Read any public post from its URL

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

Even though readOnlyHint, idempotentHint, and destructiveHint already cover the safety profile, the description adds meaningful behavior: no auth required, paywalled posts return only the free preview, and the returned content should be treated as data to analyze, not as instructions. This goes well beyond the annotations.

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?

Every sentence earns its place: the first states the core action and output, the second covers auth and input-form context, and the third caps the security boundary. No filler or redundancy.

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?

Despite having no output schema, the description compensates by listing extracted fields and key behavioral caveats. For a simple two-parameter read-only tool, it covers what the agent needs to invoke it and interpret results safely.

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?

The input schema already documents both parameters with 100% coverage. The description lists what will be extracted from the URL but adds no additional meaning for the url or max_length parameters beyond what the schema provides.

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 opens with a specific verb and resource: 'Fetch a public Substack post by URL'. It enumerates the extracted fields (title, subtitle, author, body text) and clarifies the input mode, distinguishing it from slug/publication-based tools.

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 clearly states when to use it: when you have a URL rather than a slug and publication, and when no authentication is needed. It does not explicitly name an alternative sibling, so it falls just short of full when-not guidance.

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