Stream Vessel

Start a large upload

start_multipart_upload

Destructive

Opens a multipart upload for a file too big for one PUT (S3 refuses a single PUT over 5 GiB). Returns an uploadId; nothing appears at the key until complete_multipart_upload runs, and the parts are billed until it does - so call abort_multipart_upload if you give up. Then: presign_upload_parts for URLs, PUT each part yourself keeping the ETag each returns, and complete with those ETags.

Input Schema

Name	Required	Description
`key`	Yes	Full object key.
`bucket`	Yes	Bucket name.
`overwrite`	No	Allow this upload to replace an object already at the key. Checked now and again at completion, which is when the replacement actually happens.
`contentType`	No	Content-Type of the finished object; left out, the provider picks its default, usually application/octet-stream.
`cacheControl`	No	Cache-Control to store on the object, such as max-age=3600.
`connectionId`	Yes	Connection id from list_connections.
`userMetadata`	No	User metadata to store on the object, as a map of name to value.
`contentEncoding`	No	Content-Encoding to store on the object, such as gzip for a body compressed before upload.
`contentLanguage`	No	Content-Language to store on the object, such as en-US.
`contentDisposition`	No	Content-Disposition to store on the object, such as attachment; filename="report.pdf".

Schema Changelog

Changes observed during successful MCP inspections.

Changed6 schema fields changedOct 27, 2026

addedInput schema / properties / cacheControl

Added value: +{
+  "description": "Cache-Control to store on the object, such as max-age=3600.",
+  "type": "string"
+}

addedInput schema / properties / contentDisposition

Added value: +{
+  "description": "Content-Disposition to store on the object, such as attachment; filename=\"report.pdf\".",
+  "type": "string"
+}

addedInput schema / properties / contentEncoding

Added value: +{
+  "description": "Content-Encoding to store on the object, such as gzip for a body compressed before upload.",
+  "type": "string"
+}

addedInput schema / properties / contentLanguage

Added value: +{
+  "description": "Content-Language to store on the object, such as en-US.",
+  "type": "string"
+}

changedInput schema / properties / contentType / description

Previous value: -"Content type of the finished object; defaults to application/octet-stream."New value: +"Content-Type of the finished object; left out, the provider picks its default, usually application/octet-stream."

changedInput schema / properties / userMetadata / description

Previous value: -"User metadata to store on the finished object, as a map of name to value."New value: +"User metadata to store on the object, as a map of name to value."

First observedOct 16, 2026

TDQS

A4.7/5.0

Behavior5/5

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

Goes well beyond the annotations (destructiveHint=true, non-idempotent) by disclosing that nothing appears at the key until completion, that parts incur billing until then, and that an uploadId is returned. These are non-obvious, costly-to-get-wrong behaviors that neither annotations nor schema convey.

Agents need to know what a tool does to the world before calling it. Descriptions should go beyond structured annotations to explain consequences.

Conciseness5/5

Is the description appropriately sized, front-loaded, and free of redundancy?

Four short sentences, front-loaded with the size threshold and followed by the return value, the failure/billing caveat, and the workflow. Every sentence carries distinct operational information with no padding.

Shorter descriptions cost fewer tokens and are easier for agents to parse. Every sentence should earn its place.

Completeness5/5

Given the tool's complexity, does the description cover enough for an agent to succeed on first attempt?

For a 10-parameter mutating tool with no output schema, the description supplies the key return value (uploadId) and the entire surrounding workflow plus cleanup obligation, which is exactly what an agent needs to invoke it and act on the result.

Complex tools with many parameters or behaviors need more documentation. Simple tools need less. This dimension scales expectations accordingly.

Parameters3/5

Does the description clarify parameter syntax, constraints, interactions, or defaults beyond what the schema provides?

Schema coverage is 100%, so all ten parameters, including overwrite's completion-time re-check, are already documented in the schema. The description adds no parameter-level detail beyond what is structured, which is the baseline 3 case.

Input schemas describe structure but not intent. Descriptions should explain non-obvious parameter relationships and valid value ranges.

Purpose5/5

Does the description clearly state what the tool does and how it differs from similar tools?

States a specific verb and resource ('opens a multipart upload') with the defining condition ('a file too big for one PUT, S3 refuses a single PUT over 5 GiB'). This distinguishes it cleanly from write_object, which the agent would otherwise pick for file writes.

Agents choose between tools based on descriptions. A clear purpose with a specific verb and resource helps agents select the right tool.

Usage Guidelines5/5

Does the description explain when to use this tool, when not to, or what alternatives exist?

Gives explicit when-to-use (file over 5 GiB), a full downstream recipe (presign_upload_parts, PUT each part keeping ETags, complete_multipart_upload), and a when-to-abandon path ('call abort_multipart_upload if you give up'). All alternatives are named siblings, leaving nothing to inference.

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

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