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

With readOnlyHint already declaring safety, the description still adds real context: it warns the call hits the UpCloud API and can take seconds, explains the shared-snapshot trap that would otherwise produce a false 'no GPU available' conclusion, and flags that a one-server gateway plan loses all cluster egress during a rebuild. These are behavioral consequences not derivable from annotations or 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?

Front-loaded with the what-is-returned summary before the routing rules, and every clause carries information. It is nonetheless dense and drops into parenthetical asides, so it reads as longer than strictly necessary for a two-parameter read tool.

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?

There is no output schema, so the description must carry the return shape — and it does, naming zone descriptions, per-plan vCPU/memory/storage/current_offering/EUR price/GPU count and model, and gateway_plans fields with their operational meaning. Nothing an agent needs in order to call this and interpret the result is missing.

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 adding that the two Helsinki zones are distinguishable only by description, which tells the agent how to pick a sensible zone value rather than guessing from the zone id. It does not restate the credential_id prerequisite (list_provider_credentials), but the schema already carries that.

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 ('List the LIVE UpCloud catalogue a credential can reach') and then enumerates exactly what the payload contains — zones, plans with named fields, and gateway_plans. It explicitly distinguishes itself from the sibling list_instance_types in the first sentence, so an agent can route correctly without opening either schema.

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?

Gives an explicit directive ('ALWAYS call this instead of list_instance_types before create_upcloud_cluster or before answering any question about UpCloud zones, plans, GPUs, or prices') plus the reason the alternative is unsafe: the frozen snapshot stops at PREMIUM-8xCPU-32GB and has no GPU plan. Both the when-to-use and the when-not-to-use with named alternative are present.

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