A webhook inspector gives you a place to receive a provider’s HTTP request and examine its headers, body, response, and delivery details. For local development, the provider needs a URL it can reach—typically a tunnel or hosted inspection endpoint. The crucial caveat: a readable body in a browser is not proof that the bytes your application verifies are identical to the bytes the provider signed.
What a webhook tap shows—and what it cannot prove
A webhook is an HTTP request sent by a service to a URL you configure. A request inspector, sometimes called a webhook bin or listener, receives that request and displays details such as headers and body. Depending on the tool, it may also record arrival time and response status, forward the request to another endpoint, or let you replay it.
“Raw bytes” matters because signatures are calculated over a particular representation of the request body. Keep three things distinct:
- Raw body bytes: the received byte sequence.
- Decoded text: those bytes interpreted using an encoding such as UTF-8.
- Parsed JSON: a structured object produced by interpreting the text.
A display may show decoded text or formatted JSON rather than preserve the original bytes. Even a tool that documents raw-body capture only establishes what reached its own endpoint; it does not prove that a proxy or your application’s middleware passed identical bytes onward. Postman, for example, documents a listener that displays raw headers and a raw body without reformatting, and supports replay with those captured values: Postman’s webhook listener documentation.
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How do I test webhook delivery locally?
A provider cannot normally call a server that is reachable only on your laptop or local network. Give it a public URL that routes to your development receiver, or point it at a hosted inspector and forward requests from there. OpenAI lists ngrok and cloud development environments as options for local webhook testing; Twilio likewise demonstrates exposing a local computer with ngrok. These are provider-documented approaches, not requirements that every provider mandates.
- Start your receiver. Confirm the local route is running and accepts the HTTP method the provider sends—commonly POST.
- Expose it or choose an inspector. Use a tunnel that routes a public URL to your local server, or create a hosted inspection endpoint. ngrok describes inbound webhook inspection and a gateway for forwarding provider events to services behind a firewall: ngrok’s webhook documentation.
- Set the provider’s destination URL exactly. Check protocol, domain, and path. A correct domain with a missing route path can still send the event to the wrong endpoint.
- Trigger a test event and inspect the attempt. Compare the received method, headers, body, arrival time, and response status with the provider’s delivery record where available.
- Forward or replay if needed. Forwarding can test your local or staging receiver; replay can reproduce a captured request without triggering the provider again, if the tool supports it.
Clerk recommends checking the exact URL components, whether the route accepts POST, delivery-attempt response codes, and logging the body before verification: Clerk’s webhook debugging guide. Treat public endpoints and captured payloads with care: the cited tool descriptions do not establish universal access controls or security guarantees. Avoid sending real secrets or sensitive production payloads to an endpoint unless you understand who can access it and how long it retains data.
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Why is webhook signature verification failing?
First check that verification uses the exact body representation required by that provider. GitHub documents its webhook signature as an HMAC hex digest generated from the configured secret token and payload contents. Its examples read the request body, verify the signature, and then parse the payload. GitHub recommends constant-time comparison rather than ordinary equality and discusses handling payloads as UTF-8 when the language or server specifies an encoding: GitHub’s “Validating webhook deliveries” documentation.
“The hash signature is generated using your webhook’s secret token and the payload contents.” — GitHub Docs, “Validating webhook deliveries”
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If middleware parses JSON and application code serializes it again before verification, the result may differ from what was signed. Whitespace, key ordering, or encoding can change even when the JSON represents equivalent data. Verify against the original body representation required by the provider, then parse it for application logic. GitHub’s signing format is not universal: follow the signing specification for the service sending the request.
Use this checklist to narrow down a rejection:
- Signature header: Is the expected header present, and are you extracting its value correctly?
- Verification input: Are you passing the original body representation the provider specifies, rather than re-serialized JSON?
- Secret selection: Does the receiver use the secret configured for this endpoint and environment, rather than a development/production mismatch or an outdated key?
