get_subscriber_count

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

The readOnlyHint annotation already signals a safe read operation, but the description goes beyond it by explaining the three precision modes ('exact', 'approximate' with the hedged rendering guidance, and 'unavailable' with count -1). This is genuinely useful behavioral context about return value semantics, not just repetition of the annotation. It could add what errors occur or rate limits, but for a read tool this is solid.

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 sentences, zero waste. Every sentence adds value: purpose and precision-mode semantics, and the critical warning not to treat approximate as exact. Front-loaded with the core purpose.

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 0-parameter read tool with no output schema, the description explains the return shape (precision modes and count values) thoroughly. It covers the edge case of 'unavailable' and the rendering guidance. Complete enough for an agent to invoke and interpret results 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?

With 0 parameters, there's nothing for the description to clarify. The baseline of 4 applies for zero-parameter tools; the description appropriately focuses on return-value semantics instead, which is the semantically meaningful content here.

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 clearly states the tool gets the current subscriber count for a Substack publication. The verb+resource (get subscriber count) is specific and distinguishes it from siblings, which all deal with posts, drafts, sections, and analytics.

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 does not explicitly state when to use this tool vs alternatives, but for a 0-parameter retrieval tool the usage context is largely self-evident. It doesn't provide when-not-to-use guidance or alternatives, but given there's no close sibling (no other subscriber-related tool), the lack of exclusions is acceptable. The precision-mode guidance is user-facing behavior rather than tool-selection guidance.

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