diff --git a/mercurial/httppeer.py b/mercurial/httppeer.py
--- a/mercurial/httppeer.py
+++ b/mercurial/httppeer.py
@@ -328,13 +328,24 @@
 
     return res
 
+class RedirectedRepoError(error.RepoError):
+    def __init__(self, msg, respurl):
+        super(RedirectedRepoError, self).__init__(msg)
+        self.respurl = respurl
+
 def parsev1commandresponse(ui, baseurl, requrl, qs, resp, compressible,
                            allowcbor=False):
     # record the url we got redirected to
+    redirected = False
     respurl = pycompat.bytesurl(resp.geturl())
     if respurl.endswith(qs):
         respurl = respurl[:-len(qs)]
+        qsdropped = False
+    else:
+        qsdropped = True
+
     if baseurl.rstrip('/') != respurl.rstrip('/'):
+        redirected = True
         if not ui.quiet:
             ui.warn(_('real URL is %s\n') % respurl)
 
@@ -351,10 +362,16 @@
     # application/hg-changegroup. We don't support such old servers.
     if not proto.startswith('application/mercurial-'):
         ui.debug("requested URL: '%s'\n" % util.hidepassword(requrl))
-        raise error.RepoError(
-            _("'%s' does not appear to be an hg repository:\n"
-              "---%%<--- (%s)\n%s\n---%%<---\n")
-            % (safeurl, proto or 'no content-type', resp.read(1024)))
+        msg = _("'%s' does not appear to be an hg repository:\n"
+                "---%%<--- (%s)\n%s\n---%%<---\n") % (
+            safeurl, proto or 'no content-type', resp.read(1024))
+
+        # Some servers may strip the query string from the redirect. We
+        # raise a special error type so callers can react to this specially.
+        if redirected and qsdropped:
+            raise RedirectedRepoError(msg, respurl)
+        else:
+            raise error.RepoError(msg)
 
     try:
         subtype = proto.split('-', 1)[1]
@@ -434,8 +451,6 @@
 
     # End of ipeercommands interface.
 
-    # look up capabilities only when needed
-
     def _callstream(self, cmd, _compressible=False, **args):
         args = pycompat.byteskwargs(args)
 
@@ -853,12 +868,32 @@
     req, requrl, qs = makev1commandrequest(ui, requestbuilder, caps,
                                            capable, url, 'capabilities',
                                            args)
-
     resp = sendrequest(ui, opener, req)
 
-    respurl, ct, resp = parsev1commandresponse(ui, url, requrl, qs, resp,
-                                               compressible=False,
-                                               allowcbor=advertisev2)
+    # The server may redirect us to the repo root, stripping the
+    # ?cmd=capabilities query string from the URL. The server would likely
+    # return HTML in this case and ``parsev1commandresponse()`` would raise.
+    # We catch this special case and re-issue the capabilities request against
+    # the new URL.
+    #
+    # We should ideally not do this, as a redirect that drops the query
+    # string from the URL is arguably a server bug. (Garbage in, garbage out).
+    # However,  Mercurial clients for several years appeared to handle this
+    # issue without behavior degradation. And according to issue 5860, it may
+    # be a longstanding bug in some server implementations. So we allow a
+    # redirect that drops the query string to "just work."
+    try:
+        respurl, ct, resp = parsev1commandresponse(ui, url, requrl, qs, resp,
+                                                   compressible=False,
+                                                   allowcbor=advertisev2)
+    except RedirectedRepoError as e:
+        req, requrl, qs = makev1commandrequest(ui, requestbuilder, caps,
+                                               capable, e.respurl,
+                                               'capabilities', args)
+        resp = sendrequest(ui, opener, req)
+        respurl, ct, resp = parsev1commandresponse(ui, url, requrl, qs, resp,
+                                                   compressible=False,
+                                                   allowcbor=advertisev2)
 
     try:
         rawdata = resp.read()
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
@@ -333,3 +333,394 @@
   response: [b'\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00']
 
   $ killdaemons.py
+
+HTTP client follows HTTP redirect on handshake to new repo
+
+  $ cd $TESTTMP
+
+  $ hg init redirector
+  $ hg init redirected
+  $ cd redirected
+  $ touch foo
+  $ hg -q commit -A -m initial
+  $ cd ..
