diff --git a/mercurial/help/internals/wireprotocolv2.txt b/mercurial/help/internals/wireprotocolv2.txt
--- a/mercurial/help/internals/wireprotocolv2.txt
+++ b/mercurial/help/internals/wireprotocolv2.txt
@@ -82,6 +82,11 @@
    An array of bytestrings defining the supported framing protocol
    media types. Servers will not accept media types not in this list.
 
+pathfilterprefixes
+   (set of bytestring) Matcher prefixes that are recognized when performing
+   path filtering. Specifying a path filter whose type/prefix does not
+   match one in this set will likely be rejected by the server.
+
 rawrepoformats
    An array of storage formats the repository is using. This set of
    requirements can be used to determine whether a client can read a
diff --git a/mercurial/narrowspec.py b/mercurial/narrowspec.py
--- a/mercurial/narrowspec.py
+++ b/mercurial/narrowspec.py
@@ -23,7 +23,9 @@
 # Pattern prefixes that are allowed in narrow patterns. This list MUST
 # only contain patterns that are fast and safe to evaluate. Keep in mind
 # that patterns are supplied by clients and executed on remote servers
-# as part of wire protocol commands.
+# as part of wire protocol commands. That means that changes to this
+# data structure influence the wire protocol and should not be taken
+# lightly - especially removals.
 VALID_PREFIXES = (
     b'path:',
     b'rootfilesin:',
diff --git a/mercurial/wireprotov2server.py b/mercurial/wireprotov2server.py
--- a/mercurial/wireprotov2server.py
+++ b/mercurial/wireprotov2server.py
@@ -20,6 +20,7 @@
     discovery,
     encoding,
     error,
+    narrowspec,
     pycompat,
     streamclone,
     util,
@@ -429,6 +430,7 @@
         'commands': {},
         'compression': compression,
         'framingmediatypes': [FRAMINGTYPE],
+        'pathfilterprefixes': set(narrowspec.VALID_PREFIXES),
     }
 
     # TODO expose available changesetdata fields.
diff --git a/tests/test-http-protocol.t b/tests/test-http-protocol.t
--- a/tests/test-http-protocol.t
+++ b/tests/test-http-protocol.t
@@ -313,7 +313,7 @@
   s>     Content-Type: application/mercurial-cbor\r\n
   s>     Content-Length: *\r\n (glob)
   s>     \r\n
-  s>     \xa3GapibaseDapi/Dapis\xa1Pexp-http-v2-0001\xa4Hcommands\xaaIbranchmap\xa2Dargs\xa0Kpermissions\x81DpullLcapabilities\xa2Dargs\xa0Kpermissions\x81DpullMchangesetdata\xa2Dargs\xa3Ffields\xd9\x01\x02\x82GparentsHrevisionInoderange\x82\x81J0123456...