diff --git a/gateway/platforms/api_server.py b/gateway/platforms/api_server.py
index f050682760..7483ad5827 100644
--- a/gateway/platforms/api_server.py
+++ b/gateway/platforms/api_server.py
@@ -1199,6 +1199,11 @@ class APIServerAdapter(OpenAICompatRoutesMixin, BasePlatformAdapter):
         # @mssteuer.)
         self._direct_model_requests: bool = _coerce_request_bool(
             extra.get("direct_model_requests"), default=False)
+        # ``platforms.api_server.tool_progress_events: false`` drops the custom
+        # ``hermes.tool.progress`` SSE frames from Chat Completions streams for strict OpenAI
+        # clients that choke on named events (#12020). Default on.
+        self._tool_progress_events: bool = _coerce_request_bool(
+            extra.get("tool_progress_events"), default=True)
         self._app: Optional["web.Application"] = None
         self._runner: Optional["web.AppRunner"] = None
         self._site: Optional["web.TCPSite"] = None
diff --git a/gateway/platforms/api_server_openai_routes.py b/gateway/platforms/api_server_openai_routes.py
index c0293417b1..178ebec2eb 100644
--- a/gateway/platforms/api_server_openai_routes.py
+++ b/gateway/platforms/api_server_openai_routes.py
@@ -882,6 +882,8 @@ class OpenAICompatRoutesMixin:
                     break
                 if isinstance(delta, tuple) and len(delta) == 2 and delta[0] == "__tool_progress__":
                     # Custom event: tool lifecycle for frontends without markers in history.
+                    if not self._tool_progress_events:
+                        continue  # opted out for strict OpenAI clients (#12020)
                     await response.write(_sse_frame(delta[1], event="hermes.tool.progress"))
                 elif isinstance(delta, tuple) and len(delta) == 2 and delta[0] == "__reasoning__":
                     # DeepSeek-style ``delta.reasoning_content`` (#99552), the field Open WebUI,
diff --git a/tests/gateway/test_chat_completions_tool_progress_optout.py b/tests/gateway/test_chat_completions_tool_progress_optout.py
new file mode 100644
index 0000000000..6f2dea3aa4
--- /dev/null
+++ b/tests/gateway/test_chat_completions_tool_progress_optout.py
@@ -0,0 +1,48 @@
+"""``platforms.api_server.tool_progress_events: false`` drops the custom
+``hermes.tool.progress`` SSE frames from streaming Chat Completions (#12020)."""
+
+import asyncio
+from unittest.mock import AsyncMock, MagicMock, patch
+
+
+def _stream_body(platform_cfg):
+    from aiohttp import web
+    from gateway.config import PlatformConfig
+    from gateway.platforms.api_server import APIServerAdapter, ThreadSafeAsyncQueue
+
+    # from_dict is the production loader: a top-level platform key lands in ``extra``.
+    adapter = APIServerAdapter(PlatformConfig.from_dict(platform_cfg))
+    written = []
+
+    async def fake_agent():
+        return {"final_response": "done", "completed": True}, {
+            "input_tokens": 1, "output_tokens": 1, "total_tokens": 2}
+
+    async def run():
+        stream_q = ThreadSafeAsyncQueue()
+        stream_q.put_nowait(("__tool_progress__", {"tool": "terminal", "toolCallId": "c1", "status": "running"}))
+        stream_q.put_nowait("done")
+        stream_q.put_nowait(None)
+        agent_task = asyncio.ensure_future(fake_agent())
+        resp = AsyncMock(spec=web.StreamResponse)
+        resp.write = AsyncMock(side_effect=lambda data: written.append(data))
+        resp.prepare = AsyncMock()
+        req = MagicMock()
+        req.headers = {}
+        with patch("gateway.platforms.api_server.web.StreamResponse", return_value=resp):
+            await adapter._write_sse_chat_completion(req, "cmpl-1", "m", 1, stream_q, agent_task)
+
+    asyncio.run(run())
+    return b"".join(written).decode()
+
+
+def test_tool_progress_frames_emitted_by_default():
+    body = _stream_body({"enabled": True, "token": "k"})
+    assert "event: hermes.tool.progress" in body
+    assert '"content": "done"' in body
+
+
+def test_tool_progress_events_false_suppresses_frames_but_keeps_content():
+    body = _stream_body({"enabled": True, "token": "k", "tool_progress_events": False})
+    assert "hermes.tool.progress" not in body
+    assert '"content": "done"' in body
diff --git a/website/docs/user-guide/features/api-server.md b/website/docs/user-guide/features/api-server.md
index 5db0303f4f..142caf45b0 100644
--- a/website/docs/user-guide/features/api-server.md
+++ b/website/docs/user-guide/features/api-server.md
@@ -111,7 +111,7 @@ Uploaded files (`file` / `input_file` / `file_id`) and non-image `data:` URLs re
 All SSE streams (Chat Completions, Responses, `/api/sessions/{id}/chat/stream`, `/v1/runs/{id}/events`) emit a `: keepalive` comment line whenever no event has been sent for 10 seconds, so long tool calls do not trip client idle timeouts. Standard SSE clients ignore comment lines; custom parsers must skip lines that start with `:`.
 
