diff --git a/Justfile b/Justfile
index 5d150150..c25a00aa 100644
--- a/Justfile
+++ b/Justfile
@@ -24,6 +24,15 @@ tests:
 test:
     @go test -race -covermode=atomic -coverprofile coverage.txt ./...
 
+# Build the browser WebAssembly binding (dist/numscript.wasm + wasm_exec.js).
+# wasm_exec.js is version-locked to the building toolchain, so always copy it
+# from GOROOT rather than pinning a checked-in copy.
+wasm:
+    @mkdir -p dist
+    @GOOS=js GOARCH=wasm go build -trimpath -ldflags="-s -w" -o dist/numscript.wasm ./cmd/wasm
+    @cp "$(go env GOROOT)/lib/wasm/wasm_exec.js" dist/wasm_exec.js
+    @ls -lh dist/numscript.wasm
+
 release-local:
     @goreleaser release --nightly --skip=publish --clean
 
diff --git a/cmd/wasm/main.go b/cmd/wasm/main.go
new file mode 100644
index 00000000..a914edf1
--- /dev/null
+++ b/cmd/wasm/main.go
@@ -0,0 +1,68 @@
+//go:build js && wasm
+
+// Command wasm is the browser-facing entry point of the Numscript WebAssembly
+// binding. It is a thin syscall/js shim over internal/wasmbinding: it registers
+// a single namespace object on globalThis.__numscript and then blocks forever so
+// the Go runtime (and the registered callbacks) stay alive for the lifetime of
+// the page.
+//
+// All the real work — parsing, static analysis, execution, JSON shaping, panic
+// recovery — lives in internal/wasmbinding, which is pure Go and unit-tested on
+// the host toolchain. Each registered function takes a single string argument
+// and returns a JSON string the TS binding parses.
+package main
+
+import (
+	"syscall/js"
+
+	"github.com/formancehq/numscript/internal/wasmbinding"
+)
+
+func stringArg(args []js.Value) (string, bool) {
+	if len(args) < 1 || args[0].Type() != js.TypeString {
+		return "", false
+	}
+	return args[0].String(), true
+}
+
+func main() {
+	ns := js.Global().Get("Object").New()
+
+	// check(source) -> JSON { diagnostics: [...] }
+	ns.Set("check", js.FuncOf(func(_ js.Value, args []js.Value) any {
+		source, ok := stringArg(args)
+		if !ok {
+			return `{"diagnostics":[]}`
+		}
+		return string(wasmbinding.Check(source))
+	}))
+
+	// run(inputJSON) -> JSON { ok, value } | { ok:false, error }
+	ns.Set("run", js.FuncOf(func(_ js.Value, args []js.Value) any {
+		input, ok := stringArg(args)
+		if !ok {
+			return `{"ok":false,"error":"run expects a JSON string argument"}`
+		}
+		return string(wasmbinding.Run([]byte(input)))
+	}))
+
+	// getNeededVariables(source) -> JSON { ok, variables } | { ok:false, error }
+	ns.Set("getNeededVariables", js.FuncOf(func(_ js.Value, args []js.Value) any {
+		source, ok := stringArg(args)
+		if !ok {
+			return `{"ok":false,"error":"getNeededVariables expects a string argument"}`
+		}
+		return string(wasmbinding.NeededVariables(source))
+	}))
+
+	// version() -> the binding's ABI version string
+	ns.Set("version", js.FuncOf(func(_ js.Value, _ []js.Value) any {
+		return wasmbinding.Version
+	}))
+
+	js.Global().Set("__numscript", ns)
+
+	// Block forever: if main returned, the Go runtime would exit and every
+	// registered js.Func would become invalid.
+	select {}
+}
diff --git a/internal/wasmbinding/binding.go b/internal/wasmbinding/binding.go
new file mode 100644
index 00000000..3c332cb7
--- /dev/null
+++ b/internal/wasmbinding/binding.go
@@ -0,0 +1,273 @@
+// Package wasmbinding is the host-testable core of the Numscript WebAssembly
+// binding. It is deliberately free of syscall/js so its JSON/shape logic can be
+// unit-tested with the normal Go toolchain; the js/wasm shim in ../../cmd/wasm
+// is a thin wrapper that calls these functions and marshals the result across
+// the JS boundary.
