diff --git a/internal/config/store_postgres.go b/internal/config/store_postgres.go
index 9e8c5dd6..5eb49e51 100644
--- a/internal/config/store_postgres.go
+++ b/internal/config/store_postgres.go
@@ -93,7 +93,8 @@ func getGlobalConfigFrom(ctx context.Context, q globalConfigExecutor) (*GlobalCo
 		       COALESCE(ladder_execution_enabled, false),
 		       offering_class,
 		       require_different_approver,
-		       auto_purchase_enabled
+		       auto_purchase_enabled,
+		       auto_purchase_changed_by, auto_purchase_changed_at, auto_purchase_changed_via
 		FROM global_config
 		WHERE id = 1
 	`
@@ -128,6 +129,9 @@ func getGlobalConfigFrom(ctx context.Context, q globalConfigExecutor) (*GlobalCo
 		&config.OfferingClass,
 		&config.RequireDifferentApprover,
 		&config.AutoPurchaseEnabled,
+		&config.AutoPurchaseChangedBy,
+		&config.AutoPurchaseChangedAt,
+		&config.AutoPurchaseChangedVia,
 	)
 
 	if err != nil {
@@ -261,8 +265,9 @@ func saveGlobalConfigWith(ctx context.Context, q globalConfigExecutor, config *G
 			grace_period_days,
 			recommendations_cache_stale_hours, recommendations_lookback_days,
 			purchase_delay_hours, laddering_enabled, ladder_execution_enabled, offering_class,
-			require_different_approver, auto_purchase_enabled
-		) VALUES (1, $1, $2, $3, $4, $5, $6, $7, $8, $9, $10, $11, $12, $13, $14, $15, $16, $17, $18, $19, $20, $21, $22, $23, $24, $25)
+			require_different_approver, auto_purchase_enabled,
+			auto_purchase_changed_by, auto_purchase_changed_at, auto_purchase_changed_via
+		) VALUES (1, $1, $2, $3, $4, $5, $6, $7, $8, $9, $10, $11, $12, $13, $14, $15, $16, $17, $18, $19, $20, $21, $22, $23, $24, $25, $26, $27, $28)
 		ON CONFLICT (id) DO UPDATE SET
 			enabled_providers = $1,
 			notification_email = $2,
@@ -289,6 +294,9 @@ func saveGlobalConfigWith(ctx context.Context, q globalConfigExecutor, config *G
 			offering_class = $23,
 			require_different_approver = $24,
 			auto_purchase_enabled = $25,
+			auto_purchase_changed_by = $26,
+			auto_purchase_changed_at = $27,
+			auto_purchase_changed_via = $28,
 			updated_at = NOW()
 	`
 
@@ -358,6 +366,9 @@ func saveGlobalConfigWith(ctx context.Context, q globalConfigExecutor, config *G
 		offeringClass,
 		config.RequireDifferentApprover,
 		config.AutoPurchaseEnabled,
+		config.AutoPurchaseChangedBy,
+		config.AutoPurchaseChangedAt,
+		config.AutoPurchaseChangedVia,
 	)
 
