docs(fee): document effective base-fee floor vs minBaseFee

crates/execution-config/src/gas_fee.rs

@@ -387,4 +387,39 @@ mod tests {
             );
         }
     }
+
+    /// Empty blocks stop lowering the fee once `base_fee * k_rate < 10_000`
+    /// (integer truncation on the decrease path). At production-scale gas
+    /// limits the resting value is `ceil(10000/k_rate) - 1` — matching early
+    /// testnet (`k_rate = 1250` → 7 wei) from #367.
+    #[test]
+    fn empty_blocks_rest_at_truncation_floor() {
+        let k_rate = 1250;
+        let iem = 5000; // 50% target
+        let gas_limit = 30_000_000;
+        let resting = 7u64; // ceil(10000/1250) - 1
+        assert_eq!(
+            arc_calc_next_block_base_fee(0, gas_limit, 8, k_rate, iem),
+            7,
+            "one step above resting must fall"
+        );
+        assert_eq!(
+            arc_calc_next_block_base_fee(0, gas_limit, resting, k_rate, iem),
+            resting,
+            "at resting value must stay put"
+        );
+        assert_eq!(
+            arc_calc_next_block_base_fee(0, gas_limit, resting - 1, k_rate, iem),
+            resting - 1,
+            "below resting value must stay put"
+        );
+        let mut base_fee = 1_000_000_000u64; // 1 gwei
+        for _ in 0..2_000 {
+            base_fee = arc_calc_next_block_base_fee(0, gas_limit, base_fee, k_rate, iem);
+        }
+        assert_eq!(
+            base_fee, resting,
+            "long empty-block run reaches resting value"
+        );
+    }
 }

docs/adr/0004-base-fee-validation.md

@@ -132,6 +132,7 @@ next_base_fee = arc_calc_next_block_base_fee(smoothed_gas, gas_limit, base_fee,
 if fee_params is not None:
     next_base_fee = clamp(next_base_fee, fee_params.minBaseFee, fee_params.maxBaseFee)
 
+
 # 4. Apply chainspec absolute bounds
 next_base_fee = clamp(next_base_fee, config.absolute_min_base_fee, config.absolute_max_base_fee)
 
@@ -162,6 +163,19 @@ assert decode(header.extra_data) == expected
 
 This replaces the current `parent.extra_data → child.base_fee` check with a post-execution invariant: the proposer's `extra_data` must match the deterministic output of execution.
 
+### Effective base-fee floor vs `minBaseFee`
+
+`arc_calc_next_block_base_fee` guarantees a minimum **increase** of 1 when utilization is above target, but the **decrease** path truncates to zero with no equivalent floor. For `k_rate > 0`, an empty block stops lowering the fee once `base_fee * k_rate < 10_000`. Under sustained empty blocks at production-scale gas limits, the resting value is therefore:
+
+```text
+resting_value = ceil(10000 / kRate) - 1
+effective_floor ≈ max(minBaseFee, resting_value)
+```
+
+`10000 / kRate` is the truncation *threshold*; the fee comes to rest one wei below it (e.g. `kRate = 1250` ⇒ threshold 8, rest 7; `kRate = 200` ⇒ threshold 50, rest 49). Operators reading only `feeParams.minBaseFee` can miss this interaction. It does not affect mainnet as currently configured (`minBaseFee = 20 gwei`, `kRate = 200`), but early testnet sat at 7 wei for millions of blocks against a declared `minBaseFee` of 1. A future governance update that lowers `minBaseFee` below that resting value (or lowers `kRate` enough to raise it above `minBaseFee`) would recreate the gap. See #367.
+
+Also note: the fee parameters in `assets/testnet/config.json` are **historical genesis values**. Live testnet `ProtocolConfig.feeParams()` currently matches the mainnet parameter set.
+
 ## Consequences
 
 ### Positive