diff --git a/src/paperless_ai/base_model.py b/src/paperless_ai/base_model.py
index 8af146ee5..da7ebcb5f 100644
--- a/src/paperless_ai/base_model.py
+++ b/src/paperless_ai/base_model.py
@@ -6,6 +6,7 @@ from pydantic import BaseModel
 from pydantic import Field
 from pydantic import ValidationInfo
 from pydantic import field_validator
+from pydantic import model_validator
 from pydantic.fields import FieldInfo
 
 # taxonomy.py MAX_TAG_CANDIDATES = 10, prompt is "up to 3 relevant dates"
@@ -37,6 +38,27 @@ def _truncate_to_field_limit(value: Any, field: FieldInfo) -> Any:
 class TaxonomyChoice(BaseModel):
     """One field's suggestions: existing values to reuse, plus new ones to create."""
 
+    @model_validator(mode="before")
+    @classmethod
+    def _normalize_flat_list(cls, value: Any) -> Any:
+        """Accept the flat list shape used before 3.1 and still emitted by
+        some smaller models despite the nested tool schema. Strings are new
+        names and integers are candidate IDs; the latter remain subject to
+        the shown-candidate allowlist in ai_classifier.py.
+        """
+        if not isinstance(value, list):
+            return value
+        if not all(
+            isinstance(item, str)
+            or (isinstance(item, int) and not isinstance(item, bool))
+            for item in value
+        ):
+            return value
+        return {
+            "existing_ids": [item for item in value if isinstance(item, int)],
+            "new_names": [item for item in value if isinstance(item, str)],
+        }
+
     existing_ids: list[int] = Field(
         default_factory=list,
         max_length=MAX_EXISTING_IDS,
diff --git a/src/paperless_ai/tests/test_base_model.py b/src/paperless_ai/tests/test_base_model.py
index caf6ea32a..775938ce9 100644
--- a/src/paperless_ai/tests/test_base_model.py
+++ b/src/paperless_ai/tests/test_base_model.py
@@ -43,6 +43,43 @@ def test_document_classifier_schema_declared_defaults():
     assert dumped["dates"] == []
 
 
+def test_flat_taxonomy_lists_are_normalized_for_legacy_model_responses():
+    """
+    GIVEN:
+        - A model response using the flat taxonomy lists accepted before 3.1
+        - Strings, integer candidate IDs, and a mixture of both
+    WHEN:
+        - DocumentClassifierSchema validates the response
+    THEN:
+        - Strings become new_names and integers become existing_ids
+
+    Some smaller models emit the old flat shape even when shown the nested
+    tool schema. Candidate IDs are still restricted to the IDs actually shown
+    to the model later in ai_classifier.py.
+    """
+    parsed = DocumentClassifierSchema(
+        title="Electricity Bill",
+        tags=["Utilities", "Electricity"],
+        correspondents=[12],
+        document_types=[34, "Utility Bill"],
+        storage_paths=["Finance/Utilities"],
+    )
+
+    assert parsed.tags == TaxonomyChoice(
+        existing_ids=[],
+        new_names=["Utilities", "Electricity"],
+    )
+    assert parsed.correspondents == TaxonomyChoice(existing_ids=[12], new_names=[])
+    assert parsed.document_types == TaxonomyChoice(
+        existing_ids=[34],
+        new_names=["Utility Bill"],
+    )
+    assert parsed.storage_paths == TaxonomyChoice(
+        existing_ids=[],
+        new_names=["Finance/Utilities"],
+    )
+
+
 def test_document_classifier_schema_json_schema_is_self_contained():
     """
     GIVEN: