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

Annotations declare a non-destructive write, but the description adds almost nothing beyond the name: it does not say whether joining is reversible/leavable, whether it fails when the match is full or already joined, or how idempotencyKey affects retries. Only 'with this agent identity' gives any behavioural context, which leaves significant gaps for a mutation tool.

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?

It is a single, front-loaded sentence with no filler, which is structurally sound. However, the brevity comes at the cost of under-specification rather than genuine economy.

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 3-required-parameter mutation with no output schema and no explanatory annotations, the description is too thin: it omits how to discover a match, seat/availability semantics, and what a successful join returns. An agent would need to open the schema and guess at prerequisites.

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% and the two documented parameters are tautological ('Match ID', 'Unique operation key'), while the game enum is undocumented. The description does not say where matchId comes from, what the game values mean, or what idempotencyKey governs, so it fails to compensate for the coverage gap.

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 ('Join') and resource ('available multiplayer match') and adds that the agent identity is bound to the action. It is clear what the tool does, but it never distinguishes itself from neighbours like morrowkin_create_arena_match, morrowkin_list_arena_matches or morrowkin_read_arena_match.

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?

There is no guidance on when to use this versus listing matches first (morrowkin_list_arena_matches) or creating one (morrowkin_create_arena_match), nor any stated prerequisite such as obtaining a matchId or the match being open. The word 'available' hints at a precondition but never states it or what to do when it fails.

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