diff --git a/src/aegis_gitea_mcp/security.py b/src/aegis_gitea_mcp/security.py
index a3bdafe..e8b2bb3 100644
--- a/src/aegis_gitea_mcp/security.py
+++ b/src/aegis_gitea_mcp/security.py
@@ -90,7 +90,11 @@ def sanitize_data(value: Any, mode: str = "mask") -> Any:
         Sanitized payload value.
     """
     if isinstance(value, dict):
-        return {str(key): sanitize_data(item, mode=mode) for key, item in value.items()}
+        # Keys are data too: a secret can sit in an object key (``{"<token>": 1}``).
+        return {
+            sanitize_data(str(key), mode=mode): sanitize_data(item, mode=mode)
+            for key, item in value.items()
+        }
 
     if isinstance(value, list):
         return [sanitize_data(item, mode=mode) for item in value]
diff --git a/tests/security/test_auth_required.py b/tests/security/test_auth_required.py
new file mode 100644
index 0000000..5dec5c5
--- /dev/null
+++ b/tests/security/test_auth_required.py
@@ -0,0 +1,302 @@
+"""The MCP endpoint rejects every request that does not carry a valid credential."""
+
+from __future__ import annotations
+
+import hashlib
+import hmac
+from typing import Any
+
+import pytest
+from pytest_httpx import HTTPXMock
+from starlette.applications import Starlette
+from starlette.routing import Route
+from starlette.testclient import TestClient
+
+from aegis_gitea_mcp import auth as auth_module
+from aegis_gitea_mcp.auth import StaticBearerVerifier
+from aegis_gitea_mcp.oauth.provider import TOKEN_PREFIX
+from aegis_gitea_mcp.transports import build_http_app
+from tests.test_oauth_helpers import (
+    API,
+    GITEA_TOKEN,
+    USER_TOKEN,
+    FakeClock,
+    browser,
+    build_oauth_app,
+    mcp_client,
+    mcp_status_lifespan,
+    sign_in,
+)
+from tests.test_server_helpers import BEARER_TOKEN, server_settings
+
+MCP_BODY = {"jsonrpc": "2.0", "id": 1, "method": "tools/list", "params": {}}
+HOST = "http://127.0.0.1:8080"
+
+
+@pytest.fixture(autouse=True)
+def _env(mock_env: None) -> None:
+    """The audit logger and the rate limiter read the process-wide settings."""
+
+
+def bearer_app() -> Starlette:
+    return build_http_app(server_settings(mcp_bearer_token=BEARER_TOKEN))
+
+
+def post_mcp(client: TestClient, authorization: str | None) -> Any:
+    headers = {"Content-Type": "application/json", "Accept": "application/json, text/event-stream"}
+    if authorization is not None:
+        headers["Authorization"] = authorization
+    return client.post("/mcp", json=MCP_BODY, headers=headers)
+
+
+# No credential, wrong credential
+
+
+def test_mcp_without_a_bearer_is_401_with_a_challenge_on_the_bearer_app() -> None:
+    client = TestClient(bearer_app(), base_url=HOST)
+
+    response = post_mcp(client, None)
+
+    assert response.status_code == 401
+    assert response.headers["www-authenticate"].lower().startswith("bearer")
+
+
+def test_mcp_without_a_bearer_is_401_with_a_challenge_on_the_oauth_app() -> None:
+    client = browser(build_oauth_app().app)
+
+    response = post_mcp(client, None)
+
+    assert response.status_code == 401
+    challenge = response.headers["www-authenticate"]
+    assert challenge.lower().startswith("bearer")
+    assert "resource_metadata" in challenge  # tells clients where to find the OAuth server
+
+
+@pytest.mark.parametrize("method", ["GET", "POST", "DELETE"])
+def test_every_method_on_the_endpoint_needs_authentication(method: str) -> None:
+    client = TestClient(bearer_app(), base_url=HOST)
+
+    assert client.request(method, "/mcp").status_code == 401
+
+
+WRONG_BEARERS = {
+    "same length": "x" * len(BEARER_TOKEN),
+    "one character off": BEARER_TOKEN[:-1] + ("A" if BEARER_TOKEN[-1] != "A" else "B"),
+    "shorter": BEARER_TOKEN[:-1],
+    "longer": BEARER_TOKEN + "x",
+    "prefix only": BEARER_TOKEN[:8],
+    "doubled": BEARER_TOKEN * 2,
+    "upper case": BEARER_TOKEN.upper(),
+    "trailing space": BEARER_TOKEN + " ",
+    "trailing newline": BEARER_TOKEN + "\n",
+    "another deployment": "another-deployments-bearer-0123456789abcdef",
