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A Slack slash command is an HTTP POST from Slack to a public Flask route. To build one, create a Slack app and command, point its Request URL at your deployed HTTPS endpoint, verify Slack’s signature against the raw request body, read the form-encoded text argument, and acknowledge the request within 3,000 milliseconds. Return a quick result when possible; for slow work, acknowledge first and post the result to Slack’s supplied response_url.
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The low-level Flask approach gives you complete control over HTTP, verification, parsing, authorization, and response formatting. Bolt for Python is a useful alternative when you prefer built-in command dispatch and ack()/respond() helpers. The security and timing rules are the same either way.
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A slash command belongs to a Slack app. When a user types a command such as /lookup printer, Slack sends an HTTP POST request to the command’s configured Request URL. Your Flask application is the web server that receives and processes that request.
The endpoint must be reachable from Slack over HTTPS with a publicly verifiable certificate. A local development server is not reachable by Slack unless you expose it through a secure tunnel or deploy it to a public environment. In production, use a stable URL and monitor its availability because users experience an unavailable endpoint as a failed command.
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Command names and conversation context
Choose a distinctive command name. Slash-command names are not namespaced, so a collision with another installed app can cause the most recently installed app to receive the command. Explain the exact syntax in your command’s help text and documentation.
Custom slash commands do not run in message threads. If your workflow depends on thread context, document that users must invoke the command from a supported channel or direct-message context instead of assuming Slack will provide a thread.
Set up the Slack app and Request URL
- Create a new Slack app in the Slack app configuration interface.
- Open the Slash Commands section and add a command, for example
/lookupor/weather. - Enter the complete HTTPS address of your deployed Flask route as the command’s Request URL, such as
https://example.com/slack/commands. - Save the command, install or reinstall the app in the target workspace, and note the app’s signing secret from the app’s credentials.
Keep the signing secret out of source control. Supply it through an environment variable or a secret-management system, and rotate it through that system when necessary. The signing secret replaces Slack’s older verification-token approach.
Install Flask and the Slack SDK
Flask provides the HTTP route. The official Python Slack SDK supplies helpers such as SignatureVerifier for request authentication and WebhookClient for posting webhooks. If you want a higher-level event and command framework, Bolt for Python adds command listeners plus ack() and respond() helpers.
A minimal low-level project can install Flask and the SDK with:
python -m venv .venv
. .venv/bin/activate
pip install Flask slack-sdkOn Windows PowerShell, activate the environment with .venvScriptsActivate.ps1. Pin versions in your application’s dependency file and review the installed SDK’s current reference before copying verifier code, because helper signatures can change between SDK releases.
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Verify Slack before reading the form
Every request must be authenticated before your application performs work. Slack sends two relevant headers: X-Slack-Signature and X-Slack-Request-Timestamp. Slack’s verification scheme combines a versioned timestamp and the exact raw request body to produce the signature. The body must be read before Flask parses request.form; otherwise you risk verifying a representation that is not byte-for-byte identical to what Slack sent.
Use the SDK verifier
This route shows the essential ordering. It reads the raw body, lets the SDK validate the timestamp and signature, and only then accesses form fields:
import os
from flask import Flask, request, jsonify
from slack_sdk.signature import SignatureVerifier
app = Flask(__name__)
verifier = SignatureVerifier(
signing_secret=os.environ["SLACK_SIGNING_SECRET"]
)
@app.post("/slack/commands")
def command():
raw_body = request.get_data()
if not verifier.is_valid_request(raw_body, request.headers):
return "invalid request", 401
text = request.form.get("text", "").strip()
if not text:
return jsonify(text="Usage: /lookup <query>"), 200
return jsonify(text=f"You asked for: {text}"), 200The verifier’s implementation should reject stale timestamps and compare signatures safely. Do not replace it with a plain string comparison or skip timestamp validation: accepting an old, valid request creates a replay risk. If you implement verification yourself, construct the versioned base string from the timestamp and raw body exactly as Slack specifies, calculate the expected signature with the signing secret, and use a constant-time comparison.
Keep verification ahead of application logic
- Read
request.get_data()first. - Validate the signature and timestamp.
- Return HTTP 401 for an invalid request.
- Only after successful verification, parse
request.form, authorize the caller, query a database, or call another service. - Log enough information to diagnose failures without logging the signing secret or sensitive command text.
Read the slash-command payload
Slash commands normally arrive as application/x-www-form-urlencoded data. The most important value is text, which contains everything typed after the command name. Treat it as untrusted user input: trim it, validate its syntax, enforce length limits appropriate to your operation, and escape or encode it when inserting it into another system.
