diff --git a/pm3py/sim/__init__.py b/pm3py/sim/__init__.py
index 99e08b2..23c8ead 100644
--- a/pm3py/sim/__init__.py
+++ b/pm3py/sim/__init__.py
@@ -16,68 +16,27 @@ from .mcu_bridge import McuBridge
 from .dual_session import DualInterfaceSession
 
 # --- Transponder models (canonical home: pm3py.transponders) ---
-from pm3py.transponders.hf.iso14443a.base import (
-    Tag14443A, Tag14443A_3, Tag14443A_4, Reader14443A, State14443A,
-)
-from pm3py.transponders.hf.iso14443a.nxp.crypto1 import Crypto1
-from pm3py.transponders.hf.iso14443a.nxp.mifare_classic import (
-    MifareClassicTag, MifareClassicReader, MifareAccessConditions,
-)
-from pm3py.transponders.hf.iso14443a.nxp.desfire import DesfireTag, DesfireReader
-from pm3py.transponders.hf.iso14443a.ndef import NfcType2Tag, NfcType4Tag
-from pm3py.transponders.hf.iso14443a.nxp.type2 import NxpType2Tag, Type2Config
-from pm3py.transponders.hf.iso14443a.nxp.ntag21x import (
-    Ntag21x, NTAG210, NTAG212, NTAG213, NTAG215, NTAG216, Ntag21xConfig,
-)
-from pm3py.transponders.hf.iso14443a.nxp.ultralight import (
-    MifareUltralight, MifareUltralightC, MifareUltralightEV1, MF0UL11, MF0UL21,
-    UL_C_DEFAULT_KEY, UltralightEV1Config, UltralightCConfig,
-)
-from pm3py.transponders.hf.iso14443a.nxp.ntag_i2c import (
-    NtagI2C, NT3H2111, NT3H2211, NtagI2CConfig, NtagI2CSession, NtagI2CPtI2C,
-)
-from pm3py.transponders.hf.iso14443a.st.st25tn import ST25TN, ST25TN512, ST25TN01K
-from pm3py.transponders.hf.iso14443a.infineon.optiga_nbt import OptigaAuthenticateNBT
-from pm3py.transponders.hf.iso14443a.st.st25ta import (
-    ST25TA, ST25TA512B, ST25TA02KB, ST25TA16K, ST25TA64K,
-)
-from pm3py.transponders.hf.iso15693.base import Tag15693, Reader15693, State15693
-from pm3py.transponders.hf.iso15693.type5 import NfcType5Tag, ndef_text, ndef_uri, ndef_mime
-from pm3py.transponders.hf.iso15693.nxp.nxp_icode import NxpIcodeTag
-from pm3py.transponders.hf.iso15693.nxp.icode_slix import IcodeSlixTag
-from pm3py.transponders.hf.iso15693.nxp.icode_slix2 import IcodeSlix2Tag
-from pm3py.transponders.hf.iso15693.nxp.icode3 import Icode3Tag, IcodeFeatureFlags
-from pm3py.transponders.hf.iso15693.nxp.icode_dna import IcodeDnaTag
-from pm3py.transponders.hf.iso15693.nxp.auth_aes import NxpKeyPrivileges
-from pm3py.transponders.hf.iso15693.nxp.ntag5_platform import (
-    Ntag5PlatformTag, Ntag5StatusReg, Ntag5ConfigReg,
-)
-from pm3py.transponders.hf.iso15693.nxp.ntag5_switch import Ntag5SwitchTag
-from pm3py.transponders.hf.iso15693.nxp.ntag5_link import Ntag5LinkTag
-from pm3py.transponders.hf.iso15693.nxp.ntag5_boost import Ntag5BoostTag
-from pm3py.transponders.hf.iso15693.st.st25tv import ST25TV, ST25TV512C, ST25TV02KC
-from pm3py.transponders.hf.iso15693.st.st25dv import (
