diff --git a/crates/buzz-acp/README.md b/crates/buzz-acp/README.md
index e6164b02dd3..4d84a537ff8 100644
--- a/crates/buzz-acp/README.md
+++ b/crates/buzz-acp/README.md
@@ -98,6 +98,23 @@ buzz-acp
 Older installs that still expose `claude-code-acp` are also supported. `buzz-acp`
 treats both Claude ACP command names as the same zero-arg runtime.
 
+## Running with Grok Build
+
+[Grok Build](https://docs.x.ai/build/overview) speaks ACP over `grok agent stdio`. After `grok login` (subscription / cached token in `~/.grok/auth.json`):
+
+```bash
+export BUZZ_PRIVATE_KEY="nsec1..."   # the *agent* identity, not a human's
+export BUZZ_RELAY_URL="ws://localhost:3000"
+export BUZZ_ACP_AGENT_COMMAND="grok"
+export BUZZ_ACP_AGENT_ARGS="agent,--always-approve,stdio"
+
+buzz-acp
+```
+
+Grok advertises `cached_token` on `initialize`. The harness now sends ACP `authenticate` with `_meta.headless` on spawn so the first `@mention` does not wait for a worker crash/respawn. Goose and Claude do not advertise that method and are unchanged. Do not re-run `authenticate` on every `session/new` — that resets Grok's inner worker.
+
+For CI, set `XAI_API_KEY` instead of (or in addition to) `grok login`; the harness will use `xai.api_key` when that method is advertised.
+
 ## Configuration
 
 All configuration is via environment variables (or CLI flags — every env var has a matching flag).
diff --git a/crates/buzz-acp/src/acp.rs b/crates/buzz-acp/src/acp.rs
index 0d87bca028c..959e0b3d40a 100644
--- a/crates/buzz-acp/src/acp.rs
+++ b/crates/buzz-acp/src/acp.rs
@@ -623,9 +623,32 @@ impl AcpClient {
 
     /// Send the ACP `authenticate` request for an adapter-advertised method.
     pub async fn authenticate(&mut self, method_id: &str) -> Result<serde_json::Value, AcpError> {
-        let params = serde_json::json!({
-            "methodId": method_id,
-        });
+        self.authenticate_with_meta(method_id, false).await
+    }
+
+    /// Headless authenticate. Grok's ACP example requires `_meta.headless`
+    /// before `session/new`; without it the worker dies with
+    /// `Auth(AuthorizationRequired)` on the first mention.
+    pub async fn authenticate_headless(
+        &mut self,
+        method_id: &str,
+    ) -> Result<serde_json::Value, AcpError> {
+        self.authenticate_with_meta(method_id, true).await
+    }
+
+    async fn authenticate_with_meta(
+        &mut self,
+        method_id: &str,
+        headless: bool,
+    ) -> Result<serde_json::Value, AcpError> {
+        let params = if headless {
+            serde_json::json!({
+                "methodId": method_id,
+                "_meta": { "headless": true },
+            })
+        } else {
+            serde_json::json!({ "methodId": method_id })
+        };
         self.send_request("authenticate", params).await
     }
 
diff --git a/crates/buzz-acp/src/lib.rs b/crates/buzz-acp/src/lib.rs
index 1352b31cad8..145b08770e9 100644
--- a/crates/buzz-acp/src/lib.rs
+++ b/crates/buzz-acp/src/lib.rs
@@ -4649,6 +4649,12 @@ async fn initialize_agent_pool(
                 match initialize_result {
                     Ok(Ok(init_result)) => {
                         tracing::info!(agent = i, "agent initialized: {init_result}");
+                        if let Err(e) = authenticate_if_needed(&mut acp, &init_result).await {
+                            tracing::error!(agent = i, "{e}");
+                            acp.shutdown().await;
+                            agent_slots.push(None);
+                            continue;
+                        }
                         let protocol_version =
                             init_result["protocolVersion"].as_u64().unwrap_or(1) as u32;
                         tracing::info!(
@@ -4742,6 +4748,10 @@ async fn spawn_and_init(
     match acp.initialize().await {
         Ok(init_result) => {
             tracing::info!("agent initialized: {init_result}");
+            if let Err(e) = authenticate_if_needed(&mut acp, &init_result).await {
+                acp.shutdown().await;
+                return Err(e);
+            }
             let protocol_version = init_result["protocolVersion"].as_u64().unwrap_or(1) as u32;
             acp.observe(
                 "agent_initialized",
@@ -4768,6 +4778,39 @@ async fn spawn_auth_client(agent: &AuthAgentArgs) -> Result<AcpClient, acp::AcpE
     AcpClient::spawn(&agent.agent_command, &agent_args, &[], false).await
 }
 