+
+  $ cat > paths.conf << EOF
+  > [paths]
+  > / = $TESTTMP/*
+  > EOF
+
+  $ cat > redirectext.py << EOF
+  > from mercurial import extensions, wireprotoserver
+  > def wrappedcallhttp(orig, repo, req, res, proto, cmd):
+  >     path = req.advertisedurl[len(req.advertisedbaseurl):]
+  >     if not path.startswith(b'/redirector'):
+  >         return orig(repo, req, res, proto, cmd)
+  >     relpath = path[len(b'/redirector'):]
+  >     res.status = b'301 Redirect'
+  >     newurl = b'%s/redirected%s' % (req.baseurl, relpath)
+  >     if not repo.ui.configbool('testing', 'redirectqs', True) and b'?' in newurl:
+  >         newurl = newurl[0:newurl.index(b'?')]
+  >     res.headers[b'Location'] = newurl
+  >     res.headers[b'Content-Type'] = b'text/plain'
+  >     res.setbodybytes(b'redirected')
+  >     return True
+  > 
+  > extensions.wrapfunction(wireprotoserver, '_callhttp', wrappedcallhttp)
+  > EOF
+
+  $ hg --config extensions.redirect=$TESTTMP/redirectext.py \
+  >    --config server.compressionengines=zlib \
+  >     serve --web-conf paths.conf --pid-file hg.pid -p $HGPORT -d
+  $ cat hg.pid > $DAEMON_PIDS
+
+Verify our HTTP 301 is served properly
+
+  $ hg --verbose debugwireproto --peer raw http://$LOCALIP:$HGPORT << EOF
+  > httprequest GET /redirector?cmd=capabilities
+  >     user-agent: test
+  > EOF
+  using raw connection to peer
+  s>     GET /redirector?cmd=capabilities HTTP/1.1\r\n
+  s>     Accept-Encoding: identity\r\n
+  s>     user-agent: test\r\n
+  s>     host: $LOCALIP:$HGPORT\r\n (glob)
+  s>     \r\n
+  s> makefile('rb', None)
+  s>     HTTP/1.1 301 Redirect\r\n
+  s>     Server: testing stub value\r\n
+  s>     Date: $HTTP_DATE$\r\n
+  s>     Location: http://$LOCALIP:$HGPORT/redirected?cmd=capabilities\r\n (glob)
+  s>     Content-Type: text/plain\r\n
+  s>     Content-Length: 10\r\n
+  s>     \r\n
+  s>     redirected
+  s>     GET /redirected?cmd=capabilities HTTP/1.1\r\n
+  s>     Accept-Encoding: identity\r\n
+  s>     user-agent: test\r\n
+  s>     host: $LOCALIP:$HGPORT\r\n (glob)
+  s>     \r\n
+  s> makefile('rb', None)
+  s>     HTTP/1.1 200 Script output follows\r\n
+  s>     Server: testing stub value\r\n
+  s>     Date: $HTTP_DATE$\r\n
+  s>     Content-Type: application/mercurial-0.1\r\n
+  s>     Content-Length: 453\r\n
+  s>     \r\n
+  s>     batch branchmap $USUAL_BUNDLE2_CAPS_SERVER$ 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
+
+Test with the HTTP peer
+
+  $ hg --verbose debugwireproto http://$LOCALIP:$HGPORT/redirector << EOF
+  > command heads
+  > EOF
+  s>     GET /redirector?cmd=capabilities HTTP/1.1\r\n
+  s>     Accept-Encoding: identity\r\n
+  s>     accept: application/mercurial-0.1\r\n
+  s>     host: $LOCALIP:$HGPORT\r\n (glob)
+  s>     user-agent: Mercurial debugwireproto\r\n
+  s>     \r\n
+  s> makefile('rb', None)
+  s>     HTTP/1.1 301 Redirect\r\n
+  s>     Server: testing stub value\r\n
+  s>     Date: $HTTP_DATE$\r\n
+  s>     Location: http://$LOCALIP:$HGPORT/redirected?cmd=capabilities\r\n (glob)
