fix: use default provider and model when provider in session no longer exists (#8035)

crates/goose-server/src/routes/agent.rs

@@ -385,7 +385,7 @@ async fn resume_agent(
             }
         })?;
 
-    let extension_results = if payload.load_model_and_extensions {
+    let (extension_results, session) = if payload.load_model_and_extensions {
         let agent = state
             .get_agent_for_route(payload.session_id.clone())
             .await
@@ -394,7 +394,7 @@ async fn resume_agent(
                 status: code,
             })?;
 
-        agent
+        let provider_changed = agent
             .restore_provider_from_session(&session)
             .await
             .map_err(|e| ErrorResponse {
@@ -402,6 +402,19 @@ async fn resume_agent(
                 status: StatusCode::INTERNAL_SERVER_ERROR,
             })?;
 
+        let session = if provider_changed {
+            state
+                .session_manager()
+                .get_session(&payload.session_id, true)
+                .await
+                .map_err(|err| ErrorResponse {
+                    message: format!("Failed to re-fetch session: {}", err),
+                    status: StatusCode::INTERNAL_SERVER_ERROR,
+                })?
+        } else {
+            session
+        };
+
         let extension_results =
             if let Some(results) = state.take_extension_loading_task(&payload.session_id).await {
                 tracing::debug!(
@@ -420,9 +433,9 @@ async fn resume_agent(
                 agent.load_extensions_from_session(&session).await
             };
 
-        Some(extension_results)
+        (Some(extension_results), session)
     } else {
-        None
+        (None, session)
     };
 
     Ok(Json(ResumeAgentResponse {