Slack also supplies identifiers and response information. Preserve the fields relevant to your routing and authorization:
| Field | Purpose | Recommended handling |
|---|---|---|
command |
The invoked command, such as /lookup. |
Use it to route requests if one endpoint handles multiple commands. |
text |
The argument string after the command. | Parse and validate it as user input; an empty string is valid input that may need usage help. |
response_url |
A URL Slack supplies for later responses. | Use it for asynchronous results rather than trying to hold the original request open. |
trigger_id |
A short-lived identifier for interactions such as opening a modal. | Pass it only to the Slack operation that needs it and do not treat it as a durable user identifier. |
user_id |
The stable Slack ID of the invoking user. | Use it for authorization, auditing, and user-specific routing. |
team_id |
The workspace ID. | Use it to scope configuration and permissions. |
channel_id |
The conversation ID where the command was invoked. | Use it for channel-specific policy and logging. |
Prefer stable IDs to mutable display names. Do not rely on deprecated user_name values when user_id is available. For sensitive commands, authorize by workspace, user, channel, or a combination of those IDs before starting the operation.
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Acknowledge within 3,000 milliseconds
Slack expects confirmation within 3,000 milliseconds of sending the request. If it does not receive a successful response in that window, the user sees an operation_timeout error. This deadline includes network latency, application startup, verification, database work, and calls to external services, so do not spend the acknowledgement window on a slow task.
Fast path: return an immediate answer
For a simple validation or computation, return HTTP 200 directly. An empty 200 response is a valid acknowledgement; a short plain-text or JSON message is usually more helpful:
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def command():
raw_body = request.get_data()
if not verifier.is_valid_request(raw_body, request.headers):
return "invalid request", 401
text = request.form.get("text", "").strip()
if not text:
return jsonify(text="Usage: /lookup <query>"), 200
result = lookup_in_memory(text)
return jsonify(text=result), 200Do not perform an unbounded network request, long database query, large computation, or retry loop before returning this response. Put a timeout on any operation that is safe to execute in the immediate path.
Slow path: acknowledge, then work asynchronously
If the command calls a remote API, processes a large data set, or waits for a job, acknowledge first and queue the work. A production design commonly uses a background worker and a queue, although a small service can start a carefully managed task if its process model guarantees that the task will survive after the HTTP response.
- Verify the request and validate the command arguments.
- Capture the needed IDs, text, and
response_url. - Enqueue a job with bounded retries and an execution timeout.
- Return HTTP 200 immediately, optionally with a brief “working” message.
- Have the worker post the completed or failed result to
response_url.
Never place the signing secret, raw authorization headers, or unnecessary sensitive data in a job payload. Store only what the worker needs and protect queued data according to your organization’s policy.
Choose the response visibility and format
A response can be plain text or JSON. Use plain text for a short answer. Use JSON when you need to set visibility, send blocks, or include attachments.
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Private versus channel-visible responses
An ephemeral response is visible only to the user who invoked the command and is the default private experience when no channel-wide visibility is requested. Use it for validation errors, account-specific data, and administrative output that should not clutter a channel.
Set response_type to in_channel when the result should be visible to everyone in the conversation. Make that choice deliberately: channel-visible output can expose the command argument or result to people who did not invoke it.
# Private response (explicit)
return jsonify(
response_type="ephemeral",
text="Only you can see this result."
), 200
# Channel-visible response
return jsonify(
response_type="in_channel",
text="The lookup completed successfully."
), 200Slack may render structured Block Kit content when you include a blocks array in the JSON response. Keep a readable text fallback for clients or notifications that cannot render the blocks.
Delayed responses through response_url
After the initial acknowledgement, send a JSON POST to the response_url supplied in the command payload. The archived SlackHQ Slash Commands API reference documents up to five responses through that URL within 30 minutes. Treat that as a response budget and time window, not as permission to retry indefinitely.
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import requests
def post_result(response_url, message, visible_in_channel=False):
payload = {
"response_type": "in_channel" if visible_in_channel else "ephemeral",
"text": message,
}
response = requests.post(response_url, json=payload, timeout=10)
response.raise_for_status()Use a timeout and handle non-2xx responses. If the worker fails, post a concise error through the same URL when possible, and record detailed diagnostics in your server logs rather than exposing stack traces to the channel.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.Low-level Flask route versus Bolt for Python
Both approaches can implement the same command. Choose based on how much HTTP control and framework assistance your application needs.
| Concern | Hand-written Flask route | Bolt for Python |
|---|---|---|
| HTTP control | Direct control over the Flask route, headers, raw body, status codes, and serialization. | Bolt manages command dispatch and presents a higher-level listener interface. |
| Verification | You call the SDK verifier or implement the documented ordering yourself. | Bolt handles much of the request plumbing, but signing-secret configuration still must be correct. |
| Acknowledgement | Return a Flask response within the deadline. | Call ack() promptly in the command listener. |
| Responses | Return JSON or use an HTTP client with response_url. |
Use respond() for the supplied response mechanism. |
| Testing | Test the route, headers, raw body, form parsing, status code, and queue boundary directly. | Test listener behavior as well as Bolt adapter configuration. |
| Dependency surface | Small and explicit; you own more integration code. | More framework behavior, less repetitive command plumbing. |
Do not treat Bolt as a way around the 3,000-millisecond rule. A listener that performs slow work before ack() can still produce operation_timeout.
