diff --git a/backend/app/alembic/versions/047_backfill_legacy_config_completion_type.py b/backend/app/alembic/versions/047_backfill_legacy_config_completion_type.py
new file mode 100644
index 00000000..bd243b81
--- /dev/null
+++ b/backend/app/alembic/versions/047_backfill_legacy_config_completion_type.py
@@ -0,0 +1,29 @@
+"""backfill legacy config completion type
+
+Revision ID: 047
+Revises: 046
+Create Date: 2026-02-17 00:00:00.000000
+
+"""
+from alembic import op
+
+revision = "047"
+down_revision = "046"
+branch_labels = None
+depends_on = None
+
+
+def upgrade() -> None:
+    op.execute(
+        """
+        UPDATE config_version
+        SET config_blob = jsonb_set(config_blob, '{completion,type}', '"text"')
+        WHERE config_blob->'completion' IS NOT NULL
+          AND config_blob->'completion'->>'type' IS NULL
+        """
+    )
+
+
+def downgrade() -> None:
+    # No-op: removing type='text' would drop a required field and break validation; NULL safely defaults to 'text'.
+    pass
diff --git a/backend/app/crud/config/version.py b/backend/app/crud/config/version.py
index b3da74f1..04a80693 100644
--- a/backend/app/crud/config/version.py
+++ b/backend/app/crud/config/version.py
@@ -156,6 +156,10 @@ def _validate_immutable_fields(
         existing_type = existing_completion.get("type")
         merged_type = merged_completion.get("type")
 
+        # Legacy configs predate the 'type' field; all were text-only
+        if existing_type is None:
+            existing_type = "text"
+
         if existing_type != merged_type:
             raise HTTPException(
                 status_code=400,
@@ -274,7 +278,11 @@ def _validate_config_type_unchanged(
             version_create.config_blob.model_dump().get("completion", {}).get("type")
         )
 
-        if old_type is None or new_type is None:
+        # Legacy configs predate the 'type' field; all were text-only
+        if old_type is None:
+            old_type = "text"
+
+        if new_type is None:
             logger.error(
                 f"[ConfigVersionCrud._validate_config_type_unchanged] Missing type field | "
                 f"{{'config_id': '{self.config_id}', 'old_type': {old_type}, 'new_type': {new_type}}}"
diff --git a/backend/app/tests/crud/config/test_version.py b/backend/app/tests/crud/config/test_version.py
index dfbe137a..dccb54f4 100644
--- a/backend/app/tests/crud/config/test_version.py
+++ b/backend/app/tests/crud/config/test_version.py
@@ -419,6 +419,78 @@ def test_create_version_different_configs(
     assert version2_config2.config_id == config2.id
 
 
+def test_validate_immutable_fields_legacy_config_allows_text_update(
+    db: Session, example_config_blob: ConfigBlob
+) -> None:
+    """Test that a legacy config (no type field) allows updates with type='text'."""
+    config = create_test_config(db)
+    version_crud = ConfigVersionCrud(
+        session=db, project_id=config.project_id, config_id=config.id
+    )
+
+    # Simulate a legacy config_blob without 'type' in completion
+    legacy_blob = {
+        "completion": {
+            "provider": "openai-native",
+            "params": {
+                "model": "gpt-4",
+                "temperature": 0.7,
+                "max_tokens": 1000,
+            },
+        }
+    }
+    merged_blob = {
+        "completion": {
+            "provider": "openai-native",
+            "type": "text",
+            "params": {
+                "model": "gpt-4",
+                "temperature": 0.8,
+                "max_tokens": 1500,
+            },
+        }
+    }
+
+    # Should NOT raise — legacy configs default to "text"
+    version_crud._validate_immutable_fields(legacy_blob, merged_blob)
+
+
+def test_validate_immutable_fields_legacy_config_rejects_non_text_update(
+    db: Session, example_config_blob: ConfigBlob
+) -> None:
+    """Test that a legacy config (no type field) rejects updates with type != 'text'."""
+    config = create_test_config(db)
+    version_crud = ConfigVersionCrud(
+        session=db, project_id=config.project_id, config_id=config.id
+    )
+
+    legacy_blob = {
+        "completion": {
+            "provider": "openai-native",
+            "params": {
+                "model": "gpt-4",
+                "temperature": 0.7,
+            },
+        }
+    }
+    merged_blob = {
+        "completion": {
+            "provider": "openai-native",
+            "type": "stt",
+            "params": {
+                "model": "gpt-4",
+                "temperature": 0.8,
+            },
+        }
+    }
+
+    with pytest.raises(
+        HTTPException,
+        match="Cannot change config type from 'text' to 'stt'",
+    ):
+        version_crud._validate_immutable_fields(legacy_blob, merged_blob)
+
+
 def test_read_all_versions_config_not_found(db: Session) -> None:
     """Test reading versions for a non-existent config raises HTTPException."""
     project = create_test_project(db)