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Proton Safe MCP

by fbossiere
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Upload attachment chunk

upload_attachment_chunk

Send the next base64 chunk for a staged attachment upload, strictly in index order, to continue building the file before finishing. Returns received bytes and the next chunk index to send.

Instructions

Append the next base64 chunk to an attachment upload, strictly in index order. This is step 2 of 3, repeated until every byte declared to begin_attachment_upload has been sent, then closed with finish_attachment_upload. A chunk that is empty, not valid base64, larger than the max_chunk_bytes begin_attachment_upload reported, out of order, or past the declared total size is refused without being stored; the upload stays open at its current position, so retry that same index rather than restarting. It returns received_bytes, expected_bytes, and the next_chunk index to send.

Input Schema

TableJSON Schema
NameRequiredDescriptionDefault
upload_idYesThe upload_id begin_attachment_upload returned for this file. It identifies the staged upload only and is not the attachment token a draft accepts.
chunk_indexYesZero-based position of this chunk: 0 for the first, then the next_chunk value the previous call returned. Gaps and replays are refused, so the staged bytes can only be the declared file, in order.
data_base64YesThis chunk's bytes, base64-encoded. The decoded length must not exceed the max_chunk_bytes begin_attachment_upload reported, 384 KiB by default. Send encoded bytes only: this is never a path and never a data URL.

Output Schema

TableJSON Schema
NameRequiredDescriptionDefault

No arguments

Schema Changelog

Changes observed during successful MCP inspections.

  1. Changed3 schema fields changedv2.3.0
    • addedInput schema / properties / chunk_index / description
      Added value: +"Zero-based position of this chunk: 0 for the first, then the next_chunk value the previous call returned. Gaps and replays are refused, so the staged bytes can only be the declared file, in order."
    • addedInput schema / properties / data_base64 / description
      Added value: +"This chunk's bytes, base64-encoded. The decoded length must not exceed the max_chunk_bytes begin_attachment_upload reported, 384 KiB by default. Send encoded bytes only: this is never a path and never a data URL."
    • addedInput schema / properties / upload_id / description
      Added value: +"The upload_id begin_attachment_upload returned for this file. It identifies the staged upload only and is not the attachment token a draft accepts."
  2. First observedv0.1.0

TDQS

A5/5.0
Behavior5/5

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

The description discloses stateful behavior beyond annotations: chunks are refused and not stored on validation failure, the upload position remains unchanged, and ordering is enforced. It also explains the return fields (received_bytes, expected_bytes, next_chunk), which adds important runtime context.

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?

Every sentence earns its place: core action, sequence position, failure modes, retry behavior, and return values are all packed in without redundancy. The description is dense but organized and fully front-loaded with the main purpose.

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?

The description fully covers what an agent needs to invoke this tool correctly: ordering, size limits, rejection conditions, retry semantics, and the following step. Combined with the provided output schema and parameter descriptions, nothing essential is missing.

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

Parameters5/5

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

Even though schema coverage is 100%, the description adds key semantic meaning: upload_id identifies a staged upload, not the final attachment token; chunk_index follows next_chunk returned from the previous call; data_base64 must be raw encoded bytes, never a path or data URL. This meaningfully reduces misuse risk.

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 action ('Append the next base64 chunk to an attachment upload') and the strict ordering constraint. It clearly identifies its place in a multi-step process and is distinguishable from sibling tools like begin_attachment_upload, finish_attachment_upload, and discard_attachment.

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?

Explicitly frames this as step 2 of 3, repeated until all declared bytes are sent, then closed with finish_attachment_upload. It also gives concrete retry guidance: on refusal, retry the same index rather than restarting, because the upload stays open at its current position.

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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