Send a Slack Message From PowerShell: Webhook, Bot Token and Files

To send a Slack message from PowerShell, POST a JSON body to an incoming webhook URL with Invoke-RestMethod. Three lines do it, and you need no module. For more control (any channel, Block Kit, files, replies in threads) call chat.postMessage with a bot token in the Authorization header. On 6 October 2026 we ran every script on this page in PowerShell 7.6.6 on macOS against our free-plan test workspace. The output under each script is what we got. We had no Windows machine, so Windows PowerShell 5.1 was not run; the section on special characters shows the bytes it sends and what Slack makes of them.

Send to an incoming webhook

Create the webhook first: make a Slack app, turn on Incoming Webhooks and pick a channel. Our webhook guide shows each screen. Keep the URL in an environment variable or a secret store. Anyone who has it can post to your channel.

$body = @{ text = "Build 142 passed on main" } | ConvertTo-Json
$r = Invoke-RestMethod -Uri $env:SLACK_WEBHOOK_URL -Method Post -ContentType 'application/json' -Body $body
"Response: $r"
$r.GetType().Name

Output:

Response: ok
String

A webhook answers with the plain text ok, not JSON, so $r is a string. Webhook messages appear under the app name. In the screenshot above they show as "Rename Test", while the chat.postMessage messages show the bot's name, "Rename Bot 2". A webhook always posts to the channel it was made for.

Send with a bot token and chat.postMessage

Give your app the chat:write scope, install it, and copy the xoxb- token from OAuth & Permissions. Add chat:write.public if the bot should post in public channels it has not joined.

$headers = @{ Authorization = "Bearer $env:SLACK_BOT_TOKEN" }
$body = @{ channel = $env:SLACK_CHANNEL; text = "Hello from PowerShell $($PSVersionTable.PSVersion)" } | ConvertTo-Json
$r = Invoke-RestMethod -Uri 'https://slack.com/api/chat.postMessage' -Method Post `
  -Headers $headers -ContentType 'application/json; charset=utf-8' -Body $body
"ok=$($r.ok) channel=$($r.channel) ts=$($r.ts) error=$($r.error)"

Output:

ok=True channel=C0C6UGBU151 ts=1791272338.444699

Use a channel ID such as C0C6UGBU151, not a name; this guide shows where to find it. Keep ts: pass it back as thread_ts to reply in a thread, or to chat.update to edit the message. A plain form body also worked. -Body @{ channel = ...; text = ... } with no ConvertTo-Json and no content type returned ok=True, because the Web API accepts form fields. That only works for flat fields; blocks and attachments need JSON.

Block Kit: the ConvertTo-Json depth trap

ConvertTo-Json stops at a depth of 2 by default. Block Kit payloads are deeper, so the blocks get cut off. This is the payload we sent:

$payload = @{
  channel = $env:SLACK_CHANNEL
  text    = "Deploy report"
  blocks  = @(
    @{ type = "section"; text = @{ type = "mrkdwn"; text = "*Deploy finished* in 42 s" } },
    @{ type = "context"; elements = @( @{ type = "mrkdwn"; text = "by PowerShell" } ) }
  )
}
$json2 = $payload | ConvertTo-Json            # default depth
$json10 = $payload | ConvertTo-Json -Depth 10 # deep enough

With the default depth, PowerShell printed a warning and wrote the inner objects as the text System.Collections.Hashtable:

WARNING: Resulting JSON is truncated as serialization has exceeded the set depth of 2.
{
  "text": "Deploy report",
  "blocks": [
    {
      "type": "section",
      "text": "System.Collections.Hashtable"
    },
    {
      "type": "context",
      "elements": "System.Collections.Hashtable"
    }
  ],
  "channel": "C0C6UGBU151"
}

Slack rejected it:

ok=False error=invalid_blocks [ERROR] failed to match all allowed schemas [json-pointer:/blocks/0/text] | [ERROR] must provide an object [json-pointer:/blocks/0/text] | [ERROR] must provide an array [json-pointer:/blocks/1/elements]

With -Depth 10 the same payload posted with ok=True. If you see System.Collections.Hashtable in an error or in a message, add -Depth. Our invalid_blocks guide lists the other causes of that error.

