mcp: select and selection tools for the app's viewport selection

Bridged to a running app, select and selection call ui.select and
ui.selection; headless they are errors (selection belongs to the
window).

Closes #73

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01NmAttHhUZTokgPeEJia8sm

crates/mcp/src/backend.rs

@@ -1,6 +1,6 @@
 //! Where tool calls end up: an in-process headless session, or a running app's control channel
 //! (docs/control-protocol.md: `engine.execute`, `engine.commands`, `ui.view`, `ui.camera`,
-//! `ui.render`, `ui.capture`).
+//! `ui.render`, `ui.capture`, `ui.select`, `ui.selection`).
 
 use std::io::{BufRead, BufReader, Read, Write};
 use std::net::{TcpStream, ToSocketAddrs};
@@ -43,6 +43,8 @@ pub trait Backend {
     fn commands(&mut self) -> Result<Value, String>;
     /// A PNG of the model, or of the app's window when one is attached.
     fn picture(&mut self, request: &PictureRequest) -> Result<Picture, String>;
+    /// A request to the app's window (`ui.select`, `ui.selection`): an error without one.
+    fn window(&mut self, method: &str, params: Value) -> Result<Value, String>;
     /// The folders file access is confined to (checked by the server for the files it writes
     /// itself).
     fn roots(&self) -> &Roots;
@@ -124,6 +126,10 @@ impl Backend for Headless {
         Ok(Picture { png, info })
     }
 
+    fn window(&mut self, method: &str, _params: Value) -> Result<Value, String> {
+        Err(format!("{method} needs the app's window: start the server with --connect to a running FieldCraft"))
+    }
+
     fn roots(&self) -> &Roots {
         &self.roots
     }
@@ -267,6 +273,10 @@ impl Backend for Remote {
         read
     }
 
+    fn window(&mut self, method: &str, params: Value) -> Result<Value, String> {
+        self.call(method, params)
+    }
+
     fn roots(&self) -> &Roots {
         &self.roots
     }

crates/mcp/src/tests.rs

@@ -78,6 +78,8 @@ fn initialize_lists_tools_and_resources() {
         "probe",
         "reactions",
         "screenshot",
+        "select",
+        "selection",
         "undo",
         "redo",
     ] {
@@ -181,6 +183,9 @@ fn hostile_tool_arguments_are_tool_errors() {
         ("screenshot", json!({"camera": {"yaw": 0, "pitch": 0}, "view": "top"})),
         ("batch", json!({"commands": [{"command": "model.info"}, {"params": {}}, {"command": "model.info"}]})),
         ("batch", json!({"commands": vec![json!({"command": "model.info"}); crate::tools::MAX_STEPS + 1]})),
+        ("select", json!({"filter": "faces"})),
+        ("select", json!({})),
+        ("selection", json!({})),
         ("undo", json!({})),
         ("redo", json!({})),
     ] {