+//
+// Every exported function takes and returns plain bytes/strings and never
+// panics: a panic in any of them is recovered and turned into an error
+// envelope, so a malformed input can never take down the long-lived Go runtime
+// that backs an editor.
+package wasmbinding
+
+import (
+	"context"
+	"encoding/json"
+	"fmt"
+	"math/big"
+
+	"github.com/formancehq/numscript"
+	"github.com/formancehq/numscript/internal/analysis"
+)
+
+// ---- Static analysis (check) --------------------------------------------------
+
+type position struct {
+	Line      int `json:"line"`
+	Character int `json:"character"`
+}
+
+type rng struct {
+	Start position `json:"start"`
+	End   position `json:"end"`
+}
+
+type diagnostic struct {
+	Range    rng    `json:"range"`
+	Severity byte   `json:"severity"` // LSP-aligned: 1=Error 2=Warning 3=Info 4=Hint
+	Message  string `json:"message"`
+	Id       int32  `json:"id"`
+}
+
+type checkEnvelope struct {
+	Diagnostics []diagnostic `json:"diagnostics"`
+}
+
+// Check runs the full static-analysis pass (parse + type checking) over the
+// given source and returns a JSON envelope of diagnostics. Ranges are 0-based
+// {line, character}, faithful to the Go contract; the TS binding converts them
+// to 1-based Monaco markers. Severities are the LSP-aligned values returned by
+// the analysis package verbatim.
+func Check(source string) (out []byte) {
+	defer func() {
+		if r := recover(); r != nil {
+			// Surface the internal failure as a single error diagnostic rather
+			// than silently swallowing it, so the editor still shows something.
+			out = mustMarshal(checkEnvelope{Diagnostics: []diagnostic{{
+				Severity: byte(analysis.ErrorSeverity),
+				Message:  fmt.Sprintf("internal error: %v", r),
+			}}})
+		}
+	}()
+
+	result := analysis.CheckSource(source)
+
+	diagnostics := make([]diagnostic, 0, len(result.Diagnostics))
+	for _, d := range result.Diagnostics {
+		diagnostics = append(diagnostics, diagnostic{
+			Range: rng{
+				Start: position{Line: d.Range.Start.Line, Character: d.Range.Start.Character},
+				End:   position{Line: d.Range.End.Line, Character: d.Range.End.Character},
+			},
+			Severity: d.Kind.Severity(),
+			Message:  d.Kind.Message(),
+			Id:       d.Id,
+		})
+	}
+
+	return mustMarshal(checkEnvelope{Diagnostics: diagnostics})
+}
+
+// ---- Execution (run) ----------------------------------------------------------
+
+// runInput mirrors the playground's existing request payload so the UI call
+// site barely changes. balances/metadata use the nested-object shape the
+// playground already produces (not the CLI's array shape).
+type runInput struct {
+	Script       string                            `json:"script"`
+	Variables    map[string]string                 `json:"variables"`
+	Balances     map[string]map[string]json.Number `json:"balances"`
+	Metadata     map[string]map[string]string      `json:"metadata"`
+	FeatureFlags []string                          `json:"featureFlags"`
+}
+
+// runValue is the success payload. Fields carry numscript's own JSON tags, and
+// the slices/maps are guaranteed non-nil so the JSON never contains `null`
+// where the UI expects an array/object.
+type runValue struct {
+	Postings     []numscript.Posting           `json:"postings"`
+	TxMeta       map[string]string             `json:"txMeta"`
+	AccountsMeta numscript.SetAccountsMetadata `json:"accountsMeta"`
+}
+
+type runEnvelope struct {
+	Ok    bool      `json:"ok"`
+	Value *runValue `json:"value,omitempty"`
+	Error string    `json:"error,omitempty"`
+}
+
+// Run parses and executes the given script against an in-memory store built
+// entirely from the input (no network). It returns a JSON envelope:
+//
+//	{ "ok": true,  "value": { "postings", "txMeta", "accountsMeta" } }
+//	{ "ok": false, "error": "<message>\n<annotated source>" }
+func Run(inputJSON []byte) (out []byte) {
+	defer func() {
+		if r := recover(); r != nil {
+			out = mustMarshal(runEnvelope{Ok: false, Error: fmt.Sprintf("internal error: %v", r)})
+		}
+	}()
+
+	var input runInput
+	if err := json.Unmarshal(inputJSON, &input); err != nil {
+		return mustMarshal(runEnvelope{Ok: false, Error: fmt.Sprintf("invalid input: %v", err)})
+	}
+
+	balances, err := toBalances(input.Balances)
+	if err != nil {
+		return mustMarshal(runEnvelope{Ok: false, Error: err.Error()})
+	}
+
+	store := numscript.StaticStore{
+		Balances: balances,
+		Meta:     toAccountsMetadata(input.Metadata),
+	}
+
+	flags := make(map[string]struct{}, len(input.FeatureFlags))
+	for _, flag := range input.FeatureFlags {
+		flags[flag] = struct{}{}
+	}
+
+	vars := numscript.VariablesMap(input.Variables)
+	if vars == nil {
+		vars = numscript.VariablesMap{}
+	}
+
+	parsed := numscript.Parse(input.Script)
+	result, iErr := parsed.RunWithFeatureFlags(context.Background(), vars, store, flags)
+	if iErr != nil {
+		// Reuse the CLI's annotated-source formatting: the error message
+		// followed by the offending source span. Useful for runtime errors
+		// (missing funds, etc.) that the static pass can't catch.
