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

Annotations already declare readOnlyHint=true, idempotentHint=true, openWorldHint=true and destructiveHint=false, so the safety profile is covered. The description adds useful provenance context ('linking a solved co-crystal to the potency numbers published for that target') but says nothing about result volume, pagination, or data freshness that annotations don't already imply.

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 sentences, zero filler: the first defines the data, the second defines the input and its authoritative use case. The key scoping statement is front-loaded before the parameter detail.

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 no output schema, the description does carry the burden of explaining returns, and it does so at a high level ('measured binding affinities', 'potency numbers'). It stops short of indicating what a result row contains or how large result sets behave, but for a simple two-parameter lookup tool this is close to sufficient.

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%, so both 'pdb' and 'limit' are already documented in the schema, including the 4-character format, the example 3EML, and the default/range for limit. The description only repeats the PDB ID format and example, adding no semantics beyond the schema — the baseline 3 for full coverage 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?

States a specific verb+resource ('measured binding affinities for the ligands associated with a PDB structure entry') and frames the exact question it answers. The input constraint (single 4-character PDB ID) makes it trivially separable from sibling tools like bindingdb_ligands_by_uniprot or bindingdb_targets_by_smiles, which take different key types.

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 'AUTHORITATIVE for "what is the measured affinity of the ligand in this PDB structure"' line gives an explicit routing rule that an agent can match against a user question. It lacks an explicit when-not-to-use clause or named alternative, so it falls just short of a 5.

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