charactercheck

Deterministic D&D Beyond character-sheet derivation with provenance — every stat computed and traceable, every unhandled case named, never guessed.

The D&D Beyond API returns build data — there is no computed AC, attack bonus, or save anywhere in the payload. Everyone who consumes it re-derives the math, usually inside a host app (a VTT module, a browser extension) or, worse, by letting a language model guess. charactercheck is that derivation as a standalone, dependency-free library and CLI: the character accountant for agents.

Cold-boot probe (2026-07-24): a fresh agent session given only this repo URL derived a live character correctly in 2 commands, zero failures (install → derive), ~seconds end-to-end.

Status: 0.1 — young but real. The derivation surface is the 100-question QA pass below, run in CI on synthetic fixtures. Unrecognized data is reported, never silently defaulted — that honesty contract is the product.

30 seconds to a derived character

$ pip install charactercheck        # stdlib only, no dependencies
$ charactercheck derive https://www.dndbeyond.com/characters/<id>
{
 "combat": {
  "ac": {"value": 16, "provenance": "Breastplate 14 + DEX +1 + +1 [manual adjustment]"},
  "initiative": {"bonus": 6, "provenance": "DEX +1 + 5 [bonus:initiative]"},
  "hp": {"current": 51, "max": 51, "provenance": "base 30 + CON +2×7 + 7 [per-level bonuses]"},
  "stance": {"main_hand": "Night Rapier", "off_hand": "Boot Knife (off hand)",
             "ac_states": {"current": 16,
                           "shield raised (+2)": {"ac": 18, "cost": "requires the off hand"}}},
  ...
 },
 "unhandled": {"modifier_patterns": ["munch:cookies"]},
 "lint": []
}

Works on any public D&D Beyond character (URL, bare id, or a saved character-service v5 JSON file). No login, no cookies, no API key — ever.

For agents

• tool.json at the repo root and charactercheck --schema describe the full I/O contract.
• Exit codes are the three honesty lanes: 0 = derived clean · 1 = lint findings (the sheet looks inconsistent) · 2 = unhandled content present (data the engine recognizes as there but does not model — each pattern named in unhandled with possibly_affects — exactly which derived numbers to double-check — and a verified_clean list of stat families the unknowns cannot touch (0.2.0)). ⚠ Exit 2 is NOT a failure — the derivation output is complete and usable; the nonzero code is your cue to also resolve the named unhandled items with a human. Don't retry.
• --pipe reads refs from stdin for batch runs.
• MCP server: charactercheck-mcp (stdio) exposes derive, stance, qa, report.
• Every derived number carries a provenance string — the arithmetic that produced it — so a downstream agent (or a suspicious player) can audit any value without re-deriving it.

$ charactercheck stance <ref>    # what's in each hand, AC states with costs
$ charactercheck report <ref>    # ONLY the honesty lanes — resolve these before play
$ charactercheck qa <ref>        # the 100-question pass, per-question OK/PARTIAL/NO
$ charactercheck diff <ref> --baseline intake.json   # the sheet is a LIVE state store:
                                 # classify what the player changed mid-session —
                                 # state (engine's lane) vs build (mini-intake) vs
                                 # impossible edits (equipping gear stashed elsewhere)

Why provenance and refusal, not just numbers

A character sheet is a mix of derivable core-rules content and everything else — homebrew, legacy-edition options, manual overrides someone typed in three campaigns ago. Tools that guess produce confident wrong numbers; at a real table those become wrong rulings. charactercheck's contract:

1. Derived values carry their arithmetic (AC 17 = Breastplate 14 [equipped] + DEX +1 [medium cap] + 2 [manual adjustment]).
2. Unhandled data is surfaced by name (an unknown modifier pattern, an unrecognized characterValue type) and flips the exit code — your cue to ask the player, not to guess.
3. Lint catches sheets that disagree with themselves: nothing flagged equipped, stale damage, a caster with slots and zero prepared spells, gear stashed in a container that was left somewhere else entirely (yes, the container graph is modeled — a chest labeled "stashed @ the docks" stops contributing weight and armor candidates).

The QA pass

tests/ ships a 100-question QA suite covering the surface a table actually uses — vitals, saves, all 18 skills, passives, weapons and masteries, spell slots (pact included), resources with used-counts, encumbrance, attunement. CI runs it on synthetic fixtures on every push; the scorecard is generated, never hand-edited. Current: 92 OK / 7 PARTIAL / 1 NO per fixture-class, with every PARTIAL/NO carrying a named reason in the output.

What it does NOT do (on purpose)

• No rules adjudication — "is this action legal" is a different product (srdcheck, this project's sibling: srdcheck judges actions, charactercheck derives the actor).
• No private sheets — public share links only; this tool will never ask for credentials.
• No VTT output, no homebrew content database, no character building.
• No guessing — the whole point.

As a library

from charactercheck import derive, stance, fetch

r = derive("https://www.dndbeyond.com/characters/<id>")
r["combat"]["ac"]              # {'value': 17, 'provenance': 'Breastplate 14 + ...'}
r["unhandled"]                 # what you must resolve by asking a human
stance(fetch("<id>"))          # hands / AC states / attack lines

Credits

• Schema semantics for the D&D Beyond v5 payload were partly informed by reading the source of MrPrimate/ddb-importer (MIT) — the most complete derivation math in the ecosystem, coupled to FoundryVTT. No code was copied; see NOTICE.
• The 100-question QA schema was authored for this project.
• D&D Beyond is a trademark of Wizards of the Coast. charactercheck is unofficial, unaffiliated, and reads only what a character's owner has made public.

mcp-name: io.github.chaoz23/charactercheck