diff --git a/network.h b/network.h
index 4c0b0332..57e6662d 100644
--- a/network.h
+++ b/network.h
@@ -397,8 +397,10 @@ void backend_socket_log(Seat *seat, LogContext *logctx,
 typedef struct ProxyStderrBuf {
     char buf[8192];
     size_t size;
+    const char *prefix;                /* must be statically allocated */
 } ProxyStderrBuf;
 void psb_init(ProxyStderrBuf *psb);
+void psb_set_prefix(ProxyStderrBuf *psb, const char *prefix);
 void log_proxy_stderr(
     Plug *plug, ProxyStderrBuf *psb, const void *vdata, size_t len);
 
@@ -429,4 +431,6 @@ struct DeferredSocketOpenerVtable {
 static inline void deferred_socket_opener_free(DeferredSocketOpener *dso)
 { dso->vt->free(dso); }
 
+DeferredSocketOpener *null_deferred_socket_opener(void);
+
 #endif
diff --git a/putty.h b/putty.h
index 4cdc9ae8..8f506fb6 100644
--- a/putty.h
+++ b/putty.h
@@ -2893,6 +2893,16 @@ typedef void (*toplevel_callback_notify_fn_t)(void *ctx);
 void request_callback_notifications(toplevel_callback_notify_fn_t notify,
                                     void *ctx);
 
+/*
+ * Facility provided by the platform to spawn a parallel subprocess
+ * and present its stdio via a Socket.
+ *
+ * 'prefix' indicates the prefix that should appear on messages passed
+ * to plug_log to provide stderr output from the process.
+ */
+Socket *platform_start_subprocess(const char *cmd, Plug *plug,
+                                  const char *prefix);
+
 /*
  * Define no-op macros for the jump list functions, on platforms that
  * don't support them. (This is a bit of a hack, and it'd be nicer to
diff --git a/stubs/CMakeLists.txt b/stubs/CMakeLists.txt
index e6b2fc31..dc02aca3 100644
--- a/stubs/CMakeLists.txt
+++ b/stubs/CMakeLists.txt
@@ -26,5 +26,6 @@ add_sources_from_current_dir(utils
   null-cipher.c
   null-key.c
   null-mac.c
+  null-opener.c
   null-plug.c
   null-seat.c)
diff --git a/stubs/null-opener.c b/stubs/null-opener.c
new file mode 100644
index 00000000..6fdb7c28
--- /dev/null
+++ b/stubs/null-opener.c
@@ -0,0 +1,20 @@
+/*
+ * Null implementation of DeferredSocketOpener. Doesn't even bother to
+ * allocate and free itself: there's just one static implementation
+ * which we hand out to any caller.
+ */
+
+#include "putty.h"
+
+static void null_opener_free(DeferredSocketOpener *opener) {}
+
+static const DeferredSocketOpenerVtable NullOpener_vt = {
+    .free = null_opener_free,
+};
+
+static DeferredSocketOpener null_opener = { .vt = &NullOpener_vt };
+
+DeferredSocketOpener *null_deferred_socket_opener(void)
+{
+    return &null_opener;
+}
diff --git a/unix/fd-socket.c b/unix/fd-socket.c
index 036979d3..9758a17b 100644
--- a/unix/fd-socket.c
+++ b/unix/fd-socket.c
@@ -371,6 +371,13 @@ void setup_fd_socket(Socket *s, int infd, int outfd, int inerrfd)
     queue_toplevel_callback(fdsocket_connect_success_callback, fds);
 }
 
+void fd_socket_set_psb_prefix(Socket *s, const char *prefix)
+{
+    FdSocket *fds = container_of(s, FdSocket, sock);
+    assert(fds->sock.vt == &FdSocket_sockvt);
+    psb_set_prefix(&fds->psb, prefix);
+}
+
 static FdSocket *make_fd_socket_internal(SockAddr *addr, int port, Plug *plug)
 {
     FdSocket *fds;
diff --git a/unix/local-proxy.c b/unix/local-proxy.c
index 92f2a501..157f9207 100644
--- a/unix/local-proxy.c
+++ b/unix/local-proxy.c
@@ -96,3 +96,21 @@ Socket *platform_new_connection(SockAddr *addr, const char *hostname,
         return NULL;
     }
 }
+
+Socket *platform_start_subprocess(const char *cmd, Plug *plug,
+                                  const char *prefix)
+{
+    Socket *socket = make_deferred_fd_socket(
+        null_deferred_socket_opener(),
+        sk_nonamelookup("<local command>"), 0, plug);
+    char *err = platform_setup_local_proxy(socket, cmd);
+    fd_socket_set_psb_prefix(socket, prefix);
+
+    if (err) {
+        sk_close(socket);
+        socket = new_error_socket_fmt(plug, "%s", err);
+        sfree(err);
+    }
+
+    return socket;
+}
diff --git a/unix/platform.h b/unix/platform.h
index a3cbfd13..3b1db9ba 100644
--- a/unix/platform.h
+++ b/unix/platform.h
@@ -389,6 +389,7 @@ Socket *make_fd_socket(int infd, int outfd, int inerrfd,
 Socket *make_deferred_fd_socket(DeferredSocketOpener *opener,
                                 SockAddr *addr, int port, Plug *plug);
 void setup_fd_socket(Socket *s, int infd, int outfd, int inerrfd);
+void fd_socket_set_psb_prefix(Socket *s, const char *prefix);
 
