BerriAI/litellm · error · ValueError

Event hook is not in the supported event hooks

Error message

Event hook {hook} is not in the supported event hooks {supported_event_hooks}

What it means

CustomGuardrail validates its event_hook configuration (single value, list, or Mode) against the hooks the concrete guardrail actually supports via _validate_event_hook_list_is_in_supported_event_hooks. Any hook in the list that is not in supported_event_hooks raises a ValueError naming the offending hook and the allowed set. This variant comes from the list/Mode branch of validation.

Solutions

  1. Check the guardrail class's supported_event_hooks (its docs or constructor) and set mode/event_hook to a supported value like pre_call
  2. Remove unsupported entries from the config list or Mode tags/defaults
  3. If you own the guardrail, implement the missing hook and add it to supported_event_hooks

Example fix

# before (config.yaml)
litellm_settings:
  guardrails:
    - guardrail_name: my_pre_call_guard
      mode: [pre_call, post_call_error]  # ValueError: post_call_error not supported

# after
litellm_settings:
  guardrails:
    - guardrail_name: my_pre_call_guard
      mode: pre_call
Defensive patterns

Strategy: validation

Validate before calling

supported = {'pre_call', 'during_call', 'post_call'}
requested = [h.value if hasattr(h, 'value') else h for h in (mode if isinstance(mode, list) else [mode])]
bad = [h for h in requested if h not in supported]
if bad:
    raise ConfigError(f'unsupported event hooks for this guardrail: {bad}; allowed: {sorted(supported)}')

Type guard

def is_supported_hook(hook, supported_event_hooks: list[GuardrailEventHooks]) -> bool:
    h = GuardrailEventHooks(hook) if isinstance(hook, str) else hook
    return h in supported_event_hooks

Try / catch

except ValueError as e:
    if 'not in the supported event hooks' in str(e):
        fail config load with the supported list from the message
    raise

Prevention

When it happens

Trigger: Configuring mode: [pre_call, during_call] on a guardrail whose supported_event_hooks is only ['pre_call']; a Mode object whose tags or default list reference hooks the guardrail does not implement; passing hook names as strings that resolve to unsupported GuardrailEventHooks values.

Common situations: Copy-pasting guardrail config between guardrails with different hook support; upgrading a guardrail that dropped a hook; enabling 'post_call_error' style modes on a pre-call-only guardrail.

Understand the failure class

Background: "Invalid value" and "allowed values are" config errors: what your library rejected and how to fix it — this error's family across 41 libraries.

Related errors

AI-assisted analysis of BerriAI/litellm@77b7c6c40c (2026-08-18). Data as JSON: /api/errors/1efb2b84729d4365. Report an issue: GitHub.

Appendix: source

Thrown at litellm/integrations/custom_guardrail.py:473

        and the UI is expected to fall back to the global `supported_modes`
        list client-side.
        """
        return None

    def _validate_event_hook(
        self,
        event_hook: GuardrailEventHooks | list[GuardrailEventHooks] | Mode | None,
        supported_event_hooks: list[GuardrailEventHooks],
    ) -> None:
        def _validate_event_hook_list_is_in_supported_event_hooks(
            event_hook: list[GuardrailEventHooks] | list[str],
            supported_event_hooks: list[GuardrailEventHooks],
        ) -> None:
            for hook in event_hook:
                if isinstance(hook, str):
                    hook = GuardrailEventHooks(hook)
                if hook not in supported_event_hooks:
                    raise ValueError(f"Event hook {hook} is not in the supported event hooks {supported_event_hooks}")

        if event_hook is None:
            return
        if isinstance(event_hook, str):
            event_hook = GuardrailEventHooks(event_hook)
        if isinstance(event_hook, list):
            _validate_event_hook_list_is_in_supported_event_hooks(event_hook, supported_event_hooks)
        elif isinstance(event_hook, Mode):
            tag_values_flat: Final[list] = []
            for v in event_hook.tags.values():
                if isinstance(v, list):
                    tag_values_flat.extend(v)
                else:
                    tag_values_flat.append(v)
            _validate_event_hook_list_is_in_supported_event_hooks(tag_values_flat, supported_event_hooks)
            if event_hook.default:
                default_list = event_hook.default if isinstance(event_hook.default, list) else [event_hook.default]
                _validate_event_hook_list_is_in_supported_event_hooks(default_list, supported_event_hooks)

View on GitHub (pinned to 77b7c6c40c)