Carries A2UI over the Model Context Protocol, as the A2UI project's guide "A2UI over MCP" describes: the JSON values an MCP server puts in its responses, and the A2UI values it reads from requests. It works with any MCP server implementation.
Presentation and data are separate. Layouts (createSurface and
updateComponents messages, with data bindings) are MCP resources with
the MIME type application/a2ui+json; a tool call returns the data,
usually an updateDataModel message, in an embedded resource next to a
text fallback. A tool links to its layout with _meta.ui:
A2UIAgent.MCP.tool("get_recipe", resource_uri: "a2ui://recipe-card",
description: "Returns a recipe.")
A2UIAgent.MCP.tool_result(messages,
uri: "a2ui://recipe-card/data",
resource_uri: "a2ui://recipe-card",
text: "Generated recipe: Grilled salmon")A client states its renderer capabilities once in initialize
(capabilities.a2ui.clientCapabilities), or in the _meta of each tool
call; renderer_capabilities/1 reads either. The renderer sends its
users' actions and its errors as calls of the a2ui_action and
a2ui_error tools (action_tool/0, error_tool/0).
Summary
Functions
The A2UI messages in a tool call's result: the JSON of every embedded resource with the A2UI MIME type, in order. Content that is not A2UI is skipped; A2UI text that is not JSON is an error.
The arguments of an a2ui_action tool call as an A2UI action
message of version, or an error naming the missing fields.
The definition of the a2ui_action tool, through which a renderer
sends its users' actions. Every field of an action is required, so
that an MCP SDK does not drop one.
The definition of the a2ui_error tool, through which a renderer reports errors.
The MIME type of A2UI resources: application/a2ui+json.
The renderer capabilities in the params of an initialize request
(capabilities.a2ui.clientCapabilities) or of a tool call
(_meta.a2ui.clientCapabilities), keyed by protocol version, or nil
when they carry none. Pass them to
A2UIAgent.Generator.create_processor/3.
A resource entry for resources/list. Option: :description.
The contents of an A2UI resource for resources/read: messages, as
JSON text.
A tool definition for tools/list. Options: :title,
:description, :input_schema (default: an object that takes any
arguments) and :resource_uri, the A2UI resource that presents its
results.
A tool call's result (CallToolResult) carrying A2UI messages in an
embedded resource.
The _meta that links a tool, or a tool's result, to the A2UI resource
that presents it.
Functions
@spec a2ui_messages(map()) :: {:ok, [map()]} | {:error, Jason.DecodeError.t()}
The A2UI messages in a tool call's result: the JSON of every embedded resource with the A2UI MIME type, in order. Content that is not A2UI is skipped; A2UI text that is not JSON is an error.
The arguments of an a2ui_action tool call as an A2UI action
message of version, or an error naming the missing fields.
iex> A2UIAgent.MCP.action(%{"name" => "go", "surfaceId" => "s", "sourceComponentId" => "b",
...> "timestamp" => "2026-10-16T12:00:00Z", "context" => %{}})
{:ok, %{"version" => "v1.0", "action" => %{"name" => "go", "surfaceId" => "s",
"sourceComponentId" => "b", "timestamp" => "2026-10-16T12:00:00Z", "context" => %{}}}}@spec action_tool() :: map()
The definition of the a2ui_action tool, through which a renderer
sends its users' actions. Every field of an action is required, so
that an MCP SDK does not drop one.
@spec error_tool() :: map()
The definition of the a2ui_error tool, through which a renderer reports errors.
@spec mime_type() :: String.t()
The MIME type of A2UI resources: application/a2ui+json.
The renderer capabilities in the params of an initialize request
(capabilities.a2ui.clientCapabilities) or of a tool call
(_meta.a2ui.clientCapabilities), keyed by protocol version, or nil
when they carry none. Pass them to
A2UIAgent.Generator.create_processor/3.
A resource entry for resources/list. Option: :description.
iex> A2UIAgent.MCP.resource("a2ui://form", "Form")
%{"uri" => "a2ui://form", "name" => "Form", "mimeType" => "application/a2ui+json"}The contents of an A2UI resource for resources/read: messages, as
JSON text.
A tool definition for tools/list. Options: :title,
:description, :input_schema (default: an object that takes any
arguments) and :resource_uri, the A2UI resource that presents its
results.
A tool call's result (CallToolResult) carrying A2UI messages in an
embedded resource.
Options:
:uri— the embedded resource's URI (default"a2ui://messages");:text— a text fallback, put first, for clients and models that do not render A2UI;:resource_uri— the A2UI resource that presents the result, linked with_meta.ui;:audience— who the messages are for:["user"]renders them without putting them in the model's context,["assistant"]the other way round (default: both).
The _meta that links a tool, or a tool's result, to the A2UI resource
that presents it.
iex> A2UIAgent.MCP.ui_meta("a2ui://recipe-card")
%{"ui" => %{"resourceUri" => "a2ui://recipe-card", "mimeType" => "application/a2ui+json"}}