diff --git a/assemblyai/__version__.py b/assemblyai/__version__.py
index 27729bb..0348df7 100644
--- a/assemblyai/__version__.py
+++ b/assemblyai/__version__.py
@@ -1 +1 @@
-__version__ = "0.64.25"
+__version__ = "0.64.26"
diff --git a/assemblyai/streaming/v3/models.py b/assemblyai/streaming/v3/models.py
index 879e4a7..55794f2 100644
--- a/assemblyai/streaming/v3/models.py
+++ b/assemblyai/streaming/v3/models.py
@@ -2,7 +2,16 @@
 from enum import Enum
 from typing import Any, List, Literal, Optional, Union
 
-from pydantic import BaseModel
+try:
+    # pydantic v2 import
+    from pydantic import BaseModel, model_validator
+
+    pydantic_v2 = True
+except ImportError:
+    # pydantic v1 import (fallback for Python < 3.14)
+    from pydantic import BaseModel, root_validator
+
+    pydantic_v2 = False
 
 
 class LLMGatewayMessage(BaseModel):
@@ -153,11 +162,12 @@ class StreamingSessionParameters(BaseModel):
 class Encoding(str, Enum):
     pcm_s16le = "pcm_s16le"
     pcm_mulaw = "pcm_mulaw"
-    # Raw Opus packets, one packet per binary WS message. `sample_rate` is
-    # optional and ignored — the Opus stream is self-describing.
+    # Raw Opus packets, one packet per binary WS message. `sample_rate` may be
+    # omitted — the Opus stream is self-describing and the server ignores it.
     opus = "opus"
     # Ogg-encapsulated Opus byte stream (ffmpeg, gstreamer, opusenc, browser
-    # MediaRecorder output). `sample_rate` is optional and ignored.
+    # MediaRecorder output). `sample_rate` may be omitted — the Opus stream is
+    # self-describing and the server ignores it.
     ogg_opus = "ogg_opus"
 
     def __str__(self):
@@ -263,7 +273,10 @@ def __str__(self):
 
 
 class StreamingParameters(StreamingSessionParameters):
-    sample_rate: int
+    # Required for PCM encodings. May be omitted for Opus encodings
+    # (opus, ogg_opus) — the stream is self-describing and the server
+    # ignores the value.
+    sample_rate: Optional[int] = None
     encoding: Optional[Encoding] = None
     speech_model: Optional[SpeechModel] = None
     # Deprecated: use language_codes instead (pass a single-element list, e.g.
@@ -291,6 +304,36 @@ class StreamingParameters(StreamingSessionParameters):
     redact_pii_sub: Optional[StreamingPiiSubstitution] = None
     mode: Optional[StreamingMode] = None
 
+    if pydantic_v2:
+
+        @model_validator(mode="after")
+        def _require_sample_rate_for_non_opus(self):
+            if self.sample_rate is None and self.encoding not in (
+                Encoding.opus,
+                Encoding.ogg_opus,
+            ):
+                raise ValueError(
+                    "sample_rate is required; it may only be omitted when "
+                    "encoding is 'opus' or 'ogg_opus' (the Opus stream is "
+                    "self-describing)."
+                )
+            return self
+
+    else:
+
+        @root_validator(skip_on_failure=True)
+        def _require_sample_rate_for_non_opus(cls, values):
+            if values.get("sample_rate") is None and values.get("encoding") not in (
+                Encoding.opus,
+                Encoding.ogg_opus,
+            ):
+                raise ValueError(
+                    "sample_rate is required; it may only be omitted when "
+                    "encoding is 'opus' or 'ogg_opus' (the Opus stream is "
+                    "self-describing)."
+                )
+            return values
+
 
 class UpdateConfiguration(StreamingSessionParameters):
     type: Literal["UpdateConfiguration"] = "UpdateConfiguration"
diff --git a/tests/unit/test_streaming.py b/tests/unit/test_streaming.py
index 154b7e8..cfc6ca4 100644
--- a/tests/unit/test_streaming.py
+++ b/tests/unit/test_streaming.py
@@ -3,6 +3,7 @@
 import threading
 import time
 from types import SimpleNamespace
+from typing import Optional
 from urllib.parse import urlencode
 
 import pytest
@@ -412,6 +413,53 @@ def mocked_websocket_connect(
     assert f"encoding={encoding.value}" in actual_url
 
 
+@pytest.mark.parametrize("encoding", [Encoding.opus, Encoding.ogg_opus])
+def test_client_connect_opus_without_sample_rate(
+    mocker: MockFixture, encoding: Encoding
+):
+    # Given: client + an Opus encoding and no sample_rate (the Opus stream is
+    # self-describing, so the parameter may be omitted)
+    actual_url = None
+
+    def mocked_websocket_connect(
+        url: str, additional_headers: dict, open_timeout: float
+    ):
+        nonlocal actual_url
+        actual_url = url
+
+    mocker.patch(
+        "assemblyai.streaming.v3.client.websocket_connect",
+        new=mocked_websocket_connect,
+    )
+    _disable_rw_threads(mocker)
+    client = StreamingClient(
+        StreamingClientOptions(api_key="test", api_host="api.example.com")
+    )
+    params = StreamingParameters(
+        speech_model=SpeechModel.universal_3_5_pro,
+        encoding=encoding,
+    )
+
+    # When: connect
+    client.connect(params)
+
+    # Then: the encoding is forwarded and sample_rate is absent from the URL
+    assert f"encoding={encoding.value}" in actual_url
+    assert "sample_rate" not in actual_url
+
+
+@pytest.mark.parametrize("encoding", [None, Encoding.pcm_s16le, Encoding.pcm_mulaw])
+def test_sample_rate_required_for_non_opus_encodings(encoding: Optional[Encoding]):
+    # Given/When: constructing parameters without sample_rate for a PCM (or
+    # unset) encoding
+    # Then: validation rejects it
+    with pytest.raises(ValueError, match="sample_rate is required"):
+        StreamingParameters(
+            speech_model=SpeechModel.universal_3_5_pro,
+            encoding=encoding,
+        )
+
+
 def test_noise_suppression_deprecated_alias_migrates_to_voice_focus(
     mocker: MockFixture, caplog: pytest.LogCaptureFixture
 ):