diff --git a/gateway/session.py b/gateway/session.py
index 1778c2e43dc..3dafa1781f4 100644
--- a/gateway/session.py
+++ b/gateway/session.py
@@ -328,7 +328,9 @@ def build_session_key(source: SessionSource) -> str:
 
     Group/channel rules:
       - chat_id identifies the parent group/channel.
+      - user_id/user_id_alt isolates participants within that parent chat when available.
       - thread_id differentiates threads within that parent chat.
+      - Without participant identifiers, messages fall back to one shared session per chat.
       - Without identifiers, messages fall back to one session per platform/chat_type.
     """
     platform = source.platform.value
@@ -340,13 +342,18 @@ def build_session_key(source: SessionSource) -> str:
         if source.thread_id:
             return f"agent:main:{platform}:dm:{source.thread_id}"
         return f"agent:main:{platform}:dm"
+
+    participant_id = source.user_id_alt or source.user_id
+    key_parts = ["agent:main", platform, source.chat_type]
+
     if source.chat_id:
-        if source.thread_id:
-            return f"agent:main:{platform}:{source.chat_type}:{source.chat_id}:{source.thread_id}"
-        return f"agent:main:{platform}:{source.chat_type}:{source.chat_id}"
+        key_parts.append(source.chat_id)
     if source.thread_id:
-        return f"agent:main:{platform}:{source.chat_type}:{source.thread_id}"
-    return f"agent:main:{platform}:{source.chat_type}"
+        key_parts.append(source.thread_id)
+    if participant_id:
+        key_parts.append(str(participant_id))
+
+    return ":".join(key_parts)
 
 
 class SessionStore:
diff --git a/tests/gateway/test_session.py b/tests/gateway/test_session.py
index cd0104aceb6..e9e629fe6ed 100644
--- a/tests/gateway/test_session.py
+++ b/tests/gateway/test_session.py
@@ -369,6 +369,29 @@ class TestWhatsAppDMSessionKeyConsistency:
         )
         assert store._generate_session_key(source) == build_session_key(source)
 
+    def test_store_creates_distinct_group_sessions_per_user(self, store):
+        first = SessionSource(
+            platform=Platform.DISCORD,
+            chat_id="guild-123",
+            chat_type="group",
+            user_id="alice",
+            user_name="Alice",
+        )
+        second = SessionSource(
+            platform=Platform.DISCORD,
+            chat_id="guild-123",
+            chat_type="group",
+            user_id="bob",
+            user_name="Bob",
+        )
+
+        first_entry = store.get_or_create_session(first)
+        second_entry = store.get_or_create_session(second)
+
+        assert first_entry.session_key == "agent:main:discord:group:guild-123:alice"
+        assert second_entry.session_key == "agent:main:discord:group:guild-123:bob"
+        assert first_entry.session_id != second_entry.session_id
+
     def test_telegram_dm_includes_chat_id(self):
         """Non-WhatsApp DMs should also include chat_id to separate users."""
         source = SessionSource(
@@ -398,6 +421,24 @@ class TestWhatsAppDMSessionKeyConsistency:
         key = build_session_key(source)
         assert key == "agent:main:discord:group:guild-123"
 
+    def test_group_sessions_are_isolated_per_user_when_user_id_present(self):
+        first = SessionSource(
+            platform=Platform.DISCORD,
+            chat_id="guild-123",
+            chat_type="group",
+            user_id="alice",
+        )
+        second = SessionSource(
+            platform=Platform.DISCORD,
+            chat_id="guild-123",
+            chat_type="group",
+            user_id="bob",
+        )
+
+        assert build_session_key(first) == "agent:main:discord:group:guild-123:alice"
+        assert build_session_key(second) == "agent:main:discord:group:guild-123:bob"
+        assert build_session_key(first) != build_session_key(second)
+
     def test_group_thread_includes_thread_id(self):
         """Forum-style threads need a distinct session key within one group."""
         source = SessionSource(
@@ -409,6 +450,17 @@ class TestWhatsAppDMSessionKeyConsistency:
         key = build_session_key(source)
         assert key == "agent:main:telegram:group:-1002285219667:17585"
 
+    def test_group_thread_sessions_are_isolated_per_user(self):
+        source = SessionSource(
+            platform=Platform.TELEGRAM,
+            chat_id="-1002285219667",
+            chat_type="group",
+            thread_id="17585",
+            user_id="42",
+        )
+        key = build_session_key(source)
+        assert key == "agent:main:telegram:group:-1002285219667:17585:42"
+
 
 class TestSessionStoreEntriesAttribute:
     """Regression: /reset must access _entries, not _sessions."""