-    ST25DV, ST25DV04K, ST25DV16K, ST25DV64K, St25dvRfAreaSS,
-)
-from pm3py.transponders.hf.iso15693.infineon.myd_vicinity import (
-    MydVicinity, SRF55V02P, SRF55V10P,
-)
-from pm3py.transponders.hf.iso14443b.base import Tag14443B, StateB, Tag14443B_4, StateB4
-from pm3py.transponders.hf.iso14443b.st.st25tb import (
-    ST25TB, ST25TB512AC, ST25TB512AT, ST25TB02K, ST25TB04K,
-)
-from pm3py.transponders.hf.iso14443b.ti.rf430cl330h import RF430CL330H
-from pm3py.transponders.hf.iso15693.ti.tagit import TagItHFIPlus
-from pm3py.transponders.hf.iso15693.ti.rf430frl import (
-    RF430FRL15xH, RF430FRL152H, RF430FRL153H, RF430FRL154H,
-)
-from pm3py.transponders.lf.base import TagLF, ReaderLF, Modulation
-from pm3py.transponders.lf.em.em4100 import EM4100Tag, EM4100Reader, EM4100Code
-from pm3py.transponders.lf.hid.hid import HIDProxTag, HIDReader
-from pm3py.transponders.bitfield import BitField, Register
-from pm3py.transponders.lf.atmel.t5577 import T5577Tag, T5577Reader, T5577Config
-from pm3py.transponders.lf.nxp.hitag2 import Hitag2Tag, Hitag2Reader
-from pm3py.transponders.implants import xEM, xNT, xM1, FlexDF, NExT, MagicMifareClassicTag
+# Loaded lazily (PEP 562) from ._models. Those models import back into pm3py.sim for the
+# Transponder/Reader/RFFrame/Medium primitives, so importing them at package-init time would
+# create a transponders<->sim circular import (breaking e.g. `from pm3py.transponders...` as a
+# first import). On first access they load from ._models and cache into this module's globals.
+def __getattr__(name):
+    if name.startswith("__") and name.endswith("__"):
+        raise AttributeError(name)                 # don't trigger a model import for dunder probes
+    import importlib
+
+    models = importlib.import_module(f"{__name__}._models")
+    try:
+        value = getattr(models, name)
+    except AttributeError:
+        raise AttributeError(f"module {__name__!r} has no attribute {name!r}") from None
+    globals()[name] = value                        # cache: subsequent access is a direct lookup
+    return value
+
+
+def __dir__():
+    return sorted(set(globals()) | set(__all__))
+
 
 __all__ = [
     "RFFrame",
diff --git a/pm3py/sim/_models.py b/pm3py/sim/_models.py
new file mode 100644
index 0000000..269b7f7
--- /dev/null
+++ b/pm3py/sim/_models.py
@@ -0,0 +1,71 @@
+"""Transponder-model re-exports for :mod:`pm3py.sim`.
+
+Loaded lazily by the package's PEP 562 ``__getattr__`` — these models import back into
+``pm3py.sim`` (for the ``Transponder`` / ``Reader`` / ``RFFrame`` / ``Medium`` primitives), so
+importing them at ``sim`` package-init time creates a circular import. Their canonical home is
+``pm3py.transponders``; this module just re-exposes them under ``pm3py.sim``.