 	if err != nil {
diff --git a/internal/config/store_postgres_auto_purchase_stamps_integration_test.go b/internal/config/store_postgres_auto_purchase_stamps_integration_test.go
new file mode 100644
index 00000000..dd15468a
--- /dev/null
+++ b/internal/config/store_postgres_auto_purchase_stamps_integration_test.go
@@ -0,0 +1,75 @@
+//go:build integration
+// +build integration
+
+package config
+
+import (
+	"context"
+	"net/url"
+	"os"
+	"strings"
+	"testing"
+	"time"
+
+	"github.com/LeanerCloud/cloud-commitments-platform/internal/database/postgres/migrations"
+	"github.com/jackc/pgx/v5/pgxpool"
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
+)
+
+// TestPostgresStoreDB_AutoPurchaseStamps_FreshMigrateAndRIWritePreserve runs
+// against a dedicated database named r1a_* (AUTO_PURCHASE_STAMPS_TEST_DATABASE_URL):
+// a fresh migrate leaves the kill switch OFF with NULL stamps, and the RI-exchange
+// style whole-config read-modify-write keeps the stamps instead of wiping them.
+func TestPostgresStoreDB_AutoPurchaseStamps_FreshMigrateAndRIWritePreserve(t *testing.T) {
+	dsn := os.Getenv("AUTO_PURCHASE_STAMPS_TEST_DATABASE_URL")
+	if dsn == "" {
+		t.Skip("set AUTO_PURCHASE_STAMPS_TEST_DATABASE_URL to a dedicated r1a_* test database")
+	}
+	parsed, err := url.Parse(dsn)
+	require.NoError(t, err)
+	require.True(t, strings.HasPrefix(strings.TrimPrefix(parsed.Path, "/"), "r1a_"), "database must be dedicated to R1a verification")
+
+	ctx, cancel := context.WithTimeout(context.Background(), time.Minute)
+	defer cancel()
+	pool, err := pgxpool.New(ctx, dsn)
+	require.NoError(t, err)
+	defer pool.Close()
+	require.NoError(t, migrations.RunMigrations(ctx, pool, getTestMigrationsPath(), "", ""))
+	store := &PostgresStore{db: pool}
+
+	cfg, err := store.GetGlobalConfig(ctx)
+	require.NoError(t, err)
+	assert.False(t, cfg.AutoPurchaseEnabled, "a fresh migrate leaves the kill switch OFF")
+	assert.Nil(t, cfg.AutoPurchaseChangedBy)
+	assert.Nil(t, cfg.AutoPurchaseChangedAt)
+	assert.Nil(t, cfg.AutoPurchaseChangedVia)
+
+	// R1b will write these; seed them directly, as nothing sets them yet.
+	_, err = pool.Exec(ctx, `
+		UPDATE global_config
+		SET auto_purchase_enabled = TRUE, auto_purchase_changed_by = 'user-1',
+		    auto_purchase_changed_at = '2026-10-13T01:02:03Z', auto_purchase_changed_via = 'user-session'
+		WHERE id = 1`)
+	require.NoError(t, err)
+
+	// The RI-exchange write path: read the whole config, change RI fields, upsert.
+	after, err := store.UpdateGlobalConfigAtomic(ctx, func(existing *GlobalConfig) error {
+		existing.RIExchangeEnabled = true
+		existing.RIExchangeMode = "manual"
+		return nil
+	})
+	require.NoError(t, err)
+	require.NotNil(t, after.AutoPurchaseChangedBy)
+
+	cfg, err = store.GetGlobalConfig(ctx)
+	require.NoError(t, err)
+	assert.True(t, cfg.RIExchangeEnabled)
+	assert.True(t, cfg.AutoPurchaseEnabled)
+	require.NotNil(t, cfg.AutoPurchaseChangedBy)
+	assert.Equal(t, "user-1", *cfg.AutoPurchaseChangedBy)
+	require.NotNil(t, cfg.AutoPurchaseChangedAt)
+	assert.True(t, cfg.AutoPurchaseChangedAt.Equal(time.Date(2026, 10, 13, 1, 2, 3, 0, time.UTC)))
+	require.NotNil(t, cfg.AutoPurchaseChangedVia)
+	assert.Equal(t, "user-session", *cfg.AutoPurchaseChangedVia)
+}
diff --git a/internal/config/store_postgres_coverage_test.go b/internal/config/store_postgres_coverage_test.go
index c8c14ff0..23303f85 100644
--- a/internal/config/store_postgres_coverage_test.go
+++ b/internal/config/store_postgres_coverage_test.go
@@ -602,6 +602,9 @@ func TestSaveGlobalConfig_OfferingClassBindsAt23(t *testing.T) {
 			"standard",       // $23 offering_class -- the field this test guards
 			pgxmock.AnyArg(), // $24 require_different_approver
 			pgxmock.AnyArg(), // $25 auto_purchase_enabled
+			pgxmock.AnyArg(), // $26 auto_purchase_changed_by
+			pgxmock.AnyArg(), // $27 auto_purchase_changed_at
+			pgxmock.AnyArg(), // $28 auto_purchase_changed_via
 		).
 		WillReturnResult(pgxmock.NewResult("INSERT", 1))
 
diff --git a/internal/config/store_postgres_pgxmock_test.go b/internal/config/store_postgres_pgxmock_test.go
index fecf8e93..5cdf4ff1 100644
--- a/internal/config/store_postgres_pgxmock_test.go
+++ b/internal/config/store_postgres_pgxmock_test.go
@@ -65,6 +65,7 @@ func TestPGXMock_GetGlobalConfig_Success(t *testing.T) {
 		"offering_class",
 		"require_different_approver",
 		"auto_purchase_enabled",
+		"auto_purchase_changed_by", "auto_purchase_changed_at", "auto_purchase_changed_via",
 	}
 	rows := pgxmock.NewRows(cols).AddRow(
 		[]string{"aws"}, strPtr("ops@example.com"), true,
@@ -80,6 +81,7 @@ func TestPGXMock_GetGlobalConfig_Success(t *testing.T) {
 		"convertible",
 		false,
 		false,
+		nil, nil, nil,
 	)
 	mock.ExpectQuery("SELECT").WillReturnRows(rows)
 