+    "admin gitea token": GITEA_TOKEN,
+    "null byte": BEARER_TOKEN + "\x00",
+}
+
+
+@pytest.mark.parametrize("name", WRONG_BEARERS)
+def test_a_wrong_bearer_is_401(name: str) -> None:
+    client = TestClient(bearer_app(), base_url=HOST)
+
+    try:
+        response = post_mcp(client, f"Bearer {WRONG_BEARERS[name]}")
+    except Exception as exc:  # an invalid header value is refused before it leaves the client
+        assert "header" in str(exc).lower() or isinstance(exc, (ValueError, UnicodeError))
+        return
+
+    assert response.status_code == 401
+
+
+@pytest.mark.parametrize(
+    "header",
+    [
+        "",
+        "Bearer",
+        "Bearer ",
+        f"Basic {BEARER_TOKEN}",
+        f"token {BEARER_TOKEN}",
+        f"Token {BEARER_TOKEN}",
+        BEARER_TOKEN,
+        f"Bearer  {BEARER_TOKEN} extra",
+        f"Digest {BEARER_TOKEN}",
+    ],
+)
+def test_wrong_scheme_or_shape_of_the_authorization_header_is_401(header: str) -> None:
+    client = TestClient(bearer_app(), base_url=HOST)
+
+    assert post_mcp(client, header).status_code == 401
+
+
+def test_the_right_bearer_is_not_401() -> None:
+    assert mcp_status_lifespan(bearer_app(), BEARER_TOKEN) != 401
+
+
+def test_the_bearer_is_never_accepted_from_a_query_string_or_a_cookie() -> None:
+    client = TestClient(bearer_app(), base_url=HOST)
+
+    by_query = client.post(f"/mcp?access_token={BEARER_TOKEN}&token={BEARER_TOKEN}", json=MCP_BODY)
+    client.cookies.set("token", BEARER_TOKEN)
+    by_cookie = client.post("/mcp", json=MCP_BODY)
+
+    assert by_query.status_code == 401 and by_cookie.status_code == 401
+
+
+# Constant-time comparison
+
+
+async def test_the_static_bearer_is_compared_in_constant_time_on_digests(
+    monkeypatch: pytest.MonkeyPatch,
+) -> None:
+    seen: list[tuple[Any, Any]] = []
+    real = hmac.compare_digest
+
+    def spy(a: Any, b: Any) -> bool:
+        seen.append((a, b))
+        return real(a, b)
+
+    monkeypatch.setattr(auth_module.hmac, "compare_digest", spy)
+    verifier = StaticBearerVerifier(BEARER_TOKEN)
+
+    for candidate in ("", "x", BEARER_TOKEN[:-1], BEARER_TOKEN, BEARER_TOKEN + "x" * 500):
+        accepted = await verifier.verify_token(candidate)
+        assert (accepted is not None) == (candidate == BEARER_TOKEN)
+
+    assert len(seen) == 5
+    # Digests of equal length whatever the candidate length: nothing about the length leaks.
+    assert {len(a) for a, _ in seen} == {len(b) for _, b in seen} == {32}
+    assert all(isinstance(a, bytes) and isinstance(b, bytes) for a, b in seen)
+    assert seen[3][0] == hashlib.sha256(BEARER_TOKEN.encode()).digest()
+
+
+def test_the_source_has_no_plain_equality_on_the_secret() -> None:
+    import inspect
+
+    source = inspect.getsource(StaticBearerVerifier)
+
+    assert "compare_digest" in source
+    assert "== self._token" not in source and "self._token ==" not in source
+
+
+async def test_the_access_token_of_the_static_bearer_never_carries_the_secret() -> None:
+    access = await StaticBearerVerifier(BEARER_TOKEN).verify_token(BEARER_TOKEN)
+
+    assert access is not None
+    assert BEARER_TOKEN not in repr(access) and BEARER_TOKEN not in access.model_dump_json()
+
+
+# OAuth tokens
+
+
+def test_unknown_expired_revoked_and_foreign_oauth_tokens_are_401(httpx_mock: HTTPXMock) -> None:
+    clock = FakeClock()
+    oauth = build_oauth_app(clock=clock)
+    web = browser(oauth.app)
+    flow = sign_in(web, httpx_mock)
+    access = flow.tokens["access_token"]
+    assert access.startswith(TOKEN_PREFIX)
+    assert mcp_status_lifespan(oauth.app, access) != 401
+
+    unknown = TOKEN_PREFIX + "A" * 43
+    assert post_mcp(web, f"Bearer {unknown}").status_code == 401
+    assert post_mcp(web, f"Bearer {TOKEN_PREFIX}").status_code == 401
+    assert post_mcp(web, f"Bearer {access}x").status_code == 401
+    assert post_mcp(web, f"Bearer {access[:-1]}").status_code == 401
+
+    # Bound to another resource: the token does not work here.
