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

Annotations already declare destructiveHint=true and readOnlyHint=false, so the safety profile is covered. The description adds genuinely useful nuance that tempers that hint: 'Additive and surgical: only the addressed cells change', clarifying that the write is scoped rather than wholesale. It does not mention the needs_confirmation/approvalId flow, but that is documented in the 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 core action, then behavior ('additive and surgical'), then the workflow prerequisite and construction advice. Four dense sentences with no filler, each carrying distinct 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?

For a 5-param mutation tool with no output schema, the description covers the prerequisite lookup, the write semantics, and how to compose a small table in one call. It omits any hint about the return value or the approval round-trip, but annotations plus the schema's approvalId field cover enough that the gap is minor.

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 the baseline is 3; the schema already documents the A1-keyed cells map, sheetId, tabName default, workspace rules and approvalId. The description's 'values or formulas' and A1 notes largely restate what the cells property description already gives, adding little beyond the baseline.

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 ('Sets cells in one tab of a sheet, by A1 address') and enumerates what can be written (values or formulas with an example). It also distinguishes itself from the broader sheet-level siblings like napkin_sheets_update by emphasizing the per-cell, surgical scope, so an agent can pick it 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?

Gives an explicit prerequisite ('Check napkin_sheets_schema first so you know the tab names and where data ends') and a structural convention ('put headers in row 1 when creating a new region'). It stops short of naming a competing tool to use instead for other cases (e.g. napkin_sheets_update vs this), but the context is clear.

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