diff --git a/lark_oapi/channel/normalize/registry.py b/lark_oapi/channel/normalize/registry.py
index cfdf6e144..a64b691cc 100644
--- a/lark_oapi/channel/normalize/registry.py
+++ b/lark_oapi/channel/normalize/registry.py
@@ -59,14 +59,19 @@ def _safe_json(raw: Any) -> Dict[str, Any]:
 def _flatten_post_text(post: Dict[str, Any]) -> Tuple[str, str]:
     """Return (title, plain_text) from a post AST.
 
-    Post content has locale keys (`zh_cn`, `en_us`). We pick the first locale.
+    Post content may be locale-keyed (``{"zh_cn": {...}, "en_us": {...}}``)
+    or carry ``content`` directly (the classic single-locale post shape).
+    For locale-keyed payloads the first locale is used.
     """
     if not isinstance(post, dict):
         return "", ""
-    first_key = next(iter(post), None)
-    if first_key is None:
-        return "", ""
-    locale_doc = post.get(first_key)
+    if "content" in post:
+        locale_doc = post
+    else:
+        first_key = next(iter(post), None)
+        if first_key is None:
+            return "", ""
+        locale_doc = post.get(first_key)
     if not isinstance(locale_doc, dict):
         return "", ""
     title = locale_doc.get("title") or ""
@@ -82,8 +87,19 @@ def _flatten_post_text(post: Dict[str, Any]) -> Tuple[str, str]:
             elif tag == "a":
                 chunk.append(el.get("text") or el.get("href") or "")
             elif tag == "at":
-                nm = el.get("user_name") or el.get("user_id") or ""
-                chunk.append(f"@{nm}" if nm else "@")
+                el_id = el.get("id")
+                is_all = el.get("user_id") == "all" or (
+                    isinstance(el_id, dict) and el_id.get("user_id") == "all"
+                )
+                if is_all:
+                    # Mention-all node: render the ``@_all`` placeholder so the
+                    # pipeline's mention-all probes (``text_has_mention_all`` /
+                    # ``resolve_mentions``) fire for rich-text @all messages.
+                    # See https://github.com/larksuite/oapi-sdk-python/issues/138
+                    chunk.append("@_all")
+                else:
+                    nm = el.get("user_name") or el.get("user_id") or ""
+                    chunk.append(f"@{nm}" if nm else "@")
             elif tag == "emotion":
                 chunk.append(f":{el.get('emoji_type') or ''}:")
             elif tag == "img":
diff --git a/lark_oapi/channel/tests/test_post_mention_all.py b/lark_oapi/channel/tests/test_post_mention_all.py
new file mode 100644
index 000000000..d4e62f4bb
--- /dev/null
+++ b/lark_oapi/channel/tests/test_post_mention_all.py
@@ -0,0 +1,83 @@
+"""mentioned_all for rich-text post @all messages (issue #138).
+
+A post that @-mentions everyone carries an ``at`` AST node with
+``user_id == "all"`` and Feishu does NOT populate ``mentions[]`` for @all —
+so the only signal is the rendered text. The flatten renderer must emit the
+``@_all`` placeholder so the pipeline's probes (``text_has_mention_all`` /
+``resolve_mentions``) fire.
+"""
+
+import json
+
+import pytest
+
+from lark_oapi.channel.normalize.pipeline import InboundPipeline, PipelineConfig, PipelineDeps
+
+
+def _msg(msg_type="post", content=None):
+    return {
+        "message_id": "om_1",
+        "create_time": 1000,
+        "chat_id": "oc_1",
+        "chat_type": "group",
+        "message_type": msg_type,
+        "content": json.dumps(content or {"text": "hi"}, ensure_ascii=False),
+        "mentions": [],
+    }
+
+
+def _sender(open_id="ou_sender"):
+    return {"sender_id": {"open_id": open_id, "user_id": "u1"}, "sender_type": "user"}
+
+
+@pytest.mark.asyncio
+async def test_post_mention_all_sets_mentioned_all():
+    # Covers both wire shapes: the classic single-locale ``{"content": ...}``
+    # post and locale-keyed posts (``{"zh_cn": {...}}``).
+    for post_content in (
+        {
+            "content": [
+                [
+                    {"tag": "at", "user_id": "all", "user_name": "Everyone"},
+                    {"tag": "text", "text": " heads up everyone"},
+                ]
+            ]
+        },
+        {
+            "zh_cn": {
+                "content": [
+                    [
+                        {"tag": "at", "user_id": "all", "user_name": "所有人"},
+                        {"tag": "text", "text": " 全体注意"},
+                    ]
+                ]
+            }
+        },
+    ):
+        p = InboundPipeline(PipelineConfig(), PipelineDeps())
+        inbound = await p.process(
+            event_id="e", message_event=_msg(content=post_content), sender=_sender()
+        )
+        assert inbound is not None
+        assert inbound.mentioned_all is True
+        # the @_all placeholder resolves to the human-visible form
+        assert "@all" in inbound.content.text
+
+
+@pytest.mark.asyncio
+async def test_post_regular_at_does_not_set_mentioned_all():
+    post_content = {
+        "content": [
+            [
+                {"tag": "at", "user_id": "ou_user", "user_name": "Alice"},
+                {"tag": "text", "text": " hi"},
+            ]
+        ]
+    }
+    p = InboundPipeline(PipelineConfig(), PipelineDeps())
+    inbound = await p.process(
+        event_id="e", message_event=_msg(content=post_content), sender=_sender()
+    )
+    assert inbound is not None
+    assert inbound.mentioned_all is False
+    assert "@Alice" in inbound.content.text