diff --git a/modeling-cmds/openapi/api.json b/modeling-cmds/openapi/api.json
index 8c98f1cb..f181b2c2 100644
--- a/modeling-cmds/openapi/api.json
+++ b/modeling-cmds/openapi/api.json
@@ -665,6 +665,65 @@
           }
         ]
       },
+      "AssemblyConstraint": {
+        "description": "How to constrain bodies in an assembly.",
+        "oneOf": [
+          {
+            "description": "Makes two entities occupy the same line/plane or places a point on a surface.",
+            "type": "object",
+            "properties": {
+              "coincident": {
+                "type": "object",
+                "properties": {
+                  "targets": {
+                    "description": "IDs of two mate frames in the assembly.",
+                    "type": "array",
+                    "items": {
+                      "type": "string",
+                      "format": "uuid"
+                    },
+                    "minItems": 2,
+                    "maxItems": 2
+                  }
+                },
+                "required": [
+                  "targets"
+                ]
+              }
+            },
+            "required": [
+              "coincident"
+            ],
+            "additionalProperties": false
+          },
+          {
+            "description": "Aligns cylindrical, conical, or spherical surfaces to a common center.",
+            "type": "object",
+            "properties": {
+              "concentric": {
+                "type": "object",
+                "properties": {
+                  "targets": {
+                    "description": "Which edge? IDs of two mate frames in the assembly. Must be >= 2 items.",
+                    "type": "array",
+                    "items": {
+                      "type": "string",
+                      "format": "uuid"
+                    }
+                  }
+                },
+                "required": [
+                  "targets"
+                ]
+              }
+            },
+            "required": [
+              "concentric"
+            ],
+            "additionalProperties": false
+          }
+        ]
+      },
       "Axis": {
         "description": "Co-ordinate axis specifier.\n\nSee [cglearn.eu] for background reading.\n\n[cglearn.eu]: https://cglearn.eu/pub/computer-graphics/introduction-to-geometry#material-coordinate-systems-1",
         "oneOf": [
@@ -2532,6 +2591,78 @@
         "type": "number",
         "format": "double"
       },
+      "MateFrameOn": {
+        "description": "What to create a mate frame on",
+        "oneOf": [
+          {
+            "description": "Mate frame at the centroid of this face.",
+            "type": "object",
+            "properties": {
+              "face_centroid": {
+                "type": "object",
+                "properties": {
+                  "where_on_face": {
+                    "description": "Where on the given face?",
+                    "allOf": [
+                      {
+                        "$ref": "#/components/schemas/PointOnFace"
+                      }
+                    ]
+                  },
+                  "which_face": {
+                    "description": "Which face?",
+                    "type": "string",
+                    "format": "uuid"
+                  }
+                },
+                "required": [
+                  "where_on_face",
+                  "which_face"
+                ]
+              }
+            },
+            "required": [
+              "face_centroid"
+            ],
+            "additionalProperties": false
+          },
+          {
+            "description": "Mate frame along this edge.",
+            "type": "object",
+            "properties": {
+              "edge": {
+                "type": "object",
+                "properties": {
+                  "where_on_edge": {
+                    "description": "Where on the given edge?",
+                    "allOf": [
+                      {
+                        "$ref": "#/components/schemas/WhereOnEdge"
+                      }
+                    ]
+                  },
+                  "which_edge": {
+                    "description": "Which edge?",
+                    "allOf": [
+                      {
+                        "$ref": "#/components/schemas/EdgeSpecifier"
+                      }
+                    ]
+                  }
+                },
+                "required": [
+                  "where_on_edge",
+                  "which_edge"
+                ]
+              }
+            },
+            "required": [
+              "edge"
+            ],
+            "additionalProperties": false
+          }
+        ]
+      },
       "MbdSymbol": {
         "description": "MBD symbol type",
         "type": "string",
@@ -8190,6 +8321,99 @@
               "path_id",
               "type"
             ]
+          },
+          {
