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

Adds real behavior beyond the annotations: 'Reading marks nothing seen' clarifies no read-state side effects, 'Page with next_cursor' documents pagination, and 'the feed is personal and changes between calls' warns results are volatile and not cacheable. That last clause sits in mild tension with idempotentHint=true, though it describes result variance rather than side effects, so it is not a true contradiction.

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 scope and return shape are front-loaded in the first two sentences, followed by tab semantics, exclusions, and pagination. Nearly every sentence carries information, though the opening fragment ('posts and Notes, For you or Following') is slightly clipped and the capitalization is inconsistent.

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 two-optional-param read tool with no output schema, the description covers the return shape ('split into posts and Notes in feed order'), pagination, exclusions, and volatility. Nothing an agent needs to call it correctly 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 coverage is 100%, so baseline is 3, but the description goes further by defining what each tab value actually contains and how cursor paging works ('Page with next_cursor'). It adds semantic meaning over the enum labels rather than restating them.

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 ('Returns one page of the home feed of the Substack app') and scopes it to posts and Notes, which separates it from list_notes, list_posts, and list_activity. An agent can identify the resource and its two-variant scope without opening the 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?

Explains the two modes clearly ('for_you is Substack's own mix; following holds only people and publications you follow') and notes that follow suggestions are excluded, so the agent knows what it will and won't get. It stops short of naming an explicit alternative (e.g. use list_notes for note-only browsing), so there is no routing statement beyond scope.

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