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

Annotations already indicate this is a write operation and not idempotent, so the description does not need to restate that. It adds useful behavioral nuance by noting that 'values are strings' and that 'paths are dnd5e; other systems differ', which helps an agent anticipate system-specific failures or path-format issues. The worked example also clarifies the change-mode semantics.

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 compact: two sentences plus a thin JSON example. It front-loads the core action, then provides a detailed illustrative example that anchors the parameter semantics. No words are wasted, and the system-specific caveat is packed into the final clause without bloat.

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 tool with 8 parameters, nested objects, and no output schema, the description gives enough context to invoke it correctly: it explains what the tool does, provides a realistic changes payload, and warns about system-specific path differences. It does not describe the return value or error behavior, but those are less critical for a create operation and the schema already documents all parameters.

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 description coverage is 100%, so the baseline is 3. The description adds value beyond the schema by giving a concrete example of the changes array structure, clarifying that mode 2 in the example means a +2 AC bonus, and warning that data paths are dnd5e-specific. This practical elaboration helps an agent construct valid parameters without merely restating the 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 opens with a specific verb and resource: 'Add a custom active effect to an actor', immediately distinguishing it from sibling tools like effect-update, effect-delete, and effect-list. It further clarifies scope with 'buff, debuff, custom condition' and gives a concrete dnd5e example that leaves no ambiguity about what the tool does.

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 establishes clear context: this tool is for creating custom active effects on actors, which separates it from effect-update and effect-delete. It does not explicitly name sibling tools or give when-not-to-use guidance, but the 'custom' qualifier and the create-versus-update phrasing imply the appropriate use case.

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