A webhook inspector gives you a reachable endpoint where you can see incoming HTTP headers, body, and delivery response instead of guessing what a provider sent. It is useful for finding URL, payload, and response problems—but a displayed request is not automatically proof that your application verified the exact original body bytes.
What a webhook inspector shows—and what it does not
A webhook provider sends an HTTP request to a URL you configure. During local development, that URL must be reachable from the provider, often through a tunnel or a hosted inspection endpoint. The inspector receives the request and can make its headers and body visible in a web interface.
That view helps answer practical questions: Did the event arrive? Which signature header was sent? What status did the endpoint return? But there are three distinct representations to keep straight:
- Raw body bytes: the original byte sequence received by the HTTP server.
- Decoded text: those bytes interpreted using a character encoding such as UTF-8.
- Parsed JSON: a structured object produced by a JSON parser.
A user interface may display decoded text or formatted JSON rather than preserve the original bytes. Even a tool that records raw request data only proves what reached that tool’s endpoint; proxying or application middleware may alter what your receiver later sees. Postman documents that its webhook event record includes raw headers and a raw body without reformatting, and that replay sends the captured raw headers and body as received: Postman webhook documentation.
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How do I test webhook delivery locally?
Your laptop’s local address is not ordinarily reachable from a provider on the public internet. Give the provider a public URL that routes to your local service, or use a hosted inspector to receive requests first. OpenAI lists ngrok and cloud development environments as ways to make a URL available for local webhook testing; Twilio likewise demonstrates ngrok because a local computer is not automatically reachable from Twilio. These are provider-specific setup examples, not requirements for every webhook service. OpenAI webhook guidance · Twilio webhook testing documentation
- Expose a development endpoint. Start your local app and use a tunnel, or create an endpoint with a hosted inspection service. Copy the public URL it provides.
- Configure the provider’s webhook URL. Check the protocol, domain, and path character for character. A correct host with a missing route segment still points to the wrong endpoint.
- Confirm the route accepts POST. Clerk recommends checking that the target route accepts the request method and inspecting delivery-attempt response codes. Clerk webhook debugging guide
- Trigger a test event and inspect the delivery. Check arrival time, headers, body, response status, and any attempt details the provider exposes. If using an inspector before your app, forward the request to the local or staging target when that feature is available.
- Compare the application’s view. Log the body at the receiver before signature verification, and compare it with the inspection record. Do not assume a displayed body alone proves the application received identical bytes.
Tool capabilities vary. Postman documents capture and display, forwarding—including to localhost—response configuration, signature verification options, and replay. ngrok describes inbound webhook traffic inspection and a gateway for forwarding provider events to services behind a firewall. RequestBin documents capture, inspection, replay, and forwarding; its cited documentation does not establish detailed limits or commercial terms. Check current product documentation for the behavior and access controls you need. ngrok webhook inspection · RequestBin documentation
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| Workflow capability | What to verify |
|---|---|
| Capture and display | Are headers and body visible? Is the body preserved as raw data, decoded text, or formatted JSON? |
| Forwarding | Can the captured request be sent to your local or staging receiver? |
| Replay | Can you resend a captured event without triggering the provider again? |
| Response behavior | Can you configure the status code and response body returned to the provider? |
| Signature checks | Does the tool support your provider’s signature format, or will verification happen in your application? |
| Delivery diagnosis | Are timing, response status, and attempt history shown? |
| Exposure and access | Is the endpoint public, tunneled, or hosted, and what access controls are documented? |
Why is webhook signature verification failing?
Start with the provider’s signing specification and the exact body representation your verification code receives. GitHub says its signature is an HMAC hex digest generated from the secret token and payload contents. Its guidance reads the request body, verifies the signature, and then parses the payload; it also advises using constant-time comparison rather than ordinary equality. GitHub: Validating webhook deliveries
Parsing JSON and serializing it again before verification can change whitespace, key order, or encoding. The resulting text may represent the same data while containing different bytes, so a signature computed over the provider’s original payload will not necessarily match. Preserve and verify the original body representation required by that provider, then parse it for application use. Follow the provider’s own algorithm and encoding rules; GitHub’s HMAC format is not a universal webhook standard.
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When a signature check fails, inspect these items in order:
- Header: Is the expected signature header present, and is the value being read correctly?
- Body: Is verification using the original body rather than parsed and re-serialized JSON?
- Secret: Does the application use the secret belonging to this endpoint and environment?
- Algorithm and encoding: Do they match the provider’s specification?
- Comparison: Does the implementation use a constant-time comparison where the provider recommends it?
Twilio’s diagnostic guidance also points developers to validation code, the shared key, setting names, and signature algorithm when investigating rejected signatures. Seeing a signature header in an inspector is not the same as validating it. Twilio webhook testing documentation
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What to check when deliveries fail, time out, or repeat
Inspect the provider’s delivery attempt, your endpoint’s response, and the request path together. A request may reach an inspector yet fail during forwarding, route handling, verification, or response generation. Clerk recommends checking the exact URL’s protocol, domain, and path, the attempt’s response code, whether the route accepts POST, and the body before verification. Twilio’s troubleshooting guidance recommends checking connection timing against configured timeouts. Provider status-code rules and retry schedules differ, so consult the relevant provider’s delivery documentation.
For OpenAI webhooks specifically, the endpoint should return a successful 2xx response quickly. OpenAI says it retries when the endpoint does not return a successful 2xx or fails to respond within a few seconds; attempts can continue for up to 72 hours with exponential backoff. OpenAI also warns that duplicate events can occur and identifies webhook-id as an idempotency key. Treat those as OpenAI-documented behaviors, not a general rule for other providers. OpenAI webhook guidance
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Design the receiver to acknowledge promptly and handle duplicate events safely. If processing takes longer than the provider’s response window, use the provider’s recommended pattern for accepting the event and completing work asynchronously. Record enough delivery information to match an event to its attempts, and make processing idempotent where the provider supplies a stable identifier.
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