+    (session,) = oauth.store.sessions.values()
+    real_resource = session.resource
+    session.resource = "https://other.example.com/mcp"
+    assert post_mcp(web, f"Bearer {access}").status_code == 401
+    session.resource = real_resource
+
+    # Revoked by its client.
+    revoked = web.post(
+        "/revoke",
+        data={"token": access, "client_id": flow.client_id, "client_secret": flow.client_secret},
+    )
+    assert revoked.status_code == 200
+    assert post_mcp(web, f"Bearer {access}").status_code == 401
+
+
+def test_an_expired_oauth_token_is_401(httpx_mock: HTTPXMock) -> None:
+    clock = FakeClock()
+    oauth = build_oauth_app(clock=clock)
+    web = browser(oauth.app)
+    flow = sign_in(web, httpx_mock)
+    access = flow.tokens["access_token"]
+
+    clock.advance(3600 + 1)
+
+    assert post_mcp(web, f"Bearer {access}").status_code == 401
+
+
+async def test_an_oauth_token_never_resolves_to_the_admin_identity(
+    httpx_mock: HTTPXMock,
+) -> None:
+    oauth = build_oauth_app()
+    flow = sign_in(browser(oauth.app), httpx_mock)
+    httpx_mock.add_response(url=f"{API}/repos/octo/demo", json={})
+    httpx_mock.add_response(url=f"{API}/admin/users", status_code=403, json={"message": "no"})
+
+    async with mcp_client(oauth.app, flow.tokens["access_token"]) as mcp:
+        await mcp.call_tool("gitea_repo_get", {"owner": "octo", "repo": "demo"})
+        await mcp.call_tool("gitea_admin_search_users", {})
+
+    calls = [
+        r for r in httpx_mock.get_requests() if "/repos/" in str(r.url) or "/admin/" in str(r.url)
+    ]
+    assert len(calls) == 2
+    for request in calls:
+        assert request.headers["Authorization"] == f"Bearer {USER_TOKEN}"
+        assert GITEA_TOKEN not in request.headers["Authorization"]
+
+
+def test_an_oauth_only_deployment_has_no_static_bearer_to_guess(httpx_mock: HTTPXMock) -> None:
+    from tests.test_oauth_helpers import oauth_settings
+
+    oauth = build_oauth_app(oauth_settings(gitea_token=None, mcp_bearer_token=None))
+    web = browser(oauth.app)
+
+    assert post_mcp(web, f"Bearer {BEARER_TOKEN}").status_code == 401
+    assert post_mcp(web, None).status_code == 401
+
+
+# Route inventory
+
+#: Routes reachable without authentication, with the reason each one exists.
+PUBLIC_ROUTES = {
+    "/": "service banner",
+    "/health": "container and client health check",
+    "/.well-known/oauth-protected-resource": "RFC 9728 discovery (bare path, OAuth only)",
+    "/.well-known/oauth-protected-resource/mcp": "RFC 9728 discovery",
+    "/.well-known/oauth-authorization-server": "RFC 8414 discovery (OAuth only)",
+    "/authorize": "OAuth authorization endpoint (OAuth only)",
+    "/token": "OAuth token endpoint (OAuth only)",
+    "/register": "OAuth dynamic client registration (OAuth only)",
+    "/revoke": "OAuth revocation (OAuth only)",
+    "/oauth/consent": "consent page (OAuth only)",
+    "/oauth/callback": "Gitea callback (OAuth only)",
+}
+
+
+def inventory(app: Starlette) -> tuple[set[str], set[str]]:
+    """Paths without authentication, and paths guarded by the SDK's bearer middleware."""
+    open_paths: set[str] = set()
+    guarded: set[str] = set()
+    for route in app.routes:
+        assert isinstance(route, Route)
+        endpoint = type(route.endpoint).__name__
+        (guarded if endpoint == "RequireAuthMiddleware" else open_paths).add(route.path)
+    return open_paths, guarded
+
+
+def test_the_routes_reachable_without_authentication_are_exactly_the_known_ones() -> None:
+    oauth_open, oauth_guarded = inventory(build_oauth_app().app)
+    bearer_open, bearer_guarded = inventory(bearer_app())
+
+    assert oauth_open == set(PUBLIC_ROUTES)
+    assert bearer_open <= set(PUBLIC_ROUTES)
+    assert {"/", "/health"} <= bearer_open
+    # The MCP endpoint is the only guarded route, and it is guarded in both modes.