+"""
+from __future__ import annotations
+
+from pm3py.transponders.hf.iso14443a.base import (
+    Tag14443A, Tag14443A_3, Tag14443A_4, Reader14443A, State14443A,
+)
+from pm3py.transponders.hf.iso14443a.nxp.crypto1 import Crypto1
+from pm3py.transponders.hf.iso14443a.nxp.mifare_classic import (
+    MifareClassicTag, MifareClassicReader, MifareAccessConditions,
+)
+from pm3py.transponders.hf.iso14443a.nxp.desfire import DesfireTag, DesfireReader
+from pm3py.transponders.hf.iso14443a.ndef import NfcType2Tag, NfcType4Tag
+from pm3py.transponders.hf.iso14443a.nxp.type2 import NxpType2Tag, Type2Config
+from pm3py.transponders.hf.iso14443a.nxp.ntag21x import (
+    Ntag21x, NTAG210, NTAG212, NTAG213, NTAG215, NTAG216, Ntag21xConfig,
+)
+from pm3py.transponders.hf.iso14443a.nxp.ultralight import (
+    MifareUltralight, MifareUltralightC, MifareUltralightEV1, MF0UL11, MF0UL21,
+    UL_C_DEFAULT_KEY, UltralightEV1Config, UltralightCConfig,
+)
+from pm3py.transponders.hf.iso14443a.nxp.ntag_i2c import (
+    NtagI2C, NT3H2111, NT3H2211, NtagI2CConfig, NtagI2CSession, NtagI2CPtI2C,
+)
+from pm3py.transponders.hf.iso14443a.st.st25tn import ST25TN, ST25TN512, ST25TN01K
+from pm3py.transponders.hf.iso14443a.infineon.optiga_nbt import OptigaAuthenticateNBT
+from pm3py.transponders.hf.iso14443a.st.st25ta import (
+    ST25TA, ST25TA512B, ST25TA02KB, ST25TA16K, ST25TA64K,
+)
+from pm3py.transponders.hf.iso15693.base import Tag15693, Reader15693, State15693
+from pm3py.transponders.hf.iso15693.type5 import NfcType5Tag, ndef_text, ndef_uri, ndef_mime
+from pm3py.transponders.hf.iso15693.nxp.nxp_icode import NxpIcodeTag
+from pm3py.transponders.hf.iso15693.nxp.icode_slix import IcodeSlixTag
+from pm3py.transponders.hf.iso15693.nxp.icode_slix2 import IcodeSlix2Tag
+from pm3py.transponders.hf.iso15693.nxp.icode3 import Icode3Tag, IcodeFeatureFlags
+from pm3py.transponders.hf.iso15693.nxp.icode_dna import IcodeDnaTag
+from pm3py.transponders.hf.iso15693.nxp.auth_aes import NxpKeyPrivileges
+from pm3py.transponders.hf.iso15693.nxp.ntag5_platform import (
+    Ntag5PlatformTag, Ntag5StatusReg, Ntag5ConfigReg,
+)
+from pm3py.transponders.hf.iso15693.nxp.ntag5_switch import Ntag5SwitchTag
+from pm3py.transponders.hf.iso15693.nxp.ntag5_link import Ntag5LinkTag
+from pm3py.transponders.hf.iso15693.nxp.ntag5_boost import Ntag5BoostTag
+from pm3py.transponders.hf.iso15693.st.st25tv import ST25TV, ST25TV512C, ST25TV02KC
+from pm3py.transponders.hf.iso15693.st.st25dv import (
+    ST25DV, ST25DV04K, ST25DV16K, ST25DV64K, St25dvRfAreaSS,
+)
+from pm3py.transponders.hf.iso15693.infineon.myd_vicinity import (
+    MydVicinity, SRF55V02P, SRF55V10P,
+)
+from pm3py.transponders.hf.iso14443b.base import Tag14443B, StateB, Tag14443B_4, StateB4
+from pm3py.transponders.hf.iso14443b.st.st25tb import (
+    ST25TB, ST25TB512AC, ST25TB512AT, ST25TB02K, ST25TB04K,
+)
+from pm3py.transponders.hf.iso14443b.ti.rf430cl330h import RF430CL330H
+from pm3py.transponders.hf.iso15693.ti.tagit import TagItHFIPlus
+from pm3py.transponders.hf.iso15693.ti.rf430frl import (
+    RF430FRL15xH, RF430FRL152H, RF430FRL153H, RF430FRL154H,
+)
+from pm3py.transponders.lf.base import TagLF, ReaderLF, Modulation
+from pm3py.transponders.lf.em.em4100 import EM4100Tag, EM4100Reader, EM4100Code
+from pm3py.transponders.lf.hid.hid import HIDProxTag, HIDReader
