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

With no meaningful behavioral annotation coverage beyond readOnlyHint/destructiveHint flags, the description carries the full burden and succeeds. It discloses the fixed 10MB partSize, presigned URL validity of 3600s, the requirement to PUT each slice and collect ETags, and that the upload completes via a separate call. No annotation contradiction exists.

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?

The description is compact and front-loaded: purpose first, then return values, then step-by-step workflow, then when-not-to-use guidance. Every sentence adds operational information with no filler or redundancy.

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?

There is no output schema, so the description correctly explains what the tool returns and how to use those values. It also covers size limits, chunking mechanics, URL expiration, ETag handling, and the completion step. For a tool of this complexity, nothing essential is missing.

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 schema already documents contentType and sizeBytes, including allowed sizes. The description adds the 10MB chunking context and clarifies how sizeBytes relates to the multipart flow, but this is contextual rather than necessary to understand the two parameters. A baseline of 3 is appropriate.

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?

The description states a specific verb ('Start') and resource ('chunked direct-to-storage upload'), and clearly distinguishes it from ordinary single uploads. It also frames the output (r2Key, uploadId, partSize, presigned URLs) and explicitly names the sibling complete_multipart_upload as the next step, making the tool's role unmistakable.

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?

Usage guidance is explicit: 'For ordinary files prefer upload_media' and 'Use this when a file is too large for a single upload'. It gives concrete size thresholds, the chunking workflow, and points to complete_multipart_upload as the required follow-up, leaving no ambiguity about when to choose this tool.

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