diff --git a/mercurial/hgweb/common.py b/mercurial/hgweb/common.py
--- a/mercurial/hgweb/common.py
+++ b/mercurial/hgweb/common.py
@@ -28,6 +28,7 @@
 HTTP_FORBIDDEN = 403
 HTTP_NOT_FOUND = 404
 HTTP_METHOD_NOT_ALLOWED = 405
+HTTP_NOT_ACCEPTABLE = 406
 HTTP_SERVER_ERROR = 500
 
 
diff --git a/mercurial/hgweb/protocol.py b/mercurial/hgweb/protocol.py
--- a/mercurial/hgweb/protocol.py
+++ b/mercurial/hgweb/protocol.py
@@ -11,6 +11,8 @@
 import struct
 
 from .common import (
+    ErrorResponse,
+    HTTP_NOT_ACCEPTABLE,
     HTTP_OK,
 )
 
@@ -140,8 +142,8 @@
 
                     return HGTYPE2, engine, opts
 
-            # No mutually supported compression format. Fall back to the
-            # legacy protocol.
+            raise ErrorResponse(HTTP_NOT_ACCEPTABLE,
+                                'No common compression format')
 
         # Don't allow untrusted settings because disabling compression or
         # setting a very high compression level could lead to flooding
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
@@ -78,14 +78,15 @@
   server: * (glob)
   transfer-encoding: chunked
 
-Requesting a compression format that server doesn't support results will fall back to 0.1
+Server should fail when it can't agree with client on a compression format
 
   $ get-with-headers.py --hgproto '0.2 comp=aa' --headeronly $LOCALIP:$HGPORT '?cmd=getbundle&heads=e93700bd72895c5addab234c56d4024b487a362f&common=0000000000000000000000000000000000000000' -
-  200 Script output follows
+  406 No common compression format
+  content-length: 31
   content-type: application/mercurial-0.1
   date: * (glob)
   server: * (glob)
-  transfer-encoding: chunked
+  [1]
 
 #if zstd
 zstd is used if available