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

Goes well beyond annotations (readOnly=false, destructive=false, openWorld=false) by disclosing the return buckets (added/already_member/not_found), idempotency behavior ('a repeat adds nothing'), create_if_missing semantics, the AUTO_SYNCED_LIST error for family, the undo path, cost ($0.05) and API-key requirement.

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?

Dense but front-loaded: the core action leads, followed by returns, edge cases, and cost. Parenthetical asides are efficient, though the run-on flow of error codes and undo notes slightly reduces scannability.

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 mutation tool with six params, an output schema, and non-destructive annotations, the description covers the critical behavior an agent needs: idempotency, partial results, the refusal case, reversal, auth, and cost. Nothing essential is missing.

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 100%, so the schema already documents list_id, list_name, person_ids, request_id, idempotency_key, and create_if_missing. The description reinforces list_name ('as its name or slug') and create_if_missing but adds little syntax beyond what the schema carries, so the baseline 3 applies.

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+resource (add owned contacts to a contact list) with scope that distinguishes it from siblings like remove_people_from_list and create_contact_list. The 'owned contacts' qualifier and the list_id/list_name targeting make it unambiguous.

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?

Provides clear routing for edge cases: family relationships are refused and should be handled via update_person, and additions can be reversed with undo_contact_change(change_id). It doesn't explicitly contrast the primary add-vs-remove or add-vs-create alternatives, but the context is strong.

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