diff --git a/internal/settings/postgres_settings_test.go b/internal/settings/postgres_settings_test.go
new file mode 100644
index 0000000..3906b90
--- /dev/null
+++ b/internal/settings/postgres_settings_test.go
@@ -0,0 +1,153 @@
+package settings
+
+import (
+	"database/sql"
+	"errors"
+	"fmt"
+	"os"
+	"strings"
+	"sync"
+	"testing"
+	"time"
+
+	"agent-system-backend/internal/store"
+)
+
+// These checks require a disposable database. They never select a deployment DSN.
+func isolatedSettingsPostgres(t *testing.T) *store.Store {
+	t.Helper()
+	dsn := os.Getenv("AGENT_SETTINGS_PG_TEST_DSN")
+	if dsn == "" {
+		t.Skip("NOT_RUN: AGENT_SETTINGS_PG_TEST_DSN requires an isolated PostgreSQL fixture")
+	}
+	if os.Getenv("AGENT_SETTINGS_PG_ISOLATED") != "1" {
+		t.Fatal("refusing PostgreSQL settings tests without an isolated fixture marker")
+	}
+	probe, err := sql.Open("pgx", dsn)
+	if err != nil {
+		t.Fatal("cannot open isolated PostgreSQL fixture")
+	}
+	var database string
+	err = probe.QueryRow("SELECT current_database()").Scan(&database)
+	probe.Close()
+	if err != nil || !strings.HasPrefix(database, "settings_") {
+		t.Fatal("PostgreSQL settings fixture database must have the settings_ prefix")
+	}
+	st, err := store.OpenPostgres(dsn, t.TempDir())
+	if err != nil {
+		t.Fatal("cannot initialize isolated PostgreSQL fixture")
+	}
+	t.Cleanup(func() { st.Close() })
+	return st
+}
+
+func TestPostgresSharedSettingsConcurrentCASAndPersonalBoundary(t *testing.T) {
+	first, second := isolatedSettingsPostgres(t), isolatedSettingsPostgres(t)
+	a, b := NewService(first), NewService(second)
+	project := fmt.Sprintf("pg-cas-%d", time.Now().UnixNano())
+	start := make(chan struct{})
+	results := make(chan error, 2)
+	var writers sync.WaitGroup
+	for i, service := range []*Service{a, b} {
+		writers.Add(1)
+		go func(service *Service, style string) {
+			defer writers.Done()
+			<-start
+			_, err := service.MergeSharedProject(project, map[string]any{"deliverables.style": style}, 0)
+			results <- err
+		}(service, []string{"formal", "minimal"}[i])
+	}
+	close(start)
+	writers.Wait()
+	close(results)
+	passed, conflicts := 0, 0
+	for err := range results {
+		switch {
+		case err == nil:
+			passed++
+		case errors.Is(err, ErrSettingsConflict):
+			conflicts++
+		default:
+			t.Fatalf("unexpected concurrent write result: %v", err)
+		}
+	}
+	if passed != 1 || conflicts != 1 {
+		t.Fatalf("CAS admitted %d writers with %d conflicts", passed, conflicts)
+	}
+	values, revision, err := b.SharedProjectScope(project)
+	if err != nil || revision != 1 {
+		t.Fatalf("second connection sees revision %d: %v", revision, err)
+	}
+	if err := first.MergeSettingsScope("alice", ScopeProjectPrefix+project, map[string]any{"context.userInstructions": "private", "models.chat.model": "legacy"}); err != nil {
+		t.Fatal(err)
+	}
+	if b.Effective("bob", project).GetString("context.userInstructions", "") != "" || b.Effective("alice", project).GetString("models.chat.model", "") != "" {
+		t.Fatal("legacy model override or another member's preferences became effective")
+	}
+	if got := b.Effective("bob", project).GetString("deliverables.style", ""); got != a.Effective("alice", project).GetString("deliverables.style", "") || len(values) == 0 {
+		t.Fatal("shared project configuration diverged across actors/connections")
+	}
+	if _, err := a.MergeSharedProject(project, map[string]any{}, revision, []string{"deliverables.style"}); err != nil {
+		t.Fatal(err)
+	}
+	if got := b.Effective("bob", project).GetString("deliverables.style", ""); got != "modern" {
+		t.Fatalf("removing the override did not restore the default: %s", got)
+	}
+}
+
+func TestPostgresProjectMigrationRollbackAndReplay(t *testing.T) {
+	st := isolatedSettingsPostgres(t)
+	s := NewService(st)
+	project := fmt.Sprintf("pg-migration-%d", time.Now().UnixNano())
+	if err := st.MergeSettingsScope("alice", ScopeProjectPrefix+project, map[string]any{"deliverables.style": "formal", "context.userInstructions": "private"}); err != nil {
+		t.Fatal(err)
+	}
+	preview, err := s.PreviewProjectMigration("alice", project, []MigrationChoice{{Path: "deliverables.style", SourceOwner: "alice"}}, 0)
+	if err != nil {
+		t.Fatal(err)
+	}
+	selected, _, err := s.MigrationSelection("alice", project, preview.Observation, 0)
+	if err != nil {
+		t.Fatal(err)
+	}
+	// Abort after the observation has been updated, proving that both rows roll back.
+	function := fmt.Sprintf("settings_reject_%d", time.Now().UnixNano())
+	trigger := function + "_trigger"
+	ddl := fmt.Sprintf(`CREATE FUNCTION %s() RETURNS trigger LANGUAGE plpgsql AS $$ BEGIN IF NEW.scope='%s' THEN RAISE EXCEPTION 'fixture migration target failure'; END IF; RETURN NEW; END $$`, function, ScopeSharedProjectPrefix+project)
+	if _, err := st.DB().Exec(ddl); err != nil {
+		t.Fatal(err)
+	}
+	t.Cleanup(func() {
+		st.DB().Exec("DROP TRIGGER IF EXISTS " + trigger + " ON settings")
+		st.DB().Exec("DROP FUNCTION IF EXISTS " + function + "()")
+	})
+	if _, err := st.DB().Exec(fmt.Sprintf("CREATE TRIGGER %s BEFORE INSERT OR UPDATE ON settings FOR EACH ROW EXECUTE FUNCTION %s()", trigger, function)); err != nil {
+		t.Fatal(err)
+	}
+	if _, err := s.ApplyProjectMigration("alice", project, preview.Observation, 0, selected); err == nil {
+		t.Fatal("injected PostgreSQL target failure succeeded")
+	}
+	observation, err := st.GetSettingsScope(OwnerSystemRow, observationPrefix+preview.Observation)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if _, done := observation["appliedRevision"]; done {
+		t.Fatal("rollback retained a completed observation")
+	}
+	if _, revision, err := s.SharedProjectScope(project); err != nil || revision != 0 {
+		t.Fatal("failed migration changed the project target")
+	}
+	if _, err := st.DB().Exec("DROP TRIGGER " + trigger + " ON settings"); err != nil {
+		t.Fatal(err)
+	}
+	for attempt := 0; attempt < 2; attempt++ {
+		out, err := s.ApplyProjectMigration("alice", project, preview.Observation, 0, selected)
+		if err != nil || out.ProjectRevision != 1 || out.Replayed != (attempt == 1) {
+			t.Fatalf("apply/replay attempt %d: %+v, %v", attempt, out, err)
+		}
+	}
+	old, err := st.GetSettingsScope("alice", ScopeProjectPrefix+project)
+	if err != nil || old["context.userInstructions"] != "private" || old["deliverables.style"] != "formal" {
+		t.Fatal("migration changed a legacy or private source row")
+	}
+}