diff --git a/mercurial/debugcommands.py b/mercurial/debugcommands.py
--- a/mercurial/debugcommands.py
+++ b/mercurial/debugcommands.py
@@ -2683,6 +2683,21 @@
 
     Read a line of output from the server. If there are multiple output
     pipes, reads only the main pipe.
+
+    ereadline
+    ---------
+
+    Like ``readline``, but read from the stderr pipe, if available.
+
+    read <X>
+    --------
+
+    ``read()`` N bytes from the server's main output pipe.
+
+    eread <X>
+    ---------
+
+    ``read()`` N bytes from the server's stderr pipe, if available.
     """
     opts = pycompat.byteskwargs(opts)
 
@@ -2831,6 +2846,14 @@
                 util.readpipe(stderr)
         elif action == 'readline':
             stdout.readline()
+        elif action == 'ereadline':
+            stderr.readline()
+        elif action.startswith('read '):
+            count = int(action.split(' ', 1)[1])
+            stdout.read(count)
+        elif action.startswith('eread '):
+            count = int(action.split(' ', 1)[1])
+            stderr.read(count)
         else:
             raise error.Abort(_('unknown action: %s') % action)
 
diff --git a/tests/test-ssh-proto-unbundle.t b/tests/test-ssh-proto-unbundle.t
--- a/tests/test-ssh-proto-unbundle.t
+++ b/tests/test-ssh-proto-unbundle.t
@@ -45,7 +45,7 @@
   > # This is "force" in hex.
   >     heads 666f726365
   >     PUSHFILE ../initial.v1.hg
-  > readavailable
+  > eread 115
   > EOF
   testing ssh1
   creating ssh peer from handshake results
@@ -93,7 +93,7 @@
   o> read(1) -> 1: 0
   result: 0
   remote output: 
-  e> read(-1) -> 115:
+  e> read(115) -> 115:
   e>     abort: incompatible Mercurial client; bundle2 required\n
   e>     (see https://www.mercurial-scm.org/wiki/IncompatibleClient)\n
   
@@ -143,7 +143,7 @@
   o> read(1) -> 1: 0
   result: 0
   remote output: 
-  e> read(-1) -> 115:
+  e> read(115) -> 115:
   e>     abort: incompatible Mercurial client; bundle2 required\n
   e>     (see https://www.mercurial-scm.org/wiki/IncompatibleClient)\n
 
