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

Adds two things the annotations do not: custom `fields` are MERGED with existing values (not replaced), and the write_crm scope is required. That is genuine behavioral context for a mutation. It doesn't say what happens when the contact isn't found (error vs. no-op), and idempotency is already covered by the idempotency_key schema entry plus idempotentHint.

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?

Three short sentences, zero filler. The lookup mechanism lands before the merge semantics and the auth requirement closes it out.

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 an 11-parameter mutation with no output schema, this covers lookup, merge behavior, and auth. The remaining gap is failure semantics — what an agent should expect when the id/email matches nothing, or whether omitted fields are left untouched.

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 91%, so parameters are largely self-documenting and baseline applies. The description still earns credit by clarifying merge semantics for the nested `fields` object and by explaining that id and email are alternate lookup keys rather than both being required (required count is 0).

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?

A specific verb plus resource ("Change fields of an existing contact") and a lookup mechanism ("found by id or email"). The word "existing" quietly separates it from upsert_contact, though the sibling is never named outright.

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?

Implies usage via "existing contact" suggesting it will not create records, but never states when-not, and never points to upsert_contact as the alternative. The agent has to infer which of the two write paths to use.

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