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

Annotations declare writability and open-world scope but say nothing about consequences; the description supplies the critical side effect that the message is "switched on at once, so it reaches real people from then on" and names the return payload (id, active). It also discloses the uniqueness failure mode, so the agent knows a second call will be rejected rather than silently overwritten.

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?

The riskiest fact (it goes live immediately, confirm with the user first) is placed early, and every sentence carries distinct information. Slightly dense with nested parenthetical sibling references, which costs a little readability for an otherwise four-sentence block.

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 no output schema, the description covers consequence, uniqueness constraints, return values, and sibling alternatives. Nothing needed to invoke it safely 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 coverage is 100%, so the baseline is 3; the description still adds constraint-level meaning by tying accountId to the one-per-account rule and the rejection behavior. It says nothing about idempotencyKey, but that parameter is fully documented in the schema with retry guidance, so no real 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?

States a specific verb and resource ("Create the message one account sends automatically to each person the first time they write to it") and defines the exact trigger semantics of a welcome message. This is clearly separable from update_welcome_message, delete_welcome_message, and create_reply_template among the many sibling create_* tools.

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?

Explicitly routes the agent: check list_welcome_messages because a second one is rejected, use update_welcome_message to change an existing one, and use send_welcome_to_existing_contacts to reach existing contacts. It also adds a procedural precondition — confirm the text with the user first — which is an actual when-to-use gate.

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