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

Annotations already declare non-readonly/non-idempotent, but the description adds substantial context beyond them: the one-time-per-account limit, the self-referral prohibition, irreversibility ('it cannot be undone'), the deferred welcome-gift side effect, and the error on an invalid code. This is rich behavioral disclosure.

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?

Dense and front-loaded, leading with the core action and constraints before the edge cases. Every sentence carries information, though the gift/error sentences could be slightly tighter.

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 mutation with no output schema, the description covers action, constraints, irreversibility, deferred effects, and error behavior. Nothing an agent needs to call 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 description coverage is 100% and the single 'code' parameter is fully documented in the schema. The description only echoes the LFG-… format and adds the referral-vs-alarm distinction, so it adds little beyond the structured field.

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 and resource ('Uses a friend's LFG referral code'), and explicitly distinguishes itself from the alarm-joining path ('an alarm's link or code is joined, not used here'). An agent can separate this from join_alarm without opening either 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?

Gives a clear selection condition: use this for a referral code (LFG-…), while an alarm's link/code is joined elsewhere. Constraints (once per account, never your own code) further scope usage, though it does not name the alternative sibling tool outright.

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