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

Annotations carry the safety profile (readOnlyHint=false, destructiveHint=false, idempotentHint=true), and the description adds real behavior beyond them: email is immutable, unknown fields are rejected with 400, and the return payload includes ignored_fields. The notable undisclosed behavior is whether omitted properties are left unchanged or cleared, which matters for a mutation tool.

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 sentences, no filler: what changes, what is immutable, where tags go, and failure/return behavior. The mutable surface is front-loaded and each remaining sentence handles a distinct concern.

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?

There is no output schema, but the description summarizes the response (updated record plus ignored_fields), which covers the return contract. For a nested-object mutation it is nearly complete; only the replace-vs-merge semantics of omitted properties and any auth requirements are left unstated.

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 50% (first_name and last_name are bare properties), and the description compensates by naming the three updatable properties and their meanings, specifying that fields is keyed by technical key, and stating that email is the identifier and cannot be changed. Unknown fields being rejected further clarifies how the fields object is interpreted.

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?

Specific verb (updates) plus resource (existing subscriber) and an explicit enumeration of the mutable surface: first_name, last_name, and custom fields by technical key. It also names the sibling that owns the excluded concern ('Tags are managed with tag_subscriber'), so an agent can separate this from tag_subscriber and add_subscriber without opening a 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 establishes the precondition that the subscriber must already exist and routes the tag concern to tag_subscriber, which is genuine alternative-naming. It stops short of explicit when-not guidance (e.g. use add_subscriber for creation, remove_subscriber/erase_subscriber for deletion), so it is clear context rather than full routing.

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