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mcp: avoid "null" in structuredContent

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Avoid a typed nil in structuredContent (yet another typed nil), leading
to "null" on the wire.

Fixes #417
This commit is contained in:
Rob Findley 2025-09-04 19:58:12 +00:00 • committed by Robert Findley
commit 9d34aff732
3 changed files with 54 additions and 18 deletions
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26
mcp/server.go
View file

@ -267,19 +267,21 @@ func toolForErr[In, Out any](t *Tool, h ToolHandlerFor[In, Out]) (*Tool, ToolHan
outval = elemZero outval = elemZero
} }
} }
outbytes, err := json.Marshal(outval) if outval != nil {
if err != nil { outbytes, err := json.Marshal(outval)
return nil, fmt.Errorf("marshaling output: %w", err) if err != nil {
} return nil, fmt.Errorf("marshaling output: %w", err)
res.StructuredContent = json.RawMessage(outbytes) // avoid a second marshal over the wire }
res.StructuredContent = json.RawMessage(outbytes) // avoid a second marshal over the wire
// If the Content field isn't being used, return the serialized JSON in a // If the Content field isn't being used, return the serialized JSON in a
// TextContent block, as the spec suggests: // TextContent block, as the spec suggests:
// https://modelcontextprotocol.io/specification/2025-06-18/server/tools#structured-content. // https://modelcontextprotocol.io/specification/2025-06-18/server/tools#structured-content.
if res.Content == nil { if res.Content == nil {
res.Content = []Content{&TextContent{ res.Content = []Content{&TextContent{
Text: string(outbytes), Text: string(outbytes),
}} }}
}
} }
return res, nil return res, nil
} // end of handler } // end of handler

6
mcp/streamable_test.go
View file

@ -1115,8 +1115,7 @@ func TestStreamableStateless(t *testing.T) {
}, },
wantMessages: []jsonrpc.Message{ wantMessages: []jsonrpc.Message{
resp(2, &CallToolResult{ resp(2, &CallToolResult{
Content: []Content{&TextContent{Text: "hi World"}}, Content: []Content{&TextContent{Text: "hi World"}},
StructuredContent: json.RawMessage("null"),
}, nil), }, nil),
}, },
}, },
@ -1128,8 +1127,7 @@ func TestStreamableStateless(t *testing.T) {
}, },
wantMessages: []jsonrpc.Message{ wantMessages: []jsonrpc.Message{
resp(2, &CallToolResult{ resp(2, &CallToolResult{
Content: []Content{&TextContent{Text: "hi foo"}}, Content: []Content{&TextContent{Text: "hi foo"}},
StructuredContent: json.RawMessage("null"),
}, nil), }, nil),
}, },
}, },

38
mcp/testdata/conformance/server/tools.txtar vendored
View file

@ -21,11 +21,15 @@ structured
"clientInfo": { "name": "ExampleClient", "version": "1.0.0" } "clientInfo": { "name": "ExampleClient", "version": "1.0.0" }
} }
} }
{"jsonrpc":"2.0", "method": "notifications/initialized"} { "jsonrpc":"2.0", "method": "notifications/initialized" }
{ "jsonrpc": "2.0", "id": 2, "method": "tools/list" } { "jsonrpc": "2.0", "id": 2, "method": "tools/list" }
{ "jsonrpc": "2.0", "id": 3, "method": "resources/list" } { "jsonrpc": "2.0", "id": 3, "method": "resources/list" }
{ "jsonrpc": "2.0", "id": 4, "method": "prompts/list" } { "jsonrpc": "2.0", "id": 4, "method": "prompts/list" }
{ "jsonrpc": "2.0", "id": 5, "method": "tools/call" } { "jsonrpc": "2.0", "id": 5, "method": "tools/call" }
{ "jsonrpc": "2.0", "id": 6, "method": "tools/call", "params": {"name": "greet", "arguments": {"name": "you"} } }
{ "jsonrpc": "2.0", "id": 1, "result": {} }
{ "jsonrpc": "2.0", "id": 7, "method": "tools/call", "params": {"name": "structured", "arguments": {"In": "input"} } }
-- server -- -- server --
{ {
"jsonrpc": "2.0", "jsonrpc": "2.0",
@ -119,3 +123,35 @@ structured
"message": "invalid request: missing required \"params\"" "message": "invalid request: missing required \"params\""
} }
} }
{
"jsonrpc": "2.0",
"id": 1,
"method": "ping"
}
{
"jsonrpc": "2.0",
"id": 6,
"result": {
"content": [
{
"type": "text",
"text": "hi you"
}
]
}
}
{
"jsonrpc": "2.0",
"id": 7,
"result": {
"content": [
{
"type": "text",
"text": "{\"Out\":\"Ack input\"}"
}
],
"structuredContent": {
"Out": "Ack input"
}
}
}