MCP stdio server: osw log records bypass the stdout guard #170

Once #160 and
#133 are both merged, osw log
records will be written to the JSON-RPC channel of the MCP stdio server.

Cause

• PR feat(logging): replace print statements with the logging module #160 calls enable_logging() at import time in src/osw/__init__.py. Its
  default stream is sys.stdout and its default level is INFO.
• The MCP server serves JSON-RPC over stdout
  (src/osw/mcp/server.py:170, mcp.run(transport="stdio")).
• The existing protection does not cover logging. Context.guard() wraps each
  osw call in redirect_stdout(sys.stderr) when policy.capture_stdout is set
  (src/osw/service/context.py:113-125, enabled at src/osw/mcp/server.py:134).
  A logging.StreamHandler stores the stream object at construction time, so
  rebinding sys.stdout afterwards has no effect on it.

Reproduction of the last point, independent of osw:

import io
import logging
import sys
from contextlib import redirect_stdout

handler = logging.StreamHandler(sys.stdout)  # built before the redirect
logger = logging.getLogger("probe")
logger.addHandler(handler)
logger.setLevel(logging.INFO)
logger.propagate = False

buffer = io.StringIO()
with redirect_stdout(buffer):
    logger.info("does redirect_stdout capture this?")
print("captured by redirect:", repr(buffer.getvalue()))

The record is written to the real stdout and buffer stays empty.

Effect

• The INFO banner that feat(logging): replace print statements with the logging module #160 emits when OSW_LOG_LEVEL is unset is written to
  stdout at import, before the protocol starts.
• Any osw log record emitted during a tool call is written to stdout while the
  JSON-RPC stream is live. This is the case the guard was written to prevent.

Proposed fix

Redirect the osw logger to stderr in main() in src/osw/mcp/server.py, before
mcp.run():

osw.enable_logging(stream=sys.stderr)

osw.disable_logging() is the alternative, if the server should stay silent
unless the client configures logging itself.

Neither PR can carry this alone: enable_logging only exists in #160, and
src/osw/mcp/server.py only exists in #133. Whichever merges second should add
the call, plus a test asserting that a tool call writes nothing to stdout.

Related

The osw[wikitext] hint in src/osw/wiki_tools.py is the current instance of
the same pattern. It reaches stdout through print() today, but at import time
only, so it does not corrupt the stream. #160 converts it to a logger call,
which does not change its destination as long as the default handler writes to
stdout.