`claude -p --output-format json` shares stdout with the CLI's own
subsystems, so the envelope is not always the first thing on it. An
attached MCP server that advertises no tools makes every invocation emit
Client.listTools() called but server does not advertise tools
capability - returning empty list
ahead of the envelope, and `json.loads(stdout)` fails on the whole buffer.
The failure is raised after the model has already answered, so the chunk
is discarded with its tokens spent. On a real corpus this burned 149,582
input and 52,162 output tokens on chunk 1 of 3 and returned nothing, and
the error names the JSON rather than the preamble that caused it, so the
log points at the wrong thing. Anyone with an MCP server configured hits
it on every chunk.
`_envelope_after_preamble` scans for the first `[`/`{` that begins a
valid JSON document. `raw_decode` ignores trailing bytes, so a diagnostic
on either side is tolerated. The fast path is unchanged: clean stdout
still parses on the first `json.loads`, and stdout with no JSON at all
still returns None so the caller raises -- test_raises_on_garbage_envelope
holds.
(cherry picked from commit ede3a9e0a8)
prav
17d9a71c5c