modelcontextprotocol/go-sdk
Watch
1
0
Fork
You've already forked go-sdk
 0
Code Issues Pull requests Projects Releases Packages Wiki Activity Actions 1

mcp: preserve existing Content in SetError (#864)

Browse source 
SetError now only populates the Content field when it has not already
been populated. This allows callers to provide a user-friendly message
while still recording the underlying error for middleware inspection
via GetError.
The len check (rather than nil check) ensures both nil and empty-slice
Content are treated as "not yet populated."
  Fixes #858
This commit is contained in:
Ravish Gupta 2026-04-03 11:15:24 -07:00 • committed by GitHub
commit 34a19dff46
No known key found for this signature in database
GPG key ID: B5690EEEBB952194
2 changed files with 60 additions and 3 deletions
Download patch file Download diff file Expand all files Collapse all files

41
mcp/mcp_test.go
View file

@ -2228,4 +2228,45 @@ func TestToolErrorMiddleware(t *testing.T) {
}
}
func TestSetErrorPreservesContent(t *testing.T) {
for _, tt := range []struct {
name string
content []Content
err error
wantContent string
}{
{
name: "nil content",
err: errors.New("internal failure"),
wantContent: "internal failure",
},
{
name: "empty slice content",
content: []Content{},
err: errors.New("internal failure"),
wantContent: "internal failure",
},
{
name: "existing content preserved",
content: []Content{&TextContent{Text: "user-friendly msg"}},
err: errors.New("db timeout"),
wantContent: "user-friendly msg",
},
} {
t.Run(tt.name, func(t *testing.T) {
res := CallToolResult{Content: tt.content}
res.SetError(tt.err)
if !res.IsError {
t.Fatal("want IsError=true")
}
if got := res.Content[0].(*TextContent).Text; got != tt.wantContent {
t.Errorf("Content text = %q, want %q", got, tt.wantContent)
}
if got := res.GetError(); got != tt.err {
t.Errorf("GetError() = %v, want %v", got, tt.err)
}
})
}
}
var ctrCmpOpts = []cmp.Option{cmp.AllowUnexported(CallToolResult{})}

22
mcp/protocol.go
View file

@ -10,6 +10,7 @@ import (
"maps"
internaljson "github.com/modelcontextprotocol/go-sdk/internal/json"
"github.com/modelcontextprotocol/go-sdk/internal/mcpgodebug"
)
// Optional annotations for the client. The client can use annotations to inform
@ -113,10 +114,25 @@ type CallToolResult struct {
err error
}
// SetError sets the error for the tool result and populates the Content field
// with the error text. It also sets IsError to true.
// seterroroverwrite is a compatibility parameter that restores the pre-1.6.0
// behavior of [CallToolResult.SetError], where Content was always overwritten
// with the error text. See the documentation for the mcpgodebug package for
// instructions on how to enable it.
// The option will be removed in the 1.8.0 version of the SDK.
var seterroroverwrite = mcpgodebug.Value("seterroroverwrite")
// SetError sets the error for the tool result and sets IsError to true.
// If Content has not already been populated, it is set to the error text.
// If Content has already been populated, it is left unchanged, allowing callers
// to provide a user-friendly message while still recording the underlying error
// for inspection via [GetError] in server middleware.
//
// To restore the previous behavior where Content was always overwritten,
// set MCPGODEBUG=seterroroverwrite=1.
func (r *CallToolResult) SetError(err error) {
r.Content = []Content{&TextContent{Text: err.Error()}}
if len(r.Content) == 0 || seterroroverwrite == "1" {
r.Content = []Content{&TextContent{Text: err.Error()}}
}
r.IsError = true
r.err = err
}