n8n Binary and Data

Every n8n item carries two independent slots: $json for structured data and $binary for file bytes. They travel side by side through the workflow. File contents, the actual PDF, image, or zip, live in $binary, never in $json. Get that split wrong and you read an empty field, lose a file mid-flow, or hand an AI agent a tool input it can't use.

This skill covers where binary lives, how to read and write it, how to keep it from being silently stripped, the hard wall between binary and the AI-agent tool boundary, and why chat surfaces need a URL instead of raw bytes.

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The three rules that prevent 90% of binary bugs

1. File contents are in $binary, not $json. After an HTTP download, a "Read Files", or an email-attachment trigger, the bytes sit in $binary.<key>. $json holds metadata at most. Reading $json.data for file contents gives you nothing.

2. Binary cannot cross the AI-agent tool boundary, in either direction. Tool arguments and tool return values are JSON only. An uploaded image can't be passed into a tool as a file, and a tool can't return raw bytes. Pre-stage to storage and pass a key or URL through JSON instead. See AGENT_TOOL_BINARY.md.

3. Chat surfaces render images by URL, not by $binary. Slack, Discord, Teams, Telegram, embedded webhook chat, none of them read the binary slot. The image has to live somewhere a URL can fetch it. See CDN_REQUIREMENT.md.

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The two slots

Each item is shaped like this:

{
  "json": { "customerId": 42, "status": "sent" },
  "binary": {
    "invoice": {
      "data": "<base64-encoded bytes>",
      "mimeType": "application/pdf",
      "fileName": "invoice-42.pdf",
      "fileExtension": "pdf"
    }
  }
}