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

Annotations only declare that this is a non-read-only, non-destructive, non-idempotent, closed-world call; the description supplies the effect that matters most — a mutual, permanent bar on future deals ('Neither of you can start a deal with the other again'). It does not say whether a block can be lifted, nor that the effect is symmetric for the seller's view. Mild tension exists with idempotentHint=false given the optional idempotency_key retry semantics, but that is a schema-level detail, not a contradiction in the description.

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?

Two short sentences, front-loaded with the action and its two target modes, with the consequence in the second sentence. Nothing is redundant with the title or schema.

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?

An output schema exists and annotations cover the safety profile, so the description only needs to carry the action, its target options, and the effect — all present. The notable gap is that the schema marks only owner_key as required and does not constrain the call to exactly one of deal_id/listing_id, a rule the description implies but never states.

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 earns an extra point by explaining what each identifier resolves to as a target (deal_id blocks the counterparty, listing_id blocks the seller), which the schema does not say. It says nothing about owner_key or idempotency_key semantics beyond what the schema documents.

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 (block) and names the two exact targets (the other side of a deal via deal_id, or the seller of a listing via listing_id). No sibling tool performs blocking, so an agent can identify this action unambiguously from the name plus first sentence.

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 parenthetical mapping tells the agent which identifier selects which target, which is genuine usage guidance. It stops short of stating exclusions or prerequisites — e.g. that exactly one of deal_id/listing_id should be supplied, and that owner_key must come from an authenticated identity (only the schema says this).

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