@@ -127,6 +129,7 @@ func TestPGXMock_GetGlobalConfig_GracePeriodDays(t *testing.T) {
 		"offering_class",
 		"require_different_approver",
 		"auto_purchase_enabled",
+		"auto_purchase_changed_by", "auto_purchase_changed_at", "auto_purchase_changed_via",
 	}
 	baseRow := func(graceJSON string) []any {
 		return []any{
@@ -143,6 +146,7 @@ func TestPGXMock_GetGlobalConfig_GracePeriodDays(t *testing.T) {
 			"convertible",
 			false,
 			false,
+			nil, nil, nil,
 		}
 	}
 
@@ -204,6 +208,7 @@ var globalConfigCols = []string{
 	"offering_class",
 	"require_different_approver",
 	"auto_purchase_enabled",
+	"auto_purchase_changed_by", "auto_purchase_changed_at", "auto_purchase_changed_via",
 }
 
 // TestPGXMock_UpdateGlobalConfigAtomic_LockedReadModifyWrite proves the F2
@@ -236,6 +241,7 @@ func TestPGXMock_UpdateGlobalConfigAtomic_LockedReadModifyWrite(t *testing.T) {
 		"convertible", // offering_class
 		false,         // require_different_approver
 		false,         // auto_purchase_enabled
+		nil, nil, nil, // auto_purchase_changed_by/at/via
 	)
 
 	// Strict order: the SELECT and the UPSERT must sit between the same
@@ -244,7 +250,7 @@ func TestPGXMock_UpdateGlobalConfigAtomic_LockedReadModifyWrite(t *testing.T) {
 	mock.ExpectExec("pg_advisory_xact_lock").WithArgs(pgxmock.AnyArg()).
 		WillReturnResult(pgxmock.NewResult("SELECT", 1))
 	mock.ExpectQuery("FROM global_config").WillReturnRows(seeded)
-	mock.ExpectExec("INSERT INTO global_config").WithArgs(anyArgsCfg(25)...).
+	mock.ExpectExec("INSERT INTO global_config").WithArgs(anyArgsCfg(28)...).
 		WillReturnResult(pgxmock.NewResult("INSERT", 1))
 	mock.ExpectCommit()
 
@@ -294,6 +300,7 @@ func TestPGXMock_UpdateGlobalConfigAtomic_ApplyErrorRollsBack(t *testing.T) {
 		"convertible",
 		false,
 		false,
+		nil, nil, nil,
 	)
 
