fix(stdio): bidi transport closes on parse error; strip UTF-8 BOM#797
Merged
joshrotenberg merged 1 commit intomainfrom May 7, 2026
Merged
fix(stdio): bidi transport closes on parse error; strip UTF-8 BOM#797joshrotenberg merged 1 commit intomainfrom
joshrotenberg merged 1 commit intomainfrom
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BidirectionalStdioTransport::handle_incoming_message previously propagated serde_json errors via `?`, tearing down the run loop on a single malformed line. Per JSON-RPC 2.0, malformed input should produce a -32700 Parse error response (id = null) and the transport should keep reading. The unidirectional StdioTransport already did this; bring the bidirectional path to parity. While here, add a clean_input_line helper that strips a leading UTF-8 BOM and trims whitespace, and wire it into all four stdio read sites (StdioTransport, GenericStdioTransport, SyncStdioTransport, BidirectionalStdioTransport). Windows tools that emit a BOM-prefixed first line now parse correctly across all transports. Closes #795
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Summary
Two related fixes in
transport/stdio.rs:1. Bidirectional transport tears down on parse errors
BidirectionalStdioTransport::handle_incoming_messagepropagatedserde_jsonerrors via?, which killed the entire run loop on a single malformed line. The peer couldn't tell parse failure from a normal stream close, and any subsequent valid input was lost.Now both
serde_json::from_strcall sites catch the error, emit a-32700 Parse errorJSON-RPC response withid = null, and continue reading. Mirrors the existing behavior on the unidirectionalStdioTransport::runpath. Extracted a smallwrite_parse_errorhelper to keep the two error sites consistent.2. UTF-8 BOM stripping
Added a
clean_input_linehelper that strips a leading\u{feff}BOM, then trims whitespace. Wired into all four stdio read sites:StdioTransport::runGenericStdioTransport::process_inputSyncStdioTransport::run_blockingBidirectionalStdioTransport::runWindows tools that emit a BOM-prefixed first line now parse correctly across all transports.
Mirrors rmcp #833.
Tests
5 new tests in
stdio::tests:test_clean_input_line_no_bomtest_clean_input_line_strips_leading_bomtest_clean_input_line_strips_bom_then_trimstest_clean_input_line_does_not_strip_internal_bomtest_clean_input_line_emptytest_process_line_with_bom_stripped_input_parses(BOM round-trip)Closes #795.
Test plan
cargo fmt --all -- --checkcargo clippy --all-targets --all-features -- -D warningscargo test -p tower-mcp --lib --all-featurescargo test -p tower-mcp --test '*' --all-featurescargo doc --no-deps --all-features