diff --git a/CHANGELOG.md b/CHANGELOG.md
index 7fd2718..6e57433 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -2,6 +2,17 @@
 
 ## Unreleased
 
+* Fix: `write_dlkcat_input` and the OpenKineticsPredictor input builder
+  raised `TypeError: cannot use 'list' as a set element` on any model that
+  had been saved and reloaded. cobra returns an annotation value as a list
+  for a model read from a file, so `annotation['smiles']` is `"CCO"` on a
+  model annotated in memory by `find_met_smiles` and `["CCO"]` on the same
+  model after a round trip, and the ignore-list check hashed it. The new
+  `databases.met_smiles` reads either shape; a reloaded model now writes
+  the same DLKcat input as a freshly annotated one.
+* Fix: `parse_okp_output` raised `TypeError: cannot use 'list' as a dict
+  key` on a reloaded model for the same reason, when indexing metabolites
+  by their SMILES annotation to map each prediction back to an `ec.rxn`.
 * Fix: evotune's trust tiers never applied to a fuzzy-matched kcat.
   `ec.source` reads `"brenda (wc=0, origin=1)"` where MATLAB's reads
   `"brenda"`, and `classify_kcat_source` compared the whole string against
diff --git a/src/geckopy/databases/__init__.py b/src/geckopy/databases/__init__.py
index 0a71b50..0f88da5 100644
--- a/src/geckopy/databases/__init__.py
+++ b/src/geckopy/databases/__init__.py
@@ -13,7 +13,7 @@
 from .pax_db_loader import ProtData, load_pax_db
 from .phyl_dist import PhylDist, download_phyl_dist, load_phyl_dist
 from .prot_data_loader import load_prot_data
-from .pubchem import find_met_smiles
+from .pubchem import find_met_smiles, met_smiles
 from .uniprot_download import download_uniprot
 from .uniprot_loader import UniprotDB, load_uniprot_tsv
 
@@ -41,4 +41,5 @@
     "load_phyl_dist",
     "load_prot_data",
     "load_uniprot_tsv",
+    "met_smiles",
 ]
diff --git a/src/geckopy/databases/pubchem.py b/src/geckopy/databases/pubchem.py
index 3a933a4..edcee5a 100644
--- a/src/geckopy/databases/pubchem.py
+++ b/src/geckopy/databases/pubchem.py
@@ -42,6 +42,24 @@
 _FIRST_LINE_PATTERN = re.compile(r"^(\S*)\n")
 
 
+def met_smiles(metabolite) -> str:
+    """The metabolite's SMILES, whatever shape its annotation is in.
+
+    cobra stores an annotation value as the object it was assigned, and
+    returns a *list* for a model read back from a file: a metabolite
+    annotated in memory by :func:`find_met_smiles` carries ``"CCO"``,
+    and the same metabolite after a save/load round trip carries
+    ``["CCO"]``. Readers that assume a string get a list that compares
+    equal to nothing and is not hashable.
+
+    Returns the first non-empty entry, or ``""`` when there is none.
+    """
+    value = metabolite.annotation.get("smiles", "")
+    if isinstance(value, (list, tuple, set)):
+        return next((str(v) for v in value if v), "")
+    return str(value) if value else ""
+
+
 def find_met_smiles(
     model: "cobra.Model",
     *,
diff --git a/src/geckopy/gather_kcats/open_kinetics_predictor/parse_output.py b/src/geckopy/gather_kcats/open_kinetics_predictor/parse_output.py
index fd811ef..55b8945 100644
--- a/src/geckopy/gather_kcats/open_kinetics_predictor/parse_output.py
+++ b/src/geckopy/gather_kcats/open_kinetics_predictor/parse_output.py
@@ -17,6 +17,8 @@
 
 import pandas as pd
 
+from ...databases.pubchem import met_smiles as read_met_smiles
+
 if TYPE_CHECKING:
     from ...ec_model.ec_model import EcModel
 
@@ -88,7 +90,7 @@ def parse_okp_output(
     seq_to_proteins = _index(model.ec.sequence)
     smiles_to_mets: dict[str, list] = {}
     for met in model.metabolites:
-        smiles = met.annotation.get("smiles", "")
+        smiles = read_met_smiles(met)
         if smiles:
             smiles_to_mets.setdefault(smiles, []).append(met)
 
diff --git a/src/geckopy/gather_kcats/write_dlkcat_input.py b/src/geckopy/gather_kcats/write_dlkcat_input.py
index b1700b0..ea0c1f8 100644
--- a/src/geckopy/gather_kcats/write_dlkcat_input.py
+++ b/src/geckopy/gather_kcats/write_dlkcat_input.py
@@ -14,6 +14,8 @@
 import pandas as pd
 from scipy import sparse
 
+from ..databases.pubchem import met_smiles as read_met_smiles
+
 if TYPE_CHECKING:
     from ..databases.dlkcat_ignore_lists import DLKcatIgnoreLists
     from ..ec_model.ec_model import EcModel
@@ -64,7 +66,8 @@ def write_dlkcat_input(
       SMILES are dropped from the output. Otherwise the SMILES is
       written as the literal string ``"None"``.
 
-    SMILES are read from each metabolite's ``annotation['smiles']``.
+    SMILES are read from each metabolite's ``annotation['smiles']``,
+    which cobra returns as a list for a model loaded from a file.
 
