Fix confusing error message for invalid data parameter

When AsyncClient receives invalid data like data=[{"a": "b"}] (list of dicts),
it was raising "Attempted to send a sync request with an AsyncClient instance"
which is misleading. The actual issue is invalid data format.

This fix adds early validation in encode_request() to check if data is a
list/tuple containing non-bytes objects, and raises a clear TypeError with
helpful guidance (use json= or data={...} instead).

Fixes #3471

httpx/_content.py

@@ -202,6 +202,21 @@ def encode_request(
         # However for compat with requests, we *do* still support
         # `data=<bytes...>` usages. We deal with that case here, treating it
         # as if `content=<...>` had been supplied instead.
+
+        # Validate that data is bytes-like or an iterable of bytes, not other types
+        if isinstance(data, (list, tuple)):
+            # Check if it's a list/tuple of bytes
+            try:
+                for item in data:
+                    if not isinstance(item, (bytes, bytearray, memoryview)):
+                        raise TypeError(
+                            f"Expected bytes-like object in 'data' sequence, got {type(item).__name__}. "
+                            f"Use 'json=' for JSON data or 'data={{...}}' for form data."
+                        )
+            except TypeError:
+                # Re-raise our custom error, not the iteration error
+                raise
+
         message = "Use 'content=<...>' to upload raw bytes/text content."
         warnings.warn(message, DeprecationWarning, stacklevel=2)
         return encode_content(data)

tests/test_content.py

@@ -364,6 +364,27 @@ def test_invalid_argument():
         httpx.Request(method, url, content={"a": "b"})  # type: ignore
 
 
+def test_invalid_data_list_of_dicts():
+    """Test that passing a list of dicts to data= produces a clear error message.
+
+    This was previously raising a confusing 'Attempted to send a sync request
+    with an AsyncClient instance' error with AsyncClient. Now it should give
+    a clear error for both sync and async clients.
+    """
+    with pytest.raises(TypeError, match="Expected bytes-like object.*got dict"):
+        httpx.Request(method, url, data=[{"a": "b"}])  # type: ignore
+
+
+@pytest.mark.anyio
+async def test_invalid_data_list_of_dicts_async():
+    """Test that AsyncClient produces clear error for invalid data parameter.
+
+    Regression test for issue #3471.
+    """
+    with pytest.raises(TypeError, match="Expected bytes-like object.*got dict"):
+        httpx.Request(method, url, data=[{"a": "b"}])  # type: ignore
+
+
 @pytest.mark.anyio
 async def test_multipart_multiple_files_single_input_content():
     files = [