Fix/base64 handling (Issue #342) (#343)

Co-authored-by: David Soria Parra <167242713+dsp-ant@users.noreply.github.com>

src/mcp/server/lowlevel/server.py

@@ -301,7 +301,7 @@ class Server(Generic[LifespanResultT]):
 
                             return types.BlobResourceContents(
                                 uri=req.params.uri,
-                                blob=base64.urlsafe_b64encode(data).decode(),
+                                blob=base64.b64encode(data).decode(),
                                 mimeType=mime_type or "application/octet-stream",
                             )
 

tests/issues/test_342_base64_encoding.py

@@ -0,0 +1,89 @@
+"""Test for base64 encoding issue in MCP server.
+
+This test demonstrates the issue in server.py where the server uses
+urlsafe_b64encode but the BlobResourceContents validator expects standard
+base64 encoding.
+
+The test should FAIL before fixing server.py to use b64encode instead of
+urlsafe_b64encode.
+After the fix, the test should PASS.
+"""
+
+import base64
+from typing import cast
+
+import pytest
+from pydantic import AnyUrl
+
+from mcp.server.lowlevel.helper_types import ReadResourceContents
+from mcp.server.lowlevel.server import Server
+from mcp.types import (
+    BlobResourceContents,
+    ReadResourceRequest,
+    ReadResourceRequestParams,
+    ReadResourceResult,
+    ServerResult,
+)
+
+
+@pytest.mark.anyio
+async def test_server_base64_encoding_issue():
+    """Tests that server response can be validated by BlobResourceContents.
+
+    This test will:
+    1. Set up a server that returns binary data
+    2. Extract the base64-encoded blob from the server's response
+    3. Verify the encoded data can be properly validated by BlobResourceContents
+
+    BEFORE FIX: The test will fail because server uses urlsafe_b64encode
+    AFTER FIX: The test will pass because server uses standard b64encode
+    """
+    server = Server("test")
+
+    # Create binary data that will definitely result in + and / characters
+    # when encoded with standard base64
+    binary_data = bytes([x for x in range(255)] * 4)
+
+    # Register a resource handler that returns our test data
+    @server.read_resource()
+    async def read_resource(uri: AnyUrl) -> list[ReadResourceContents]:
+        return [
+            ReadResourceContents(
+                content=binary_data, mime_type="application/octet-stream"
+            )
+        ]
+
+    # Get the handler directly from the server
+    handler = server.request_handlers[ReadResourceRequest]
+
+    # Create a request
+    request = ReadResourceRequest(
+        method="resources/read",
+        params=ReadResourceRequestParams(uri=AnyUrl("test://resource")),
+    )
+
+    # Call the handler to get the response
+    result: ServerResult = await handler(request)
+
+    # After (fixed code):
+    read_result: ReadResourceResult = cast(ReadResourceResult, result.root)
+    blob_content = read_result.contents[0]
+
+    # First verify our test data actually produces different encodings
+    urlsafe_b64 = base64.urlsafe_b64encode(binary_data).decode()
+    standard_b64 = base64.b64encode(binary_data).decode()
+    assert urlsafe_b64 != standard_b64, "Test data doesn't demonstrate"
+    " encoding difference"
+
+    # Now validate the server's output with BlobResourceContents.model_validate
+    # Before the fix: This should fail with "Invalid base64" because server
+    # uses urlsafe_b64encode
+    # After the fix: This should pass because server will use standard b64encode
+    model_dict = blob_content.model_dump()
+
+    # Direct validation - this will fail before fix, pass after fix
+    blob_model = BlobResourceContents.model_validate(model_dict)
+
+    # Verify we can decode the data back correctly
+    decoded = base64.b64decode(blob_model.blob)
+    assert decoded == binary_data