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

Annotations already indicate this is not read-only, is idempotent, and is not destructive, so the bar is lower. The description adds valuable behavioral context: it is a partial update ('Pass only the fields to change') and that criteria overwrites existing rules ('criteria replaces the existing rules'). This goes beyond the annotation hints without contradicting them.

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 two tight sentences (18 words), front-loaded with the action and resource, followed by two crucial hints. There is no filler, redundant schema repetition, or ambiguity. Every word earns its place.

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 tool has 5 parameters with a complex criteria object, the description plus full schema coverage provides enough information to select and invoke it correctly. It explains the partial-update semantics and criteria replacement, leaving no major gaps. The presence of an output schema further reduces the need to explain return values.

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?

The schema has 100% coverage with descriptive parameter docs, so the baseline is 3. The description adds extra meaning by explaining the overall update pattern (only specified fields change) and highlighting the criteria replacement behavior, which complements the schema's per-field descriptions and helps the AI understand how to construct a valid request.

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 'Update an existing customer segment by id' – a specific verb ('Update') and resource ('customer segment') with a unique identifier, clearly distinguishing it from create/delete/get segment siblings. It also specifies the partial-update behavior ('Pass only the fields to change'), reinforcing the exact operation.

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 on when to use the tool: any time you need to modify an existing segment. It states the partial-update pattern ('Pass only the fields to change') and highlights the critical replacement semantics for criteria, but it does not explicitly mention alternative tools (e.g., set_segment_members) or exclusions. Still, the usage intent is unambiguous.

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