PostPen

Create media upload

create_upload

Returns a private, short-lived upload URL for a file that has no public HTTPS link (a carousel PDF, an image, or an mp4). The file is sent with an HTTP PUT of its raw bytes to upload_url using the returned headers; the returned upload_id is then accepted by save_draft as media: [{ upload_id }] (an image item can add alt_text there), with id to attach it to an existing draft or scheduled post. The URL expires soon (expires_in gives the seconds left; it can be as little as 60), so upload right away. Limits: images 5MB, video 75KB–200MB mp4, PDF 100MB.

Tool call template

Replace each placeholder with a value matching the input schema. Only required parameters are included; add optional parameters as needed.

{
  "id": 1,
  "jsonrpc": "2.0",
  "method": "tools/call",
  "params": {
    "arguments": {
      "type": "<type>"
    },
    "name": "create_upload"
  }
}

Input Schema

Name	Required	Description
`type`	Yes	What you will upload: image (jpeg, png, webp, gif), video (mp4), or document (pdf).
`size_bytes`	No	Optional file size in bytes, checked against the limit before you upload.
`content_type`	No	Optional MIME type, for example image/png, video/mp4, or application/pdf. Defaults from type.

Schema Changelog

Changes observed during successful MCP inspections.

First observedOct 5, 2026

TDQS

A4.9/5.0

Behavior5/5

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

Annotations declare the mutation/non-idempotent/safety profile, but the description adds what they cannot: the URL is private and short-lived, expires_in can be as little as 60 seconds so the upload must happen immediately, and per-type size ceilings (5MB images, 75KB–200MB mp4, 100MB PDF). That is exactly the operational context an agent needs before calling.

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?

One dense paragraph, front-loaded with the return value and purpose, then the call procedure, then the expiry warning, then the hard limits. Every clause carries information an agent would otherwise have to discover by error.

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?

No output schema exists, yet the description names every returned field (upload_url, headers, upload_id, expires_in) and explains how each is consumed. Combined with the size limits and expiry timing, nothing needed to invoke this correctly is missing.

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

Parameters4/5

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

With 100% schema coverage the baseline is 3, and the description earns above it by supplying the actual size limits that 'size_bytes' only vaguely references ('checked against the limit'), plus the concrete enum examples (carousel PDF, image, mp4). It stops short of explaining content_type defaults beyond what the schema already says.

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 ('Returns a private, short-lived upload URL for a file') and immediately scopes it to files with no public HTTPS link, which distinguishes it from list/publish siblings. The named artifact types (carousel PDF, image, mp4) make the resource concrete.

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 an explicit when-to-use condition ('a file that has no public HTTPS link'), the exact next action (HTTP PUT of raw bytes to upload_url with the returned headers), and the downstream handoff to save_draft via media: [{ upload_id }] with optional alt_text. It even routes the agent to the 'id' field when attaching to an existing draft or scheduled post.

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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