diff --git a/backend/app/gateway/services.py b/backend/app/gateway/services.py
index da4a06824..ea9e8662b 100644
--- a/backend/app/gateway/services.py
+++ b/backend/app/gateway/services.py
@@ -10,6 +10,7 @@ from __future__ import annotations
 import asyncio
 import json
 import logging
+import re
 import time
 from typing import Any
 
@@ -93,20 +94,56 @@ def normalize_input(raw_input: dict[str, Any] | None) -> dict[str, Any]:
     return raw_input
 
 
+_DEFAULT_ASSISTANT_ID = "lead_agent"
+
+
 def resolve_agent_factory(assistant_id: str | None):
-    """Resolve the agent factory callable from config."""
+    """Resolve the agent factory callable from config.
+
+    Custom agents are implemented as ``lead_agent`` + an ``agent_name``
+    injected into ``configurable`` — see :func:`build_run_config`.  All
+    ``assistant_id`` values therefore map to the same factory; the routing
+    happens inside ``make_lead_agent`` when it reads ``cfg["agent_name"]``.
+    """
     from deerflow.agents.lead_agent.agent import make_lead_agent
 
-    if assistant_id and assistant_id != "lead_agent":
-        logger.info("assistant_id=%s requested; falling back to lead_agent", assistant_id)
     return make_lead_agent
 
 
-def build_run_config(thread_id: str, request_config: dict[str, Any] | None, metadata: dict[str, Any] | None) -> dict[str, Any]:
-    """Build a RunnableConfig dict for the agent."""
-    configurable = {"thread_id": thread_id}
+def build_run_config(
+    thread_id: str,
+    request_config: dict[str, Any] | None,
+    metadata: dict[str, Any] | None,
+    *,
+    assistant_id: str | None = None,
+) -> dict[str, Any]:
+    """Build a RunnableConfig dict for the agent.
+
+    When *assistant_id* refers to a custom agent (anything other than
+    ``"lead_agent"`` / ``None``), the name is forwarded as
+    ``configurable["agent_name"]``.  ``make_lead_agent`` reads this key to
+    load the matching ``agents/<name>/SOUL.md`` and per-agent config —
+    without it the agent silently runs as the default lead agent.
+
+    This mirrors the channel manager's ``_resolve_run_params`` logic so that
+    the LangGraph Platform-compatible HTTP API and the IM channel path behave
+    identically.
+    """
+    configurable: dict[str, Any] = {"thread_id": thread_id}
     if request_config:
         configurable.update(request_config.get("configurable", {}))
+
+    # Inject custom agent name when the caller specified a non-default assistant.
+    # Honour an explicit configurable["agent_name"] in the request if already set.
+    if assistant_id and assistant_id != _DEFAULT_ASSISTANT_ID and "agent_name" not in configurable:
+        # Normalize the same way ChannelManager does: strip, lowercase,
+        # replace underscores with hyphens, then validate to prevent path
+        # traversal and invalid agent directory lookups.
+        normalized = assistant_id.strip().lower().replace("_", "-")
+        if not normalized or not re.fullmatch(r"[a-z0-9-]+", normalized):
+            raise ValueError(f"Invalid assistant_id {assistant_id!r}: must contain only letters, digits, and hyphens after normalization.")
+        configurable["agent_name"] = normalized
+
     config: dict[str, Any] = {"configurable": configurable, "recursion_limit": 100}
     if request_config:
         for k, v in request_config.items():
@@ -233,7 +270,7 @@ async def start_run(
 
     agent_factory = resolve_agent_factory(body.assistant_id)
     graph_input = normalize_input(body.input)
-    config = build_run_config(thread_id, body.config, body.metadata)
+    config = build_run_config(thread_id, body.config, body.metadata, assistant_id=body.assistant_id)
     stream_modes = normalize_stream_modes(body.stream_mode)
 
     task = asyncio.create_task(
diff --git a/backend/tests/test_gateway_services.py b/backend/tests/test_gateway_services.py
index 730616027..3921a2b34 100644
--- a/backend/tests/test_gateway_services.py
+++ b/backend/tests/test_gateway_services.py
@@ -100,3 +100,63 @@ def test_build_run_config_with_overrides():
     assert config["configurable"]["model_name"] == "gpt-4"
     assert config["tags"] == ["test"]
     assert config["metadata"]["user"] == "alice"
+
+
+# ---------------------------------------------------------------------------
+# Regression tests for issue #1644:
+# assistant_id not mapped to agent_name → custom agent SOUL.md never loaded
+# ---------------------------------------------------------------------------
+
+
+def test_build_run_config_custom_agent_injects_agent_name():
+    """Custom assistant_id must be forwarded as configurable['agent_name'].
+
+    Regression test for #1644: when the LangGraph Platform-compatible
+    /runs endpoint receives a custom assistant_id (e.g. 'finalis'), the
+    Gateway must inject configurable['agent_name'] so that make_lead_agent
+    loads the correct agents/finalis/SOUL.md.
+    """
+    from app.gateway.services import build_run_config
+
+    config = build_run_config("thread-1", None, None, assistant_id="finalis")
+    assert config["configurable"]["agent_name"] == "finalis", "Custom assistant_id must be forwarded as configurable['agent_name'] so that make_lead_agent loads the correct SOUL.md"
+
+
+def test_build_run_config_lead_agent_no_agent_name():
+    """'lead_agent' assistant_id must NOT inject configurable['agent_name']."""
+    from app.gateway.services import build_run_config
+
+    config = build_run_config("thread-1", None, None, assistant_id="lead_agent")
+    assert "agent_name" not in config["configurable"]
+
+
+def test_build_run_config_none_assistant_id_no_agent_name():
+    """None assistant_id must NOT inject configurable['agent_name']."""
+    from app.gateway.services import build_run_config
+
+    config = build_run_config("thread-1", None, None, assistant_id=None)
+    assert "agent_name" not in config["configurable"]
+
+
+def test_build_run_config_explicit_agent_name_not_overwritten():
+    """An explicit configurable['agent_name'] in the request must take precedence."""
+    from app.gateway.services import build_run_config
+
+    config = build_run_config(
+        "thread-1",
+        {"configurable": {"agent_name": "explicit-agent"}},
+        None,
+        assistant_id="other-agent",
+    )
+    assert config["configurable"]["agent_name"] == "explicit-agent", "An explicit configurable['agent_name'] in the request body must not be overwritten by the assistant_id mapping"
+
+
+def test_resolve_agent_factory_returns_make_lead_agent():
+    """resolve_agent_factory always returns make_lead_agent regardless of assistant_id."""
+    from app.gateway.services import resolve_agent_factory
+    from deerflow.agents.lead_agent.agent import make_lead_agent
+
+    assert resolve_agent_factory(None) is make_lead_agent
+    assert resolve_agent_factory("lead_agent") is make_lead_agent
+    assert resolve_agent_factory("finalis") is make_lead_agent
+    assert resolve_agent_factory("custom-agent-123") is make_lead_agent