A2UI Atomic Catalog

render_surface

Read-only

Render an A2UI payload INSIDE the chat as a live interactive view (MCP Apps hosts only — on other hosts this returns the payload as text). No URL, no size ceiling, nothing stored: the payload travels in the tool result and renders in the a2ui_catalog_view. Accepts the blocks dialect or a v1.0 createSurface envelope (templates + dataModel decode in the view). Prefer this over preview_url when the host supports MCP Apps.

If the user names one of the Workspace's own pre-built views — Wall Builder, profile, history, links, composer — call open_workspace(view: ...) instead of composing one here yourself; those are wired demos with their own state, not something to reconstruct block by block. And never invent a generic block type (there is no "text" or "button" atom) — call get_catalog / get_atom_schema for the real vocabulary first; an unrecognised type is caught and replaced with a visible notice rather than silently dropped, but the block still won't render as intended.

Tool call template

Replace each placeholder with a value matching the input schema. Only required parameters are included; add optional parameters as needed.

{
  "id": 1,
  "jsonrpc": "2.0",
  "method": "tools/call",
  "params": {
    "arguments": {
      "payload": "<payload>"
    },
    "name": "render_surface"
  }
}

Input Schema

Name	Required	Description	Default
`payload`	Yes	The A2UI payload to render in the chat view: blocks dialect {title, theme, blocks:[...]} or a v1.0 createSurface envelope.

Schema Changelog

Changes observed during successful MCP inspections.

First observedNov 10, 2026

TDQS

A4.6/5.0

Behavior4/5

Does the description disclose side effects, auth requirements, rate limits, or destructive behavior?

Annotations already cover read-only, non-destructive, closed-world semantics, yet the description adds real behavioral detail beyond them: no URL, no size ceiling, nothing stored, payload travels in the tool result, and non-MCP-Apps hosts degrade to plain text. It also discloses failure behavior (unrecognised block types are replaced with a visible notice rather than silently dropped). It stops short of describing the rendered result's lifecycle or update semantics, so not a 5.

Agents need to know what a tool does to the world before calling it. Descriptions should go beyond structured annotations to explain consequences.

Conciseness4/5

Is the description appropriately sized, front-loaded, and free of redundancy?

The core capability and host caveat are front-loaded in the first sentence. The second paragraph is dense and packs three separate rules (pre-built views, vocabulary lookup, unknown-type fallback), which slightly hurts scannability but every sentence carries information.

Shorter descriptions cost fewer tokens and are easier for agents to parse. Every sentence should earn its place.

Completeness5/5

Given the tool's complexity, does the description cover enough for an agent to succeed on first attempt?

For a single-parameter rendering tool with no output schema, the definition covers host compatibility, fallback behavior, dialect variants, alternative tools, and the vocabulary prerequisite. An agent has everything needed to call it correctly or route elsewhere.

Complex tools with many parameters or behaviors need more documentation. Simple tools need less. This dimension scales expectations accordingly.

Parameters4/5

Does the description clarify parameter syntax, constraints, interactions, or defaults beyond what the schema provides?

Schema coverage is 100% and the payload structure is already documented, so the baseline is 3. The description still adds meaning by accepting two dialects (blocks dialect or a v1.0 createSurface envelope) and noting that templates and dataModel are decoded in the view, which the schema only hints at.

Input schemas describe structure but not intent. Descriptions should explain non-obvious parameter relationships and valid value ranges.

Purpose5/5

Does the description clearly state what the tool does and how it differs from similar tools?

States a specific verb and resource ('Render an A2UI payload') plus the exact destination ('INSIDE the chat as a live interactive view'), and names the render surface a2ui_catalog_view. It is immediately distinguishable from preview_url (URL-based) and open_workspace (pre-built views).

Agents choose between tools based on descriptions. A clear purpose with a specific verb and resource helps agents select the right tool.

Usage Guidelines5/5

Does the description explain when to use this tool, when not to, or what alternatives exist?

Explicit routing rules: prefer this over preview_url when the host supports MCP Apps, use open_workspace(view: ...) when the user names a pre-built view, and call get_catalog / get_atom_schema before inventing block types. Both when-to-use and when-not-to-use are covered with named alternatives.

Agents often have multiple tools that could apply. Explicit usage guidance like "use X instead of Y when Z" prevents misuse.

All A2UI Atomic Catalog tools Inspect

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