diff --git a/.github/workflows/build.yml b/.github/workflows/build.yml
index 49a7b5594..be8f2d415 100644
--- a/.github/workflows/build.yml
+++ b/.github/workflows/build.yml
@@ -239,6 +239,97 @@ jobs:
           name: Hang Interop Test Reports
           path: nestsClient/build/reports/tests/jvmTest/
 
+  # Phase 4 of T16: browser-side cross-stack interop. Drives a headless
+  # Chromium (via Playwright) running @moq/lite + @moq/hang against
+  # the same moq-relay subprocess the hang-interop job uses. Linux-only:
+  # Chromium QUIC behaviour is consistent across platforms in the
+  # scenarios we care about, and macOS/Windows would double the matrix
+  # cost without catching new defects.
+  #
+  # Inherits the cargo cache from `hang-interop` because the browser
+  # path still needs `moq-relay` + `hang-listen` built (the harness
+  # boots the same Rust subprocess). Adds bun + node_modules + Playwright
+  # browser caches. See:
+  #   nestsClient/plans/2026-05-06-phase4-browser-harness.md
+  browser-interop:
+    needs: lint
+    runs-on: ubuntu-latest
+    timeout-minutes: 30
+    steps:
+      - name: Checkout code
+        uses: actions/checkout@v6
+
+      - name: Set up JDK 21
+        uses: actions/setup-java@v5
+        with:
+          distribution: 'zulu'
+          java-version: 21
+
+      - name: Set up Gradle
+        uses: gradle/actions/setup-gradle@v4
+        with:
+          cache-read-only: ${{ github.ref != 'refs/heads/main' }}
+
+      - name: Set up Rust
+        uses: dtolnay/rust-toolchain@stable
+
+      - name: Cache cargo
+        uses: actions/cache@v4
+        with:
+          path: |
+            ~/.cargo/registry
+            ~/.cargo/git
+            nestsClient/tests/hang-interop/target
+            ~/.cache/amethyst-nests-interop/hang-interop-cargo
+          key: ${{ runner.os }}-cargo-${{ hashFiles('nestsClient/tests/hang-interop/Cargo.lock', 'nestsClient/tests/hang-interop/REV') }}
+          restore-keys: |
+            ${{ runner.os }}-cargo-
+
+      # bun is a separate install — Playwright + the bun harness build
+      # both go through it. Pin the version that matches the local agent
+      # tooling so a CI/local skew can't surface a wire-format change
+      # in `bun build` output.
+      - name: Set up bun
+        uses: oven-sh/setup-bun@v2
+        with:
+          bun-version: 1.3.11
+
+      # Cache bun-installed node_modules for the browser harness.
+      # Keyed on package.json + bun.lock so a dep bump invalidates.
+      - name: Cache node_modules
+        uses: actions/cache@v4
+        with:
+          path: |
+            nestsClient-browser-interop/node_modules
+            nestsClient-browser-interop/dist
+          key: ${{ runner.os }}-bun-${{ hashFiles('nestsClient-browser-interop/package.json', 'nestsClient-browser-interop/bun.lock') }}
+
+      # Cache Playwright's browser cache (~/.cache/ms-playwright/chromium-*).
+      # The cold install of Chromium is ~200 MB and 30–60 s; cached runs
+      # are essentially instant.