 	mock.ExpectBegin()
@@ -331,6 +338,7 @@ func TestPGXMock_UpdateGlobalConfigAtomic_PropagatesOnlyChangedDefaults(t *testi
 			"convertible",
 			false,
 			false,
+			nil, nil, nil,
 		)
 	}
 	setCoverage := func(existing *GlobalConfig) error {
@@ -345,7 +353,7 @@ func TestPGXMock_UpdateGlobalConfigAtomic_PropagatesOnlyChangedDefaults(t *testi
 		mock.ExpectExec("pg_advisory_xact_lock").WithArgs(pgxmock.AnyArg()).
 			WillReturnResult(pgxmock.NewResult("SELECT", 1))
 		mock.ExpectQuery("FROM global_config").WillReturnRows(seeded())
-		mock.ExpectExec("INSERT INTO global_config").WithArgs(anyArgsCfg(25)...).
+		mock.ExpectExec("INSERT INTO global_config").WithArgs(anyArgsCfg(28)...).
 			WillReturnResult(pgxmock.NewResult("INSERT", 1))
 		mock.ExpectExec("UPDATE service_configs").
 			WithArgs(false, 1, false, "no-upfront", true, 70.0, false, RampImmediate).
@@ -364,7 +372,7 @@ func TestPGXMock_UpdateGlobalConfigAtomic_PropagatesOnlyChangedDefaults(t *testi
 		mock.ExpectExec("pg_advisory_xact_lock").WithArgs(pgxmock.AnyArg()).
 			WillReturnResult(pgxmock.NewResult("SELECT", 1))
 		mock.ExpectQuery("FROM global_config").WillReturnRows(seeded())
-		mock.ExpectExec("INSERT INTO global_config").WithArgs(anyArgsCfg(25)...).
+		mock.ExpectExec("INSERT INTO global_config").WithArgs(anyArgsCfg(28)...).
 			WillReturnResult(pgxmock.NewResult("INSERT", 1))
 		dbErr := errors.New("service_configs update failed")
 		mock.ExpectExec("UPDATE service_configs").WithArgs(anyArgsCfg(8)...).WillReturnError(dbErr)
@@ -3264,3 +3272,49 @@ func TestPGXMock_ReserveApprovedRIExchange_RejectsRowThatIsNotProcessing(t *test
 	require.ErrorContains(t, err, "is not processing")
 	assert.NoError(t, mock.ExpectationsWereMet())
 }
+
+// TestPGXMock_UpdateGlobalConfigAtomic_PreservesAutoPurchaseStamps proves the
+// whole-config upsert writes back the stamps it read (migration 000115). The RI
+// exchange config path is a read-modify-write of the whole row, so an upsert that
+// omitted them would reset them to NULL on any unrelated config save.
+func TestPGXMock_UpdateGlobalConfigAtomic_PreservesAutoPurchaseStamps(t *testing.T) {
+	mock := newMock(t)
+	store := storeWith(mock)
+	ctx := context.Background()
+
+	by, via := "user-1", "user-session"
+	at := time.Date(2026, 10, 13, 1, 2, 3, 0, time.UTC)
+	seeded := pgxmock.NewRows(globalConfigCols).AddRow(
+		[]string{"aws"}, strPtr("ops@example.com"), true,
+		3, "all-upfront", 80.0, RampImmediate,
+		false, "manual", 95.0,
+		0.0, 0.0, 30,
+		true, "daily", 3,
+		"{}",
+		24, 7,
+		0,
+		false,
+		false,
+		"convertible",
+		false,
+		true,
+		&by, &at, &via,
+	)
+	args := anyArgsCfg(28)
+	args[25], args[26], args[27] = &by, &at, &via
+
+	mock.ExpectBegin()
+	mock.ExpectExec("pg_advisory_xact_lock").WithArgs(pgxmock.AnyArg()).
+		WillReturnResult(pgxmock.NewResult("SELECT", 1))
+	mock.ExpectQuery("FROM global_config").WillReturnRows(seeded)
+	mock.ExpectExec("INSERT INTO global_config").WithArgs(args...).
+		WillReturnResult(pgxmock.NewResult("INSERT", 1))
+	mock.ExpectCommit()
+
+	_, err := store.UpdateGlobalConfigAtomic(ctx, func(existing *GlobalConfig) error {
+		existing.RIExchangeEnabled = true
+		return nil
+	})
+	require.NoError(t, err)
+	require.NoError(t, mock.ExpectationsWereMet())
+}
diff --git a/internal/config/types.go b/internal/config/types.go
index 501a9968..65dca4c9 100644
--- a/internal/config/types.go
+++ b/internal/config/types.go
@@ -85,6 +85,14 @@ type GlobalConfig struct {
 	// Manual purchases (Opportunities, token approval) are unaffected.
 	AutoPurchaseEnabled bool `json:"auto_purchase_enabled" db:"auto_purchase_enabled"`
 
