PAC engine differences

The same proxy.pac can return different answers in Chrome, Firefox and pactester. This table puts the newest version of each engine side by side. Every cell says where the claim comes from.

Values describe the newest version range of each engine. Older ranges, for example Chrome 51 and older, which pass the full URL to the script, are listed on each engine page. code means read in the engine’s source code at a pinned commit, doc means vendor documentation, lab means observed in a lab run, and expert means practitioner knowledge that is not verified yet.

Test how your own file behaves in each engine with the online PAC file tester.

Behaviour	Chrome ≥ 52	Firefox	pacparser ≥ 1.5.1
alert() output	`netlog` code, verified 2026-10-04	`console` code, verified 2026-10-04	`console` alert() is not defined before 1.5.1, so calling it throws. code, verified 2026-10-04
Non-ASCII result rejected	`true` code, verified 2026-10-04	`false` The returned string is used as is; non-ASCII characters are not rejected. code, verified 2026-10-04	`false` code, verified 2026-10-04
DNS lookups	`async` Chrome resolves names asynchronously and may run the script again once the answer arrives; the checker answers from your DNS table. expert, unverified	`sync` code, verified 2026-10-04	`sync` code, verified 2026-10-04
dnsResolve is IPv4 only	`true` code, verified 2026-10-04	`false` Whether dnsResolve can return IPv6 addresses in current Firefox has not been checked. expert, unverified	`true` code, verified 2026-10-04
JavaScript level accepted	`latest` code, verified 2026-10-04	`latest` code, verified 2026-10-04	`latest` SpiderMonkey 1.7 accepts ES3 syntax only; the checker approximates it with an ES3 parser. doc, verified 2026-10-04
Calls FindProxyForURLEx if defined	`false` code, verified 2026-10-04	`false` code, verified 2026-10-04	`true` code, verified 2026-10-04
Microsoft *Ex functions	`true` code, verified 2026-10-04	`false` code, verified 2026-10-04	`true` doc, verified 2026-10-04
Global variables persist between calls	`true` code, verified 2026-10-04	`true` expert, unverified	`true` code, verified 2026-10-04
Host lower-cased by the engine	`true` The host comes from the browser's canonicalised URL, so it is already lower-case. expert, unverified	`true` The host comes from the parsed request URL, which is lower-case. expert, unverified	`false` code, verified 2026-10-04
myIpAddress() source	`routing_ipv4` expert, unverified	`routing_ipv4` code, verified 2026-10-04	`user_supplied` code, verified 2026-10-04
Native (C++) bindings	`alert, myIpAddress, dnsResolve, isPlainHostName, dnsResolveEx, myIpAddressEx, sortIpAddressList, isInNetEx` Read from the current Chromium source; assumed to be the same in old versions. code, verified 2026-10-04	`dnsResolve, myIpAddress, alert` code, verified 2026-10-04	`dnsResolve, myIpAddress, dnsResolveEx, myIpAddressEx, alert` code, verified 2026-10-04
On exception	`direct` code, verified 2026-10-04	`direct` If FindProxyForURL throws, the request goes DIRECT. code, verified 2026-10-04	`error` code, verified 2026-10-04
On non-string / undefined result	`direct` code, verified 2026-10-04	`direct` A result that is not a string (for example a missing return) makes the request go DIRECT. code, verified 2026-10-04	`stringify` A missing return value becomes the string "undefined". code, verified 2026-10-04
URL passed to FindProxyForURL	`https_path_query` For https:// and wss:// URLs the script sees only scheme, host and port; user name, password and fragment are always removed. code, verified 2026-10-04	`all_path_query` Firefox removes path and query for every scheme, not only https (preference network.proxy.autoconfig_url.include_path, default false). code, verified 2026-10-04	`none` code, verified 2026-10-04

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