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

Annotations already declare readOnlyHint, idempotentHint, destructiveHint=false and openWorldHint=false, so safety is covered. The description adds genuinely non-obvious behavior: counts carry over during migration, and email audiences include corporate subscribers only by default under PECR, excluding sole traders and ordinary partnerships unless the toggle is changed. The UK GDPR Article 6(1)(f) sentence is boilerplate that does not change how the tool is invoked.

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?

The deprecation notice and the replacement tool are correctly front-loaded in the first two sentences. The final sentence is overlong, bundling lawful basis, PECR composition rules and a migration link, and the legal citation dilutes rather than sharpens the call guidance.

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 deprecated zero-argument read tool with an output schema, everything an agent needs is present: deprecation status, replacement, sunset window, migration mapping URL, and a caveat about result composition. Return values are covered by the output schema, so no description of them is required.

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?

There are zero parameters, so the baseline is 4. The schema description explains that old arguments are unchanged and mapped by the adapter, and points to the migration URL, which is sufficient for a no-argument call.

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 ('lists the saved lists of the organisation') and immediately names the superseding tool, list_audiences. The parenthetical that saved lists carry over as aud_ audiences removes any doubt about what is being listed. An agent can distinguish it from list_audiences and save_selections without opening either 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?

Explicit routing instruction: 'Use list_audiences instead', plus a stated sunset ('stops working 12 months after the new tools launch'). This tells the agent both the preferred alternative and the condition under which the old tool remains callable.

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