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

Annotations already declare readOnly, idempotent, openWorld, and non-destructive, so the safety profile is covered. The description reinforces this ('Only reads... Changes nothing') and adds non-obvious behavior the annotations do not: it accepts an entire message, not just a URL, and it degrades gracefully ('A page with no readable start time says so').

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?

Front-loaded with the core action, then the return taxonomy, then the read-only caveat — a sensible ordering. The middle clause is a long parenthetical chain that is dense but each element (what each link type yields) earns its place for a tool with no output schema.

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?

There is no output schema, so the description must carry return-value semantics, and it does so by enumerating the three link kinds and their reported fields, plus the edge case of an unparseable start time. For a single-param read tool, an agent has everything needed to call it and interpret the result.

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?

With one parameter at 100% schema description coverage, the schema already carries the semantics ('The link, or the message it came in'). The description only lightly reinforces that a full message is acceptable, which is essentially the same information; no additional format, length, or validation guidance is added beyond the schema's minLength/maxLength.

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 names a specific verb ('reads') and resource ('a link... or a whole message with a link in it') and then enumerates exactly what it reports back per link kind: alarm details, invite with referral code, or event page fields. That distinguishes it clearly from siblings like get_invite_card, show_alarm, and join_alarm without requiring the schema.

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?

It states the triggering condition (a link the person was sent or pasted, or a message containing one) and explicitly excludes follow-up actions: 'joining an alarm or using an invite code are separate steps.' Those alternatives are described functionally rather than by tool name, so the routing is clear but requires the agent to map to join_alarm/use_invite_code itself.

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