diff --git a/gateway/platforms/api_server.py b/gateway/platforms/api_server.py
index 7efb756c9c..8bbf16e17e 100644
--- a/gateway/platforms/api_server.py
+++ b/gateway/platforms/api_server.py
@@ -469,6 +469,7 @@ class _IdempotencyCache:
     def __init__(self, max_items: int = 1000, ttl_seconds: int = 300):
         from collections import OrderedDict
         self._store = OrderedDict()
+        self._inflight: Dict[tuple[str, str], "asyncio.Task[Any]"] = {}
         self._ttl = ttl_seconds
         self._max = max_items
 
@@ -486,11 +487,27 @@ class _IdempotencyCache:
         item = self._store.get(key)
         if item and item["fp"] == fingerprint:
             return item["resp"]
-        resp = await compute_coro()
-        import time as _t
-        self._store[key] = {"resp": resp, "fp": fingerprint, "ts": _t.time()}
-        self._purge()
-        return resp
+
+        inflight_key = (key, fingerprint)
+        task = self._inflight.get(inflight_key)
+        if task is None:
+            async def _compute_and_store():
+                resp = await compute_coro()
+                import time as _t
+                self._store[key] = {"resp": resp, "fp": fingerprint, "ts": _t.time()}
+                self._purge()
+                return resp
+
+            task = asyncio.create_task(_compute_and_store())
+            self._inflight[inflight_key] = task
+
+            def _clear_inflight(done_task: "asyncio.Task[Any]") -> None:
+                if self._inflight.get(inflight_key) is done_task:
+                    self._inflight.pop(inflight_key, None)
+
+            task.add_done_callback(_clear_inflight)
+
+        return await asyncio.shield(task)
 
 
 _idem_cache = _IdempotencyCache()
diff --git a/tests/gateway/test_api_server.py b/tests/gateway/test_api_server.py
index d0cebacb88..ca229f26f7 100644
--- a/tests/gateway/test_api_server.py
+++ b/tests/gateway/test_api_server.py
@@ -12,6 +12,7 @@ Tests cover:
 - Error handling (invalid JSON, missing fields)
 """
 
+import asyncio
 import json
 import time
 import uuid
@@ -25,6 +26,7 @@ from gateway.config import GatewayConfig, Platform, PlatformConfig
 from gateway.platforms.api_server import (
     APIServerAdapter,
     ResponseStore,
+    _IdempotencyCache,
     _CORS_HEADERS,
     _derive_chat_session_id,
     check_api_server_requirements,
@@ -104,6 +106,95 @@ class TestResponseStore:
         assert store.delete("resp_missing") is False
 
 
+# ---------------------------------------------------------------------------
+# _IdempotencyCache
+# ---------------------------------------------------------------------------
+
+
+class TestIdempotencyCache:
+    @pytest.mark.asyncio
+    async def test_concurrent_same_key_and_fingerprint_runs_once(self):
+        cache = _IdempotencyCache()
+        gate = asyncio.Event()
+        started = asyncio.Event()
+        calls = 0
+
+        async def compute():
+            nonlocal calls
+            calls += 1
+            started.set()
+            await gate.wait()
+            return ("response", {"total_tokens": 1})
+
+        first = asyncio.create_task(cache.get_or_set("idem-key", "fp-1", compute))
+        second = asyncio.create_task(cache.get_or_set("idem-key", "fp-1", compute))
+
+        await started.wait()
+        assert calls == 1
+
+        gate.set()
+        first_result, second_result = await asyncio.gather(first, second)
+
+        assert first_result == second_result == ("response", {"total_tokens": 1})
+
+    @pytest.mark.asyncio
+    async def test_different_fingerprint_does_not_reuse_inflight_task(self):
+        cache = _IdempotencyCache()
+        gate = asyncio.Event()
+        started = asyncio.Event()
+        calls = 0
+
+        async def compute():
+            nonlocal calls
+            calls += 1
+            result = calls
+            if calls == 2:
+                started.set()
+            await gate.wait()
+            return result
+
+        first = asyncio.create_task(cache.get_or_set("idem-key", "fp-1", compute))
+        second = asyncio.create_task(cache.get_or_set("idem-key", "fp-2", compute))
+
+        await started.wait()
+        assert calls == 2
+
+        gate.set()
+        results = await asyncio.gather(first, second)
+
+        assert sorted(results) == [1, 2]
+
+    @pytest.mark.asyncio
+    async def test_cancelled_waiter_does_not_drop_shared_inflight_task(self):
+        cache = _IdempotencyCache()
+        gate = asyncio.Event()
+        started = asyncio.Event()
+        calls = 0
+
+        async def compute():
+            nonlocal calls
+            calls += 1
+            started.set()
+            await gate.wait()
+            return "response"
+
+        first = asyncio.create_task(cache.get_or_set("idem-key", "fp-1", compute))
+
+        await started.wait()
+        assert calls == 1
+
+        first.cancel()
+        with pytest.raises(asyncio.CancelledError):
+            await first
+
+        second = asyncio.create_task(cache.get_or_set("idem-key", "fp-1", compute))
+        await asyncio.sleep(0)
+        assert calls == 1
+
+        gate.set()
+        assert await second == "response"
+
+
 # ---------------------------------------------------------------------------
 # Adapter initialization
 # ---------------------------------------------------------------------------