollout path and using explicit resume API.
     let NewConversation {
         conversation_id: _,
         conversation,
         session_configured,
-    } = conversation_manager.new_conversation(config).await?;
+    } = if let Some(ExecCommand::Resume(args)) = command {
+        let resume_path = resolve_resume_path(&config, &args).await?;
+
+        if let Some(path) = resume_path {
+            conversation_manager
+                .resume_conversation_from_rollout(
+                    config.clone(),
+                    path,
+                    AuthManager::shared(config.codex_home.clone()),
+                )
+                .await?
+        } else {
+            conversation_manager.new_conversation(config).await?
+        }
+    } else {
+        conversation_manager.new_conversation(config).await?
+    };
     info!("Codex initialized with event: {session_configured:?}");
 
     let (tx, mut rx) = tokio::sync::mpsc::unbounded_channel::<Event>();
@@ -279,3 +307,23 @@ pub async fn run_main(cli: Cli, codex_linux_sandbox_exe: Option<PathBuf>) -> any
 
     Ok(())
 }
+
+async fn resolve_resume_path(
+    config: &Config,
+    args: &crate::cli::ResumeArgs,
+) -> anyhow::Result<Option<PathBuf>> {
+    if args.last {
+        match codex_core::RolloutRecorder::list_conversations(&config.codex_home, 1, None).await {
+            Ok(page) => Ok(page.items.first().map(|it| it.path.clone())),
+            Err(e) => {
+                error!("Error listing conversations: {e}");
+                Ok(None)
+            }
+        }
+    } else if let Some(id_str) = args.session_id.as_deref() {
+        let path = find_conversation_path_by_id_str(&config.codex_home, id_str).await?;
+        Ok(path)
+    } else {
+        Ok(None)
+    }
+}