PrefectHQ/fastmcp · error · TypeError

Elicitation schema field '{prop_name}' is an array, but arra

Error message

Elicitation schema field '{prop_name}' is an array, but arrays are only allowed when items are enums (for multi-select). Only enum arrays are supported in elicitation schemas.

What it means

Beyond arrays of objects, elicitation arrays must follow the enum multi-select pattern: items with 'enum', or anyOf/oneOf branches that are all const/enum values. Any other array (e.g. list[int], list[str] of free-form strings) is rejected with this error.

Source

Thrown at fastmcp_slim/fastmcp/server/elicitation.py:499

                raise TypeError(
                    f"Elicitation schema field '{prop_name}' is an array of objects, but arrays of objects are not allowed. "
                    "Elicitation schemas must be flat objects with primitive properties only."
                )

            # Allow arrays with enum patterns (for multi-select)
            if "enum" in items_schema:
                continue  # Allowed: {"type": "array", "items": {"enum": [...]}}

            # Allow arrays with oneOf/anyOf const patterns (SEP-1330)
            if "oneOf" in items_schema or "anyOf" in items_schema:
                union_schemas = items_schema.get("oneOf", []) + items_schema.get(
                    "anyOf", []
                )
                if union_schemas and all("const" in s for s in union_schemas):
                    continue  # Allowed: {"type": "array", "items": {"anyOf": [{"const": ...}, ...]}}

            # Reject other array types (e.g., arrays of primitives without enum pattern)
            raise TypeError(
                f"Elicitation schema field '{prop_name}' is an array, but arrays are only allowed "
                "when items are enums (for multi-select). Only enum arrays are supported in elicitation schemas."
            )

        # Check for nested objects (not allowed)
        if prop_type == "object":
            raise TypeError(
                f"Elicitation schema field '{prop_name}' is an object, but nested objects are not allowed. "
                "Elicitation schemas must be flat objects with primitive properties only."
            )

        # Check if it's a primitive type
        if prop_type not in ALLOWED_TYPES:
            raise TypeError(
                f"Elicitation schema field '{prop_name}' has type '{prop_type}' which is not "
                f"a primitive type. Only {ALLOWED_TYPES} are allowed in elicitation schemas."
            )

View on GitHub (pinned to 1f02114297)

Solutions

Change the field to list[Literal[...]] or list[SomeEnum] so items are enum-like
If free-form strings are needed, use a single string field with a separator, or multiple named primitive fields
Collect arbitrary lists via a tool call instead of elicitation

Example fix

// before
class Prefs(BaseModel):
    scores: list[int]
// after
class Prefs(BaseModel):
    tier: list[Literal["basic", "pro"]]  # enum multi-select

Defensive patterns

Strategy: validation

Validate before calling

def check_enum_array(schema: dict) -> None:
    for prop in schema.get("properties", {}).values():
        if prop.get("type") != "array":
            continue
        items = prop.get("items", {})
        if "enum" in items:
            continue
        branches = items.get("anyOf") or items.get("oneOf") or []
        if branches and all("const" in b for b in branches):
            continue
        raise TypeError("arrays in elicitation must have enum/const items (multi-select only)")

Type guard

def is_enum_array(prop_schema: dict) -> bool:
    if not isinstance(prop_schema, dict) or prop_schema.get("type") != "array":
        return False
    items = prop_schema.get("items", {})
    if "enum" in items:
        return True
    branches = items.get("anyOf") or items.get("oneOf") or []
    return bool(branches) and all("const" in b for b in branches)

Try / catch

try:
    schema = get_elicitation_schema(response_type)
except TypeError as e:
    if "arrays are only allowed" in str(e):
        logger.error("use list[Literal[...]] or list[Enum] for multi-select fields")
        return None
    raise

Prevention

Use list[Literal["a","b"]] or list[MyEnum] — not list[str]/list[int] — for multi-select
Free-form string lists are not supported; use a delimited string field or multiple fields
Collect arbitrary lists with a tool call, not elicitation
Lint response models for non-enum list fields before deployment

When it happens

Trigger: A response type like list[int], list[str], or list[float] that isn't a list[Enum]/list[Literal] — validated via get_elicitation_schema.

Common situations: Asking the user for a list of numbers or free-form tags; assuming list[str] is a multi-select (it must be Literal/Enum values); migrating from a generic JSON-form tool to elicitation.

Understand the failure class

Background: Schema validation failed / invalid input schema: payload rejected because its shape doesn't match the expected schema — this error's family across 28 libraries.

Related errors

AI-assisted analysis of PrefectHQ/fastmcp@1f02114297 (2026-08-29). Data as JSON: /api/errors/036c54ba6a93bea0. Report an issue: GitHub.