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

Annotations already mark the operation as a write with idempotent and non-destructive hints; the description adds meaningful detail by stating the source character is never written, the state is replaced wholesale, maximums are dropped, and a stale baseUpdatedAt triggers OUT_OF_SYNC with no write. No contradiction with 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?

Every sentence carries a distinct, necessary fact, and the most important scoping statement is front-loaded. The length is justified by the tool's complexity; there is no filler or 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?

For a write operation with conditional applicability and a concurrency guard, the description covers what is written, what is not written, the required workflow, the failure mode, and the alternative tool. With an output schema present and the mutating behavior covered by annotations, nothing essential is missing for correct selection and invocation.

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?

Although the input schema declares zero properties, the description names two effective parameters (`state` and `baseUpdatedAt`) and explains their semantics — full replacement, no maximums, and the need to copy updatedAt from a fresh read. It does not supply exact types or a full state shape, so it stops short of a 5, but it meaningfully compensates for the empty schema.

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?

Effectively identifies the operation as writing a LINK-mode companion row's transient overlay for locally-tracked values, naming concrete fields (hit points, temp HP, hit dice, etc.) and explicitly distinguishing from owned/clone mode rows. This clearly differentiates it from sibling vault_update_character.

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?

Provides an explicit routing rule: LINK-mode companions use this tool, while 'owned' (or legacy 'clone') rows should go to vault_update_character instead. It also prescribes the correct call sequence — vault_get_character the Main first, modify its state, send it back complete with a fresh updatedAt — leaving no ambiguity about when and how to use it.

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