diff --git a/.github/workflows/build.yml b/.github/workflows/build.yml
index 66201ae..87db682 100644
--- a/.github/workflows/build.yml
+++ b/.github/workflows/build.yml
@@ -89,6 +89,62 @@ jobs:
           revive -config revive.toml -formatter friendly ./... >> $GITHUB_STEP_SUMMARY
           printf '```\n\n' >> $GITHUB_STEP_SUMMARY
 
+  wasm:
+    name: Cross-build ${{ matrix.goos }}/${{ matrix.goarch }} with ${{ matrix.go-version }} on ${{ matrix.vm-os }}
+    runs-on: ${{ matrix.vm-os }}
+    strategy:
+      max-parallel: 10
+      fail-fast: false
+      matrix:
+        vm-os: [
+          ubuntu-22.04,
+          macos-14,
+          windows-2022
+        ]
+        go-version: [
+          1.19.x,
+          1.25.x,
+        ]
+        goos: [
+          js,
+          wasip1
+        ]
+        goarch: [
+          wasm
+        ]
+        exclude:
+          # WASI support starts after Go 1.19; keep the compatibility floor
+          # on browser js/wasm and verify WASI on the current toolchain.
+          - go-version: 1.19.x
+            goos: wasip1
+    permissions:
+      contents: read
+    steps:
+      - uses: actions/checkout@v5
+      - name: Set up Go
+        uses: actions/setup-go@v6
+        with:
+          go-version: ${{ matrix.go-version }}
+          cache: true
+      - name: Cross-build package
+        env:
+          GOOS: ${{ matrix.goos }}
+          GOARCH: ${{ matrix.goarch }}
+        run: |
+          go version
+          go tool dist list | grep -Fx "${GOOS}/${GOARCH}"
+          go mod download
+          go build .
+      - name: Guard REPL terminal dependencies
+        env:
+          GOOS: ${{ matrix.goos }}
+          GOARCH: ${{ matrix.goarch }}
+        run: |
+          if go list -deps . | grep -E '^(go\.starlark\.net/repl|github\.com/chzyer/readline)$'; then
+            echo "terminal-only REPL dependency leaked into ${GOOS}/${GOARCH}"
+            exit 1
+          fi
+
   # cmd/starlet is a separate Go module with its own (older) pins and extra
   # third-party dependencies; build it as a non-gating signal so a CLI-side
   # breakage is visible without blocking library PRs.
diff --git a/run.go b/run.go
index 55f6bfd..202b069 100644
--- a/run.go
+++ b/run.go
@@ -11,20 +11,16 @@ import (
 
 	"github.com/1set/starlet/lib/goidiomatic"
 	"github.com/1set/starlight/convert"
-	"go.starlark.net/repl"
 	"go.starlark.net/starlark"
 	"go.starlark.net/syntax"
 )
 
-// REPL is a Read-Eval-Print-Loop for Starlark.
-// It loads the predeclared symbols and modules into the global environment,
-func (m *Machine) REPL() {
-	if err := m.prepareThread(nil); err != nil {
-		repl.PrintError(err)
-		return
-	}
-	repl.REPLOptions(m.getFileOptions(), m.thread, m.predeclared)
-}
+// REPL is defined in run_repl.go (terminal targets) / run_repl_stub.go
+// (non-terminal targets): the
+// interactive REPL pulls go.starlark.net/repl -> chzyer/readline, a terminal
+// library that does not compile for browser js/wasm or WASI. Isolating it
+// behind a build tag keeps the library core (and every consumer, e.g. a WASM
+// playground) free of that terminal dependency.
 
