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

The description adds substantial behavioral context beyond the annotations: only supplied fields are sent, baseUpdatedAt must be copied from a fresh play_get_session read, stale or fabricated values are rejected with OUT_OF_SYNC, and nothing is written on rejection. It also clarifies that null untags an imagineAdventureId. This is exactly the kind of procedural detail an agent needs.

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: usage, supported operations, partial-update behavior, permission requirement, and the critical baseUpdatedAt precondition are all front-loaded. The REQUIRES emphasis is justified because it prevents a common failure mode.

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?

The description covers what the tool does, what fields it can update, the permission needed, the optimistic-concurrency requirement, and the error behavior. Since an output schema exists, the description does not need to explain return values, and nothing essential for invoking the tool correctly is missing.

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 lists no properties, the description gives meaningful parameter semantics: baseUpdatedAt must be the updatedAt from a fresh read, imagineAdventureId must come from the campaign's imagineAdventures list, and null untags. This far exceeds the baseline for a zero-parameter 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?

The description states a specific verb and resource ('Update a session by id') and enumerates concrete use cases: appending session notes, changing the date, renaming, and tagging with an Imagine adventure. It clearly separates this from sibling session tools like play_get_session, play_create_session, and play_delete_session.

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 explains when to use the tool ('Use this to append session notes, change the date, rename, or tag the session') and adds important preconditions such as requiring 'contribute' permission and a fresh baseUpdatedAt. It does not explicitly call out when-not-to-use or name alternative tools, but the use cases are concrete enough for an agent.

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