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

Annotations already declare idempotentHint=true, destructiveHint=false and openWorldHint=false, so the safety profile is covered. The description adds genuinely non-structured behavior: it may create a lead as a side effect ("Adds the lead to the CRM if needed") and it validates keys, rejecting undefined ones. It does not describe the response or partial-failure behavior, keeping it short of 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?

Front-loaded with the restrictive routing guidance, then the action, then the argument shape, then the validation and side-effect caveats. Four short sentences, each carrying a distinct decision-relevant fact, with no repetition of the schema.

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 and annotations covering the safety profile, the description supplies the enforcement rules (defined keys only), the alternative routes, and the upsert side effect. It stops short of explaining what happens when an unknown key is passed or how the update is reported, but an agent has enough to call it correctly.

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 100%, so baseline is 3, but the description adds real meaning: `fields` is a { field_key: value } map and only keys registered in CRM are accepted, routing the agent to list_crm_fields for discovery. contact_id being a lead identifier rather than a contact-edit target is also clarified by context.

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 (set custom field values on a lead, keyed by contact_id) and explicitly distinguishes itself from siblings set_deal_stage and update_contact. An agent can select this tool without opening any 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?

"Use only for custom CRM fields" gives an explicit when-to-use, and the two named alternatives (set_deal_stage for stage, update_contact for contact details) supply when-not-to-use with the routing condition attached. Nothing is left to inference.

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