     Parameters
     ----------
@@ -194,7 +197,7 @@ def extract_enzyme_substrate_pairs(
     metabolites = list(model.metabolites)
     met_ids = [m.id for m in metabolites]
     met_names = [m.name for m in metabolites]
-    met_smiles = [m.annotation.get("smiles", "") for m in metabolites]
+    met_smiles = [read_met_smiles(m) for m in metabolites]
     met_normalized = [_normalize(n) for n in met_names]
 
     # Build the ignore mask.
diff --git a/tests/test_open_kinetics_predictor.py b/tests/test_open_kinetics_predictor.py
index 3077627..a34fd14 100644
--- a/tests/test_open_kinetics_predictor.py
+++ b/tests/test_open_kinetics_predictor.py
@@ -169,6 +169,22 @@ def test_parse_maps_back_to_ec_rxns():
     assert set(df["rxn_id"]) == {"r1", "r2"}
 
 
+def test_parse_maps_back_from_a_reloaded_models_list_valued_smiles():
+    """cobra returns annotation values as lists for a model read from a
+    file, which is what `save_ec_model` + `load_ec_model` produces. The
+    SMILES index has to key on the string inside the list, not on the
+    list itself."""
+    model = _ec_model()
+    for met in model.metabolites:
+        if met.annotation.get("smiles"):
+            met.annotation["smiles"] = [met.annotation["smiles"]]
+
+    df = parse_okp_output(model, _OKP_RESULT_CSV)
+    assert len(df) == 3
+    assert set(df["rxn_id"]) == {"r1", "r2"}
+    assert all(s in (["ala"], ["beta"]) for s in df["substrates"])
+
+
 def test_parse_source_provenance_stripped_and_verbatim():
     df = parse_okp_output(_ec_model(), _OKP_RESULT_CSV)
     sources = set(df["source"])
diff --git a/tests/test_pubchem.py b/tests/test_pubchem.py
index 168f60a..4ba33d2 100644
--- a/tests/test_pubchem.py
+++ b/tests/test_pubchem.py
@@ -343,3 +343,28 @@ def test_logs_summary_after_run(monkeypatch, tmp_path, caplog):
         find_met_smiles(model, cache_path=cache_path)
 
     assert "SMILES found for" in caplog.text
+
+
+# --------------------------------------------------------------------------- #
+# Reading the annotation back
+# --------------------------------------------------------------------------- #
+
+def test_met_smiles_accepts_the_shape_a_loaded_model_carries():
+    """cobra returns a list for a model read from a file, a string for one
+    annotated in memory. Both mean the same molecule."""
+    import cobra
+
+    from geckopy.databases import met_smiles
+
+    met = cobra.Metabolite("etoh")
+    met.annotation["smiles"] = "CCO"
+    assert met_smiles(met) == "CCO"
+
+    met.annotation["smiles"] = ["CCO"]
+    assert met_smiles(met) == "CCO"
+
+    met.annotation["smiles"] = []
+    assert met_smiles(met) == ""
+
+    del met.annotation["smiles"]
+    assert met_smiles(met) == ""
diff --git a/tests/test_write_dlkcat_input.py b/tests/test_write_dlkcat_input.py
index ea2e773..cf9593f 100644
--- a/tests/test_write_dlkcat_input.py
+++ b/tests/test_write_dlkcat_input.py
@@ -417,3 +417,40 @@ def test_written_file_has_no_header(tmp_path):
     first_line = out.read_text(encoding="utf-8").splitlines()[0]
     # Should be the data row, NOT a header.
     assert first_line.startswith("r1\t")
+
+
+def test_a_reloaded_model_carries_its_smiles_as_a_list(tmp_path):
+    """cobra returns annotation values as lists for a model read from a
+    file, which is what `save_ec_model` + `load_ec_model` produces. The
+    SMILES has to survive that, and the ignore-list check has to not
+    choke on it."""
+    model = _ec_model(
+        [("r1", [("A", -1.0, "alpha"), ("B", 1.0, "beta")])],
+        genes_per_rxn={"r1": ["g1"]},
+        gene_sequences={"g1": "MASEQ"},
+        met_smiles={"A": "C(C)O"},
+    )
+    for met in model.metabolites:
+        if met.annotation.get("smiles"):
+            met.annotation["smiles"] = [met.annotation["smiles"]]
+
+    df = write_dlkcat_input(model, tmp_path / "DLKcat.tsv", _ignore_lists())
+    assert len(df) == 1
+    assert df.iloc[0]["smiles"] == "C(C)O"
+
+
+def test_a_list_valued_smiles_is_still_matched_against_the_ignore_list(tmp_path):
+    model = _ec_model(
+        [("r1", [("A", -1.0, "alpha"), ("B", -1.0, "beta"), ("C", 1.0, "gamma")])],
+        genes_per_rxn={"r1": ["g1"]},
+        gene_sequences={"g1": "MASEQ"},
+        met_smiles={"A": "SMI_A", "B": "O"},
+    )
+    for met in model.metabolites:
+        if met.annotation.get("smiles"):
+            met.annotation["smiles"] = [met.annotation["smiles"]]
+
+    df = write_dlkcat_input(
+        model, tmp_path / "DLKcat.tsv", _ignore_lists(ignore_smiles=["O"]),
+    )
+    assert set(df["substrate"]) == {"alpha"}