+    assert oauth_guarded == bearer_guarded == {"/mcp"}
diff --git a/tests/security/test_redaction.py b/tests/security/test_redaction.py
new file mode 100644
index 0000000..eedf5e8
--- /dev/null
+++ b/tests/security/test_redaction.py
@@ -0,0 +1,306 @@
+"""Credentials never reach results, errors, logs, the audit file or ``repr`` output.
+
+Every secret the server knows (the Gitea token, the MCP bearer, the OAuth client secret,
+the store key, a user's Gitea access and refresh tokens and our own issued tokens) is
+planted in Gitea's answers, successful and failed, and in the incoming requests.
+"""
+
+from __future__ import annotations
+
+import json
+import logging
+from typing import Any
+
+import pytest
+from pytest_httpx import HTTPXMock
+
+from aegis_gitea_mcp.audit import get_audit_logger
+from aegis_gitea_mcp.identity import Identity
+from aegis_gitea_mcp.oauth.provider import PendingAuthorization
+from aegis_gitea_mcp.oauth.store import SessionRecord
+from aegis_gitea_mcp.oauth.upstream import UpstreamTokens
+from aegis_gitea_mcp.transports import AccessLogQueryFilter, run_http
+from tests.test_oauth_helpers import (
+    API,
+    APP_CLIENT_SECRET,
+    BEARER_TOKEN,
+    ENCRYPTION_KEY,
+    GITEA_TOKEN,
+    USER_REFRESH,
+    USER_TOKEN,
+    browser,
+    build_oauth_app,
+    mcp_client,
+    oauth_settings,
+    sign_in,
+)
+
+
+@pytest.fixture(autouse=True)
+def _env(mock_env: None) -> None:
+    """The audit logger and the rate limiter read the process-wide settings."""
+
+
+def static_secrets() -> list[str]:
+    return [GITEA_TOKEN, BEARER_TOKEN, APP_CLIENT_SECRET, ENCRYPTION_KEY, USER_TOKEN, USER_REFRESH]
+
+
+def everything_secret(extra: list[str]) -> str:
+    return " ".join([*static_secrets(), *extra])
+
+
+def assert_clean(text: str, secrets: list[str]) -> None:
+    for secret in secrets:
+        assert secret not in text, f"secret leaked: {secret[:6]}..."
+
+
+@pytest.mark.parametrize("mode", ["off", "mask", "block"])
+async def test_no_secret_reaches_a_result_the_audit_file_or_the_logs(
+    httpx_mock: HTTPXMock,
+    caplog: pytest.LogCaptureFixture,
+    capsys: pytest.CaptureFixture[str],
+    mode: str,
+) -> None:
+    caplog.set_level(logging.DEBUG)
+    oauth = build_oauth_app(oauth_settings(secret_detection_mode=mode))
+    flow = sign_in(browser(oauth.app), httpx_mock)
+    # The client secret the SDK issues at registration is plain hex without a recognisable
+    # prefix: it is stored (encrypted) but cannot be pattern masked, and Gitea never sees it.
+    issued = [flow.tokens["access_token"], flow.tokens["refresh_token"]]
+    # SECRET_DETECTION_MODE=off disables pattern matching, which is what protects our own
+    # issued agm_ tokens (only hashes of them live on the server, so they cannot be redacted
+    # exactly); every other secret is redacted exactly in all modes.
+    secrets = static_secrets() if mode == "off" else [*static_secrets(), *issued]
+    # The admin identity is not the OAuth user: Gitea never shows it that user's tokens, so
+    # only the server's own secrets (and, with pattern masking, our issued tokens) apply.
+    admin_secrets = [GITEA_TOKEN, BEARER_TOKEN, APP_CLIENT_SECRET, ENCRYPTION_KEY]
+    blob = everything_secret(issued)
+
+    # The admin identity's lookup, then success and failure bodies that echo every secret.
+    httpx_mock.add_response(url=f"{API}/user", json={"login": "admin"}, is_reusable=True)
+    echo = {"is_reusable": True}
+    httpx_mock.add_response(
+        url=f"{API}/repos/octo/ok", json={"description": blob, blob: "key"}, **echo
+    )
+    httpx_mock.add_response(
+        url=f"{API}/repos/octo/bad",
+        status_code=422,
+        json={"message": blob, "errors": [blob]},
+        **echo,
+    )
+    httpx_mock.add_response(
+        url=f"{API}/repos/octo/text", text=blob, headers={"content-type": "text/plain"}, **echo
+    )
+    httpx_mock.add_response(url=f"{API}/repos/octo/boom", status_code=500, text=blob, **echo)
+
+    results: list[Any] = []
+    # One app per identity: the SDK's session manager can only run once per app.