crates/mcp/src/tools.rs

@@ -290,6 +290,41 @@ pub fn tool_definitions() -> Vec<Value> {
             true,
             false,
         ),
+        tool(
+            "select",
+            "Select in the viewport",
+            "Change the app window's viewport selection (needs --connect; ui.select): `filter` (faces, edges, vertices, \
+             bodies, nodes, element_faces, elements), `mode` (replace, add, remove, toggle), `clear`, items by name and \
+             label as model.set.create takes them (`geometry`, `nodes`, `elements`, `faces`), a pick at a window point \
+             `at` [x, y] or the visible items in a box `box` [x0, y0, x1, y1]. Returns the selection; its `params` are \
+             a set's members.",
+            obj(
+                json!({
+                    "filter": {"type": "string", "enum": ["faces", "edges", "vertices", "bodies", "nodes", "element_faces", "elements"]},
+                    "box_mode": {"type": "boolean", "description": "Box selection on or off in the window"},
+                    "mode": {"type": "string", "enum": ["replace", "add", "remove", "toggle"]},
+                    "clear": {"type": "boolean", "description": "Empty the selection first"},
+                    "geometry": {"type": "array", "items": {"type": "object"}, "description": "[{body, face|edge|vertex}]"},
+                    "nodes": {"type": "array", "items": {"type": "integer"}},
+                    "elements": {"type": "array", "items": {"type": "integer"}},
+                    "faces": {"type": "array", "items": {"type": "object"}, "description": "[{element, face}]"},
+                    "at": {"type": "array", "items": {"type": "number"}, "description": "Window point [x, y] to pick at"},
+                    "box": {"type": "array", "items": {"type": "number"}, "description": "Window box [x0, y0, x1, y1]"},
+                }),
+                &[],
+            ),
+            false,
+            false,
+        ),
+        tool(
+            "selection",
+            "Read the viewport selection",
+            "The app window's viewport selection (needs --connect; ui.selection): filter, items, `params` (the \
+             selection as a set's members, ready for model.set.create) and what is under the pointer.",
+            obj(json!({}), &[]),
+            true,
+            false,
+        ),
         tool("undo", "Undo", "Take back the last command that changed the model.", obj(json!({}), &[]), false, false),
         tool("redo", "Redo", "Run the last undone change again.", obj(json!({}), &[]), false, false),
     ]
@@ -440,6 +475,14 @@ fn dispatch(b: &mut dyn Backend, name: &str, a: &Map<String, Value>) -> Result<V
         "show_results" => show_results(b, a),
         "probe" => b.execute("solve.probe", pick(a, &["step", "nodes", "elements", "points", "forces", "mode", "frame"])),
         "reactions" => b.execute("results.reactions", pick(a, &["step", "sets", "about"])),
+        "select" => {
+            let p = pick(a, &["filter", "box_mode", "mode", "clear", "geometry", "nodes", "elements", "faces", "at", "box"]);
+            if p.as_object().is_none_or(|o| o.is_empty()) {
+                return Err("select: give `filter`, `mode`, `clear`, items, `at` or `box`".into());
+            }
+            b.window("ui.select", p)
+        }
+        "selection" => b.window("ui.selection", json!({})),
         "undo" => b.execute("edit.undo", json!({})),
         "redo" => b.execute("edit.redo", json!({})),
         other => Err(format!("unknown tool `{}`", other.chars().take(100).collect::<String>())),

docs/mcp.md

@@ -105,6 +105,8 @@ software on the computer.
 | `probe` | `step?`, `nodes?`, `elements?`, `points?`, `forces?`, `mode?`, `frame?` | `solve.probe`: stored results at nodes, elements (explicit) or points (fluid). |
 | `reactions` | `step?`, `sets?`, `about?` | `results.reactions`: reaction force and moment per node set and in total. |
 | `screenshot` | `view?`, `camera?`, `source?` (window, model), `width?`, `height?`, `path?` | A PNG image content block and what it shows. Headless: a CPU render framing the model, the geometry, the analysis setup's symbols (boundary conditions, loads, reference points, couplings; `sampled_symbols` lists definitions drawn with a sample) and the results display (deformed shape, contour bands). Bridged: the whole app window drawn offscreen (`ui.capture`, the default) or a model render with the app's camera (`ui.render`). `view` turns the camera to a standard view first; `camera` `{yaw, pitch, distance?}` to any orientation, in degrees (yaw 0 looks along +Y, −45 is the iso view's; pitch is the elevation, clamped to ±90) and model units (`ui.camera`; headless, the picture is framed unless `distance` is given). Give one of the two; the orientation is kept for the next picture, so a sequence of `camera` angles orbits smoothly. |
+| `select` | `filter?`, `box_mode?`, `mode?`, `clear?`, `geometry?`, `nodes?`, `elements?`, `faces?`, `at?`, `box?` | Bridged only: changes the app's viewport selection (`ui.select`; control-protocol.md, Selection) and returns it. Headless it is an error: selection belongs to the window. |
+| `selection` | — | Bridged only: the viewport selection (`ui.selection`), with `params`, the selection as a set's members for `model.set.create`. |
 | `undo`, `redo` | — | `edit.undo`, `edit.redo`. |
 
 Resources: `fieldcraft://model` (`model.info`) and `fieldcraft://commands` (the command catalog).