diff --git a/sdk/agent_session.py b/sdk/agent_session.py
index 6f90e3f..8b7c7b3 100644
--- a/sdk/agent_session.py
+++ b/sdk/agent_session.py
@@ -10,7 +10,7 @@ from sdk.interface import MessageChunk, MessageResponse, PlatformError
 
 
 def build_thread_key(platform: str, user_id: str, chat_id: str) -> str:
-    return f"{platform}:{user_id}:{chat_id}"
+    return f"{len(platform)}:{platform}{len(user_id)}:{user_id}{len(chat_id)}:{chat_id}"
 
 
 @dataclass(frozen=True, slots=True)
@@ -75,6 +75,11 @@ class AgentSessionClient:
                             payload.get("details", "Agent error"),
                             code=payload.get("code", "AGENT_ERROR"),
                         )
+                    elif msg_type == "GRACEFUL_DISCONNECT":
+                        raise PlatformError(
+                            "Agent disconnected gracefully",
+                            code="GRACEFUL_DISCONNECT",
+                        )
                     else:
                         raise PlatformError(
                             f"Unexpected agent message: {payload}",
diff --git a/tests/platform/test_agent_session.py b/tests/platform/test_agent_session.py
index a1d9dd6..bd38b27 100644
--- a/tests/platform/test_agent_session.py
+++ b/tests/platform/test_agent_session.py
@@ -6,7 +6,14 @@ from sdk.agent_session import AgentSessionClient, AgentSessionConfig, build_thre
 
 
 def test_build_thread_key_uses_platform_user_and_chat_id():
-    assert build_thread_key("matrix", "@alice:example.org", "C1") == "matrix:@alice:example.org:C1"
+    assert build_thread_key("matrix", "@alice:example.org", "C1") == "6:matrix18:@alice:example.org2:C1"
+
+
+def test_build_thread_key_does_not_collide_when_user_id_contains_colons():
+    left = build_thread_key("matrix", "@alice:example.org", "C1")
+    right = build_thread_key("matrix", "@alice", "example.org:C1")
+
+    assert left != right
 
 
 @pytest.mark.asyncio