+    apps = [
+        (oauth.app, flow.tokens["access_token"]),
+        (build_oauth_app(oauth.settings).app, BEARER_TOKEN),
+    ]
+    for app, token in apps:
+        async with mcp_client(app, token) as mcp:
+            for repo in ("ok", "bad", "boom"):
+                results.append(
+                    await mcp.call_tool("gitea_repo_get", {"owner": "octo", "repo": repo})
+                )
+            results.append(
+                await mcp.call_tool(
+                    "gitea_api_request", {"method": "GET", "path": "/repos/octo/text"}
+                )
+            )
+
+    texts = [result.content[0].text for result in results]
+    per_identity = [
+        (results[:4], secrets),
+        (results[4:], [*admin_secrets, *(issued if mode != "off" else [])]),
+    ]
+    for group, forbidden in per_identity:
+        for result in group:
+            assert_clean(result.content[0].text, forbidden)
+            assert_clean(json.dumps(result.structured_content), forbidden)
+    assert any("[REDACTED]" in text for text in texts)  # the redaction happened, not an empty body
+    everything = [*secrets, *admin_secrets]
+    assert_clean(get_audit_logger().log_path.read_text(encoding="utf-8"), everything)
+    assert_clean(caplog.text, everything)
+    captured = capsys.readouterr()
+    assert_clean(captured.out + captured.err, everything)
+
+
+async def test_exceptions_during_a_call_do_not_leak_secrets(
+    httpx_mock: HTTPXMock, caplog: pytest.LogCaptureFixture, monkeypatch: pytest.MonkeyPatch
+) -> None:
+    caplog.set_level(logging.DEBUG)
+    oauth = build_oauth_app()
+    secrets = static_secrets()
+
+    async def explode(*args: Any, **kwargs: Any) -> None:
+        raise RuntimeError("failure carrying " + " ".join(secrets))
+
+    monkeypatch.setattr(oauth.extras.executor, "execute", explode)
+    httpx_mock.add_response(url=f"{API}/user", json={"login": "admin"}, is_reusable=True)
+
+    async with mcp_client(oauth.app, BEARER_TOKEN) as mcp:
+        result = await mcp.call_tool("gitea_repo_get", {"owner": "o", "repo": "r"})
+
+    assert result.is_error
+    assert_clean(result.content[0].text, secrets)
+    assert_clean(caplog.text, secrets)
+    assert_clean(get_audit_logger().log_path.read_text(encoding="utf-8"), secrets)
+    assert "RuntimeError" not in result.content[0].text  # no exception details by default
+
+
+async def test_validation_errors_never_contain_argument_values(httpx_mock: HTTPXMock) -> None:
+    oauth = build_oauth_app()
+    planted = "sekret-value-" + GITEA_TOKEN
+
+    async with mcp_client(oauth.app, BEARER_TOKEN) as mcp:
+        results = [
+            await mcp.call_tool("gitea_repo_get", {"owner": planted + "?x#y", "repo": "r"}),
+            await mcp.call_tool("gitea_repo_get", {"owner": 12345, "repo": planted}),
+            await mcp.call_tool("gitea_repo_get", {"owner": "o", "repo": "r", "extra": planted}),
+            await mcp.call_tool("gitea_api_request", {"method": planted, "path": "/x"}),
+            await mcp.call_tool("gitea_api_request", {"method": "GET", "path": planted}),
+            await mcp.call_tool(
+                "gitea_api_request",
+                {"method": "GET", "path": "/user", "sudo": "not valid " + planted},
+            ),
+        ]
+
+    for result in results:
+        assert result.is_error
+        assert planted not in result.content[0].text
+        assert GITEA_TOKEN not in result.content[0].text
+    assert planted not in get_audit_logger().log_path.read_text(encoding="utf-8")
+
+
+# Access logs
+
+
+def uvicorn_access_record(path_with_query: str) -> logging.LogRecord:
+    return logging.LogRecord(
+        name="uvicorn.access",
+        level=logging.INFO,
+        pathname=__file__,
+        lineno=1,
+        msg='%s - "%s %s HTTP/%s" %d',
+        args=("127.0.0.1:5000", "GET", path_with_query, "1.1", 302),
+        exc_info=None,
+    )
+
+
+@pytest.mark.parametrize(
+    "path",
+    [
+        "/oauth/callback?code=one-time-gitea-code&state=state-secret-value",
