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

With no annotations available, the description carries the full burden of behavioral disclosure and delivers: it flags that this is a write operation, that it incurs no credit cost, and most importantly, that it will not send invites and only writes to the internal calendar. These are non-obvious side-effect characteristics that an agent must know before invoking.

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?

Three short sentences are front-loaded with the core purpose and follow with the cost and side-effect caveat; every sentence carries distinct information without redundancy. This is an efficiently sized description.

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 moderate complexity of five parameters and the absence of both annotations and an output schema, the description covers purpose, cost, and key side-effect limitations, which is strong. It is slightly incomplete in not stating what the tool returns after creation and not specifying any required credentials or permissions, though the schema covers required 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 coverage is only 40% (only starts_at and ends_at have 'ISO datetime' descriptions), yet the description adds no parameter-level meaning—title, notes, and all_day remain unexplained. The tool description doesn't mention any of the five parameters, so it fails to compensate for the schema's gaps.

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 the specific verb-resource pairing 'Add a custom event to the CRM planner,' which clearly names the action, the object, and the destination. It also sets the tool apart from sibling update/delete/list calendar tools by framing this as a creation action, and the no-invite caveat differentiates it from meeting/connect tools.

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 gives clear context for when this tool is appropriate: creating an internal CRM calendar event, and it explicitly excludes the use case of sending invitations ('Does NOT send invites to anyone'). No alternative sibling is named, so the guidance is strong but not fully explicit.

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