+/// Call ACP `authenticate` when the agent advertised a non-interactive method.
+///
+/// The harness already exposes `buzz-acp authenticate` as a one-shot CLI, but
+/// the live pool never sent it. Agents that require auth before `session/new`
+/// (Grok's `cached_token`) then fail the first turn. Goose and Claude advertise
+/// no such method and skip this path.
+async fn authenticate_if_needed(
+    acp: &mut AcpClient,
+    init_result: &serde_json::Value,
+) -> Result<()> {
+    let Some(method_id) = preferred_headless_auth_method(init_result) else {
+        tracing::debug!("ACP headless auth skipped (no cached_token / xai.api_key)");
+        return Ok(());
+    };
+    match tokio::time::timeout(
+        Duration::from_secs(30),
+        acp.authenticate_headless(&method_id),
+    )
+    .await
+    {
+        Ok(Ok(_)) => {
+            tracing::info!(method_id, "agent authenticated");
+            Ok(())
+        }
+        Ok(Err(e)) => Err(anyhow::anyhow!(
+            "agent authenticate ({method_id}) failed: {e}"
+        )),
+        Err(_) => Err(anyhow::anyhow!(
+            "agent authenticate ({method_id}) timed out (30s)"
+        )),
+    }
+}
+
 fn extract_auth_methods(init_result: &serde_json::Value) -> Vec<serde_json::Value> {
     init_result
         .get("authMethods")
@@ -4776,6 +4819,72 @@ fn extract_auth_methods(init_result: &serde_json::Value) -> Vec<serde_json::Valu
         .unwrap_or_default()
 }
 
+/// Pick a non-interactive ACP auth method advertised on `initialize`.
+///
+/// Prefer `cached_token` (Grok subscription / `~/.grok/auth.json`), then
+/// `xai.api_key` when `XAI_API_KEY` is set. Never `grok.com` — that opens a
+/// browser. Returns `None` for adapters that do not require authenticate
+/// (Goose, Claude) so their session/new path is unchanged.
+fn preferred_headless_auth_method(init_result: &serde_json::Value) -> Option<String> {
+    let ids: Vec<String> = extract_auth_methods(init_result)
+        .iter()
+        .filter_map(|method| {
+            method
+                .get("id")
+                .and_then(|id| id.as_str())
+                .map(str::to_string)
+        })
+        .collect();
+    if ids.iter().any(|id| id == "cached_token") {
+        return Some("cached_token".into());
+    }
+    if std::env::var_os("XAI_API_KEY").is_some() && ids.iter().any(|id| id == "xai.api_key") {
+        return Some("xai.api_key".into());
+    }
+    None
+}
+
+#[cfg(test)]
+mod headless_auth_method_tests {
+    use super::preferred_headless_auth_method;
+
+    fn init_with_methods(ids: &[&str]) -> serde_json::Value {
+        serde_json::json!({
+            "authMethods": ids.iter().map(|id| serde_json::json!({"id": id})).collect::<Vec<_>>(),
+        })
+    }
+
+    #[test]
+    fn prefers_cached_token_over_browser_login() {
+        let init = init_with_methods(&["grok.com", "cached_token"]);
+        assert_eq!(
+            preferred_headless_auth_method(&init).as_deref(),
+            Some("cached_token")
+        );
+    }
+
+    #[test]
+    fn skips_when_only_browser_login_is_advertised() {
+        let init = init_with_methods(&["grok.com"]);
+        assert_eq!(preferred_headless_auth_method(&init), None);
+    }
+
+    #[test]
+    fn skips_adapters_with_no_auth_methods() {
+        let init = serde_json::json!({});
+        assert_eq!(preferred_headless_auth_method(&init), None);
+    }
+
+    #[test]
+    fn skips_api_key_when_env_is_unset() {
+        if std::env::var_os("XAI_API_KEY").is_some() {
+            return;
+        }
+        let init = init_with_methods(&["xai.api_key"]);
+        assert_eq!(preferred_headless_auth_method(&init), None);
+    }
+}
+
 /// `buzz-acp auth-methods` — spawn an adapter, initialize it, print authMethods.
 async fn run_auth_methods(args: AuthMethodsArgs) -> Result<()> {
     let mut client = match spawn_auth_client(&args.agent).await {