+        "/oauth/consent?request=pending-request-secret",
+        "/authorize?client_id=c&code_challenge=abc&state=client-state-secret",
+    ],
+)
+def test_access_log_lines_drop_the_query_string(path: str) -> None:
+    record = uvicorn_access_record(path)
+
+    assert AccessLogQueryFilter().filter(record)
+
+    line = record.getMessage()
+    assert "[redacted]" in line
+    assert path.split("?")[0] in line
+    for secret in (
+        "one-time-gitea-code",
+        "state-secret-value",
+        "pending-request-secret",
+        "client-state-secret",
+        "code_challenge",
+    ):
+        assert secret not in line
+
+
+def test_the_runner_installs_the_access_log_filter(
+    monkeypatch: pytest.MonkeyPatch, caplog: pytest.LogCaptureFixture
+) -> None:
+    from aegis_gitea_mcp import transports
+
+    access_logger = logging.getLogger("uvicorn.access")
+    monkeypatch.setattr(access_logger, "filters", [])
+    monkeypatch.setattr(transports.uvicorn, "run", lambda *a, **k: None)
+    caplog.set_level(logging.INFO, logger="uvicorn.access")
+
+    assert run_http(oauth_settings()) == 0
+    access_logger.info(
+        '%s - "%s %s HTTP/%s" %d',
+        "127.0.0.1:5000",
+        "GET",
+        "/oauth/callback?code=one-time-gitea-code&state=state-secret-value",
+        "1.1",
+        302,
+    )
+
+    assert "one-time-gitea-code" not in caplog.text
+    assert "[redacted]" in caplog.text
+
+
+# repr
+
+
+def test_repr_of_settings_and_secret_holding_objects_contains_no_secret() -> None:
+    settings = oauth_settings()
+    identity = Identity(login="alice", mode="oauth", token=USER_TOKEN, redact=(USER_REFRESH,))
+    session = SessionRecord(
+        session_id="session-id-value",
+        client_id="c1",
+        login="alice",
+        is_admin=False,
+        gitea_access_token=USER_TOKEN,
+        gitea_refresh_token=USER_REFRESH,
+        gitea_expires_at=None,
+        scopes=["gitea"],
+        resource="https://mcp.example.com/mcp",
+        created_at=0.0,
+        previous_refresh_hash="hash-of-previous-refresh",
+    )
+    upstream = UpstreamTokens(access_token=USER_TOKEN, refresh_token=USER_REFRESH, expires_at=None)
+    pending = PendingAuthorization(
+        client_id="c1",
+        client_name="Client",
+        redirect_uri="https://client.example/cb",
+        redirect_uri_provided_explicitly=True,
+        code_challenge="challenge",
+        client_state="client-state-secret",
+        scopes=["gitea"],
+        resource="https://mcp.example.com/mcp",
+        code_verifier="pkce-verifier-secret",
+        csrf_hash="csrf-hash-secret",
+        binding_hash="binding-hash-secret",
+    )
+
+    rendered = [
+        repr(settings),
+        str(settings),
+        repr(identity),
+        repr(session),
+        repr(upstream),
+        repr(pending),
+        repr(settings.gitea_token),
+        repr(settings.model_dump()),
+    ]
+
+    for text in rendered:
+        assert_clean(text, [*static_secrets(), "client-state-secret", "pkce-verifier-secret"])
+        assert "csrf-hash-secret" not in text and "binding-hash-secret" not in text
+        assert "hash-of-previous-refresh" not in text
+    assert "**********" in repr(settings.gitea_token)
+
+
+def test_secrets_in_object_keys_are_masked_like_values() -> None:
+    from aegis_gitea_mcp.security import sanitize_data
+
+    token = "agm_" + "A1b2C3d4E5f6G7h8I9j0K1l2M3n4O5p6Q7r8S9t0U1v"
+    nested = {"outer": [{token: "value", "plain": token}]}
+
+    masked = sanitize_data(nested, mode="mask")
+    blocked = sanitize_data(nested, mode="block")
+
+    assert token not in json.dumps(masked) and token not in json.dumps(blocked)
+    assert masked["outer"][0]["plain"] != token
+    assert blocked != nested
diff --git a/tests/security/test_startup_fail_closed.py b/tests/security/test_startup_fail_closed.py
new file mode 100644
index 0000000..ee948ac
--- /dev/null
+++ b/tests/security/test_startup_fail_closed.py
@@ -0,0 +1,223 @@
+"""The server refuses to start unless it is safe to: authentication off loopback, valid settings."""
