diff --git a/internal/bundler/web_bundle_test.go b/internal/bundler/web_bundle_test.go
index 0863101..95070d0 100644
--- a/internal/bundler/web_bundle_test.go
+++ b/internal/bundler/web_bundle_test.go
@@ -122,6 +122,38 @@ func TestBundleWeb_IsDeterministic(t *testing.T) {
 	}
 }
 
+// TestBundleWeb_InjectsSeedDefine asserts that a SANDERLING_SEED define is
+// inlined into the bundle so --seed reaches the web runtime PRNG. esbuild
+// replaces process.env.SANDERLING_SEED with the quoted literal at build time.
+func TestBundleWeb_InjectsSeedDefine(t *testing.T) {
+	directory := t.TempDir()
+	runtimePath := filepath.Join(directory, "web-runtime.ts")
+	specPath := filepath.Join(directory, "spec.ts")
+	runtime := fakeRuntime + "\n(globalThis as Record<string, unknown>).__seed = process.env.SANDERLING_SEED;\n"
+	if err := os.WriteFile(runtimePath, []byte(runtime), 0o644); err != nil {
+		t.Fatal(err)
+	}
+	if err := os.WriteFile(specPath, []byte(fakeSpec), 0o644); err != nil {
+		t.Fatal(err)
+	}
+
+	result, err := BundleWeb(WebOptions{
+		EntryFile:      specPath,
+		WebRuntimeFile: runtimePath,
+		Defines:        map[string]string{"SANDERLING_SEED": "8675309"},
+	})
+	if err != nil {
+		t.Fatalf("BundleWeb: %v", err)
+	}
+	source := string(result.JavaScript)
+	if !strings.Contains(source, "8675309") {
+		t.Errorf("seed literal not inlined into bundle\nsource head:\n%s", head(source, 500))
+	}
+	if strings.Contains(source, "process.env.SANDERLING_SEED") {
+		t.Errorf("define left unsubstituted in bundle")
+	}
+}
+
 func head(text string, max int) string {
 	if len(text) <= max {
 		return text
diff --git a/internal/testrun/testrun_test.go b/internal/testrun/testrun_test.go
index 9611150..5cd63bf 100644
--- a/internal/testrun/testrun_test.go
+++ b/internal/testrun/testrun_test.go
@@ -3,9 +3,31 @@ package testrun
 import (
 	"os"
 	"path/filepath"
+	"strconv"
 	"testing"
 )
 
+func TestResolveSeed_UsesConfiguredWhenNonZero(t *testing.T) {
+	if got := resolveSeed(42); got != 42 {
+		t.Fatalf("got %d, want 42", got)
+	}
+}
+
+func TestResolveSeed_DerivesWhenZero(t *testing.T) {
+	if got := resolveSeed(0); got == 0 {
+		t.Fatal("expected a non-zero time-derived seed")
+	}
+}
+
+// TestSeedDefineFormatsAsDecimal guards the contract that Execute uses to put
+// the seed into the web bundle: strconv.FormatInt base 10, which web-runtime
+// folds to a 32-bit PRNG seed.
+func TestSeedDefineFormatsAsDecimal(t *testing.T) {
+	if got := strconv.FormatInt(resolveSeed(8675309), 10); got != "8675309" {
+		t.Fatalf("got %q, want 8675309", got)
+	}
+}
+
 func TestResolveSpecAPIPath_FindsUpwardSibling(t *testing.T) {
 	root := t.TempDir()
 	apiPath := filepath.Join(root, "pkg", "spec", "src", "index.ts")
diff --git a/pkg/spec/test/web-runtime.test.ts b/pkg/spec/test/web-runtime.test.ts
new file mode 100644
index 0000000..4c65158
--- /dev/null
+++ b/pkg/spec/test/web-runtime.test.ts
@@ -0,0 +1,69 @@
+import assert from "node:assert/strict";
+import { test } from "node:test";
+
+import { __testing__ } from "../src/web-runtime.ts";
+
+const { mulberry32, deriveSeed32, pickWeighted, WEB_PRESS_KEYS } = __testing__;
+
+function entry(weight: number, kind: string) {
+  return [weight, { __sanderlingActionGenerator: true, __sanderlingKind: kind }] as const;
+}
+
+test("mulberry32 is deterministic for the same seed", () => {
+  const a = mulberry32(deriveSeed32("12345"));
+  const b = mulberry32(deriveSeed32("12345"));
+  const first = Array.from({ length: 8 }, () => a());
+  const second = Array.from({ length: 8 }, () => b());
+  assert.deepEqual(first, second);
+});
+
+test("mulberry32 diverges for different seeds", () => {
+  const a = Array.from({ length: 8 }, mulberry32(deriveSeed32("1")));
+  const b = Array.from({ length: 8 }, mulberry32(deriveSeed32("2")));
+  assert.notDeepEqual(a, b);
+});
+
+test("mulberry32 stays in [0, 1)", () => {
+  const next = mulberry32(deriveSeed32("seed"));
+  for (let i = 0; i < 1000; i++) {
+    const value = next();
+    assert.ok(value >= 0 && value < 1, `value out of range: ${value}`);
+  }
+});
+
+test("deriveSeed32 folds a 64-bit seed without precision loss", () => {
+  // Two distinct 64-bit values that collide as a JS Number must not collide here.
+  const a = deriveSeed32("9007199254740993");
+  const b = deriveSeed32("9007199254740992");
+  assert.notEqual(a, b);
+  assert.equal(deriveSeed32(undefined), 0);
+});
+
+test("pickWeighted selects by cumulative ascending scan", () => {
+  const handle = { __sanderlingActionGenerator: true, __sanderlingKind: "weighted", entries: [entry(1, "a"), entry(3, "b")] } as const;
+  // total = 4. pick = 0.1 -> 0.4 lands in [0,1) -> first; pick = 0.5 -> 2.0 lands in [1,4) -> second.
+  assert.equal(pickWeighted(handle as never, () => 0.1)?.__sanderlingKind, "a");
+  assert.equal(pickWeighted(handle as never, () => 0.5)?.__sanderlingKind, "b");
+});
+
+test("pickWeighted never selects a zero-weight entry when pick lands before it", () => {
+  const handle = { __sanderlingActionGenerator: true, __sanderlingKind: "weighted", entries: [entry(1, "a"), entry(0, "zero"), entry(1, "b")] } as const;
+  // total = 2. Any pick < 0.5 lands strictly inside the first entry's [0,1) band.
+  for (const r of [0, 0.1, 0.25, 0.49]) {
+    assert.equal(pickWeighted(handle as never, () => r)?.__sanderlingKind, "a");
+  }
+  // A zero-weight entry adds 0 to the cumulative, so it can never win.
+  for (let i = 0; i < 100; i++) {
+    const r = i / 100;
+    assert.notEqual(pickWeighted(handle as never, () => r)?.__sanderlingKind, "zero");
+  }
+});
+
+test("seeded press-key draw is reproducible", () => {
+  const draw = (seed: string) => {
+    const next = mulberry32(deriveSeed32(seed));
+    return Array.from({ length: 12 }, () => WEB_PRESS_KEYS[Math.floor(next() * WEB_PRESS_KEYS.length)]);
+  };
+  assert.deepEqual(draw("777"), draw("777"));
+  assert.notDeepEqual(draw("777"), draw("778"));
+});