diff --git a/crates/sprout-acp/README.md b/crates/sprout-acp/README.md
index a869bbe33f..ecd66bd66e 100644
--- a/crates/sprout-acp/README.md
+++ b/crates/sprout-acp/README.md
@@ -105,7 +105,7 @@ All configuration is via environment variables (or CLI flags — every env var h
 | `SPROUT_ACP_AGENT_COMMAND` | no | `goose` | Agent binary to spawn. |
 | `SPROUT_ACP_AGENT_ARGS` | no | `acp` | Agent arguments (comma-separated). |
 | `SPROUT_ACP_MCP_COMMAND` | no | `sprout-mcp-server` | Path to the Sprout MCP server binary. |
-| `SPROUT_ACP_IDLE_TIMEOUT` | no | `300` | Idle timeout: max seconds of silence before cancelling a turn. Resets on any agent stdout activity. |
+| `SPROUT_ACP_IDLE_TIMEOUT` | no | `620` | Idle timeout: max seconds of silence before cancelling a turn. Resets on any agent stdout activity. |
 | `SPROUT_ACP_MAX_TURN_DURATION` | no | `3600` | Absolute wall-clock cap per turn (safety valve). |
 | `SPROUT_API_TOKEN` | no | — | API token (required if relay enforces token auth). |
 
diff --git a/crates/sprout-acp/src/config.rs b/crates/sprout-acp/src/config.rs
index ed354526dc..f7cf492324 100644
--- a/crates/sprout-acp/src/config.rs
+++ b/crates/sprout-acp/src/config.rs
@@ -13,6 +13,17 @@ use uuid::Uuid;
 
 use crate::filter::SubscriptionRule;
 
+// ── Constants ─────────────────────────────────────────────────────────────────
+
+/// Default idle timeout (seconds) when neither `--idle-timeout` nor the
+/// deprecated `--turn-timeout` is set.
+///
+/// Sized for slow turns where the agent may go silent on its outer ACP channel
+/// while running long sub-tools (e.g. a sprout-agent running another agent, or
+/// codex/claude doing multi-minute single tool calls). 600s working budget +
+/// 20s buffer. Override via `--idle-timeout` / `SPROUT_ACP_IDLE_TIMEOUT`.
+pub(crate) const DEFAULT_IDLE_TIMEOUT_SECS: u64 = 620;
+
 // ── Errors ────────────────────────────────────────────────────────────────────
 
 #[derive(Debug, Error)]
@@ -597,7 +608,7 @@ impl Config {
         };
 
         // Resolve idle_timeout_secs with deprecation handling.
-        // Precedence: explicit --idle-timeout > --turn-timeout (deprecated) > default 320.
+        // Precedence: explicit --idle-timeout > --turn-timeout (deprecated) > `DEFAULT_IDLE_TIMEOUT_SECS`.
         let idle_timeout_secs = {
             let raw = match (args.idle_timeout, args.turn_timeout) {
                 (Some(idle), Some(_turn)) => {
@@ -615,7 +626,7 @@ impl Config {
                     );
                     turn
                 }
-                (None, None) => 320, // default: 20s buffer over goose's 300s turn timeout
+                (None, None) => DEFAULT_IDLE_TIMEOUT_SECS,
             };
             if raw == 0 {
                 tracing::warn!("idle timeout of 0 is invalid — using 1s minimum");
@@ -1092,7 +1103,7 @@ mod tests {
             agent_command: "goose".into(),
             agent_args: vec!["acp".into()],
             mcp_command: "sprout-mcp-server".into(),
-            idle_timeout_secs: 320,
+            idle_timeout_secs: DEFAULT_IDLE_TIMEOUT_SECS,
             max_turn_duration_secs: 3600,
             agents: 1,
             heartbeat_interval_secs: 0,
@@ -1800,13 +1811,13 @@ channels = "ALL"
     // ── Idle timeout config precedence ─────────────────────────────────────
 
     /// Helper: resolve idle_timeout_secs using the same precedence logic as Config::from_args.
-    /// Precedence: explicit --idle-timeout > --turn-timeout (deprecated) > default 320.
+    /// Precedence: explicit --idle-timeout > --turn-timeout (deprecated) > `DEFAULT_IDLE_TIMEOUT_SECS`.
     fn resolve_idle_timeout(idle: Option<u64>, turn: Option<u64>) -> u64 {
         let raw = match (idle, turn) {
             (Some(idle), Some(_)) => idle,
             (Some(idle), None) => idle,
             (None, Some(turn)) => turn,
-            (None, None) => 320,
+            (None, None) => DEFAULT_IDLE_TIMEOUT_SECS,
         };
         if raw == 0 {
             1
@@ -1826,8 +1837,8 @@ channels = "ALL"
     }
 
     #[test]
-    fn idle_timeout_defaults_to_320_when_neither_set() {
-        assert_eq!(resolve_idle_timeout(None, None), 320);
+    fn idle_timeout_defaults_to_constant_when_neither_set() {
+        assert_eq!(resolve_idle_timeout(None, None), DEFAULT_IDLE_TIMEOUT_SECS);
     }
 
     #[test]
@@ -1844,9 +1855,10 @@ channels = "ALL"
     fn test_config_summary_includes_idle_and_max_turn() {
         let config = test_config(SubscribeMode::Mentions);
         let summary = config.summary();
+        let expected_idle = format!("idle_timeout={DEFAULT_IDLE_TIMEOUT_SECS}s");
         assert!(
-            summary.contains("idle_timeout=320s"),
-            "summary should include idle_timeout: {summary}"
+            summary.contains(&expected_idle),
+            "summary should include {expected_idle}: {summary}"
         );
         assert!(
             summary.contains("max_turn=3600s"),
diff --git a/crates/sprout-acp/src/main.rs b/crates/sprout-acp/src/main.rs
index 33e1944961..6ad750d672 100644
--- a/crates/sprout-acp/src/main.rs
+++ b/crates/sprout-acp/src/main.rs
@@ -2729,7 +2729,7 @@ mod build_mcp_servers_tests {
             agent_command: "goose".into(),
             agent_args: vec!["acp".into()],
             mcp_command: "sprout-mcp-server".into(),
-            idle_timeout_secs: 320,
+            idle_timeout_secs: config::DEFAULT_IDLE_TIMEOUT_SECS,
             max_turn_duration_secs: 3600,
             agents: 1,
             heartbeat_interval_secs: 0,