diff --git a/central/alembic/versions/0020_not_null_timestamps.py b/central/alembic/versions/0020_not_null_timestamps.py
new file mode 100644
index 0000000..abe0241
--- /dev/null
+++ b/central/alembic/versions/0020_not_null_timestamps.py
@@ -0,0 +1,63 @@
+"""Enforce NOT NULL on server-defaulted timestamps the models already require.
+
+0011, 0012, 0014 and 0017 created these columns with ``server_default=now()``
+but without ``nullable=False``, so the database accepted NULL while the models
+declare them non-optional. No application path writes NULL to them (inserts
+omit them and rely on the default; updates set ``now``), and before this
+revision was written no NULL existed in production or in the development
+database.
+
+The upgrade refuses to run — changing nothing — if any NULL is found, instead
+of guessing a value for it.
+
+Revision ID: 0020_not_null_timestamps
+Revises: 0019_fix_timestamp_defaults
+"""
+from alembic import op
+import sqlalchemy as sa
+
+revision = "0020_not_null_timestamps"
+down_revision = "0019_fix_timestamp_defaults"
+branch_labels = None
+depends_on = None
+
+COLUMNS = (
+    ("admin_users", "created_at"),
+    ("agent_events", "occurred_at"),
+    ("agent_snapshots", "updated_at"),
+    ("ai_eval_cache", "created_at"),
+    ("ai_runs", "started_at"),
+    ("ai_work_items", "created_at"),
+    ("ai_work_items", "updated_at"),
+    ("audit_events", "occurred_at"),
+    ("fiscal_context_entries", "created_at"),
+    ("fiscal_context_entries", "updated_at"),
+    ("installations", "created_at"),
+    ("installations", "updated_at"),
+    ("kb_knowledge", "created_at"),
+    ("kb_observations", "observed_at"),
+    ("kb_products", "created_at"),
+    ("kb_products", "updated_at"),
+    ("tenant_product_context", "created_at"),
+    ("tenant_product_context", "updated_at"),
+    ("web_sessions", "created_at"),
+    ("web_sessions", "last_seen_at"),
+)
+
+
+def upgrade():
+    bind = op.get_bind()
+    offenders = []
+    for table, column in COLUMNS:
+        nulls = bind.execute(sa.text(f'SELECT count(*) FROM "{table}" WHERE "{column}" IS NULL')).scalar()
+        if nulls:
+            offenders.append(f"{table}.{column}={nulls}")
+    if offenders:
+        raise RuntimeError("0020 aborted, NULL values present (fix them explicitly first): " + ", ".join(offenders))
+    for table, column in COLUMNS:
+        op.alter_column(table, column, nullable=False, existing_type=sa.DateTime(timezone=True), existing_server_default=sa.func.now())
+
+
+def downgrade():
+    for table, column in reversed(COLUMNS):
+        op.alter_column(table, column, nullable=True, existing_type=sa.DateTime(timezone=True), existing_server_default=sa.func.now())