+from pm3py.transponders.bitfield import BitField, Register
+from pm3py.transponders.lf.atmel.t5577 import T5577Tag, T5577Reader, T5577Config
+from pm3py.transponders.lf.nxp.hitag2 import Hitag2Tag, Hitag2Reader
+from pm3py.transponders.implants import xEM, xNT, xM1, FlexDF, NExT, MagicMifareClassicTag
diff --git a/pm3py/sim/fuzzer.py b/pm3py/sim/fuzzer.py
index 77442e8..c9f9e39 100644
--- a/pm3py/sim/fuzzer.py
+++ b/pm3py/sim/fuzzer.py
@@ -6,7 +6,6 @@ from typing import Iterator
 
 from bitarray import bitarray
 from .frame import RFFrame
-from pm3py.transponders.hf.iso14443a.base import CL1, NVB_SELECT, _compute_bcc
 
 
 class MutationFuzzer:
@@ -76,6 +75,9 @@ class GrammarFuzzer:
         return None
 
     def _gen_14443a(self, command: str, **overrides) -> RFFrame | None:
+        # deferred: iso14443a.base imports back into pm3py.sim, so a module-level import here would
+        # create a transponders<->sim circular import when 14443a is the first thing imported
+        from pm3py.transponders.hf.iso14443a.base import CL1, NVB_SELECT, _compute_bcc
         match command:
             case "REQA":
                 return RFFrame.from_bytes(b"\x26")
diff --git a/tests/test_import_hygiene.py b/tests/test_import_hygiene.py
new file mode 100644
index 0000000..23c260a
--- /dev/null
+++ b/tests/test_import_hygiene.py
@@ -0,0 +1,46 @@
+"""Guards against the transponders<->sim circular import.
+
+Transponder models import ``pm3py.sim`` primitives (Transponder/Reader/RFFrame/Medium), so
+``pm3py.sim`` must NOT eagerly re-import transponder models at package-init time — its model
+re-exports are lazy (see ``pm3py/sim/_models.py`` + the package ``__getattr__``), and ``fuzzer.py``
+defers its ``iso14443a.base`` import. Each family must therefore import as a *fresh* first import.
+These run in subprocesses because import-order bugs only show in a clean interpreter.
+"""
+import subprocess
+import sys
+
+import pytest
+
+# each of these, imported first in a clean interpreter, used to trigger a circular import
+_FRESH_IMPORTS = [
+    "pm3py.transponders.hf.iso15693.type5",   # the reported failure
+    "pm3py.transponders.hf.iso14443a.base",   # the fuzzer-side failure
+    "pm3py.transponders.hf.iso14443b.base",
+    "pm3py.transponders.lf.atmel.t5577",
+    "pm3py.transponders.implants",
+    "pm3py.sim.fuzzer",
+    "pm3py.sim",
+]
+
+
+@pytest.mark.parametrize("module", _FRESH_IMPORTS)
+def test_fresh_import_has_no_cycle(module):
+    r = subprocess.run([sys.executable, "-c", f"import {module}"],
+                       capture_output=True, text=True)
+    assert r.returncode == 0, f"importing {module} first failed:\n{r.stderr}"
+
+
+def test_sim_lazy_reexports_resolve():
+    import pm3py.sim as sim
+    from pm3py.transponders.hf.iso15693.type5 import NfcType5Tag
+
+    assert sim.NfcType5Tag is NfcType5Tag                    # lazy re-export == canonical class
+    assert "NfcType5Tag" in dir(sim)
+    assert all(getattr(sim, n, None) is not None for n in sim.__all__)
+
+
+def test_fuzzer_deferred_import_works():
+    from pm3py.sim.fuzzer import GrammarFuzzer
+
+    frame = GrammarFuzzer("14443a").generate("SELECT", uid=b"\x04\x11\x22\x33")
+    assert frame.data.hex() == "93700411223304"             # CL1, NVB_SELECT, uid, bcc