diff --git a/bin/chat-fastapi.py b/bin/chat-fastapi.py
index 85321c6..50bfa1b 100644
--- a/bin/chat-fastapi.py
+++ b/bin/chat-fastapi.py
@@ -69,6 +69,27 @@ async def lifespan(_app: FastAPI) -> AsyncIterator[None]:
 CHAINLIT_URL = os.getenv("CHAINLIT_URL")
 
 CLOUDFLARE_SECRET_KEY = get_secret("CLOUDFLARE_SECRET_KEY")
+
+# A human check on the chat is required unless a deployment says otherwise, and
+# saying otherwise is explicit. Absence used to mean "no captcha to enforce", so
+# a deployment that simply had no key served an ungated chat and reported
+# nothing -- satisfied on paper, off in practice.
+#
+# Refusing to start rather than serving ungated is the same trade the answer
+# endpoint's verifying key already makes: losing the feature is the correct
+# failure, serving it unprotected is not.
+CHAT_REQUIRES_HUMAN = os.getenv("CHAT_REQUIRES_HUMAN", "1").strip() not in {
+    "0",
+    "false",
+    "no",
+}
+if CHAT_REQUIRES_HUMAN and not CLOUDFLARE_SECRET_KEY:
+    raise RuntimeError(
+        "CHAT_REQUIRES_HUMAN is on and no CLOUDFLARE_SECRET_KEY is configured, "
+        "so the chat would be served with no human check at all. Provide the "
+        "Turnstile keys, or set CHAT_REQUIRES_HUMAN=0 to say deliberately that "
+        "this deployment does not want one."
+    )
 CLOUDFLARE_SITE_KEY = os.getenv("CLOUDFLARE_SITE_KEY")
 
 ERROR_PAGE_TEMPLATE = Template(
diff --git a/src/util/captcha_scope.py b/src/util/captcha_scope.py
index f4d54f7..9e1349e 100644
--- a/src/util/captcha_scope.py
+++ b/src/util/captcha_scope.py
@@ -49,8 +49,16 @@ def is_captcha_exempt(
         return True
     if any(path.startswith(prefix) for prefix in extra_prefixes):
         return True
-    # No captcha configured means no captcha to enforce. Deliberate: beta runs
-    # without one because the Turnstile site key is bound to reactome.org.
+    # A missing key is NOT a reason to skip the check. It used to be, and that
+    # is the failure this exists to prevent: "there is captcha middleware" and
+    # "the chat is gated" were different statements, and nothing said which one
+    # was true of a given deployment.
+    #
+    # `captcha_configured` is still taken, because a deployment that has
+    # deliberately opted out (CHAT_REQUIRES_HUMAN=0) passes False and expects to
+    # be let through. What changed is where that decision is made: in
+    # configuration someone wrote, rather than inferred from a value being
+    # absent.
     if not captcha_configured:
         return True
     # Anything outside the Chainlit app is not ours to guard.
diff --git a/tests/api/test_chat_requires_human.py b/tests/api/test_chat_requires_human.py
new file mode 100644
index 0000000..f17112c
--- /dev/null
+++ b/tests/api/test_chat_requires_human.py
@@ -0,0 +1,75 @@
+"""The chat must not be served without a human check by accident.
+
+Adam asked for a human check on the regular chat, not only the search-page
+endpoint. The obstacle was not the check -- the middleware existed -- but that a
+deployment with no Turnstile key skipped it and said nothing, so "there is
+captcha middleware" and "the chat is gated" were different statements with
+nothing to tell them apart.
+
+This pins the decision being explicit in both directions: no key means refuse to
+start, and a deployment that genuinely wants no check must say so.
+"""
+
+import subprocess
+import sys
+from pathlib import Path
+
+import pytest
+
+REPO = Path(__file__).parent.parent.parent
+SNIPPET = """
+import os, sys
+sys.path.insert(0, "src")
+required = os.getenv("CHAT_REQUIRES_HUMAN", "1").strip() not in {"0", "false", "no"}
+secret = os.getenv("CLOUDFLARE_SECRET_KEY") or ""
+if required and not secret:
+    raise SystemExit("REFUSED")
+print("STARTED")
+"""
+
+
+def _run(env: dict[str, str]) -> str:
+    """The startup decision, in isolation.
+
+    Importing bin/chat-fastapi.py itself would build a graph and need an OpenAI
+    key, so the guard's logic is exercised rather than the module. The test for
+    the two staying in step is below.
+    """
+    result = subprocess.run(  # noqa: S603 - fixed argv, no shell, no user input
+        [sys.executable, "-c", SNIPPET],
+        capture_output=True,
+        text=True,
+        cwd=REPO,
+        env={"PATH": "/usr/bin:/bin", **env},
+    )
+    return (result.stdout + result.stderr).strip()
+
+
+def test_no_key_and_no_opt_out_refuses_to_start() -> None:
+    assert "REFUSED" in _run({})
+
+
+def test_a_key_starts_the_chat() -> None:
+    assert "STARTED" in _run({"CLOUDFLARE_SECRET_KEY": "0x-secret"})
+
+
+@pytest.mark.parametrize("value", ["0", "false", "no"])
+def test_an_explicit_opt_out_starts_without_a_key(value: str) -> None:
+    """Beta ran ungated deliberately. That stays possible -- it just has to be
+    written down rather than inferred from an absent value."""
+    assert "STARTED" in _run({"CHAT_REQUIRES_HUMAN": value})
+
+
+@pytest.mark.parametrize("value", ["1", "true", "yes", "", "anything-else"])
+def test_anything_but_an_explicit_off_still_requires_a_key(value: str) -> None:
+    """A typo in the opt-out must not silently disable the check."""
+    assert "REFUSED" in _run({"CHAT_REQUIRES_HUMAN": value})
+
+
+def test_the_guard_in_the_app_matches_the_one_tested_here() -> None:
+    """The snippet above is a copy, so pin that it has not drifted."""
+    source = (REPO / "bin" / "chat-fastapi.py").read_text()
+    assert 'os.getenv("CHAT_REQUIRES_HUMAN", "1").strip() not in {' in source
+    for off in ('"0",', '"false",', '"no",'):
+        assert off in source
+    assert "if CHAT_REQUIRES_HUMAN and not CLOUDFLARE_SECRET_KEY:" in source