diff --git a/internal/cli/up.go b/internal/cli/up.go
index ade660c..5cf8cae 100644
--- a/internal/cli/up.go
+++ b/internal/cli/up.go
@@ -3,6 +3,7 @@ package cli
 import (
 	"context"
 	"fmt"
+	"io"
 	"os"
 	"path/filepath"
 
@@ -14,6 +15,7 @@ import (
 	"github.com/open-source-cloud/devstack/internal/hooks"
 	"github.com/open-source-cloud/devstack/internal/lock"
 	"github.com/open-source-cloud/devstack/internal/orchestrate"
+	"github.com/open-source-cloud/devstack/internal/profile"
 	"github.com/open-source-cloud/devstack/internal/state"
 	"github.com/open-source-cloud/devstack/internal/workspace"
 	"github.com/open-source-cloud/devstack/internal/xdg"
@@ -48,6 +50,15 @@ func newUpCmd(g *GlobalOpts) *cobra.Command {
 			d.NoPreflight = noPreflight
 			d.Profiles = profiles
 
+			// Memory-budget warning (spec 12 §budget): opt-in — only when the
+			// workspace declares memoryBudgetMB and the active slice exceeds it. Never
+			// fatal; suppressed under --quiet/--json.
+			if !g.Quiet && !g.JSON {
+				if b := profile.CheckBudget(d.Model, profile.Resolve(d.Model, profiles)); b.Over {
+					warnMemoryBudget(cmd.ErrOrStderr(), b)
+				}
+			}
+
 			// Self-healing reconcile before the saga (spec 09): prune ref rows for
 			// projects no longer live. Best-effort — never blocks `up`.
 			_, _ = d.Manager.Reconcile(cmd.Context())
@@ -84,6 +95,16 @@ func newUpCmd(g *GlobalOpts) *cobra.Command {
 	return cmd
 }
 
+// warnMemoryBudget prints the spec-12 over-budget warning naming the offending
+// active services and pointing at a lighter slice.
+func warnMemoryBudget(w io.Writer, b profile.Budget) {
+	fmt.Fprintf(w, "[warn]    active profile needs ~%d MB but memoryBudgetMB is %d MB:\n", b.TotalMB, b.BudgetMB)
+	for _, s := range b.Services {
+		fmt.Fprintf(w, "            %s/%s: %d MB\n", s.Project, s.Service, s.MemoryMB)
+	}
+	fmt.Fprintln(w, "          consider a smaller slice, e.g. `--profile minimal`")
+}
+
 // newDownCmd wires `devstack down [project...]` — stop project stacks, run
 // preDown hooks first, drop their ref rows. The external network and volumes are
 // never touched; shared services are left running (autostop is X1 config + spec 03).
diff --git a/internal/profile/profile.go b/internal/profile/profile.go
index be22480..bfd8c5a 100644
--- a/internal/profile/profile.go
+++ b/internal/profile/profile.go
@@ -65,6 +65,43 @@ func Resolve(m *config.Model, requested []string) Active {
 	return out
 }
 
+// ServiceMem is one active service's declared memory hint (spec 12/18).
+type ServiceMem struct {
+	Project  string `json:"project"`
+	Service  string `json:"service"`
+	MemoryMB int    `json:"memoryMB"`
+}
+
+// Budget is the result of checking the active set against the workspace memory
+// budget (spec 12 §budget). Over is false whenever no budget is configured.
+type Budget struct {
+	BudgetMB int          `json:"budgetMB"` // workspace.memoryBudgetMB (0 → no check)
+	TotalMB  int          `json:"totalMB"`  // sum of active services' memoryMB
+	Over     bool         `json:"over"`     // TotalMB > BudgetMB (only when BudgetMB > 0)
+	Services []ServiceMem `json:"services"` // active services that declared a memoryMB, sorted
+}
+
+// CheckBudget sums the active services' declared memoryMB and compares it to the
+// workspace memoryBudgetMB. With no budget configured (0), it never reports Over —
+// the check is opt-in (spec 12 acceptance). Services with no memoryMB contribute
+// nothing and are omitted from the breakdown.
+func CheckBudget(m *config.Model, a Active) Budget {
+	b := Budget{BudgetMB: m.Workspace.MemoryBudgetMB}
+	for _, project := range sortedKeys(a.Services) {
+		p := m.Projects[project]
+		for _, sname := range a.Services[project] {
+			mb := p.Services[sname].MemoryMB
+			if mb <= 0 {
+				continue
+			}
+			b.TotalMB += mb
+			b.Services = append(b.Services, ServiceMem{Project: project, Service: sname, MemoryMB: mb})
+		}
+	}
+	b.Over = b.BudgetMB > 0 && b.TotalMB > b.BudgetMB
+	return b
+}
+
 // serviceActive reports whether a service is in any active group or carries an
 // active profile tag.
 func serviceActive(m *config.Model, profiles map[string]bool, svc config.Service, sname string) bool {
diff --git a/internal/profile/profile_test.go b/internal/profile/profile_test.go
index 138bed9..b43566f 100644
--- a/internal/profile/profile_test.go
+++ b/internal/profile/profile_test.go
@@ -84,6 +84,39 @@ func TestResolveNoConfigDefaultIsAll(t *testing.T) {
 	}
 }
 
+func TestCheckBudgetOptInAndOver(t *testing.T) {
+	m := sliceModel()
+	// No budget configured → never Over, regardless of usage.
+	if CheckBudget(m, Resolve(m, []string{"all"})).Over {
+		t.Error("no memoryBudgetMB → must never report Over")
+	}
+
+	// Give the frontend services memory hints and a tight budget.
+	app := m.Projects["app"]
+	web := app.Services["web"]
+	web.MemoryMB = 800
+	app.Services["web"] = web
+	worker := app.Services["worker"]
+	worker.MemoryMB = 400
+	app.Services["worker"] = worker
+	m.Projects["app"] = app
+	m.Workspace.MemoryBudgetMB = 1000
+
+	// frontend slice = web(800)+worker(400)=1200 > 1000 → Over, both named.
+	b := CheckBudget(m, Resolve(m, []string{"frontend"}))
+	if !b.Over || b.TotalMB != 1200 {
+		t.Fatalf("frontend budget = %+v, want Over with total 1200", b)
+	}
+	if len(b.Services) != 2 {
+		t.Errorf("offenders = %v, want web+worker", b.Services)
+	}
+
+	// core slice = api(no memoryMB)=0 ≤ 1000 → not Over.
+	if CheckBudget(m, Resolve(m, []string{"core"})).Over {
+		t.Error("core slice (0 MB) must be under budget")
+	}
+}
+
 func TestResolveHasAndShared(t *testing.T) {
 	a := Resolve(sliceModel(), []string{"core"})
 	if !a.Has("app", "api") || a.Has("app", "web") {