diff --git a/Include/firmament2.h b/Include/firmament2.h
new file mode 100644
index 00000000000000..2d99b593b7fb64
--- /dev/null
+++ b/Include/firmament2.h
@@ -0,0 +1,71 @@
+#ifndef Py_FIRMAMENT2_H
+#define Py_FIRMAMENT2_H
+#ifdef __cplusplus
+extern "C" {
+#endif
+
+#include "Python.h"
+#include <pythread.h>
+#include <stdint.h>
+#include <stdlib.h>   /* getenv */
+#include <stdio.h>    /* printf, fflush */
+#include <string.h>   /* strlen, memcpy */
+#include <stdatomic.h>
+#if defined(_WIN32)
+  #include <windows.h>
+#else
+  #include <unistd.h> /* getpid */
+  #include <time.h>   /* clock_gettime */
+#endif
+
+/* ---------------- Global gate ---------------- */
+static inline int _firm2_enabled(void) {
+    static int cached = -1;
+    if (cached == -1) {
+        const char *env = getenv("FIRMAMENT2_ENABLE");
+        cached = (env && env[0] != '\0') ? 1 : 0;
+    }
+    return cached;
+}
+
+/* ---------------- Envelope helpers ---------------- */
+static inline unsigned long _firm2_pid(void) {
+#if defined(_WIN32)
+    return (unsigned long)GetCurrentProcessId();
+#else
+    return (unsigned long)getpid();
+#endif
+}
+
+static inline unsigned long long _firm2_tid(void) {
+    /* PyThread_get_thread_ident() returns a long. Cast to 64-bit. */
+    return (unsigned long long)PyThread_get_thread_ident();
+}
+
+static inline long long _firm2_now_ns(void) {
+#if !defined(_WIN32)
+    struct timespec ts;
+    if (clock_gettime(CLOCK_REALTIME, &ts) == 0) {
+        return ((long long)ts.tv_sec * 1000000000LL) + (long long)ts.tv_nsec;
+    }
+#endif
+    return 0;
+}
+
+static inline unsigned long long _firm2_next_eid(void) {
+    static _Atomic unsigned long long ctr = 0;
+    return ++ctr;  /* monotonic per-process event id */
+}
+
+/* ---------------- Per-source (compilation unit) TLS ----------------
+ * Implemented in ceval.c so all core files can link against them.
+ */
+void _firm2_source_begin(const char *filename_utf8);
+void _firm2_source_end(void);
+const char *_firm2_current_filename(void);
+const char *_firm2_current_source_id_hex(void);
+
+#ifdef __cplusplus
+}
+#endif
+#endif /* Py_FIRMAMENT2_H */