+            "description": "Create a new assembly",
+            "type": "object",
+            "properties": {
+              "children": {
+                "description": "Which entities, if any, to move into this assembly.",
+                "type": "array",
+                "items": {
+                  "type": "string",
+                  "format": "uuid"
+                }
+              },
+              "type": {
+                "type": "string",
+                "enum": [
+                  "assembly_create"
+                ]
+              }
+            },
+            "required": [
+              "type"
+            ]
+          },
+          {
+            "description": "Add children to the assembly",
+            "type": "object",
+            "properties": {
+              "children": {
+                "description": "Which entities, to move into this assembly.",
+                "type": "array",
+                "items": {
+                  "type": "string",
+                  "format": "uuid"
+                }
+              },
+              "type": {
+                "type": "string",
+                "enum": [
+                  "assembly_add_children"
+                ]
+              }
+            },
+            "required": [
+              "type"
+            ]
+          },
+          {
+            "description": "Create a mate frame",
+            "type": "object",
+            "properties": {
+              "on": {
+                "description": "What to create the mate frame on.",
+                "allOf": [
+                  {
+                    "$ref": "#/components/schemas/MateFrameOn"
+                  }
+                ]
+              },
+              "type": {
+                "type": "string",
+                "enum": [
+                  "mate_frame_create"
+                ]
+              }
+            },
+            "required": [
+              "on",
+              "type"
+            ]
+          },
+          {
+            "description": "Add constraints to an assembly.",
+            "type": "object",
+            "properties": {
+              "constraints": {
+                "description": "Which constraints to apply.",
+                "type": "array",
+                "items": {
+                  "$ref": "#/components/schemas/AssemblyConstraint"
+                }
+              },
+              "type": {
+                "type": "string",
+                "enum": [
+                  "assembly_add_constraints"
+                ]
+              }
+            },
+            "required": [
+              "constraints",
+              "type"
+            ]
           }
         ]
       },
@@ -9118,6 +9342,57 @@
           "z"
         ]
       },
+      "PointOnFace": {
+        "description": "Identifies a point on a specific face.",
+        "oneOf": [
+          {
+            "description": "Centermost point of the face.",
+            "type": "object",
+            "properties": {
+              "centroid": {
+                "type": "object"
+              }
+            },
+            "required": [
+              "centroid"
+            ],
+            "additionalProperties": false
+          },
+          {
+            "description": "Somewhere along the face.",
+            "type": "object",
+            "properties": {
+              "at_parameter": {
+                "type": "object",
+                "properties": {
+                  "u": {
+                    "description": "Value for the face's 2D parametric equation.",
+                    "type": "number",
+                    "format": "double",
+                    "minimum": 0.0,
+                    "maximum": 1.0
+                  },
+                  "v": {
+                    "description": "Value for the face's 2D parametric equation.",
+                    "type": "number",
+                    "format": "double",
+                    "minimum": 0.0,
+                    "maximum": 1.0
+                  }
+                },
+                "required": [
+                  "u",
+                  "v"
+                ]
+              }
+            },
+            "required": [
+              "at_parameter"
+            ],
+            "additionalProperties": false
+          }
+        ]
+      },
       "PrimitiveTopologyFallback": {
         "description": "Optional fallback when primary UUIDs are missing from the client artifact graph (e.g. stale or engine-only ids). Identifies the same topology via a **parent** entity UUID and a **primitive index** on that parent.\n\nSemantics by selection kind (aligned with engine BREP topology):\n\n- **Face / Edge (3D)**: `parent_id` is the owning [`EntityType::Solid3D`] body UUID; `primitive_index` matches the index returned by **EntityGetPrimitiveIndex** for that face or edge (and matches **EntityGetParentId** → parent + **EntityGetPrimitiveIndex** → index). - **Vertex (3D)**: same `parent_id` (solid); `primitive_index` is the BREP vertex index on that solid. - **Solid2dEdge**: `parent_id` is the **Solid2D** profile UUID; `primitive_index` is the curve index within that profile. - **Segment**: `parent_id` is the **Path** UUID; `primitive_index` is the curve index within that path.\n\nOther [`EntityReference`] variants may omit this field or leave it unset when not applicable.",
