diff --git a/reversal/multiplier.go b/reversal/multiplier.go
new file mode 100644
index 0000000..d621f71
--- /dev/null
+++ b/reversal/multiplier.go
@@ -0,0 +1,258 @@
+package reversal
+
+import (
+	"strings"
+	"unicode"
+
+	i18n "forge.lthn.ai/core/go-i18n"
+)
+
+// Multiplier generates deterministic grammatical variants of text
+// for training data augmentation. Zero API calls.
+type Multiplier struct {
+	tokeniser *Tokeniser
+}
+
+// NewMultiplier creates a Multiplier using the default English tokeniser.
+func NewMultiplier() *Multiplier {
+	return &Multiplier{tokeniser: NewTokeniser()}
+}
+
+// NewMultiplierForLang creates a Multiplier for the specified language.
+func NewMultiplierForLang(lang string) *Multiplier {
+	return &Multiplier{tokeniser: NewTokeniserForLang(lang)}
+}
+
+// Expand produces: original + tense flips (past, gerund) + number flips (plural toggle) + combinations.
+// All output is deterministic and grammatically correct.
+func (m *Multiplier) Expand(text string) []string {
+	text = strings.TrimSpace(text)
+	if text == "" {
+		return nil
+	}
+
+	tokens := m.tokeniser.Tokenise(text)
+	if len(tokens) == 0 {
+		return nil
+	}
+
+	// Collect indices of verbs and nouns for targeted replacement.
+	var verbIndices []int
+	var nounIndices []int
+	for i, tok := range tokens {
+		switch tok.Type {
+		case TokenVerb:
+			verbIndices = append(verbIndices, i)
+		case TokenNoun:
+			nounIndices = append(nounIndices, i)
+		}
+	}
+
+	// Build the list of variants in deterministic order:
+	// 1. Original
+	// 2. Single verb transforms (past, gerund) for each verb
+	// 3. Single noun transforms (plural toggle) for each noun
+	// 4. Combined transforms (verb transform + noun transform)
+	seen := make(map[string]bool)
+	var results []string
+
+	addVariant := func(s string) {
+		if !seen[s] {
+			seen[s] = true
+			results = append(results, s)
+		}
+	}
+
+	// 1. Original text
+	addVariant(text)
+
+	// 2. Verb transforms: for each verb, produce past and gerund variants
+	for _, vi := range verbIndices {
+		pastTokens := m.applyVerbTransform(tokens, vi, "past")
+		addVariant(reconstruct(pastTokens))
+
+		gerundTokens := m.applyVerbTransform(tokens, vi, "gerund")
+		addVariant(reconstruct(gerundTokens))
+
+		baseTokens := m.applyVerbTransform(tokens, vi, "base")
+		addVariant(reconstruct(baseTokens))
+	}
+
+	// 3. Noun transforms: for each noun, toggle plural/singular
+	for _, ni := range nounIndices {
+		pluralTokens := m.applyNounTransform(tokens, ni)
+		addVariant(reconstruct(pluralTokens))
+	}
+
+	// 4. Combinations: each verb transform + each noun transform
+	for _, vi := range verbIndices {
+		for _, ni := range nounIndices {
+			// past + noun toggle
+			pastTokens := m.applyVerbTransform(tokens, vi, "past")
+			pastPluralTokens := m.applyNounTransformOnTokens(pastTokens, ni)
+			addVariant(reconstruct(pastPluralTokens))
+
+			// gerund + noun toggle
+			gerundTokens := m.applyVerbTransform(tokens, vi, "gerund")
+			gerundPluralTokens := m.applyNounTransformOnTokens(gerundTokens, ni)
+			addVariant(reconstruct(gerundPluralTokens))
+
+			// base + noun toggle
+			baseTokens := m.applyVerbTransform(tokens, vi, "base")
+			basePluralTokens := m.applyNounTransformOnTokens(baseTokens, ni)
+			addVariant(reconstruct(basePluralTokens))
+		}
+	}
+
+	return results
+}
+
+// applyVerbTransform returns a copy of tokens with the verb at index vi
+// transformed to the specified tense ("past", "gerund", or "base").
+func (m *Multiplier) applyVerbTransform(tokens []Token, vi int, targetTense string) []Token {
+	result := make([]Token, len(tokens))
+	copy(result, tokens)
+
+	tok := tokens[vi]
+	base := tok.VerbInfo.Base
+	currentTense := tok.VerbInfo.Tense
+
+	if currentTense == targetTense {
+		return result
+	}
+
+	var newForm string
+	switch targetTense {
+	case "past":
+		newForm = i18n.PastTense(base)
+	case "gerund":
+		newForm = i18n.Gerund(base)
+	case "base":
+		newForm = base
+	}
+
+	if newForm == "" {
+		return result
+	}
+
+	// Preserve capitalisation of the original token.
+	newForm = preserveCase(tok.Raw, newForm)
+
+	result[vi] = Token{
+		Raw:   newForm,
+		Lower: strings.ToLower(newForm),
+		Type:  TokenVerb,
+		VerbInfo: VerbMatch{
+			Base:  base,
+			Tense: targetTense,
+			Form:  newForm,
+		},
+	}
+
+	return result
+}
+
+// applyNounTransform returns a copy of tokens with the noun at index ni
+// toggled between singular and plural.
