diff --git a/mercurial/wireprotoframing.py b/mercurial/wireprotoframing.py
--- a/mercurial/wireprotoframing.py
+++ b/mercurial/wireprotoframing.py
@@ -269,12 +269,26 @@
     wantframe
        Indicates that nothing of interest happened and the server is waiting on
        more frames from the client before anything interesting can be done.
+
+    noop
+       Indicates no additional action is required.
     """
 
-    def __init__(self, ui, repo):
+    def __init__(self, ui, repo, deferoutput=False):
+        """Construct a new server reactor.
+
+        ``deferoutput`` can be used to indicate that no output frames should be
+        instructed to be sent until input has been exhausted. In this mode,
+        events that would normally generate output frames (such as a command
+        response being ready) will instead defer instructing the consumer to
+        send those frames. This is useful for half-duplex transports where the
+        sender cannot receive until all data has been transmitted.
+        """
         self._ui = ui
         self._repo = repo
+        self._deferoutput = deferoutput
         self._state = 'idle'
+        self._bufferedframegens = []
         self._activecommand = None
         self._activeargs = None
         self._activedata = None
@@ -307,8 +321,33 @@
 
         The raw bytes response is passed as an argument.
         """
+        framegen = createbytesresponseframesfrombytes(data)
+
+        if self._deferoutput:
+            self._bufferedframegens.append(framegen)
+            return 'noop', {}
+        else:
+            return 'sendframes', {
+                'framegen': framegen,
+            }
+
+    def oninputeof(self):
+        """Signals that end of input has been received.
+
+        No more frames will be received. All pending activity should be
+        completed.
+        """
+        if not self._deferoutput:
+            return 'noop', {}
+
+        # If we buffered all our responses, emit those.
+        def makegen():
+            for gen in self._bufferedframegens:
+                for frame in gen:
+                    yield frame
+
         return 'sendframes', {
-            'framegen': createbytesresponseframesfrombytes(data),
+            'framegen': makegen(),
         }
 
     def onapplicationerror(self, msg):
diff --git a/mercurial/wireprotoserver.py b/mercurial/wireprotoserver.py
--- a/mercurial/wireprotoserver.py
+++ b/mercurial/wireprotoserver.py
@@ -401,6 +401,10 @@
         meta['action'] = action
         states.append(json.dumps(meta, sort_keys=True, separators=(', ', ': ')))
 
+    action, meta = reactor.oninputeof()
+    meta['action'] = action
+    states.append(json.dumps(meta, sort_keys=True, separators=(', ',': ')))
+
     res.status = b'200 OK'
     res.headers[b'Content-Type'] = b'text/plain'
     res.setbodybytes(b'\n'.join(states))
@@ -411,7 +415,11 @@
     Called when the HTTP request contains unified frame-based protocol
     frames for evaluation.
     """
-    reactor = wireprotoframing.serverreactor(ui, repo)
+    # TODO Some HTTP clients are full duplex and can receive data before
+    # the entire request is transmitted. Figure out a way to indicate support
+    # for that so we can opt into full duplex mode.
+    reactor = wireprotoframing.serverreactor(ui, repo,
+                                             deferoutput=True)
     seencommand = False
 
     while True:
@@ -448,6 +456,19 @@
             raise error.ProgrammingError(
                 'unhandled action from frame processor: %s' % action)
 
+    action, meta = reactor.oninputeof()
+    if action == 'sendframes':
+        # We assume we haven't started sending the response yet. If we're
+        # wrong, the response type will raise an exception.
+        res.status = b'200 OK'
+        res.headers[b'Content-Type'] = FRAMINGTYPE
+        res.setbodygen(meta['framegen'])
+    elif action == 'noop':
+        pass
+    else:
+        raise error.ProgrammingError('unhandled action from frame processor: %s'
+                                     % action)
+
 def _httpv2runcommand(ui, repo, req, res, authedperm, reqcommand, reactor,
                       command):
     """Dispatch a wire protocol command made from HTTPv2 requests.
@@ -504,6 +525,8 @@
 
