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

Annotations only cover the generic mutation profile (readOnly=false, idempotent=false, destructive=false), and the description adds substantial behavioral context beyond that: the email is created unverified, the agent key is returned only once, the person must store it in client settings, and the agent must not repeat the key in conversation. These are non-obvious, high-stakes operating details.

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?

Everything is packed into a single dense sentence with the gating condition front-loaded, which is appropriate given the title marks the tool optional. It is slightly run-on, but nearly every clause (consent gating, one-time key, storage instruction, newsletter condition) carries necessary information.

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?

With no output schema, the description explains what is returned (an agent key, once) and what the caller must do with it, covering the consent gate, the side effect of account creation, and the downstream benefits. An agent has enough to invoke this correctly without further inference.

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 0%, so the description carries the burden and does much of it: it explains that consent must be the explicit-when condition, and that newsletter should only be set on request. Email semantics (unverified, client-side storage) are also implied, though format/length constraints 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?

The description states a concrete action and resource: it creates a free account for the person's email and returns an agent key. The tool name 'sign_in' is slightly misleading since the actual behavior is account creation, but the description corrects this immediately and names the concrete benefits (saved tools, history, personal savings, higher limit) that distinguish it from siblings like list_saved_tools or savings.

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 an explicit precondition ('Only when the person explicitly asks for a free account (consent:true)') and a conditional for the optional flag ('newsletter:true only if they ask'). This tells the agent both when to call and when not to call, which is exactly the routing guidance needed.

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