- Algorithm and comparison: Does the implementation match the provider’s algorithm and encoding, and use a constant-time comparison where recommended?
- Route and delivery: Is the request reaching the intended URL, and what status or timeout appears in the delivery attempt?
Twilio’s troubleshooting guidance points developers investigating rejected signatures to their validation code, shared key, setting names, and signature algorithm; for timeouts, it advises checking connection timing against configured timeouts: Twilio’s webhook testing and security documentation. Seeing a request in an inspector helps diagnose what arrived there; it does not authenticate the sender or replace verification in your receiver.
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Choose an inspector by the job you need it to do
Tools that receive webhooks can differ substantially. Compare documented capabilities rather than assuming every request bin can forward, replay, configure responses, or verify signatures.
| Capability | Why it matters | Documented examples |
|---|---|---|
| Capture and display | Lets you inspect headers, body, and delivery metadata. Check whether the tool preserves raw body data or only displays a parsed or transformed representation. | Postman documents raw headers, an un-reformatted raw body, arrival time, and response status. ngrok describes inspection of inbound webhook headers and payload. Postman; ngrok |
| Forwarding | Routes a captured event to a local or staging receiver so you can debug the application that handles it. | Postman documents forwarding to a target including localhost; ngrok describes forwarding events to services behind a firewall. Postman; ngrok |
| Replay | Resends an event for repeatable debugging without asking the provider to trigger a new one. | Postman documents replay with raw headers and body as received; RequestBin documents replay. Postman; RequestBin |
| Response behavior | Lets you see or configure the status and response body returned to the provider. This helps distinguish a delivery problem from application processing problems. | Postman documents response configuration and recording response status. Postman |
| Signature checks | May help test a provider-specific verification flow, but support for one signing format does not imply support for another. | Postman documents signature-verification options. Confirm the currently supported providers and formats in its documentation. Postman |
| Delivery diagnosis | Shows whether a request arrived and what status or timing was recorded, where the provider or tool exposes those details. | Postman documents arrival time and response status. Clerk recommends examining provider delivery-attempt response codes. Postman; Clerk |
| Endpoint exposure and access | Clarifies whether requests go to a public hosted endpoint or through a tunnel, and what access controls apply. | ngrok describes a webhook gateway; Clerk describes tunnels and hosted inspection services. Review each product’s current access and retention details before sending sensitive payloads. ngrok; Clerk |
RequestBin documents capture, inspection, replay, and forwarding, but its cited documentation does not establish detailed limits or commercial terms. Check its current documentation for the behavior you need: RequestBin documentation.
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What to check when an event is missing, rejected, or repeated
No request appears in the inspector
- Confirm the provider is configured with the inspector’s current URL, including protocol and path.
- Check the provider’s delivery attempt record, if available, for a connection error, timeout, or rejected status.
- Make sure you triggered an event that should match the configured webhook subscription.
The request arrives but your receiver does not handle it
- Verify the HTTP method and route; a receiver expecting a different path or method may reject the request.
- Check the response status and timing from the actual receiver, not just the inspector. A tool’s successful receipt does not show that a later forwarding step or your application succeeded.
- Compare the forwarded request with the captured request if your tool exposes both, especially when debugging body transformations or signature failures.
The provider retries or sends duplicates
A webhook sender may retry when it does not receive an acceptable response promptly, but status-code rules and retry schedules vary by provider. OpenAI’s documentation says its webhook deliveries are retried if the endpoint does not return a successful 2xx or does not respond within a few seconds; attempts continue for up to 72 hours with exponential backoff. OpenAI also notes that duplicate events can occur and identifies webhook-id as an idempotency key. These are OpenAI-specific behaviors, not a universal webhook standard: OpenAI’s webhook documentation.
Return the successful acknowledgment your provider expects promptly, and design processing to tolerate duplicate delivery. For OpenAI, use the documented event identifier to recognize repeats; for other providers, consult their documentation for the appropriate identifier and retry rules. Do not assume that responding successfully means an event will never be delivered again.
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