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

Goes beyond the annotations by disclosing the two consequences that matter for a write: immediacy and visibility to other people. That is real value on top of readOnlyHint=false and openWorldHint=true. It omits anything about repeated sends creating duplicates (idempotentHint=false already covers that) and does not mention the sending identity's auth requirements beyond 'as the user'.

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 sentences, no padding. The capability statement comes first and the risk/confirmation guidance second, which is the correct order for a write tool the agent may call mid-conversation.

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 three-parameter mutation with full schema coverage, rich annotations, and no output schema, this covers what an agent needs: what it does, that it is irreversible in the social sense, and that confirmation is required. Missing only minor detail such as whether the response returns the created message id.

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%, so space, text, and thread are all documented at the field level, including that thread should be omitted for a top-level message. The description reinforces the space-versus-thread choice but adds no format, length, or mention semantics beyond the schema. Baseline 3 applies when the schema carries the parameter burden.

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 (send a message) plus the two mutually exclusive modes: new top-level message in a space/DM, or a reply in a thread. All siblings are read/lookup tools, so the write tool is unambiguously distinguishable from list_spaces, read_messages, read_thread, and search.

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 precondition and operating context: it posts immediately, others will see it, and the agent should confirm text and conversation with the user first. It does not explicitly say when to prefer this over a sibling, but no sibling sends messages, so no disambiguation is needed. The confirm-first instruction is the key usage guidance and it is stated plainly.

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