@@ -212,7 +212,7 @@
   > # This is "force" in hex.
   >     heads 666f726365
   >     PUSHFILE ../initial.v1.hg
-  > readavailable
+  > eread 196
   > EOF
   testing ssh1
   creating ssh peer from handshake results
@@ -260,7 +260,7 @@
   o> read(1) -> 1: 0
   result: 0
   remote output: 
-  e> read(-1) -> 196:
+  e> read(196) -> 196:
   e>     adding changesets\n
   e>     adding manifests\n
   e>     adding file changes\n
@@ -316,7 +316,7 @@
   o> read(1) -> 1: 0
   result: 0
   remote output: 
-  e> read(-1) -> 196:
+  e> read(196) -> 196:
   e>     adding changesets\n
   e>     adding manifests\n
   e>     adding file changes\n
@@ -338,7 +338,7 @@
   > # This is "force" in hex.
   >     heads 666f726365
   >     PUSHFILE ../initial.v1.hg
-  > readavailable
+  > eread 218
   > EOF
   testing ssh1
   creating ssh peer from handshake results
@@ -386,7 +386,7 @@
   o> read(1) -> 1: 0
   result: 0
   remote output: 
-  e> read(-1) -> 218:
+  e> read(218) -> 218:
   e>     adding changesets\n
   e>     adding manifests\n
   e>     adding file changes\n
@@ -443,7 +443,7 @@
   o> read(1) -> 1: 0
   result: 0
   remote output: 
-  e> read(-1) -> 218:
+  e> read(218) -> 218:
   e>     adding changesets\n
   e>     adding manifests\n
   e>     adding file changes\n
@@ -466,7 +466,7 @@
   > # This is "force" in hex.
   >     heads 666f726365
   >     PUSHFILE ../initial.v1.hg
-  > readavailable
+  > eread 202
   > EOF
   testing ssh1
   creating ssh peer from handshake results
@@ -514,7 +514,7 @@
   o> read(1) -> 1: 0
   result: 0
   remote output: 
-  e> read(-1) -> 202:
+  e> read(202) -> 202:
   e>     adding changesets\n
   e>     adding manifests\n
   e>     adding file changes\n
@@ -570,7 +570,7 @@
   o> read(1) -> 1: 0
   result: 0
   remote output: 
-  e> read(-1) -> 202:
+  e> read(202) -> 202:
   e>     adding changesets\n
   e>     adding manifests\n
   e>     adding file changes\n
@@ -592,7 +592,7 @@
   > # This is "force" in hex.
   >     heads 666f726365
   >     PUSHFILE ../initial.v1.hg
-  > readavailable
+  > eread 206
   > EOF
   testing ssh1
   creating ssh peer from handshake results
@@ -640,7 +640,7 @@
   o> read(1) -> 1: 0
   result: 0
   remote output: 
-  e> read(-1) -> 206:
+  e> read(206) -> 206:
   e>     adding changesets\n
   e>     adding manifests\n
   e>     adding file changes\n
@@ -697,7 +697,7 @@
   o> read(1) -> 1: 0
   result: 0
   remote output: 
-  e> read(-1) -> 206:
+  e> read(206) -> 206:
   e>     adding changesets\n
   e>     adding manifests\n
   e>     adding file changes\n
@@ -720,7 +720,7 @@
   > # This is "force" in hex.
   >     heads 666f726365
   >     PUSHFILE ../initial.v1.hg
-  > readavailable
+  > eread 232
   > EOF
   testing ssh1
   creating ssh peer from handshake results
@@ -768,7 +768,7 @@
   o> read(1) -> 1: 0
   result: 0
   remote output: 
-  e> read(-1) -> 232:
+  e> read(232) -> 232:
   e>     adding changesets\n
   e>     adding manifests\n
   e>     adding file changes\n
@@ -827,7 +827,7 @@
   o> read(1) -> 1: 0
   result: 0
   remote output: 
-  e> read(-1) -> 232:
+  e> read(232) -> 232:
   e>     adding changesets\n
   e>     adding manifests\n
   e>     adding file changes\n
@@ -852,7 +852,7 @@
   > # This is "force" in hex.
   >     heads 666f726365
   >     PUSHFILE ../initial.v1.hg
-  > readavailable
+  > eread 193
   > EOF
   testing ssh1
   creating ssh peer from handshake results
@@ -900,7 +900,7 @@
   o> read(1) -> 1: 0
   result: 0
   remote output: 
-  e> read(-1) -> 193:
+  e> read(193) -> 193:
   e>     adding changesets\n
   e>     adding manifests\n
   e>     adding file changes\n
@@ -956,7 +956,7 @@
   o> read(1) -> 1: 0
   result: 0
   remote output: 
-  e> read(-1) -> 193:
+  e> read(193) -> 193:
   e>     adding changesets\n
   e>     adding manifests\n
   e>     adding file changes\n
@@ -978,7 +978,7 @@
   > # This is "force" in hex.
   >     heads 666f726365
   >     PUSHFILE ../initial.v1.hg
-  > readavailable
+  > eread 218
   > EOF
   testing ssh1
   creating ssh peer from handshake results
@@ -1026,7 +1026,7 @@
   o> read(1) -> 1: 0
   result: 0
   remote output: 
-  e> read(-1) -> 218:
+  e> read(218) -> 218:
   e>     adding changesets\n
   e>     adding manifests\n
   e>     adding file changes\n
@@ -1085,7 +1085,7 @@
   o> read(1) -> 1: 0
   result: 0
   remote output: 
-  e> read(-1) -> 218:
+  e> read(218) -> 218:
   e>     adding changesets\n
   e>     adding manifests\n
   e>     adding file changes\n
@@ -1110,7 +1110,7 @@
   > # This is "force" in hex.
   >     heads 666f726365
   >     PUSHFILE ../initial.v1.hg
-  > readavailable
+  > eread 216
   > EOF
   testing ssh1
   creating ssh peer from handshake results
@@ -1158,7 +1158,7 @@
   o> read(1) -> 1: 0
   result: 0
   remote output: 
