diff --git a/tests/tui_gateway/test_session_id_injection.py b/tests/tui_gateway/test_session_id_injection.py
new file mode 100644
index 000000000000..069b3a5a5d15
--- /dev/null
+++ b/tests/tui_gateway/test_session_id_injection.py
@@ -0,0 +1,80 @@
+"""Contract test: tui_gateway._set_session_context must inject the live
+session id into HERMES_SESSION_ID so terminal/execute_code subprocesses can
+read the current session's id.
+
+Regression for the bug where _set_session_context called set_session_vars
+WITHOUT session_id, leaving the contextvar as "" (explicitly empty). Because
+the session-context bridge treats an explicit "" as authoritative and does NOT
+fall back to os.environ, every terminal command in a dashboard/TUI/web session
+saw an empty HERMES_SESSION_ID even though agent_init had set it via
+set_current_session_id().
+"""
+import pytest
+
+from gateway.session_context import (
+    get_session_env,
+    _VAR_MAP,
+    _UNSET,
+)
+import tui_gateway.server as server
+
+
+@pytest.fixture(autouse=True)
+def _reset_contextvars():
+    """Reset all session contextvars to _UNSET between tests.
+
+    In production each asyncio.Task/worker thread gets a fresh context copy
+    where the defaults are _UNSET. In tests functions share one thread
+    context, so a value set by test A would leak into test B without this.
+    """
+    yield
+    for var in _VAR_MAP.values():
+        var.set(_UNSET)
+
+
+class _FakeAgent:
+    def __init__(self, session_id):
+        self.session_id = session_id
+
+
+def _install_session(monkeypatch, *, session_key, agent_session_id, source="cli"):
+    """Register a fake session in server._sessions for the duration of a test."""
+    sess = {
+        "session_key": session_key,
+        "source": source,
+        "agent": _FakeAgent(agent_session_id) if agent_session_id is not None else None,
+        "cwd": "/home/user",
+    }
+    monkeypatch.setattr(server, "_sessions", {session_key: sess}, raising=False)
+    return sess
+
+
+def test_set_session_context_injects_agent_session_id(monkeypatch):
+    """HERMES_SESSION_ID must equal the live agent.session_id after binding."""
+    _install_session(
+        monkeypatch, session_key="skey-abc", agent_session_id="20260722_deadbeef"
+    )
+
+    server._set_session_context("skey-abc", ui_session_id="ui-123")
+
+    assert get_session_env("HERMES_SESSION_ID") == "20260722_deadbeef"
+
+
+def test_set_session_context_falls_back_to_session_key(monkeypatch):
+    """When the agent has no session_id yet, fall back to the session_key
+    (never leave HERMES_SESSION_ID empty for an identified session)."""
+    _install_session(monkeypatch, session_key="skey-xyz", agent_session_id=None)
+
+    server._set_session_context("skey-xyz")
+
+    assert get_session_env("HERMES_SESSION_ID") == "skey-xyz"
+
+
+def test_set_session_context_unknown_key_uses_key(monkeypatch):
+    """An ephemeral/unknown session_key (not in _sessions) still binds the key
+    itself rather than leaving the id empty."""
+    monkeypatch.setattr(server, "_sessions", {}, raising=False)
+
+    server._set_session_context("ephemeral-key")
+
+    assert get_session_env("HERMES_SESSION_ID") == "ephemeral-key"
diff --git a/tui_gateway/server.py b/tui_gateway/server.py
index c5ad49720dcb..06c15641d97a 100644
--- a/tui_gateway/server.py
+++ b/tui_gateway/server.py
@@ -2573,13 +2573,27 @@ def _set_session_context(
         # it instead of falling back to the gateway launch dir.
         resolved = cwd if cwd is not None else _cwd_for_session_key(session_key)
         source = _resolve_session_platform()
+        # Derive the live conversation id so terminal/execute_code subprocesses
+        # can read HERMES_SESSION_ID. Without this, set_session_vars leaves the
+        # session-id contextvar as "" (explicitly empty), and the subprocess-env
+        # bridge treats that as authoritative — NOT falling back to os.environ —
+        # so every command in a dashboard/TUI/web session saw an empty
+        # HERMES_SESSION_ID even though agent_init set it via
+        # set_current_session_id(). Prefer the agent's durable session_id, then
+        # fall back to the session_key (matching the id derivation used at
+        # session-finalize), so an identified session is never left blank.
+        session_id = session_key
         with _sessions_lock:
             for sess in list(_sessions.values()):
                 if sess.get("session_key") == session_key:
                     source = _session_source(sess)
+                    session_id = (
+                        getattr(sess.get("agent"), "session_id", None) or session_key
+                    )
                     break
         return set_session_vars(
             session_key=session_key,
+            session_id=session_id,
             source=source,
             cwd=resolved,
             ui_session_id=ui_session_id,