diff --git a/crates/mcp/src/tests.rs b/crates/mcp/src/tests.rs
index 9140ae7..48725c9 100644
--- a/crates/mcp/src/tests.rs
+++ b/crates/mcp/src/tests.rs
@@ -163,6 +163,7 @@ fn hostile_tool_arguments_are_tool_errors() {
         ("open", json!({"path": "/etc/passwd"})),
         ("open", json!({"path": "/no/such/model.fieldcraft"})),
         ("open", json!({"path": huge.clone() + ".solidcraft"})),
+        ("open", json!({"path": "/no/such/model.fieldcraft", "model": "new"})),
         ("save", json!({})),
         ("write_deck", json!({"format": "abaqus", "path": "/no/such/dir/x.inp"})),
         ("mesh", json!({"method": "tet", "params": {"body": "Nope", "tolerance": 0.5}})),
@@ -270,6 +271,7 @@ fn file_access_stays_inside_the_roots() {
     refused(&mut s, "open", json!({"path": input.join("link.solidcraft")}));
     refused(&mut s, "execute", json!({"command": "geometry.open", "params": {"path": input.join("../outside/c.solidcraft")}}));
     ok(&mut s, "open", json!({"path": input.join("c.solidcraft")}));
+    ok(&mut s, "open", json!({"path": input.join("c.solidcraft"), "model": "new"}));
     refused(&mut s, "save", json!({"path": outside.join("m.fieldcraft")}));
     refused(&mut s, "save", json!({"path": input.join("m.fieldcraft")}));
     refused(&mut s, "execute", json!({"command": "file.save_as", "params": {"path": outside.join("m.fieldcraft")}}));
diff --git a/crates/mcp/src/tools.rs b/crates/mcp/src/tools.rs
index ac7132a..fbedf79 100644
--- a/crates/mcp/src/tools.rs
+++ b/crates/mcp/src/tools.rs
@@ -139,9 +139,17 @@ pub fn tool_definitions() -> Vec<Value> {
             "open",
             "Open a file",
             "Open by extension: a .fieldcraft model (file.open: replaces the model; its design and meshes open again), a \
-             SolidCraft design .solidcraft (geometry.open: its bodies become the geometry) or a face-tagged .3mf mesh \
-             (geometry.import.mesh: added as a surface).",
-            obj(json!({"path": string("Path to a .fieldcraft, .solidcraft or .3mf file")}), &["path"]),
+             SolidCraft design .solidcraft (geometry.open: its bodies become the geometry; `model` \"keep\", the default, \
+             keeps the mesh and definitions for a reopen after an edit, \"new\" starts a new model; the reply's `related` \
+             and `stale` say what no longer resolves on another part) or a face-tagged .3mf mesh (geometry.import.mesh: \
+             added as a surface).",
+            obj(
+                json!({
+                    "path": string("Path to a .fieldcraft, .solidcraft or .3mf file"),
+                    "model": {"type": "string", "enum": ["keep", "new"], "description": "For a .solidcraft design: keep the current model (default) or start a new one"},
+                }),
+                &["path"],
+            ),
             false,
             true,
         ),
@@ -450,7 +458,14 @@ fn dispatch(b: &mut dyn Backend, name: &str, a: &Map<String, Value>) -> Result<V
                 Some("3mf") => "geometry.import.mesh",
                 _ => return Err("open: a .fieldcraft model, a .solidcraft design or a .3mf mesh".into()),
             };
-            let r = b.execute(command, json!({"path": path}))?;
+            let mut params = json!({"path": path});
+            if let Some(model) = a.get("model").filter(|m| !m.is_null()) {
+                if command != "geometry.open" {
+                    return Err("open: `model` applies to a .solidcraft design only".into());
+                }
+                params["model"] = model.clone();
+            }
+            let r = b.execute(command, params)?;
             Ok(json!({"command": command, "result": r}))
         }
         "save" => b.execute("file.save", pick(a, &["path"])),
diff --git a/docs/mcp.md b/docs/mcp.md
index 3a4d0a3..4ea5701 100644
--- a/docs/mcp.md
+++ b/docs/mcp.md
@@ -96,7 +96,7 @@ software on the computer.
 | `batch` | `commands: [{command, params}]` | Run steps in order, stop at the first error; the reply has every result and the failing step (`failed.index`, `failed.command`, `failed.error`). Steps before it stay applied (`undo` takes them back). |
 | `inspect_model` | — | `model.info` (counts, units, definitions), `model.activity` (the step table) and `geometry.bodies` (bodies with measures and persistent face names, which sets name as `{body, face}`). |
 | `check_model` | — | `model.check`: errors and warnings, each naming its item. |
-| `open` | `path` | By extension: `.fieldcraft` (`file.open`), `.solidcraft` (`geometry.open`), `.3mf` (`geometry.import.mesh`). |
+| `open` | `path`, `model?` | By extension: `.fieldcraft` (`file.open`), `.solidcraft` (`geometry.open`), `.3mf` (`geometry.import.mesh`). For a design, `model` is `keep` (default: the mesh and definitions stay, as a reopen after an edit wants) or `new` (a new model with the design); the reply's `related` is false when the design has none of the bodies the model refers to, and `stale` lists the mesh bodies and sets that no longer resolve. |
 | `save` | `path?` | `file.save`: to `path` or the model's own file. |
 | `write_deck` | `format` (abaqus, nastran, lsdyna), `path`, `steps?` | `deck.write`. |
 | `mesh` | `method`, `params?` | Runs the mesher (`tet`: `mesh.tet.generate`; `planar`: `mesh.planar`; the hex and block meshers as they land) and replies with its result, `model.info` and `mesh.quality`. |