Test the command before releasing it
Unit and route tests
- Send a correctly signed request with an empty
textfield and verify that usage help returns HTTP 200. - Send a valid argument containing spaces, Unicode, punctuation, and an unexpectedly long value; verify your parser’s limits and error messages.
- Change one byte of the body and confirm that verification returns HTTP 401.
- Send a stale timestamp and confirm that the request is rejected.
- Confirm that the application reads the raw body before accessing parsed form data.
- Mock the queue and
response_urlclient so tests do not call external services. - Assert that an immediate response uses the intended
ephemeralorin_channelvisibility.
End-to-end checks in a test workspace
- Deploy the route behind the same HTTPS and proxy configuration used in production.
- Install the app in a test workspace and invoke the command with no argument.
- Invoke it with a valid argument and verify the visible audience of the response.
- Exercise a deliberately slow operation and confirm that Slack receives a quick acknowledgement followed by the delayed result.
- Check application logs for request IDs, workspace and user IDs, queue status, and response failures without recording secrets.
Troubleshoot common failures
operation_timeout appears in Slack
The route did not acknowledge within 3,000 milliseconds. Measure total request time, including cold starts, signature verification, database connections, DNS, and external API calls. Move slow work behind a queue, return 200 first, and post the result through response_url.
Every request returns 401
Check that the configured signing secret belongs to the installed app, that a reverse proxy has not altered the body, and that your code passes the raw bytes from request.get_data() to the verifier. Verify that your server clock is accurate enough for timestamp validation.
The command works locally but Slack cannot reach it
Confirm that the Request URL is publicly reachable over HTTPS, the certificate is trusted, DNS resolves publicly, and firewalls or access-control rules allow Slack’s request. A URL that works only on localhost cannot receive Slack traffic.
The argument is always empty
Inspect the incoming content type and confirm that the route uses request.form.get("text", ""). Test the command with text after the command name, and avoid reading JSON from a request that Slack sent as form-encoded data.
Users see information they should not see
Set response_type explicitly, use stable IDs for authorization, and audit which operations are allowed for each workspace, user, and channel. An in_channel response is public to the conversation; use ephemeral for private results.
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- Use a public HTTPS Request URL with a verifiable certificate.
- Keep the signing secret in environment or secret-management settings, never in source control.
- Verify the raw body and reject stale timestamps before parsing or processing.
- Validate command syntax, length, and authorization using workspace, user, and channel IDs.
- Return HTTP 200 within 3,000 milliseconds.
- Queue slow operations and send later results through
response_url. - Document whether each command response is private or channel-visible.
- Document that custom slash commands cannot be invoked in message threads.
- Use bounded timeouts, retries, and logging for downstream services.
- Test invalid signatures, stale requests, empty arguments, delayed jobs, and visibility settings before release.
A complete minimal example
The following example combines verification, argument handling, and a fast response. Replace the in-memory lookup with a queue when the operation can exceed the acknowledgement deadline.
import os
from flask import Flask, request, jsonify
from slack_sdk.signature import SignatureVerifier
app = Flask(__name__)
verifier = SignatureVerifier(
signing_secret=os.environ["SLACK_SIGNING_SECRET"]
)
def lookup_in_memory(query):
return f"Lookup result for: {query}"
@app.post("/slack/commands")
def command():
# Verify the exact raw body before Flask form parsing.
if not verifier.is_valid_request(
request.get_data(), request.headers
):
return "invalid request", 401
command_name = request.form.get("command", "")
text = request.form.get("text", "").strip()
user_id = request.form.get("user_id", "")
team_id = request.form.get("team_id", "")
channel_id = request.form.get("channel_id", "")
if command_name != "/lookup":
return jsonify(text="Unknown command."), 200
if not text:
return jsonify(
response_type="ephemeral",
text="Usage: /lookup <query>",
), 200
# Authorize with team_id, user_id, and channel_id as required.
result = lookup_in_memory(text)
return jsonify(
response_type="ephemeral",
text=result,
), 200The exact verifier API can vary by installed SDK version, so consult that version’s reference when adapting the example. The important invariants are stable: verify the raw request before using parsed fields, authenticate every request, acknowledge within Slack’s deadline, and move slow work to an asynchronous response.
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