+func (m *Multiplier) applyNounTransform(tokens []Token, ni int) []Token {
+	return m.applyNounTransformOnTokens(tokens, ni)
+}
+
+// applyNounTransformOnTokens returns a copy of the given tokens with the
+// noun at index ni toggled between singular and plural.
+func (m *Multiplier) applyNounTransformOnTokens(tokens []Token, ni int) []Token {
+	result := make([]Token, len(tokens))
+	copy(result, tokens)
+
+	tok := tokens[ni]
+	base := tok.NounInfo.Base
+	isPlural := tok.NounInfo.Plural
+
+	var newForm string
+	var newPlural bool
+
+	if isPlural {
+		// Already plural, revert to singular (base form).
+		newForm = base
+		newPlural = false
+	} else {
+		// Singular, generate plural.
+		newForm = i18n.PluralForm(base)
+		newPlural = true
+	}
+
+	if newForm == "" {
+		return result
+	}
+
+	// Preserve capitalisation.
+	newForm = preserveCase(tok.Raw, newForm)
+
+	result[ni] = Token{
+		Raw:   newForm,
+		Lower: strings.ToLower(newForm),
+		Type:  TokenNoun,
+		NounInfo: NounMatch{
+			Base:   base,
+			Plural: newPlural,
+			Form:   newForm,
+		},
+	}
+
+	return result
+}
+
+// reconstruct joins tokens back into a string, preserving spacing.
+func reconstruct(tokens []Token) string {
+	var b strings.Builder
+	for i, tok := range tokens {
+		if i > 0 {
+			// Punctuation tokens that were split from the previous word
+			// should not have a leading space.
+			if tok.Type == TokenPunctuation {
+				b.WriteString(tok.Raw)
+				continue
+			}
+			b.WriteByte(' ')
+		}
+		b.WriteString(tok.Raw)
+	}
+	return b.String()
+}
+
+// preserveCase applies the capitalisation pattern of the original word
+// to the replacement word. If the original started with an uppercase
+// letter, the replacement will too.
+func preserveCase(original, replacement string) string {
+	if len(original) == 0 || len(replacement) == 0 {
+		return replacement
+	}
+
+	origRunes := []rune(original)
+	repRunes := []rune(replacement)
+
+	// If the original is all uppercase (like "DELETE"), make replacement all uppercase.
+	if isAllUpper(original) && len(original) > 1 {
+		return strings.ToUpper(replacement)
+	}
+
+	// If the first character of the original is uppercase, capitalise the replacement.
+	if unicode.IsUpper(origRunes[0]) {
+		repRunes[0] = unicode.ToUpper(repRunes[0])
+		return string(repRunes)
+	}
+
+	// Otherwise, ensure the replacement starts lowercase.
+	repRunes[0] = unicode.ToLower(repRunes[0])
+	return string(repRunes)
+}
+
+// isAllUpper returns true if every letter in the string is uppercase.
+func isAllUpper(s string) bool {
+	for _, r := range s {
+		if unicode.IsLetter(r) && !unicode.IsUpper(r) {
+			return false
+		}
+	}
+	return true
+}
diff --git a/reversal/multiplier_test.go b/reversal/multiplier_test.go
new file mode 100644
index 0000000..844b13b
--- /dev/null
+++ b/reversal/multiplier_test.go
@@ -0,0 +1,67 @@
+package reversal
+
+import (
+	"testing"
+
+	i18n "forge.lthn.ai/core/go-i18n"
+)
+
+func TestMultiplier_Expand(t *testing.T) {
+	svc, _ := i18n.New()
+	i18n.SetDefault(svc)
+	m := NewMultiplier()
+	variants := m.Expand("Delete the configuration file")
+
+	if len(variants) < 4 {
+		t.Errorf("Expand() returned %d variants, want >= 4", len(variants))
+	}
+
+	expected := map[string]bool{
+		"Delete the configuration file":   true, // original
+		"Deleted the configuration file":  true, // past
+		"Deleting the configuration file": true, // gerund
+		"Delete the configuration files":  true, // plural
+	}
+	for _, v := range variants {
+		delete(expected, v)
+	}
+	for missing := range expected {
+		t.Errorf("Expand() missing expected variant: %q", missing)
+	}
+}
+
+func TestMultiplier_Expand_NoVerbs(t *testing.T) {
+	svc, _ := i18n.New()
+	i18n.SetDefault(svc)
+	m := NewMultiplier()
+	variants := m.Expand("the configuration file")
+	if len(variants) < 2 {
+		t.Errorf("Expand() returned %d variants, want >= 2", len(variants))
+	}
+}
+
+func TestMultiplier_Expand_Empty(t *testing.T) {
+	svc, _ := i18n.New()
+	i18n.SetDefault(svc)
+	m := NewMultiplier()
+	variants := m.Expand("")
+	if len(variants) != 0 {
+		t.Errorf("Expand(\"\") returned %d variants, want 0", len(variants))
+	}
+}
+
+func TestMultiplier_Expand_Deterministic(t *testing.T) {
+	svc, _ := i18n.New()
+	i18n.SetDefault(svc)
+	m := NewMultiplier()
+	v1 := m.Expand("Delete the file")
+	v2 := m.Expand("Delete the file")
+	if len(v1) != len(v2) {
+		t.Fatalf("Non-deterministic: %d vs %d variants", len(v1), len(v2))
+	}
+	for i := range v1 {
+		if v1[i] != v2[i] {
+			t.Errorf("Non-deterministic at [%d]: %q vs %q", i, v1[i], v2[i])
+		}
+	}
+}