list_subscribers

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

With no annotations provided, the description carries the full burden and does well by disclosing non-obvious behaviors: the count is independent of limit (making limit=1 a segment-size probe), engagement columns can be filtered on but are not part of returned records, and filters combine with AND. It lacks explicit read-only/permission notes, but the read-style action is implied and the disclosed quirks are highly valuable.

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 tight sentences: purpose, feature set, count behavior, and an alternative-tool pointer. Each sentence carries distinct information with no filler or repetition. The structure front-loads the core purpose and then adds operational details in a logical order.

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?

Given the tool's complexity (6 params, nested filters with 48 columns) and the very rich schema, the description supplies enough high-level context: it explains the AND limitation, the count behavior, and the division of labor with export_subscribers. It does not enumerate the returned record fields, but the schema and the explicit exclusions make the tool usable without that detail.

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?

The schema already covers all parameters with descriptions (100% coverage), so the baseline is 3. The description adds meaningful extra semantics: the limit parameter's use for cheap segment sizing via count, and the clarification that the filters parameter can include engagement columns even though they are not returned. These enrich the parameter understanding beyond the schema.

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?

The description opens with a clear verb-resource pair: 'List and filter the subscribers of your Substack publication.' It further distinguishes itself from sibling tools, especially export_subscribers, by explicitly noting that engagement columns are filterable here but returned only via export_subscribers. This makes the tool's role 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?

The description gives explicit when-to-use context: it supports dashboard-like filtering, AND-only combination, and a cheap count-based segment sizing pattern. It also provides an explicit alternative: 'use export_subscribers to read their values' for engagement metrics, clearly directing the agent away from this tool for that need.

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