+
+from __future__ import annotations
+
+from pathlib import Path
+from typing import Any
+
+import pytest
+
+from aegis_gitea_mcp import cli, transports
+from aegis_gitea_mcp.cli import main
+from aegis_gitea_mcp.transports import run_http, run_stdio
+from tests.test_oauth_helpers import oauth_settings
+from tests.test_server_helpers import BEARER_TOKEN, GITEA_TOKEN, server_settings
+
+WILDCARD_V4 = "0.0.0.0"  # noqa: S104 (a bind address used as test input, never bound)
+WILDCARD_V6 = "::"
+NON_LOOPBACK = [
+    WILDCARD_V4,
+    WILDCARD_V6,
+    "[::]",
+    "192.168.1.10",
+    "10.0.0.5",
+    "203.0.113.5",
+    "2001:db8::1",
+    "mcp.example.com",
+    "localhost.evil.example",
+    "gitea.internal",
+    "127.1",  # not a canonical loopback address: refused rather than guessed
+    "0",
+    "128.0.0.1",
+]
+LOOPBACK = ["127.0.0.1", "127.0.0.2", "127.255.255.254", "::1", "[::1]", "localhost", "LOCALHOST"]
+
+
+class Served:
+    """Replaces ``uvicorn.run`` and records whether anything was started."""
+
+    def __init__(self) -> None:
+        self.calls: list[dict[str, Any]] = []
+
+    def __call__(self, app: Any, **kwargs: Any) -> None:
+        self.calls.append(kwargs)
+
+
+@pytest.fixture(autouse=True)
+def served(mock_env: None, monkeypatch: pytest.MonkeyPatch) -> Served:
+    spy = Served()
+    monkeypatch.setattr(transports.uvicorn, "run", spy)
+    monkeypatch.setattr(cli, "configure_logging", lambda level: None)
+    return spy
+
+
+@pytest.mark.parametrize("host", NON_LOOPBACK)
+def test_a_non_loopback_bind_without_authentication_exits_2(
+    host: str, served: Served, capsys: pytest.CaptureFixture[str]
+) -> None:
+    assert run_http(server_settings(mcp_host=host)) == 2
+
+    captured = capsys.readouterr()
+    assert served.calls == []
+    assert "without HTTP authentication" in captured.err
+    assert captured.out == ""
+
+
+@pytest.mark.parametrize("host", NON_LOOPBACK)
+def test_the_serve_command_refuses_the_same_binds(
+    host: str, served: Served, monkeypatch: pytest.MonkeyPatch, capsys: pytest.CaptureFixture[str]
+) -> None:
+    monkeypatch.setenv("GITEA_URL", "https://gitea.example.com")
+    monkeypatch.setenv("GITEA_TOKEN", GITEA_TOKEN)
+
+    assert main(["serve", "--transport", "http", "--host", host]) == 2
+
+    assert served.calls == []
+    assert "without HTTP authentication" in capsys.readouterr().err
+
+
+@pytest.mark.parametrize("host", NON_LOOPBACK[:4])
+def test_a_bearer_is_enough_to_bind_publicly(host: str, served: Served) -> None:
+    assert run_http(server_settings(mcp_host=host, mcp_bearer_token=BEARER_TOKEN)) == 0
+
+    assert len(served.calls) == 1
+
+
+@pytest.mark.parametrize("host", NON_LOOPBACK[:4])
+def test_complete_oauth_is_enough_to_bind_publicly(host: str, served: Served) -> None:
+    settings = oauth_settings(mcp_host=host, mcp_bearer_token=None, gitea_token=None)
+
+    assert run_http(settings) == 0
+    assert len(served.calls) == 1
+
+
+@pytest.mark.parametrize("host", LOOPBACK)
+def test_loopback_binds_may_run_without_authentication(host: str, served: Served) -> None:
+    assert run_http(server_settings(mcp_host=host)) == 0
+
+    assert len(served.calls) == 1
+
+
+# Half-configured authentication
+
+
+@pytest.mark.parametrize(
+    "missing",
+    [
+        "gitea_oauth_client_id",
+        "gitea_oauth_client_secret",
+        "token_encryption_key",
+        "public_base_url",
+    ],
+)
+def test_half_configured_oauth_exits_2_and_never_falls_back_to_no_auth(
+    missing: str, served: Served, capsys: pytest.CaptureFixture[str]
+) -> None:
+    settings = oauth_settings(mcp_host=WILDCARD_V4, mcp_bearer_token=None, **{missing: None})
+
+    assert run_http(settings) == 2
+
+    assert served.calls == []
+    assert missing.upper() in capsys.readouterr().err
+
+
+def test_oauth_with_a_bearer_but_no_admin_token_exits_2(
+    served: Served, capsys: pytest.CaptureFixture[str]
+) -> None:
+    assert run_http(oauth_settings(gitea_token=None)) == 2
+
+    assert served.calls == []
+    assert "GITEA_TOKEN" in capsys.readouterr().err
+
+
+@pytest.mark.parametrize("bearer", ["", "short", "x" * 31])
+def test_a_bearer_shorter_than_32_characters_is_rejected_at_configuration(
+    bearer: str, served: Served, monkeypatch: pytest.MonkeyPatch, capsys: pytest.CaptureFixture[str]
+) -> None:
+    monkeypatch.setenv("GITEA_URL", "https://gitea.example.com")
+    monkeypatch.setenv("GITEA_TOKEN", GITEA_TOKEN)
+    monkeypatch.setenv("MCP_BEARER_TOKEN", bearer)
+    monkeypatch.setenv("MCP_HOST", WILDCARD_V4)
+
+    code = main(["serve", "--transport", "http"])
+
+    if bearer == "":
+        # A blank value counts as unset, so the unauthenticated public bind is refused instead.
