[bug] Runtime inputs exceeding field prime are silently reduced without warning

[chamitro]

Summary

When runtime inputs (via input.json) exceed the field prime p, the witness generator silently reduces them modulo p without any warning or error. A developer passing a = p+5 gets the same result as a = 5 with no indication that the input was modified.

Environment

• Circom: 2.2.3 (latest master)
• Node: v18.17.1
• snarkjs: 0.7.6
• OS: Ubuntu

Reproducer

pragma circom 2.0.0;

template Test() {
    signal input a;
    signal output out;
    out <== a * a + a + 1;
}

component main = Test();

circom test.circom --r1cs --wasm --output .

# Normal input — correct
echo '{"a": "5"}' > input.json
node test_js/generate_witness.js test_js/test.wasm input.json w.wtns
snarkjs wtns export json w.wtns w.json
cat w.json  # output = 31 ✓

# Input exceeding field prime — silently wrong!
echo '{"a": "21888242871839275222246405745257275088548364400416034343698204186575808495622"}' > input.json
# This is p+5, should warn but silently becomes 5
node test_js/generate_witness.js test_js/test.wasm input.json w.wtns
snarkjs wtns export json w.wtns w.json
cat w.json  # output = 31 (same as a=5!) — no warning!

Results

Circuit: out = a² + a + 1

Input (a)	Expected behavior	Actual output
5	31	✅ 31
p+5	Warning or error	❌ 31 (silently treated as a=5)
2p+5	Warning or error	❌ 31 (silently treated as a=5)
100p+5	Warning or error	❌ 31 (silently treated as a=5)
p	Warning or error	❌ 1 (silently treated as a=0)
p²	Warning or error	❌ 1 (silently treated as a=0)
2²⁵⁶	Warning or error	❌ Silent reduction
10¹⁰⁰	Warning or error	❌ Silent reduction

Expected Behavior

The witness generator should either:

1. Reject inputs >= p with an error: Error: input value exceeds field prime
2. Warn the user: Warning: input 'a' exceeds field prime, reducing modulo p

Comparison with Noir

Noir (nargo) correctly rejects oversized inputs:

Failed to deserialize inputs: The value exceeds the field prime

[chamitro]

@clararod9

[joshturner35654-lang]

Hi

[joshturner35654-lang]

Hi