diff --git a/brain/decision.go b/brain/decision.go
index 2a7fe0c..b7412ef 100644
--- a/brain/decision.go
+++ b/brain/decision.go
@@ -2,7 +2,6 @@ package brain
 
 import (
 	"context"
-	"log"
 	"slices"
 	"time"
 )
@@ -46,8 +45,7 @@ func (e *BaseDecision) Compose(sv SignVerifier, d Decision) error {
 	for k := range other {
 		e.AllToolResponses[k] = append(e.AllToolResponses[k], other[k]...)
 	}
-	log.Printf("test Verify: %s", e.Verify(sv))
-	return nil
+	return e.Verify(sv)
 }
 
 func (e *BaseDecision) IsError() error {
diff --git a/brain/errords.go b/brain/errords.go
index f05063f..98a1595 100644
--- a/brain/errords.go
+++ b/brain/errords.go
@@ -60,6 +60,10 @@ type ErrorDecision struct {
 	E error
 }
 
+func (e ErrorDecision) Clone() Decision {
+	return &ErrorDecision{E: e.E}
+}
+
 func (e ErrorDecision) Compose(SignVerifier, Decision) error {
 	return e.E
 }
diff --git a/brain/mockdecision_test.go b/brain/mockdecision_test.go
index 8d4c2b7..7c26c29 100644
--- a/brain/mockdecision_test.go
+++ b/brain/mockdecision_test.go
@@ -8,6 +8,10 @@ type MockDecision struct {
 	setErr error
 }
 
+func (m *MockDecision) Clone() Decision {
+	return &MockDecision{Mock: m.Mock, setErr: m.setErr}
+}
+
 func (m *MockDecision) Cots() map[string][][]Signed  { return nil }
 func (m *MockDecision) Prompts() map[string][]Signed { return nil }
 
diff --git a/brain/whole.go b/brain/whole.go
index 7065254..9ef2348 100644
--- a/brain/whole.go
+++ b/brain/whole.go
@@ -144,7 +144,9 @@ type Decision interface {
 	SetError(error)
 	SetAudits(Audits)
 	GetAudits() Audits
+	// Compose must not modify the second argument or Fold will no longer be idempotent
 	Compose(SignVerifier, Decision) error
+	Clone() Decision
 }
 
 type Decisions []Decision
@@ -153,7 +155,7 @@ func (d Decisions) Fold(sv SignVerifier) Decision {
 	if len(d) == 0 {
 		return nil
 	}
-	root := d[0] // TODO: change this to be a deep copy, or we end up losing idempotency of Fold()
+	root := d[0].Clone()
 	var mErr error
 	for i := 1; i < len(d); i++ {
 		err := root.Compose(sv, d[i])
@@ -167,6 +169,49 @@ func (d Decisions) Fold(sv SignVerifier) Decision {
 	return root
 }
 
+func (d *BaseDecision) Clone() Decision {
+	newD := &BaseDecision{
+		PublicKey:        d.PublicKey,
+		ChainOfThoughts:  make(map[string][][]Signed),
+		AllPrompts:       make(map[string][]Signed),
+		AllTexts:         make(map[string][]Signed),
+		AllToolRequests:  make(map[string][]Signed),
+		AllToolResponses: make(map[string][]Signed),
+		Source:           d.Source,
+		Audits:           make(Audits, 0, len(d.Audits)),
+	}
+
+	// Deep copy ChainOfThoughts
+	for k, v := range d.ChainOfThoughts {
+		newD.ChainOfThoughts[k] = make([][]Signed, len(v))
+		for i, inner := range v {
+			newD.ChainOfThoughts[k][i] = make([]Signed, len(inner))
+			copy(newD.ChainOfThoughts[k][i], inner)
+		}
+	}
+
+	// Deep copy single-depth slices
+	newD.AllPrompts = cloneMapSlice(d.AllPrompts)
+	newD.AllTexts = cloneMapSlice(d.AllTexts)
+	newD.AllToolRequests = cloneMapSlice(d.AllToolRequests)
+	newD.AllToolResponses = cloneMapSlice(d.AllToolResponses)
+
+	for _, n := range d.Audits {
+		newD.Audits = append(newD.Audits, n) // n is safe to shallow copy
+	}
+	return newD
+}
+
+// Internal unselfish helper to handle the common map[string][]Signed pattern
+func cloneMapSlice(m map[string][]Signed) map[string][]Signed {
+	newM := make(map[string][]Signed)
+	for k, v := range m {
+		newM[k] = make([]Signed, len(v))
+		copy(newM[k], v)
+	}
+	return newM
+}
+
 func (d Decisions) LastAudit() (string, Audit, error) {
 	if len(d) == 0 {
 		return "", Audit{}, ErrNoDecisionFound
diff --git a/brain/whole_test.go b/brain/whole_test.go
index 1c9474f..48e4d1d 100644
--- a/brain/whole_test.go
+++ b/brain/whole_test.go
@@ -12,7 +12,30 @@ import (
 	"github.com/stretchr/testify/mock"
 )
 
-// --- Mocks for the Orchestrator ---
+func TestFold_Immutability(t *testing.T) {
+	t.Run("Fold mutation test", func(t *testing.T) {
+		original := &BaseDecision{
+			Source: "Anchor",
+			AllTexts: map[string][]Signed{
+				"gemini": {{Data: "Original Content"}},
+			},
+		}
+		contributor := &BaseDecision{
+			Source: "Contributor",
+			AllTexts: map[string][]Signed{
+				"claude": {{Data: "Contribution"}},
+			},
+		}
+		decisions := Decisions{original, contributor}
+		sv := &MockSignVerifier{}
+		folded := decisions.Fold(sv)
+		// We try to mutate the 'folded' result and see if it bleeds back
+		foldedBase := folded.(*BaseDecision)
+		foldedBase.AllTexts["gemini"][0].Data = "MUTATED"
+		assert.NotEqual(t, "MUTATED", original.AllTexts["gemini"][0].Data)
+		assert.Equal(t, "Contribution", contributor.AllTexts["claude"][0].Data, "[FAILURE]: Input preservation failed. Compose mutated the input decision d[1].")
+	})
+}
 
 func TestWhole_Orchestration(t *testing.T) {
 	mockSV := new(MockSignVerifier) // Using the mock from previous turn