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

Annotations already declare readOnlyHint=true, openWorldHint=true and destructiveHint=false, so safety is covered. The description adds genuinely useful non-schema behavior: paid posts return only the free preview, the response includes paywall status, and the call is billed $0.003 per request. That pricing and paywall disclosure is real added value, though no auth or rate-limit notes.

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?

Two sentences, front-loaded with the core action and identifier contract, followed by the return-field inventory. The second sentence is a dense list but every item is informative; slight density is the only cost.

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?

With no output schema, the description carries the return-value burden and does so thoroughly: body formats, metadata, engagement, paywall status, media and tags. Combined with the pricing note and identifier rules, an agent has enough to invoke it correctly, though it omits any note on error behavior for invalid identifiers.

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 description coverage is 100%, so each parameter is already documented in the schema, making 3 the baseline. The description restates the identifier requirement and lists what fields come back, but adds no syntax or format detail beyond the schema (e.g., no guidance on precedence between url and post_id).

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 ('Get a Substack post') plus the accepted identifier forms, and the resource scope (a single post) clearly separates it from siblings like substack_publication_posts, substack_post_comments, or search_substack. An agent can distinguish it 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?

It gives the identifier contract ('provide url, post_id, or publication together with slug'), which implies usage, but never names an alternative tool or states when-not to use it (e.g., versus search_substack or substack_publication_posts). Usage is inferable but not routed.

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