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---
name: necessity-evaluation
description: Evaluate the probability of necessity (PN) for a causal factor — would the conclusion fail if this factor were absent?
execution: subagent
prompt: ./prompt.md
input: artifact (string), factor (string), conclusion (string)
used-by: [counterfactual-probing]
---
# Necessity Evaluation
Scores probability of necessity: P(Y would not hold | X absent).
## Execution
Subagent — spawned via subagent-spawning/spawn-agent.
## Why Subagent
Necessity evaluation requires careful counterfactual reasoning about absence, isolated from sufficiency judgments.
## Input
- **artifact**: The original artifact
- **factor**: The factor to evaluate for necessity
- **conclusion**: The conclusion that may depend on this factor
## Output
- **pn_score**: Probability of necessity (0.0–1.0)
- **reasoning**: Why this factor is/isn't necessary
- **alternative_paths**: Other ways the conclusion could hold without this factor
- **confidence**: Confidence in the PN estimate
## Budget
One unit = one necessity evaluation per factor.
