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

No annotations are provided, so the description carries the full burden, and it delivers a lot: required API key location, required payment header, two concrete price/unit tiers, and the guarantee that 'the units land in that workspace and nothing new is created.' It stops short of stating failure/partial-payment or refund behavior, so not quite a 5.

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 that front-load the core action before the credential/payment logistics. Minor redundancy exists between the description and schema on the api_key/_meta preference, but every sentence contributes.

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 one-parameter purchase tool with no annotations and no output schema, the description covers the preconditions, the credential and payment headers, the price tiers, and the post-condition on the ledger. An agent has everything needed to call it correctly.

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% and the single api_key parameter is already fully documented in the schema, including the '_meta preferred because arguments are logged' guidance. The description's mention of the key largely repeats that, adding only the framing that the key identifies the target workspace. Baseline 3 for a fully covered schema.

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?

Specific verb+resource: 'Add units to a workspace you already hold.' It explicitly differentiates from provision_workspace by stating that buying a workspace 'gives you an empty ledger under a different key.' An agent can distinguish this from all siblings without opening a 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?

States the precondition ('a workspace you already hold'), the credential source (_meta or argument), the payment mechanism, and the exact alternative (buy a workspace instead) with the reason it differs. Nothing about when to pick this vs a sibling is left to inference.

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