diff --git a/src/proofbundle/anchors_rfc3161.py b/src/proofbundle/anchors_rfc3161.py
index 4aa476b..a66f0b7 100644
--- a/src/proofbundle/anchors_rfc3161.py
+++ b/src/proofbundle/anchors_rfc3161.py
@@ -46,6 +46,40 @@ def verify_rfc3161(proof: bytes, canonical_root: bytes, *, frozen: dict, now: Op
     stricter-only pin via ``frozen.policyOid``; either way the token's ``TSTInfo.policy`` MUST match or it
     fails closed.
     """
+    # TYPE FLOOR on the trust-config arguments (self-gate finding F3, 2026-07-31; still reproducing on
+    # main sixteen days later, re-measured 2026-08-16). `register_anchor_type` prescribes to THIRD-PARTY
+    # authors that a verifier "MUST be fail-closed … never raise for an ordinary bad proof". This
+    # first-party implementation did not hold its own rule: `frozen` and `rp_trust` are consumed with
+    # `.get(...)` below, so a non-dict raised a raw `AttributeError` out of a verdict-returning surface.
+    #
+    # Measured before the fix, with `[anchors]` installed so the code actually reaches these lines (without
+    # it the function returns at the import guard above and the probe is green for a reason that has
+    # nothing to do with the defence it names):
+    #   rp_trust=123 -> AttributeError: 'int' object has no attribute 'get'
+    #   frozen=123   -> AttributeError: 'int' object has no attribute 'get'
+    #
+    # Why the floor and not a wider except: the same reason the evalcard/prereg floors (L1-01) exist. An
+    # `except AttributeError` would close this one shape and let the next type-confusion sibling through,
+    # and it would swallow a genuine internal AttributeError too. `BundleFormatError` is in the family's
+    # accepted, fail-closed set, so the surface still DECIDES instead of crashing.
+    #
+    # Honest severity, unchanged from the finding: this surface is not exported at package level and
+    # `verify_anchor` wraps every verifier in `except Exception`, so nothing leaked over the public path.
+    # The contradiction is what mattered — the project's own implementation not keeping the rule it
+    # prescribes to others.
+    # The check is `Mapping`, not `dict`, and the difference is not pedantry. The counter-read rejected
+    # the first attempt for testing the IMPLEMENTATION instead of the INTERFACE, and measuring settled it:
+    # every use of these two arguments in this function is `.get(...)` — six call sites, no subscript, no
+    # dict-only method, no mutation. `.get` is part of the `Mapping` protocol, so `MappingProxyType`,
+    # `OrderedDict` and any dict-like object work here and were being rejected for no reason. A floor that
+    # refuses valid input is a defect of its own, just a quieter one than the crash it replaced.
+    from collections.abc import Mapping as _Mapping  # noqa: PLC0415
+    if frozen is not None and not isinstance(frozen, _Mapping):
+        from .errors import BundleFormatError as _BFE  # noqa: PLC0415
+        raise _BFE(f"frozen must be a mapping, got {type(frozen).__name__} (fail-closed)")
+    if rp_trust is not None and not isinstance(rp_trust, _Mapping):
+        from .errors import BundleFormatError as _BFE  # noqa: PLC0415
+        raise _BFE(f"rp_trust must be a mapping, got {type(rp_trust).__name__} (fail-closed)")
     try:
         import rfc3161_client as tsp  # noqa: PLC0415
     except ImportError:
diff --git a/tests/test_anchors_rfc3161_type_floor.py b/tests/test_anchors_rfc3161_type_floor.py
new file mode 100644
index 0000000..9b4eea9
--- /dev/null
+++ b/tests/test_anchors_rfc3161_type_floor.py
@@ -0,0 +1,124 @@
+"""`verify_rfc3161` must hold the never-raise rule it prescribes to third-party authors.
+
+`register_anchor_type` documents that a verifier "MUST be fail-closed … never raise for an ordinary
+bad proof". The self-gate run of 2026-07-31 recorded as F3 that this FIRST-PARTY implementation did
+not hold it: `frozen` and `rp_trust` are consumed with `.get(...)`, so a non-dict raised a raw
+`AttributeError` out of a verdict-returning surface. Re-measured against `main` on 2026-08-16, sixteen
+days later, it still reproduced.
+
+WHY THE FAMILY PROPERTY NEVER CAUGHT IT, and why this file has to exist separately. Two axes of the
+same instrument were blind here:
+
+  * the MODULE axis — `anchors_rfc3161` was outside `_MODULES`, so the property never entered it.
+    That is closed by tests/test_never_raise_population_guard.py.
+  * the ARGUMENT axis — the property fuzzes only the PRIMARY parameter. `frozen` and `rp_trust` are
+    keyword-only, so even inside the population the property does not reach them. That axis is the
+    finding the self-gate recorded as F2 and it is NOT closed; this file covers this one surface
+    directly rather than pretending the general sweep does.