     if action == 'sendframes':
         res.setbodygen(meta['framegen'])
+    elif action == 'noop':
+        pass
     else:
         raise error.ProgrammingError('unhandled event from reactor: %s' %
                                      action)
diff --git a/tests/test-http-api-httpv2.t b/tests/test-http-api-httpv2.t
--- a/tests/test-http-api-httpv2.t
+++ b/tests/test-http-api-httpv2.t
@@ -384,11 +384,12 @@
   s>     Server: testing stub value\r\n
   s>     Date: $HTTP_DATE$\r\n
   s>     Content-Type: text/plain\r\n
-  s>     Content-Length: 117\r\n
+  s>     Content-Length: 136\r\n
   s>     \r\n
   s>     received: 1 1 command1\n
   s>     {"action": "runcommand", "args": {}, "command": "command1", "data": null}\n
-  s>     received: <no frame>
+  s>     received: <no frame>\n
+  s>     {"action": "noop"}
 
 Single argument frame is received
 
@@ -415,13 +416,14 @@
   s>     Server: testing stub value\r\n
   s>     Date: $HTTP_DATE$\r\n
   s>     Content-Type: text/plain\r\n
-  s>     Content-Length: 217\r\n
+  s>     Content-Length: 236\r\n
   s>     \r\n
   s>     received: 1 2 command1\n
   s>     {"action": "wantframe", "state": "command-receiving-args"}\n
   s>     received: 2 2 \x03\x00\x05\x00foovalue\n
   s>     {"action": "runcommand", "args": {"foo": "value"}, "command": "command1", "data": null}\n
-  s>     received: <no frame>
+  s>     received: <no frame>\n
+  s>     {"action": "noop"}
 
 Multiple argument frames are received
 
@@ -449,7 +451,7 @@
   s>     Server: testing stub value\r\n
   s>     Date: $HTTP_DATE$\r\n
   s>     Content-Type: text/plain\r\n
-  s>     Content-Length: 319\r\n
+  s>     Content-Length: 338\r\n
   s>     \r\n
   s>     received: 1 2 command1\n
   s>     {"action": "wantframe", "state": "command-receiving-args"}\n
@@ -457,7 +459,8 @@
   s>     {"action": "wantframe", "state": "command-receiving-args"}\n
   s>     received: 2 2 \x04\x00\x04\x00foo1val1\n
   s>     {"action": "runcommand", "args": {"foo": "value", "foo1": "val1"}, "command": "command1", "data": null}\n
-  s>     received: <no frame>
+  s>     received: <no frame>\n
+  s>     {"action": "noop"}
 
 Command with single data frame
 
@@ -484,13 +487,14 @@
   s>     Server: testing stub value\r\n
   s>     Date: $HTTP_DATE$\r\n
   s>     Content-Type: text/plain\r\n
-  s>     Content-Length: 199\r\n
+  s>     Content-Length: 218\r\n
   s>     \r\n
   s>     received: 1 4 command1\n
   s>     {"action": "wantframe", "state": "command-receiving-data"}\n
   s>     received: 3 2 data!\n
   s>     {"action": "runcommand", "args": {}, "command": "command1", "data": "data!"}\n
-  s>     received: <no frame>
+  s>     received: <no frame>\n
+  s>     {"action": "noop"}
 
 Command with multiple data frames
 
@@ -519,7 +523,7 @@
   s>     Server: testing stub value\r\n
   s>     Date: $HTTP_DATE$\r\n
   s>     Content-Type: text/plain\r\n
-  s>     Content-Length: 367\r\n
+  s>     Content-Length: 386\r\n
   s>     \r\n
   s>     received: 1 4 command1\n
   s>     {"action": "wantframe", "state": "command-receiving-data"}\n
@@ -529,7 +533,8 @@
   s>     {"action": "wantframe", "state": "command-receiving-data"}\n
   s>     received: 3 2 data3\n
   s>     {"action": "runcommand", "args": {}, "command": "command1", "data": "data1data2data3"}\n
-  s>     received: <no frame>
+  s>     received: <no frame>\n
+  s>     {"action": "noop"}
 
 Unexpected frame type
 
@@ -556,13 +561,14 @@
   s>     Server: testing stub value\r\n
   s>     Date: $HTTP_DATE$\r\n
   s>     Content-Type: text/plain\r\n
-  s>     Content-Length: 181\r\n
+  s>     Content-Length: 200\r\n
   s>     \r\n
   s>     received: 2 0 0\n
   s>     {"action": "error", "message": "expected command frame; got 2"}\n
   s>     received: 1 1 mycommand\n
   s>     {"action": "error", "message": "server already errored"}\n
-  s>     received: <no frame>
+  s>     received: <no frame>\n
+  s>     {"action": "noop"}
 
