feat(mcp): add robust error handling per JSON-RPC 2.0 spec

Implement comprehensive error handling:
- errors.go: Standard and custom error codes
- errors_test.go: Full test coverage

Error codes:
- -32700: Parse error
- -32600: Invalid Request
- -32601: Method not found
- -32602: Invalid params
- -32603: Internal error
- -32001: Symbol not found (with suggestions)
- -32002: Index not ready
- -32003: Query timeout

Features:
- Parameter validation helpers (ValidateStringParam, ValidateIntParam)
- Structured error responses with data field
- Tool error format with code and details
- Request validation (JSONRPC version, method required)

Coverage: errors.go 100%, total 91.6%

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <[email protected]>

Author: shivasurya

sast-engine/mcp/errors.go

@@ -0,0 +1,187 @@
+package mcp
+
+import (
+	"encoding/json"
+	"fmt"
+)
+
+// Standard JSON-RPC 2.0 error codes.
+const (
+	ErrCodeParseError     = -32700
+	ErrCodeInvalidRequest = -32600
+	ErrCodeMethodNotFound = -32601
+	ErrCodeInvalidParams  = -32602
+	ErrCodeInternalError  = -32603
+
+	// Custom server error codes (-32000 to -32099).
+	ErrCodeSymbolNotFound   = -32001
+	ErrCodeIndexNotReady    = -32002
+	ErrCodeQueryTimeout     = -32003
+	ErrCodeResultsTruncated = -32004
+)
+
+// errorMessages maps error codes to default messages.
+var errorMessages = map[int]string{
+	ErrCodeParseError:       "Parse error",
+	ErrCodeInvalidRequest:   "Invalid Request",
+	ErrCodeMethodNotFound:   "Method not found",
+	ErrCodeInvalidParams:    "Invalid params",
+	ErrCodeInternalError:    "Internal error",
+	ErrCodeSymbolNotFound:   "Symbol not found",
+	ErrCodeIndexNotReady:    "Index not ready",
+	ErrCodeQueryTimeout:     "Query timeout",
+	ErrCodeResultsTruncated: "Results truncated",
+}
+
+// Error implements the error interface for RPCError.
+func (e *RPCError) Error() string {
+	return fmt.Sprintf("[%d] %s", e.Code, e.Message)
+}
+
+// NewRPCError creates a new RPC error with optional data.
+func NewRPCError(code int, data interface{}) *RPCError {
+	message := errorMessages[code]
+	if message == "" {
+		message = "Unknown error"
+	}
+	return &RPCError{
+		Code:    code,
+		Message: message,
+		Data:    data,
+	}
+}
+
+// NewRPCErrorWithMessage creates an RPC error with custom message.
+func NewRPCErrorWithMessage(code int, message string, data interface{}) *RPCError {
+	return &RPCError{
+		Code:    code,
+		Message: message,
+		Data:    data,
+	}
+}
+
+// ParseError creates a parse error response.
+func ParseError(detail string) *RPCError {
+	return NewRPCErrorWithMessage(ErrCodeParseError, "Parse error: "+detail, nil)
+}
+
+// InvalidRequestError creates an invalid request error.
+func InvalidRequestError(detail string) *RPCError {
+	return NewRPCErrorWithMessage(ErrCodeInvalidRequest, "Invalid request: "+detail, nil)
+}
+
+// MethodNotFoundError creates a method not found error.
+func MethodNotFoundError(method string) *RPCError {
+	return NewRPCErrorWithMessage(ErrCodeMethodNotFound,
+		fmt.Sprintf("Method not found: %s", method),
+		map[string]string{"method": method})
+}
+
+// InvalidParamsError creates an invalid params error.
+func InvalidParamsError(detail string) *RPCError {
+	return NewRPCErrorWithMessage(ErrCodeInvalidParams, "Invalid params: "+detail, nil)
+}
+
+// InternalError creates an internal error.
+func InternalError(detail string) *RPCError {
+	return NewRPCErrorWithMessage(ErrCodeInternalError, "Internal error: "+detail, nil)
+}
+
+// SymbolNotFoundError creates a symbol not found error with suggestions.
+func SymbolNotFoundError(symbol string, suggestions []string) *RPCError {
+	data := map[string]interface{}{
+		"symbol": symbol,
+	}
+	if len(suggestions) > 0 {
+		data["suggestions"] = suggestions
+	}
+	return NewRPCErrorWithMessage(ErrCodeSymbolNotFound,
+		fmt.Sprintf("Symbol not found: %s", symbol), data)
+}
+
+// IndexNotReadyError creates an index not ready error.
+func IndexNotReadyError() *RPCError {
+	return NewRPCError(ErrCodeIndexNotReady, nil)
+}
+
+// QueryTimeoutError creates a query timeout error.
+func QueryTimeoutError(timeout string) *RPCError {
+	return NewRPCErrorWithMessage(ErrCodeQueryTimeout,
+		fmt.Sprintf("Query timed out after %s", timeout),
+		map[string]string{"timeout": timeout})
+}
+
+// MakeErrorResponse creates a JSON-RPC error response from an RPCError.
+func MakeErrorResponse(id interface{}, err *RPCError) *JSONRPCResponse {
+	return &JSONRPCResponse{
+		JSONRPC: "2.0",
+		ID:      id,
+		Error:   err,
+	}
+}
+
+// ToolError represents a structured tool error response.
+type ToolError struct {
+	Error   string      `json:"error"`
+	Code    int         `json:"code,omitempty"`
+	Details interface{} `json:"details,omitempty"`
+}
+
+// NewToolError creates a JSON-formatted tool error response.
+func NewToolError(message string, code int, details interface{}) string {
+	te := ToolError{
+		Error:   message,
+		Code:    code,
+		Details: details,
+	}
+	bytes, _ := json.Marshal(te)
+	return string(bytes)
+}
+
+// ValidateRequiredParams checks for required parameters.
+func ValidateRequiredParams(args map[string]interface{}, required []string) *RPCError {
+	missing := []string{}
+	for _, param := range required {
+		if _, ok := args[param]; !ok {
+			missing = append(missing, param)
+		}
+	}
+	if len(missing) > 0 {
+		return InvalidParamsError(fmt.Sprintf("missing required parameters: %v", missing))
+	}
+	return nil
+}
+
+// ValidateStringParam validates a string parameter.
+func ValidateStringParam(args map[string]interface{}, name string) (string, *RPCError) {
+	val, ok := args[name]
+	if !ok {
+		return "", InvalidParamsError(fmt.Sprintf("missing required parameter: %s", name))
+	}
+	str, ok := val.(string)
+	if !ok {
+		return "", InvalidParamsError(fmt.Sprintf("parameter %s must be a string", name))
+	}
+	if str == "" {
+		return "", InvalidParamsError(fmt.Sprintf("parameter %s cannot be empty", name))
+	}
+	return str, nil
+}
+
+// ValidateIntParam validates an integer parameter with optional default.
+func ValidateIntParam(args map[string]interface{}, name string, defaultVal int) (int, *RPCError) {
+	val, ok := args[name]
+	if !ok {
+		return defaultVal, nil
+	}
+
+	// JSON numbers come as float64.
+	switch v := val.(type) {
+	case float64:
+		return int(v), nil
+	case int:
+		return v, nil
+	default:
+		return 0, InvalidParamsError(fmt.Sprintf("parameter %s must be a number", name))
+	}
+}