\x81Iabcdef...Enodes\x81J0123456...Kpermissions\x81DpullHfiledata\xa2Dargs\xa4Ffields\xd9\x01\x02\x82GparentsHrevisionKhaveparents\xf5Enodes\x81J0123456...DpathGfoo.txtKpermissions\x81DpullEheads\xa2Dargs\xa1Jpubliconly\xf4Kpermissions\x81DpullEknown\xa2Dargs\xa1Enodes\x81HdeadbeefKpermissions\x81DpullHlistkeys\xa2Dargs\xa1InamespaceBnsKpermissions\x81DpullFlookup\xa2Dargs\xa1CkeyCfooKpermissions\x81DpullLmanifestdata\xa2Dargs\xa4Ffields\xd9\x01\x02\x82GparentsHrevisionKhaveparents\xf5Enodes\x81J0123456...Dtree@Kpermissions\x81DpullGpushkey\xa2Dargs\xa4CkeyCkeyInamespaceBnsCnewCnewColdColdKpermissions\x81DpushKcompression\x81\xa1DnameDzlibQframingmediatypes\x81X&application/mercurial-exp-framing-0005Nrawrepoformats\x82LgeneraldeltaHrevlogv1Nv1capabilitiesY\x01\xd3batch branchmap $USUAL_BUNDLE2_CAPS$ changegroupsubset compression=$BUNDLE2_COMPRESSIONS$ getbundle httpheader=1024 httpmediatype=0.1rx,0.1tx,0.2tx known lookup pushkey streamreqs=generaldelta,revlogv1 unbundle=HG10GZ,HG10BZ,HG10UN unbundlehash
+  s>     \xa3GapibaseDapi/Dapis\xa1Pexp-http-v2-0001\xa5Hcommands\xaaIbranchmap\xa2Dargs\xa0Kpermissions\x81DpullLcapabilities\xa2Dargs\xa0Kpermissions\x81DpullMchangesetdata\xa2Dargs\xa3Ffields\xd9\x01\x02\x82GparentsHrevisionInoderange\x82\x81J0123456...\x81Iabcdef...Enodes\x81J0123456...Kpermissions\x81DpullHfiledata\xa2Dargs\xa4Ffields\xd9\x01\x02\x82GparentsHrevisionKhaveparents\xf5Enodes\x81J0123456...DpathGfoo.txtKpermissions\x81DpullEheads\xa2Dargs\xa1Jpubliconly\xf4Kpermissions\x81DpullEknown\xa2Dargs\xa1Enodes\x81HdeadbeefKpermissions\x81DpullHlistkeys\xa2Dargs\xa1InamespaceBnsKpermissions\x81DpullFlookup\xa2Dargs\xa1CkeyCfooKpermissions\x81DpullLmanifestdata\xa2Dargs\xa4Ffields\xd9\x01\x02\x82GparentsHrevisionKhaveparents\xf5Enodes\x81J0123456...Dtree@Kpermissions\x81DpullGpushkey\xa2Dargs\xa4CkeyCkeyInamespaceBnsCnewCnewColdColdKpermissions\x81DpushKcompression\x81\xa1DnameDzlibQframingmediatypes\x81X&application/mercurial-exp-framing-0005Rpathfilterprefixes\xd9\x01\x02\x82Epath:Lrootfilesin:Nrawrepoformats\x82LgeneraldeltaHrevlogv1Nv1capabilitiesY\x01\xd3batch branchmap $USUAL_BUNDLE2_CAPS$ changegroupsubset compression=$BUNDLE2_COMPRESSIONS$ getbundle httpheader=1024 httpmediatype=0.1rx,0.1tx,0.2tx known lookup pushkey streamreqs=generaldelta,revlogv1 unbundle=HG10GZ,HG10BZ,HG10UN unbundlehash
   sending heads command
   s>     POST /api/exp-http-v2-0001/ro/heads HTTP/1.1\r\n
   s>     Accept-Encoding: identity\r\n
diff --git a/tests/test-wireproto-command-capabilities.t b/tests/test-wireproto-command-capabilities.t
--- a/tests/test-wireproto-command-capabilities.t
+++ b/tests/test-wireproto-command-capabilities.t
@@ -212,7 +212,7 @@
   s>     Content-Type: application/mercurial-cbor\r\n
   s>     Content-Length: *\r\n (glob)
   s>     \r\n
