Add design containers with deduplicated blobs and JSON compatibility (#327)

* Add fast design containers with lossless deduplicated payloads

* Keep display caching opt-in to avoid default save overhead

crates/mcp/src/tests.rs

@@ -178,6 +178,38 @@ fn agent_builds_box_fillet_hole() {
     let _ = std::fs::remove_dir_all(&dir);
 }
 
+#[test]
+fn save_selects_container_or_json_and_reopens_by_contents() {
+    let mut s = server();
+    tool(&mut s, "execute", json!({"command": "solid.box", "params": {"length": 10, "width": 20, "height": 30}}));
+    let dir = std::env::temp_dir().join(format!("solidcraft-mcp-encodings-{}", std::process::id()));
+    std::fs::create_dir_all(&dir).unwrap();
+    for (name, format, is_json) in [
+        ("default.solidcraft", None, false),
+        ("explicit.solidcraft", Some("json"), true),
+        ("legacy.solvecraft", None, false),
+        ("inferred.json", None, true),
+        ("container.json", Some("container"), false),
+    ] {
+        let path = dir.join(name);
+        let mut args = json!({"path": path.to_string_lossy()});
+        if let Some(format) = format {
+            args["format"] = json!(format);
+        }
+        tool(&mut s, "save", args);
+        let bytes = std::fs::read(&path).unwrap();
+        if is_json {
+            assert!(serde_json::from_slice::<Value>(&bytes).unwrap().is_object());
+        } else {
+            assert!(bytes.starts_with(b"PK\x03\x04"));
+        }
+        tool(&mut s, "new_design", json!({}));
+        tool(&mut s, "open", json!({"path": path.to_string_lossy()}));
+        assert!(rel(volume(&mut s), 6000.0) < 1e-9);
+    }
+    std::fs::remove_dir_all(&dir).unwrap();
+}
+
 #[test]
 fn parameters_drive_the_design() {
     let mut s = server();

crates/mcp/src/tools.rs

@@ -224,8 +224,11 @@ pub fn tool_definitions() -> Vec<Value> {
         tool(
             "save",
             "Save the design",
-            "Save the design as a .solidcraft file (to `path`, or to the file it came from).",
-            obj(json!({"path": string("Path to save to (default: the current file)")}), &[]),
+            "Save the design to `path` or its current file. The default is a design container; a .json path selects plain JSON. Set format to container or json to override the path.",
+            obj(
+                json!({"path": string("Path to save to (default: the current file)"), "format": {"type": "string", "enum": ["container", "json"], "description": "Design encoding (default: JSON for .json, container otherwise)"}}),
+                &[],
+            ),
             false,
             false,
         ),
@@ -378,7 +381,7 @@ fn dispatch(b: &mut dyn Backend, name: &str, a: &Map<String, Value>) -> Result<V
         "redo" => exec(b, "edit.redo", json!({})),
         "new_design" => exec(b, "file.new", pick(a, &["name"])),
         "open" => exec(b, "doc.open", pick(a, &["path"])),
-        "save" => exec(b, "file.save", pick(a, &["path"])),
+        "save" => exec(b, "file.save", pick(a, &["path", "format"])),
         other => Err(format!("unknown tool `{other}`")),
     }
 }