+  s>     Content-Type: text/plain\r\n
+  s>     Content-Length: 10\r\n
+  s>     \r\n
+  s>     redirected
+  s>     GET /redirected?cmd=capabilities HTTP/1.1\r\n
+  s>     Accept-Encoding: identity\r\n
+  s>     accept: application/mercurial-0.1\r\n
+  s>     host: $LOCALIP:$HGPORT\r\n (glob)
+  s>     user-agent: Mercurial debugwireproto\r\n
+  s>     \r\n
+  s> makefile('rb', None)
+  s>     HTTP/1.1 200 Script output follows\r\n
+  s>     Server: testing stub value\r\n
+  s>     Date: $HTTP_DATE$\r\n
+  s>     Content-Type: application/mercurial-0.1\r\n
+  s>     Content-Length: 453\r\n
+  s>     \r\n
+  real URL is http://$LOCALIP:$HGPORT/redirected (glob)
+  s>     batch branchmap $USUAL_BUNDLE2_CAPS_SERVER$ 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>     GET /redirected?cmd=heads HTTP/1.1\r\n
+  s>     Accept-Encoding: identity\r\n
+  s>     vary: X-HgProto-1\r\n
+  s>     x-hgproto-1: 0.1 0.2 comp=$USUAL_COMPRESSIONS$ partial-pull\r\n
+  s>     accept: application/mercurial-0.1\r\n
+  s>     host: $LOCALIP:$HGPORT\r\n (glob)
+  s>     user-agent: Mercurial debugwireproto\r\n
+  s>     \r\n
+  s> makefile('rb', None)
+  s>     HTTP/1.1 200 Script output follows\r\n
+  s>     Server: testing stub value\r\n
+  s>     Date: $HTTP_DATE$\r\n
+  s>     Content-Type: application/mercurial-0.1\r\n
+  s>     Content-Length: 41\r\n
+  s>     \r\n
+  s>     96ee1d7354c4ad7372047672c36a1f561e3a6a4c\n
+  response: [b'\x96\xee\x1dsT\xc4\xadsr\x04vr\xc3j\x1fV\x1e:jL']
+
+  $ killdaemons.py
+
+Now test a variation where we strip the query string from the redirect URL.
+(SCM Manager apparently did this and clients would recover from it)
+
+  $ hg --config extensions.redirect=$TESTTMP/redirectext.py \
+  >    --config server.compressionengines=zlib \
+  >    --config testing.redirectqs=false \
+  >     serve --web-conf paths.conf --pid-file hg.pid -p $HGPORT -d
+  $ cat hg.pid > $DAEMON_PIDS
+
+  $ hg --verbose debugwireproto --peer raw http://$LOCALIP:$HGPORT << EOF
+  > httprequest GET /redirector?cmd=capabilities
+  >     user-agent: test
+  > EOF
+  using raw connection to peer
+  s>     GET /redirector?cmd=capabilities HTTP/1.1\r\n
+  s>     Accept-Encoding: identity\r\n
+  s>     user-agent: test\r\n
+  s>     host: $LOCALIP:$HGPORT\r\n (glob)
+  s>     \r\n
+  s> makefile('rb', None)
+  s>     HTTP/1.1 301 Redirect\r\n
+  s>     Server: testing stub value\r\n
+  s>     Date: $HTTP_DATE$\r\n
+  s>     Location: http://$LOCALIP:$HGPORT/redirected\r\n (glob)
+  s>     Content-Type: text/plain\r\n
+  s>     Content-Length: 10\r\n
+  s>     \r\n
+  s>     redirected
+  s>     GET /redirected HTTP/1.1\r\n
+  s>     Accept-Encoding: identity\r\n
+  s>     user-agent: test\r\n
+  s>     host: $LOCALIP:$HGPORT\r\n (glob)
+  s>     \r\n
+  s> makefile('rb', None)
+  s>     HTTP/1.1 200 Script output follows\r\n
+  s>     Server: testing stub value\r\n
+  s>     Date: $HTTP_DATE$\r\n
+  s>     ETag: W/"*"\r\n (glob)
+  s>     Content-Type: text/html; charset=ascii\r\n