 **Tool progress in streams**:
-- **Chat Completions**: Hermes emits `event: hermes.tool.progress` for tool-start visibility without polluting persisted assistant text.
+- **Chat Completions**: Hermes emits `event: hermes.tool.progress` for tool-start visibility without polluting persisted assistant text. Strict OpenAI clients that choke on named SSE events can turn these frames off with `gateway.platforms.api_server.tool_progress_events: false` (default `true`); content chunks are unaffected. The opt-out applies only to Chat Completions — `/v1/runs/{id}/events` always emits tool events, which is what the `tool_progress_events` feature in `/v1/capabilities` describes.
 - **Responses**: Hermes emits spec-native `function_call` and `function_call_output` output items during the SSE stream, so clients can render structured tool UI in real time.
 
 **Model reasoning** (emitted only when the model actually produces reasoning and the resolved `reasoning` config allows it; the input-side opt-out is `model_options.reasoning.enabled: false`):
diff --git a/website/i18n/zh-Hans/docusaurus-plugin-content-docs/current/user-guide/features/api-server.md b/website/i18n/zh-Hans/docusaurus-plugin-content-docs/current/user-guide/features/api-server.md
index b9dde680b0..1bfa3909a7 100644
--- a/website/i18n/zh-Hans/docusaurus-plugin-content-docs/current/user-guide/features/api-server.md
+++ b/website/i18n/zh-Hans/docusaurus-plugin-content-docs/current/user-guide/features/api-server.md
@@ -109,7 +109,7 @@ curl http://localhost:8642/v1/chat/completions \
 **流式传输**（`"stream": true`）：返回逐 token 响应块的 Server-Sent Events（SSE）。对于 **Chat Completions**，流使用标准 `chat.completion.chunk` 事件，以及 Hermes 自定义的 `hermes.tool.progress` 事件用于工具启动的 UX 展示。对于 **Responses**，流使用 OpenAI Responses 事件类型，如 `response.created`、`response.output_text.delta`、`response.output_item.added`、`response.output_item.done` 和 `response.completed`。
 
 **流中的工具进度：**
-- **Chat Completions**：Hermes 发出 `event: hermes.tool.progress` 以提供工具启动可见性，同时不污染持久化的 assistant 文本。
+- **Chat Completions**：Hermes 发出 `event: hermes.tool.progress` 以提供工具启动可见性，同时不污染持久化的 assistant 文本。无法处理具名 SSE 事件的严格 OpenAI 客户端可设置 `gateway.platforms.api_server.tool_progress_events: false`（默认 `true`）关闭这些帧；内容块不受影响。该开关仅作用于 Chat Completions——`/v1/runs/{id}/events` 始终发出工具事件，`/v1/capabilities` 中的 `tool_progress_events` 功能描述的正是这一点。
 - **Responses**：Hermes 在 SSE 流期间发出符合规范的 `function_call` 和 `function_call_output` 输出项，让客户端能够实时渲染结构化工具 UI。
 **模型推理**（仅当模型确实产生了推理内容且解析后的 `reasoning` 配置允许时才会发出；输入侧的关闭方式是 `model_options.reasoning.enabled: false`）：
 - **Chat Completions**：推理增量以 `choices[0].delta.reasoning_content` 块的形式到达（DeepSeek 风格的字段，Open WebUI、opencode 和 Vercel AI SDK 会将其渲染为思考块）；回答文本仍留在 `delta.content` 中。