timeline.inspect returns a feature's definition in the shape timeline.edit's set takes, so agents no longer guess field names: timeline.edit had no documented format ({"extent": {"distance": …}} worked, but only by trying). The new read-only command (feature: id or name) returns type, definition (the editable fields and their current values; bookkeeping and large payloads such as sketch geometry and imported STEP text are left out and listed under omitted), inputs (label, parameter name, expression, unit) and the feature's error and warning. timeline.edit's params doc points to it, and the book's Solid features chapter documents the edit format with the fields of the common features. Tests: inspect, edit by the field names it shows, inspect again (extrude, fillet, sketch); no undo step; hostile feature arguments are refused

crates/engine/src/tests.rs

@@ -407,6 +407,46 @@ fn sample_edges_chain_and_faces() {
     assert!(m.edge_faces.iter().all(|f| !f.is_empty()));
 }
 
+/// timeline.inspect returns a feature's definition in the shape timeline.edit's `set` takes, so
+/// an agent can read the field names instead of guessing them.
+#[test]
+fn timeline_inspect_shows_what_timeline_edit_changes() {
+    let mut s = Session::default();
+    run(&mut s, "sketch.create", json!({"plane": "XY", "name": "Base"}));
+    run(&mut s, "sketch.rectangle.two_point", json!({"p0": [0, 0], "p1": [40, 30]}));
+    run(&mut s, "sketch.finish", json!({}));
+    run(&mut s, "solid.extrude", json!({"distance": 10, "name": "Slab"}));
+    run(&mut s, "solid.fillet", json!({"edges": [[20, 0, 10]], "radius": 2, "name": "Round"}));
+    let i = run(&mut s, "timeline.inspect", json!({"feature": "Slab"}));
+    assert_eq!(i["type"], "ExtrudeFeature", "{i}");
+    assert_eq!(i["definition"]["extent"]["distance"], "10", "{i}");
+    assert_eq!(i["definition"]["operation"], "new_body", "{i}");
+    assert!(i["definition"].get("type").is_none() && i["definition"].get("param_names").is_none(), "{i}");
+    assert_eq!(i["inputs"][0]["label"], "Distance", "{i}");
+    assert_eq!(i["inputs"][0]["unit"], "mm", "{i}");
+    let d = i["inputs"][0]["param"].as_str().unwrap().to_string();
+    // The definition's field names are what timeline.edit takes.
+    run(&mut s, "timeline.edit", json!({"feature": "Slab", "set": {"extent": {"distance": "12"}}}));
+    let i = run(&mut s, "timeline.inspect", json!({"feature": i["id"]}));
+    assert_eq!(i["definition"]["extent"]["distance"], "12", "{i}");
+    assert_eq!(i["inputs"][0]["param"], d.as_str(), "{i}");
+    let f = run(&mut s, "timeline.inspect", json!({"feature": "Round"}));
+    assert_eq!(f["definition"]["radius"], "2", "{f}");
+    run(&mut s, "timeline.edit", json!({"feature": "Round", "set": {"radius": "3"}}));
+    assert_eq!(run(&mut s, "timeline.inspect", json!({"feature": "Round"}))["definition"]["radius"], "3");
+    // A sketch leaves its geometry out (sketch.inspect has it) and says so.
+    let k = run(&mut s, "timeline.inspect", json!({"feature": "Base"}));
+    assert!(k["definition"].get("sketch").is_none() && k["definition"].get("plane").is_some(), "{k}");
+    assert_eq!(k["omitted"][0]["field"], "sketch", "{k}");
+    // Read-only: no undo step; bad input is refused.
+    let undo = s.undo.len();
+    run(&mut s, "timeline.inspect", json!({"feature": "Slab"}));
+    assert_eq!(s.undo.len(), undo);
+    for bad in [json!({}), json!({"feature": "Nope"}), json!({"feature": [1]}), json!({"feature": 1e300}), json!({"feature": null})] {
+        assert!(s.execute("timeline.inspect", &bad).is_err(), "{bad}");
+    }
+}
+
 #[test]
 fn timeline_edits_reresolve_references() {
     let mut s = Session::default();