substack_user_activity

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

With no annotations, the description carries the full behavioral burden. It discloses that it returns one page at a time, that pagination is cursor-based, and that the upstream's kind labels are finer-grained (e.g., 'restack' yields both 'comment_restack' and 'post_restack'). It also implies read-only semantics via 'List' and 'public activity', but does not explicitly state there are no side effects or mention rate limits. Overall, it adds meaningful behavioral context beyond the schema.

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 description is three sentences with no redundancy. The first sentence states the purpose, the second explains the filter, and the third details a subtle behavior. Every sentence earns its place, and the most critical information is front-loaded. It is appropriately concise for the tool's complexity.

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 simplicity (one required param, two optional), the description covers the essential aspects: what it returns, filtering behavior, and pagination. It does not explicitly describe the response structure (e.g., array of items with kind fields) but hints at it via the 'each item's kind' note. It omits details like page size limits or error cases, but these are not critical for a basic list operation. Overall, it is complete enough for an agent to invoke 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?

Schema coverage is 100%, so the baseline is 3. The description adds value by explaining the types filter accepts comma-separated values, lists the valid options, notes the default mixed feed, and clarifies the granularity of the upstream's kind labels. It also mentions cursor-based pagination, reinforcing the cursor parameter's purpose. This exceeds the baseline by providing nuance not present in the schema alone.

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 states a specific verb ('List') and resource ('a Substack user's public activity') and enumerates the exact content (notes, posts, likes, replies, restacks). It clearly distinguishes this from sibling tools like substack_user (profile), substack_notes (notes only), and substack_post (single post) by focusing on aggregated activity. The purpose is unambiguous and actionable.

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 explains how to use the tool (optional types filter, cursor-based pagination, default mixed feed) but does not explicitly state when to choose this over alternatives. It implies this is for a user's activity feed, but it does not mention when not to use it, such as when a specific note or post is needed. The guidance is sufficient for basic use but lacks explicit alternative routing.

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