feat: add ACP server skeleton with stdout-purity test

src/acp/mod.rs

@@ -0,0 +1,4 @@
+mod server;
+mod types;
+
+pub use server::run_acp_server;

src/acp/server.rs

@@ -0,0 +1,150 @@
+use super::types::{METHOD_NOT_FOUND, PARSE_ERROR, Request, Response};
+use anyhow::Result;
+use serde_json::json;
+use tokio::io::{AsyncBufReadExt, AsyncWrite, AsyncWriteExt, BufReader};
+
+pub async fn run_acp_server() -> Result<()> {
+    run_acp_server_on(tokio::io::stdin(), tokio::io::stdout()).await
+}
+
+pub(crate) async fn run_acp_server_on<R, W>(reader: R, mut writer: W) -> Result<()>
+where
+    R: tokio::io::AsyncRead + Unpin,
+    W: AsyncWrite + Unpin,
+{
+    let reader = BufReader::new(reader);
+    let mut lines = reader.lines();
+
+    while let Some(line) = lines.next_line().await? {
+        let line = line.trim().to_string();
+        if line.is_empty() {
+            continue;
+        }
+        if let Some(response) = dispatch(&line) {
+            emit(&mut writer, &response).await?;
+        }
+    }
+
+    Ok(())
+}
+
+fn dispatch(raw: &str) -> Option<Response> {
+    let req: Request = match serde_json::from_str(raw) {
+        Ok(r) => r,
+        Err(_) => return Some(Response::err(None, PARSE_ERROR, "Parse error")),
+    };
+
+    req.id.as_ref()?;
+
+    Some(match req.method.as_str() {
+        "initialize" => handle_initialize(req),
+        _ => Response::err(
+            req.id,
+            METHOD_NOT_FOUND,
+            format!("Method not found: {}", req.method),
+        ),
+    })
+}
+
+fn handle_initialize(req: Request) -> Response {
+    Response::ok(
+        req.id,
+        json!({
+            "name": "coyote",
+            "version": env!("CARGO_PKG_VERSION"),
+            "protocolVersion": "1",
+        }),
+    )
+}
+
+async fn emit<W: AsyncWrite + Unpin>(writer: &mut W, response: &Response) -> Result<()> {
+    let mut line = serde_json::to_string(response)?;
+    line.push('\n');
+    writer.write_all(line.as_bytes()).await?;
+    writer.flush().await?;
+    Ok(())
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[tokio::test]
+    async fn all_stdout_is_valid_json_rpc() {
+        let input = concat!(
+            r#"{"jsonrpc":"2.0","id":1,"method":"initialize","params":{"name":"test","version":"0.1.0"}}"#,
+            "\n",
+        );
+        let mut output = Vec::new();
+        run_acp_server_on(input.as_bytes(), &mut output)
+            .await
+            .unwrap();
+
+        for line in output.split(|&b| b == b'\n').filter(|l| !l.is_empty()) {
+            let s = std::str::from_utf8(line).expect("non-UTF8 in ACP stdout");
+            let _: serde_json::Value = serde_json::from_str(s)
+                .unwrap_or_else(|_| panic!("ACP stdout not valid JSON: {s}"));
+        }
+    }
+
+    #[tokio::test]
+    async fn unknown_method_returns_method_not_found() {
+        let input = concat!(
+            r#"{"jsonrpc":"2.0","id":2,"method":"nonexistent","params":{}}"#,
+            "\n",
+        );
+        let mut output = Vec::new();
+        run_acp_server_on(input.as_bytes(), &mut output)
+            .await
+            .unwrap();
+
+        let s = String::from_utf8(output).unwrap();
+        let v: serde_json::Value = serde_json::from_str(s.trim()).unwrap();
+        assert_eq!(v["error"]["code"], METHOD_NOT_FOUND);
+        assert_eq!(v["id"], 2);
+    }
+
+    #[tokio::test]
+    async fn invalid_json_returns_parse_error() {
+        let input = "not json\n";
+        let mut output = Vec::new();
+        run_acp_server_on(input.as_bytes(), &mut output)
+            .await
+            .unwrap();
+
+        let s = String::from_utf8(output).unwrap();
+        let v: serde_json::Value = serde_json::from_str(s.trim()).unwrap();
+        assert_eq!(v["error"]["code"], PARSE_ERROR);
+    }
+
+    #[tokio::test]
+    async fn notification_without_id_produces_no_output() {
+        let input = concat!(
+            r#"{"jsonrpc":"2.0","method":"session/cancel","params":{}}"#,
+            "\n",
+        );
+        let mut output = Vec::new();
+        run_acp_server_on(input.as_bytes(), &mut output)
+            .await
+            .unwrap();
+        assert!(output.is_empty());
+    }
+
+    #[tokio::test]
+    async fn initialize_returns_server_info() {
+        let input = concat!(
+            r#"{"jsonrpc":"2.0","id":1,"method":"initialize","params":{"name":"test","version":"0.1.0"}}"#,
+            "\n",
+        );
+        let mut output = Vec::new();
+        run_acp_server_on(input.as_bytes(), &mut output)
+            .await
+            .unwrap();
+
+        let s = String::from_utf8(output).unwrap();
+        let v: serde_json::Value = serde_json::from_str(s.trim()).unwrap();
+        assert_eq!(v["id"], 1);
+        assert_eq!(v["result"]["name"], "coyote");
+        assert!(v["result"]["version"].is_string());
+    }
+}