+	// AutoPurchaseChanged{By,At,Via} stamp the last change of the switch
+	// (migration 000115): actor, time, credential kind. NULL = never changed.
+	// json:"-" so PUT /api/config cannot forge them by unmarshalling onto this
+	// struct; the whole-config upsert round-trips them (read, modify, write).
+	AutoPurchaseChangedBy  *string    `json:"-" db:"auto_purchase_changed_by"`
+	AutoPurchaseChangedAt  *time.Time `json:"-" db:"auto_purchase_changed_at"`
+	AutoPurchaseChangedVia *string    `json:"-" db:"auto_purchase_changed_via"`
+
 	// OfferingClass controls the EC2 Reserved Instance offering class used
 	// during purchase. Accepted values: "convertible" (default) and
 	// "standard". Convertible RIs can be exchanged for a different
diff --git a/internal/database/postgres/migrations/000115_auto_purchase_switch_stamps.down.sql b/internal/database/postgres/migrations/000115_auto_purchase_switch_stamps.down.sql
new file mode 100644
index 00000000..ea68171c
--- /dev/null
+++ b/internal/database/postgres/migrations/000115_auto_purchase_switch_stamps.down.sql
@@ -0,0 +1,4 @@
+ALTER TABLE global_config
+    DROP COLUMN IF EXISTS auto_purchase_changed_by,
+    DROP COLUMN IF EXISTS auto_purchase_changed_at,
+    DROP COLUMN IF EXISTS auto_purchase_changed_via;
diff --git a/internal/database/postgres/migrations/000115_auto_purchase_switch_stamps.up.sql b/internal/database/postgres/migrations/000115_auto_purchase_switch_stamps.up.sql
new file mode 100644
index 00000000..b497c2a6
--- /dev/null
+++ b/internal/database/postgres/migrations/000115_auto_purchase_switch_stamps.up.sql
@@ -0,0 +1,8 @@
+-- Migration 000115 (platform#631 R1a): who last changed the auto-purchase
+-- kill switch (global_config.auto_purchase_enabled), when, and with what kind
+-- of credential. Nullable, additive, no backfill: NULL means "never changed
+-- since the column existed". Nothing writes these until R1b. Idempotent.
+ALTER TABLE global_config
+    ADD COLUMN IF NOT EXISTS auto_purchase_changed_by TEXT,
+    ADD COLUMN IF NOT EXISTS auto_purchase_changed_at TIMESTAMPTZ,
+    ADD COLUMN IF NOT EXISTS auto_purchase_changed_via TEXT;
diff --git a/internal/database/postgres/migrations/000115_auto_purchase_switch_stamps_test.go b/internal/database/postgres/migrations/000115_auto_purchase_switch_stamps_test.go
new file mode 100644
index 00000000..297247f7
--- /dev/null
+++ b/internal/database/postgres/migrations/000115_auto_purchase_switch_stamps_test.go
@@ -0,0 +1,58 @@
+//go:build integration
+// +build integration
+
+package migrations_test
+
+import (
+	"context"
+	"os"
+	"path/filepath"
+	"testing"
+
+	"github.com/LeanerCloud/cloud-commitments-platform/internal/database/postgres/migrations"
+	"github.com/LeanerCloud/cloud-commitments-platform/internal/database/postgres/testhelpers"
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
+)
+
+// TestMigration_AutoPurchaseSwitchStamps pins migration 000115 against a real
+// PostgreSQL: an existing global_config row keeps the kill switch OFF and gets
+// NULL stamps, the up statement is idempotent, and the down migration drops
+// the three columns.
+func TestMigration_AutoPurchaseSwitchStamps(t *testing.T) {
+	ctx := context.Background()
+	migrationsPath := getMigrationsPath()
+
+	container, err := testhelpers.SetupPostgresContainer(ctx, t)
+	require.NoError(t, err)
+	defer container.Cleanup(ctx)
+	pool := container.DB.Pool()
+
+	require.NoError(t, migrations.MigrateToVersion(ctx, pool, migrationsPath, previousMigrationVersion(t, 115)))
+	_, err = pool.Exec(ctx, `INSERT INTO global_config (id) VALUES (1) ON CONFLICT (id) DO NOTHING`)
+	require.NoError(t, err)
+
+	require.NoError(t, migrations.MigrateToVersion(ctx, pool, migrationsPath, 115))
+	var enabled bool
+	var by, via *string
+	var hasAt bool
+	require.NoError(t, pool.QueryRow(ctx, `
+		SELECT auto_purchase_enabled, auto_purchase_changed_by, auto_purchase_changed_at IS NOT NULL, auto_purchase_changed_via
+		FROM global_config WHERE id = 1`).Scan(&enabled, &by, &hasAt, &via))
+	assert.False(t, enabled, "the kill switch stays OFF across the migration")
+	assert.Nil(t, by)
+	assert.False(t, hasAt)
+	assert.Nil(t, via)
+
+	up, err := os.ReadFile(filepath.Join(migrationsPath, "000115_auto_purchase_switch_stamps.up.sql"))
+	require.NoError(t, err)
+	_, err = pool.Exec(ctx, string(up))
+	require.NoError(t, err, "the up statement must be idempotent")
+
+	require.NoError(t, migrations.MigrateToVersion(ctx, pool, migrationsPath, 114))
+	var cols int
+	require.NoError(t, pool.QueryRow(ctx, `
+		SELECT count(*) FROM information_schema.columns
+		WHERE table_name = 'global_config' AND column_name LIKE 'auto_purchase_changed_%'`).Scan(&cols))
+	assert.Equal(t, 0, cols, "down migration must drop the columns")
+}