diff --git a/tests/e2e/c/.gitignore b/tests/e2e/c/.gitignore
new file mode 100644
index 0000000..7c54a42
--- /dev/null
+++ b/tests/e2e/c/.gitignore
@@ -0,0 +1,4 @@
+/stress
+/libmy_timer.dylib
+/libmy_timer.so
+/build/
diff --git a/tests/e2e/c/Makefile b/tests/e2e/c/Makefile
new file mode 100644
index 0000000..35de7b1
--- /dev/null
+++ b/tests/e2e/c/Makefile
@@ -0,0 +1,48 @@
+# Build + run the native/CBOR concurrency stress test for the timer example.
+#
+#   make stress                 # plain build + run
+#   make stress SAN=address     # AddressSanitizer
+#   make stress SAN=thread      # ThreadSanitizer
+#   make clean
+#
+# Drives the real exported ABI of the timer example library, so it links the
+# same dylib the C/Go consumers do. The library is built from the repo root so
+# its vendored Nimble dependencies resolve.
+
+REPO_ROOT := $(abspath ../../..)
+NIM_SRC   := $(REPO_ROOT)/examples/timer/timer.nim
+HDR_DIR   := $(REPO_ROOT)/examples/timer/c_bindings
+
+UNAME_S := $(shell uname -s)
+ifeq ($(UNAME_S),Darwin)
+  LIBNAME := libmy_timer.dylib
+  RPATH   := -Wl,-rpath,.
+else
+  LIBNAME := libmy_timer.so
+  RPATH   := -Wl,-rpath,'$$ORIGIN'
+endif
+
+CC      ?= cc
+CFLAGS  ?= -std=c11 -Wall -Wextra -O1 -g -I$(HDR_DIR)
+NIMFLAGS := --mm:orc -d:chronicles_log_level=WARN --app:lib --noMain \
+            --nimMainPrefix:libmy_timer
+
+# Optional sanitizer: SAN=address | thread | undefined
+ifneq ($(SAN),)
+  SANFLAGS  := -fsanitize=$(SAN) -fno-omit-frame-pointer
+  CFLAGS    += $(SANFLAGS)
+  LDSAN     := $(SANFLAGS)
+  NIMFLAGS  += -d:useMalloc --passC:-fsanitize=$(SAN) --passL:-fsanitize=$(SAN)
+endif
+
+.PHONY: stress clean
+
+$(LIBNAME):
+	cd $(REPO_ROOT) && nim c $(NIMFLAGS) -o:$(CURDIR)/$(LIBNAME) $(NIM_SRC)
+
+stress: stress.c $(HDR_DIR)/my_timer.h $(LIBNAME)
+	$(CC) $(CFLAGS) stress.c -L. -lmy_timer -lpthread $(RPATH) $(LDSAN) -o stress
+	./stress
+
+clean:
+	rm -f stress $(LIBNAME)
diff --git a/tests/e2e/c/stress.c b/tests/e2e/c/stress.c
new file mode 100644
index 0000000..449f5b8
--- /dev/null
+++ b/tests/e2e/c/stress.c
@@ -0,0 +1,202 @@
+// Concurrency + memory stress for BOTH ABIs of one library:
+//   - native (pure C): EchoRequest struct in, typed EchoResponse* back
+//   - CBOR: encoded request in, CBOR EchoResponse map back
+//
+// Many threads hammer a shared context with both call shapes, each verifying
+// the echoed message round-trips. Built to run under ASAN/TSAN to flush out
+// leaks, use-after-free in the POD deep-copy/free paths, and data races.
