diff --git a/hugr-py/tests/conftest.py b/hugr-py/tests/conftest.py
index 909d0a8bfd..e8fdf9dd7b 100644
--- a/hugr-py/tests/conftest.py
+++ b/hugr-py/tests/conftest.py
@@ -10,7 +10,7 @@
 from typing_extensions import Self
 
 from hugr import ext, tys
-from hugr.envelope import EnvelopeConfig
+from hugr.envelope import EnvelopeConfig, EnvelopeFormat
 from hugr.hugr import Hugr
 from hugr.ops import AsExtOp, Command, Const, Custom, DataflowOp, ExtOp, RegisteredOp
 from hugr.package import Package
@@ -181,20 +181,26 @@ def validate(
         if os.environ.get("HUGR_RENDER_DOT"):
             dot.pipe("svg")
 
-    # Encoding formats to test, indexed by the format name as used by
-    # `hugr convert --format`.
+    # Encoding formats to test. Note that these include other formats than
+    # those supported by `hugr convert`.
     FORMATS = {
         "json": EnvelopeConfig.TEXT,
+        "json-compressed": EnvelopeConfig(format=EnvelopeFormat.JSON, zstd=0),
         "model-exts": EnvelopeConfig.BINARY,
+        "model-exts-no-compression": EnvelopeConfig(
+            format=EnvelopeFormat.MODEL_WITH_EXTS, zstd=None
+        ),
     }
     # Envelope formats used when exporting test hugrs.
-    WRITE_FORMATS = ["json", "model-exts"]
-    # Envelope formats used as target for `hugr convert` before loading back the
-    # test hugrs.
-    #
-    # Model envelopes cannot currently be loaded from python.
-    # TODO: Add model envelope loading to python, and add it to the list.
-    LOAD_FORMATS = ["json"]
+    WRITE_FORMATS = [
+        "json",
+        "json-compressed",
+        "model-exts",
+        "model-exts-no-compression",
+    ]
+    # Envelope formats used as target before loading back the test hugrs.
+    # These should correspond to the formats supported by `hugr convert`.
+    LOAD_FORMATS = ["json", "model-exts"]
 
     cmd = [*_base_command(), "validate", "-"]
 
@@ -324,7 +330,7 @@ def _run_hugr_cmd(serial: bytes, cmd: list[str]) -> subprocess.CompletedProcess[
     The `serial` argument is the serialized HUGR to pass to the command via stdin.
     """
     try:
-        return subprocess.run(cmd, check=True, input=serial, capture_output=True)  # noqa: S603
+        return subprocess.run(cmd, check=True, input=serial, capture_output=True)
     except subprocess.CalledProcessError as e:
         error = e.stderr.decode()
         raise RuntimeError(error) from e
diff --git a/hugr-py/tests/test_envelope.py b/hugr-py/tests/test_envelope.py
index b42d4729fc..eb517bde3a 100644
--- a/hugr-py/tests/test_envelope.py
+++ b/hugr-py/tests/test_envelope.py
@@ -30,18 +30,27 @@ def package() -> Package:
     return Package([mod.hugr, mod2.hugr])
 
 
-def test_envelope(package: Package):
-    # Binary compression roundtrip
-    for format in [
+@pytest.mark.parametrize(
+    "compression", [None, 0], ids=["compression:None", "compression:0"]
+)
+@pytest.mark.parametrize(
+    "format",
+    [
         EnvelopeFormat.JSON,
         EnvelopeFormat.MODEL,
         EnvelopeFormat.MODEL_WITH_EXTS,
-    ]:
-        for compression in [None, 0]:
-            encoded = package.to_bytes(EnvelopeConfig(format=format, zstd=compression))
-            decoded = Package.from_bytes(encoded)
-            assert decoded == package
+    ],
+)
+def test_envelope_binary(
+    package: Package, compression: int | None, format: EnvelopeFormat
+):
+    # Binary compression roundtrip
+    encoded = package.to_bytes(EnvelopeConfig(format=format, zstd=compression))
+    decoded = Package.from_bytes(encoded)
+    assert decoded == package
+
 
+def test_envelope_text(package: Package):
     # String roundtrip
     encoded_str = package.to_str(EnvelopeConfig.TEXT)
     decoded = Package.from_str(encoded_str)