+      - name: Cache Playwright browsers
+        uses: actions/cache@v4
+        with:
+          path: ~/.cache/ms-playwright
+          key: ${{ runner.os }}-playwright-${{ hashFiles('nestsClient-browser-interop/package.json') }}
+
+      - name: Run browser cross-stack interop suite
+        run: |
+          ./gradlew :nestsClient:jvmTest \
+              --tests "com.vitorpamplona.nestsclient.interop.native.BrowserInteropTest" \
+              -DnestsHangInterop=true \
+              -DnestsBrowserInterop=true
+
+      - name: Upload browser interop test report
+        uses: actions/upload-artifact@v7
+        if: failure()
+        with:
+          name: Browser Interop Test Reports
+          path: |
+            nestsClient/build/reports/tests/jvmTest/
+            nestsClient-browser-interop/test-results/
+            nestsClient-browser-interop/playwright-report/
+
   test-and-build-android:
     needs: lint
     runs-on: ubuntu-latest
diff --git a/nestsClient/plans/2026-05-06-phase4-browser-harness-results.md b/nestsClient/plans/2026-05-06-phase4-browser-harness-results.md
new file mode 100644
index 000000000..aba42caa8
--- /dev/null
+++ b/nestsClient/plans/2026-05-06-phase4-browser-harness-results.md
@@ -0,0 +1,133 @@
+# Plan: Phase 4 (browser harness) — landed results
+
+**Status:** 4.A scaffold + 4.B Playwright driver + first Kotlin
+test green; 4.C ships I15; 4.D ships the CI workflow job. Tracks
+the spec at `nestsClient/plans/2026-05-06-phase4-browser-harness.md`.
+
+## Where it landed
+
+- New top-level `nestsClient-browser-interop/` workspace:
+  - `package.json` pins `@moq/lite@0.2.2`, `@moq/hang@0.2.4`,
+    `@moq/watch@0.2.10`, `@moq/publish@0.2.6`, `@playwright/test@1.56.1`.
+  - `REV` documents the pinned versions next to the
+    `nestsClient/tests/hang-interop/REV`.
+  - `src/listen.html` + `src/listen.ts` — Watch path, uses
+    `Container.Legacy.Consumer` and WebCodecs `AudioDecoder`
+    directly (the published `@moq/hang` 0.2.4 doesn't expose the
+    higher-level `Container.Consumer` from upstream HEAD; we wire
+    its data path manually).
+  - `src/publish.html` + `src/publish.ts` — symmetric publisher
+    scaffold for the I4-reverse / I14-decoder-warmup scenarios
+    Phase 4.C extension can pick up.
+  - `src/server.ts` — bun static + WebSocket back-channel; the
+    listener page posts Float32 LE PCM frames as binary messages,
+    a textual `done` message flips the server's `done` flag.
+  - `tests/harness.spec.ts` — single Playwright spec the Kotlin
+    driver invokes per scenario; reads `NESTS_HARNESS_URL`
+    + `NESTS_TIMEOUT_MS` from env.
+  - `playwright.config.ts` — Chromium with `--enable-quic`,
+    `--ignore-certificate-errors`, AutoplayPolicy override.
+- `nestsClient/src/jvmTest/.../interop/native/PlaywrightDriver.kt`
+  — Kotlin shim that spawns the bun server + `bun x playwright
+  test` per test, returns a `HarnessRun(pcmFile, stdout, exit)`.
+  Includes a `CertCapturingValidator` that pulls the relay's leaf
+  cert during the speaker's QUIC handshake so we can pass its
+  SHA-256 to Chromium via `serverCertificateHashes`.
+- `nestsClient/src/jvmTest/.../interop/native/BrowserInteropTest.kt`
+  — two scenarios:
+  - **I1 forward (browser)**: Amethyst Kotlin speaker → Chromium
+    `@moq/lite` listener; asserts FFT 440 Hz on the captured tail.
+  - **I15 (WT-Protocol round-trip)**: asserts Chromium's
+    `Connection.version` starts with `moq-lite-`.
+- `nestsClient/build.gradle.kts` — two new tasks:
+  - `interopBuildBrowserHarness` (bun install + bun build → dist/),
+  - `interopInstallPlaywrightChromium` (skipped if
+    `PLAYWRIGHT_BROWSERS_PATH` already points at a chromium build).
+  Both gated on `-DnestsBrowserInterop=true` like the hang tier
+  is gated on `-DnestsHangInterop=true`.
+- `.github/workflows/build.yml` — new `browser-interop` job
+  parallel to `hang-interop`, with bun + node_modules + Playwright
+  caches.
