Remove dual JSON encoding in pyodide message callback (#6350)

The pyodide bootstrap was encoding kernel messages twice. First via
`encode_json_bytes()` for the data payload, then again via
`encode_json_str()` for the entire message. This resulted in the data
field being base64-encoded instead of a proper JSON object.

Fixed by manually constructing the JSON string to avoid the second
encoding, and added a test to prevent regression.

Author: manzt

marimo/_pyodide/bootstrap.py

@@ -5,10 +5,7 @@
 from typing import TYPE_CHECKING, Callable
 
 from marimo._config.config import merge_config
-from marimo._messaging.msgspec_encoder import (
-    encode_json_bytes,
-    encode_json_str,
-)
+from marimo._messaging.msgspec_encoder import encode_json_bytes
 from marimo._messaging.ops import KernelCapabilities, KernelReady
 from marimo._runtime.requests import (
     AppMetadata,
@@ -81,7 +78,9 @@ def create_session(
     """
 
     def write_kernel_message(op: KernelMessage) -> None:
-        message_callback(encode_json_str({"op": op[0], "data": op[1]}))
+        op_name, data_bytes = op
+        text = f'{{"op": "{op_name}", "data": {data_bytes.decode("utf-8")}}}'
+        message_callback(text)
 
     # Lazy import to decrease startup time
     from marimo._config.config import merge_default_config

tests/_pyodide/test_bootstrap.py

@@ -1,6 +1,7 @@
 # Copyright 2024 Marimo. All rights reserved.
 from __future__ import annotations
 
+import json
 from typing import TYPE_CHECKING, Any, Callable
 
 import pytest
@@ -176,3 +177,22 @@ def test_save_file(
     # Verify the file was saved correctly
     saved_content = mock_app_file.read_text()
     assert "print('hello')" in saved_content
+
+
+async def test_message_callback_format(
+    mock_app_file: Path,
+) -> None:
+    """Test that message_callback receives properly formatted JSON."""
+    received_messages: list[str] = []
+
+    session, _ = create_session(
+        filename=str(mock_app_file),
+        query_params={},
+        message_callback=lambda text: received_messages.append(text),
+        user_config=DEFAULT_CONFIG,
+    )
+
+    assert len(received_messages) >= 1
+    parsed = json.loads(received_messages[0])
+    assert parsed["op"] == "kernel-ready"
+    assert isinstance(parsed["data"], dict)