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

The description adds behavior beyond the annotations: it discloses that an admin API key is required, that an unknown deployment_id fails with not_found, and explicitly notes 'Read-only.' Since annotations already declare readOnlyHint=true, idempotentHint=true, and destructiveHint=false, the description's added context about auth and error handling is valuable. There is no contradiction with annotations.

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 three sentences with zero filler. The first sentence front-loads the core purpose and returned fields, the second covers auth and error behavior, and the third gives the prerequisite and read-only note. Every sentence 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?

For a single-parameter tool with no output schema, the description is complete: it states what is returned (enumerated fields), the required auth, the failure mode, and the prerequisite sibling call. An agent has everything needed to invoke it correctly and interpret the result at a high level.

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 input schema already provides a full description for deployment_id: 'Deployment id from get_deployment.' With 100% schema coverage, the description adds little new semantic detail—it reinforces that the id refers to a deployment from get_deployment and calls it 'active,' but this is marginal. The baseline of 3 applies because the schema carries the parameter documentation burden.

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 clearly identifies the action (get), the resource (current-month cost details of a deployment by id), and enumerates the returned fields (provider, region, cost_usd_month, billable_cost_usd_month, billing_markup_pct, last_updated). It names get_deployment as a prerequisite, which implicitly differentiates it from that sibling, though it doesn't explicitly contrast with other cost-related tools like get_billing_info. Overall the verb and resource are specific enough for an agent to understand 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 gives clear preconditions: requires an admin API key, and the agent should 'Use get_deployment to find the active deployment first.' It also states the error outcome for an unknown deployment_id (not_found), which helps the agent anticipate failure. It doesn't explicitly say when not to use this tool or name alternative cost/billing tools, but the context provided is sufficient for a single-purpose lookup.

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