github/github-mcp-server · error

parameter %s is not of type %T, is %T

Error message

parameter %s is not of type %T, is %T

What it means

Thrown by OptionalParamOK in pkg/github/params.go when an optional argument IS present in the tool request but its JSON type does not match the Go type the handler expects (e.g. a string is sent where a float64/int is expected, or a number where a string is expected). It is a strict type assertion: unlike toInt/toInt64 it does not coerce numeric strings to numbers.

Source

Thrown at pkg/github/params.go:27

	"github.com/google/go-github/v89/github"
	"github.com/google/jsonschema-go/jsonschema"
)

// OptionalParamOK is a helper function that can be used to fetch a requested parameter from the request.
// It returns the value, a boolean indicating if the parameter was present, and an error if the type is wrong.
func OptionalParamOK[T any, A map[string]any](args A, p string) (value T, ok bool, err error) {
	// Check if the parameter is present in the request
	val, exists := args[p]
	if !exists {
		// Not present, return zero value, false, no error
		return
	}

	// Check if the parameter is of the expected type
	value, ok = val.(T)
	if !ok {
		// Present but wrong type
		err = fmt.Errorf("parameter %s is not of type %T, is %T", p, value, val)
		ok = true // Set ok to true because the parameter *was* present, even if wrong type
		return
	}

	// Present and correct type
	ok = true
	return
}

// OptionalNullableStringParam preserves omitted, null, and non-empty string values.
func OptionalNullableStringParam(args map[string]any, p string) (*string, bool, error) {
	value, ok := args[p]
	if !ok {
		return nil, false, nil
	}
	if value == nil {
		return nil, true, nil
	}

View on GitHub (pinned to 0ea1f775a7)

Solutions

1. Send the argument with the exact JSON type shown in the tool's inputSchema: strings unquoted-as-numbers for numeric fields, plain strings for text fields.
2. Remove the argument if you don't need it — optional parameters that are absent never trigger this error.
3. For numeric fields you must send as strings, check whether the tool uses RequiredInt/OptionalInt-style helpers (which accept numeric strings) instead of the generic path.
4. Inspect the full error text: it names the parameter and both the expected and actual types.

Example fix

// before
arguments: { owner, repo, per_page: "30" }
// after
arguments: { owner, repo, per_page: 30 }

function checkOptional(args, name, kind) {
  const v = args[name];
  if (v === undefined) return; // absent is fine
  const ok = kind === "number" ? typeof v === "number" : typeof v === kind;
  if (!ok) throw new Error(`${name} must be a JSON ${kind}, got ${typeof v}`);
}

function isJSONNumber(v: unknown): v is number {
  return typeof v === "number" && Number.isFinite(v);
}
function isJSONString(v: unknown): v is string {
  return typeof v === "string";
}

Check the tool result's error text: parse 'parameter X is not of type A, is B' to learn both expected and actual types, then coerce or drop the parameter and retry once.

When it happens

Trigger: Passing per_page: "30" (string) to a tool whose handler extracts it via OptionalParamOK[float64]; passing owner: 42 (number) where string is expected; passing a JSON array or object where a scalar was expected.

Common situations: MCP clients or LLMs that serialize all arguments as strings; hand-built JSON requests where numbers are quoted; schema drift after upgrading the server so a parameter changed type; proxies that mangle JSON types.

Related errors

• parameter %s is not of type string or null, is %T
• expected number, got %T
• parameter %s is not of type %T
• invalid detail %q: must be one of "none", "stats", "full_pat
• parameter %s must not be empty

AI-assisted analysis of github/github-mcp-server@0ea1f775a7 (2026-08-15). Data as JSON: /api/errors/54904419ca16295c. Report an issue: GitHub.