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

Annotations already signal mutation and idempotency, but the description adds important non-obvious behavior: unsubscribing frees market data lineshare and 'all=true' also stops streams opened by other clients of the same server. It does not contradict any annotation, and the server-wide side effect is valuable context the agent could not infer from the schema.

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 tight sentences: what it does, how to invoke it, and the critical all=true caveat. No fluff, no repetition of the schema, and the most important decision (which parameter to use) is front-loaded in the second sentence.

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 an output schema present and annotations covering idempotency and world effects, the description is largely complete for correct invocation. It covers single vs. all scopebinary behavior and the server-wide consequenceholistic. The main gaps are the unspecified behavior when both or neither parameter is supplied, but these are minor against an otherwise solid definition.

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 100%, so the baseline is 3. The description adds value by framing the two parameters as mutually exclusive alternatives and by explicitly tying 'works for every kind of subscription' to the cancellation scope. That goes slightly beyond the schema's per-parameter descriptions.

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 a specific action and resource: 'Cancel one subscription, or all of them', followed by the consequence of freeing IBKR market data lines. It makes the tool's scope explicit and distinguishes it from the many subscribe_* and list_subscriptions siblings.

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?

It explicitly states the two invocation modes ('Pass either subscription_id or all=true') and clarifies when all=true is the right choice, including the cross-client side effect. It does not explicitly discuss what happens if both are passed or neither is provided, but there are no competing cancellation tools to distinguish between.

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