fix(acp): allow custom model as default for non-local providers (#10438)

crates/goose/src/acp/server/config.rs

@@ -214,14 +214,22 @@ impl GooseAcpAgent {
                 .data(format!("Provider is not configured: {provider_id}")));
         }
 
-        if let Some(model_id) = model_id.as_deref() {
-            let model_exists = entry.default_model == model_id
-                || entry.models.iter().any(|model| model.id == model_id)
-                || (provider_id == "local" && local_inference_model_exists(model_id)?);
-            if !model_exists {
-                return Err(agent_client_protocol::Error::invalid_params().data(format!(
-                    "Model '{model_id}' is not available for provider '{provider_id}'"
-                )));
+        // Custom/unlisted model entry is always allowed (#7255), matching the CLI
+        // and in-session model-selection paths, so a model that is simply absent
+        // from the provider's (often non-exhaustive) inventory must still be
+        // accepted as the default here. Local inference is the exception: its
+        // models cannot be fetched on demand, so they are validated against the
+        // inventory or the on-disk registry.
+        if provider_id == "local" {
+            if let Some(model_id) = model_id.as_deref() {
+                let model_exists = entry.default_model == model_id
+                    || entry.models.iter().any(|model| model.id == model_id)
+                    || local_inference_model_exists(model_id)?;
+                if !model_exists {
+                    return Err(agent_client_protocol::Error::invalid_params().data(format!(
+                        "Model '{model_id}' is not available for provider '{provider_id}'"
+                    )));
+                }
             }
         }
 

crates/goose/tests/acp_custom_requests_test.rs

@@ -901,6 +901,47 @@ fn test_custom_defaults_read() {
     });
 }
 
+// Regression test for #10401: a custom model that is not present in a provider's
+// (non-exhaustive) inventory must still be accepted when saved as the default from
+// a brand-new chat, matching the CLI and in-session model-selection paths where
+// custom/unlisted model entry is always allowed (#7255).
+#[test]
+#[serial]
+fn test_custom_defaults_save_allows_unlisted_model() {
+    let _env = env_lock::lock_env([("ANTHROPIC_API_KEY", Some("test-key"))]);
+    let config_dir = write_acp_global_config(
+        "GOOSE_MODEL: claude-3-5-haiku-latest\nGOOSE_PROVIDER: anthropic\n",
+    );
+
+    run_test(async move {
+        let openai = OpenAiFixture::new(vec![], Arc::new(EnforceSessionId::default())).await;
+        let config = TestConnectionConfig {
+            data_root: config_dir,
+            ..Default::default()
+        };
+        let conn = AcpServerConnection::new(config, openai).await;
+
+        let response = send_custom(
+            conn.cx(),
+            "_goose/unstable/defaults/save",
+            serde_json::json!({
+                "providerId": "anthropic",
+                "modelId": "custom-unlisted-model",
+            }),
+        )
+        .await
+        .expect("saving an unlisted custom model as the default should succeed");
+
+        assert_eq!(
+            response,
+            serde_json::json!({
+                "providerId": "anthropic",
+                "modelId": "custom-unlisted-model",
+            })
+        );
+    });
+}
+
 #[test]
 #[serial]
 fn test_custom_dictation_secret_save_delete() {