+        assert code == 2 and "without HTTP authentication" in capsys.readouterr().err
+    else:
+        assert code == 2
+        assert "invalid configuration" in capsys.readouterr().err
+    assert served.calls == []
+
+
+def test_an_http_public_base_url_on_a_public_host_is_rejected(
+    served: Served, monkeypatch: pytest.MonkeyPatch, capsys: pytest.CaptureFixture[str]
+) -> None:
+    monkeypatch.setenv("GITEA_URL", "https://gitea.example.com")
+    monkeypatch.setenv("GITEA_TOKEN", GITEA_TOKEN)
+    monkeypatch.setenv("PUBLIC_BASE_URL", "http://mcp.example.com")
+
+    assert main(["serve", "--transport", "http"]) == 2
+
+    assert served.calls == []
+    assert "PUBLIC_BASE_URL" in capsys.readouterr().err
+
+
+def test_a_missing_gitea_token_exits_2_for_admin_token_mode_and_stdio(
+    served: Served, capsys: pytest.CaptureFixture[str]
+) -> None:
+    assert run_http(server_settings(gitea_token=None, mcp_bearer_token=BEARER_TOKEN)) == 2
+    assert run_http(server_settings(gitea_token=None)) == 2
+    assert run_stdio(server_settings(gitea_token=None)) == 2
+
+    captured = capsys.readouterr()
+    assert served.calls == []
+    assert "GITEA_TOKEN" in captured.err
+    assert captured.out == ""  # stdout is reserved for JSON-RPC
+
+
+def test_an_unwritable_audit_log_exits_2_before_serving(
+    tmp_path: Path,
+    served: Served,
+    monkeypatch: pytest.MonkeyPatch,
+    capsys: pytest.CaptureFixture[str],
+) -> None:
+    blocker = tmp_path / "blocker"
+    blocker.write_text("a file where a directory is needed")
+    path = blocker / "audit.log"
+    monkeypatch.setenv("AUDIT_LOG_PATH", str(path))
+
+    assert run_http(server_settings(audit_log_path=path)) == 2
+    assert run_stdio(server_settings(audit_log_path=path)) == 2
+
+    assert served.calls == []
+    assert "audit log" in capsys.readouterr().err
+
+
+def test_a_symlinked_audit_path_exits_2(
+    tmp_path: Path,
+    served: Served,
+    monkeypatch: pytest.MonkeyPatch,
+    capsys: pytest.CaptureFixture[str],
+) -> None:
+    target = tmp_path / "elsewhere.log"
+    target.write_text("")
+    link = tmp_path / "audit.log"
+    link.symlink_to(target)
+    monkeypatch.setenv("AUDIT_LOG_PATH", str(link))
+
+    assert run_http(server_settings(audit_log_path=link)) == 2
+
+    assert served.calls == []
+    assert "symbolic link" in capsys.readouterr().err
+    assert target.read_text() == ""
+
+
+def test_unknown_disabled_tools_stop_startup(
+    served: Served, capsys: pytest.CaptureFixture[str]
+) -> None:
+    assert run_http(server_settings(disabled_tools=["gitea_no_such_tool"])) == 2
+    assert run_stdio(server_settings(disabled_tools=["gitea_no_such_tool"])) == 2
+
+    assert served.calls == []
+    assert "gitea_no_such_tool" in capsys.readouterr().err