diff --git a/src/crates/assembly/core/src/agentic/coordination/coordinator.rs b/src/crates/assembly/core/src/agentic/coordination/coordinator.rs
index b25bded9e7..aa70669d52 100644
--- a/src/crates/assembly/core/src/agentic/coordination/coordinator.rs
+++ b/src/crates/assembly/core/src/agentic/coordination/coordinator.rs
@@ -4264,6 +4264,11 @@ Update the persona files and delete BOOTSTRAP.md as soon as bootstrap is complet
             )
             .await?;
         let session_id = session.session_id.clone();
+        // Sync context window from AI config so subagents with large-context
+        // models are not prematurely capped at SessionConfig::default()'s 128128.
+        self.session_manager
+            .refresh_session_context_window(&session_id)
+            .await?;
         if let Some(source_session_id) = prompt_cache_source_session_id.as_deref() {
             let copied = self
                 .session_manager
diff --git a/src/crates/assembly/core/src/agentic/session/session_manager.rs b/src/crates/assembly/core/src/agentic/session/session_manager.rs
index c9aeba967f..d88353d0dc 100644
--- a/src/crates/assembly/core/src/agentic/session/session_manager.rs
+++ b/src/crates/assembly/core/src/agentic/session/session_manager.rs
@@ -2166,6 +2166,33 @@ impl SessionManager {
         Ok(())
     }
 
+    /// Sync session context window from AI config without requiring an explicit model_id.
+    ///
+    /// Subagent sessions created via `build_session_config_for_workspace` use
+    /// `SessionConfig::default()` which hardcodes `max_context_tokens: 128128`.
+    /// This method reloads the AI config and updates `max_context_tokens` to the
+    /// model's actual configured `context_window`, so subagents with large-context
+    /// models are not prematurely capped.
+    pub async fn refresh_session_context_window(
+        &self,
+        session_id: &str,
+    ) -> BitFunResult<()> {
+        if let Some(ai_config) = Self::load_ai_config_for_model_resolution().await {
+            if let Some(mut session) = self.sessions.get_mut(session_id) {
+                let previous = session.config.max_context_tokens;
+                Self::sync_session_context_window_from_ai_config(&mut session, &ai_config);
+                let updated = session.config.max_context_tokens;
+                if updated != previous {
+                    debug!(
+                        "Refreshed session context window: session_id={}, previous={}, updated={}",
+                        session_id, previous, updated
+                    );
+                }
+            }
+        }
+        Ok(())
+    }
+
     /// Update session activity time
     pub fn touch_session(&self, session_id: &str) {
         if let Some(mut session) = self.sessions.get_mut(session_id) {