Scripting Settings and Limits
Fiddler Everywhere runs only one active script at a time, but that script's hooks can execute concurrently (for example, OnBeforeRequest for several in-flight sessions at once). To keep the application responsive even when a script misbehaves, Fiddler Everywhere applies the runtime limits described below.
Execution Timeout
Every hook invocation is bounded by a timeout, measured in milliseconds. If a hook does not complete within the timeout:
- Its execution is cancelled cooperatively (see Loop Cancellation below).
- The invocation is treated as a failure and counts toward the consecutive-error kill switch.
The default timeout is 3000 ms.
Concurrency Limit
Up to four hook invocations can run at the same time. If all execution slots are busy, additional dispatches wait for a free slot until the configured timeout elapses; if none becomes free in time, that particular hook invocation is skipped for that session/event, and a warning diagnostic is raised, but the traffic pipeline is not blocked.
Consecutive-Error Kill Switch
Fiddler Everywhere tracks consecutive failures per hook (compile errors surfaced at runtime, unhandled exceptions thrown by the script, and timeouts). Once a hook accumulates a configurable number of consecutive failures:
- The kill switch trips: the active script is automatically disabled (both in memory and in its saved settings entry), and scripting stops dispatching to it.
- An error notification is shown, indicating how many consecutive errors occurred and in which hook.
The default threshold is 5 consecutive errors. A successful invocation of a hook resets its consecutive-error counter to zero.
This is a fail-open design: a throwing or hanging script can never break the live traffic pipeline—at worst, Fiddler Everywhere stops calling that script and continues processing traffic normally.
Runtime Diagnostics
While a script is active, non-fatal problems (a single failed hook invocation, or a hook skipped because the concurrency limit was busy until timeout) are surfaced as toast notifications in the Scripting tab, without necessarily disabling the script (that only happens once the kill switch trips).
Loop Cancellation
To make the timeout effective even inside a script's own loops, Fiddler Everywhere automatically injects a cancellation checkpoint at the start of every for, while, do, and foreach loop body before compiling the script. This lets a long-running loop be interrupted once its hook's timeout is reached, instead of running indefinitely.
This mechanism only protects cooperative loop constructs recognized at compile time. It does not prevent every possible way a script could hang the process (for example, a single very expensive synchronous call, or deep/unbounded recursion). See Scripting Security Considerations for the full set of caveats.