diff --git a/src/anthropic/_base_client.py b/src/anthropic/_base_client.py
index f7d910e4..fbfba66f 100644
--- a/src/anthropic/_base_client.py
+++ b/src/anthropic/_base_client.py
@@ -531,9 +531,17 @@ def _process_response(
         # in the response, e.g. application/json; charset=utf-8
         content_type, *_ = response.headers.get("content-type").split(";")
         if content_type != "application/json":
-            raise ValueError(
-                f"Expected Content-Type response header to be `application/json` but received {content_type} instead."
-            )
+            if self._strict_response_validation:
+                raise exceptions.APIResponseValidationError(
+                    response=response,
+                    request=response.request,
+                    message=f"Expected Content-Type response header to be `application/json` but received `{content_type}` instead.",
+                )
+
+            # If the API responds with content that isn't JSON then we just return
+            # the (decoded) text without performing any parsing so that you can still
+            # handle the response however you need to.
+            return response.text  # type: ignore
 
         data = response.json()
         return self._process_response_data(data=data, cast_to=cast_to, response=response)
diff --git a/src/anthropic/_base_exceptions.py b/src/anthropic/_base_exceptions.py
index aac00103..e2ba6aa4 100644
--- a/src/anthropic/_base_exceptions.py
+++ b/src/anthropic/_base_exceptions.py
@@ -1,3 +1,5 @@
+from __future__ import annotations
+
 from typing_extensions import Literal
 
 from httpx import Request, Response
@@ -17,8 +19,8 @@ class APIResponseValidationError(APIError):
     response: Response
     status_code: int
 
-    def __init__(self, request: Request, response: Response) -> None:
-        super().__init__("Data returned by API invalid for expected schema.", request)
+    def __init__(self, request: Request, response: Response, *, message: str | None = None) -> None:
+        super().__init__(message or "Data returned by API invalid for expected schema.", request)
         self.response = response
         self.status_code = response.status_code
 
diff --git a/tests/test_client.py b/tests/test_client.py
index 55db3be0..d6fae54c 100644
--- a/tests/test_client.py
+++ b/tests/test_client.py
@@ -12,7 +12,7 @@
 import pytest
 from respx import MockRouter
 
-from anthropic import Anthropic, AsyncAnthropic
+from anthropic import Anthropic, AsyncAnthropic, APIResponseValidationError
 from anthropic._types import Omit
 from anthropic._models import BaseModel, FinalRequestOptions
 from anthropic._streaming import Stream, AsyncStream
@@ -383,6 +383,23 @@ class Model(BaseModel):
         response = self.client.post("/foo", cast_to=Model, stream=True)
         assert isinstance(response, Stream)
 
+    @pytest.mark.respx(base_url=base_url)
+    def test_received_text_for_expected_json(self, respx_mock: MockRouter) -> None:
+        class Model(BaseModel):
+            name: str
+
+        respx_mock.get("/foo").mock(return_value=httpx.Response(200, text="my-custom-format"))
+
+        strict_client = Anthropic(base_url=base_url, api_key=api_key, _strict_response_validation=True)
+
+        with pytest.raises(APIResponseValidationError):
+            strict_client.get("/foo", cast_to=Model)
+
+        client = Anthropic(base_url=base_url, api_key=api_key, _strict_response_validation=False)
+
+        response = client.get("/foo", cast_to=Model)
+        assert isinstance(response, str)  # type: ignore[unreachable]
+
 
 class TestAsyncAnthropic:
     client = AsyncAnthropic(base_url=base_url, api_key=api_key, _strict_response_validation=True)
@@ -740,3 +757,21 @@ class Model(BaseModel):
 
         response = await self.client.post("/foo", cast_to=Model, stream=True)
         assert isinstance(response, AsyncStream)
+
+    @pytest.mark.respx(base_url=base_url)
+    @pytest.mark.asyncio
+    async def test_received_text_for_expected_json(self, respx_mock: MockRouter) -> None:
+        class Model(BaseModel):
+            name: str
+
+        respx_mock.get("/foo").mock(return_value=httpx.Response(200, text="my-custom-format"))
+
+        strict_client = AsyncAnthropic(base_url=base_url, api_key=api_key, _strict_response_validation=True)
+
+        with pytest.raises(APIResponseValidationError):
+            await strict_client.get("/foo", cast_to=Model)
+
+        client = AsyncAnthropic(base_url=base_url, api_key=api_key, _strict_response_validation=False)
+
+        response = await client.get("/foo", cast_to=Model)
+        assert isinstance(response, str)  # type: ignore[unreachable]