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

Annotations already declare the safety profile (not read-only, idempotent, non-destructive, closed-world), and the description adds genuinely new context: mutation is partial/merge-based, tags accumulate rather than replace, and the call costs $0.01 with no charge on failure. It still omits error behavior for unknown ids and what the response returns, which is why it is not a 5.

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, with the core operation stated first and the cost note last. Every sentence carries information an agent cannot get from the schema or annotations.

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 13-parameter mutation with no output schema, the description covers merge semantics and pricing but says nothing about the result payload (does it return the updated contact?) or failure modes. Annotations cover safety, so the remaining gap is moderate rather than severe.

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 only 23% across 13 parameters, so the description has to compensate and largely does not: it says nothing about the meaning of name/email/phone/notes/title/language/location/timezone or the single enum (lifecycle_stage). The only semantics it adds — tags appended, custom fields merged — are already documented in the schema property descriptions, so it adds little beyond structured data.

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 ('Change fields on an existing contact by id') and scopes it to existing records, so an agent will not confuse it with creation. However, it never distinguishes itself from the closely related crm_save_contact / crm_save_contacts siblings, which likely cover overlapping intent.

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?

'Only the fields you send change' implies this is the partial-update path, which nudges usage, but there is no explicit when-to-use versus crm_save_contact or versus crm_tag_records for tag removal (the latter is only mentioned in the schema). Usage is inferable rather than stated.

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