diff --git a/src/api/inference.py b/src/api/inference.py
index f30e9af..7a6a98b 100644
--- a/src/api/inference.py
+++ b/src/api/inference.py
@@ -38,11 +38,16 @@ def _zip_path(model_id: str) -> Path:
     # the server's own volume listing; the user value is used solely in
     # an equality comparison, never in path construction (CWE-22)
     import glob
+    import os
+    import sys
+
+    sys.path.insert(0, "/root/interscript-ml")
+    from src.api.model_resolution import load_index, resolve_zip_filename
 
-    wanted = f"{model_id}-fp32.zip"
-    for z in glob.glob("/v/imf/*/*-fp32.zip"):
-        if z.rsplit("/", 1)[1] == wanted:
-            return Path(z)
+    volume_files = [os.path.basename(z) for z in glob.glob("/v/imf/*/*.zip")]
+    filename = resolve_zip_filename(model_id, load_index("models.yaml"), volume_files)
+    for z in glob.glob(f"/v/imf/*/{filename}"):
+        return Path(z)
     raise KeyError(model_id)
 
 
@@ -124,9 +129,15 @@ async def _run_infer(body):
     @api.get("/health")
     def health() -> dict:
         import glob
+        import os
 
         models_volume.reload()
-        return {"ok": True, "models": len(glob.glob("/v/imf/*/*-fp32.zip"))}
+        zips = glob.glob("/v/imf/*/*.zip")
+        return {
+            "ok": True,
+            "zips": len(zips),
+            "model_dirs": len({os.path.dirname(z).rsplit("/", 1)[1] for z in zips}),
+        }
 
     return api
 
diff --git a/src/api/model_resolution.py b/src/api/model_resolution.py
new file mode 100644
index 0000000..629faeb
--- /dev/null
+++ b/src/api/model_resolution.py
@@ -0,0 +1,47 @@
+"""Index-driven model-id resolution: the models.yaml contract, shared
+by every consumer. A model id resolves to the exact index filename
+when the volume carries it, falling back to the precision convention
+(``{id}-{precision}.zip``) and then ``{id}-fp32.zip`` — volume copies
+have historically landed under convention names even when the release
+artifact uses another name (heb-diac-1.1 ships as heb.zip but the
+volume copy is heb-diac-1.1-fp32.zip).
+
+Callers pass the volume listing; candidate names are built server-side
+and matched by equality only — user input never touches path
+construction (CWE-22).
+"""
+
+from __future__ import annotations
+
+import yaml
+from pathlib import Path
+
+
+def load_index(path: str | Path = "models.yaml") -> dict:
+    with open(path, encoding="utf-8") as fh:
+        return yaml.safe_load(fh)["models"]
+
+
+def resolve_zip_filename(model_id: str, index: dict, volume_files: list[str]) -> str:
+    """Return the volume filename serving `model_id`.
+
+    Raises KeyError when the id is unknown or no volume file matches.
+    """
+    entry = index.get(model_id)
+    if entry is None:
+        raise KeyError(model_id)
+
+    candidates: list[str] = []
+    filename = entry.get("filename")
+    if filename:
+        candidates.append(filename)
+    precision = entry.get("precision")
+    if precision:
+        candidates.append(f"{model_id}-{precision}.zip")
+    candidates.append(f"{model_id}-fp32.zip")
+
+    available = set(volume_files)
+    for candidate in candidates:
+        if candidate in available:
+            return candidate
+    raise KeyError(model_id)
diff --git a/tests/test_model_resolution.py b/tests/test_model_resolution.py
new file mode 100644
index 0000000..c72bc8a
--- /dev/null
+++ b/tests/test_model_resolution.py
@@ -0,0 +1,44 @@
+"""Index-driven model-id → volume-filename resolution (the harness
+contract: model ids resolve through models.yaml, never hardcoded
+naming conventions)."""
+
+from pathlib import Path
+
+import yaml
+
+from src.api.model_resolution import resolve_zip_filename
+
+INDEX = yaml.safe_load(Path("models.yaml").read_text())["models"]
+
+
+def test_exact_index_filename_wins():
+    volume = ["ara-diac-small-1.0-fp32.zip", "ara-diac-small-1.0-int8.zip"]
+    assert resolve_zip_filename("ara-diac-small-1.0-int8", INDEX, volume) == (
+        "ara-diac-small-1.0-int8.zip"
+    )
+
+
+def test_precision_fallback_when_volume_name_differs_from_index():
+    # heb-diac-1.1 ships as heb.zip on GH Releases but the volume copy
+    # landed as heb-diac-1.1-fp32.zip
+    volume = ["heb-diac-1.1-fp32.zip", "heb-diac-1.1-fp16.zip"]
+    assert resolve_zip_filename("heb-diac-1.1", INDEX, volume) == "heb-diac-1.1-fp32.zip"
+
+
+def test_int4_variant_resolves():
+    volume = ["tha-g2p-small-1.0-int8.zip", "tha-g2p-small-1.0-int4.zip"]
+    assert resolve_zip_filename("tha-g2p-small-1.0-int4", INDEX, volume) == (
+        "tha-g2p-small-1.0-int4.zip"
+    )
+
+
+def test_fp32_convention_fallback():
+    volume = ["ara-diac-1.0-fp32.zip"]
+    assert resolve_zip_filename("ara-diac-1.0", INDEX, volume) == "ara-diac-1.0-fp32.zip"
+
+
+def test_unknown_id_raises_keyerror():
+    import pytest
+
+    with pytest.raises(KeyError):
+        resolve_zip_filename("nope-1.0", INDEX, ["a-fp32.zip"])