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mcp: tool.InputSchema and tool.OutputSchema validation (SEP-2106) (#1009)

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Fixes #990
This commit is contained in:
Guglielmo Colombo 2026-06-16 13:55:47 +02:00 • committed by GitHub
commit 79e1a13dcf
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1 changed files with 377 additions and 40 deletions
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32
mcp/server.go
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@ -232,7 +232,8 @@ func (s *Server) RemovePrompts(names ...string) {
// that takes no input, or one where any input is valid, set [Tool.InputSchema] to
// `{"type": "object"}`, using your preferred library or `json.RawMessage`.
//
// If present, [Tool.OutputSchema] must also have type "object".
// If present, [Tool.OutputSchema] may be any valid JSON Schema (object, array,
// primitive, or composition).
//
// When the handler is invoked as part of a CallTool request, req.Params.Arguments
// will be a json.RawMessage.
@ -279,16 +280,10 @@ func (s *Server) AddTool(t *Tool, h ToolHandler) {
if s == nil {
panic(fmt.Errorf("AddTool %q: output schema is nil", t.Name))
}
if s.Type != "object" {
panic(fmt.Errorf(`AddTool %q: output schema must have type "object"`, t.Name))
}
} else {
var m map[string]any
var m any
if err := remarshal(t.OutputSchema, &m); err != nil {
panic(fmt.Errorf("AddTool %q: can't marshal output schema to a JSON object: %v", t.Name, err))
}
if typ := m["type"]; typ != "object" {
panic(fmt.Errorf(`AddTool %q: output schema must have type "object" (got %v)`, t.Name, typ))
panic(fmt.Errorf("AddTool %q: can't marshal output schema to JSON: %v", t.Name, err))
}
}
}
@ -340,7 +335,7 @@ func toolForErr[In, Out any](t *Tool, h ToolHandlerFor[In, Out], cache *SchemaCa
}
// Validate input and apply defaults.
var err error
input, err = applySchema(input, inputResolved)
input, err = applySchema(input, inputResolved, false)
if err != nil {
var errRes CallToolResult
errRes.SetError(fmt.Errorf("validating \"arguments\": %v", err))
@ -402,7 +397,7 @@ func toolForErr[In, Out any](t *Tool, h ToolHandlerFor[In, Out], cache *SchemaCa
//
// We validate against the JSON, rather than the output value, as
// some types may have custom JSON marshalling (issue #447).
outJSON, err = applySchema(outJSON, outputResolved)
outJSON, err = applySchema(outJSON, outputResolved, true)
if err != nil {
return nil, fmt.Errorf("validating tool output: %w", err)
}
@ -411,10 +406,18 @@ func toolForErr[In, Out any](t *Tool, h ToolHandlerFor[In, Out], cache *SchemaCa
// If the Content field isn't being used, return the serialized JSON in a
// TextContent block, as the spec suggests:
// https://modelcontextprotocol.io/specification/2025-06-18/server/tools#structured-content.
//
// Ensure a serialized-JSON TextContent fallback is present in case of servers using array
// or primitive structuredContent, so that pre-SEP-2106 clients can recover the structured
// payload from unstructured content.
if res.Content == nil {
res.Content = []Content{&TextContent{
Text: string(outJSON),
}}
} else if !isObjectJSON(outJSON) {
res.Content = append(res.Content, &TextContent{
Text: string(outJSON),
})
}
}
return res, nil
@ -525,9 +528,10 @@ func setSchema[T any](sfield *any, rfield **jsonschema.Resolved, cache *SchemaCa
// empty object schema value.
//
// If the tool's output schema is nil, and the Out type is not 'any', the
// output schema is set to the schema inferred from the Out type argument,
// which must also be a map or struct. If the Out type is 'any', the output
// schema is omitted.
// output schema is set to the schema inferred from the Out type argument.
// Per SEP-2106, the Out type may be any Go type whose inferred schema is a
// valid JSON Schema (struct, map, slice, primitive, etc.). If the Out type is
// 'any', the output schema is omitted.
//
// Unlike [Server.AddTool], AddTool does a lot automatically, and forces
// tools to conform to the MCP spec. See [ToolHandlerFor] for a detailed