Comfy-Org/ComfyUI · error · Exception

Field '{field_name}' cannot be shorter than {min_length} cha

Error message

Field '{field_name}' cannot be shorter than {min_length} characters; was {len(string)} characters long.

What it means

Raised by validate_string() in comfy_api_nodes/util/validation_utils.py when, after optional whitespace stripping, the string is shorter than the node's min_length. Used by dozens of API nodes to enforce provider minimums (usually min_length=1, i.e. non-empty) before spending a request. Note the truthiness of min_length: a min_length of 0 disables the check entirely.

Source

Thrown at comfy_api_nodes/util/validation_utils.py:186

    if min_duration is not None and dur + eps < min_duration:
        raise ValueError(f"Audio duration must be at least {min_duration}s, got {dur + eps:.2f}s")
    if max_duration is not None and dur - eps > max_duration:
        raise ValueError(f"Audio duration must be at most {max_duration}s, got {dur - eps:.2f}s")


def validate_string(
    string: str,
    strip_whitespace=True,
    field_name="prompt",
    min_length=None,
    max_length=None,
):
    if string is None:
        raise Exception(f"Field '{field_name}' cannot be empty.")
    if strip_whitespace:
        string = string.strip()
    if min_length and len(string) < min_length:
        raise Exception(
            f"Field '{field_name}' cannot be shorter than {min_length} characters; was {len(string)} characters long."
        )
    if max_length and len(string) > max_length:
        raise Exception(
            f" Field '{field_name} cannot be longer than {max_length} characters; was {len(string)} characters long."
        )


def validate_container_format_is_mp4(video: Input.Video) -> None:
    """Validates video container format is MP4."""
    container_format = video.get_container_format()
    if container_format not in ["mp4", "mov,mp4,m4a,3gp,3g2,mj2"]:
        raise ValueError(f"Only MP4 container format supported. Got: {container_format}")


def _ratio_from_tuple(r: tuple[float, float]) -> float:
    a, b = r
    if a <= 0 or b <= 0:

View on GitHub (pinned to 1c6d8d45b3)

Solutions

1. Type actual prompt text into the node's input field or connect a text node with content.
2. If the prompt is assembled dynamically, add a guard upstream that falls back to a default string when the result is blank.
3. Check for whitespace-only content (tabs/newlines) — it passes visual inspection but fails after .strip().
4. Inspect the wire from any text-joining/template node for empty segments producing ''.

Example fix

# before
validate_string('   ', min_length=1)  # Exception: cannot be shorter than 1 characters

# after
validate_string('a cinematic shot of a fox', min_length=1)

text = (prompt or '').strip()
if not text:
    raise ValueError('prompt is empty after stripping whitespace')

def is_nonempty_text(v) -> bool:
    return isinstance(v, str) and len(v.strip()) > 0

try: validate_string(prompt, min_length=1) except Exception as e: raise UserVisibleError(str(e)) from e

When it happens

Trigger: Passing an empty or whitespace-only string to any node calling validate_string(prompt, min_length=1) — Gemini, Kling, LTX, Recraft, Ideogram, Anthropic, etc. Because strip_whitespace defaults to True, a string of only spaces/newlines also triggers it.

Common situations: A text widget left blank; a template/prompt-building node emitted only whitespace; a conditioning-to-text conversion produced an empty string; or a user pasted a prompt that was entirely invisible characters. Also common when a workflow's injected variable is empty at runtime.

Related errors

• Field '{field_name} cannot be longer than {max_length} char
• The prompt references @Audio{max_tag}, but reference mode is
• Field '{field_name}' cannot be empty.
• Connect at least one keyframe image.
• Spreading {len(images)} images across the clip needs an expl

AI-assisted analysis of Comfy-Org/ComfyUI@1c6d8d45b3 (2026-08-14). Data as JSON: /api/errors/1cca7bd5fef422f6. Report an issue: GitHub.