From fe162f8f2326c89e8adf0d6743fd24ecfa44e0dc Mon Sep 17 00:00:00 2001
From: MarekSpodBiedry <73122968+MarekSpodBiedry@users.noreply.github.com>
Date: Sat, 10 Oct 2026 19:04:03 +0200
Subject: [PATCH] Report an error when file commands get no file in MCP and CLI
 (#135)

---
 apps/gridcraft-cli/src/main.rs |  3 +--
 crates/engine/src/lib.rs       | 19 +++++++++++++++++++
 crates/mcp/src/headless.rs     |  8 ++------
 crates/mcp/src/tests.rs        | 11 +++++++++++
 4 files changed, 33 insertions(+), 8 deletions(-)

diff --git a/apps/gridcraft-cli/src/main.rs b/apps/gridcraft-cli/src/main.rs
index c97b1d637..452448e4f 100644
--- a/apps/gridcraft-cli/src/main.rs
+++ b/apps/gridcraft-cli/src/main.rs
@@ -145,8 +145,7 @@ impl Args {
 // ---------------------------------------------------------------- helpers
 
 fn exec(s: &mut Session, id: &str, params: Value) -> Result<Value, String> {
-    let r = s.execute(id, params).map_err(|e| e.to_string());
-    s.take_ui_requests();
+    let r = s.execute_headless(id, params).map_err(|e| e.to_string());
     if id == "file.open"
         && let Ok(value) = &r
         && let Some(warnings) = value.get("warnings").and_then(Value::as_array)
diff --git a/crates/engine/src/lib.rs b/crates/engine/src/lib.rs
index a13e5cef1..de786a1f4 100644
--- a/crates/engine/src/lib.rs
+++ b/crates/engine/src/lib.rs
@@ -311,6 +311,25 @@ impl Session {
     pub fn take_ui_requests(&mut self) -> Vec<UiRequest> {
         std::mem::take(&mut self.ui_requests)
     }
+
+    /// Runs a command for a caller with no UI (MCP, CLI): queued UI requests are dropped, and a command that only
+    /// queued a file dialog because it got no file (`file.open`, `file.saveAs`, `file.save` on a workbook with no
+    /// file yet, `data.getData`) is an error instead of a silent success.
+    pub fn execute_headless(&mut self, id: &str, params: Json) -> Result<Json> {
+        let r = self.execute(id, params);
+        let requests = self.take_ui_requests();
+        let asked_for_file = requests.iter().any(|q| matches!(q, UiRequest::Dialog(name, _) if name == "open" || name == "saveAs"));
+        if r.is_ok() && asked_for_file {
+            let what = match id {
+                "file.open" => "`path` or `base64`",
+                "data.getData" | "data.fromTextCsv" => "`path` or `text`",
+                _ => "`path`",
+            };
+            let msg = format!("missing {what}: programmatic calls never open dialogs");
+            return Err(EngineError::BadParams { cmd: id.to_string(), msg });
+        }
+        r
+    }
 }
 
 /// Convenience: the active workbook's active sheet.
diff --git a/crates/mcp/src/headless.rs b/crates/mcp/src/headless.rs
index ec649ea08..5f96fbd25 100644
--- a/crates/mcp/src/headless.rs
+++ b/crates/mcp/src/headless.rs
@@ -34,12 +34,8 @@ impl Headless {
     }
 
     fn execute(&mut self, id: &str, params: Value) -> Result<Value, String> {
-        let r = self.session.execute(id, params).map_err(|e| e.to_string());
-        // Programmatic calls never open dialogs; drop anything a command queued for a UI.
-        for req in self.session.take_ui_requests() {
-            log::debug!("headless: ignoring UI request {req:?}");
-        }
-        r
+        // Programmatic calls never open dialogs; anything a command queued for a UI is dropped.
+        self.session.execute_headless(id, params).map_err(|e| e.to_string())
     }
 
     fn commands(&self, params: &Value) -> Value {
diff --git a/crates/mcp/src/tests.rs b/crates/mcp/src/tests.rs
index 8e37a21e7..cf590bedb 100644
--- a/crates/mcp/src/tests.rs
+++ b/crates/mcp/src/tests.rs
@@ -381,3 +381,14 @@ fn remote_backend_over_tcp() {
     handle.join().unwrap();
     assert!(Remote::connect("127.0.0.1:1").is_err());
 }
+
+#[test]
+fn file_commands_without_a_file_fail() {
+    let mut s = server();
+    // `file.save` on Book1, which has no file yet, would ask for one too.
+    for command in ["file.open", "file.saveAs", "file.save", "data.getData", "data.fromTextCsv"] {
+        let msg = fails(&mut s, "execute_command", json!({"command": command}));
+        assert!(msg.contains("missing") && msg.contains("never open dialogs"), "{command}: {msg}");
+        fails(&mut s, "execute_command", json!({"command": command, "params": {}}));
+    }
+}