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

Annotations already declare readOnly/idempotent/non-destructive. The description adds genuinely non-derivable behavior: it renders a widget, the card contains username, human/agent badge, purpose and functional Tip/View-page buttons, and clients without widget support fall back to the same data as JSON. That fallback/interactivity disclosure is real added value beyond annotations.

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

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

Three tight sentences: action first, then prerequisite/routing, then client-compatibility fallback. Every sentence carries information an agent needs and nothing is repeated.

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

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

No output schema exists, and the description compensates by enumerating the card contents and the non-widget JSON fallback, so an agent knows what comes back. It omits failure behavior (e.g. unknown username) and any client-detection caveat, leaving a small gap.

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

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

Schema coverage is 100% and the sole 'username' parameter is documented in the schema (with an example). The description only implies that username selects the blockpage to preview; it adds no format, ID-style, or validation nuance beyond the schema. Baseline 3 applies.

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

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

Specific verb+resource: 'Render an interactive blockpage preview card inside the chat (MCP Apps widget)'. It is clearly distinguishable from lookup_blockpage (data lookup) and render_blockpage_image (image variant), and names the sibling it depends on.

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

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

Explicit sequencing: 'Look up the blockpage first with lookup_blockpage, then call this to show the human a visual card.' This tells the agent the prerequisite step and the intent (show the human). It does not state when to prefer render_blockpage_image or when-not to render at all, so it stops short of full routing guidance.

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