Function argumentValidationMiddleware

pkg/cli/mcp_argument_validation.go:41–94

argumentValidationMiddleware returns a mcp.Middleware that intercepts tool-call results containing "unexpected additional properties" validation errors and replaces them with a helpful message that names the unknown parameter, suggests a close match, and points to the tool's --help output. toolPara

(toolParams map[string]toolParamEntry)

// toolParams maps tool names to their list of valid JSON parameter names.  It is
// provided by the caller as a hardcoded registry (see mcpToolParams).
func argumentValidationMiddleware(toolParams map[string]toolParamEntry) mcp.Middleware {
	return func(next mcp.MethodHandler) mcp.MethodHandler {
		return func(ctx context.Context, method string, req mcp.Request) (mcp.Result, error) {
			result, err := next(ctx, method, req)
			if err != nil || method != "tools/call" {
				return result, err
			}

			// Check whether the result is a tool error containing a schema
			// "additional properties" validation message.
			toolResult, ok := result.(*mcp.CallToolResult)
			if !ok || !toolResult.IsError {
				return result, err
			}

			// Extract the error text from the first TextContent element.
			if len(toolResult.Content) == 0 {
				return result, err
			}
			textContent, ok := toolResult.Content[0].(*mcp.TextContent)
			if !ok {
				return result, err
			}
			errMsg := textContent.Text

			if !strings.Contains(errMsg, "unexpected additional properties") {
				return result, err
			}

			// Parse the unknown parameter names from the error text.
			unknownParams := extractUnknownParams(errMsg)
			if len(unknownParams) == 0 {
				return result, err
			}

			// Determine the tool name from the request so we can look up valid params.
			toolName := extractMCPToolName(req)
			validParams, ok := toolParams[toolName]
			if !ok {
				return nil, newMCPError(jsonrpc.CodeMethodNotFound, fmt.Sprintf("unknown MCP tool: %q", toolName), nil)
			}

			mcpArgValidationLog.Printf("Intercepted unknown param error: tool=%s, unknown_params=%v", toolName, unknownParams)

			// Build a helpful replacement message.
			helpMsg := buildHelpfulParamError(toolName, unknownParams, validParams)

			// Return a modified tool result with the helpful message, preserving IsError.
			replaced := *toolResult
			replaced.Content = []mcp.Content{&mcp.TextContent{Text: helpMsg}}
			return &replaced, nil
		}
	}
}

// extractMCPToolName retrieves the tool name from a MCP Request by casting the
// request params to *mcp.CallToolParamsRaw.  Returns an empty string if the cast