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Best answer

Sending files from Slack to ChatGPT

  • February 26, 2026
  • 5 replies
  • 39 views
B

Hi everyone,

I’m building a workflow to analyze creative briefings (PDF/Docx) using a ChatGPT Assistant (File Search), but I’m stuck on a file-transfer issue between Slack and OpenAI.

Goal: Automatically send files from a Slack message to an OpenAI Assistant for deep analysis.

The Setup:

  1. Trigger: Slack (New Message Posted to Channel).

  2. Action: OpenAI "Upload File" using the Url Private Download from Slack.

  3. Delay: 1 minute for indexing.

  4. Action: "Conversation with Assistant" (File Search enabled).

The Problem: The Assistant gives very generic "hallucinated" output. When I check the Data Out of the "Upload File" step, it shows the file size is only 63,654 bytes. However, the actual file in Slack is 18,810,724 bytes (18.8MB).

It seems OpenAI is only "downloading" a Slack 403 error page or a login redirect instead of the actual binary PDF content.

What I’ve tried:

  • Using Url Private Download instead of Permalink.

  • Both ChatGPT Assistant and regular Conversation give error regarding the files.

  • Clearing Conversation ID to ensure a fresh session.

  • Ensuring File Search is ON in the OpenAI Dashboard.

Question: How do I force Zapier to pass the actual binary File Object (stream) to OpenAI instead of just the URL string, so the Assistant receives all 18.8MB of data?

Any tips would be greatly appreciated!

Best answer by SamB

Hi there ​@Biek  👋

You’d need to set up a Slack app in order to get a bot token. If it helps Slack has a guide with more details on how to do that: https://docs.slack.dev/app-management/quickstart-app-settings

That said, I saw you reached out to our Support team about this as well and you’d since changed from using the Url Private Download field to use the individual Files X File fields. For example: 

c60547256bb6ad1d63097d9bf87f6dde.png
 

This fixed the error however, you then ran into a new error which was related to the size of the PDF files as smaller files (~5MB) were being processed fine. Our Support team suggested increasing the value in the max_tokens field to allow it to process the larger files.

Can you let us know whether increasing the max_tokens does the trick? Want to make sure you’re all set! 🙂

5 replies

drtanvisachar
Forum|alt.badge.img+5
  • drtanvisacharZapier Solution Partner
  • Zapier Solution Partner
  • February 26, 2026

Hey Biek!

That 63 KB “file” is almost certainly an HTML page (Slack 403 or a redirect), not your PDF.

Slack’s url_private_download only works if the request includes a Slack token in the Authorization header. If you just pass the URL to another app, Slack will block it.

What to do in Zapier:

  1. Trigger on the Slack message that contains the file.

  2. Add a step that actually downloads the file bytes from Slack:

    • Use Webhooks by Zapier, Custom Request

    • Method: GET

    • URL: the file’s url_private_download

    • Headers: Authorization: Bearer xoxb-your-slack-bot-token
      This is the part that turns the URL into a real binary file.

  3. Then feed that output into OpenAI Upload File as a file object (not a link). OpenAI’s upload endpoint expects the actual file content.

  4. After upload, pass the returned file id into the Assistant thread/tool resources for File Search.

Two quick checks that usually matter:

  • Make sure the Slack bot token you use can access that channel and file. Private channels often fail if the bot is not in the channel.

  • In Zap history, confirm the webhook response “Content Type” looks like application/pdf and the byte size matches what Slack shows.

    Dr. Tanvi Sachar
    Monday Certified Partner, Monday Wizard

Zapier Solutions Partner-Tuesday Wizard
B
  • BiekBeginner
  • Author
  • Beginner
  • February 27, 2026

Hi, Dr. Tanvi Sachar,

Thank you for the reply. Just to double check, where would I find the Slack bot token? Since the message in Slack with the files is send by a user, and not a bot.

 

Thanks.

SamB
Community Manager
Forum|alt.badge.img+11
  • SamBCommunity Manager
  • Community Manager
  • Answer
  • March 2, 2026

Hi there ​@Biek  👋

You’d need to set up a Slack app in order to get a bot token. If it helps Slack has a guide with more details on how to do that: https://docs.slack.dev/app-management/quickstart-app-settings

That said, I saw you reached out to our Support team about this as well and you’d since changed from using the Url Private Download field to use the individual Files X File fields. For example: 

c60547256bb6ad1d63097d9bf87f6dde.png
 

This fixed the error however, you then ran into a new error which was related to the size of the PDF files as smaller files (~5MB) were being processed fine. Our Support team suggested increasing the value in the max_tokens field to allow it to process the larger files.

Can you let us know whether increasing the max_tokens does the trick? Want to make sure you’re all set! 🙂

B
  • BiekBeginner
  • Author
  • Beginner
  • March 2, 2026

Hi Sam. Making sure the file size wasn’t larger than 5mb has indeed resolved the issue! Thanks for the reply.

    SamB
    Community Manager
    Forum|alt.badge.img+11
    • SamBCommunity Manager
    • Community Manager
    • March 3, 2026

    Yay! 🎉 That's so great to hear, ​@Biek. Really glad keeping the files under 5MB helped to sort it. Thanks for letting us know.

    Hope you have a great rest of your week! 😁

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