diff --git a/api/routes.py b/api/routes.py
index 08e7a358..06db3f69 100644
--- a/api/routes.py
+++ b/api/routes.py
@@ -512,6 +512,50 @@ def _session_visible_to_active_profile(session_profile, handler=None) -> bool:
     return _profiles_match(session_profile, active_profile)
 
 
+def _request_session_visibility_exempt(method: str, path: str | None) -> bool:
+    if not path:
+        return False
+    if method != "POST":
+        return False
+    # Import routes create/claim sessions before normal ownership exists, and
+    # chat/start has inline placeholder-retag rules that must run before the
+    # generic request-session guard.
+    return path in {
+        "/api/session/import",
+        "/api/session/import_cli",
+        "/api/chat/start",
+    }
+
+
+def _session_id_visible_to_request_profile(handler, sid) -> bool:
+    """Return whether ``sid`` belongs to the active profile."""
+    if not isinstance(sid, str) or not sid:
+        return True
+    if not is_safe_session_id(sid):
+        return True
+    try:
+        session = get_session(sid, metadata_only=True)
+    except KeyError:
+        return True
+    if not _session_visible_to_active_profile(getattr(session, "profile", None), handler):
+        bad(handler, "Session not found", 404)
+        return False
+    return True
+
+
+def _guard_request_session_visibility(handler, parsed, body=None, method="GET") -> bool:
+    """Apply request session-profile visibility check to request-supplied IDs."""
+    method = str(method).upper()
+    if _request_session_visibility_exempt(method, getattr(parsed, "path", "")):
+        return True
+    sid = parse_qs(parsed.query).get("session_id", [None])[0]
+    if not _session_id_visible_to_request_profile(handler, sid):
+        return False
+    if isinstance(body, dict) and not _session_id_visible_to_request_profile(handler, body.get("session_id")):
+        return False
+    return True
+
+
 def _active_skills_dir() -> Path:
     """Return the skills directory for the request's active Hermes profile.
 
@@ -10012,6 +10056,9 @@ def handle_get(handler, parsed) -> bool:
             handler.end_headers()
         return True
 
+    if parsed.path.startswith("/api/") and not _guard_request_session_visibility(handler, parsed, method="GET"):
+        return True
+
     # ── Insights / knowledge status ──
     if parsed.path == "/api/insights":
         return _handle_insights(handler, parsed)
@@ -11605,6 +11652,10 @@ def handle_post(handler, parsed) -> bool:
         if diag:
             diag.finish()
         raise
+    if not _guard_request_session_visibility(handler, parsed, body=body, method="POST"):
+        if diag:
+            diag.finish()
+        return True
 
     if parsed.path == "/api/escape/authorize":
         return _handle_escape_authorize(handler, parsed, body)
@@ -13758,6 +13809,8 @@ def handle_patch(handler, parsed) -> bool:
     if not _check_csrf(handler):
         return j(handler, {"error": _csrf_rejection_error(handler)}, status=403)
     body = read_body(handler)
+    if not _guard_request_session_visibility(handler, parsed, body=body, method="PATCH"):
+        return True
     if parsed.path.startswith("/api/mcp/servers/"):
         name = parsed.path[len("/api/mcp/servers/"):]
         return _handle_mcp_server_toggle(handler, name, body)
@@ -13776,6 +13829,8 @@ def handle_delete(handler, parsed) -> bool:
     if not _check_csrf(handler):
         return j(handler, {"error": _csrf_rejection_error(handler)}, status=403)
     body = read_body(handler)
+    if not _guard_request_session_visibility(handler, parsed, body=body, method="DELETE"):
+        return True
     if parsed.path.startswith("/api/mcp/servers/"):
         name = parsed.path[len("/api/mcp/servers/"):]
         return _handle_mcp_server_delete(handler, name)
@@ -13802,6 +13857,8 @@ def handle_put(handler, parsed) -> bool:
     if not _check_csrf(handler):
