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

Beyond the annotations, it discloses the authorization model (owner/coordinator, viewers rejected), the exact sending identity and reply-to routing the web app uses, and flags that the body may contain health-related content — a compliance-relevant trait no structured field carries. 'This is a write' mildly restates readOnlyHint=false, but the added context is substantial.

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?

Four short sentences, front-loaded with verb and resource, and every clause except the slightly redundant 'This is a write' contributes a distinct operational fact. Sizing is appropriate for a mutation with an access gate.

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 all five parameters documented in the schema, annotations covering safety hints, and no output schema to explain, the description supplies everything else an agent needs: access requirements, identity/addressing behavior, and data sensitivity. Nothing material 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 description coverage is 100%, so subject, toEmail constraints, messageId provenance, and lovedOneId ownership check are all already documented in the input schema. The description adds no parameter-level detail (e.g., length limits or formatting) beyond what the schema provides, so baseline 3 applies.

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 opening clause gives a specific verb (Sends a plaintext reply) and resource (a Family Inbox message), immediately distinguishing it from read-oriented siblings such as get_inbox_message and list_inbox_messages. It further pins down the mechanism by naming the from address and reply-to identity used.

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?

The description states a hard precondition — 'Owner or coordinator only — viewers are rejected' — which tells the agent exactly when the call will fail. It does not, however, name the sibling to use for the read side or say what to do if the agent only has viewer rights, so the routing guidance stops short of explicit alternatives.

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