apache/shardingsphere · error · MCPInvalidRequestException

object_types must be an array.

Error message

object_types must be an array.

What it means

Thrown by MCPToolArguments.getObjectTypes when the object_types argument is present but is not a JSON array/Collection (e.g. a plain string or number). The metadata tool API expects object_types to be an array of strings; a scalar is treated as a malformed request (MCPInvalidRequestException), not coerced.

Source

Thrown at mcp/core/src/main/java/org/apache/shardingsphere/mcp/core/tool/request/MCPToolArguments.java:56

@RequiredArgsConstructor
public final class MCPToolArguments {
    
    private final Map<String, Object> arguments;
    
    /**
     * Get object types.
     *
     * @param supportedObjectTypes supported object types
     * @return object types
     * @throws MCPInvalidRequestException object types is malformed or unsupported
     */
    public Set<SupportedMCPMetadataObjectType> getObjectTypes(final Set<SupportedMCPMetadataObjectType> supportedObjectTypes) {
        Object rawValue = arguments.get("object_types");
        if (null == rawValue) {
            return Collections.emptySet();
        }
        if (!(rawValue instanceof Collection)) {
            throw new MCPInvalidRequestException("object_types must be an array.");
        }
        Collection<?> objectTypes = (Collection<?>) rawValue;
        if (objectTypes.isEmpty()) {
            return Collections.emptySet();
        }
        Set<SupportedMCPMetadataObjectType> result = new LinkedHashSet<>(objectTypes.size(), 1F);
        for (Object each : objectTypes) {
            result.add(resolveObjectType(each, supportedObjectTypes));
        }
        return result;
    }
    
    private SupportedMCPMetadataObjectType resolveObjectType(final Object objectType, final Set<SupportedMCPMetadataObjectType> supportedObjectTypes) {
        String actualValue = Objects.toString(objectType, "").trim();
        if (actualValue.isEmpty()) {
            throw new MCPInvalidRequestException("object_types cannot contain blank values.");
        }
        try {

View on GitHub (pinned to e952770a21)

Solutions

1. Send object_types as a JSON array of strings: {"object_types": ["table", "view"]}
2. Split comma-joined values into array elements on the client before sending
3. Validate the argument shape in your client schema before invoking the tool

Example fix

// before
list_metadata({"object_types": "table"})

// after
list_metadata({"object_types": ["table"]})

Object o = arguments.get("object_types");
if (o != null && !(o instanceof Collection)) {
    arguments.put("object_types", o instanceof String s && s.contains(",")
            ? Arrays.asList(s.split(",")) : List.of(o));
}

// TypeScript — narrow before sending
function isStringArray(v: unknown): v is string[] {
  return Array.isArray(v) && v.every(x => typeof x === "string");
}
if (!isStringArray(args.object_types)) args.object_types = [String(args.object_types)];

try {
    listMetadata(arguments);
} catch (MCPInvalidRequestException e) {
    if (e.getMessage().contains("must be an array")) listMetadata(Map.of("object_types", List.of(String.valueOf(arguments.get("object_types")))));
}

When it happens

Trigger: arguments.get("object_types") is non-null but not an instanceof Collection — e.g. {"object_types": "table"} instead of {"object_types": ["table"]}, or object_types: 5. JSON arrays arrive as List, scalars do not.

Common situations: LLM agents collapsing a single-value list into a bare string; clients built from loosely typed maps; passing a comma-joined string "table,view" instead of an array.

Related errors

• object_types cannot contain blank values.
• Unsupported object_types value `%s`.
• Statement is not a transaction command.
• %s must be an integer between %d and %d.
• Metadata introspection SQL should use MCP metadata resources

AI-assisted analysis of apache/shardingsphere@e952770a21 (2026-08-14). Data as JSON: /api/errors/5bdbf749f3ae3021. Report an issue: GitHub.