src/acp/types.rs

@@ -0,0 +1,59 @@
+use serde::{Deserialize, Serialize};
+use serde_json::Value;
+
+pub const METHOD_NOT_FOUND: i32 = -32601;
+pub const PARSE_ERROR: i32 = -32700;
+
+#[derive(Debug, Deserialize)]
+#[allow(dead_code)]
+pub struct Request {
+    pub jsonrpc: String,
+    pub id: Option<Value>,
+    pub method: String,
+    pub params: Option<Value>,
+}
+
+#[derive(Debug, Serialize)]
+pub struct Response {
+    pub jsonrpc: &'static str,
+    #[serde(skip_serializing_if = "Option::is_none")]
+    pub id: Option<Value>,
+    #[serde(flatten)]
+    pub body: ResponseBody,
+}
+
+#[derive(Debug, Serialize)]
+#[serde(untagged)]
+pub enum ResponseBody {
+    Ok { result: Value },
+    Err { error: RpcError },
+}
+
+#[derive(Debug, Serialize)]
+pub struct RpcError {
+    pub code: i32,
+    pub message: String,
+}
+
+impl Response {
+    pub fn ok(id: Option<Value>, result: Value) -> Self {
+        Self {
+            jsonrpc: "2.0",
+            id,
+            body: ResponseBody::Ok { result },
+        }
+    }
+
+    pub fn err(id: Option<Value>, code: i32, message: impl Into<String>) -> Self {
+        Self {
+            jsonrpc: "2.0",
+            id,
+            body: ResponseBody::Err {
+                error: RpcError {
+                    code,
+                    message: message.into(),
+                },
+            },
+        }
+    }
+}

src/cli/mod.rs

@@ -234,6 +234,10 @@ pub struct Cli {
     /// prompting. Implies --dangerously-skip-permissions. Incompatible with REPL mode (requires a prompt).
     #[arg(long, help_heading = "Sandbox")]
     pub headless: bool,
+    /// Run as an ACP agent server over stdio (JSON-RPC 2.0). Every stdout byte must be valid JSON-RPC.
+    /// Implies --headless. Single session per process.
+    #[arg(long, help_heading = "Sandbox")]
+    pub acp_server: bool,
     /// Display information
     #[arg(long, help_heading = "Diagnostics & Tools")]
     pub info: bool,
@@ -505,6 +509,17 @@ mod tests {
         assert!(!parse(&[]).headless);
     }
 
+    #[test]
+    fn parse_acp_server_flag() {
+        let cli = parse(&["--acp-server"]);
+        assert!(cli.acp_server);
+    }
+
+    #[test]
+    fn parse_acp_server_default_off() {
+        assert!(!parse(&[]).acp_server);
+    }
+
     #[test]
     fn parse_sync_models_flag() {
         let cli = parse(&["--sync-models"]);

src/main.rs

@@ -1,3 +1,4 @@
+mod acp;
 mod cli;
 mod client;
 mod config;
@@ -82,8 +83,8 @@ async fn main() -> Result<()> {
         WorkingMode::Cmd
     };
 
-    if cli.headless {
+    if cli.headless || cli.acp_server {
-        if text.is_none() && cli.file.is_empty() {
+        if cli.headless && !cli.acp_server && text.is_none() && cli.file.is_empty() {
             bail!("--headless requires a prompt argument; REPL mode is not supported");
         }
         unsafe {
@@ -107,7 +108,11 @@ async fn main() -> Result<()> {
         || cli.delete_secret.is_some()
         || cli.list_secrets;
 
-    let log_path = setup_logger()?;
+    let log_path = setup_logger(cli.acp_server)?;
+
+    if cli.acp_server {
+        return acp::run_acp_server().await;
+    }
 
     if let Some(version) = &cli.update {
         let version = version.clone();
@@ -694,7 +699,7 @@ async fn create_input(
     Ok(input)
 }
 
-fn setup_logger() -> Result<Option<PathBuf>> {
+fn setup_logger(acp_mode: bool) -> Result<Option<PathBuf>> {
     let (log_level, log_path) = paths::log_config()?;
     if log_level == LevelFilter::Off {
         return Ok(None);
@@ -705,7 +710,11 @@ fn setup_logger() -> Result<Option<PathBuf>> {
     let log_filter = env::var(get_env_name("log_filter")).ok();
     match log_path.clone() {
         None => {
-            let console_appender = ConsoleAppender::builder().encoder(encoder).build();
+            let mut builder = ConsoleAppender::builder().encoder(encoder);
+            if acp_mode {
+                builder = builder.target(log4rs::append::console::Target::Stderr);
+            }
+            let console_appender = builder.build();
             log4rs::init_config(init_console_logger(log_level, log_filter, console_appender))?;
         }
         Some(path) => {