diff --git a/windows/agent-client.c b/windows/agent-client.c
index 4eb0bcfb..408480df 100644
--- a/windows/agent-client.c
+++ b/windows/agent-client.c
@@ -12,8 +12,6 @@
 #include "security-api.h"
 #include "cryptoapi.h"
 
-#define AGENT_COPYDATA_ID 0x804e50ba   /* random goop */
-
 static bool wm_copydata_agent_exists(void)
 {
     HWND hwnd;
diff --git a/windows/pageant.c b/windows/pageant.c
index 6fbbf883..4bf9ea3b 100644
--- a/windows/pageant.c
+++ b/windows/pageant.c
@@ -30,8 +30,6 @@
 #define WM_SYSTRAY   (WM_APP + 6)
 #define WM_SYSTRAY2  (WM_APP + 7)
 
-#define AGENT_COPYDATA_ID 0x804e50ba   /* random goop */
-
 #define APPNAME "Pageant"
 
 /* Titles and class names for invisible windows. IPCWINTITLE and
diff --git a/windows/platform.h b/windows/platform.h
index 78ebf188..454ae2f0 100644
--- a/windows/platform.h
+++ b/windows/platform.h
@@ -35,6 +35,10 @@
 #define BUILDINFO_PLATFORM "Windows"
 #endif
 
+/* Randomly-chosen dwData value identifying a WM_COPYDATA message as
+ * being a Pageant transaction */
+#define AGENT_COPYDATA_ID 0x804e50ba
+
 struct Filename {
     char *path;
 };