feat(process): add StreamMode for real-time stdout tee to stderr

crates/cli-sub-agent/src/batch.rs

@@ -585,6 +585,7 @@ async fn execute_task(
         extra_env_ref,
         Some("batch"),
         None, // batch does not use tier-based selection
+        csa_process::StreamMode::BufferOnly,
     )
     .await;
 

crates/cli-sub-agent/src/claude_sub_agent_cmd.rs

@@ -75,6 +75,7 @@ pub(crate) async fn handle_claude_sub_agent(
         extra_env,
         Some("run"),
         None, // claude-sub-agent does not use tier-based selection
+        csa_process::StreamMode::BufferOnly,
     )
     .await?;
 

crates/cli-sub-agent/src/cli.rs

@@ -67,6 +67,10 @@ pub enum Commands {
         /// Block-wait for a free slot instead of failing when all slots are occupied
         #[arg(long)]
         wait: bool,
+
+        /// Stream child stdout to stderr in real-time (prefix: [stdout])
+        #[arg(long)]
+        stream_stdout: bool,
     },
 
     /// Manage sessions

crates/cli-sub-agent/src/debate_cmd.rs

@@ -69,6 +69,7 @@ pub(crate) async fn handle_debate(args: DebateArgs, current_depth: u32) -> Resul
         extra_env,
         Some("debate"),
         None, // debate does not use tier-based selection
+        csa_process::StreamMode::BufferOnly,
     )
     .await?;
 

crates/cli-sub-agent/src/main.rs

@@ -1,3 +1,5 @@
+use std::io::IsTerminal;
+
 use anyhow::Result;
 use clap::Parser;
 use tempfile::TempDir;
@@ -71,7 +73,17 @@ async fn main() -> Result<()> {
             thinking,
             no_failover,
             wait,
+            stream_stdout,
         } => {
+            // Determine stream mode: explicit flag > auto-detect (text format + stderr is TTY)
+            let stream_mode = if stream_stdout
+                || (matches!(output_format, OutputFormat::Text) && std::io::stderr().is_terminal())
+            {
+                csa_process::StreamMode::TeeToStderr
+            } else {
+                csa_process::StreamMode::BufferOnly
+            };
+
             let exit_code = handle_run(
                 tool,
                 prompt,
@@ -88,6 +100,7 @@ async fn main() -> Result<()> {
                 wait,
                 current_depth,
                 output_format,
+                stream_mode,
             )
             .await?;
             std::process::exit(exit_code);
@@ -283,6 +296,7 @@ async fn handle_run(
     wait: bool,
     current_depth: u32,
     output_format: OutputFormat,
+    stream_mode: csa_process::StreamMode,
 ) -> Result<i32> {
     // 1. Determine project root
     let project_root = pipeline::determine_project_root(cd.as_deref())?;
@@ -556,7 +570,12 @@ async fn handle_run(
             let temp_dir = TempDir::new()?;
             info!("Ephemeral session in: {:?}", temp_dir.path());
             executor
-                .execute_in(&prompt_text, temp_dir.path(), extra_env.as_ref())
+                .execute_in(
+                    &prompt_text,
+                    temp_dir.path(),
+                    extra_env.as_ref(),
+                    stream_mode,
+                )
                 .await?
         } else {
             // Persistent session
@@ -572,6 +591,7 @@ async fn handle_run(
                 extra_env.as_ref(),
                 Some("run"),
                 resolved_tier_name.as_deref(),
+                stream_mode,
             )
             .await
             {

crates/cli-sub-agent/src/mcp_server.rs

@@ -574,7 +574,12 @@ async fn handle_run_tool(args: Value) -> Result<Value> {
         // Ephemeral: use temp directory
         let temp_dir = TempDir::new()?;
         executor
-            .execute_in(prompt, temp_dir.path(), extra_env_ref)
+            .execute_in(
+                prompt,
+                temp_dir.path(),
+                extra_env_ref,
+                csa_process::StreamMode::BufferOnly,
+            )
             .await?
     } else {
         // Persistent session
@@ -590,6 +595,7 @@ async fn handle_run_tool(args: Value) -> Result<Value> {
             extra_env_ref,
             Some("run"),
             None, // MCP server does not use tier-based selection
+            csa_process::StreamMode::BufferOnly,
         )
         .await?
     };

