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Fiddler Classic’s AutoResponder lets you return a local file or a previously captured response when a request matches a rule, instead of sending that request to the destination server. To use it, enable automatic responses, add and enable a matching rule, then trigger the request and inspect the resulting session. Fiddler Everywhere has a separate Rules workflow; its controls and rule syntax should not be assumed to match Classic’s.
What AutoResponder does in Fiddler Classic
AutoResponder is a Fiddler Classic feature that pairs a request-matching condition with an action. When a captured request URI matches, Classic performs the action—for example, returning a response file from local disk. This is useful for testing how a client behaves with a controlled response without sending the matching request to the server. See the Fiddler Classic AutoResponder reference.
How to create a basic Classic rule
- Open the AutoResponder tab and check Enable automatic responses. Without this setting, rules will not take effect.
- In the Rule Editor, enter the request match rule in the top field and the action string in the bottom field. A match can use a URL string or a regular expression; Classic documents case-insensitive string matching.
- Add the rule, confirm it is enabled, and place it where it should run in the rule list. Classic processes rules in displayed order.
- Generate a request that should match, then inspect the session list to verify what happened.
For an action that returns a local response file, use the exact action syntax documented for your installed Classic version in the official reference. Do not treat the match condition and action as interchangeable: one identifies the request, the other specifies what Classic should do.
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How to replay a captured response
If you already captured the response you need, Classic can create a replay rule from that session:
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- Enable automatic responses.
- Select one or more sessions in the Web Sessions list.
- Drag the selected sessions into the AutoResponder rules list.
- Generate the matching request and verify the returned response in the session list.
The resulting rules use the stored captured responses, so the matching request is answered locally rather than transmitted to its destination. Classic also supports importing SAZ captures or FARX rule archives, exporting rules to FARX, editing a stored response, and generating rules by dragging files from Windows Explorer. Consult the AutoResponder reference for the supported workflow.
Choose a response source
| Approach | Use it when | What to keep in mind |
|---|---|---|
| Local response file | You want to supply a response file you control. | Create a rule with a request match and the documented file action. Confirm the match is narrow enough for the request you intend to intercept. |
| Captured session | A useful response already exists in the Web Sessions list. | Drag the session into the rules list to seed a replay rule. |
| Fiddler Everywhere Rules | You are using Everywhere and want to build a rule with its Rule Builder. | This is a separate workflow with distinct controls; do not assume Classic syntax or action parity. |
Rule order, multiple actions, and response delay
Order rules deliberately
Classic evaluates rules in the order shown. A rule higher in the list can take effect before a later rule, so promote or demote rules when a broad match is catching requests intended for a more specific one.
Allow more than one rule to act
The Classic reference documents non-final actions, which let processing continue so a request can match more than one rule. A final action ends rule processing. Use the documented action syntax rather than assuming every action terminates evaluation.
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Classic can delay a response for selected rules. Enable the latency option and set the delay in milliseconds for those rules to test client behavior under slower response conditions. This is a configured simulation, not a measurement of real network latency.
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Fiddler Everywhere uses a separate Rules workflow
In Fiddler Everywhere, create rules in the Rules tab using the Rule Builder: add matching conditions and actions, save the rule, and enable the Rules tab. Trigger it with a matching request or execute it against saved sessions. The documented feature supports mocking client requests or server responses and sharing, importing, or exporting rules. See Fiddler Everywhere Rules.
Do not transfer Classic instructions directly into Everywhere: the documented workflows are distinct, and matching-grammar parity is not established. Also, modifying a response body through Rules in Everywhere requires Streaming mode to be disabled. The setting is documented in Fiddler Everywhere Connections.
Troubleshoot a rule that does not match
- Check the global switch: In Classic, verify Enable automatic responses is checked.
- Check the rule itself: Make sure the rule is enabled in the list.
- Check the request condition: Compare the actual captured request URI with the configured string or regular expression. Confirm the rule targets the URI you intended.
- Check rule position: A higher-priority rule may act first. Reorder rules and test again.
- Check which product you are using: Classic AutoResponder and Everywhere Rules have separate workflows; verify instructions against the correct product.
- For Everywhere response-body edits: Confirm Streaming mode is off.
Version and documentation context
The Fiddler Classic AutoResponder reference identifies product version 5.0.20253 and was updated August 3, 2026. The Fiddler Everywhere Rules and Connections pages were updated September 29, 2026. Interface labels and behavior can differ by product and version, so use the documentation for the Fiddler edition you have installed.
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