diff --git a/src/tpmstream/io/auto/marshal.py b/src/tpmstream/io/auto/marshal.py
index cb80900..783a9af 100644
--- a/src/tpmstream/io/auto/marshal.py
+++ b/src/tpmstream/io/auto/marshal.py
@@ -1,12 +1,12 @@
 import binascii
+import re
 
-from ...spec.structures.interface_types import TPMI_ST_COMMAND_TAG
 from ..binary import Binary
 from ..hex import Hex
 from ..pcapng import Pcapng
 
 
-def detect_format_and_yield_buffer(buffer):
+def detect_format_and_yield_buffer(buffer, strict=True):
     """First yield is format. Rest is buffer bytewise."""
     buffer_iter = iter(buffer)
 
@@ -19,26 +19,29 @@ def detect_format_and_yield_buffer(buffer):
         ) from e
 
     if look_ahead == b"\x0a\x0d":
-        # TODO use enum or some sort of canonical mapping?
         yield "pcapng"
-    elif look_ahead == b"80":
-        yield "hex"
-    elif look_ahead in (
-        tag.to_bytes() for tag in TPMI_ST_COMMAND_TAG._valid_values._values
-    ):
-        yield "binary"
     else:
-        raise IOError(
-            f"Unknown detect input format: magic number is {binascii.hexlify(look_ahead).decode()}"
-        )
+        if re.match(b"[0-9a-fA-F]{2}", look_ahead):
+            # look ahead is valid hex, so it's MAYBE hex
+            if not strict:
+                yield "hex"
+            else:
+                raise IOError(
+                    f"Ambiguous input format: magic number is {binascii.hexlify(look_ahead).decode()}. Could be binary or hex."
+                )
+        else:
+            # not valid hex, so it must be binary
+            yield "binary"
 
     yield from look_ahead
     yield from buffer_iter
 
 
-def marshal(tpm_type, buffer, root_path=None, command_code=None, **kwargs):
+def marshal(
+    tpm_type, buffer, root_path=None, command_code=None, strict=False, **kwargs
+):
     """Generator. Take iterable which yields single bytes. Yield MarshalEvents. Be smart about format."""
-    format_buffer_iter = detect_format_and_yield_buffer(buffer)
+    format_buffer_iter = detect_format_and_yield_buffer(buffer, strict=strict)
 
     format = next(format_buffer_iter)