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

With only readOnlyHint=false available, the description carries real weight: it discloses MIME-type inspection, the authoritative descriptor returned, the no-send boundary, and that normal recipient permissions and human review still apply. It omits idempotency, deduplication, and retention behavior, keeping it short of 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?

Four tight sentences, front-loaded with the size limit that most affects a calling decision, and every sentence contributes a distinct fact (limit, alternative, MIME handling, side-effect boundary).

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?

An output schema exists, so return values need not be spelled out, and the description still names the descriptor return. For a two-parameter upload tool the remaining gaps are negligible.

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 both parameters are fully documented in the schema, so the baseline is 3. The description's 2 MB ceiling loosely corroborates the base64 maxLength and restates the canonical-base64 requirement, but adds no new syntax or format detail.

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 (upload) plus resource (base64 file bytes) with an explicit scope limit and names the alternative route for oversized files. An agent can tell immediately what this tool does and which transport it uses versus the multipart REST path.

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 a clear size-based decision rule (up to 2 MB here, 10 MB via authenticated multipart REST) and clarifies that uploading does not send a message. It does not, however, differentiate from the sibling documents.upload, which an agent could reasonably confuse with this one.

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