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mcp: avoid "null" when tools fail to return content

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For some reason, this was a regression from v0.3.1, for some of our
examples. Fix it and address the existing TODO.
This commit is contained in:
Rob Findley 2025-09-05 20:48:32 +00:00 • committed by Robert Findley
commit 4656930de8
2 changed files with 13 additions and 9 deletions
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10
mcp/server.go
View file

@ -496,9 +496,13 @@ func (s *Server) callTool(ctx context.Context, req *CallToolRequest) (*CallToolR
Message: fmt.Sprintf("unknown tool %q", req.Params.Name),
}
}
// TODO: if handler returns nil content, it will serialize as null.
// Add a test and fix.
return st.handler(ctx, req)
res, err := st.handler(ctx, req)
if err == nil && res != nil && res.Content == nil {
res2 := *res
res2.Content = []Content{} // avoid "null"
res = &res2
}
return res, err
}
func (s *Server) listResources(_ context.Context, req *ListResourcesRequest) (*ListResourcesResult, error) {

12
mcp/streamable_test.go
View file

@ -520,7 +520,7 @@ func TestStreamableServerTransport(t *testing.T) {
method: "POST",
messages: []jsonrpc.Message{req(2, "tools/call", &CallToolParams{Name: "tool"})},
wantStatusCode: http.StatusOK,
wantMessages: []jsonrpc.Message{resp(2, &CallToolResult{}, nil)},
wantMessages: []jsonrpc.Message{resp(2, &CallToolResult{Content: []Content{}}, nil)},
},
},
},
@ -547,14 +547,14 @@ func TestStreamableServerTransport(t *testing.T) {
headers: http.Header{"Accept": {"text/plain", "*/*"}},
messages: []jsonrpc.Message{req(4, "tools/call", &CallToolParams{Name: "tool"})},
wantStatusCode: http.StatusOK,
wantMessages: []jsonrpc.Message{resp(4, &CallToolResult{}, nil)},
wantMessages: []jsonrpc.Message{resp(4, &CallToolResult{Content: []Content{}}, nil)},
},
{
method: "POST",
headers: http.Header{"Accept": {"text/*, application/*"}},
messages: []jsonrpc.Message{req(4, "tools/call", &CallToolParams{Name: "tool"})},
wantStatusCode: http.StatusOK,
wantMessages: []jsonrpc.Message{resp(4, &CallToolResult{}, nil)},
wantMessages: []jsonrpc.Message{resp(4, &CallToolResult{Content: []Content{}}, nil)},
},
},
},
@ -592,7 +592,7 @@ func TestStreamableServerTransport(t *testing.T) {
wantStatusCode: http.StatusOK,
wantMessages: []jsonrpc.Message{
req(0, "notifications/progress", &ProgressNotificationParams{}),
resp(2, &CallToolResult{}, nil),
resp(2, &CallToolResult{Content: []Content{}}, nil),
},
},
},
@ -624,7 +624,7 @@ func TestStreamableServerTransport(t *testing.T) {
wantStatusCode: http.StatusOK,
wantMessages: []jsonrpc.Message{
req(1, "roots/list", &ListRootsParams{}),
resp(2, &CallToolResult{}, nil),
resp(2, &CallToolResult{Content: []Content{}}, nil),
},
},
},
@ -674,7 +674,7 @@ func TestStreamableServerTransport(t *testing.T) {
},
wantStatusCode: http.StatusOK,
wantMessages: []jsonrpc.Message{
resp(2, &CallToolResult{}, nil),
resp(2, &CallToolResult{Content: []Content{}}, nil),
},
},
{