 Missing flags on command name frame
 
@@ -588,11 +594,12 @@
   s>     Server: testing stub value\r\n
   s>     Date: $HTTP_DATE$\r\n
   s>     Content-Type: text/plain\r\n
-  s>     Content-Length: 114\r\n
+  s>     Content-Length: 133\r\n
   s>     \r\n
   s>     received: 1 0 command1\n
   s>     {"action": "error", "message": "missing frame flags on command frame"}\n
-  s>     received: <no frame>
+  s>     received: <no frame>\n
+  s>     {"action": "noop"}
 
 Missing argument frame
 
@@ -619,13 +626,14 @@
   s>     Server: testing stub value\r\n
   s>     Date: $HTTP_DATE$\r\n
   s>     Content-Type: text/plain\r\n
-  s>     Content-Length: 197\r\n
+  s>     Content-Length: 216\r\n
   s>     \r\n
   s>     received: 1 2 command1\n
   s>     {"action": "wantframe", "state": "command-receiving-args"}\n
   s>     received: 1 0 ignored\n
   s>     {"action": "error", "message": "expected command argument frame; got 1"}\n
-  s>     received: <no frame>
+  s>     received: <no frame>\n
+  s>     {"action": "noop"}
 
 Argument frame with incomplete name
 
@@ -652,13 +660,14 @@
   s>     Server: testing stub value\r\n
   s>     Date: $HTTP_DATE$\r\n
   s>     Content-Type: text/plain\r\n
-  s>     Content-Length: 206\r\n
+  s>     Content-Length: 225\r\n
   s>     \r\n
   s>     received: 1 2 command1\n
   s>     {"action": "wantframe", "state": "command-receiving-args"}\n
   s>     received: 2 2 \x04\x00\xde\xadfoo\n
   s>     {"action": "error", "message": "malformed argument frame: partial argument name"}\n
-  s>     received: <no frame>
+  s>     received: <no frame>\n
+  s>     {"action": "noop"}
 
 Argument frame with incomplete value
 
@@ -685,13 +694,14 @@
   s>     Server: testing stub value\r\n
   s>     Date: $HTTP_DATE$\r\n
   s>     Content-Type: text/plain\r\n
-  s>     Content-Length: 219\r\n
+  s>     Content-Length: 238\r\n
   s>     \r\n
   s>     received: 1 2 command1\n
   s>     {"action": "wantframe", "state": "command-receiving-args"}\n
   s>     received: 2 2 \x03\x00\xaa\xaafoopartialvalue\n
   s>     {"action": "error", "message": "malformed argument frame: partial argument value"}\n
-  s>     received: <no frame>
+  s>     received: <no frame>\n
+  s>     {"action": "noop"}
 
 Missing command data frame
 
@@ -718,13 +728,14 @@
   s>     Server: testing stub value\r\n
   s>     Date: $HTTP_DATE$\r\n
   s>     Content-Type: text/plain\r\n
-  s>     Content-Length: 194\r\n
+  s>     Content-Length: 213\r\n
   s>     \r\n
   s>     received: 1 4 command1\n
   s>     {"action": "wantframe", "state": "command-receiving-data"}\n
   s>     received: 1 1 command2\n
   s>     {"action": "error", "message": "expected command data frame; got 1"}\n
-  s>     received: <no frame>
+  s>     received: <no frame>\n
+  s>     {"action": "noop"}
 
 No flags on command data frame
 
@@ -751,12 +762,13 @@
   s>     Server: testing stub value\r\n
   s>     Date: $HTTP_DATE$\r\n
   s>     Content-Type: text/plain\r\n
-  s>     Content-Length: 188\r\n
+  s>     Content-Length: 207\r\n
   s>     \r\n
   s>     received: 1 4 command1\n
   s>     {"action": "wantframe", "state": "command-receiving-data"}\n
   s>     received: 3 0 data\n
   s>     {"action": "error", "message": "command data frame without flags"}\n
-  s>     received: <no frame>
+  s>     received: <no frame>\n
+  s>     {"action": "noop"}
 
   $ cat error.log