+  s>     Transfer-Encoding: chunked\r\n
+  s>     \r\n
+  s>     414\r\n
+  s>     <!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.1//EN" "http://www.w3.org/TR/xhtml11/DTD/xhtml11.dtd">\n
+  s>     <html xmlns="http://www.w3.org/1999/xhtml" xml:lang="en-US">\n
+  s>     <head>\n
+  s>     <link rel="icon" href="/redirected/static/hgicon.png" type="image/png" />\n
+  s>     <meta name="robots" content="index, nofollow" />\n
+  s>     <link rel="stylesheet" href="/redirected/static/style-paper.css" type="text/css" />\n
+  s>     <script type="text/javascript" src="/redirected/static/mercurial.js"></script>\n
+  s>     \n
+  s>     <title>redirected: log</title>\n
+  s>     <link rel="alternate" type="application/atom+xml"\n
+  s>        href="/redirected/atom-log" title="Atom feed for redirected" />\n
+  s>     <link rel="alternate" type="application/rss+xml"\n
+  s>        href="/redirected/rss-log" title="RSS feed for redirected" />\n
+  s>     </head>\n
+  s>     <body>\n
+  s>     \n
+  s>     <div class="container">\n
+  s>     <div class="menu">\n
+  s>     <div class="logo">\n
+  s>     <a href="https://mercurial-scm.org/">\n
+  s>     <img src="/redirected/static/hglogo.png" alt="mercurial" /></a>\n
+  s>     </div>\n
+  s>     <ul>\n
+  s>     <li class="active">log</li>\n
+  s>     <li><a href="/redirected/graph/tip">graph</a></li>\n
+  s>     <li><a href="/redirected/tags">tags</a></li>\n
+  s>     <li><a href="
+  s>     \r\n
+  s>     810\r\n
+  s>     /redirected/bookmarks">bookmarks</a></li>\n
+  s>     <li><a href="/redirected/branches">branches</a></li>\n
+  s>     </ul>\n
+  s>     <ul>\n
+  s>     <li><a href="/redirected/rev/tip">changeset</a></li>\n
+  s>     <li><a href="/redirected/file/tip">browse</a></li>\n
+  s>     </ul>\n
+  s>     <ul>\n
+  s>     \n
+  s>     </ul>\n
+  s>     <ul>\n
+  s>      <li><a href="/redirected/help">help</a></li>\n
+  s>     </ul>\n
+  s>     <div class="atom-logo">\n
+  s>     <a href="/redirected/atom-log" title="subscribe to atom feed">\n
+  s>     <img class="atom-logo" src="/redirected/static/feed-icon-14x14.png" alt="atom feed" />\n
+  s>     </a>\n
+  s>     </div>\n
+  s>     </div>\n
+  s>     \n
+  s>     <div class="main">\n
+  s>     <h2 class="breadcrumb"><a href="/">Mercurial</a> &gt; <a href="/redirected">redirected</a> </h2>\n
+  s>     <h3>log</h3>\n
+  s>     \n
+  s>     \n
+  s>     <form class="search" action="/redirected/log">\n
+  s>     \n
+  s>     <p><input name="rev" id="search1" type="text" size="30" value="" /></p>\n
+  s>     <div id="hint">Find changesets by keywords (author, files, the commit message), revision\n
+  s>     number or hash, or <a href="/redirected/help/revsets">revset expression</a>.</div>\n
+  s>     </form>\n
+  s>     \n
+  s>     <div class="navigate">\n
+  s>     <a href="/redirected/shortlog/tip?revcount=30">less</a>\n
+  s>     <a href="/redirected/shortlog/tip?revcount=120">more</a>\n
