diff --git a/surfsense_backend/alembic/versions/134_add_thread_auto_model_pinning_fields.py b/surfsense_backend/alembic/versions/134_add_thread_auto_model_pinning_fields.py
new file mode 100644
index 000000000..ab1643b02
--- /dev/null
+++ b/surfsense_backend/alembic/versions/134_add_thread_auto_model_pinning_fields.py
@@ -0,0 +1,63 @@
+"""134_add_thread_auto_model_pinning_fields
+
+Revision ID: 134
+Revises: 133
+Create Date: 2026-04-29
+
+Add thread-level fields to persist Auto (Fastest) model pinning metadata:
+- pinned_llm_config_id: concrete resolved config id used for this thread
+- pinned_auto_mode: auto policy identifier (currently "auto_fastest")
+- pinned_at: timestamp when the pin was created/refreshed
+"""
+
+from __future__ import annotations
+
+from collections.abc import Sequence
+
+import sqlalchemy as sa
+
+from alembic import op
+
+revision: str = "134"
+down_revision: str | None = "133"
+branch_labels: str | Sequence[str] | None = None
+depends_on: str | Sequence[str] | None = None
+
+
+def upgrade() -> None:
+    op.add_column(
+        "new_chat_threads",
+        sa.Column("pinned_llm_config_id", sa.Integer(), nullable=True),
+    )
+    op.add_column(
+        "new_chat_threads",
+        sa.Column("pinned_auto_mode", sa.String(length=32), nullable=True),
+    )
+    op.add_column(
+        "new_chat_threads",
+        sa.Column("pinned_at", sa.TIMESTAMP(timezone=True), nullable=True),
+    )
+
+    op.create_index(
+        "ix_new_chat_threads_pinned_llm_config_id",
+        "new_chat_threads",
+        ["pinned_llm_config_id"],
+        unique=False,
+    )
+    op.create_index(
+        "ix_new_chat_threads_pinned_auto_mode",
+        "new_chat_threads",
+        ["pinned_auto_mode"],
+        unique=False,
+    )
+
+
+def downgrade() -> None:
+    op.drop_index("ix_new_chat_threads_pinned_auto_mode", table_name="new_chat_threads")
+    op.drop_index(
+        "ix_new_chat_threads_pinned_llm_config_id", table_name="new_chat_threads"
+    )
+
+    op.drop_column("new_chat_threads", "pinned_at")
+    op.drop_column("new_chat_threads", "pinned_auto_mode")
+    op.drop_column("new_chat_threads", "pinned_llm_config_id")
diff --git a/surfsense_backend/app/db.py b/surfsense_backend/app/db.py
index 75342a8e1..f8b1390d9 100644
--- a/surfsense_backend/app/db.py
+++ b/surfsense_backend/app/db.py
@@ -638,6 +638,13 @@ class NewChatThread(BaseModel, TimestampMixin):
         default=False,
         server_default="false",
     )
+    # Auto model pinning metadata:
+    # - pinned_llm_config_id stores the concrete resolved model config id.
+    # - pinned_auto_mode indicates which auto policy produced the pin.
+    # This allows Auto (Fastest) to resolve once per thread and stay stable.
+    pinned_llm_config_id = Column(Integer, nullable=True, index=True)
+    pinned_auto_mode = Column(String(32), nullable=True, index=True)
+    pinned_at = Column(TIMESTAMP(timezone=True), nullable=True)
 
     # Relationships
     search_space = relationship("SearchSpace", back_populates="new_chat_threads")