diff --git a/tensorrt_llm/serve/openai_protocol.py b/tensorrt_llm/serve/openai_protocol.py
index 46cc34cea210..b65882eded07 100644
--- a/tensorrt_llm/serve/openai_protocol.py
+++ b/tensorrt_llm/serve/openai_protocol.py
@@ -1103,6 +1103,10 @@ class ChatCompletionRequest(OpenAIBaseModel):
     # emits per turn, so parallel emission remains model behavior either way.
     parallel_tool_calls: Optional[bool] = None
     user: Optional[str] = None
+    # Standard OpenAI field, accepted for compatibility and otherwise ignored:
+    # KV cache reuse needs no client hint, and cache_salt is the field that
+    # isolates cached prefixes between users.
+    prompt_cache_key: Optional[str] = None
     reasoning_effort: Optional[ReasoningEffort | Literal[
         "low", "medium", "high", "max", "none"]] = Field(
             default=ReasoningEffort.LOW,
diff --git a/tests/integration/test_lists/test-db/l0_cpu.yml b/tests/integration/test_lists/test-db/l0_cpu.yml
index b7bb5eeb30ad..85eee2eed5b5 100644
--- a/tests/integration/test_lists/test-db/l0_cpu.yml
+++ b/tests/integration/test_lists/test-db/l0_cpu.yml
@@ -208,6 +208,7 @@ l0_cpu:
   - unittest/llmapi/apps/test_openai_chat_disagg_multimodal.py
   - unittest/llmapi/apps/test_openai_chat_multimodal_prompt.py
   - unittest/llmapi/apps/test_openai_protocol_chat_store.py
+  - unittest/llmapi/apps/test_openai_protocol_prompt_cache_key.py
   - unittest/llmapi/apps/test_openai_server_iteration_stats.py
   - unittest/llmapi/apps/test_reasoning_prompt_resolution.py
   - unittest/llmapi/apps/test_render_callers.py
diff --git a/tests/unittest/api_stability/references/trtllm_serve_api.yaml b/tests/unittest/api_stability/references/trtllm_serve_api.yaml
index 3f2568cf1394..4d8b4b458312 100644
--- a/tests/unittest/api_stability/references/trtllm_serve_api.yaml
+++ b/tests/unittest/api_stability/references/trtllm_serve_api.yaml
@@ -646,6 +646,12 @@ models:
         default: null
         status: stable
         required: false
+      prompt_cache_key:
+        kind: openai
+        type: Optional[str]
+        default: null
+        status: stable
+        required: false
       reasoning_effort:
         kind: openai
         type: Optional[ReasoningEffort | Literal['low', 'medium', 'high', 'max', 'none']]
diff --git a/tests/unittest/llmapi/apps/test_openai_protocol_prompt_cache_key.py b/tests/unittest/llmapi/apps/test_openai_protocol_prompt_cache_key.py
new file mode 100644
index 000000000000..22a4c43ba618
--- /dev/null
+++ b/tests/unittest/llmapi/apps/test_openai_protocol_prompt_cache_key.py
@@ -0,0 +1,73 @@
+# SPDX-FileCopyrightText: Copyright (c) 2026 NVIDIA CORPORATION & AFFILIATES. All rights reserved.
+# SPDX-License-Identifier: Apache-2.0
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#     http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+"""ChatCompletionRequest must accept the standard `prompt_cache_key` field.
+
+The request schema is extra="forbid", so the field must exist for requests
+from OpenAI clients that set it (for example agent frameworks that pass a
+per-session key) to validate. The value is not used.
+"""
+
+import pytest
+from pydantic import ValidationError
+
+from tensorrt_llm.serve.openai_protocol import ChatCompletionRequest
+
+pytestmark = pytest.mark.cpu_only
+
+
+def _request(**extra) -> ChatCompletionRequest:
+    return ChatCompletionRequest(
+        model="test-model",
+        messages=[{"role": "user", "content": "hi"}],
+        **extra,
+    )
+
+
+@pytest.mark.parametrize("value", ["session-1234", ""])
+def test_prompt_cache_key_accepted(value):
+    req = _request(prompt_cache_key=value)
+    assert req.prompt_cache_key == value
+
+
+def test_prompt_cache_key_defaults_to_none():
+    assert _request().prompt_cache_key is None
+
+
+def test_prompt_cache_key_accepted_from_json():
+    req = ChatCompletionRequest.model_validate_json(
+        '{"model": "test-model", "messages": [{"role": "user", "content": "hi"}],'
+        ' "prompt_cache_key": "session-1234"}'
+    )
+    assert req.prompt_cache_key == "session-1234"
+
+
+@pytest.mark.parametrize("value", [1, ["session"], {"key": "session"}])
+def test_prompt_cache_key_must_be_a_string(value):
+    with pytest.raises(ValidationError):
+        _request(prompt_cache_key=value)
+
+
+def test_prompt_cache_key_leaves_other_fields_unchanged():
+    plain = _request(cache_salt="tenant")
+    keyed = _request(cache_salt="tenant", prompt_cache_key="session-1234")
+    assert keyed.cache_salt == "tenant"
+    assert keyed.model_dump(exclude={"prompt_cache_key"}) == plain.model_dump(
+        exclude={"prompt_cache_key"}
+    )
+
+
+def test_unknown_fields_remain_rejected():
+    with pytest.raises(ValidationError):
+        _request(prompt_cache_keys="typo")