-  e> read(-1) -> 216:
+  e> read(216) -> 216:
   e>     adding changesets\n
   e>     adding manifests\n
   e>     adding file changes\n
@@ -1217,7 +1217,7 @@
   o> read(1) -> 1: 0
   result: 0
   remote output: 
-  e> read(-1) -> 216:
+  e> read(216) -> 216:
   e>     adding changesets\n
   e>     adding manifests\n
   e>     adding file changes\n
@@ -1248,7 +1248,7 @@
   > # This is "force" in hex.
   >     heads 666f726365
   >     PUSHFILE ../initial.v1.hg
-  > readavailable
+  > eread 212
   > EOF
   testing ssh1
   creating ssh peer from handshake results
@@ -1296,7 +1296,7 @@
   o> read(1) -> 1: 0
   result: 0
   remote output: 
-  e> read(-1) -> 212:
+  e> read(212) -> 212:
   e>     adding changesets\n
   e>     adding manifests\n
   e>     adding file changes\n
@@ -1353,7 +1353,7 @@
   o> read(1) -> 1: 0
   result: 0
   remote output: 
-  e> read(-1) -> 212:
+  e> read(212) -> 212:
   e>     adding changesets\n
   e>     adding manifests\n
   e>     adding file changes\n
@@ -1377,7 +1377,7 @@
   > # This is "force" in hex.
   >     heads 666f726365
   >     PUSHFILE ../initial.v1.hg
-  > readavailable
+  > eread 212
   > EOF
   testing ssh1
   creating ssh peer from handshake results
@@ -1425,7 +1425,7 @@
   o> read(1) -> 1: 0
   result: 0
   remote output: 
-  e> read(-1) -> 212:
+  e> read(212) -> 212:
   e>     adding changesets\n
   e>     adding manifests\n
   e>     adding file changes\n
@@ -1482,7 +1482,7 @@
   o> read(1) -> 1: 0
   result: 0
   remote output: 
-  e> read(-1) -> 212:
+  e> read(212) -> 212:
   e>     adding changesets\n
   e>     adding manifests\n
   e>     adding file changes\n
@@ -1508,7 +1508,7 @@
   > # This is "force" in hex.
   >     heads 666f726365
   >     PUSHFILE ../initial.v1.hg
-  > readavailable
+  > eread 230
   > EOF
   testing ssh1
   creating ssh peer from handshake results
@@ -1556,7 +1556,7 @@
   o> read(1) -> 1: 0
   result: 0
   remote output: 
-  e> read(-1) -> 230:
+  e> read(230) -> 230:
   e>     adding changesets\n
   e>     adding manifests\n
   e>     adding file changes\n
@@ -1615,7 +1615,7 @@
   o> read(1) -> 1: 0
   result: 0
   remote output: 
-  e> read(-1) -> 230:
+  e> read(230) -> 230:
   e>     adding changesets\n
   e>     adding manifests\n
   e>     adding file changes\n
@@ -1649,7 +1649,7 @@
   > # This is "force" in hex.
   >     heads 666f726365
   >     PUSHFILE ../initial.v1.hg
-  > readavailable
+  > eread 273
   > EOF
   testing ssh1
   creating ssh peer from handshake results
@@ -1697,7 +1697,7 @@
   o> read(1) -> 1: 0
   result: 0
   remote output: 
-  e> read(-1) -> 273:
+  e> read(273) -> 273:
   e>     adding changesets\n
   e>     adding manifests\n
   e>     adding file changes\n
@@ -1760,7 +1760,7 @@
   o> read(1) -> 1: 0
   result: 0
   remote output: 
-  e> read(-1) -> 273:
+  e> read(273) -> 273:
   e>     adding changesets\n
   e>     adding manifests\n
   e>     adding file changes\n
@@ -1789,7 +1789,7 @@
   > # This is "force" in hex.
   >     heads 666f726365
   >     PUSHFILE ../initial.v1.hg
-  > readavailable
+  > eread 100
   > EOF
   testing ssh1
   creating ssh peer from handshake results
@@ -1837,7 +1837,7 @@
   o> read(1) -> 1: 1
   result: 1
   remote output: 
-  e> read(-1) -> 100:
+  e> read(100) -> 100:
   e>     adding changesets\n
   e>     adding manifests\n
   e>     adding file changes\n
@@ -1889,7 +1889,7 @@
   o> read(1) -> 1: 1
   result: 1
   remote output: 
-  e> read(-1) -> 100:
+  e> read(100) -> 100:
   e>     adding changesets\n
   e>     adding manifests\n
   e>     adding file changes\n
@@ -1919,7 +1919,7 @@
   > # This is "force" in hex.
   >     heads 666f726365
   >     PUSHFILE ../initial.v1.hg
-  > readavailable
+  > eread 152
   > EOF
   testing ssh1
   creating ssh peer from handshake results
@@ -1967,7 +1967,7 @@
   o> read(1) -> 1: 1
   result: 1
   remote output: 
-  e> read(-1) -> 152:
+  e> read(152) -> 152:
   e>     adding changesets\n
   e>     adding manifests\n
   e>     adding file changes\n
@@ -2023,7 +2023,7 @@
   o> read(1) -> 1: 1
   result: 1
   remote output: 
-  e> read(-1) -> 152:
+  e> read(152) -> 152:
   e>     adding changesets\n
   e>     adding manifests\n
   e>     adding file changes\n
diff --git a/tests/test-ssh-proto.t b/tests/test-ssh-proto.t
--- a/tests/test-ssh-proto.t
+++ b/tests/test-ssh-proto.t
@@ -1118,7 +1118,8 @@
   >     upgrade another-token proto=irrelevant\n
   >     hello\n
   > readline
-  > readavailable
+  > ereadline
+  > ereadline
   > EOF
   using raw connection to peer
   i> write(153) -> 153:
@@ -1138,8 +1139,9 @@
   i>     hello\n
   o> readline() -> 1:
   o>     \n
-  e> read(-1) -> 42:
+  e> readline() -> 40:
   e>     cannot upgrade protocols multiple times\n
+  e> readline() -> 2:
   e>     -\n
 
