diff --git a/internal/driver/chrome/driver.go b/internal/driver/chrome/driver.go
index 63aa2b2..220e7a6 100644
--- a/internal/driver/chrome/driver.go
+++ b/internal/driver/chrome/driver.go
@@ -21,6 +21,7 @@ import (
 	"github.com/chromedp/chromedp/kb"
 
 	"github.com/priyanshujain/sanderling/internal/driver"
+	"github.com/priyanshujain/sanderling/internal/verifier"
 )
 
 // Driver implements DeviceDriver via chromedp for web platform testing.
@@ -1055,10 +1056,19 @@ func (d *Driver) runCtx(ctx context.Context) (context.Context, context.CancelFun
 // InstallBundle registers the source so it runs at every freshly-navigated
 // document context, then immediately evaluates it against the current page so
 // the very first tick has access to the registered globals.
+//
+// The installed runtime then has to declare the action encoding it serializes,
+// and it has to be the one this binary decodes. The web bundle is resolved from
+// whatever @sanderling/spec the spec's project has installed, so a page can
+// carry a runtime that encodes an action differently from the runner that
+// dispatches it: both halves then run to completion, every action reports
+// success, and the gestures are wrong. Verifier.checkActionEncoding applies the
+// same rule to the goja host.
 func (d *Driver) InstallBundle(ctx context.Context, source []byte) error {
 	runCtx, cancel := d.runCtx(ctx)
 	defer cancel()
-	return chromedp.Run(runCtx,
+	var declaration actionEncodingDeclaration
+	if err := chromedp.Run(runCtx,
 		chromedp.ActionFunc(func(ctx context.Context) error {
 			if _, err := page.AddScriptToEvaluateOnNewDocument(string(source)).Do(ctx); err != nil {
 				return fmt.Errorf("addScriptToEvaluateOnNewDocument: %w", err)
@@ -1072,9 +1082,36 @@ func (d *Driver) InstallBundle(ctx context.Context, source []byte) error {
 			}
 			return nil
 		}),
-	)
+		chromedp.Evaluate(actionEncodingScript, &declaration),
+	); err != nil {
+		return err
+	}
+	if !declaration.InstallsPicker {
+		return nil
+	}
+	if declaration.Encoding != verifier.ActionWireContract {
+		return verifier.ActionEncodingError(declaration.Encoding)
+	}
+	return nil
 }
 
+type actionEncodingDeclaration struct {
+	InstallsPicker bool   `json:"installsPicker"`
+	Encoding       string `json:"encoding"`
+}
+
+// actionEncodingScript reports both halves of the rule in one round trip. A
+// bundle that installs no picker generates no actions, so it has no encoding to
+// disagree about; a bundle that installs a picker and declares nothing is an
+// @sanderling/spec older than the declaration, so an absent declaration is a
+// mismatch rather than a default. Coercing to a string keeps that case
+// decodable as "".
+const actionEncodingScript = `({
+	installsPicker: window.__sanderlingNextAction__ !== undefined &&
+		window.__sanderlingNextAction__ !== null,
+	encoding: String(window.__sanderlingActionEncoding__ ?? ""),
+})`
+
 // EvaluateExtractors invokes the bundle-installed extractor table and returns
 // each extractor's JSON-encoded current value keyed by its registration index.
 //
diff --git a/internal/driver/chrome/driver_test.go b/internal/driver/chrome/driver_test.go
index f18f03e..f612010 100644
--- a/internal/driver/chrome/driver_test.go
+++ b/internal/driver/chrome/driver_test.go
@@ -10,13 +10,17 @@ import (
 	"net/http"
 	"net/http/httptest"
 	"os"
+	"path/filepath"
+	"strconv"
 	"strings"
 	"testing"
 	"time"
 
 	"github.com/chromedp/chromedp"
+	"github.com/priyanshujain/sanderling/internal/bundler"
 	"github.com/priyanshujain/sanderling/internal/driver"
 	"github.com/priyanshujain/sanderling/internal/hierarchy"
+	"github.com/priyanshujain/sanderling/internal/verifier"
 )
 
