diff --git a/api/profiles.py b/api/profiles.py
index fe0e1023..f9744c27 100644
--- a/api/profiles.py
+++ b/api/profiles.py
@@ -679,13 +679,18 @@ def switch_profile(name: str, *, process_wide: bool = True) -> dict:
     # Import here to avoid circular import at module load
     from api.config import STREAMS, STREAMS_LOCK, reload_config
 
-    # Block if agent is running
-    with STREAMS_LOCK:
-        if len(STREAMS) > 0:
-            raise RuntimeError(
-                'Cannot switch profiles while an agent is running. '
-                'Cancel or wait for it to finish.'
-            )
+    # Process-wide profile switches mutate HERMES_HOME, module-level path caches,
+    # os.environ-backed .env keys, and the global config cache. Keep those blocked
+    # while any agent stream is active. Per-client WebUI switches are cookie/TLS
+    # scoped (process_wide=False) and do not mutate those globals, so users can
+    # leave a running session in one profile and start work in another (#1700).
+    if process_wide:
+        with STREAMS_LOCK:
+            if len(STREAMS) > 0:
+                raise RuntimeError(
+                    'Cannot switch profiles while an agent is running. '
+                    'Cancel or wait for it to finish.'
+                )
 
     # Resolve profile directory
     if _is_root_profile(name):
diff --git a/docs/pr-media/1700/profile-switch-away-from-running-session.png b/docs/pr-media/1700/profile-switch-away-from-running-session.png
new file mode 100644
index 00000000..a6c1f178
Binary files /dev/null and b/docs/pr-media/1700/profile-switch-away-from-running-session.png differ
diff --git a/static/panels.js b/static/panels.js
index c07e6229..12531130 100644
--- a/static/panels.js
+++ b/static/panels.js
@@ -3766,7 +3766,9 @@ window.addEventListener('resize',()=>{
 });
 
 async function switchToProfile(name) {
-  if (S.busy) { showToast(t('profiles_busy_switch')); return; }
+  // Profile switches are per-client cookie/TLS scoped, so a running stream in
+  // the current session can safely continue while this tab moves to another
+  // profile. The in-flight session stays attached to its original profile.
 
   // ── Loading indicator ───────────────────────────────────────────────────
   // Show spinner on the profile chip immediately so the user gets visual
@@ -3781,7 +3783,11 @@ async function switchToProfile(name) {
   // Determine whether the current session has any messages.
   // A session with messages is "in progress" and belongs to the current profile —
   // we must not retag it.  We'll start a fresh session for the new profile instead.
-  const sessionInProgress = S.session && S.messages && S.messages.length > 0;
+  const sessionInProgress = S.session && (
+    (S.messages && S.messages.length > 0) ||
+    S.session.active_stream_id ||
+    S.session.pending_user_message
+  );
 
   try {
     const data = await api('/api/profile/switch', { method: 'POST', body: JSON.stringify({ name }) });
diff --git a/tests/test_issue1700_parallel_profile_switch.py b/tests/test_issue1700_parallel_profile_switch.py
new file mode 100644
index 00000000..7928b9a7
--- /dev/null
+++ b/tests/test_issue1700_parallel_profile_switch.py
@@ -0,0 +1,95 @@
+"""Regression coverage for issue #1700 parallel profile switching.
+
+A WebUI profile switch uses cookie/thread-local profile state, so it should be
+allowed while another session is streaming. Only process-wide profile switches
+must remain blocked because they mutate global Hermes runtime state.
+"""
+from pathlib import Path
+
+import pytest
+
+REPO_ROOT = Path(__file__).parent.parent.resolve()
+PANELS_JS = (REPO_ROOT / "static" / "panels.js").read_text(encoding="utf-8")
+
+
+def _extract_switch_to_profile() -> str:
+    marker = "async function switchToProfile(name) {"
+    idx = PANELS_JS.find(marker)
+    assert idx != -1, "switchToProfile() not found in static/panels.js"
+    depth = 0
+    for i, ch in enumerate(PANELS_JS[idx:], idx):
+        if ch == "{":
+            depth += 1
+        elif ch == "}":
+            depth -= 1
+            if depth == 0:
+                return PANELS_JS[idx : i + 1]
+    raise AssertionError("Could not extract switchToProfile() body")
+
+
+def _prepare_profile_tree(tmp_path, monkeypatch):
+    import api.profiles as profiles
+
+    default_home = tmp_path / ".hermes"
+    target_home = default_home / "profiles" / "writer"
+    target_workspace = tmp_path / "writer-workspace"
+    target_workspace.mkdir(parents=True)
+    target_home.mkdir(parents=True)
+    (target_home / "config.yaml").write_text(
+        f"model:\n  provider: openai-codex\n  default: gpt-5.5\n"
+        f"terminal:\n  cwd: {target_workspace}\n",
+        encoding="utf-8",
+    )
+
+    monkeypatch.setattr(profiles, "_DEFAULT_HERMES_HOME", default_home)
+    monkeypatch.setattr(profiles, "_active_profile", "default")
+    monkeypatch.setattr(profiles, "list_profiles_api", lambda: [{"name": "default"}, {"name": "writer"}])
+    profiles._tls.profile = None
+    return profiles
+
+
+def test_process_wide_switch_still_blocks_when_stream_is_active(tmp_path, monkeypatch):
+    profiles = _prepare_profile_tree(tmp_path, monkeypatch)
+    from api.config import STREAMS
+
+    STREAMS.clear()
+    STREAMS["stream-default"] = object()
+    try:
+        with pytest.raises(RuntimeError, match="Cannot switch profiles while an agent is running"):
+            profiles.switch_profile("writer", process_wide=True)
+    finally:
+        STREAMS.clear()
+        profiles._tls.profile = None
+
+
+def test_per_client_switch_allowed_when_stream_is_active(tmp_path, monkeypatch):
+    profiles = _prepare_profile_tree(tmp_path, monkeypatch)
+    from api.config import STREAMS
+
+    STREAMS.clear()
+    STREAMS["stream-default"] = object()
+    try:
+        result = profiles.switch_profile("writer", process_wide=False)
+    finally:
+        STREAMS.clear()
+        profiles._tls.profile = None
+
+    assert result["active"] == "writer"
+    assert result["default_model"] == "gpt-5.5"
+
+
+def test_frontend_profile_switch_no_longer_blocks_on_busy_state():
+    fn = _extract_switch_to_profile()
+
+    assert "profiles_busy_switch" not in fn
+    assert "if (S.busy)" not in fn
+    assert "Profile switches are per-client cookie/TLS scoped" in fn
+
+
+def test_frontend_treats_active_or_pending_session_as_in_progress():
+    fn = _extract_switch_to_profile()
+    session_block = fn[fn.find("const sessionInProgress") : fn.find("try {", fn.find("const sessionInProgress"))]
+
+    assert "S.session.active_stream_id" in session_block
+    assert "S.session.pending_user_message" in session_block
+    assert "S.messages.length > 0" in session_block