         "type": "object",
@@ -10178,6 +10453,49 @@
           }
         ]
       },
+      "WhereOnEdge": {
+        "description": "Identifies a point on a specific edge.",
+        "oneOf": [
+          {
+            "description": "Midpoint along the edge.",
+            "type": "object",
+            "properties": {
+              "midpoint": {
+                "type": "object"
+              }
+            },
+            "required": [
+              "midpoint"
+            ],
+            "additionalProperties": false
+          },
+          {
+            "description": "Somewhere along the edge's length.",
+            "type": "object",
+            "properties": {
+              "at_parameter": {
+                "type": "object",
+                "properties": {
+                  "t": {
+                    "description": "Value which, substituted into the edge's parametric equation, gives a point. Must be between 0 and 1 (inclusive).",
+                    "type": "number",
+                    "format": "double",
+                    "minimum": 0.0,
+                    "maximum": 1.0
+                  }
+                },
+                "required": [
+                  "t"
+                ]
+              }
+            },
+            "required": [
+              "at_parameter"
+            ],
+            "additionalProperties": false
+          }
+        ]
+      },
       "WorldCoordinateSystem": {
         "type": "string",
         "enum": [
diff --git a/modeling-cmds/src/def_enum.rs b/modeling-cmds/src/def_enum.rs
index df4fce5f..76d2dead 100644
--- a/modeling-cmds/src/def_enum.rs
+++ b/modeling-cmds/src/def_enum.rs
@@ -25,6 +25,8 @@ define_modeling_cmd_enum! {
             length_unit::LengthUnit,
             shared::{
                 Angle,
+                AssemblyConstraint,
+                MateFrameOn,
                 RegionVersion,
                 BlendType,
                 BodyType,
@@ -2422,7 +2424,7 @@ define_modeling_cmd_enum! {
         ///
         /// Most successful unions come from solids who's faces do not overlap
         /// aka non-coplanar.
-        /// 
+        ///
         /// Failure cases:
         /// * A common failure is unsupported coincident faces try to be unioned.
         ///
@@ -2491,7 +2493,7 @@ define_modeling_cmd_enum! {
         /// aka non-coplanar.
         ///
         /// Prefer one `tool` over multiple when calling this feature.
-        /// 
+        ///
         /// Failure cases:
         /// * A common failure is unsupported coplanar faces try to be unioned.
         ///
@@ -2838,6 +2840,53 @@ define_modeling_cmd_enum! {
             /// Which path to query
             pub path_id: ModelingCmdId,
         }
+
+        /// Create a new assembly
+        #[derive(Debug, Clone, PartialEq, Serialize, Deserialize, JsonSchema, ModelingCmdVariant, Builder)]
+        #[cfg_attr(feature = "ts-rs", derive(ts_rs::TS))]
+        #[cfg_attr(feature = "arbitrary", derive(arbitrary::Arbitrary))]
+        #[cfg_attr(feature = "ts-rs", ts(export_to = "ModelingCmd.ts"))]
+        #[cfg_attr(not(feature = "unstable_exhaustive"), non_exhaustive)]
+        pub struct AssemblyCreate {
+            /// Which entities, if any, to move into this assembly.
+            #[serde(default, skip_serializing_if = "Vec::is_empty")]
+            pub children: Vec<Uuid>,
+        }
+
+        /// Add children to the assembly
+        #[derive(Debug, Clone, PartialEq, Serialize, Deserialize, JsonSchema, ModelingCmdVariant, Builder)]
+        #[cfg_attr(feature = "ts-rs", derive(ts_rs::TS))]
+        #[cfg_attr(feature = "arbitrary", derive(arbitrary::Arbitrary))]
+        #[cfg_attr(feature = "ts-rs", ts(export_to = "ModelingCmd.ts"))]
+        #[cfg_attr(not(feature = "unstable_exhaustive"), non_exhaustive)]
+        pub struct AssemblyAddChildren {
+            /// Which entities, to move into this assembly.
+            #[serde(default, skip_serializing_if = "Vec::is_empty")]
+            pub children: Vec<Uuid>,
+        }
+
+        /// Create a mate frame
+        #[derive(Debug, Clone, PartialEq, Serialize, Deserialize, JsonSchema, ModelingCmdVariant, Builder)]
+        #[cfg_attr(feature = "ts-rs", derive(ts_rs::TS))]
+        #[cfg_attr(feature = "arbitrary", derive(arbitrary::Arbitrary))]
+        #[cfg_attr(feature = "ts-rs", ts(export_to = "ModelingCmd.ts"))]
+        #[cfg_attr(not(feature = "unstable_exhaustive"), non_exhaustive)]
+        pub struct MateFrameCreate {
+            /// What to create the mate frame on.