 // TestLaunch_ClearStateWipesStorageForTheTargetOrigin covers the CLI's default
@@ -1570,3 +1574,115 @@ func TestSwipe_DeliversATrustedDragToARowHandler(t *testing.T) {
 		t.Errorf("row status = %q, want %q", status, "dismissed left trusted")
 	}
 }
+
+// TestInstallBundle_RefusesARuntimeThatDeclaresADifferentActionEncoding covers
+// the pairing that voided a whole campaign: a spec bundled by an older
+// @sanderling/spec, run by this binary. internal/testrun resolves the web
+// runtime from whatever the spec's project has installed, so the page's picker
+// and the runner that dispatches its actions can encode a gesture differently.
+// Neither half fails: every action dispatches successfully and executes the
+// wrong gesture, and the run reports a full step count of results that mean
+// nothing.
+func TestInstallBundle_RefusesARuntimeThatDeclaresADifferentActionEncoding(t *testing.T) {
+	server := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, _ *http.Request) {
+		w.Header().Set("Content-Type", "text/html")
+		_, _ = w.Write([]byte(`<body><div id="app">app</div></body>`))
+	}))
+	defer server.Close()
+
+	d := New()
+	defer d.Terminate(context.Background())
+	ctx, cancel := context.WithTimeout(context.Background(), 60*time.Second)
+	defer cancel()
+	if err := d.Launch(ctx, server.URL, false, nil); err != nil {
+		t.Fatalf("Launch: %v", err)
+	}
+
+	const legacyRuntime = `window.__sanderlingNextAction__ = function () { return null; };`
+	if err := d.InstallBundle(ctx, []byte(legacyRuntime)); err == nil {
+		t.Error("InstallBundle accepted a runtime that declares no action encoding; " +
+			"the run would dispatch every action successfully and execute the wrong gesture")
+	} else if !strings.Contains(err.Error(), verifier.ActionWireContract) {
+		t.Errorf("InstallBundle error %q does not name the encoding this binary implements", err)
+	}
+
+	currentRuntime := legacyRuntime +
+		`window.__sanderlingActionEncoding__ = ` + strconv.Quote(verifier.ActionWireContract) + `;`
+	if err := d.InstallBundle(ctx, []byte(currentRuntime)); err != nil {
+		t.Fatalf("InstallBundle rejected a runtime on this binary's encoding: %v", err)
+	}
+}
+
+// TestInstallBundle_AcceptsTheWebRuntimeThisCheckoutShips is the other half of
+// the gate: the encoding pkg/spec/src/web-runtime.ts declares has to be the one
+// this binary decodes, or every web run refuses to start.
+func TestInstallBundle_AcceptsTheWebRuntimeThisCheckoutShips(t *testing.T) {
+	directory := t.TempDir()
+	specPath := filepath.Join(directory, "spec.ts")
+	const spec = `
+import { actions, Wait } from "@sanderling/spec";
+export const actionsRoot = actions(Wait({ durationMillis: 1 }));
+export const properties = {};
+`
+	if err := os.WriteFile(specPath, []byte(spec), 0o600); err != nil {
+		t.Fatal(err)
+	}
+	apiPath, err := filepath.Abs("../../../pkg/spec/src/index.ts")
+	if err != nil {
+		t.Fatal(err)
+	}
+	runtimePath, err := filepath.Abs("../../../pkg/spec/src/web-runtime.ts")
+	if err != nil {
+		t.Fatal(err)
+	}
+	bundle, err := bundler.BundleWeb(bundler.WebOptions{
+		EntryFile:      specPath,
+		WebRuntimeFile: runtimePath,
+		Aliases:        map[string]string{"@sanderling/spec": apiPath},
+	})
+	if err != nil {
+		t.Fatalf("BundleWeb: %v", err)
+	}
+
+	server := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, _ *http.Request) {
+		w.Header().Set("Content-Type", "text/html")
+		_, _ = w.Write([]byte(`<body><div id="app">app</div></body>`))
+	}))
+	defer server.Close()
+
+	d := New()
+	defer d.Terminate(context.Background())
+	ctx, cancel := context.WithTimeout(context.Background(), 60*time.Second)
+	defer cancel()
+	if err := d.Launch(ctx, server.URL, false, nil); err != nil {
+		t.Fatalf("Launch: %v", err)
+	}
+	if err := d.InstallBundle(ctx, bundle.JavaScript); err != nil {
+		t.Fatalf("InstallBundle rejected this checkout's own web runtime: %v", err)
+	}
+}
+
+// TestInstallBundle_AcceptsAPageThatInstallsNoPicker pins the other half of the
+// rule Verifier.checkActionEncoding states: a bundle that installs no picker
+// generates no actions, so it has no encoding to disagree about and demanding a
+// declaration from it would refuse a spec that was never going to dispatch.
+func TestInstallBundle_AcceptsAPageThatInstallsNoPicker(t *testing.T) {
+	server := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, _ *http.Request) {
+		w.Header().Set("Content-Type", "text/html")
+		_, _ = w.Write([]byte(`<body><div id="app">app</div></body>`))
+	}))
+	defer server.Close()
+
+	d := New()
+	defer d.Terminate(context.Background())
+	ctx, cancel := context.WithTimeout(context.Background(), 60*time.Second)
+	defer cancel()
+	if err := d.Launch(ctx, server.URL, false, nil); err != nil {
+		t.Fatalf("Launch: %v", err)
+	}
+
+	const pickerFree = `window.__sanderlingBundleCheck__ = true;`
+	if err := d.InstallBundle(ctx, []byte(pickerFree)); err != nil {
+		t.Fatalf("InstallBundle refused a bundle that generates no actions: %v", err)
+	}
+}