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

Annotations already provide the safety profile (readOnlyHint=false, idempotentHint=true, destructiveHint=false), and the description adds genuine value beyond them: it details the idempotent skip behavior ('leads already in the list are skipped') and discloses the return payload (counts of newly added and already-present leads). This is useful behavioral context a schema cannot convey, and there is no contradiction with the annotations.

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?

Five sentences, each earning its place: purpose, idempotency behavior, parameter requirements, return value, and prerequisite tool routing. The core purpose is front-loaded in the first sentence and there is no filler or redundancy.

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?

Strong completeness for a two-parameter mutation: the safety profile is covered by annotations, the return format is disclosed because no output schema exists, and prerequisite lookup tools are explicitly named despite a very large sibling set. The main remaining gap is the meaning of the leadSource enum values ('global' vs 'private') and error behavior for non-existent lists.

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 0%, so the description carries the burden for parameter meaning. It does restate the leadRef structure and the leadSource default ('global'), and it routes the agent to tools that produce valid IDs, which is helpful. However, it does not explain the semantic difference between 'global' and 'private' lead sources or what leadListId refers to beyond its name, leaving a real gap.

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 ('Add') and resource ('leads to an existing lead list'), making the operation unambiguous. The qualifier 'existing' implicitly distinguishes it from list-creation tools like campaignstack_create_lead_list. However, it does not explicitly differentiate from similar add-to-target siblings such as campaignstack_add_lead_to_external_list or campaignstack_queue_leads.

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?

Gives clear workflow context by naming the exact prerequisite tools: campaignstack_list_lead_lists for lead list IDs and campaignstack_search_leads for lead IDs. This tells an agent how to assemble a valid call. It stops short of stating when NOT to use this tool or naming an alternative operation, so exclusions are left to inference.

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