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mcp: validate tool output #352

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mcp: validate tool output #352

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mcp: validate tool output
jba Aug 22, 2025
074f692
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jba Aug 22, 2025
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8 changes: 6 additions & 2 deletions mcp/server.go
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Original file line number Diff line number Diff line change
Expand Up @@ -240,8 +240,12 @@ func toolForErr[In, Out any](t *Tool, h ToolHandlerFor[In, Out]) (*Tool, ToolHan
}, nil
}

// TODO(v0.3.0): Validate out.
_ = outputResolved
// Validate output schema, if any.
// Skip if out is nil: we've removed "null" from the output schema, so nil won't validate.
if v := reflect.ValueOf(out); v.Kind() == reflect.Pointer && v.IsNil() {
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But reflect.ValueOf(untyped nil) is invalid, no? Should you also add a v.IsValid() check here?
In that case, I'm concerned as to why our tests pass.

} else if err := validateSchema(outputResolved, &out); err != nil {
return nil, err
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Wrap this error to say that the tool output failed to validate.

}

// TODO: return the serialized JSON in a TextContent block, as per spec?
// https://modelcontextprotocol.io/specification/2025-06-18/server/tools#structured-content
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15 changes: 10 additions & 5 deletions mcp/tool.go
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Expand Up @@ -9,6 +9,7 @@ import (
"context"
"encoding/json"
"fmt"
// "log"

"github.com/google/jsonschema-go/jsonschema"
)
Expand Down Expand Up @@ -42,13 +43,17 @@ func unmarshalSchema(data json.RawMessage, resolved *jsonschema.Resolved, v any)
if err := dec.Decode(v); err != nil {
return fmt.Errorf("unmarshaling: %w", err)
}
// TODO: test with nil args.
return validateSchema(resolved, v)
}

// TODO: test with nil args.
func validateSchema(resolved *jsonschema.Resolved, value any) error {
if resolved != nil {
if err := resolved.ApplyDefaults(v); err != nil {
return fmt.Errorf("applying defaults from \n\t%s\nto\n\t%s:\n%w", schemaJSON(resolved.Schema()), data, err)
if err := resolved.ApplyDefaults(value); err != nil {
return fmt.Errorf("applying defaults from \n\t%s\nto\n\t%v:\n%w", schemaJSON(resolved.Schema()), value, err)
}
if err := resolved.Validate(v); err != nil {
return fmt.Errorf("validating\n\t%s\nagainst\n\t %s:\n %w", data, schemaJSON(resolved.Schema()), err)
if err := resolved.Validate(value); err != nil {
return fmt.Errorf("validating\n\t%v\nagainst\n\t %s:\n %w", value, schemaJSON(resolved.Schema()), err)
}
}
return nil
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