diff --git a/replace.go b/replace.go
index c199974..af37041 100644
--- a/replace.go
+++ b/replace.go
@@ -1,7 +1,9 @@
 package managers
 
 import (
+	"bytes"
 	"context"
+	"encoding/json"
 	"errors"
 	"fmt"
 	"net/url"
@@ -183,9 +185,17 @@ func buildComposerReplaceInputs(pkg string, mode replaceMode, opts ReplaceOption
 	}
 	switch mode {
 	case replaceModePath:
-		input.Args["payload"] = fmt.Sprintf(`{"type":"path","url":%q}`, opts.Path)
+		payload, err := composerRepositoryPayload("path", opts.Path)
+		if err != nil {
+			return nil, err
+		}
+		input.Args["payload"] = payload
 	case replaceModeGit:
-		input.Args["payload"] = fmt.Sprintf(`{"type":"vcs","url":%q}`, opts.Git)
+		payload, err := composerRepositoryPayload("vcs", opts.Git)
+		if err != nil {
+			return nil, err
+		}
+		input.Args["payload"] = payload
 		inputs := []replaceInput{{operation: opReplace, input: input}}
 		if opts.Ref != "" {
 			inputs = append(inputs, replaceInput{
@@ -205,6 +215,20 @@ func buildComposerReplaceInputs(pkg string, mode replaceMode, opts ReplaceOption
 	return []replaceInput{{operation: opReplace, input: input}}, nil
 }
 
+func composerRepositoryPayload(kind, url string) (string, error) {
+	var buf bytes.Buffer
+	enc := json.NewEncoder(&buf)
+	enc.SetEscapeHTML(false)
+	err := enc.Encode(struct {
+		Type string `json:"type"`
+		URL  string `json:"url"`
+	}{kind, url})
+	if err != nil {
+		return "", err
+	}
+	return strings.TrimSuffix(buf.String(), "\n"), nil
+}
+
 func (m *GenericManager) replaceInManifest(file, pkg string, mode replaceMode, opts ReplaceOptions) (*Result, error) {
 	start := time.Now()
 	path := filepath.Join(m.dir, file)
diff --git a/replace_manifest.go b/replace_manifest.go
index 1656aeb..cc523d0 100644
--- a/replace_manifest.go
+++ b/replace_manifest.go
@@ -183,7 +183,7 @@ func editGemfileReplaceEntry(path, pkg string, mode replaceMode, opts ReplaceOpt
 		if mode == replaceModeDrop {
 			return fmt.Errorf("gem %q not found in Gemfile", pkg)
 		}
-		lines = append(lines, fmt.Sprintf("gem %q%s\n", pkg, gemReplacementSuffix(mode, opts)))
+		lines = append(lines, fmt.Sprintf("gem %s%s\n", quoteRubyString(pkg), gemReplacementSuffix(mode, opts)))
 	}
 
 	return os.WriteFile(path, []byte(strings.Join(lines, "")), manifestPerm)
@@ -208,26 +208,26 @@ func updateGemfileLine(line, pkg string, mode replaceMode, opts ReplaceOptions)
 
 	switch mode {
 	case replaceModePath:
-		args = append(args, fmt.Sprintf("path: %q", opts.Path))
+		args = append(args, "path: "+quoteRubyString(opts.Path))
 	case replaceModeGit:
-		args = append(args, fmt.Sprintf("git: %q", opts.Git))
+		args = append(args, "git: "+quoteRubyString(opts.Git))
 		if opts.Ref != "" {
-			args = append(args, fmt.Sprintf("branch: %q", opts.Ref))
+			args = append(args, "branch: "+quoteRubyString(opts.Ref))
 		}
 	case replaceModeDrop:
 	}
 
-	return fmt.Sprintf("%sgem %q%s%s", indent, pkg, formatGemfileArgs(args, comment), newline), nil
+	return fmt.Sprintf("%sgem %s%s%s", indent, quoteRubyString(pkg), formatGemfileArgs(args, comment), newline), nil
 }
 
 func gemReplacementSuffix(mode replaceMode, opts ReplaceOptions) string {
 	switch mode {
 	case replaceModePath:
-		return fmt.Sprintf(", path: %q", opts.Path)
+		return ", path: " + quoteRubyString(opts.Path)
 	case replaceModeGit:
-		s := fmt.Sprintf(", git: %q", opts.Git)
+		s := ", git: " + quoteRubyString(opts.Git)
 		if opts.Ref != "" {
-			s += fmt.Sprintf(", branch: %q", opts.Ref)
+			s += ", branch: " + quoteRubyString(opts.Ref)
 		}
 		return s
 	default:
@@ -235,6 +235,28 @@ func gemReplacementSuffix(mode replaceMode, opts ReplaceOptions) string {
 	}
 }
 
+func quoteRubyString(s string) string {
+	var quoted strings.Builder
+	quoted.WriteByte('"')
+	for i := range len(s) {
+		c := s[i]
+		switch c {
+		case '\\', '"', '#':
+			quoted.WriteByte('\\')
+			quoted.WriteByte(c)
+		default:
+			if c < ' ' || c >= '\x7f' {
+				// Byte escapes preserve invalid UTF-8 and avoid Go-only Unicode escapes.
+				fmt.Fprintf(&quoted, `\x%02x`, c)
+			} else {
+				quoted.WriteByte(c)
+			}
+		}
+	}
+	quoted.WriteByte('"')
+	return quoted.String()
+}
+
 func parseGemfileArgs(tail string) ([]string, string) {
 	argsText, comment := splitRubyLineComment(tail)
 	parts := splitRubyArgs(argsText)
diff --git a/replace_test.go b/replace_test.go
index 516cfe7..b381572 100644
--- a/replace_test.go
+++ b/replace_test.go
@@ -2,6 +2,7 @@ package managers
 
 import (
 	"context"
+	"encoding/json"
 	"errors"
 	"os"
 	"path/filepath"
@@ -226,6 +227,34 @@ func TestReplaceComposerGitRefRunsRequire(t *testing.T) {
 	}
 }
 