crates/cli-sub-agent/src/pipeline.rs

@@ -207,6 +207,7 @@ pub(crate) async fn execute_with_session(
     extra_env: Option<&std::collections::HashMap<String, String>>,
     task_type: Option<&str>,
     tier_name: Option<&str>,
+    stream_mode: csa_process::StreamMode,
 ) -> Result<ExecutionResult> {
     let execution = execute_with_session_and_meta(
         executor,
@@ -220,6 +221,7 @@ pub(crate) async fn execute_with_session(
         extra_env,
         task_type,
         tier_name,
+        stream_mode,
     )
     .await?;
 
@@ -239,6 +241,7 @@ pub(crate) async fn execute_with_session_and_meta(
     extra_env: Option<&std::collections::HashMap<String, String>>,
     task_type: Option<&str>,
     tier_name: Option<&str>,
+    stream_mode: csa_process::StreamMode,
 ) -> Result<SessionExecutionResult> {
     // Check for parent session violation: a child process must not operate on its own session
     if let Some(ref session_id) = session_arg {
@@ -395,7 +398,7 @@ pub(crate) async fn execute_with_session_and_meta(
     let mut sigint = signal(SignalKind::interrupt()).context("Failed to install SIGINT handler")?;
 
     // Wait for either child completion or signal
-    let wait_future = csa_process::wait_and_capture(child);
+    let wait_future = csa_process::wait_and_capture(child, stream_mode);
     tokio::pin!(wait_future);
 
     let result = tokio::select! {

crates/cli-sub-agent/src/review_cmd.rs

@@ -226,6 +226,7 @@ async fn execute_review(
         extra_env,
         Some("review"),
         None,
+        csa_process::StreamMode::BufferOnly,
     )
     .await
 }

crates/csa-executor/src/agent_backend_adapter.rs

@@ -219,7 +219,12 @@ impl AgentSession for ExecutorAgentSession {
 
         match self
             .executor
-            .execute_in(&prompt, &self.cwd, extra_env)
+            .execute_in(
+                &prompt,
+                &self.cwd,
+                extra_env,
+                csa_process::StreamMode::BufferOnly,
+            )
             .await
         {
             Ok(result) => {

crates/csa-executor/src/executor.rs

@@ -207,9 +207,10 @@ impl Executor {
         tool_state: Option<&ToolState>,
         session: &MetaSessionState,
         extra_env: Option<&HashMap<String, String>>,
+        stream_mode: csa_process::StreamMode,
     ) -> Result<ExecutionResult> {
         let (cmd, stdin_data) = self.build_command(prompt, tool_state, session, extra_env);
-        csa_process::run_and_capture_with_stdin(cmd, stdin_data).await
+        csa_process::run_and_capture_with_stdin(cmd, stdin_data, stream_mode).await
     }
 
     /// Execute in a specific directory (for ephemeral sessions).
@@ -220,6 +221,7 @@ impl Executor {
         prompt: &str,
         work_dir: &Path,
         extra_env: Option<&HashMap<String, String>>,
+        stream_mode: csa_process::StreamMode,
     ) -> Result<ExecutionResult> {
         let mut cmd = Command::new(self.executable_name());
         cmd.current_dir(work_dir);
@@ -242,7 +244,7 @@ impl Executor {
         } else {
             self.append_prompt_args_with_transport(&mut cmd, prompt, prompt_transport);
         }
-        csa_process::run_and_capture_with_stdin(cmd, stdin_data).await
+        csa_process::run_and_capture_with_stdin(cmd, stdin_data, stream_mode).await
     }
 
     /// Build base command with session environment variables.