diff --git a/.github/workflows/test.yml b/.github/workflows/test.yml
index 1853a47..dc4e038 100644
--- a/.github/workflows/test.yml
+++ b/.github/workflows/test.yml
@@ -27,3 +27,39 @@ jobs:
     - run: pytest tests/ -v
       env:
         INTERSCRIPT_MAPS_PATH: ${{ github.workspace }}/maps/maps
+
+  conformance-matrix:
+    name: Conformance matrix (frozen fixtures)
+    runs-on: ubuntu-latest
+    steps:
+    - uses: actions/checkout@v4
+    - uses: actions/setup-python@v5
+      with:
+        python-version: "3.12"
+    - run: pip install -e ".[dev]"
+    - name: Download fixtures (GitHub Releases channel)
+      run: |
+        mkdir -p fixtures && cd fixtures
+        BASE="https://github.com/interscript/interscript-models/releases/download"
+        curl -fsSL -o models-index.yaml "$BASE/index-v12/models-index.yaml"
+        curl -fsSL -o models-index.yaml.sha256 "$BASE/index-v12/models-index.yaml.sha256"
+        echo "$(cat models-index.yaml.sha256)" | sha256sum -c
+        for v in urd-diac-base fas-diac-base urd-g2p-base; do
+          curl -fsSL -O "$BASE/index-v12/$v-vectors.json"
+        done
+        # the two single-zip models; urd-g2p (split parts) resolves
+        # via the runtime resolver in a later matrix pass
+        for m in urd-diac-base-1.0 fas-diac-base-1.0; do
+          url=$(sed -n "/^  $m:/,/^  [a-z]/p" models-index.yaml | grep -m1 'url:' | awk '{print $2}')
+          curl -fsSL -o "$m.zip" "$url"
+        done
+    - name: Run the matrix
+      run: |
+        for pair in "urd-diac-base-1.0 urd-diac-base" "fas-diac-base-1.0 fas-diac-base"; do
+          set -- $pair
+          echo "::group::$1"
+          SECRYST_E2E_ZIP="$PWD/fixtures/$1.zip" \
+            CONFORMANCE_VECTORS="$PWD/fixtures/$2-vectors.json" \
+            pytest tests_ml/test_conformance.py -v
+          echo "::endgroup::"
+        done
diff --git a/src/interscript/ml/model.py b/src/interscript/ml/model.py
index fcc084e..9677a29 100644
--- a/src/interscript/ml/model.py
+++ b/src/interscript/ml/model.py
@@ -31,6 +31,10 @@ def __init__(self, zip_path: Path | str):
 
         options = ort.SessionOptions()
         options.graph_optimization_level = ort.GraphOptimizationLevel.ORT_ENABLE_ALL
+        # single-thread decode: the conformance contract is byte-exact
+        # across environments (WO79); multi-thread fp16 ties flip rows
+        options.intra_op_num_threads = 1
+        options.inter_op_num_threads = 1
         graphs = verify_and_read(self.zip_path)
         self._encoder = ort.InferenceSession(
             graphs["encoder.onnx"], options, providers=_providers()
diff --git a/tests_ml/test_conformance.py b/tests_ml/test_conformance.py
new file mode 100644
index 0000000..ea48d50
--- /dev/null
+++ b/tests_ml/test_conformance.py
@@ -0,0 +1,36 @@
+"""The frozen-fixture contract (interscript-models/conformance/) — the
+python runtime must reproduce a model's vectors byte-exactly under the
+declared decode mode, or the change does not merge.
+
+Gated on env: SECRYST_E2E_ZIP (the artifact) + CONFORMANCE_VECTORS
+(the fixture file), so CI runs without downloads."""
+
+from __future__ import annotations
+
+import json
+import os
+
+import pytest
+
+ort = pytest.importorskip("onnxruntime")
+
+from interscript.ml import Model  # noqa: E402
+
+
+def test_conformance_vectors_byte_exact() -> None:
+    vectors_path = os.environ.get("CONFORMANCE_VECTORS")
+    zip_path = os.environ.get("SECRYST_E2E_ZIP")
+    if not vectors_path or not zip_path:
+        pytest.skip("set CONFORMANCE_VECTORS and SECRYST_E2E_ZIP")
+    vectors = json.loads(open(vectors_path, encoding="utf-8").read())
+    assert vectors["decode"]["mode"] == "single-row-greedy"
+    model = Model.load(zip_path)
+    failures = []
+    for v in vectors["vectors"]:
+        got = model.translate(v["src"])
+        if got != v["out"]:
+            failures.append(f"{v['src']!r} -> {got!r} (expected {v['out']!r})")
+    assert not failures, (
+        f"{vectors['model']}: {len(failures)}/{len(vectors['vectors'])} "
+        f"mismatched\n" + "\n".join(failures[:3])
+    )