         return j(handler, {"error": "Cross-origin request rejected"}, status=403)
     body = read_body(handler)
+    if not _guard_request_session_visibility(handler, parsed, body=body, method="PUT"):
+        return True
     if parsed.path.startswith("/api/mcp/servers/"):
         name = parsed.path[len("/api/mcp/servers/"):]
         return _handle_mcp_server_update(handler, name, body)
@@ -18049,6 +18106,7 @@ def _handle_chat_start(handler, body, diag=None):
             return bad(handler, "Read-only imported sessions cannot be continued from WebUI", 403)
         diag.stage("validate_profile") if diag else None
         requested_profile = str(body.get("profile") or "").strip()
+        active_profile = _get_active_profile_name()
         if requested_profile:
             try:
                 from api.profiles import _PROFILE_ID_RE
@@ -18057,17 +18115,34 @@ def _handle_chat_start(handler, body, diag=None):
                     return bad(handler, "invalid profile", 400)
             except ImportError:
                 requested_profile = ""
-        if requested_profile and not _profiles_match(getattr(s, "profile", None), requested_profile):
-            has_persisted_turns = bool(
-                getattr(s, "messages", None)
-                or getattr(s, "context_messages", None)
-                or getattr(s, "pending_user_message", None)
-            )
+        session_profile = getattr(s, "profile", None)
+        has_persisted_turns = bool(
+            getattr(s, "messages", None)
+            or getattr(s, "context_messages", None)
+            or getattr(s, "pending_user_message", None)
+        )
+        if not _session_visible_to_active_profile(session_profile, handler):
+            if (
+                requested_profile
+                and _profiles_match(requested_profile, active_profile)
+                and not has_persisted_turns
+            ):
+                # Empty placeholders can still be retagged when the
+                # requested profile matches the active request profile.
+                s.profile = requested_profile
+            else:
+                return bad(handler, "Session not found", 404)
+        elif (
+            requested_profile
+            and _profiles_match(requested_profile, active_profile)
+            and not _profiles_match(session_profile, requested_profile)
+        ):
+            # Empty sessions are placeholders. If the user switches profiles
+            # before sending the first turn, run the placeholder under the
+            # active request profile instead of the stale one stamped at
+            # creation time. The request body alone must not retag a session to
+            # a different profile than the server-selected active profile.
             if not has_persisted_turns:
-                # Empty sessions are placeholders. If the user switches profiles
-                # before sending the first turn, run the placeholder under the
-                # currently-selected profile instead of the stale one stamped at
-                # creation time.
                 s.profile = requested_profile
         diag.stage("normalize_message") if diag else None
         msg = str(body.get("message", "")).strip()
diff --git a/api/upload.py b/api/upload.py
index c9239518..5cd87ae2 100644
--- a/api/upload.py
+++ b/api/upload.py
@@ -11,6 +11,7 @@ from pathlib import Path
 from api.config import MAX_UPLOAD_BYTES, STATE_DIR
 from api.helpers import j, bad
 from api.models import get_session
+from api.profiles import _profiles_match, get_active_profile_name as _get_active_profile_name
 from api.workspace import (
     safe_resolve_ws,
     resolve_trusted_workspace,
@@ -150,6 +151,21 @@ def _session_attachment_dir(session_id: str, *, root: Path | None = None) -> Pat
     return dest_dir
 
 
+def _session_visible_to_active_profile(session) -> bool:
+    """Return whether an upload target session belongs to the active profile."""
