diff --git a/src/endpoints.py b/src/endpoints.py
index dd9785a..65cddf2 100644
--- a/src/endpoints.py
+++ b/src/endpoints.py
@@ -92,10 +92,20 @@ async def read_item(request: LinkRequest, dep: BrowserDep) -> LinkResponse:
         page_html=page_html,
     )
 
+    user_agent = (
+        page_request.request.headers.get("user-agent") if page_request else None
+    )
+    if user_agent is None:
+        try:
+            user_agent = await dep.page.evaluate("navigator.userAgent")
+        except Exception:
+            logger.warning("Could not determine User-Agent via page.evaluate")
+            user_agent = ""
+
     return LinkResponse(
         message="Success",
         solution=Solution(
-            user_agent=await dep.page.evaluate("navigator.userAgent"),
+            user_agent=user_agent,
             url=final_url if final_url is not None else dep.page.url,
             status=status,
             cookies=cookies,
diff --git a/src/utils.py b/src/utils.py
index e34bca9..abc4ff2 100644
--- a/src/utils.py
+++ b/src/utils.py
@@ -96,6 +96,7 @@ async def get_browser(
         proxy=proxy_config,
         humanize=True,
         locale=BROWSER_LOCALE or "auto",
+        extra_prefs={"devtools.jsonview.enabled": False},
     ) as browser_raw:
         # InvisiblePlaywright yields a Browser instance
         browser = cast("Browser", browser_raw)
diff --git a/tests/main_test.py b/tests/main_test.py
index 17d211b..b3136ac 100644
--- a/tests/main_test.py
+++ b/tests/main_test.py
@@ -59,6 +59,34 @@ def test_bypass(website: str):
     assert response.status_code == HTTPStatus.OK
 
 
+def test_json_api():
+    """JSON APIs must return 200, not crash on the UA evaluate.
+
+    Firefox renders application/json in a built-in viewer whose CSP blocks
+    Playwright's eval-based evaluate() (issue #394). The browser must be
+    launched with the viewer disabled so /v1 works and returns the raw JSON.
+    """
+    url = "https://api.ipify.org?format=json"
+    test_request = httpx2.get(url)
+    if test_request.status_code >= HTTPStatus.INTERNAL_SERVER_ERROR:
+        pytest.skip(
+            f"Skipping JSON API test - upstream error ({test_request.status_code})"
+        )
+
+    response = client.post(
+        "/v1",
+        json=LinkRequest.model_construct(url=url, cmd="request.get").model_dump(),
+    )
+
+    if response.status_code == HTTPStatus.REQUEST_TIMEOUT:
+        pytest.skip("Skipping JSON API test - timed out (upstream issue)")
+
+    assert response.status_code == HTTPStatus.OK
+    solution = response.json()["solution"]
+    assert solution["userAgent"]
+    assert '"ip"' in solution["response"]
+
+
 def test_health_check():
     """
     Tests the health check endpoint.
@@ -111,7 +139,9 @@ def fake_dep(*, fail_states: set[str] | None = None) -> BrowserDepClass:
     page = AsyncMock()
     page.url = "https://example.test/login"
     page.goto.return_value = MagicMock(
-        status=HTTPStatus.OK, headers={"content-type": "text/html"}
+        status=HTTPStatus.OK,
+        headers={"content-type": "text/html"},
+        request=MagicMock(headers={"user-agent": "UnitTestBrowser/1.0"}),
     )
     page.title.return_value = "Login"
     page.evaluate.return_value = "UnitTestBrowser/1.0"
@@ -158,3 +188,22 @@ async def test_domcontentloaded_timeout_returns_408():
         )
 
     assert exc.value.status_code == HTTPStatus.REQUEST_TIMEOUT
+
+
+@pytest.mark.asyncio
+async def test_user_agent_survives_csp_blocked_evaluate():
+    """UA comes from request headers when page CSP blocks evaluate (#394).
+
+    No CSP configuration (header, meta tag, or internal viewer document) may
+    turn /v1 into a 500.
+    """
+    dep = fake_dep()
+    dep.page.evaluate.side_effect = Exception("call to eval() blocked by CSP")
+
+    response = await read_item(
+        LinkRequest(url="https://example.test/login"),
+        dep,
+    )
+
+    assert response.status == "ok"
+    assert response.solution.user_agent == "UnitTestBrowser/1.0"