+		msg := iErr.Error() + "\n" + iErr.GetRange().ShowOnSource(input.Script)
+		return mustMarshal(runEnvelope{Ok: false, Error: msg})
+	}
+
+	value := &runValue{
+		Postings:     result.Postings,
+		TxMeta:       result.Metadata,
+		AccountsMeta: result.AccountsMetadata,
+	}
+	if value.Postings == nil {
+		value.Postings = []numscript.Posting{}
+	}
+	if value.TxMeta == nil {
+		value.TxMeta = map[string]string{}
+	}
+	if value.AccountsMeta == nil {
+		value.AccountsMeta = numscript.SetAccountsMetadata{}
+	}
+
+	return mustMarshal(runEnvelope{Ok: true, Value: value})
+}
+
+// toBalances flattens the nested {account: {asset: amount}} object into the
+// interpreter's row shape, preserving integer precision via big.Int (the
+// amounts arrive as json.Number, never float64).
+func toBalances(nested map[string]map[string]json.Number) (numscript.Balances, error) {
+	balances := numscript.Balances{}
+	for account, assets := range nested {
+		for asset, amount := range assets {
+			n, ok := parseIntAmount(amount.String())
+			if !ok {
+				return nil, fmt.Errorf("invalid balance amount for %s %s: %q", account, asset, amount.String())
+			}
+			balances = append(balances, numscript.BalanceRow{
+				Account: account,
+				Asset:   asset,
+				Amount:  n,
+			})
+		}
+	}
+	return balances, nil
+}
+
+// parseIntAmount parses an integer balance amount. Plain base-10 integers are
+// taken exactly. A JS number can also reach us in scientific notation (e.g.
+// "1e+21" for a large integer that overflowed float64's exact range), so we fall
+// back to high-precision float parsing and accept it only when it is exactly
+// integral — genuine fractional amounts like "10.5" are still rejected.
+func parseIntAmount(s string) (*big.Int, bool) {
+	if n, ok := new(big.Int).SetString(s, 10); ok {
+		return n, true
+	}
+
+	f, _, err := big.ParseFloat(s, 10, 256, big.ToNearestEven)
+	if err != nil {
+		return nil, false
+	}
+	n, acc := f.Int(nil)
+	if acc != big.Exact {
+		return nil, false
+	}
+	return n, true
+}
+
+// toAccountsMetadata flattens the nested {account: {key: value}} object into the
+// interpreter's row shape.
+func toAccountsMetadata(nested map[string]map[string]string) numscript.AccountsMetadata {
+	meta := numscript.AccountsMetadata{}
+	for account, kv := range nested {
+		for key, value := range kv {
+			meta = append(meta, numscript.AccountMetadataRow{
+				Account: account,
+				Key:     key,
+				Value:   value,
+			})
+		}
+	}
+	return meta
+}
+
+// ---- Needed variables ---------------------------------------------------------
+
+type neededVarsEnvelope struct {
+	Ok        bool              `json:"ok"`
+	Variables map[string]string `json:"variables,omitempty"`
+	Error     string            `json:"error,omitempty"`
+}
+
+// NeededVariables returns the variables a script declares but does not define
+// inline (name -> type), so a host can pre-populate an inputs editor.
