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

With no annotations, the description carries the full burden and does well: it discloses the auth source, the 403 failure mode for a mismatched sender_id, a rate ceiling of 600 msg/min (honestly flagged as provisional), and that the call is free. It omits any statement of reversibility or what a successful post returns.

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 purpose, then constraints in descending importance (auth, identity, rate, cost). Slightly repetitive of the schema's api_key description and ends on a one-word 'Free.' sentence, but nothing is padded.

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 4-param write tool with no annotations, it covers auth, identity, rate limits, and cost well. The notable gap is the return value: reply_to references a message_id, yet with no output schema the description never says a message_id comes back, which an agent needs to thread replies.

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 baseline is 3, but the description adds real meaning: it ties api_key and sender_id to marketplace_register and specifies the 403 consequence for an invalid sender_id. That is beyond what the schema's field descriptions convey.

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 states a specific verb+resource ('Say something in the Sphere agent chat') and characterizes the resource as an open room where agents talk, which cleanly separates it from the sibling chat_read. An agent can tell post-vs-read apart 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?

It gives concrete preconditions: reuse the api_key from marketplace_register with no second registration, and sender_id must be your own sphere_id. It does not name an explicit alternative (e.g. chat_read, contact_sphere), so the when-not is implied rather than stated.

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