UMG MCP Authoring

UMG MCP Authoring

Use MCP tools as the hands and this skill as the experience. Do not mutate .uasset files directly and do not bypass MCP with raw socket scripts when validating the public protocol.

For the five-system login demo and concrete script workflow, read login-demo.md.

Workflow

1. Set the attention target first with set_target_umg_asset.

   • Passing a missing /Game/... UMG path creates a WidgetBlueprint.
   • Use set_target_widget only when the next operation is intentionally scoped to a widget.
   • Prefer omitted/default path arguments after target is established.

2. Build widget structure with panel parents.

   • CanvasPanel, Overlay, VerticalBox, and HorizontalBox are reliable authoring parents.
   • Current create_widget attaches children through UPanelWidget; do not create children directly under Button or Border.
   • To put text on a button or content over a border, create an Overlay and add the visual widget plus the text as siblings.

3. Write properties as union updates.

   • set_widget_properties should set or overwrite supplied properties and leave unspecified properties intact.
   • Use Slot.Position and Slot.Size aliases for CanvasPanel children when possible.
   • Use nested Slot objects for panel-slot alignment and padding.
   • Use delete_widget only for explicit deletion and pass confirm_delete=true.
   • Prefer apply_layout for bulk layout authoring; export_umg_to_json and apply_json_to_umg are hidden compatibility/debug paths.

4. Use HLSL materials for UI visual richness.

   • Prefer hlsl_set_target, hlsl_set, and hlsl_compile.
   • New HLSL targets default to UI material behavior.
   • For detailed material rules, use the umg-material-mcp skill.

5. Animate widgets through Sequencer.

   • Create or select an animation with create_animation and set_animation_scope.
   • Focus the animated widget with set_target_widget or set_widget_scope.
   • Use animation_append_widget_tracks for widget-oriented union key writes.
   • Use animation_append_time_slice when authoring a single moment across one or more widgets.
   • Read back with animation_overview or animation_time_properties; do not trust write responses alone.
   • Treat get_animation_keyframes, get_animation_full_data, set_property_keys, remove_property_track, and remove_keys as hidden compatibility commands, not default authoring tools.

6. Wire Blueprint through BlueCode.

   • Use bluecode_set_function("WidgetName.EventName") for component event scope.
   • Default bluecode_read_function(detail="semantic") is the attention-compressed comprehension view and intentionally omits node/pin sidecars. Before editing an existing graph, re-read with detail="roundtrip"; use detail="debug" or include_connections=true only when raw nodes, pins, or edges are actually needed.
   • Preserve base_revision, source_map, action_hints, expression_hints, and action_hints_by_line from the round-trip read and pass them back to bluecode_apply. A stale base_revision must fail instead of rematching changed graph state.
   • When editing a readback line, keep the corresponding hint. Hints with node_id cause bluecode_apply to update the existing node's supplied input pins/defaults instead of appending a duplicate node, including variable-set and Branch nodes; Pin=A+B and Pin=value("Select", ...) can create data expression nodes for that existing pin. Unmentioned pins and links are preserved.
   • When reconnecting or deleting edges, read connections via detail="debug" or include_connections=true; use each entry's connect as a bluecode_connect reference and delete_target as a bluecode_delete(..., confirm_delete=true) target.
   • Write with default bluecode_apply(mode="union") and bluecode_apply_variables; use dry_run=true when the insertion plan should be inspected first. Union mode aligns semantic statements, inserts unmatched statements before the next matched right anchor, otherwise appends at the block end, and never deletes omitted graph content. Use FunctionName(Pin=value, ...) for normal calls, node("Action Menu Name", Pin=value, ...) for generic exec/action nodes, and value("Action Menu Name", Pin=value, ...) for pure/data nodes.
   • Pin names in Pin=value and bluecode_connect endpoints may use internal pin names, editor display names, or FriendlyName. When omitting an endpoint pin, rely on automatic resolution only if the node has one matching data input/output.
   • Pin values may be strings, numbers, booleans, or structured objects such as {path:"/Game/..."}, {class:"/Script/..."}, {object:"..."}, {text:"..."}, or {literal:"(X=0,Y=0)"}. Check input_warnings after writes.
   • Pin values may also be data expressions: Pin=value("Select", Index=ActiveIndex), Pin=PureFunction(A, B), or Pin=A + B. These create pure/data nodes and connect their compatible output.
   • For custom pure/data expression nodes, pass action hints for the inner expression as well, using the inner Action Menu name or full expression key, such as expression_hints={"My Custom Pure": {...}} for Pin=value("My Custom Pure", Input=A). Readback returns these dependency hints in expression_hints, mirrors them into action_hints, and bluecode_apply accepts either object directly.
   • If a custom, plugin, or macro node stores behavior in UObject node properties rather than pins, pass node_properties to bluecode_apply using the same statement/name keys as action_hints, or put node_properties/properties inside the matching hint object. Treat operations[].result.node_properties.failures as a failed semantic write.
   • If a data expression cannot connect to the target pin, the temporary expression nodes are rolled back and the reason appears in input_warnings.
   • Variable-input nodes that implement UE's native add-pin interface can grow before BlueCode writes values, such as value("Make Array", 0=First, 1=Second, 2=Third) or multi-input math nodes. Array-style pins accept either [0] or 0.
   • Format Text is preconfigured from its Format pin before the rest of the inputs are applied, so value("Format Text", Format="Hello {Name}", Name=PlayerName) can generate the Name argument pin.
   • Treat input_warnings as write feedback that must be read and handled; warnings mean part of the requested semantic write did not map cleanly.
   • Add alias=Name inside bluecode_apply statements, or pass node_aliases, when newly created nodes must be referenced later. Pass the returned aliases map into bluecode_connect and use Name:Pin endpoints.
   • Treat each successful bluecode_apply.operations[].result as immediate context for the next operation: it returns nodeId/node_id, node title/class, is_exec, pin_counts, and inputs/outputs pin evidence using the same endpoint format as readback hints.
   • Use bluecode_connect only when a complex custom, plugin, or macro node needs explicit pin links that cannot be inferred from bluecode_apply inputs. Treat it as union-only connection append, not as a reconnect/delete tool.
   • For custom, plugin, macro, or ambiguous nodes, call bluecode_search_nodes first and pass its returned handle as an action_handle, or place the returned search result object directly in bluecode_apply.action_hints so signature/category/node_class/node_class_path remain available for fallback matching. Use is_exec to choose node(...) versus value(...); use include_pins=true only when pin_counts is not enough.
   • Delete nodes, variables, or existing pin connections only with bluecode_delete(..., confirm_delete=true). For connection deletes, use {"kind":"connection","source":"NodeA:Then","target":"NodeB:Execute"} or "NodeA:Then -> NodeB:Execute"; if a node pair has multiple links, provide the exact pin names instead of guessing.
   • Treat add_step, prepare_value, connect_data_to_pin, get_function_nodes, delete_node, and delete_variable as backend compatibility tools, not default authoring tools.

7. Validate with read operations.

   • Use get_widget_tree after structure writes.
   • Use query_widget_properties for precise property checks.
   • Use export_umg_to_json only when that advanced tool is enabled and a full file snapshot is needed.
   • Use save_asset only after the target has passed focused readback checks.