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

Annotations already declare readOnlyHint=true and idempotentHint=true, so the safety profile is covered. The description adds valuable behavioral context: nothing is rendered now, image is generated on crawler fetch, cached at edge for a day, and each uncached fetch spends one image of monthly quota. This goes beyond annotations and helps the agent understand side effects (quota consumption) despite readOnlyHint.

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?

The description is well-structured and front-loaded: it states the core function first, then key behavioral facts (deferred rendering, caching, quota), then usage context, then return value. Every sentence earns its place, and it's appropriately sized for the tool's complexity.

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?

The description covers the essential context: what it does, when to use it, behavioral traits (deferred rendering, caching, quota), and return value (URL and meta tag). It doesn't explicitly mention the API key requirement in the schema, but the description does. Given the rich schema and annotations, this is nearly complete. Minor gap: no explicit mention of error cases or rate limits beyond quota, but that's acceptable.

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 description coverage is 100%, so the schema already documents all parameters thoroughly. The description adds context about the template being from list_templates or save_template, and mentions the fields object semantics (omit to keep template's own text), but this is largely redundant with the schema. Baseline 3 is appropriate.

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?

The description clearly states the tool's function: building a signed image URL for a saved template, ready for an og:image meta tag. It distinguishes itself from siblings by emphasizing the zero-infrastructure, URL-only approach (no rendering now, crawler fetches later), which differentiates it from render_card, preview_card, and card_from_url.

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?

The description explicitly says when to use this tool: in page templates, CMS hooks, and static site generators, as the zero-infrastructure way to give every page its own card. It also implies when not to use it (when you need immediate rendering, use render_card/preview_card instead), and mentions the API key requirement. This is strong usage guidance.

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