diff --git a/routstr/proxy.py b/routstr/proxy.py
index 9d2dbf09..ad301a45 100644
--- a/routstr/proxy.py
+++ b/routstr/proxy.py
@@ -1,8 +1,9 @@
 import json
+from pathlib import Path
 from typing import Any
 
 from fastapi import APIRouter, Depends, HTTPException, Request
-from fastapi.responses import Response, StreamingResponse
+from fastapi.responses import HTMLResponse, JSONResponse, Response, StreamingResponse
 from sqlmodel import select
 
 from .algorithm import create_model_mappings
@@ -150,10 +151,51 @@ async def refresh_model_maps_periodically() -> None:
             )
 
 
+_API_PATH_PREFIXES = ("v1/", "responses")
+
+_NOT_FOUND_HTML_FILE = Path(__file__).parent.parent / "ui_out" / "404.html"
+
+
+def _read_not_found_html() -> str | None:
+    try:
+        return _NOT_FOUND_HTML_FILE.read_text(encoding="utf-8")
+    except OSError:
+        return None
+
+
+_NOT_FOUND_HTML: str | None = _read_not_found_html()
+
+
+def _build_not_found_response(request: Request, path: str) -> Response:
+    """Return a 404 for unknown paths.
+    """
+    accept = request.headers.get("accept", "").lower()
+    prefers_json = "application/json" in accept and "text/html" not in accept
+    request_id = getattr(request.state, "request_id", "unknown")
+
+    if not prefers_json and _NOT_FOUND_HTML is not None:
+        return HTMLResponse(content=_NOT_FOUND_HTML, status_code=404)
+
+    return JSONResponse(
+        status_code=404,
+        content={
+            "error": {
+                "message": f"Path '/{path}' not found",
+                "type": "not_found",
+                "code": 404,
+            },
+            "request_id": request_id,
+        },
+    )
+
+
 @proxy_router.api_route("/{path:path}", methods=["GET", "POST"], response_model=None)
 async def proxy(
     request: Request, path: str, session: AsyncSession = Depends(get_session)
 ) -> Response | StreamingResponse:
+    if not path.startswith(_API_PATH_PREFIXES):
+        return _build_not_found_response(request, path)
+
     headers = dict(request.headers)
 
     is_responses_api = path.startswith("v1/responses") or path.startswith("responses")
diff --git a/tests/unit/test_proxy_not_found.py b/tests/unit/test_proxy_not_found.py
new file mode 100644
index 00000000..54edd29c
--- /dev/null
+++ b/tests/unit/test_proxy_not_found.py
@@ -0,0 +1,64 @@
+"""Tests for the built-in 404 handler in routstr.proxy."""
+
+from __future__ import annotations
+
+import pytest
+from fastapi import FastAPI
+from fastapi.testclient import TestClient
+
+from routstr import proxy
+from routstr.proxy import proxy_router
+
+
+def _make_app() -> FastAPI:
+    app = FastAPI()
+    app.include_router(proxy_router)
+    return app
+
+
+@pytest.mark.skipif(
+    proxy._NOT_FOUND_HTML is None,
+    reason="UI bundle (ui_out/404.html) not present in this environment",
+)
+def test_unknown_path_returns_html_404_for_browser() -> None:
+    client = TestClient(_make_app())
+    response = client.get("/some/random/page", headers={"accept": "text/html"})
+    assert response.status_code == 404
+    assert response.headers["content-type"].startswith("text/html")
+    assert "404" in response.text
+
+
+def test_unknown_path_returns_json_404_for_api_client() -> None:
+    client = TestClient(_make_app())
+    response = client.get(
+        "/some/random/page", headers={"accept": "application/json"}
+    )
+    assert response.status_code == 404
+    assert response.headers["content-type"].startswith("application/json")
+    payload = response.json()
+    assert payload["error"]["type"] == "not_found"
+    assert payload["error"]["code"] == 404
+    assert "/some/random/page" in payload["error"]["message"]
+
+
+def test_root_path_returns_404_for_proxy_router() -> None:
+    client = TestClient(_make_app())
+    response = client.get("/", headers={"accept": "application/json"})
+    assert response.status_code == 404
+
+
+def test_v1_path_is_not_intercepted_by_404_handler() -> None:
+    """Paths starting with v1/ must reach the proxy logic, not the 404 handler."""
+    client = TestClient(_make_app(), raise_server_exceptions=False)
+    response = client.get("/v1/anything")
+    if response.status_code == 404:
+        # Any 404 here must come from inner proxy logic, not our HTML page.
+        assert "<!DOCTYPE html>" not in response.text
+
+
+def test_json_returned_when_ui_html_missing(monkeypatch: pytest.MonkeyPatch) -> None:
+    monkeypatch.setattr(proxy, "_NOT_FOUND_HTML", None)
+    client = TestClient(_make_app())
+    response = client.get("/some/random/page", headers={"accept": "text/html"})
+    assert response.status_code == 404
+    assert response.headers["content-type"].startswith("application/json")