 /*
  * Default font setting, which can vary depending on NOT_X_WINDOWS.
diff --git a/utils/log_proxy_stderr.c b/utils/log_proxy_stderr.c
index 90025e08..2a84173a 100644
--- a/utils/log_proxy_stderr.c
+++ b/utils/log_proxy_stderr.c
@@ -7,6 +7,12 @@
 void psb_init(ProxyStderrBuf *psb)
 {
     psb->size = 0;
+    psb->prefix = "proxy";
+}
+
+void psb_set_prefix(ProxyStderrBuf *psb, const char *prefix)
+{
+    psb->prefix = prefix;
 }
 
 void log_proxy_stderr(Plug *plug, ProxyStderrBuf *psb,
@@ -58,7 +64,7 @@ void log_proxy_stderr(Plug *plug, ProxyStderrBuf *psb,
                                     psb->buf[endpos-1] == '\r'))
                 endpos--;
             char *msg = dupprintf(
-                "proxy: %.*s", (int)(endpos - pos), psb->buf + pos);
+                "%s: %.*s", psb->prefix, (int)(endpos - pos), psb->buf + pos);
             plug_log(plug, PLUGLOG_PROXY_MSG, NULL, 0, msg, 0);
             sfree(msg);
 
@@ -73,7 +79,8 @@ void log_proxy_stderr(Plug *plug, ProxyStderrBuf *psb,
          */
         if (pos == 0 && psb->size == lenof(psb->buf)) {
             char *msg = dupprintf(
-                "proxy (partial line): %.*s", (int)psb->size, psb->buf);
+                "%s (partial line): %.*s", psb->prefix, (int)psb->size,
+                psb->buf);
             plug_log(plug, PLUGLOG_PROXY_MSG, NULL, 0, msg, 0);
             sfree(msg);
 
diff --git a/windows/handle-socket.c b/windows/handle-socket.c
index 0b0e60a9..2820975c 100644
--- a/windows/handle-socket.c
+++ b/windows/handle-socket.c
@@ -388,6 +388,13 @@ Socket *make_handle_socket(HANDLE send_H, HANDLE recv_H, HANDLE stderr_H,
     return &hs->sock;
 }
 
+void handle_socket_set_psb_prefix(Socket *s, const char *prefix)
+{
+    HandleSocket *hs = container_of(s, HandleSocket, sock);
+    assert(hs->sock.vt == &HandleSocket_sockvt);
+    psb_set_prefix(&hs->psb, prefix);
+}
+
 static void sk_handle_deferred_close(Socket *s)
 {
     HandleSocket *hs = container_of(s, HandleSocket, sock);
diff --git a/windows/local-proxy.c b/windows/local-proxy.c
index 93baad6b..55e9cbf3 100644
--- a/windows/local-proxy.c
+++ b/windows/local-proxy.c
@@ -98,3 +98,21 @@ Socket *platform_new_connection(SockAddr *addr, const char *hostname,
     local_proxy_opener_set_socket(opener, socket);
     return socket;
 }
+
+Socket *platform_start_subprocess(const char *cmd, Plug *plug,
+                                  const char *prefix)
+{
+    Socket *socket = make_deferred_handle_socket(
+        null_deferred_socket_opener(),
+        sk_nonamelookup("<local command>"), 0, plug);
+    char *err = platform_setup_local_proxy(socket, cmd);
+    handle_socket_set_psb_prefix(socket, prefix);
+
+    if (err) {
+        sk_close(socket);
+        socket = new_error_socket_fmt(plug, "%s", err);
+        sfree(err);
+    }
+
+    return socket;
+}
diff --git a/windows/platform.h b/windows/platform.h
index 60b8db77..bb190ad2 100644
--- a/windows/platform.h
+++ b/windows/platform.h
@@ -354,6 +354,7 @@ Socket *make_deferred_handle_socket(DeferredSocketOpener *opener,
                                     SockAddr *addr, int port, Plug *plug);
 void setup_handle_socket(Socket *s, HANDLE send_H, HANDLE recv_H,
                          HANDLE stderr_H, bool overlapped);
+void handle_socket_set_psb_prefix(Socket *s, const char *prefix);
 Socket *new_named_pipe_client(const char *pipename, Plug *plug); /* winnpc */
 Socket *new_named_pipe_listener(const char *pipename, Plug *plug); /* winnps */