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

Annotations already declare readOnly, idempotent and non-destructive, yet the description adds non-obvious behavior: reading marks nothing seen, suggestions are omitted, and the feed is personal and changes between calls. That is real value beyond the structured fields; only the shape of returned items is left unstated.

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?

Four short sentences, front-loaded with what the feed is, followed by mode semantics, exclusions, and pagination. Nothing is repeated and every sentence carries information.

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?

There is no output schema, and the description covers the return split (posts and Notes in feed order), pagination, and the personal/ephemeral nature of results. Only the concrete item structure is unspecified, which is a minor gap for a read tool with full annotation coverage.

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 both parameters are already documented. The description restates the tab semantics (Substack's mix vs. whom you follow) and pagination via next_cursor, adding only marginal meaning over the schema, so baseline 3 applies.

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 first line names a specific resource (Substack home feed) and its contents (posts and Notes) plus the two modes, so an agent can distinguish it from list_posts, list_notes, or search_substack 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?

The description explains what for_you and following each contain and that suggestions are excluded, which shapes selection, but it never states when to prefer this tool over siblings like list_notes or list_posts. Usage is implied rather than prescribed.

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