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

Annotations already declare destructive=true, idempotent=true and readOnly=false, so the safety profile is covered. The description adds real value on top: it discloses the terminal effect ('Nothing further starts') and the exact failure mode for a repeat call (409 returning the real status), which an agent cannot infer from the annotations alone.

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, front-loaded with the action and its credential, then the terminal effect, then the error semantics. The closing 'No API key, no account and no sign up' is more onboarding copy than tool guidance but is at least short and placed last.

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?

With no output schema, the description carries the return burden and does address the failure path (409 with the real status). It is silent on the success response body, but for a two-parameter terminal action with full schema coverage that is a minor gap rather than a blocking one.

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% and both parameters are documented as the control_token and run_id returned at creation. The description only echoes the token requirement ('with its control token') and adds no format or sourcing detail beyond the schema, so the baseline 3 applies.

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?

States a specific verb and resource ('End a run') plus the required credential ('with its control token'), which separates it from read_agent_run, create_agent_run and complete_agent_run_step among the siblings. It stops short of naming the sibling it displaces, so it is clear but not maximally differentiated.

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?

Usage is implied rather than stated: you call it to terminate a run, and the note that a cancel after the end answers 409 tells the agent it should not be used to poll status. No explicit when-not or named alternative (e.g. complete_agent_run_step for normal finishing) is given.

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