+
+## Deviations from the spec
+
+1. **Source layout: `@moq/lite` + `@moq/hang` direct, NOT
+   `@moq/watch` `Watch.Broadcast`.** The spec called for mirroring
+   NostrNests's `transport/moq-transport.ts` `Watch.Broadcast`
+   verbatim; in practice `@moq/watch` 0.2.x bakes in a heavy
+   reactive `Effect`/`Signal` layer that's unwieldy for a one-shot
+   capture page. The lower-level `connection.consume(path) →
+   broadcast.subscribe(track) → track.readFrame()` pipeline is
+   what the watch decoder uses internally, so this is a
+   functionally equivalent path. NostrNests-side regressions in
+   `Watch.Broadcast` plumbing aren't in scope of T16.
+2. **Cert pinning via `serverCertificateHashes`, not
+   `--ignore-certificate-errors`.** Chromium's
+   `--ignore-certificate-errors` flag does NOT bypass QUIC cert
+   validation — reproduced as `net::ERR_QUIC_PROTOCOL_ERROR.
+   QUIC_TLS_CERTIFICATE_UNKNOWN`. The spec mentioned
+   `--ignore-certificate-errors-spki-list` as a "preferred long-
+   term form"; we use `serverCertificateHashes` (Web-API equivalent),
+   which works because moq-relay's `--tls-generate` produces a
+   14-day ECDSA P-256 cert — exactly what the WebTransport spec
+   requires for a serverCertificateHashes pin. The
+   `CertCapturingValidator` snags the cert during the speaker's
+   QUIC handshake so we don't need a separate fingerprint endpoint.
+3. **I1 sample-count assertion loosened.** Hang-tier I1 asserts
+   `assertSampleCount(expected = 5 s, tolerance = 0.20)` — the
+   browser path can't hit that because Chromium cold-launch +
+   Playwright runner setup eats 3–10 s before the page starts
+   capturing, by which time the `framesPerGroup = 5`
+   per-subscriber forward cliff means only the latest cached
+   group is replayable. The browser I1 instead asserts ≥ 1 s of
+   decoded audio + FFT peak at 440 Hz. The FFT peak is the
+   load-bearing assertion (catches downmix / channel-swap /
+   OpusHead-leak regressions); the sample-count threshold is just
+   a sanity floor.
+4. **Phase 4.C scenarios I2/I3/I4/I13/I14 deferred.** I2
+   late-join collapses into "tail capture" anyway given the
+   Chromium boot lag, so it's not adding signal beyond I1. I3
+   mute-window has the same visibility issue. I4 needs the
+   reverse publisher path wired up end-to-end (a stub publish.ts
+   landed but isn't exercised by a Kotlin test yet). I13 long
+   broadcast and I14 CSD-skip are runtime-of-test concerns the
+   I1 path already exercises implicitly. Tracked as a follow-up
+   on a separate plan if/when the gap matters.
+
+## Verification
+
+```bash
+./gradlew :nestsClient:jvmTest \
+    --tests "com.vitorpamplona.nestsclient.interop.native.BrowserInteropTest" \
+    -DnestsHangInterop=true \
+    -DnestsBrowserInterop=true
+```
+
+Both `amethyst_speaker_to_chromium_listener_static_tone_440` and
+`chromium_round_trips_a_moq_lite_session` pass in isolation.
+
+## Follow-ups
+
+- **I4-reverse**: wire `BrowserInteropTest` to drive
+  `PlaywrightDriver.openPublishPage` (the Kotlin side already
+  exposes the entry point) → Amethyst Kotlin listener decodes;
+  assert per-channel FFT peaks. Needs the publish.ts harness
+  graduated from scaffold to a fully-working pump (the
+  `MediaStreamTrackProcessor` → `AudioEncoder` → `Container.Legacy.
+  Producer` chain compiles but isn't yet validated end-to-end).
+- **I3 mute-window**: works on the Kotlin speaker side, but the
+  short browser tail capture window means the mute-gap deficit
+  isn't observable. Would need either a longer broadcast (60 s+)
+  or a tighter capture window that brackets the mute schedule
+  reliably. Low priority — the hang-tier I3 already validates the
+  speaker-side mute behaviour against a parser-correct watcher.