+
+MEASUREMENT PRECONDITION, learned the hard way on 2026-08-16. Without `rfc3161_client` installed the
+function returns at its optional-import guard BEFORE reaching the lines under test, and a probe reads
+green for a reason that has nothing to do with the defence it names — the exact class the fixture
+manifest calls `vacuous_seam_passes_for_a_reason_other_than_the_defence_it_names`. These tests
+therefore SKIP honestly when the extra is absent instead of passing vacuously.
+"""
+from __future__ import annotations
+
+import unittest
+
+from proofbundle.anchors_rfc3161 import verify_rfc3161
+from proofbundle.errors import BundleFormatError
+
+
+def _anchors_extra_present() -> bool:
+    try:
+        import rfc3161_client  # noqa: F401, PLC0415
+        return True
+    except ImportError:
+        return False
+
+
+@unittest.skipUnless(_anchors_extra_present(),
+                     "needs proofbundle[anchors]: without it the function returns at its import guard "
+                     "before the lines under test, and a pass here would be vacuous")
+class TestRfc3161TrustConfigTypeFloor(unittest.TestCase):
+    """Every hostile TYPE on the trust-config arguments must yield a typed error, never a raw one."""
+
+    HOSTILE = [None, 123, 1.5, True, b"bytes", "a string", ["a", "list"], ("t", "u"), object()]
+
+    def test_non_mapping_frozen_is_a_typed_error(self):
+        for bad in self.HOSTILE:
+            if bad is None:
+                continue          # None is the documented "absent" case, handled below
+            with self.subTest(bad=type(bad).__name__):
+                with self.assertRaises(BundleFormatError):
+                    verify_rfc3161(b"", b"", frozen=bad)
+
+    def test_non_mapping_rp_trust_is_a_typed_error(self):
+        for bad in self.HOSTILE:
+            if bad is None:
+                continue          # None is the documented default
+            with self.subTest(bad=type(bad).__name__):
+                with self.assertRaises(BundleFormatError):
+                    verify_rfc3161(b"", b"", frozen={}, rp_trust=bad)
+
+    def test_no_raw_attributeerror_survives(self):
+        """The regression this closes, stated as the shape it had rather than as a description.
+
+        `AttributeError` is in the property's `_FORBIDDEN` set: it is a type-confusion crash signature,
+        which is precisely what a verdict-returning surface must not emit.
+        """
+        for arg, bad in (("frozen", 123), ("rp_trust", 123)):
+            with self.subTest(arg=arg):
+                kwargs = {"frozen": {}, "rp_trust": None}
+                kwargs[arg] = bad
+                try:
+                    verify_rfc3161(b"", b"", **kwargs)
+                except BundleFormatError:
+                    pass                                  # typed, fail-closed: accepted
+                except AttributeError as exc:             # the regression
+                    self.fail(f"{arg}={bad!r} still raises a raw AttributeError: {exc}")
+
+    def test_the_documented_defaults_still_reach_a_verdict(self):
+        """Bidirectional validation: the floor must not turn a legitimate call into an error.
+
+        `rp_trust=None` is the documented default and `frozen={}` is a legitimate empty block. Both must
+        still return the `needs_rp_trust` verdict rather than raise — otherwise the floor has broken the
+        surface it was meant to protect.
+        """
+        res = verify_rfc3161(b"", b"", frozen={}, rp_trust=None)
+        self.assertIsInstance(res, dict)
+        self.assertIs(res.get("ok"), False)
+        self.assertEqual(res.get("status"), "needs_rp_trust")
+
+    def test_a_valid_mapping_is_not_rejected_by_the_floor(self):
+        """A dict passes the floor and the function proceeds to its real work (and fails closed there)."""
+        res = verify_rfc3161(b"", b"", frozen={"rootCertsDerB64": []},
+                             rp_trust={"trusted_tsa_roots": []})
+        self.assertIsInstance(res, dict)
+        self.assertIs(res.get("ok"), False)
+
+    def test_a_mapping_that_is_not_a_dict_is_accepted(self):
+        """The floor tests the INTERFACE, not the implementation — and that distinction was earned.
+
+        The first version of this floor checked `isinstance(x, dict)`. The counter-read rejected it for
+        programming to an implementation, and measuring settled it: every use of these two arguments in
+        `verify_rfc3161` is `.get(...)` — six call sites, no subscript, no dict-only method, no mutation.
+        `.get` belongs to the `Mapping` protocol, so a `MappingProxyType` or an `OrderedDict` works fine
+        and was being refused for no reason. A floor that rejects valid input is a defect of its own,
+        just a quieter one than the crash it replaced.
+        """
+        from collections import OrderedDict
+        from types import MappingProxyType
+        for name, mapping in (("MappingProxyType", MappingProxyType({"rootCertsDerB64": []})),
+                              ("OrderedDict", OrderedDict(rootCertsDerB64=[]))):
+            with self.subTest(mapping=name):
+                res = verify_rfc3161(b"", b"", frozen=mapping, rp_trust=None)
+                self.assertIsInstance(res, dict)
+                self.assertIs(res.get("ok"), False)
+                self.assertEqual(res.get("status"), "needs_rp_trust",
+                                 f"a {name} was rejected by the floor instead of reaching the verdict")
+
+
+if __name__ == "__main__":  # pragma: no cover
+    unittest.main()