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

Annotations already declare readOnly/idiompotent/non-destructive, but the description adds value beyond them: it discloses that this is a UI render surface in MCP Apps hosts, that calling it redundantly duplicates cards, and that it never publishes, schedules, or enables a public link. It does not describe what the rendered card contains or any host-specific failure modes, so it falls short of 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.

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

Four compact sentences, front-loaded with the usage trigger and followed by the anti-pattern, the parameter source, and the side-effect boundary. Every sentence carries distinct information, though the host/browser distinction sentence is slightly compressed and could be tighter.

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?

With an output schema present, return values need no explanation, and the description still covers invocation triggers, the sibling to avoid, how to source the required ID, and the exact non-side-effects. Nothing an agent needs to call this correctly is missing.

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 0%, so the description must carry the load for the single postId parameter — and it does, telling the agent exactly which tools (search_posts/search_post_drafts) to use to obtain a valid ID. It adds no format/validation detail beyond the schema's length constraints, keeping it at 4 rather than 5.

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?

States a specific action (open/render an existing post card) on a specific resource, and immediately differentiates itself from create_post_draft, preview_post, and get_post by naming the duplicate-card failure mode and the host-rendering behavior. An agent can pick this over its many siblings without opening their schemas.

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?

Gives explicit when-to-use triggers (author asks to open a card, requests another card, reports a missing card), an explicit when-not-to (after create_post_draft, which already attaches a card), a prerequisite (resolve postId via search_posts/search_post_drafts), and a disambiguation (browser preview link is different). Nothing is left to inference.

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