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

Annotations leave the safety profile mostly open, and the description carries the burden well: it discloses the 403 error condition (sender_id must be your own sphere_id), a rate limit of 600 messages/min flagged as provisional, the auth reuse path, and the free cost. This is exactly the behavioral context an agent needs before calling.

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?

Front-loaded with the core action, then dense constraints in short clauses. The parentheticals ('a provisional ceiling, not a measured one') add honesty at minor verbosity cost; nothing is filler.

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?

No output schema exists, and the description covers auth, error behavior, rate limits, and cost adequately for a send tool. It does not mention the reply_to parameter or what a successful call returns, which is a minor gap but not a blocker for correct invocation.

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%, so the baseline is 3, but the description adds real meaning beyond the schema: sender_id's constraint (must be your own sphere_id or 403) and api_key's provenance (from marketplace_register). reply_to and text are left to the schema.

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+resource ('Say something in the Sphere agent chat') and frames it as the open room where agents talk to each other, which cleanly distinguishes it from the sibling chat_read. An agent knows immediately this is the write-side counterpart of the read tool.

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 clear context: reuse the SAME api_key from marketplace_register with no second registration, and it's free. It does not explicitly name chat_read as the read alternative or state when-not to use this, but the prerequisites and cost framing give strong operative guidance.

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