diff --git a/src/ratatoskr/tts.py b/src/ratatoskr/tts.py
index e98492d..6be3030 100644
--- a/src/ratatoskr/tts.py
+++ b/src/ratatoskr/tts.py
@@ -14,6 +14,7 @@ Foot-guns (verified live 2026-08-02):
 
 from __future__ import annotations
 
+import struct
 from collections.abc import Mapping
 from dataclasses import dataclass
 
@@ -97,6 +98,27 @@ class EmotionDials:
         }
 
 
+def _finalize_wav_header(data: bytes) -> bytes:
+    """Rewrite the RIFF + data chunk sizes with the real byte counts.
+
+    The Zonos gateway emits a STREAMING wav header — the RIFF chunk size (offset 4)
+    and the data chunk size are both 0xFFFFFFFF ("unknown length"), because it can
+    stream. A browser <audio> element playing a fully-downloaded blob needs a finite,
+    correctly-sized WAV to decode it; a 0xFFFFFFFF length reads as raw/streaming PCM
+    and won't play (operator-reported). Now that the whole clip is buffered we know the
+    real sizes, so patch them in. Idempotent — writing an already-correct size is a
+    no-op; safe if the data chunk isn't found (leaves those bytes untouched).
+    """
+    if len(data) < 44:  # shorter than a canonical PCM header — nothing to patch
+        return data
+    buf = bytearray(data)
+    struct.pack_into("<I", buf, 4, len(buf) - 8)  # RIFF chunk size = file len - 8
+    data_pos = buf.find(b"data", 12)  # first "data" after the fmt chunk = the data chunk
+    if data_pos != -1 and data_pos + 8 <= len(buf):
+        struct.pack_into("<I", buf, data_pos + 4, len(buf) - (data_pos + 8))
+    return bytes(buf)
+
+
 def _clamp(x: float, lo: float, hi: float) -> float:
     """Clamp to [lo, hi], NaN-safe: a NaN axis (a dead/malformed signal) → 0.0
     rather than propagating through max/min into the gateway."""
@@ -153,4 +175,6 @@ async def tts_synthesize(
     # WAVE form tag (8:12); a `b"RIFF..."` body that isn't WAVE would still fail decode.
     if data[:4] != b"RIFF" or data[8:12] != b"WAVE":
         raise TtsUnavailable("gateway returned a non-wav body")
-    return data
+    # The gateway's streaming header carries 0xFFFFFFFF sizes; rewrite them with the
+    # real byte counts so a browser <audio> element can decode the finite clip.
+    return _finalize_wav_header(data)
diff --git a/tests/test_tts.py b/tests/test_tts.py
index af6d712..b6cdd5d 100644
--- a/tests/test_tts.py
+++ b/tests/test_tts.py
@@ -8,7 +8,10 @@ DEC-3: response_format is ALWAYS "wav". DEC-7: live PAD → emotion dials.
 INV-TTS-4: a gateway failure / non-wav body → TtsUnavailable (caller degrades).
 """
 
+import io
 import math
+import struct
+import wave
 
 import httpx
 import pytest
@@ -22,9 +25,20 @@ from ratatoskr.tts import (
     tts_synthesize,
 )
 
-# A minimally-valid wav body: the DEC-3 guard checks the RIFF magic (0:4) + WAVE
-# form tag (8:12).
-_WAV = b"RIFF\x00\x00\x00\x00WAVE" + b"\x00" * 32
+
+def _wav(*, streaming: bool = False, nsamples: int = 8) -> bytes:
+    """A canonical PCM mono/44.1k/16-bit WAV. `streaming=True` mimics the Zonos
+    gateway's placeholder header (RIFF + data sizes = 0xFFFFFFFF)."""
+    samples = b"\x11\x22" * nsamples
+    fmt = struct.pack("<HHIIHH", 1, 1, 44100, 88200, 2, 16)
+    body = b"WAVE" + b"fmt " + struct.pack("<I", 16) + fmt + b"data"
+    data_size = 0xFFFFFFFF if streaming else len(samples)
+    riff_size = 0xFFFFFFFF if streaming else (len(body) + 4 + len(samples))
+    return b"RIFF" + struct.pack("<I", riff_size) + body + struct.pack("<I", data_size) + samples
+
+
+# A proper finite WAV — the header-finalize step is idempotent on it (sizes already real).
+_WAV = _wav()
 
 
 class TestPadState:
@@ -182,6 +196,29 @@ class TestTtsSynthesize:
                     url="http://tts.example/v1/audio/speech",
                 )
 
+    @respx.mock
+    async def test_finalizes_streaming_wav_header_to_real_sizes(self) -> None:
+        # The Zonos gateway returns a streaming header (RIFF + data sizes = 0xFFFFFFFF);
+        # the browser <audio> element can't play that. tts_synthesize must rewrite both
+        # to the real byte counts so the result is a decodable finite WAV.
+        streaming = _wav(streaming=True)
+        assert struct.unpack("<I", streaming[4:8])[0] == 0xFFFFFFFF  # precondition
+        respx.post("http://tts.example/v1/audio/speech").mock(
+            return_value=httpx.Response(200, content=streaming)
+        )
+        async with httpx.AsyncClient() as client:
+            out = await tts_synthesize(
+                "hi", voice="Cora", dials=pad_to_dials(None), client=client,
+                url="http://tts.example/v1/audio/speech",
+            )
+        # RIFF size + data size are now real, not 0xFFFFFFFF.
+        assert struct.unpack("<I", out[4:8])[0] == len(out) - 8
+        dpos = out.find(b"data", 12)
+        assert struct.unpack("<I", out[dpos + 4:dpos + 8])[0] == len(out) - (dpos + 8)
+        # …and Python's wave module (a real decoder) can now open it as a finite clip.
+        with wave.open(io.BytesIO(out)) as w:
+            assert w.getnchannels() == 1 and w.getsampwidth() == 2 and w.getframerate() == 44100
+
     @respx.mock
     async def test_riff_but_not_wave_raises_tts_unavailable(self) -> None:
         # A body with the RIFF magic but a non-WAVE form tag (e.g. RIFF/AVI, or a
diff --git a/tests/test_web_server.py b/tests/test_web_server.py
index 1b44f58..88aa551 100644
--- a/tests/test_web_server.py
+++ b/tests/test_web_server.py
@@ -7,6 +7,7 @@ AsyncClient.
 """
 
 import json
+import struct
 
 import httpx
 import pytest
@@ -1464,7 +1465,14 @@ class TestTtsEndpoint:
     (DEC-8); emotion dials map the browser-sent live PAD (DEC-7); a gateway failure
     degrades to 503 (INV-TTS-4)."""
 
-    _WAV = b"RIFF\x00\x00\x00\x00WAVE" + b"\x00" * 32
+    # A proper finite PCM WAV (correct RIFF + data sizes) so the header-finalize step is
+    # idempotent — the endpoint returns these bytes unchanged.
+    _WAV = (
+        b"RIFF" + (36 + 16).to_bytes(4, "little") + b"WAVE"
+        + b"fmt " + (16).to_bytes(4, "little")
+        + struct.pack("<HHIIHH", 1, 1, 44100, 88200, 2, 16)
+        + b"data" + (16).to_bytes(4, "little") + b"\x11\x22" * 8
+    )
     _TTS = "http://tts.example/v1/audio/speech"
 
     @respx.mock