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

Annotations already declare readOnlyHint=true, so the safety profile is covered. The description adds value beyond that by disclosing the concrete return facets (capabilities, expiry, usage) and pinning the exact REST call GET /tailnet/{tailnet}/keys/{keyId}, which no structured field provides.

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?

One sentence, zero filler, purpose front-loaded before the endpoint reference. Nothing is repeated from structured fields and every clause earns its place.

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?

For a one-parameter read-only lookup with no output schema, the description covers purpose, identifier, return facets, and the underlying endpoint. Minor gaps remain (error behavior for an invalid/expired keyId, expected auth scope), but nothing essential for calling it 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 100% with a single documented keyId, so the schema carries the parameter burden. The description's 'by id' restates that without adding format, source, or format examples, matching the baseline-3 case where the schema does the heavy lifting.

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 verb (Get) and resource (a single auth key / API access token) and clarifies it is a by-id single-record fetch, which distinguishes it from tailscale_list_keys. It also names the returned facets (capabilities, expiry, usage) so the agent knows what the call produces.

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?

Usage is only implied: 'by id' signals that a keyId (presumably from tailscale_list_keys) is a prerequisite, and 'single' contrasts with the list variant. There is no explicit when-to-use/when-not statement or named alternative, so guidance stays at the implied level.

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