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Blades in the Dark — action-result categories

R18 · Intermediate · Scoped procedure: base action-result bands

Question and scope

For a supplied number of dice, how often is an action roll bad, partial, clean, or critical? Take the highest d6 unless two or more sixes produce a critical. At zero dice, roll two and take the lower, with no critical.

Zero dice is not desperate position. Position sets consequences; effect sets what is accomplished. Neither is computed here. An action critical increases effect, not a separate universal damage bonus. This component does not assemble the pool, spend stress, resist consequences, or apply special abilities.

Inputs

This uses probability decomposition instead of enumerating every face tuple. The tutorial callback is useful for learning shared-face rules, but its interactive limit is five dice. Seven dice took roughly 13 seconds in a debug-WASM test; the engine's tuple guard is not a speed guarantee.

Model

Bad means no die reaches 4. Partial means no six, minus the bad cases. Clean means exactly one six; critical means at least two. These are mutually exclusive and exhaustive categories, so this particular table does sum to one. pool.count(6) constructs the count distribution efficiently without a custom callback. The zero-dice branch uses the lower of two d6 explicitly.

dice = 2
if type(dice) != "int" or dice < 0 or dice > 20:
    fail("Use an integer dice count 0–20")
if dice == 0:
    lowest = keep_lowest(2, 6, 1)
    bad = lowest.cdf(3)
    partial = lowest.cdf(5) - bad
    clean = lowest.pmf(6)
    critical = 0.0
else:
    pool = dice_pool(dice, 6)
    sixes = pool.count(6)
    bad = pool.p_none([4, 5, 6])
    partial = pool.p_none(6) - bad
    clean = sixes.pmf(1)
    critical = sixes.p_ge(2)
rows = [("Bad", bad), ("Partial", partial), ("Clean", clean), ("Critical", critical)]
output("Blades action categories", prob_table(rows))

Blades action categories · Table

Reading and independent checks

At two dice, the rows are 9/36, 16/36, 10/36, and 1/36. At zero dice they are 27/36, 8/36, 1/36, and zero. At one die, bad is 1/2 and partial is 1/3: partial is not the most common band. At four dice, clean plus critical is 1 − (5/6)^4 = 671/1296, over half.

For positive n, an independent oracle is:

• bad = (1/2)^n;
• partial = (5/6)^n − (1/2)^n;
• clean = n × (1/6) × (5/6)^(n−1);
• critical = 1 − (5/6)^n − clean.

Tests compare this decomposition against a separate face count for small pools and these formulas through the documented upper bound.

Adaptations and limits

Change dice without changing the resolution rule. Do not treat a probability of a clean result as a probability of a particular fictional consequence: that also depends on position and the situation. Do not multiply these rows together; they are exclusive categories from the same roll.

Rules and related lessons

Blades core system and action roll, source checked 2026-09-15. Independent human rules/novice review remains pending. Read L15 for counts and L23 for the equivalent callback approach.

Content ID: R18 · Review status: pilot

Prerequisites: L23