Upload a file

files.upload no longer works (details). The new way takes three calls. The bot needs files:write and has to be in the channel.

$headers = @{ Authorization = "Bearer $env:SLACK_BOT_TOKEN" }
$path = Join-Path $PSScriptRoot 'report.csv'
"name,status`nbuild-142,passed`nbuild-143,failed" | Set-Content -Path $path -NoNewline
$size = (Get-Item $path).Length
# 1. ask for an upload URL (form fields, not JSON)
$u = Invoke-RestMethod -Uri 'https://slack.com/api/files.getUploadURLExternal' -Method Post -Headers $headers `
  -Body @{ filename = 'report.csv'; length = $size }
"step1 ok=$($u.ok) file_id=$($u.file_id) error=$($u.error)"
# 2. send the bytes to that URL
$up = Invoke-WebRequest -Uri $u.upload_url -Method Post -InFile $path -ContentType 'application/octet-stream'
"step2 status=$($up.StatusCode) body=$($up.Content)"
# 3. finish and share in the channel
$done = @{ files = @(@{ id = $u.file_id; title = 'Build report' }); channel_id = $env:SLACK_CHANNEL; initial_comment = 'Nightly build report' } | ConvertTo-Json -Depth 5
$c = Invoke-RestMethod -Uri 'https://slack.com/api/files.completeUploadExternal' -Method Post -Headers $headers -ContentType 'application/json; charset=utf-8' -Body $done
"step3 ok=$($c.ok) files=$($c.files.Count) error=$($c.error)"

The first run, before the bot had joined the channel:

step1 ok=True file_id=F0C6VN0C6J1 error=
step2 status=200 body=OK - 45
step3 ok=False files=0 error=not_in_channel

Steps 1 and 2 pass and only the last call fails, so the file is uploaded but never shared. chat:write.public does not help here. After we added the bot to the channel (conversations.join), the same script returned step3 ok=True files=1, and Slack showed the CSV as a table. The not_in_channel guide covers the ways to add a bot.

Errors: when Invoke-RestMethod throws and when it does not

The Web API returns HTTP 200 even when the call fails, so Invoke-RestMethod does not throw. Always check $r.ok. A webhook returns a real HTTP error, so there you need try/catch.

This is how we read the status and the body in the catch block:

try { Invoke-RestMethod -Uri $env:SLACK_WEBHOOK_URL -Method Post -ContentType 'application/json' -Body '{"text":""}' }
catch { "status=$([int]$_.Exception.Response.StatusCode) body=$($_.ErrorDetails.Message)" }

The webhook errors by status code are in our webhook troubleshooting page.

Emoji and accented letters show as question marks

In PowerShell 7.6.6, a string body reached Slack as correct UTF-8 with or without charset=utf-8 in the content type: "Deploy done ✅ café 한국어 🚀" arrived the same all three ways we sent it. Windows PowerShell 5.1 encodes a string body as ISO-8859-1 when the content type has no charset. We could not run 5.1, so we sent the same text as ISO-8859-1 bytes from PowerShell 7. Slack posted Latin-1 bytes: Deploy done ? caf? ???: the emoji, the é and the Korean all became question marks, as in the last line of the screenshot. The fix works in both versions: send the body as UTF-8 bytes.

$bytes = [System.Text.Encoding]::UTF8.GetBytes($body)
Invoke-RestMethod -Uri $env:SLACK_WEBHOOK_URL -Method Post -ContentType 'application/json; charset=utf-8' -Body $bytes

For another PowerShell script we tested, see how to keep a Windows PC awake with PowerShell.

FAQ

Do I need the PSSlack module?

No. Every script here uses only Invoke-RestMethod and Invoke-WebRequest, which ship with PowerShell. A module adds nothing you need for sending messages.

Can I mention a user from PowerShell?

Yes. Put the member ID in angle brackets, such as <@U0B7L4YK420>, in the text. In our webhook test that showed as a linked @sieun mention, while the typed @sieun next to it stayed plain text with no link.