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mcp: wrap unmarshalParams with jsonrpc error (#1087)

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Fixes #976
This commit is contained in:
Guglielmo Colombo 2026-07-14 16:26:18 +02:00 • committed by GitHub
commit 9576afc114
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2 changed files with 63 additions and 1 deletions
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24
mcp/shared.go
View file

@ -26,9 +26,21 @@ import (
"github.com/modelcontextprotocol/go-sdk/auth"
internaljson "github.com/modelcontextprotocol/go-sdk/internal/json"
"github.com/modelcontextprotocol/go-sdk/internal/jsonrpc2"
"github.com/modelcontextprotocol/go-sdk/internal/mcpgodebug"
"github.com/modelcontextprotocol/go-sdk/jsonrpc"
)
// nowrapinvalidparams is a compatibility parameter that restores the previous
// behavior of [methodInfo.unmarshalParams]. When unset (the default), a
// params-decoding failure is wrapped with [jsonrpc2.ErrInvalidParams] so the
// wire response carries error code -32602 ("invalid params") rather than the
// zero-value code 0. See:
// https://github.com/modelcontextprotocol/go-sdk/issues/976#issuecomment-4829124838.
//
// See the documentation for the mcpgodebug package for instructions how to enable it.
// The option will be removed in a future version of the SDK.
var nowrapinvalidparams = mcpgodebug.Value("nowrapinvalidparams")
const (
// latestProtocolVersion is the latest protocol version that this version of
// the SDK supports.
@ -304,7 +316,17 @@ func newMethodInfo[P paramsPtr[T], R Result, T any](flags methodFlags) methodInf
var p P
if m != nil {
if err := internaljson.Unmarshal(m, &p); err != nil {
return nil, fmt.Errorf("unmarshaling %q into a %T: %w", m, p, err)
// Legacy behavior: pre-fix versions surfaced this as a
// plain wrapped error, which caused the wire response to
// carry code 0 instead of -32602. Restore via
// MCPGODEBUG=nowrapinvalidparams=1.
if nowrapinvalidparams == "1" {
return nil, fmt.Errorf("unmarshaling %q into a %T: %w", m, p, err)
}
// Wrap jsonrpc2.ErrInvalidParams so toWireError surfaces
// code -32602 ("invalid params") while preserving the
// descriptive message.
return nil, fmt.Errorf("%w: unmarshaling %q into a %T: %w", jsonrpc2.ErrInvalidParams, m, p, err)
}
}
// We must check missingParamsOK here, in addition to checkRequest, to

26
mcp/shared_test.go
View file

@ -512,3 +512,29 @@ func TestImplementationDescriptionJSON(t *testing.T) {
// }
// })
// }
// TestUnmarshalParamsInvalidParamsCode is a regression test for
// https://github.com/modelcontextprotocol/go-sdk/issues/976#issuecomment-4829124838.
// Malformed params for a registered method must surface as a JSON-RPC error
// whose code is CodeInvalidParams (-32602) rather than the zero-value 0.
func TestUnmarshalParamsInvalidParamsCode(t *testing.T) {
info, ok := serverMethodInfos[methodPing]
if !ok {
t.Fatalf("no methodInfo for %q", methodPing)
}
// Array where a struct is expected.
_, err := info.unmarshalParams(json.RawMessage(`["a","b"]`))
if err == nil {
t.Fatal("unmarshalParams returned nil error for malformed params")
}
var jerr *jsonrpc.Error
if !errors.As(err, &jerr) {
t.Fatalf("expected *jsonrpc.Error in chain, got %T: %v", err, err)
}
if jerr.Code != jsonrpc.CodeInvalidParams {
t.Errorf("error code = %d, want %d (CodeInvalidParams)", jerr.Code, jsonrpc.CodeInvalidParams)
}
if !strings.Contains(err.Error(), "unmarshaling") {
t.Errorf("error message = %q, want it to mention unmarshaling", err.Error())
}
}