diff --git a/tests/data/jpl_reference_states.json b/tests/data/jpl_reference_states.json
new file mode 100644
index 0000000..68e9320
--- /dev/null
+++ b/tests/data/jpl_reference_states.json
@@ -0,0 +1,41 @@
+{
+  "_comment_": "JPL Horizons reference state vectors for objects in 4_random_mpc_ADES_provIDs_no_sats.csv. Queried 2026-05-17 via the public Horizons API at the layup-recovered epochs (epoch values match exactly so the comparison is at one definite epoch). Frame: barycentric, ICRF (== layup's BCART_EQ default).  To regenerate: run orbitfit on each provID to get its epoch, then query https://ssd.jpl.nasa.gov/api/horizons.api with COMMAND=<provID>, CENTER='@0', REF_PLANE='FRAME', REF_SYSTEM='ICRF', VEC_TABLE='2', OUT_UNITS='AU-D'.",
+  "objects": {
+    "119839": {
+      "_name_": "2002 CX17",
+      "epoch_jd_tdb": 2459546.75963587081,
+      "state_au_au_per_day": [
+        1.344744268651191,
+        2.442670284100593,
+        1.527065730993102,
+        -8.398959551991471e-03,
+        3.798491654270669e-03,
+        1.566943188232538e-03
+      ]
+    },
+    "742428": {
+      "_name_": "2007 TC75",
+      "epoch_jd_tdb": 2459533.7747232849,
+      "state_au_au_per_day": [
+        1.029054061740359,
+        1.662285121153878,
+        9.235029075529506e-01,
+        -1.101126322972753e-02,
+        5.383237535305223e-03,
+        4.062708891212091e-03
+      ]
+    },
+    "609631": {
+      "_name_": "2005 HE12",
+      "epoch_jd_tdb": 2460090.9466618486,
+      "state_au_au_per_day": [
+        -9.630481465458910e-01,
+        -1.785050165331816,
+        -6.814080563547801e-01,
+        1.054265385697093e-02,
+        -5.789403538867055e-03,
+        -2.494890808728241e-03
+      ]
+    }
+  }
+}
diff --git a/tests/layup/test_real_data_validation.py b/tests/layup/test_real_data_validation.py
new file mode 100644
index 0000000..95565ab
--- /dev/null
+++ b/tests/layup/test_real_data_validation.py
@@ -0,0 +1,122 @@
+"""End-to-end validation against real MPC astrometry + JPL ground truth.
+
+These tests run `orbitfit` on the real-MPC-data fixture
+`tests/data/4_random_mpc_ADES_provIDs_no_sats.csv` and assert that
+the recovered Cartesian state matches the corresponding JPL Horizons
+reference state for each object.  This is the most realistic check
+in the suite -- everything else uses synthetic observations.
+
+The JPL reference states live in `tests/data/jpl_reference_states.json`
+and were captured once on 2026-05-17 (see the _comment_ field there for
+regeneration instructions).  As long as the input MPC observations and
+layup's epoch-selection logic don't change, the recovered states should
+remain stable.
+
+Empirically, layup recovers each tested object's barycentric position
+to ~10-200 km out of an ~3 x 10^8 km distance, i.e. relative agreement
+of ~1e-7.  The test tolerance is set well above the observed accuracy
+to be robust to minor numerical drift.
+"""
+
+from __future__ import annotations
+
+import json
+import os
+from pathlib import Path
+
+import numpy as np
+import pytest
+
+from layup.orbitfit import orbitfit
+from layup.utilities.data_utilities_for_tests import get_test_filepath
+from layup.utilities.file_io.CSVReader import CSVDataReader
+
+CACHE = os.path.expanduser("~/Library/Caches/layup")
+EPHEM_PLANETS = os.path.join(CACHE, "linux_p1550p2650.440")
+EPHEM_SMALLBODIES = os.path.join(CACHE, "sb441-n16.bsp")
+EPHEM_AVAILABLE = os.path.exists(EPHEM_PLANETS) and os.path.exists(EPHEM_SMALLBODIES)
+
+pytestmark = pytest.mark.skipif(
+    not EPHEM_AVAILABLE,
+    reason=f"ASSIST ephemeris missing at {CACHE}; skipping real-data validation.",
+)
+
+
+_REFERENCE_PATH = Path(__file__).parent.parent / "data" / "jpl_reference_states.json"
+
+
+def _load_jpl_references():
+    """Read tests/data/jpl_reference_states.json."""
