fix(acp): resolve provider default model instead of leaking sentinel "current" (#10305)

Co-authored-by: Jack Amadeo <jackamadeo@squareup.com>

crates/goose/src/acp/server.rs

@@ -3056,7 +3056,11 @@ impl GooseAcpAgent {
                 .get_goose_model()
                 .internal_err_ctx("Failed to resolve default model from config")?
         } else if is_changing_provider {
-            ACP_CURRENT_MODEL.to_string()
+            crate::providers::get_from_registry(&resolved_provider_name)
+                .await
+                .ok()
+                .map(|entry| entry.metadata().default_model.clone())
+                .unwrap_or(ACP_CURRENT_MODEL.to_string())
         } else {
             current_model
         };

crates/goose/src/agents/agent.rs

@@ -2897,7 +2897,7 @@ impl Agent {
             .or_else(|| config.get_goose_provider().ok())
             .ok_or_else(|| anyhow!("Could not configure agent: missing provider"))?;
 
-        let model_config = match session.model_config.clone() {
+        let mut model_config = match session.model_config.clone() {
             Some(saved_config) => saved_config,
             None => {
                 let model_name = config
@@ -2909,6 +2909,20 @@ impl Agent {
             }
         };
 
+        // if the saved model is the ACP sentinel "current", only preserve this if the provider
+        // uses this sentinel to indicate it's an ACP provider that manages its model
+        if model_config.model_name == crate::acp::ACP_CURRENT_MODEL {
+            if let Ok(entry) = crate::providers::get_from_registry(&provider_name).await {
+                if entry.metadata().default_model != crate::acp::ACP_CURRENT_MODEL {
+                    model_config = crate::model_config::model_config_from_user_config(
+                        &provider_name,
+                        &entry.metadata().default_model,
+                    )
+                    .map_err(|e| anyhow!("Could not resolve default model: {}", e))?;
+                }
+            }
+        }
+
         let extensions =
             EnabledExtensionsState::extensions_or_default(Some(&session.extension_data), config);