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

With no annotations, the description carries the full burden, and it does add real value: free of cost, no external lookup (deterministic, local computation, no data leaves the system), and the specific algorithmic standard applied. It is silent, however, on what happens with an invalid input or what the validation result conveys (pass/fail only, or a reason code), which matters most for an agent deciding whether to call it.

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 clauses, front-loaded with the free/no-lookup qualifiers before the core purpose. No sentence is padding; the examples of modulus variants are the only elaboration and they are the informative part.

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 deliberately narrow two-parameter validator with full schema coverage and no output schema, the description supplies purpose, cost model, and dependency profile. The remaining gap is return semantics, which the absence of an output schema leaves unaddressed.

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%, with 'sort_code: 6 digits, e.g. 20-00-00' and 'account_number: 6-10 digits' already spelling out format and length. The description adds nothing about parameter semantics beyond the schema, so the baseline of 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?

Names a specific verb and resource ('Validate one UK sort code + account number') plus the exact rule set applied (Vocalink/Pay.UK modulus checking, MOD10/MOD11/double-alternate and exceptions). The word 'one' cleanly separates it from the sibling sort_code_batch_check.

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 only implied: 'one' signals single-record scope versus the batch sibling, and 'No outside lookup' signals a self-contained check. It never states when to prefer it over sort_code_batch_check or what prerequisite state an agent needs, so the routing decision 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.