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

The description adds a meaningful behavioral detail beyond what annotations provide: it explicitly states that only the supplied fields are updated, implying omitted fields remain unchanged. This valuable PATCH-like behavior is not fully expressed by the readOnly hint or destructive hint and helps reduce risk during invocation.

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 a single front-loaded sentence with no filler: 'Updates only the supplied plan-item fields.' The phrasing is compact and immediately readable while still conveying the core operation and its partial-update scope.

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?

This is a simple update operation with a 4-field schema, 100% schema coverage, and annotations for safety and idempotency. The only notable gap is missing guidance about how this tool relates to sibling operations like update_campaign or create_campaign_plan_item, but the description plus schema gives enough information to plausibly select and call it in most contexts.

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?

Since the schema covers 100% of parameters, the description's role is smaller. However, 'only the supplied fields' clarifies that optional parameters are truly optional and that omitting them leaves their existing value unchanged, highlighting a semantic nuance that would otherwise be guessed.

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 uses a specific verb 'Updates' and resource 'campaign plan', then adds an important scoping qualifier: 'only the supplied fields.' This conveys partial update semantics and differentiates the tool from create/list plan-item operations, even though it does not explicitly name the alternative tool.

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?

There is no guidance on when to use this tool versus alternatives such as invoke_create_campaign_plan_item or invoke_update_campaign. The reader must infer the appropriate selection solely from the tool name and sibling list, providing no confidence about when this is the right choice.

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