 // RunScript initiates a Machine, executes a script with extra variables, and returns the Machine and the execution result.
 func RunScript(content []byte, extras StringAnyMap) (*Machine, StringAnyMap, error) {
diff --git a/run_repl.go b/run_repl.go
new file mode 100644
index 0000000..787fc48
--- /dev/null
+++ b/run_repl.go
@@ -0,0 +1,19 @@
+//go:build !js && !wasip1
+
+package starlet
+
+import "go.starlark.net/repl"
+
+// REPL is a Read-Eval-Print-Loop for Starlark. It loads the predeclared
+// symbols and modules into the global environment and reads from the terminal.
+//
+// This lives in a terminal-target file because go.starlark.net/repl pulls in
+// chzyer/readline, a terminal library that does not compile for browser
+// js/wasm or WASI; see run_repl_stub.go for the no-terminal stub.
+func (m *Machine) REPL() {
+	if err := m.prepareThread(nil); err != nil {
+		repl.PrintError(err)
+		return
+	}
+	repl.REPLOptions(m.getFileOptions(), m.thread, m.predeclared)
+}
diff --git a/run_repl_stub.go b/run_repl_stub.go
new file mode 100644
index 0000000..c9488a7
--- /dev/null
+++ b/run_repl_stub.go
@@ -0,0 +1,12 @@
+//go:build js || wasip1
+
+package starlet
+
+// REPL is unavailable on no-terminal wasm targets: the upstream
+// go.starlark.net/repl pulls in chzyer/readline, which depends on terminal
+// primitives that are not available in browser js/wasm or WASI. Drive the
+// Machine with Run/RunScript instead. This stub keeps the method present so
+// consumer code referencing REPL still compiles for wasm.
+func (m *Machine) REPL() {
+	// no-op on no-terminal wasm targets; use Run/RunScript
+}
diff --git a/run_test.go b/run_test.go
index 8ead922..04a1125 100644
--- a/run_test.go
+++ b/run_test.go
@@ -6,7 +6,9 @@ import (
 	"fmt"
 	"io/fs"
 	"os"
+	"os/exec"
 	"reflect"
+	"strings"
 	"testing"
 	"time"
 
@@ -2140,6 +2142,53 @@ func TestMachine_REPL_Error(t *testing.T) {
 	m.REPL()
 }
 
+// TestBuildsForWASM guards the wasm portability of the library core.
+// The interactive REPL pulls go.starlark.net/repl -> github.com/chzyer/readline,
+// a terminal library that does not compile for no-terminal wasm targets; it is
+// therefore isolated behind build tags (run_repl.go for terminal targets,
+// run_repl_stub.go for js/wasm and WASI). Re-importing a terminal dependency
+// into an always-compiled file would silently break wasm consumers (e.g. a
+// browser playground).
+func TestBuildsForWASM(t *testing.T) {
+	goBin, err := exec.LookPath("go")
+	if err != nil {
+		t.Skip("go toolchain not found; skipping wasm build check")
+	}
+
+	targets := []struct {
+		goos   string
+		goarch string
+	}{
+		{goos: "js", goarch: "wasm"},
+		{goos: "wasip1", goarch: "wasm"},
+	}
+	for _, target := range targets {
+		target := target
+		t.Run(target.goos+"/"+target.goarch, func(t *testing.T) {
+			if !goTargetSupported(t, goBin, target.goos, target.goarch) {
+				t.Skipf("go toolchain does not support %s/%s", target.goos, target.goarch)
+			}
+			cmd := exec.Command(goBin, "build", ".")
+			cmd.Env = append(os.Environ(), "GOOS="+target.goos, "GOARCH="+target.goarch)
+			if out, err := cmd.CombinedOutput(); err != nil {
+				t.Fatalf("starlet must compile for GOOS=%s GOARCH=%s — did a terminal "+
+					"dependency (e.g. chzyer/readline via go.starlark.net/repl) leak into "+
+					"an always-compiled file?\n%s\n%v", target.goos, target.goarch, out, err)
+			}
+		})
+	}
+}
+
+func goTargetSupported(t *testing.T, goBin, goos, goarch string) bool {
+	t.Helper()
+	out, err := exec.Command(goBin, "tool", "dist", "list").Output()
+	if err != nil {
+		t.Logf("warning: failed to list go targets: %v", err)
+		return false
+	}
+	return strings.Contains(string(out), goos+"/"+goarch+"\n")
+}
+
 func Test_Machine_Run_LoadTypedErrors(t *testing.T) {
 	// an unknown name surfaces as ModuleNotFoundError through the chain
 	m := starlet.NewDefault()