diff --git a/internal/cli/gen.go b/internal/cli/gen.go
index 6908c0f..ac04427 100644
--- a/internal/cli/gen.go
+++ b/internal/cli/gen.go
@@ -25,6 +25,12 @@ func (mfa *CLIApp) generateManifestOperation(ctx *cli.Context) error {
 		Fs:              mfa.Fs,
 	}
 
+	// Set seed for deterministic UUID if provided
+	if seed := ctx.String("seed"); seed != "" {
+		opts.Seed = seed
+		log.Infof("using deterministic seed for manifest UUID")
+	}
+
 	// Set up signing options if sign-key is provided
 	if signKey := ctx.String("sign-key"); signKey != "" {
 		opts.SigningOptions = &mfer.SigningOptions{
diff --git a/internal/cli/mfer.go b/internal/cli/mfer.go
index e99dd7e..2277e8c 100644
--- a/internal/cli/mfer.go
+++ b/internal/cli/mfer.go
@@ -154,6 +154,11 @@ func (mfa *CLIApp) run(args []string) {
 						Usage:   "GPG key ID to sign the manifest with",
 						EnvVars: []string{"MFER_SIGN_KEY"},
 					},
+					&cli.StringFlag{
+						Name:    "seed",
+						Usage:   "Seed value for deterministic manifest UUID (hashed 150M times with SHA-256, ~5-10s)",
+						EnvVars: []string{"MFER_SEED"},
+					},
 				),
 			},
 			{
diff --git a/mfer/builder.go b/mfer/builder.go
index 7864897..2696744 100644
--- a/mfer/builder.go
+++ b/mfer/builder.go
@@ -5,6 +5,7 @@ import (
 	"errors"
 	"fmt"
 	"io"
+	"sort"
 	"strings"
 	"sync"
 	"time"
@@ -88,6 +89,28 @@ type Builder struct {
 	files          []*MFFilePath
 	createdAt      time.Time
 	signingOptions *SigningOptions
+	fixedUUID      []byte // if set, use this UUID instead of generating one
+}
+
+// seedIterations is the number of SHA-256 rounds used to derive a UUID from a seed.
+// Tuned to take approximately 5-10 seconds on modern hardware.
+const seedIterations = 150_000_000
+
+// SetSeed derives a deterministic UUID from the given seed string.
+// The seed is hashed 150,000,000 times with SHA-256 to produce
+// 16 bytes used as a fixed UUID for the manifest (~5-10s on modern hardware).
+func (b *Builder) SetSeed(seed string) {
+	b.fixedUUID = deriveSeedUUID(seed, seedIterations)
+}
+
+// deriveSeedUUID hashes the seed string n times with SHA-256
+// and returns the first 16 bytes as a UUID.
+func deriveSeedUUID(seed string, iterations int) []byte {
+	hash := sha256.Sum256([]byte(seed))
+	for i := 1; i < iterations; i++ {
+		hash = sha256.Sum256(hash[:])
+	}
+	return hash[:16]
 }
 
 // NewBuilder creates a new Builder.
@@ -222,6 +245,11 @@ func (b *Builder) Build(w io.Writer) error {
 	b.mu.Lock()
 	defer b.mu.Unlock()
 
+	// Sort files by path for deterministic output
+	sort.Slice(b.files, func(i, j int) bool {
+		return b.files[i].Path < b.files[j].Path
+	})
+
 	// Create inner manifest
 	inner := &MFFile{
 		Version:   MFFile_VERSION_ONE,
@@ -233,6 +261,7 @@ func (b *Builder) Build(w io.Writer) error {
 	m := &manifest{
 		pbInner:        inner,
 		signingOptions: b.signingOptions,
+		fixedUUID:      b.fixedUUID,
 	}
 
 	// Generate outer wrapper
diff --git a/mfer/builder_test.go b/mfer/builder_test.go
index 761b2d5..5c1fb11 100644
--- a/mfer/builder_test.go
+++ b/mfer/builder_test.go
@@ -115,6 +115,52 @@ func TestNewTimestampFromTimeExtremeDate(t *testing.T) {
 	}
 }
 