-  s>     \xa3GapibaseDapi/Dapis\xa1Pexp-http-v2-0001\xa4Hcommands\xaaIbranchmap\xa2Dargs\xa0Kpermissions\x81DpullLcapabilities\xa2Dargs\xa0Kpermissions\x81DpullMchangesetdata\xa2Dargs\xa3Ffields\xd9\x01\x02\x82GparentsHrevisionInoderange\x82\x81J0123456...\x81Iabcdef...Enodes\x81J0123456...Kpermissions\x81DpullHfiledata\xa2Dargs\xa4Ffields\xd9\x01\x02\x82GparentsHrevisionKhaveparents\xf5Enodes\x81J0123456...DpathGfoo.txtKpermissions\x81DpullEheads\xa2Dargs\xa1Jpubliconly\xf4Kpermissions\x81DpullEknown\xa2Dargs\xa1Enodes\x81HdeadbeefKpermissions\x81DpullHlistkeys\xa2Dargs\xa1InamespaceBnsKpermissions\x81DpullFlookup\xa2Dargs\xa1CkeyCfooKpermissions\x81DpullLmanifestdata\xa2Dargs\xa4Ffields\xd9\x01\x02\x82GparentsHrevisionKhaveparents\xf5Enodes\x81J0123456...Dtree@Kpermissions\x81DpullGpushkey\xa2Dargs\xa4CkeyCkeyInamespaceBnsCnewCnewColdColdKpermissions\x81DpushKcompression\x81\xa1DnameDzlibQframingmediatypes\x81X&application/mercurial-exp-framing-0005Nrawrepoformats\x82LgeneraldeltaHrevlogv1Nv1capabilitiesY\x01\xd3batch branchmap $USUAL_BUNDLE2_CAPS$ changegroupsubset compression=$BUNDLE2_COMPRESSIONS$ getbundle httpheader=1024 httpmediatype=0.1rx,0.1tx,0.2tx known lookup pushkey streamreqs=generaldelta,revlogv1 unbundle=HG10GZ,HG10BZ,HG10UN unbundlehash
+  s>     \xa3GapibaseDapi/Dapis\xa1Pexp-http-v2-0001\xa5Hcommands\xaaIbranchmap\xa2Dargs\xa0Kpermissions\x81DpullLcapabilities\xa2Dargs\xa0Kpermissions\x81DpullMchangesetdata\xa2Dargs\xa3Ffields\xd9\x01\x02\x82GparentsHrevisionInoderange\x82\x81J0123456...\x81Iabcdef...Enodes\x81J0123456...Kpermissions\x81DpullHfiledata\xa2Dargs\xa4Ffields\xd9\x01\x02\x82GparentsHrevisionKhaveparents\xf5Enodes\x81J0123456...DpathGfoo.txtKpermissions\x81DpullEheads\xa2Dargs\xa1Jpubliconly\xf4Kpermissions\x81DpullEknown\xa2Dargs\xa1Enodes\x81HdeadbeefKpermissions\x81DpullHlistkeys\xa2Dargs\xa1InamespaceBnsKpermissions\x81DpullFlookup\xa2Dargs\xa1CkeyCfooKpermissions\x81DpullLmanifestdata\xa2Dargs\xa4Ffields\xd9\x01\x02\x82GparentsHrevisionKhaveparents\xf5Enodes\x81J0123456...Dtree@Kpermissions\x81DpullGpushkey\xa2Dargs\xa4CkeyCkeyInamespaceBnsCnewCnewColdColdKpermissions\x81DpushKcompression\x81\xa1DnameDzlibQframingmediatypes\x81X&application/mercurial-exp-framing-0005Rpathfilterprefixes\xd9\x01\x02\x82Epath:Lrootfilesin:Nrawrepoformats\x82LgeneraldeltaHrevlogv1Nv1capabilitiesY\x01\xd3batch branchmap $USUAL_BUNDLE2_CAPS$ changegroupsubset compression=$BUNDLE2_COMPRESSIONS$ getbundle httpheader=1024 httpmediatype=0.1rx,0.1tx,0.2tx known lookup pushkey streamreqs=generaldelta,revlogv1 unbundle=HG10GZ,HG10BZ,HG10UN unbundlehash
   cbor> {
     b'apibase': b'api/',
     b'apis': {
@@ -338,6 +338,10 @@
         b'framingmediatypes': [
           b'application/mercurial-exp-framing-0005'
         ],
+        b'pathfilterprefixes': set([
+          b'path:',
+          b'rootfilesin:'
+        ]),
         b'rawrepoformats': [
           b'generaldelta',
           b'revlogv1'
@@ -369,7 +373,7 @@
   s>     Content-Type: application/mercurial-cbor\r\n
   s>     Content-Length: *\r\n (glob)
   s>     \r\n
