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

Annotations already declare readOnly, idempotent, non-destructive and closed-world, so the safety profile is covered. The description adds real value beyond that: it discloses the access path (relay route vs client-side scan) and the exact result contract, including the isError row_not_found case, which annotations do not convey.

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, no filler, and the core action is front-loaded before the mechanism and return detail. Every clause carries information.

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 steps in to describe the return shape ('{ row }') and the error case ('isError row_not_found'), which is the key missing piece for this tool. Minor gaps remain (e.g. permission requirements, behavior on partial keys), but nothing essential for a correct call is absent.

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 app_id, collection and key are already documented in the schema; the description only echoes 'by its key'. It adds no format, scoping or lookup semantics beyond the structured fields, so the baseline 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?

States a specific verb and resource ('Fetch a single row by its key'), the domain ('v2 app collection'), and the mechanism ('dedicated relay route rather than a client-side scan'). This clearly separates it from list_rows, count_rows, and update_row without needing to open any 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?

The phrase 'by its key' and the contrast with a client-side scan imply the correct usage (you already have the key), which is useful context. However, it never names a sibling alternative or an explicit when-not-to-use condition, so the routing guidance is only implied.

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