diff --git a/codex-rs/exec/src/cli.rs b/codex-rs/exec/src/cli.rs
index 1c2a9eb8aa..310fd0c5af 100644
--- a/codex-rs/exec/src/cli.rs
+++ b/codex-rs/exec/src/cli.rs
@@ -46,7 +46,16 @@ pub struct Cli {
     pub last_message_file: Option<PathBuf>,
 
     /// Initial instructions for the agent.
-    pub prompt: String,
+    ///
+    /// Behaviour:
+    ///  • If a string is provided, that is used as the prompt.
+    ///  • If omitted, the prompt is read from stdin *only when* stdin is not a TTY
+    ///    (i.e. another process is piping data). Otherwise the CLI exits with an
+    ///    error explaining that a prompt is required.
+    ///  • Supplying `-` explicitly forces reading the prompt from stdin even when
+    ///    stdin **is** a TTY.
+    #[arg(value_name = "PROMPT")]
+    pub prompt: Option<String>,
 }
 
 #[derive(Debug, Clone, Copy, Default, PartialEq, Eq, ValueEnum)]
diff --git a/codex-rs/exec/src/lib.rs b/codex-rs/exec/src/lib.rs
index 8c94fe5dc9..db4fb6aff6 100644
--- a/codex-rs/exec/src/lib.rs
+++ b/codex-rs/exec/src/lib.rs
@@ -2,6 +2,7 @@ mod cli;
 mod event_processor;
 
 use std::io::IsTerminal;
+use std::io::Read;
 use std::path::Path;
 use std::path::PathBuf;
 use std::sync::Arc;
@@ -40,6 +41,41 @@ 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 {
+        Some(p) if p != "-" => p,
+        // Either `-` was passed or no positional arg.
+        maybe_dash => {
+            // When no arg (None) **and** stdin is a TTY, bail out early – unless the
+            // user explicitly forced reading via `-`.
+            let force_stdin = matches!(maybe_dash.as_deref(), Some("-"));
+
+            if std::io::stdin().is_terminal() && !force_stdin {
+                eprintln!(
+                    "No prompt provided. Either specify one, use '-' to read from stdin, or pipe the prompt into stdin."
+                );
+                std::process::exit(1);
+            }
+
+            // Ensure the user knows we are waiting on stdin, as they may
+            // have gotten into this state by mistake. If so, and they are not
+            // writing to stdin, Codex will hang indefinitely, so this should
+            // help them debug in that case.
+            if !force_stdin {
+                eprintln!("Reading prompt from stdin...");
+            }
+            let mut buffer = String::new();
+            if let Err(e) = std::io::stdin().read_to_string(&mut buffer) {
+                eprintln!("Failed to read prompt from stdin: {e}");
+                std::process::exit(1);
+            } else if buffer.trim().is_empty() {
+                eprintln!("No prompt provided via stdin.");
+                std::process::exit(1);
+            }
+            buffer
+        }
+    };
+
     let (stdout_with_ansi, stderr_with_ansi) = match color {
         cli::Color::Always => (true, true),
         cli::Color::Never => (false, false),