+    session_profile = getattr(session, 'profile', None)
+    if not isinstance(session_profile, str):
+        session_profile = None
+    return _profiles_match(session_profile, _get_active_profile_name())
+
+
+def _reject_invisible_session(handler, session) -> bool:
+    if _session_visible_to_active_profile(session):
+        return False
+    j(handler, {'error': 'Session not found'}, status=404)
+    return True
+
+
 def handle_upload(handler):
     import traceback as _tb
     try:
@@ -168,6 +184,8 @@ def handle_upload(handler):
             s = get_session(session_id)
         except KeyError:
             return j(handler, {'error': 'Session not found'}, status=404)
+        if _reject_invisible_session(handler, s):
+            return True
         safe_name = _sanitize_upload_name(filename)
         dest = _upload_destination(session_id, safe_name)
         dest.write_bytes(file_bytes)
@@ -344,6 +362,8 @@ def handle_upload_extract(handler):
             s = get_session(session_id)
         except KeyError:
             return j(handler, {'error': 'Session not found'}, status=404)
+        if _reject_invisible_session(handler, s):
+            return True
         session_dir = _session_attachment_dir(session_id)
         session_dir.mkdir(parents=True, exist_ok=True)
         result = extract_archive(file_bytes, filename, session_dir)
@@ -563,6 +583,8 @@ def handle_workspace_upload(handler):
             session = get_session(session_id)
         except KeyError:
             return j(handler, {'error': 'Session not found'}, status=404)
+        if _reject_invisible_session(handler, session):
+            return True
 
         # Resolve workspace root from session
         workspace = resolve_trusted_workspace(session.workspace)
diff --git a/tests/test_session_active_profile_authorization.py b/tests/test_session_active_profile_authorization.py
new file mode 100644
index 00000000..a2872c0d
--- /dev/null
+++ b/tests/test_session_active_profile_authorization.py
@@ -0,0 +1,261 @@
+"""Regression tests for session-profile visibility on request-scoped session_id uses.
+
+The security contract is now enforced with a generic preflight for request-supplied
+session IDs. These tests cover the most sensitive paths that previously loaded
+foreign-profile sessions directly: duplicate, file reads, and chat/start.
+"""
+
+from __future__ import annotations
+
+import io
+from urllib.parse import urlparse
+
+import api.routes as routes
+import api.upload as upload
+
+
+class _FakeHandler:
+    def __init__(self):
+        self.status = None
+        self.headers = {"Content-Type": "multipart/form-data; boundary=test", "Content-Length": "1"}
+        self.rfile = io.BytesIO(b"")
+        self.wfile = io.BytesIO()
+        self.command = "GET"
+        self.path = "/"
+        self.client_address = ("127.0.0.1", 12345)
+
+    def send_response(self, status):
+        self.status = status
+
+    def send_header(self, key, value):
+        self.headers[key] = value
+
+    def end_headers(self):
+        pass
+
+
+def _capture(monkeypatch):
+    cap = {}
+
+    def _j(_h, obj, *_, **__):
+        cap["ok"] = obj
+        return True
+
+    def _bad(_h, msg, code=400):
+        cap["bad"] = (msg, code)
+        return True
+
+    monkeypatch.setattr(routes, "j", _j)
+    monkeypatch.setattr(routes, "bad", _bad)
+    return cap
+
+
+def _capture_upload(monkeypatch):
+    cap = {}
+
+    def _j(_h, obj, *_, **kwargs):
+        cap["ok"] = obj
+        cap["status"] = kwargs.get("status", 200)
+        return True
+
+    monkeypatch.setattr(upload, "j", _j)
+    return cap
+
+
+class _SimpleSession:
+    def __init__(self, sid, profile="default", workspace="/workspace", messages=None, context_messages=None, pending_user_message=None):
+        self.session_id = sid
+        self.profile = profile
+        self.workspace = workspace
+        self.model = "test-model"
+        self.model_provider = None
+        self.title = "Test"
+        self.messages = messages or []
+        self.tool_calls = []
+        self.project_id = None
+        self.context_messages = context_messages
+        self.pending_user_message = pending_user_message