+//
+//   make stress && ./stress           # plain
+//   make stress SAN=address           # -fsanitize=address
+//   make stress SAN=thread            # -fsanitize=thread
+#include "my_timer.h"
+#include "my_timer_cbor.h"
+#include <pthread.h>
+#include <stdatomic.h>
+#include <stdio.h>
+#include <stdlib.h>
+#include <string.h>
+
+#ifndef STRESS_THREADS
+#define STRESS_THREADS 6
+#endif
+#ifndef STRESS_ITERS
+#define STRESS_ITERS 1500
+#endif
+
+static atomic_long g_ok;
+static atomic_long g_bad;
+
+// --- per-call blocking capture ----------------------------------------------
+typedef struct {
+  int ret, done;
+  char echoed[128]; // copied out of the typed return / decoded from CBOR
+  pthread_mutex_t mu;
+  pthread_cond_t cv;
+} Cap;
+static void cap_init(Cap *c) {
+  memset(c, 0, sizeof(*c));
+  pthread_mutex_init(&c->mu, NULL);
+  pthread_cond_init(&c->cv, NULL);
+}
+static void cap_destroy(Cap *c) {
+  pthread_mutex_destroy(&c->mu);
+  pthread_cond_destroy(&c->cv);
+}
+static void cap_reset(Cap *c) {
+  pthread_mutex_lock(&c->mu);
+  c->done = 0;
+  c->echoed[0] = 0;
+  pthread_mutex_unlock(&c->mu);
+}
+static void cap_wait(Cap *c) {
+  pthread_mutex_lock(&c->mu);
+  while (!c->done) pthread_cond_wait(&c->cv, &c->mu);
+  pthread_mutex_unlock(&c->mu);
+}
+
+// Native EchoResponse return: read the typed struct in-callback.
+static void native_cb(int ret, const char *msg, size_t len, void *ud) {
+  Cap *c = ud;
+  pthread_mutex_lock(&c->mu);
+  c->ret = ret;
+  if (ret == RET_OK) {
+    const EchoResponse *r = (const EchoResponse *)msg;
+    strncpy(c->echoed, r->echoed, sizeof(c->echoed) - 1);
+  }
+  c->done = 1;
+  pthread_cond_signal(&c->cv);
+  pthread_mutex_unlock(&c->mu);
+  (void)len;
+}
+
+// --- minimal CBOR map text reader (for the CBOR EchoResponse) ---------------
+static size_t cbor_item_len(const uint8_t *p, size_t len) {
+  if (len < 1) return 0;
+  uint8_t major = p[0] >> 5, info = p[0] & 0x1f;
+  size_t head = 1;
+  uint64_t arg = info;
+  if (info == 24) { if (len < 2) return 0; arg = p[1]; head = 2; }
+  else if (info == 25) { if (len < 3) return 0; arg = ((uint64_t)p[1] << 8) | p[2]; head = 3; }
+  else if (info == 26) { if (len < 5) return 0; arg = ((uint64_t)p[1] << 24)|((uint64_t)p[2] << 16)|((uint64_t)p[3] << 8)|p[4]; head = 5; }
+  else if (info == 27) { if (len < 9) return 0; arg = 0; for (int i = 1; i <= 8; i++) arg = (arg << 8) | p[i]; head = 9; }
+  else if (info >= 28) return 0;
+  switch (major) {
+  case 0: case 1: case 7: return head;
+  case 2: case 3: return head + (size_t)arg;
+  case 4: { size_t off = head; for (uint64_t i = 0; i < arg; i++) { size_t n = cbor_item_len(p+off, len-off); if (!n) return 0; off += n; } return off; }
+  case 5: { size_t off = head; for (uint64_t i = 0; i < arg*2; i++) { size_t n = cbor_item_len(p+off, len-off); if (!n) return 0; off += n; } return off; }
+  default: return 0;
+  }
+}
+static int cbor_get_text(const uint8_t *p, size_t len, const char *key, char *out, size_t cap) {
+  if (len < 1 || (p[0] >> 5) != 5) return 0;
+  uint8_t info = p[0] & 0x1f;
+  if (info >= 24) return 0;
+  size_t off = 1, klen = strlen(key);
+  for (uint64_t i = 0; i < info; i++) {
+    const uint8_t *k = p + off;
+    if ((k[0] >> 5) != 3) return 0;
+    uint8_t ki = k[0] & 0x1f;
+    if (ki >= 24) return 0;
+    const uint8_t *v = p + off + 1 + ki;
+    if ((size_t)ki == klen && memcmp(k + 1, key, klen) == 0 && (v[0] >> 5) == 3) {
+      uint8_t vi = v[0] & 0x1f;
+      if (vi >= 24) return 0;
+      size_t n = vi < cap - 1 ? vi : cap - 1;
+      memcpy(out, v + 1, n);
+      out[n] = 0;
+      return 1;
+    }
+    size_t vn = cbor_item_len(v, len - (off + 1 + ki));
+    if (!vn) return 0;
+    off += 1 + ki + vn;
+  }
+  return 0;
+}
+
+// CBOR EchoResponse return: decode the "echoed" field in-callback.