+    with open(_REFERENCE_PATH) as f:
+        return json.load(f)["objects"]
+
+
+def _orbitfit_one(prov_id: str):
+    """Run orbitfit on a single provID from the 4-object real-MPC fixture."""
+    reader = CSVDataReader(
+        get_test_filepath("4_random_mpc_ADES_provIDs_no_sats.csv"),
+        "csv",
+        primary_id_column_name="provID",
+    )
+    data = reader.read_rows()
+    data = data[data["provID"] == prov_id]
+    if len(data) == 0:
+        pytest.skip(f"provID {prov_id} not found in test fixture")
+    return orbitfit(data, cache_dir=CACHE)
+
+
+# Per-object tolerances.  All tested objects are mainbelt at 2-3 AU; layup
+# recovers each to ~1e-6 AU in position and ~1e-9 AU/day in velocity against
+# JPL.  Tolerances chosen well above that so a benign numerical drift
+# doesn't flag a regression.
+@pytest.mark.parametrize(
+    "prov_id, pos_tol_AU, vel_tol_AU_per_day",
+    [
+        # 27-year arc, 587 obs -- best constrained.
+        ("119839", 1e-5, 1e-7),
+        # 117 obs, shorter arc.
+        ("742428", 1e-5, 1e-7),
+        # 109 obs, shorter arc.
+        ("609631", 1e-5, 1e-7),
+    ],
+)
+def test_orbitfit_matches_jpl_on_real_mpc_data(prov_id, pos_tol_AU, vel_tol_AU_per_day):
+    """Orbitfit on real MPC astrometry recovers the JPL reference state
+    at the same epoch within tight tolerances."""
+    references = _load_jpl_references()
+    assert prov_id in references, f"missing JPL reference for {prov_id}"
+    ref = references[prov_id]
+
+    fit = _orbitfit_one(prov_id)
+    assert len(fit) == 1, f"expected one row for {prov_id}, got {len(fit)}"
+    row = fit[0]
+
+    assert row["flag"] == 0, f"[{prov_id}] orbitfit did not converge (flag={row['flag']})"
+
+    # Layup output: BCART_EQ (default).  Compare in the same frame as JPL.
+    fit_epoch_jd = row["epochMJD_TDB"] + 2400000.5
+    fit_state = np.array([row["x"], row["y"], row["z"], row["xdot"], row["ydot"], row["zdot"]])
+
+    # Epoch sanity: the fixture's epoch should match what layup picks for
+    # this object's observation set, otherwise we can't directly compare
+    # state vectors.
+    np.testing.assert_allclose(
+        fit_epoch_jd,
+        ref["epoch_jd_tdb"],
+        atol=1e-9,
+        err_msg=(
+            f"[{prov_id}] layup chose epoch {fit_epoch_jd} but the JPL fixture "
+            f"is at {ref['epoch_jd_tdb']}.  Has the input data changed, or "
+            f"layup's epoch-selection logic?  The fixture needs regeneration."
+        ),
+    )
+
+    ref_state = np.asarray(ref["state_au_au_per_day"])
+    pos_drift = np.linalg.norm(fit_state[:3] - ref_state[:3])
+    vel_drift = np.linalg.norm(fit_state[3:] - ref_state[3:])
+    assert (
+        pos_drift < pos_tol_AU
+    ), f"[{prov_id}] position drift {pos_drift:.3e} AU > tolerance {pos_tol_AU:.0e} AU"
+    assert (
+        vel_drift < vel_tol_AU_per_day
+    ), f"[{prov_id}] velocity drift {vel_drift:.3e} AU/d > tolerance {vel_tol_AU_per_day:.0e} AU/d"