 Malformed upgrade request line (not exactly 3 space delimited tokens)
@@ -1221,7 +1223,8 @@
   >     upgrade token proto=exp-ssh-v2-0001\n
   >     invalid\n
   > readline
-  > readavailable
+  > ereadline
+  > ereadline
   > EOF
   using raw connection to peer
   i> write(44) -> 44:
@@ -1229,8 +1232,9 @@
   i>     invalid\n
   o> readline() -> 1:
   o>     \n
-  e> read(-1) -> 46:
+  e> readline() -> 44:
   e>     malformed handshake protocol: missing hello\n
+  e> readline() -> 2:
   e>     -\n
 
   $ hg debugwireproto --localssh --peer raw << EOF
@@ -1239,7 +1243,8 @@
   >     hello\n
   >     invalid\n
   > readline
-  > readavailable
+  > ereadline
+  > ereadline
   > EOF
   using raw connection to peer
   i> write(50) -> 50:
@@ -1248,8 +1253,9 @@
   i>     invalid\n
   o> readline() -> 1:
   o>     \n
-  e> read(-1) -> 48:
+  e> readline() -> 46:
   e>     malformed handshake protocol: missing between\n
+  e> readline() -> 2:
   e>     -\n
 
   $ hg debugwireproto --localssh --peer raw << EOF
@@ -1259,7 +1265,8 @@
   >     between\n
   >     invalid\n
   > readline
-  > readavailable
+  > ereadline
+  > ereadline
   > EOF
   using raw connection to peer
   i> write(58) -> 58:
@@ -1269,8 +1276,9 @@
   i>     invalid\n
   o> readline() -> 1:
   o>     \n
-  e> read(-1) -> 49:
+  e> readline() -> 47:
   e>     malformed handshake protocol: missing pairs 81\n
+  e> readline() -> 2:
   e>     -\n
 
 Legacy commands are not exposed to version 2 of protocol