+- **I15 strict pin**: when moq-relay 0.10.x ships with both
+  `moq-lite-03` and `moq-lite-04` ALPN advertisement, tighten the
+  assertion from `startsWith("moq-lite-")` to exact-match
+  `moq-lite-03` (or whichever the production stack runs). Right
+  now the relay we boot lands `moq-lite-02` over the legacy
+  `moql` ALPN.
+- **CI cold-cache time**: cold `npx playwright install chromium`
+  takes ~60 s on a fresh GitHub runner. The `actions/cache@v4`
+  hits keyed on `package.json` should make warm runs near-zero,
+  but the first run on a new branch will be slow.
diff --git a/nestsClient/src/jvmTest/kotlin/com/vitorpamplona/nestsclient/interop/native/BrowserInteropTest.kt b/nestsClient/src/jvmTest/kotlin/com/vitorpamplona/nestsclient/interop/native/BrowserInteropTest.kt
index a64cb33a5..1393e4548 100644
--- a/nestsClient/src/jvmTest/kotlin/com/vitorpamplona/nestsclient/interop/native/BrowserInteropTest.kt
+++ b/nestsClient/src/jvmTest/kotlin/com/vitorpamplona/nestsclient/interop/native/BrowserInteropTest.kt
@@ -83,6 +83,34 @@ class BrowserInteropTest {
         }
     }
 
+    /**
+     * **I15 (WT-Protocol round-trip)** — assert Chromium's WebTransport
+     * round-trip with `moq-relay 0.10.x` produces a known-good moq-lite
+     * version on the `Connection`. The harness page exposes
+     * `connection.version` at `window.__moqVersion`; the Playwright
+     * spec bundles it in the trailing JSON line on stdout.
+     *
+     * The assertion accepts any of the moq-lite draft versions the
+     * relay advertises through SETUP — Chromium's `@moq/lite` 0.2.x
+     * client offers `moq-lite-04`, `moq-lite-03`, `moql` (legacy)
+     * ALPNs in that priority. moq-relay 0.10.x's choice depends on
+     * its build flags, but the `moq-lite-` prefix is invariant. A
+     * regression that breaks ALPN negotiation entirely or
+     * downgrades to a non-lite version (`draft-17` etc.) is caught
+     * here even if I1 audio assertions still pass via a fallback path.
+     */
+    @Test
+    fun chromium_round_trips_a_moq_lite_session() =
+        runBlocking {
+            val out = runSpeakerToBrowserListen(speakerSeconds = 5)
+            val moqVersion = parseMoqVersionFromStdout(out.stdout)
+            assertTrue(
+                moqVersion != null && moqVersion.startsWith("moq-lite-"),
+                "expected Chromium to round-trip a moq-lite-* version; got '$moqVersion'.\n" +
+                    "playwright stdout:\n${out.stdout}",
+            )
+        }
+
     /**
      * **I1 forward (browser)** — Amethyst Kotlin speaker → Chromium
      * `@moq/lite` listener with `@moq/hang` `Container.Legacy.Consumer`.
@@ -318,6 +346,23 @@ private object StaticTokenNestsClientForBrowser : NestsClient {
     ): String = ""
 }
 
+/**
+ * Pull the `meta.moqVersion` field out of the trailing JSON line
+ * the Playwright spec emits to stdout. The spec writes a single
+ * `{"state":"done","meta":{"moqVersion":"moq-lite-03",...}}` line
+ * per run; we substring-search for it rather than wiring up a JSON
+ * dependency just for this one helper.
+ */
+private fun parseMoqVersionFromStdout(stdout: String): String? {
+    val needle = "\"moqVersion\":\""
+    val start = stdout.indexOf(needle)
+    if (start < 0) return null
+    val valueStart = start + needle.length
+    val valueEnd = stdout.indexOf('"', valueStart)
+    if (valueEnd < 0) return null
+    return stdout.substring(valueStart, valueEnd)
+}
+
 /**
  * Read a file of native-endian Float32 LE PCM into a [FloatArray].
  * Matches the format the bun WS server appends per binary frame —