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

The description's only behavioral claim, 'Read-only', merely restates annotations that already declare readOnlyHint=true, idempotentHint=true, and destructiveHint=false. It adds no extra context such as what is returned if no companion is set, though for a trivial zero-param read this is a modest gap.

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 with the purpose front-loaded and no filler. The trailing 'Read-only' sentence is redundant with the annotations, which is the only wasted element.

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 parameterless, read-only lookup with no output schema, the description gives enough to call it correctly. The one missing piece is the shape of the return value (e.g., what happens if no companion is chosen), since no output schema exists to cover that.

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 no parameters, so there is nothing for the description to disambiguate; baseline 4 applies. The schema (additionalProperties: false, empty properties) is fully consistent with a parameterless call.

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 ('Returns') and resource ('which companion character the user has chosen'), which is clearly distinct from sibling get_* tools covering courses, habits, meals, plans, etc. It doesn't explicitly name those siblings, but the resource noun alone is unambiguous.

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?

The purpose implies when to call it (retrieving the chosen companion), and with zero parameters there's no input decision to guide. However, there is no explicit 'use this when...' framing or reference to any alternative tool, so guidance is implied rather than stated.

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