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

Annotations already declare this is a non-read-only, idempotent, non-destructive, open-world operation, so the description need not restate that. It usefully adds the eligible-space constraint (public/unlisted only) and the read-context prerequisite, but says nothing about what joining creates, whether it can be reversed (e.g. via manage_membership), or authority requirements — the op_key retry semantics are left entirely to the schema.

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?

Two short sentences, front-loaded with the action and eligibility rule, followed by the one prerequisite action. Nothing is padded or 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?

For a simple two-parameter mutation with full schema coverage, complete annotations, and no output schema, the description gives an agent enough to invoke it correctly. It is only slightly thin on the post-join side effects that would help an agent reason about follow-up actions.

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%, and the schema already explains both space (ID) and op_key (idempotent retry semantics) in detail. The description adds no parameter-level information beyond that, so 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?

States a specific verb (join) and resource (space) plus the concrete scope of eligible targets — public or unlisted spaces — which immediately separates it from create_space, invite, and claim. An agent can distinguish it from siblings without opening any 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?

The 'public or unlisted' qualifier implicitly tells the agent when this tool applies and when an invitation-based route (claim, invite, list_invitations) is needed instead, and 'Read its context before contributing' gives a prerequisite. It stops short of naming those alternatives explicitly.

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