-  s>     \xa3GapibaseDapi/Dapis\xa1Pexp-http-v2-0001\xa4Hcommands\xaaIbranchmap\xa2Dargs\xa0Kpermissions\x81DpullLcapabilities\xa2Dargs\xa0Kpermissions\x81DpullMchangesetdata\xa2Dargs\xa3Ffields\xd9\x01\x02\x82GparentsHrevisionInoderange\x82\x81J0123456...\x81Iabcdef...Enodes\x81J0123456...Kpermissions\x81DpullHfiledata\xa2Dargs\xa4Ffields\xd9\x01\x02\x82GparentsHrevisionKhaveparents\xf5Enodes\x81J0123456...DpathGfoo.txtKpermissions\x81DpullEheads\xa2Dargs\xa1Jpubliconly\xf4Kpermissions\x81DpullEknown\xa2Dargs\xa1Enodes\x81HdeadbeefKpermissions\x81DpullHlistkeys\xa2Dargs\xa1InamespaceBnsKpermissions\x81DpullFlookup\xa2Dargs\xa1CkeyCfooKpermissions\x81DpullLmanifestdata\xa2Dargs\xa4Ffields\xd9\x01\x02\x82GparentsHrevisionKhaveparents\xf5Enodes\x81J0123456...Dtree@Kpermissions\x81DpullGpushkey\xa2Dargs\xa4CkeyCkeyInamespaceBnsCnewCnewColdColdKpermissions\x81DpushKcompression\x81\xa1DnameDzlibQframingmediatypes\x81X&application/mercurial-exp-framing-0005Nrawrepoformats\x82LgeneraldeltaHrevlogv1Nv1capabilitiesY\x01\xd3batch branchmap $USUAL_BUNDLE2_CAPS$ changegroupsubset compression=$BUNDLE2_COMPRESSIONS$ getbundle httpheader=1024 httpmediatype=0.1rx,0.1tx,0.2tx known lookup pushkey streamreqs=generaldelta,revlogv1 unbundle=HG10GZ,HG10BZ,HG10UN unbundlehash
+  s>     \xa3GapibaseDapi/Dapis\xa1Pexp-http-v2-0001\xa5Hcommands\xaaIbranchmap\xa2Dargs\xa0Kpermissions\x81DpullLcapabilities\xa2Dargs\xa0Kpermissions\x81DpullMchangesetdata\xa2Dargs\xa3Ffields\xd9\x01\x02\x82GparentsHrevisionInoderange\x82\x81J0123456...\x81Iabcdef...Enodes\x81J0123456...Kpermissions\x81DpullHfiledata\xa2Dargs\xa4Ffields\xd9\x01\x02\x82GparentsHrevisionKhaveparents\xf5Enodes\x81J0123456...DpathGfoo.txtKpermissions\x81DpullEheads\xa2Dargs\xa1Jpubliconly\xf4Kpermissions\x81DpullEknown\xa2Dargs\xa1Enodes\x81HdeadbeefKpermissions\x81DpullHlistkeys\xa2Dargs\xa1InamespaceBnsKpermissions\x81DpullFlookup\xa2Dargs\xa1CkeyCfooKpermissions\x81DpullLmanifestdata\xa2Dargs\xa4Ffields\xd9\x01\x02\x82GparentsHrevisionKhaveparents\xf5Enodes\x81J0123456...Dtree@Kpermissions\x81DpullGpushkey\xa2Dargs\xa4CkeyCkeyInamespaceBnsCnewCnewColdColdKpermissions\x81DpushKcompression\x81\xa1DnameDzlibQframingmediatypes\x81X&application/mercurial-exp-framing-0005Rpathfilterprefixes\xd9\x01\x02\x82Epath:Lrootfilesin:Nrawrepoformats\x82LgeneraldeltaHrevlogv1Nv1capabilitiesY\x01\xd3batch branchmap $USUAL_BUNDLE2_CAPS$ changegroupsubset compression=$BUNDLE2_COMPRESSIONS$ getbundle httpheader=1024 httpmediatype=0.1rx,0.1tx,0.2tx known lookup pushkey streamreqs=generaldelta,revlogv1 unbundle=HG10GZ,HG10BZ,HG10UN unbundlehash
   sending capabilities command
   s>     POST /api/exp-http-v2-0001/ro/capabilities HTTP/1.1\r\n
   s>     Accept-Encoding: identity\r\n
@@ -392,11 +396,11 @@
   s>     \xa1FstatusBok
   s>     \r\n
