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mcp: keep request log levels scoped to each request (#1168)

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Fixes a request-scoping bug in the new-protocol logging path.
`ServerSession.handle` decoded
`_meta[io.modelcontextprotocol/logLevel]`, but stored it in
`ServerSessionState.LogLevel`. Concurrent new-protocol requests on the
same session could therefore clear or inherit another request's log
threshold before the original handler emitted logs.

This keeps the new-protocol log level on the handler context instead.
`ServerSession.Log` and `LoggingHandler.Enabled` read that context value
first, while legacy `logging/setLevel` behavior still uses session
state.

Tests:

- `go test ./mcp -run
TestServerLogLevelDoesNotLeakBetweenNewProtocolRequests -count=1`
- `go test -race ./mcp -run
TestServerLogLevelDoesNotLeakBetweenNewProtocolRequests -count=1`
- `go test ./mcp -run TestEndToEnd -count=1`
- `go test ./mcp -run TestValidateRequestMeta -count=1`
- `go test ./mcp -run
TestStreamableStateless_NewProtocolSession_NoFakeInit -count=1`
- `go test ./... -skip
TestSSELocalhostProtection|TestStreamableLocalhostProtection`

A full `go test ./...` run in my local environment still fails the two
localhost protection cases for `0.0.0.0 via localhost rejects evil.com`;
those same two tests also fail when run directly and are unrelated to
this logging change.

Co-authored-by: Guglielmo Colombo <guglielmoc@google.com>
This commit is contained in:
King Star 2026-08-17 20:00:41 +08:00 • committed by GitHub
commit ba1496a9aa
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1 changed files with 93 additions and 5 deletions
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10
mcp/logging.go
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@ -68,6 +68,13 @@ func compareLevels(l1, l2 LoggingLevel) int {
return cmp.Compare(mcpLevelToSlog(l1), mcpLevelToSlog(l2))
}
type logLevelContextKey struct{}
func logLevelFromContext(ctx context.Context) (LoggingLevel, bool) {
v, ok := ctx.Value(logLevelContextKey{}).(LoggingLevel)
return v, ok
}
// LoggingHandlerOptions are options for a LoggingHandler.
//
// Deprecated: the logging feature is deprecated as of protocol version
@ -146,6 +153,9 @@ func NewLoggingHandler(ss *ServerSession, opts *LoggingHandlerOptions) *LoggingH
func (h *LoggingHandler) Enabled(ctx context.Context, level slog.Level) bool {
// This is also checked in ServerSession.LoggingMessage, so checking it here
// is just an optimization that skips building the JSON.
if mcpLevel, ok := logLevelFromContext(ctx); ok {
return mcpLevel != "" && level >= mcpLevelToSlog(mcpLevel)
}
h.ss.mu.Lock()
mcpLevel := h.ss.state.LogLevel
h.ss.mu.Unlock()