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

Annotations already declare readOnlyHint=true, idempotentHint=true, and destructiveHint=false, so the safety profile is covered. The description adds substantial behavioral context beyond annotations: monster-typed companions are excluded server-side, one entry exists per (Main, companion) pair so duplicates appear, and each entry contains companionOf plus display fields and a projected primaryToken slice or null. This is exactly the kind of nuance agents need.

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?

The description is dense but every sentence earns its place: it captures purpose, read-only status, tie management, server-side filtering, cardinality, output shape, and a useful follow-up call. The essential identity of the tool is front-loaded in the first sentence, and the additional clauses layer detail without repetition.

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?

Given the one-parameter schema, rich output schema, and annotations already conveying safety, the description covers all operational behavior an agent needs: duplicate handling, monster exclusion, output fields, and the companionOf structure. It also connects to vault_get_character for deeper retrieval, making the tool fully self-contained in context.

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 only parameter, campaignId, has 100% schema coverage with a description identifying it as a Mongo _id. The description does not add input-format detail beyond that, but with a single fully documented parameter there is little left to compensate for. The reference to companionCharacterId is about output pairing, not parameter semantics, so a baseline 3 is appropriate.

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 clearly states a specific verb and resource: 'List the companions (buddies) attached to a play campaign's characters.' It also distinguishes this from roster listing by calling the result 'the supporting cast a table sees beyond the roster' and by naming the attachment tools vault_attach_companion / vault_detach_companion as managing the tie itself. This leaves no ambiguity about what the tool returns.

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 description explicitly routes mutation work elsewhere: 'the tie itself is managed on the vault character sheet (vault_attach_companion / vault_detach_companion).' It also recommends pairing with vault_get_character for full companion sheets. This gives clear when-to-use context and names concrete alternatives, even without a separate 'use this when' phrase.

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