+func NeededVariables(source string) (out []byte) {
+	defer func() {
+		if r := recover(); r != nil {
+			out = mustMarshal(neededVarsEnvelope{Ok: false, Error: fmt.Sprintf("internal error: %v", r)})
+		}
+	}()
+
+	vars := numscript.Parse(source).GetNeededVariables()
+	if vars == nil {
+		vars = map[string]string{}
+	}
+	return mustMarshal(neededVarsEnvelope{Ok: true, Variables: vars})
+}
+
+// ---- Version ------------------------------------------------------------------
+
+// Version reports the binding's own ABI version. It is independent of the
+// numscript library version and lets a host detect an incompatible wasm asset.
+const Version = "1"
+
+// ---- helpers ------------------------------------------------------------------
+
+// mustMarshal marshals values whose shape is fully controlled here, so an error
+// is a programmer mistake, not a runtime condition. If it somehow fails we still
+// return a valid JSON error envelope rather than panicking.
+func mustMarshal(v any) []byte {
+	b, err := json.Marshal(v)
+	if err != nil {
+		return fmt.Appendf(nil, `{"ok":false,"error":%q}`, "failed to encode result: "+err.Error())
+	}
+	return b
+}
diff --git a/internal/wasmbinding/binding_test.go b/internal/wasmbinding/binding_test.go
new file mode 100644
index 00000000..15f83667
--- /dev/null
+++ b/internal/wasmbinding/binding_test.go
@@ -0,0 +1,198 @@
+package wasmbinding_test
+
+import (
+	"encoding/json"
+	"strings"
+	"testing"
+
+	"github.com/formancehq/numscript/internal/wasmbinding"
+	"github.com/stretchr/testify/require"
+)
+
+// ---- Check --------------------------------------------------------------------
+
+type checkEnvelope struct {
+	Diagnostics []struct {
+		Range struct {
+			Start struct {
+				Line      int `json:"line"`
+				Character int `json:"character"`
+			} `json:"start"`
+			End struct {
+				Line      int `json:"line"`
+				Character int `json:"character"`
+			} `json:"end"`
+		} `json:"range"`
+		Severity byte   `json:"severity"`
+		Message  string `json:"message"`
+		Id       int32  `json:"id"`
+	} `json:"diagnostics"`
+}
+
+func TestCheckValidScriptHasNoDiagnostics(t *testing.T) {
+	out := wasmbinding.Check(`send [COIN 10] (
+	source = @world
+	destination = @dest
+)`)
+
+	var env checkEnvelope
+	require.NoError(t, json.Unmarshal(out, &env))
+	require.Empty(t, env.Diagnostics)
+}
+
+func TestCheckUnboundVariableReportsErrorWithRange(t *testing.T) {
+	out := wasmbinding.Check(`send [COIN 10] (
+	source = $unbound
+	destination = @dest
+)`)
+
+	var env checkEnvelope
+	require.NoError(t, json.Unmarshal(out, &env))
+	require.NotEmpty(t, env.Diagnostics)
+
+	d := env.Diagnostics[0]
+	require.EqualValues(t, 1, d.Severity, "unbound variable should be an Error")
+	require.Contains(t, strings.ToLower(d.Message), "unbound")
+	// The range must be non-degenerate and sit on the second line (0-based).
+	require.Equal(t, 1, d.Range.Start.Line)
+	require.Greater(t, d.Range.End.Character, d.Range.Start.Character)
+}
+
+// ---- Run ----------------------------------------------------------------------
+
+type runEnvelope struct {
+	Ok    bool `json:"ok"`
+	Value *struct {
+		Postings []struct {
+			Source      string      `json:"source"`
+			Destination string      `json:"destination"`
+			Amount      json.Number `json:"amount"`
+			Asset       string      `json:"asset"`
+		} `json:"postings"`
+		TxMeta       map[string]string `json:"txMeta"`
+		AccountsMeta []struct {
+			Account string `json:"account"`
+			Key     string `json:"key"`
+			Value   string `json:"value"`
+		} `json:"accountsMeta"`
+	} `json:"value"`
+	Error string `json:"error"`
+}
+
+func TestRunSimpleSendFromWorld(t *testing.T) {
+	out := wasmbinding.Run([]byte(`{
+		"script": "send [COIN 10] (\n source = @world\n destination = @dest\n)",
+		"variables": {},
+		"balances": {},
+		"metadata": {},
+		"featureFlags": []
+	}`))
+
+	var env runEnvelope
+	require.NoError(t, json.Unmarshal(out, &env))
+	require.True(t, env.Ok, "unexpected error: %s", env.Error)
+	require.NotNil(t, env.Value)
+	require.Len(t, env.Value.Postings, 1)
+
+	p := env.Value.Postings[0]
+	require.Equal(t, "world", p.Source)
+	require.Equal(t, "dest", p.Destination)
+	require.Equal(t, "COIN", p.Asset)
+	require.Equal(t, "10", p.Amount.String())
+
+	// Non-nil JSON containers so the UI never sees `null`.
