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

Annotations already carry the read-only/idempotent/non-destructive profile, and the description adds substantial information beyond them: monster-typed companions are excluded server-side, entries are per (Main, companion) pair so duplicates are expected, and it decodes permission-bearing fields (canEdit, canOpenInVault) including who qualifies for vault access. That is exactly the kind of edge-case and authorization context annotations can't express.

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?

Purpose and scope are front-loaded and every sentence carries some information, but the later half enumerates return fields (companionOf, canEdit, canOpenInVault, display fields, primaryToken) in a single dense paragraph — much of which an output schema already covers. Slightly overwritten, though not wasteful enough to penalize heavily.

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 one-parameter list tool, the description covers scope, sibling routing, permission semantics, duplication behavior, server-side filtering, and the natural follow-up call. Nothing an agent needs to invoke or interpret the result is missing, even setting the output schema aside.

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 coverage is 100% and the single parameter (campaignId) is already documented in the schema as the campaign's Mongo _id. The description adds no further meaning about the parameter, 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?

States a specific verb and resource ('List the companions (buddies) attached to a play campaign's characters') and immediately scopes it ('the supporting cast a table sees beyond the roster'), which cleanly separates it from the sibling play_list_campaign_characters. An agent can identify the tool's job without opening the 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?

It names the management counterparts (vault_attach_companion / vault_detach_companion) so the agent knows this tool isn't the place to mutate ties, and it prescribes the follow-up call ('Pair with vault_get_character on companionCharacterId'). It stops short of an explicit when-not-to-use statement or a direct contrast with sibling list tools, so it's strong context without full routing.

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