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

Annotations already declare readOnlyHint and openWorldHint, so the safety profile is covered. The description adds two genuinely non-obvious behaviors beyond that: the 1-credit cost and the paywall truncation rule where `text` becomes a public preview and `truncated` flips true. It does not cover caching or rate-limit behavior.

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 compact sentences, no padding, and the most decision-relevant caveat (paywall truncation) is placed early. The opening 'Post.' is wasted space and slightly redundant with the title.

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-shape burden and does so by naming the returned fields and the `truncated` flag. Cost and paywall semantics are covered. The remaining gap is the undocumented `cache_max_age` parameter.

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 only 50%: `id` is documented in the schema, but `cache_max_age` is undocumented in both schema and description, and the description never explains it. It also adds nothing about the id format beyond what the schema already says. With a low-coverage schema, the description should compensate and does not.

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 it returns one Substack post and enumerates the payload (text, reactions, comments, restacks, word count), which distinguishes it from the list-oriented substack_posts sibling. The leading fragment 'Post.' is a weak, near-tautological opener, but the sentence that follows resolves the ambiguity.

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?

There is no explicit guidance on when to use this versus substack_posts, substack_comments, or substack_profile, and no mention of prerequisites such as needing an id obtained elsewhere. The only implied signal is 'one Substack post', which hints at single-item retrieval but does not route the agent.

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