diff --git a/mypyc/lib-rt/librt_internal.c b/mypyc/lib-rt/librt_internal.c
index ada2dfeb39a5..22d54de40f08 100644
--- a/mypyc/lib-rt/librt_internal.c
+++ b/mypyc/lib-rt/librt_internal.c
@@ -962,6 +962,11 @@ NativeInternal_ABI_Version(void) {
     return LIBRT_INTERNAL_ABI_VERSION;
 }
 
+static int
+NativeInternal_API_Version(void) {
+    return LIBRT_INTERNAL_API_VERSION;
+}
+
 static int
 librt_internal_module_exec(PyObject *m)
 {
@@ -999,6 +1004,7 @@ librt_internal_module_exec(PyObject *m)
         (void *)cache_version_internal,
         (void *)ReadBuffer_type_internal,
         (void *)WriteBuffer_type_internal,
+        (void *)NativeInternal_API_Version,
     };
     PyObject *c_api_object = PyCapsule_New((void *)NativeInternal_API, "librt.internal._C_API", NULL);
     if (PyModule_Add(m, "_C_API", c_api_object) < 0) {
diff --git a/mypyc/lib-rt/librt_internal.h b/mypyc/lib-rt/librt_internal.h
index bef9e196d2c1..501162a27980 100644
--- a/mypyc/lib-rt/librt_internal.h
+++ b/mypyc/lib-rt/librt_internal.h
@@ -1,8 +1,19 @@
 #ifndef LIBRT_INTERNAL_H
 #define LIBRT_INTERNAL_H
 
-#define LIBRT_INTERNAL_ABI_VERSION 1
-#define LIBRT_INTERNAL_API_LEN 19
+// ABI version -- only an exact match is compatible. This will only be changed in
+// very exceptional cases (likely never) due to strict backward compatibility
+// requirements.
+#define LIBRT_INTERNAL_ABI_VERSION 2
+
+// API version -- more recent versions must maintain backward compatibility, i.e.
+// we can add new features but not remove or change existing features (unless
+// ABI version is changed, but see the comment above).
+ #define LIBRT_INTERNAL_API_VERSION 0
+
+// Number of functions in the capsule API. If you add a new function, also increase
+// LIBRT_INTERNAL_API_VERSION.
+#define LIBRT_INTERNAL_API_LEN 20
 
 #ifdef LIBRT_INTERNAL_MODULE
 
@@ -27,6 +38,7 @@ static PyObject *read_bytes_internal(PyObject *data);
 static uint8_t cache_version_internal(void);
 static PyTypeObject *ReadBuffer_type_internal(void);
 static PyTypeObject *WriteBuffer_type_internal(void);
+static int NativeInternal_API_Version(void);
 
 #else
 
@@ -51,6 +63,7 @@ static void *NativeInternal_API[LIBRT_INTERNAL_API_LEN];
 #define cache_version_internal (*(uint8_t (*)(void)) NativeInternal_API[16])
 #define ReadBuffer_type_internal (*(PyTypeObject* (*)(void)) NativeInternal_API[17])
 #define WriteBuffer_type_internal (*(PyTypeObject* (*)(void)) NativeInternal_API[18])
+#define NativeInternal_API_Version (*(int (*)(void)) NativeInternal_API[19])
 
 static int
 import_librt_internal(void)
@@ -72,6 +85,16 @@ import_librt_internal(void)
         PyErr_SetString(PyExc_ValueError, err);
         return -1;
     }
+    if (NativeInternal_API_Version() < LIBRT_INTERNAL_API_VERSION) {
+        char err[128];
+        snprintf(err, sizeof(err),
+                 "API version conflict for librt.internal, expected %d or newer, found %d (hint: upgrade librt)",
+            LIBRT_INTERNAL_API_VERSION,
+            NativeInternal_API_Version()
+        );
+        PyErr_SetString(PyExc_ValueError, err);
+        return -1;
+    }
     return 0;
 }