+	require.NotNil(t, env.Value.TxMeta)
+	require.NotNil(t, env.Value.AccountsMeta)
+}
+
+func TestRunPreservesLargeIntegerPrecision(t *testing.T) {
+	// 10^21 overflows float64's exact-integer range (2^53); json.Number + big.Int
+	// must carry it through losslessly.
+	const huge = "1000000000000000000000"
+	out := wasmbinding.Run([]byte(`{
+		"script": "send [COIN ` + huge + `] (\n source = @alice\n destination = @dest\n)",
+		"variables": {},
+		"balances": { "alice": { "COIN": ` + huge + ` } },
+		"metadata": {},
+		"featureFlags": []
+	}`))
+
+	var env runEnvelope
+	require.NoError(t, json.Unmarshal(out, &env))
+	require.True(t, env.Ok, "unexpected error: %s", env.Error)
+	require.Len(t, env.Value.Postings, 1)
+	require.Equal(t, huge, env.Value.Postings[0].Amount.String())
+}
+
+func TestRunAcceptsScientificNotationBalance(t *testing.T) {
+	// A JS number for a large integer stringifies as "1e+21"; the binding must
+	// accept it (exactly, since it is integral) rather than hard-erroring.
+	out := wasmbinding.Run([]byte(`{
+		"script": "send [COIN 1000000000000000000000] (\n source = @alice\n destination = @dest\n)",
+		"balances": { "alice": { "COIN": 1e21 } }
+	}`))
+
+	var env runEnvelope
+	require.NoError(t, json.Unmarshal(out, &env))
+	require.True(t, env.Ok, "unexpected error: %s", env.Error)
+	require.Len(t, env.Value.Postings, 1)
+	require.Equal(t, "1000000000000000000000", env.Value.Postings[0].Amount.String())
+}
+
+func TestRunMissingFundsReturnsAnnotatedError(t *testing.T) {
+	out := wasmbinding.Run([]byte(`{
+		"script": "send [COIN 10] (\n source = @alice\n destination = @dest\n)",
+		"variables": {},
+		"balances": {},
+		"metadata": {},
+		"featureFlags": []
+	}`))
+
+	var env runEnvelope
+	require.NoError(t, json.Unmarshal(out, &env))
+	require.False(t, env.Ok)
+	require.NotEmpty(t, env.Error)
+}
+
+func TestRunInvalidJSONReturnsErrorEnvelope(t *testing.T) {
+	out := wasmbinding.Run([]byte(`not json`))
+
+	var env runEnvelope
+	require.NoError(t, json.Unmarshal(out, &env))
+	require.False(t, env.Ok)
+	require.Contains(t, env.Error, "invalid input")
+}
+
+func TestRunInvalidBalanceAmountReturnsErrorEnvelope(t *testing.T) {
+	out := wasmbinding.Run([]byte(`{
+		"script": "send [COIN 10] (\n source = @alice\n destination = @dest\n)",
+		"balances": { "alice": { "COIN": 10.5 } }
+	}`))
+
+	var env runEnvelope
+	require.NoError(t, json.Unmarshal(out, &env))
+	require.False(t, env.Ok)
+	require.Contains(t, env.Error, "invalid balance amount")
+}
+
+// ---- NeededVariables ----------------------------------------------------------
+
+func TestNeededVariables(t *testing.T) {
+	out := wasmbinding.NeededVariables(`vars {
+	monetary $amt
+	account $dest
+}
+send $amt (
+	source = @world
+	destination = $dest
+)`)
+
+	var env struct {
+		Ok        bool              `json:"ok"`
+		Variables map[string]string `json:"variables"`
+		Error     string            `json:"error"`
+	}
+	require.NoError(t, json.Unmarshal(out, &env))
+	require.True(t, env.Ok)
+	require.Equal(t, map[string]string{"amt": "monetary", "dest": "account"}, env.Variables)
+}