+func TestReplaceComposerPayloadIsJSON(t *testing.T) {
+	values := []string{
+		`../a"b\c`,
+		"../\x00\a\v\x1b\x7f",
+		"../é\U000e0001",
+		"https://example.com/pkg?a=1&b=<2>",
+	}
+	for _, value := range values {
+		for _, opts := range []ReplaceOptions{{Path: value}, {Git: value}} {
+			runner := NewMockRunner()
+			mgr := newReplaceTestManager(t, "composer", "/test/dependent", runner)
+			if _, err := mgr.Replace(context.Background(), "vendor/pkg", opts); err != nil {
+				t.Fatalf("Replace: %v", err)
+			}
+			payload := runner.Captured[0][3]
+			var got struct {
+				URL string `json:"url"`
+			}
+			if err := json.Unmarshal([]byte(payload), &got); err != nil {
+				t.Fatalf("payload %q is not JSON: %v", payload, err)
+			}
+			if got.URL != value {
+				t.Errorf("payload url = %q, want %q", got.URL, value)
+			}
+		}
+	}
+}
+
 func TestReplaceComposerDrop(t *testing.T) {
 	runner := NewMockRunner()
 	mgr := newReplaceTestManager(t, "composer", "/test/dependent", runner)
@@ -393,6 +422,85 @@ func TestReplaceGemfileAppendsMissingGem(t *testing.T) {
 	}
 }
 
+func TestReplaceGemfileQuotesReplacementValues(t *testing.T) {
+	tests := []struct {
+		name   string
+		value  string
+		quoted string
+	}{
+		{"expression", `../#{raise('interpolated')}`, `"../\#{raise('interpolated')}"`},
+		{"variables", `../#@path/#@@path/#$path`, `"../\#@path/\#@@path/\#$path"`},
+		{"quotes and backslashes", `../a\"b'c`, `"../a\\\"b'c"`},
+		{"backslash before interpolation", `../\#{1+1}`, `"../\\\#{1+1}"`},
+		{"controls", "../\x00\a\b\t\n\v\f\r\x1b\x7f", `"../\x00\x07\x08\x09\x0a\x0b\x0c\x0d\x1b\x7f"`},
+		{"unicode", "../é\U000e0001", `"../\xc3\xa9\xf3\xa0\x80\x81"`},
+		{"invalid UTF-8", "../\xff", `"../\xff"`},
+	}
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			for _, variant := range []struct {
+				name    string
+				initial string
+				git     bool
+			}{
+				{"path/append", "", false},
+				{"path/edit", "gem \"rake\"\n", false},
+				{"git/append", "", true},
+				{"git/edit", "gem \"rake\"\n", true},
+			} {
+				t.Run(variant.name, func(t *testing.T) {
+					dir := t.TempDir()
+					gemfile := filepath.Join(dir, "Gemfile")
+					writeFile(t, gemfile, variant.initial)
+					mgr := newReplaceTestManager(t, "bundler", dir, NewMockRunner())
+					opts := ReplaceOptions{Path: tt.value}
+					want := `gem "rake", path: ` + tt.quoted + "\n"
+					if variant.git {
+						opts = ReplaceOptions{Git: tt.value, Ref: tt.value}
+						want = `gem "rake", git: ` + tt.quoted + ", branch: " + tt.quoted + "\n"
+					}
+					_, err := mgr.Replace(context.Background(), "rake", opts)
+					if err != nil {
+						t.Fatalf("Replace: %v", err)
+					}
+					if got := readFile(t, gemfile); got != want {
+						t.Fatalf("Gemfile = %q, want %q", got, want)
+					}
+				})
+			}
+		})
+	}
+}
+
+func TestReplaceGemfileQuotesGemName(t *testing.T) {
+	const pkg = `rake#{raise('interpolated')}`
+	tests := []struct {
+		name    string
+		initial string
+		opts    ReplaceOptions
+		want    string
+	}{
+		{"append", "", ReplaceOptions{Path: "../rake"}, `gem "rake\#{raise('interpolated')}", path: "../rake"` + "\n"},
+		{"edit", "gem '" + pkg + "'\n", ReplaceOptions{Path: "../rake"}, `gem "rake\#{raise('interpolated')}", path: "../rake"` + "\n"},
+		{"drop", "gem '" + pkg + "', path: '../rake'\n", ReplaceOptions{Drop: true}, `gem "rake\#{raise('interpolated')}"` + "\n"},
+	}
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			dir := t.TempDir()
+			gemfile := filepath.Join(dir, "Gemfile")
+			writeFile(t, gemfile, tt.initial)
+			mgr := newReplaceTestManager(t, "bundler", dir, NewMockRunner())
+			_, err := mgr.Replace(context.Background(), pkg, tt.opts)
+			if err != nil {
+				t.Fatalf("Replace: %v", err)
+			}
+			if got := readFile(t, gemfile); got != tt.want {
+				t.Fatalf("Gemfile = %q, want %q", got, tt.want)
+			}
+		})
+	}
+}
+
 func TestUpdateTOMLSectionEntry(t *testing.T) {
 	in := "[patch.crates-io] # local overrides\nfoo = { path = \"../foo\" }\n\n[other]\nx = 1\n"