diff --git a/README.md b/README.md
index f463cfd..33e0c43 100644
--- a/README.md
+++ b/README.md
@@ -48,7 +48,9 @@ Webhook(
 ```
 
 Inject a shared `session` for throughput (one session per process); without one, each
-send opens and closes its own.
+send opens and closes its own. A single `Webhook` instance is safe to drive from many
+concurrent `send()` calls — each call tracks its own attempt count, so concurrent sends
+don't corrupt each other's `WebhookResult.attempts`.
 
 ## WebhookResult
 
diff --git a/src/aiowebhooks/sender.py b/src/aiowebhooks/sender.py
index a926bcb..ee8f81b 100644
--- a/src/aiowebhooks/sender.py
+++ b/src/aiowebhooks/sender.py
@@ -136,10 +136,10 @@ class Webhook:
         synthetic status-0). proxy rotation on connection errors lives inside the
         attempt and is capped separately.
         """
-        self._attempt_no = 0
+        counter = [0]
         try:
             return await aretry(
-                lambda: self._attempt(session, url, payload),
+                lambda: self._attempt(session, url, payload, counter),
                 attempts=self.max_retries + 1,
                 on=(_Retryable,),
             )
@@ -147,7 +147,7 @@ class Webhook:
             return exhausted.result
 
     async def _attempt(
-        self, session: aiohttp.ClientSession, url: str, payload: Dict
+        self, session: aiohttp.ClientSession, url: str, payload: Dict, counter: List[int]
     ) -> WebhookResult:
         """one logical send: proxy rotation + a single POST; may raise _Retryable
 
@@ -155,14 +155,18 @@ class Webhook:
         aretry applies backoff; honors an explicit 429 retry_after by sleeping it
         before signalling. returns a final WebhookResult on success or a terminal
         (non-retryable) failure — never lets a provider/connection error escape.
+
+        `counter` is a per-call mutable cell ([0]) owned by the calling `_send_loop`,
+        so the attempt count is local to one `send()` and concurrent sends on the
+        same instance don't corrupt each other's tally.
         """
         timeout = aiohttp.ClientTimeout(total=self.timeout)
         last_proxy: Optional[str] = None
         proxy_tries = 0
 
         while True:
-            self._attempt_no += 1
-            attempts = self._attempt_no
+            counter[0] += 1
+            attempts = counter[0]
             proxy_url = None
             if self._proxies is not None:
                 try: