/n8n-binary-and-data

name: n8n-binary-and-data
description: Handle files and binary data in n8n correctly. Use when working with files, images, PDFs, attachments, uploads or downloads, base64, vision/multimodal input, or when an AI agent needs a file as tool input or output — and whenever the user mentions $binary, binaryPropertyName, "read the PDF", "attach the file", "send the image", Merge losing binary, or a CDN for chat images. Covers the $binary vs $json split, reading/writing binary, keeping binary alive across transforms with Merge, the agent-tool binary boundary, and the CDN/URL requirement for chat surfaces.

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.

---

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`.

---

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"
    }
  }
}

The key inside `binary` (`invoice` here) is the **binary property name**. Most file-handling nodes have a `binaryPropertyName` parameter that points at it — the producer names the slot, the consumer references it by that name. The default key across most nodes is `data`, so when nothing tells you otherwise, assume `$binary.data`.

`$json` and `$binary` are separate namespaces. An expression like `{{ $binary.invoice.fileName }}` reads file metadata; `{{ $json.customerId }}` reads data. They never mix.

This split also explains a webhook gotcha: a Webhook trigger receiving `multipart/form-data` puts the uploaded file in `$binary` and the accompanying form fields in `$json.body` — so an uploaded file is not somewhere under `$json` at all. (The `$json.body` nesting for webhooks is **n8n-expression-syntax** territory.)

See `BINARY_BASICS.md` for the full slot anatomy, mime types, and size limits.

---

Producing binary

You rarely build a `$binary` slot by hand — nodes populate it for you:

| Source | How binary appears | |---|---| | HTTP Request with `responseFormat: "file"` | Response body lands in `$binary.data` (or the name you set) | | Read/Write Files from Disk | File contents read into `$binary` | | Storage downloads (S3, Google Drive, Dropbox, etc.) | Downloaded file in `$binary.<key>` | | Email triggers with attachments | Each attachment arrives in `$binary` | | Provider AI media nodes (image/audio gen) | Set `options.binaryPropertyOutput` so the bytes land where the next node looks |

For an HTTP download, the one field that matters is `responseFormat`. Confirm it with `get_node` on `nodes-base.httpRequest` — leaving it as the default JSON/string format is the classic reason a downloaded file ends up as garbled text in `$json` instead of clean bytes in `$binary`.

---

Reading and writing binary in a Code node

Most workflows never need to crack open the bytes — they just pass binary through to a consumer (email attachment, file upload, Slack file). Most byte jobs have a native node: **Extract From File** (CSV, XLSX, PDF text, JSON, plain text → items or a field), **Convert To File** (items → CSV / XLSX / JSON / text file), **Crypto** (hash a file with `binaryData: true`), **Compression** (zip / gzip). Read or write raw bytes in a Code node only when no node handles the format, with a `Why Code:` note.

**Read** with `getBinaryDataBuffer` — do not try to base64-decode `$binary.<key>.data` by hand:

// Code node, "Run Once for Each Item"
const buffer = await this.helpers.getBinaryDataBuffer(0, 'data'); // (itemIndex, propertyName)
const text = buffer.toString('utf-8');
const length = buffer.length;

return [{
  json: { ...$json, length },
  binary: $input.item.binary,   // pass the binary through, or it's gone
}];

**Write** by building the slot yourself — base64 the bytes plus a mime type and file name:

const text = 'Hello, world!';
return [{
  json: { ok: true },
  binary: {
    report: {
      data: Buffer.from(text).toString('base64'),
      mimeType: 'text/plain',
      fileName: 'report.txt',
      fileExtension: 'txt',
    },
  },
}];

The Code-node sandbox, helpers, and execution modes are the domain of **n8n-code-javascript** (and **n8n-code-python**) — use those for the language-level detail. The one binary-specific thing to remember here: a Code node that returns `[{ json: {...} }]` without re-attaching `binary` **silently drops the file**. See `BINARY_BASICS.md`.

---

Keeping binary alive across transforms

JSON-only nodes — Edit Fields (Set), Code, IF, and others — can drop the `$binary` slot