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

Annotations already declare readOnlyHint=true, so the safety profile is covered. The description adds only that the call takes no arguments and repeats sibling routing; it adds little else about determinism or that the result is a fixed public href (which sits in the schema description). Adequate but thin 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?

Two short clauses, front-loaded with WHAT and the routing hint; every word earns its place with no padding.

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 zero-argument, read-only tool with an output schema, the description plus schema fully cover what an agent needs to invoke it correctly. The only marginal gap is not restating the exact returned value, which the output schema already handles.

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 tool takes zero parameters, which is the baseline-4 case. 'Call with {}' reinforces the empty-argument contract and matches the schema and annotations with no conflict.

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 resource and output ('tel: href for the landline (Festnetz)') and explicitly differentiates itself from the nearest sibling by name ('Mobile/24h emergency = compose_tel_mobile'). An agent can tell it apart from compose_tel_mobile without opening either 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?

Names the alternative sibling and the condition that selects it (mobile/24h emergency), which is exactly the routing decision an agent needs. It does not spell out the positive 'use when' case for the landline explicitly, but for a zero-arg compose tool the alternative routing is the useful part.

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