+static void cbor_cb(int ret, const char *msg, size_t len, void *ud) {
+  Cap *c = ud;
+  pthread_mutex_lock(&c->mu);
+  c->ret = ret;
+  if (ret == RET_OK)
+    cbor_get_text((const uint8_t *)msg, len, "echoed", c->echoed, sizeof(c->echoed));
+  c->done = 1;
+  pthread_cond_signal(&c->cv);
+  pthread_mutex_unlock(&c->mu);
+}
+
+struct Args { void *ctx; int id; };
+
+static void *worker(void *p) {
+  struct Args *a = p;
+  Cap cap;
+  cap_init(&cap);
+  char want[128];
+  for (int i = 0; i < STRESS_ITERS; i++) {
+    snprintf(want, sizeof(want), "t%d-i%d", a->id, i);
+    if (i & 1) {
+      // native: EchoRequest in, typed EchoResponse* back
+      cap_reset(&cap);
+      EchoRequest req = {.message = want, .delayMs = 0};
+      if (my_timer_echo(a->ctx, native_cb, &cap, req) == RET_OK) cap_wait(&cap);
+    } else {
+      // CBOR: { "req": { "message": want, "delayMs": 0 } }
+      cap_reset(&cap);
+      FfiCbor e = ffi_cbor_new();
+      ffi_cbor_map(&e, 1);
+      ffi_cbor_text(&e, "req");
+      ffi_cbor_map(&e, 2);
+      ffi_cbor_kv_text(&e, "message", want);
+      ffi_cbor_kv_int(&e, "delayMs", 0);
+      if (my_timer_echo_cbor(a->ctx, cbor_cb, &cap, e.buf, e.len) == RET_OK)
+        cap_wait(&cap);
+      ffi_cbor_free(&e);
+    }
+    if (cap.ret == RET_OK && strcmp(cap.echoed, want) == 0)
+      atomic_fetch_add(&g_ok, 1);
+    else
+      atomic_fetch_add(&g_bad, 1);
+  }
+  cap_destroy(&cap);
+  return NULL;
+}
+
+static void create_cb(int ret, const char *msg, size_t len, void *ud) {
+  Cap *c = ud;
+  pthread_mutex_lock(&c->mu);
+  c->ret = ret;
+  c->done = 1;
+  pthread_cond_signal(&c->cv);
+  pthread_mutex_unlock(&c->mu);
+  (void)msg; (void)len;
+}
+
+int main(void) {
+  Cap cc;
+  cap_init(&cc);
+  TimerConfig cfg = {.name = "stress"};
+  void *ctx = my_timer_create(cfg, create_cb, &cc);
+  cap_wait(&cc);
+  if (!ctx) { fprintf(stderr, "create failed\n"); return 1; }
+
+  pthread_t th[STRESS_THREADS];
+  struct Args args[STRESS_THREADS];
+  for (int i = 0; i < STRESS_THREADS; i++) {
+    args[i].ctx = ctx;
+    args[i].id = i;
+    pthread_create(&th[i], NULL, worker, &args[i]);
+  }
+  for (int i = 0; i < STRESS_THREADS; i++) pthread_join(th[i], NULL);
+
+  my_timer_destroy(ctx);
+  cap_destroy(&cc);
+
+  long ok = atomic_load(&g_ok), bad = atomic_load(&g_bad);
+  long total = (long)STRESS_THREADS * STRESS_ITERS;
+  printf("native+cbor stress: %ld/%ld ok, %ld bad (%d threads x %d iters)\n", ok,
+         total, bad, STRESS_THREADS, STRESS_ITERS);
+  printf(bad == 0 && ok == total ? "PASSED\n" : "FAILED\n");
+  return (bad == 0 && ok == total) ? 0 : 1;
+}