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

The readOnlyHint annotation already establishes that this is a safe read, so the description's main additional contribution is confirming the concrete HTTP GET endpoint. It says nothing about behavior on a missing/invalid id (e.g., 404 handling), authentication, or rate limits, which is a modest but acceptable gap given annotation coverage.

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 short sentences, front-loaded with the action and followed by the concrete endpoint. No wasted prose, though the generic opening clause is 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?

For a single-resource getter with one fully documented parameter and readOnlyHint set, the essentials are covered. With no output schema, the description gives no indication of what the returned primary IP object contains (address, assignment, datacenter), which leaves a real gap for a fetch tool.

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% and there is exactly one required parameter documented as 'The resource's numeric id.' The description's 'by id' adds nothing beyond the schema, so the 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 description names a specific verb (fetch) and, via the endpoint reference 'GET /primary_ips/{id}', pins the resource to primary IPs, which distinguishes it from the sibling list_primary_ips. However, the opening phrase 'Fetch a single resource by id' is generic and relies on the endpoint string plus the title to identify the resource, so it is clear but not crisp on its own.

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 statement of when to use this versus alternatives. The obvious sibling, hetzner_list_primary_ips, is never mentioned, nor is any condition such as 'use this when you already have an id.' Usage is only inferable from the name pattern.

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