+func TestBuilderDeterministicOutput(t *testing.T) {
+	buildManifest := func() []byte {
+		b := NewBuilder()
+		// Use a fixed createdAt and UUID so output is reproducible
+		b.createdAt = time.Date(2025, 1, 1, 0, 0, 0, 0, time.UTC)
+		b.fixedUUID = make([]byte, 16) // all zeros
+
+		mtime := ModTime(time.Date(2025, 6, 1, 0, 0, 0, 0, time.UTC))
+
+		// Add files in reverse order to test sorting
+		files := []struct {
+			path    string
+			content string
+		}{
+			{"c/file.txt", "content c"},
+			{"a/file.txt", "content a"},
+			{"b/file.txt", "content b"},
+		}
+		for _, f := range files {
+			r := bytes.NewReader([]byte(f.content))
+			_, err := b.AddFile(RelFilePath(f.path), FileSize(len(f.content)), mtime, r, nil)
+			require.NoError(t, err)
+		}
+
+		var buf bytes.Buffer
+		err := b.Build(&buf)
+		require.NoError(t, err)
+		return buf.Bytes()
+	}
+
+	out1 := buildManifest()
+	out2 := buildManifest()
+	assert.Equal(t, out1, out2, "two builds with same input should produce byte-identical output")
+}
+
+func TestDeriveSeedUUID(t *testing.T) {
+	// Use a small iteration count for testing (production uses 1B)
+	uuid1 := deriveSeedUUID("test-seed-value", 1000)
+	uuid2 := deriveSeedUUID("test-seed-value", 1000)
+	assert.Equal(t, uuid1, uuid2, "same seed should produce same UUID")
+	assert.Len(t, uuid1, 16, "UUID should be 16 bytes")
+
+	uuid3 := deriveSeedUUID("different-seed", 1000)
+	assert.NotEqual(t, uuid1, uuid3, "different seeds should produce different UUIDs")
+}
+
 func TestBuilderBuildEmpty(t *testing.T) {
 	b := NewBuilder()
 
diff --git a/mfer/manifest.go b/mfer/manifest.go
index 203c79c..bea4fa1 100644
--- a/mfer/manifest.go
+++ b/mfer/manifest.go
@@ -17,6 +17,7 @@ type manifest struct {
 	pbOuter        *MFFileOuter
 	output         *bytes.Buffer
 	signingOptions *SigningOptions
+	fixedUUID      []byte // if set, use this UUID instead of generating one
 }
 
 func (m *manifest) String() string {
diff --git a/mfer/scanner.go b/mfer/scanner.go
index df0df11..84eeabd 100644
--- a/mfer/scanner.go
+++ b/mfer/scanner.go
@@ -47,6 +47,7 @@ type ScannerOptions struct {
 	FollowSymLinks  bool            // Resolve symlinks instead of skipping them
 	Fs              afero.Fs        // Filesystem to use, defaults to OsFs if nil
 	SigningOptions  *SigningOptions // GPG signing options (nil = no signing)
+	Seed            string          // If set, derive a deterministic UUID from this seed
 }
 
 // FileEntry represents a file that has been enumerated.
@@ -276,6 +277,9 @@ func (s *Scanner) ToManifest(ctx context.Context, w io.Writer, progress chan<- S
 	if s.options.SigningOptions != nil {
 		builder.SetSigningOptions(s.options.SigningOptions)
 	}
+	if s.options.Seed != "" {
+		builder.SetSeed(s.options.Seed)
+	}
 
 	var scannedFiles FileCount
 	var scannedBytes FileSize
diff --git a/mfer/serialize.go b/mfer/serialize.go
index 4804901..3d712a6 100644
--- a/mfer/serialize.go
+++ b/mfer/serialize.go
@@ -49,8 +49,13 @@ func (m *manifest) generateOuter() error {
 		return errors.New("internal error")
 	}
 
-	// Generate UUID and set on inner message
-	manifestUUID := uuid.New()
+	// Use fixed UUID if provided, otherwise generate a new one
+	var manifestUUID uuid.UUID
+	if len(m.fixedUUID) == 16 {
+		copy(manifestUUID[:], m.fixedUUID)
+	} else {
+		manifestUUID = uuid.New()
+	}
 	m.pbInner.Uuid = manifestUUID[:]
 
 	innerData, err := proto.MarshalOptions{Deterministic: true}.Marshal(m.pbInner)