diff --git a/src/mcp/client/auth/oauth2.py b/src/mcp/client/auth/oauth2.py
index 7dc62b52b9..454fb2e498 100644
--- a/src/mcp/client/auth/oauth2.py
+++ b/src/mcp/client/auth/oauth2.py
@@ -598,9 +598,13 @@ async def async_auth_flow(self, request: httpx2.Request) -> AsyncGenerator[httpx
             if self.context.is_token_valid():
                 self._add_auth_header(request)
 
-            response = yield request
+        # Released before the request goes out: the lock serialises token acquisition, and
+        # holding it for the lifetime of the response would stall every other request on
+        # this provider until the response ends - unbounded for the standalone GET SSE stream.
+        response = yield request
 
-            if response.status_code == 401:
+        if response.status_code == 401:
+            async with self.context.lock:
                 # Perform full OAuth flow
                 try:
                     # OAuth flow must be inline due to generator constraints
@@ -751,8 +755,10 @@ async def async_auth_flow(self, request: httpx2.Request) -> AsyncGenerator[httpx
 
                 # Retry with new tokens
                 self._add_auth_header(request)
-                yield request
-            elif response.status_code == 403:
+
+            yield request
+        elif response.status_code == 403:
+            async with self.context.lock:
                 # Step 1: Extract error field from WWW-Authenticate header
                 error = extract_field_from_www_auth(response, "error")
 
@@ -782,4 +788,5 @@ async def async_auth_flow(self, request: httpx2.Request) -> AsyncGenerator[httpx
 
                 # Retry with new tokens
                 self._add_auth_header(request)
-                yield request
+
+            yield request
diff --git a/tests/client/test_auth.py b/tests/client/test_auth.py
index be96cc8eec..cdb1f4d39d 100644
--- a/tests/client/test_auth.py
+++ b/tests/client/test_auth.py
@@ -6,6 +6,7 @@
 from unittest import mock
 from urllib.parse import parse_qs, quote, unquote, urlparse
 
+import anyio
 import httpx2
 import pytest
 from inline_snapshot import Is, snapshot
@@ -3253,3 +3254,44 @@ async def echo_callback() -> AuthorizationCodeResult:
         await auth_flow.asend(httpx2.Response(200, request=final_req))
     except StopAsyncIteration:
         pass
+
+
+@pytest.mark.anyio
+async def test_in_flight_request_does_not_block_a_concurrent_request(
+    oauth_provider: OAuthClientProvider, valid_tokens: OAuthToken
+):
+    """A request still in flight must not hold up the next one on the same provider.
+
+    The standalone GET SSE stream lives as long as the server keeps it open, so holding
+    ``context.lock`` until its response arrived stalled the first ``tools/call`` for that
+    whole time (#3209).
+    """
+    oauth_provider.context.current_tokens = valid_tokens
+    oauth_provider.context.token_expiry_time = time.time() + 1800
+    oauth_provider._initialized = True
+
+    sse_sent = anyio.Event()
+    call_done = anyio.Event()
+
+    async def get_sse_stream() -> None:
+        flow = oauth_provider.async_auth_flow(httpx2.Request("GET", "https://api.example.com/v1/mcp"))
+        request = await flow.__anext__()
+        sse_sent.set()
+        # The server holds the stream open, so the response lands after the call is answered.
+        await call_done.wait()
+        with pytest.raises(StopAsyncIteration):
+            await flow.asend(httpx2.Response(200, request=request))
+
+    async def call_tool() -> None:
+        await sse_sent.wait()
+        flow = oauth_provider.async_auth_flow(httpx2.Request("POST", "https://api.example.com/v1/mcp"))
+        with anyio.fail_after(5):
+            request = await flow.__anext__()
+        assert request.headers["Authorization"] == "Bearer test_access_token"
+        with pytest.raises(StopAsyncIteration):
+            await flow.asend(httpx2.Response(200, request=request))
+        call_done.set()
+
+    async with anyio.create_task_group() as tg:
+        tg.start_soon(get_sse_stream)
+        tg.start_soon(call_tool)