+            pub on: MateFrameOn,
+        }
+
+        /// Add constraints to an assembly.
+        #[derive(Debug, Clone, PartialEq, Serialize, Deserialize, JsonSchema, ModelingCmdVariant, Builder)]
+        #[cfg_attr(feature = "ts-rs", derive(ts_rs::TS))]
+        #[cfg_attr(feature = "arbitrary", derive(arbitrary::Arbitrary))]
+        #[cfg_attr(feature = "ts-rs", ts(export_to = "ModelingCmd.ts"))]
+        #[cfg_attr(not(feature = "unstable_exhaustive"), non_exhaustive)]
+        pub struct AssemblyAddConstraints {
+            /// Which constraints to apply.
+            pub constraints: Vec<AssemblyConstraint>,
+        }
+
     }
 }
 
diff --git a/modeling-cmds/src/ok_response.rs b/modeling-cmds/src/ok_response.rs
index 779b6c19..6179bd5f 100644
--- a/modeling-cmds/src/ok_response.rs
+++ b/modeling-cmds/src/ok_response.rs
@@ -25,6 +25,7 @@ define_ok_modeling_cmd_response_enum! {
             CameraViewState,
             BodyType,
             EntityReference,
+            SolvedAssemblyItem,
         };
         use std::collections::HashMap;
 
@@ -1427,5 +1428,31 @@ define_ok_modeling_cmd_response_enum! {
             /// How many regions the Toolpaths library thinks exist
             pub region_count: u16,
         }
+
+        /// The response from the 'AssemblyCreate'.
+        #[derive(Debug, Serialize, Deserialize, Clone, PartialEq, JsonSchema, ModelingCmdOutput)]
+        #[cfg_attr(not(feature = "unstable_exhaustive"), non_exhaustive)]
+        pub struct AssemblyCreate {
+        }
+
+        /// The response from the 'AssemblyAddChildren'.
+        #[derive(Debug, Serialize, Deserialize, Clone, PartialEq, JsonSchema, ModelingCmdOutput)]
+        #[cfg_attr(not(feature = "unstable_exhaustive"), non_exhaustive)]
+        pub struct AssemblyAddChildren {
+        }
+
+        /// The response from the 'MateFrameCreate'.
+        #[derive(Debug, Serialize, Deserialize, Clone, PartialEq, JsonSchema, ModelingCmdOutput)]
+        #[cfg_attr(not(feature = "unstable_exhaustive"), non_exhaustive)]
+        pub struct MateFrameCreate {
+        }
+
+        /// The response from the 'AssemblyAddConstraints'.
+        #[derive(Debug, Serialize, Deserialize, Clone, PartialEq, JsonSchema, ModelingCmdOutput)]
+        #[cfg_attr(not(feature = "unstable_exhaustive"), non_exhaustive)]
+        pub struct AssemblyAddConstraints {
+            /// For each item in the assembly, map its ID to its solution
+            pub solution: HashMap<Uuid, SolvedAssemblyItem>,
+        }
     }
 }
diff --git a/modeling-cmds/src/shared.rs b/modeling-cmds/src/shared.rs
index c1c6e20a..09a643c8 100644
--- a/modeling-cmds/src/shared.rs
+++ b/modeling-cmds/src/shared.rs
@@ -2248,3 +2248,109 @@ pub enum CurveTypeDebug {
     /// Circle with a center and radius.
     Circle,
 }
+
+/// What to create a mate frame on
+#[derive(Debug, Serialize, Deserialize, Clone, PartialEq, JsonSchema)]
+#[serde(rename_all = "snake_case")]
+#[cfg_attr(feature = "ts-rs", derive(ts_rs::TS))]
+#[cfg_attr(feature = "arbitrary", derive(arbitrary::Arbitrary))]
+#[cfg_attr(feature = "ts-rs", ts(export_to = "ModelingCmd.ts"))]
+#[cfg_attr(not(feature = "unstable_exhaustive"), non_exhaustive)]
+pub enum MateFrameOn {
+    /// Mate frame at the centroid of this face.
+    FaceCentroid {
+        /// Which face?
+        which_face: Uuid,
+        /// Where on the given face?
+        where_on_face: PointOnFace,
+    },
+    /// Mate frame along this edge.
+    Edge {
+        /// Which edge?
+        which_edge: EdgeSpecifier,
+        /// Where on the given edge?
+        where_on_edge: WhereOnEdge,
+    },
+}
+
+/// Identifies a point on a specific face.
+#[derive(Debug, Serialize, Deserialize, Clone, PartialEq, JsonSchema)]
+#[serde(rename_all = "snake_case")]
+#[cfg_attr(feature = "ts-rs", derive(ts_rs::TS))]
+#[cfg_attr(feature = "arbitrary", derive(arbitrary::Arbitrary))]
+#[cfg_attr(feature = "ts-rs", ts(export_to = "ModelingCmd.ts"))]
+#[cfg_attr(not(feature = "unstable_exhaustive"), non_exhaustive)]
+pub enum PointOnFace {
+    /// Centermost point of the face.
+    Centroid {},
+    /// Somewhere along the face.
+    AtParameter {
+        /// Value for the face's 2D parametric equation.
+        #[schemars(range(min = 0.0, max = 1.0))]
+        u: f64,
+        /// Value for the face's 2D parametric equation.
+        #[schemars(range(min = 0.0, max = 1.0))]
+        v: f64,
+    },
+}
+
+/// Identifies a point on a specific edge.
+#[derive(Debug, Serialize, Deserialize, Clone, PartialEq, JsonSchema)]
+#[serde(rename_all = "snake_case")]
+#[cfg_attr(feature = "ts-rs", derive(ts_rs::TS))]
+#[cfg_attr(feature = "arbitrary", derive(arbitrary::Arbitrary))]
+#[cfg_attr(feature = "ts-rs", ts(export_to = "ModelingCmd.ts"))]
+#[cfg_attr(not(feature = "unstable_exhaustive"), non_exhaustive)]
+pub enum WhereOnEdge {
+    /// Midpoint along the edge.
+    Midpoint {},
+    /// Somewhere along the edge's length.
+    AtParameter {
+        /// Value which, substituted into the edge's parametric equation,
+        /// gives a point. Must be between 0 and 1 (inclusive).
+        #[schemars(range(min = 0.0, max = 1.0))]
+        t: f64,
+    },
+}
+
+/// How to constrain bodies in an assembly.
+#[derive(Debug, Serialize, Deserialize, Clone, PartialEq, JsonSchema)]
+#[serde(rename_all = "snake_case")]
+#[cfg_attr(feature = "ts-rs", derive(ts_rs::TS))]
+#[cfg_attr(feature = "arbitrary", derive(arbitrary::Arbitrary))]
+#[cfg_attr(feature = "ts-rs", ts(export_to = "ModelingCmd.ts"))]
+#[cfg_attr(not(feature = "unstable_exhaustive"), non_exhaustive)]
+pub enum AssemblyConstraint {
+    /// Makes two entities occupy the same line/plane or places a point on a surface.
+    Coincident {
+        /// IDs of two mate frames in the assembly.
+        targets: [Uuid; 2],
+    },
+    /// Aligns cylindrical, conical, or spherical surfaces to a common center.
+    Concentric {
+        /// Which edge?
+        /// IDs of two mate frames in the assembly.
+        /// Must be >= 2 items.
+        #[schemars(range(min = 2))]
+        targets: Vec<Uuid>,
+    },
+}
+
+/// Result of constraining an item in an assembly.
+/// This is in the coordinate system of the assembly's parent.
+#[derive(Debug, Serialize, Deserialize, Clone, PartialEq, JsonSchema)]
+#[serde(rename_all = "snake_case")]
+#[cfg_attr(feature = "ts-rs", derive(ts_rs::TS))]
+#[cfg_attr(feature = "arbitrary", derive(arbitrary::Arbitrary))]
+#[cfg_attr(feature = "ts-rs", ts(export_to = "ModelingCmd.ts"))]
+#[cfg_attr(not(feature = "unstable_exhaustive"), non_exhaustive)]
+pub struct SolvedAssemblyItem {
+    /// How to translate the item.
+    pub translate_by: Point3d<LengthUnit>,
+    /// How to rotate the item.
+    pub roll: Angle,
+    /// How to rotate the item.
+    pub pitch: Angle,
+    /// How to rotate the item.
+    pub yaw: Angle,
+}