Start media upload

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

Annotations only carry readOnlyHint=false and destructiveHint=false, so the description does most of the work. It adds genuinely useful behavior context—this tool reserves a URL rather than transferring the file itself. But it omits consequential traits: presigned URL expiration, whether finalize_media_upload must follow, and what happens to an unused reservation.

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 with the core purpose front-loaded. The second sentence is redundant—'Use this when you need a presigned URL for a large direct upload' restates the reservation concept, and 'prefer upload_media for normal files' duplicates 'For scripted uploads use upload_media instead'—so it could be tightened without losing 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 leaves the return value undescribed, even though the presigned URL (and any reservation ID or expiration) is the entire point of the call. The multi-step flow is also incomplete: the finalize_media_upload sibling suggests a follow-up step that the agent is never told about. For a 2-param tool this is adequate but not complete.

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%: both sizeBytes and contentType have descriptive text in the input schema. The description adds no parameter-level meaning (no size limits, no allowed MIME types, no constraints), so the baseline 3 applies with the schema doing the heavy lifting.

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: 'Reserve a presigned R2 upload URL' for a 'direct browser upload,' and anchors it to the composer UI. It explicitly distinguishes from the nearest sibling by adding 'For scripted uploads use upload_media instead,' so an agent can tell this tool apart without opening either 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 when-to-use ('Use this when you need a presigned URL for a large direct upload') and an explicit exclusion ('prefer upload_media for normal files'). However, it never addresses the multipart upload siblings (create_multipart_upload, finalize_media_upload), so an agent has no guidance on when the multipart flow should be chosen instead.

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