+  s>     | rev 0: <a href="/redirected/shortlog/96ee1d7354c4">(0)</a> <a href="/redirected/shortlog/tip">tip</a> \n
+  s>     </div>\n
+  s>     \n
+  s>     <table class="bigtable">\n
+  s>     <thead>\n
+  s>      <tr>\n
+  s>       <th class="age">age</th>\n
+  s>       <th class="author">author</th>\n
+  s>       <th class="description">description</th>\n
+  s>      </tr>\n
+  s>     </thead>\n
+  s>     <tbody class="stripes2">\n
+  s>      <tr>\n
+  s>       <td class="age">Thu, 01 Jan 1970 00:00:00 +0000</td>\n
+  s>       <td class="author">test</td>\n
+  s>       <td class="description">\n
+  s>        <a href="/redirected/rev/96ee1d7354c4">initial</a>\n
+  s>        <span class="phase">draft</span> <span class="branchhead">default</span> <span class="tag">tip</span> \n
+  s>       </td>\n
+  s>      </tr>\n
+  s>     \n
+  s>     </tbody>\n
+  s>     </table>\n
+  s>     \n
+  s>     <div class="navigate">\n
+  s>     <a href="/redirected/shortlog/tip?revcount=30">less</a>\n
+  s>     <a href="/redirected/shortlog/tip?revcount=120">more</a>\n
+  s>     | rev 0: <a href="/redirected/shortlog/96ee1d7354c4">(0)</a> <a href="/redirected/shortlog/tip">tip</a> \n
+  s>     </div>\n
+  s>     \n
+  s>     <script type="text/javascript">\n
+  s>         ajaxScrollInit(\n
+  s>                 \'/redirected/shortlog/%next%\',\n
+  s>                 \'\', <!-- NEXTHASH\n
+  s>                 function (htmlText) {
+  s>     \r\n
+  s>     14a\r\n
+  s>     \n
+  s>                     var m = htmlText.match(/\'(\\w+)\', <!-- NEXTHASH/);\n
+  s>                     return m ? m[1] : null;\n
+  s>                 },\n
+  s>                 \'.bigtable > tbody\',\n
+  s>                 \'<tr class="%class%">\\\n
+  s>                 <td colspan="3" style="text-align: center;">%text%</td>\\\n
+  s>                 </tr>\'\n
+  s>         );\n
+  s>     </script>\n
+  s>     \n
+  s>     </div>\n
+  s>     </div>\n
+  s>     \n
+  s>     \n
+  s>     \n
+  s>     </body>\n
+  s>     </html>\n
+  s>     \n
+  s>     \r\n
+  s>     0\r\n
+  s>     \r\n
+
+  $ hg --verbose debugwireproto http://$LOCALIP:$HGPORT/redirector << EOF
+  > command heads
+  > EOF
+  s>     GET /redirector?cmd=capabilities HTTP/1.1\r\n
+  s>     Accept-Encoding: identity\r\n
+  s>     accept: application/mercurial-0.1\r\n
+  s>     host: $LOCALIP:$HGPORT\r\n (glob)
+  s>     user-agent: Mercurial debugwireproto\r\n
+  s>     \r\n
+  s> makefile('rb', None)
+  s>     HTTP/1.1 301 Redirect\r\n
+  s>     Server: testing stub value\r\n
+  s>     Date: $HTTP_DATE$\r\n
+  s>     Location: http://$LOCALIP:$HGPORT/redirected\r\n (glob)
+  s>     Content-Type: text/plain\r\n
+  s>     Content-Length: 10\r\n
+  s>     \r\n
+  s>     redirected
+  s>     GET /redirected HTTP/1.1\r\n
+  s>     Accept-Encoding: identity\r\n
+  s>     accept: application/mercurial-0.1\r\n
+  s>     host: $LOCALIP:$HGPORT\r\n (glob)
+  s>     user-agent: Mercurial debugwireproto\r\n
+  s>     \r\n
+  s> makefile('rb', None)
+  s>     HTTP/1.1 200 Script output follows\r\n
