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

Annotations only declare readOnlyHint=false, openWorldHint=true, idempotentHint=false and destructiveHint=false; the description adds real context on top: it does not charge or issue a key, the token must be kept private, and only onboarding_url and user_code should be shown to the human. That is meaningful operational and safety context beyond the structured hints, though auth requirements and link expiry/lifetime are not covered.

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 sentences, roughly 40 words, with the core action front-loaded and no filler. The opening sentence is a heavy noun pile ('verified sign-in, recurring Starter/Pro plan and explicit agent access approval link') that costs a little readability but nothing is redundant.

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, and the description usefully names the returned fields (onboarding_url, user_code, token) and their disclosure rules, which an agent would otherwise have to discover at runtime. Combined with the no-charge/no-key boundary, this is close to complete for a non-idempotent, open-world link-creation tool; only follow-up flow and auth preconditions are absent.

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 67%: harness and client_name are documented in the schema, while plan is only an enum. The description reinforces the plan parameter ('Starter/Pro plan') and frames client_name as the human-readable name, but adds no new meaning for harness or client_name beyond what the schema already states, so the 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?

The description gives a specific verb (Create) and a scoped resource: an approval link tied to a verified sign-in and a recurring Starter/Pro plan. It is clear enough to distinguish from read-style siblings like get_onboarding_status or get_setup_instructions, though it never names an alternative and the compound object ('sign-in, ... plan and ... link') makes it slightly unclear whether one artifact or several are produced.

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 via the title ('Start ... onboarding') and the boundary sentence 'This does not itself charge or issue a key,' which hints this is a first step that must be followed by another call. There is no explicit when-to-use, when-not, or reference to sibling tools such as complete_onboarding or get_onboarding_status.

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