Fail Fast Dependency Errors

When resolving dependencies or other derived configuration, never “log-and-continue” into an inconsistent internal state. Apply this standard: 1. **Validate at the source (fail fast):** validate `depends_on` (types/contents) in `AgentMiddleware.__init__()` so errors point to the middleware that provided bad input.

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When resolving dependencies or other derived configuration, never “log-and-continue” into an inconsistent internal state.

Apply this standard:

  1. Validate at the source (fail fast): validate depends_on (types/contents) in AgentMiddleware.__init__() so errors point to the middleware that provided bad input.
  2. Use narrow exception handling: during auto-instantiation, catch only expected exception types (e.g., TypeError/ValueError from constructor/args). Avoid broad except Exception.
  3. Preserve invariants: if auto-instantiation fails, do not proceed to dereference structures that assume the dependency exists. Either:
    • raise a clear error immediately, or
    • explicitly check presence before access.
  4. Make errors visible: use warning/error (or raise) rather than DEBUG-only logs so failures are actionable in real runs.

Example pattern (inspired by the discussed failure mode):

instance_by_class: dict[type, Middleware] = {}

try:
    instance_by_class[dep_class] = dep_class()  # auto-instantiation
except TypeError as e:  # narrow expected failures
    # Fail fast with context; do not `continue` and later risk KeyError
    raise ValueError(
        f"Failed to auto-instantiate middleware dependency {dep_class.__name__} "
        f"required by {type(instance).__name__}: {e}"
    ) from e

# Safe access: only when guaranteed present
dep_instance = instance_by_class[dep_class]

This prevents delayed, confusing errors (like KeyError) and ensures failures are reported with precise, fixable context.

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