+        self.personality = None
+        self.enabled_toolsets = None
+        self.context_length = None
+        self.threshold_tokens = None
+        self.truncation_watermark = None
+        self.truncation_boundary = None
+        self.gateway_routing = None
+        self.gateway_routing_history = []
+        self.llm_title_generated = False
+        self.manual_title = False
+        self.composer_draft = {}
+        self.context_engine = None
+        self.context_engine_state = {}
+        self.input_tokens = 0
+        self.output_tokens = 0
+        self.estimated_cost = 0.0
+        self.cache_read_tokens = 0
+        self.cache_write_tokens = 0
+
+
+def test_session_duplicate_foreign_profile_session_blocked_by_visibility_guard(monkeypatch):
+    handler = _FakeHandler()
+    foreign = _SimpleSession("foreign_duplicate", profile="other")
+    monkeypatch.setattr(routes, "get_session", lambda sid, metadata_only=False: foreign)
+    monkeypatch.setattr(routes, "_get_active_profile_name", lambda: "default")
+    monkeypatch.setattr(routes, "_check_csrf", lambda _handler: True)
+    monkeypatch.setattr(routes, "read_body", lambda _handler: {"session_id": "foreign_duplicate"})
+    monkeypatch.setattr(routes.Session, "load", staticmethod(lambda sid: (_ for _ in ()).throw(AssertionError("duplicate should not materialize foreign session"))))
+
+    cap = _capture(monkeypatch)
+    routes.handle_post(handler, urlparse("/api/session/duplicate"))
+
+    assert cap["bad"] == ("Session not found", 404)
+
+
+def test_session_duplicate_same_profile_still_duplicates(monkeypatch):
+    handler = _FakeHandler()
+    source = _SimpleSession("session_visible", profile="default")
+    calls = {"load": 0, "save": 0}
+
+    def _load(_sid):
+        calls["load"] += 1
+        return source
+
+    monkeypatch.setattr(routes.Session, "load", staticmethod(_load))
+    monkeypatch.setattr(routes, "get_session", lambda sid, metadata_only=False: source)
+    monkeypatch.setattr(routes, "_get_active_profile_name", lambda: "default")
+    monkeypatch.setattr(routes, "_check_csrf", lambda _handler: True)
+    monkeypatch.setattr(routes, "read_body", lambda _handler: {"session_id": "session_visible"})
+
+    def _session_save(_self, *_, **__):
+        calls["save"] += 1
+
+    monkeypatch.setattr(routes.Session, "save", _session_save)
+    monkeypatch.setattr(routes, "publish_session_list_changed", lambda *_, **__: None)
+
+    cap = _capture(monkeypatch)
+    routes.handle_post(handler, urlparse("/api/session/duplicate"))
+
+    assert calls["load"] == 1
+    assert calls["save"] == 1
+    assert "bad" not in cap
+    assert cap["ok"]["session"]["session_id"] != "session_visible"
+
+
+def test_file_read_foreign_profile_session_returns_404_before_file_ops(monkeypatch):
+    handler = _FakeHandler()
+    foreign = _SimpleSession("foreign_file", profile="other", workspace="/workspace")
+    monkeypatch.setattr(routes, "get_session", lambda sid, metadata_only=False: foreign)
+    monkeypatch.setattr(routes, "_get_active_profile_name", lambda: "default")
+    monkeypatch.setattr(routes, "get_session_for_file_ops", lambda sid: (_ for _ in ()).throw(AssertionError("file ops should not run")))
+    cap = _capture(monkeypatch)
+
+    routes.handle_get(handler, urlparse("/api/file?session_id=foreign_file&path=notes.txt"))
+
+    assert cap["bad"] == ("Session not found", 404)
+
+
+def test_chat_start_foreign_persisted_session_returns_404_before_start_run(monkeypatch):
+    handler = _FakeHandler()
+    persisted_foreign = _SimpleSession(
+        "chat_foreign",
+        profile="other",
+        messages=[{"role": "user", "content": "first"}],
+    )
+
+    monkeypatch.setattr(routes, "_check_csrf", lambda _handler: True)
+    monkeypatch.setattr(routes, "read_body", lambda _handler: {"session_id": "chat_foreign", "message": "hello"})
+    monkeypatch.setattr(routes, "_get_or_materialize_session", lambda sid: persisted_foreign)
+    monkeypatch.setattr(routes, "_get_active_profile_name", lambda: "default")
+    monkeypatch.setattr(routes, "_start_run", lambda *_, **__: (_ for _ in ()).throw(AssertionError("_start_run should not run")))