   received frame(size=11; request=1; stream=2; streamflags=stream-begin; type=command-response; flags=continuation)
-  s>     314\r\n
-  s>     \x0c\x03\x00\x01\x00\x02\x001
-  s>     \xa4Hcommands\xaaIbranchmap\xa2Dargs\xa0Kpermissions\x81DpullLcapabilities\xa2Dargs\xa0Kpermissions\x81DpullMchangesetdata\xa2Dargs\xa3Ffields\xd9\x01\x02\x82GparentsHrevisionInoderange\x82\x81J0123456...\x81Iabcdef...Enodes\x81J0123456...Kpermissions\x81DpullHfiledata\xa2Dargs\xa4Ffields\xd9\x01\x02\x82GparentsHrevisionKhaveparents\xf5Enodes\x81J0123456...DpathGfoo.txtKpermissions\x81DpullEheads\xa2Dargs\xa1Jpubliconly\xf4Kpermissions\x81DpullEknown\xa2Dargs\xa1Enodes\x81HdeadbeefKpermissions\x81DpullHlistkeys\xa2Dargs\xa1InamespaceBnsKpermissions\x81DpullFlookup\xa2Dargs\xa1CkeyCfooKpermissions\x81DpullLmanifestdata\xa2Dargs\xa4Ffields\xd9\x01\x02\x82GparentsHrevisionKhaveparents\xf5Enodes\x81J0123456...Dtree@Kpermissions\x81DpullGpushkey\xa2Dargs\xa4CkeyCkeyInamespaceBnsCnewCnewColdColdKpermissions\x81DpushKcompression\x81\xa1DnameDzlibQframingmediatypes\x81X&application/mercurial-exp-framing-0005Nrawrepoformats\x82LgeneraldeltaHrevlogv1
+  s>     33e\r\n
+  s>     6\x03\x00\x01\x00\x02\x001
+  s>     \xa5Hcommands\xaaIbranchmap\xa2Dargs\xa0Kpermissions\x81DpullLcapabilities\xa2Dargs\xa0Kpermissions\x81DpullMchangesetdata\xa2Dargs\xa3Ffields\xd9\x01\x02\x82GparentsHrevisionInoderange\x82\x81J0123456...\x81Iabcdef...Enodes\x81J0123456...Kpermissions\x81DpullHfiledata\xa2Dargs\xa4Ffields\xd9\x01\x02\x82GparentsHrevisionKhaveparents\xf5Enodes\x81J0123456...DpathGfoo.txtKpermissions\x81DpullEheads\xa2Dargs\xa1Jpubliconly\xf4Kpermissions\x81DpullEknown\xa2Dargs\xa1Enodes\x81HdeadbeefKpermissions\x81DpullHlistkeys\xa2Dargs\xa1InamespaceBnsKpermissions\x81DpullFlookup\xa2Dargs\xa1CkeyCfooKpermissions\x81DpullLmanifestdata\xa2Dargs\xa4Ffields\xd9\x01\x02\x82GparentsHrevisionKhaveparents\xf5Enodes\x81J0123456...Dtree@Kpermissions\x81DpullGpushkey\xa2Dargs\xa4CkeyCkeyInamespaceBnsCnewCnewColdColdKpermissions\x81DpushKcompression\x81\xa1DnameDzlibQframingmediatypes\x81X&application/mercurial-exp-framing-0005Rpathfilterprefixes\xd9\x01\x02\x82Epath:Lrootfilesin:Nrawrepoformats\x82LgeneraldeltaHrevlogv1
   s>     \r\n
-  received frame(size=780; request=1; stream=2; streamflags=; type=command-response; flags=continuation)
+  received frame(size=822; request=1; stream=2; streamflags=; type=command-response; flags=continuation)
   s>     8\r\n
   s>     \x00\x00\x00\x01\x00\x02\x002
   s>     \r\n
@@ -526,6 +530,10 @@
       b'framingmediatypes': [
         b'application/mercurial-exp-framing-0005'
       ],
+      b'pathfilterprefixes': set([
+        b'path:',
+        b'rootfilesin:'
+      ]),
       b'rawrepoformats': [
         b'generaldelta',
         b'revlogv1'