+  s>     Server: testing stub value\r\n
+  s>     Date: $HTTP_DATE$\r\n
+  s>     ETag: W/"*"\r\n (glob)
+  s>     Content-Type: text/html; charset=ascii\r\n
+  s>     Transfer-Encoding: chunked\r\n
+  s>     \r\n
+  real URL is http://$LOCALIP:$HGPORT/redirected (glob)
+  s>     414\r\n
+  s>     <!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.1//EN" "http://www.w3.org/TR/xhtml11/DTD/xhtml11.dtd">\n
+  s>     <html xmlns="http://www.w3.org/1999/xhtml" xml:lang="en-US">\n
+  s>     <head>\n
+  s>     <link rel="icon" href="/redirected/static/hgicon.png" type="image/png" />\n
+  s>     <meta name="robots" content="index, nofollow" />\n
+  s>     <link rel="stylesheet" href="/redirected/static/style-paper.css" type="text/css" />\n
+  s>     <script type="text/javascript" src="/redirected/static/mercurial.js"></script>\n
+  s>     \n
+  s>     <title>redirected: log</title>\n
+  s>     <link rel="alternate" type="application/atom+xml"\n
+  s>        href="/redirected/atom-log" title="Atom feed for redirected" />\n
+  s>     <link rel="alternate" type="application/rss+xml"\n
+  s>        href="/redirected/rss-log" title="RSS feed for redirected" />\n
+  s>     </head>\n
+  s>     <body>\n
+  s>     \n
+  s>     <div class="container">\n
+  s>     <div class="menu">\n
+  s>     <div class="logo">\n
+  s>     <a href="https://mercurial-scm.org/">\n
+  s>     <img src="/redirected/static/hglogo.png" alt="mercurial" /></a>\n
+  s>     </div>\n
+  s>     <ul>\n
+  s>     <li class="active">log</li>\n
+  s>     <li><a href="/redirected/graph/tip">graph</a></li>\n
+  s>     <li><a href="/redirected/tags">tags</a
+  s>     GET /redirected?cmd=capabilities HTTP/1.1\r\n
+  s>     Accept-Encoding: identity\r\n
+  s>     accept: application/mercurial-0.1\r\n
+  s>     host: $LOCALIP:$HGPORT\r\n (glob)
+  s>     user-agent: Mercurial debugwireproto\r\n
+  s>     \r\n
+  s> makefile('rb', None)
+  s>     HTTP/1.1 200 Script output follows\r\n
+  s>     Server: testing stub value\r\n
+  s>     Date: $HTTP_DATE$\r\n
+  s>     Content-Type: application/mercurial-0.1\r\n
+  s>     Content-Length: 453\r\n
+  s>     \r\n
+  real URL is http://$LOCALIP:$HGPORT/redirected (glob)
+  s>     batch branchmap $USUAL_BUNDLE2_CAPS_SERVER$ 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>     GET /redirected?cmd=heads HTTP/1.1\r\n
+  s>     Accept-Encoding: identity\r\n
+  s>     vary: X-HgProto-1\r\n
+  s>     x-hgproto-1: 0.1 0.2 comp=$USUAL_COMPRESSIONS$ partial-pull\r\n
+  s>     accept: application/mercurial-0.1\r\n
+  s>     host: $LOCALIP:$HGPORT\r\n (glob)
+  s>     user-agent: Mercurial debugwireproto\r\n
+  s>     \r\n
+  s> makefile('rb', None)
+  s>     HTTP/1.1 200 Script output follows\r\n
+  s>     Server: testing stub value\r\n
+  s>     Date: $HTTP_DATE$\r\n
+  s>     Content-Type: application/mercurial-0.1\r\n
+  s>     Content-Length: 41\r\n
+  s>     \r\n
+  s>     96ee1d7354c4ad7372047672c36a1f561e3a6a4c\n
+  response: [b'\x96\xee\x1dsT\xc4\xadsr\x04vr\xc3j\x1fV\x1e:jL']