+
+    cap = _capture(monkeypatch)
+    routes.handle_post(handler, urlparse("/api/chat/start"))
+
+    assert cap["bad"] == ("Session not found", 404)
+
+
+def test_chat_start_body_profile_cannot_retag_visible_empty_session_without_active_profile(monkeypatch):
+    handler = _FakeHandler()
+    empty_visible = _SimpleSession("chat_empty", profile="default", messages=[])
+    captured = {}
+
+    monkeypatch.setattr(routes, "_check_csrf", lambda _handler: True)
+    monkeypatch.setattr(
+        routes,
+        "read_body",
+        lambda _handler: {"session_id": "chat_empty", "message": "hello", "profile": "other"},
+    )
+    monkeypatch.setattr(routes, "_get_or_materialize_session", lambda sid: empty_visible)
+    monkeypatch.setattr(routes, "_get_active_profile_name", lambda: "default")
+    monkeypatch.setattr(routes, "_resolve_chat_workspace_with_recovery", lambda *_args, **_kwargs: "/workspace")
+    monkeypatch.setattr(routes, "_read_profile_model_config", lambda *_args, **_kwargs: (None, None))
+    monkeypatch.setattr(
+        routes,
+        "_resolve_compatible_session_model_state",
+        lambda *_args, **_kwargs: ("test-model", None, "test-model"),
+    )
+
+    def _start_run(session, **_kwargs):
+        captured["profile"] = session.profile
+        return {"ok": True, "stream_id": "stream-test", "session_id": session.session_id}
+
+    monkeypatch.setattr(routes, "_start_run", _start_run)
+
+    cap = _capture(monkeypatch)
+    routes.handle_post(handler, urlparse("/api/chat/start"))
+
+    assert "bad" not in cap
+    assert captured["profile"] == "default"
+
+
+def test_attachment_upload_foreign_profile_session_returns_404_before_write(monkeypatch):
+    handler = _FakeHandler()
+    foreign = _SimpleSession("upload_foreign", profile="other")
+    monkeypatch.setattr(upload, "parse_multipart", lambda *_args: ({"session_id": "upload_foreign"}, {"file": ("note.txt", b"x")}))
+    monkeypatch.setattr(upload, "get_session", lambda sid: foreign)
+    monkeypatch.setattr(upload, "_get_active_profile_name", lambda: "default")
+    monkeypatch.setattr(
+        upload,
+        "_upload_destination",
+        lambda *_args, **_kwargs: (_ for _ in ()).throw(AssertionError("upload destination should not be resolved")),
+    )
+
+    cap = _capture_upload(monkeypatch)
+    upload.handle_upload(handler)
+
+    assert cap == {"ok": {"error": "Session not found"}, "status": 404}
+
+
+def test_archive_upload_extract_foreign_profile_session_returns_404_before_extract(monkeypatch):
+    handler = _FakeHandler()
+    foreign = _SimpleSession("extract_foreign", profile="other")
+    monkeypatch.setattr(upload, "parse_multipart", lambda *_args: ({"session_id": "extract_foreign"}, {"file": ("archive.zip", b"zip")}))
+    monkeypatch.setattr(upload, "get_session", lambda sid: foreign)
+    monkeypatch.setattr(upload, "_get_active_profile_name", lambda: "default")
+    monkeypatch.setattr(
+        upload,
+        "extract_archive",
+        lambda *_args, **_kwargs: (_ for _ in ()).throw(AssertionError("archive extraction should not run")),
+    )
+
+    cap = _capture_upload(monkeypatch)
+    upload.handle_upload_extract(handler)
+
+    assert cap == {"ok": {"error": "Session not found"}, "status": 404}
+
+
+def test_workspace_upload_foreign_profile_session_returns_404_before_workspace_resolution(monkeypatch):
+    handler = _FakeHandler()
+    foreign = _SimpleSession("workspace_foreign", profile="other", workspace="/workspace/foreign")
+    monkeypatch.setattr(upload, "parse_multipart", lambda *_args: ({"session_id": "workspace_foreign"}, {"file": ("note.txt", b"x")}))
+    monkeypatch.setattr(upload, "get_session", lambda sid: foreign)
+    monkeypatch.setattr(upload, "_get_active_profile_name", lambda: "default")
+    monkeypatch.setattr(
+        upload,
+        "resolve_trusted_workspace",
+        lambda *_args, **_kwargs: (_ for _ in ()).throw(AssertionError("workspace should not be resolved")),
+    )
+
+    cap = _capture_upload(monkeypatch)
+    upload.handle_workspace_upload(handler)
+
+    assert cap == {"ok": {"error": "Session not found"}, "status": 404}