LemmaScript (Tech Preview)

A verification toolchain for TypeScript. Write ordinary TypeScript with //@ specification annotations. The toolchain generates verifiable code from your TypeScript — either in Dafny or Lean 4 (with Velvet/Loom).

See SPEC.md, DESIGN.md, and GETTING_STARTED.md.

This is a Tech Preview: the core idea is there, but support, semantics, and ergonomics are still evolving.

See our blog post.

Examples and Case Studies

Each example and case study is verified in Lean 4 and/or Dafny from the same annotated TypeScript source.

See the internal examples.

See the external case studies:

• collab-todo-lemmascript — collaborative task management web app (React + Supabase) with a verified domain model. Single domain.ts imported directly by the UI, hooks, and edge functions — no adapter layer. 123 Dafny lemmas (120 in a separate domain.proofs.dfy): 16-conjunct invariant preserved across 25 single-project + 3 cross-project actions, NoOp completeness/soundness, initialization. Dafny only.
• colorwheel-lemmascript — verified color palette generator with mood + harmony constraints. 31 Lean proofs + 18 behavioral properties, 115 Dafny lemmas (invariant preservation, commutativity, NoOp completeness).
• clear-split-lemmascript — greenfield verified expense splitting web app. Conservation theorem, invariant preservation, delta laws — all proven in both Lean (no sorry) and Dafny (56 lemmas).
• eventab-lemmascript — greenfield verified group bill-splitter, shipped as a single self-contained client-side page. Largest-remainder allocation in integer cents proves the money never leaks: per-person shares sum exactly to the tab, non-claimers pay nothing, and the settlement nets to zero even when payments are rounded to the dollar (the payer absorbs the difference). Dafny only.
• github-star-checker-lemmascript — small verified CLI that tracks GitHub star counts across repos and reports per-run deltas. Verifies: per-row diff correctness and totalDiff == sumDiffs(rows) (via an inductive SumDiffs_append lemma); three sign-classified extractors (gainers / losers / unchanged) with soundness, completeness, ordered completeness (gainers appear in the notification in the same order they were listed on the command line), and count/sum equalities against prefix-indexed upTo helpers; conservation theorem decompose(r) — the three splits partition every row exactly once, and sumDiffs(increases) + sumDiffs(decreases) == totalDiff. 33 Dafny VCs, 0 errors; proof additions include a head/tail bridge (sumDiffs ↔ sumDiffsUpTo) and two partition-on-n inductions. Dafny only.
• equality-game-lemmascript — greenfield verified arithmetic equality card game (React + Tailwind). Sound + complete decision procedure for "can these two card lists be combined into equal expressions": canEqualize(L, R) ⟺ ∃ eL, eR. eval(eL) == eval(eR) ∧ multiset(leaves(eL)) == multiset(L) ∧ same for R. Algorithm is subset-DP over a bitmask m ∈ [1, 2^n − 1); the proof composes a PopCount upper/lower bound chain (with stdlib LemmaDivDenominator / LemmaFundamentalDivModConverse), a splitLeft/splitRight ↔ imperative-loop connection, a WitnessCombine lemma threading existential Expr witnesses through the cross-product loops, and a ChooseMask combinatorial constructor that, given any sub-multiset of cards, produces the realizing mask. Capped by CompletenessFromMaskCoverage. 753 verification conditions, 0 errors, 0 assumes, 0 axioms under --isolate-assertions --verification-time-limit 180. Dafny only.
• talktimer-lemmascript — verified talk timer React app, ported from a Dafny-only talktimer-lemmafit twin. 17-variant Action state machine + verified History (undo/redo/preview/commitFrom) all in one domain.ts — the original Dafny's Domain refines Kernel abstract-module pattern inlined since LS has no abstract modules. 108 VCs in domain.dfy (invariant preservation) + 123 in domain.proofs.dfy (behavioral lemmas + Kernel round-trip). Dafny only.
• node-casbin-lemmascript — brownfield verification of node-casbin. 5 functions verified, 217 existing tests pass. End-to-end correctness and order independence for all 4 effect modes in both Lean and Dafny (39 lemmas).
• hono-lemmascript — brownfield verification of hono's security middleware. Four CVEs covered: IP restriction bypass (CVE-2026-39409) and cookie name bypass (CVE-2026-39410) — 51 Dafny lemmas, cookie verification in-place; plus serveStatic's URL-encoded directory traversal (CVE-2024-32869) + repeated-slash bypass (CVE-2026-39407), proved as a composition — decode(rawPath) before check(decoded), so a buggy implementation that reordered the steps would fail the proof. First use of //@ assume + //@ havoc-on-assign. Dafny only.
• hono-rate-limiter-with-lemmascript — a greenfield verified feature shipped through a brownfield host's middleware seam: a standalone Hono sliding-window rate-limit middleware whose admission decision is a machine-checked theorem. SlidingWindowBound proves no half-open window (s, s+W] ever admits more than limit, so the boundary-straddling 2× burst that every fixed-window limiter leaks is impossible (the naive cousin is refuted in the same file, FixedWindowLeaks); the proof is wired into a live Hono server, with the clock's monotonicity and the per-key store's atomicity as the named trust boundary. 23 Dafny VCs, 0 errors. Dafny only.
• charmchat — brownfield verification of an AI agent orchestration backend. isEmptyResult (string emptiness predicate, 8 postconditions, <1s) and topologicalSort (Kahn's algorithm — memory safety, output bounds, completeness via acyclicity ranking witness, termination). Full completeness proof: 23 helper lemmas, 14 opaque ghost predicates, 115 loop invariants; 736 VCs verified under --isolate-assertions --verification-time-limit 600. Key technique: snapshot-based inner invariants (ghost var originalRemDeps := remDeps) replace the mid-iteration SEEN/UNSEEN split so preservation is frame reasoning against a ghost-constant rather than set-subtraction against mutating state. Dafny only.
• xyflow-lemmascript — brownfield verification of xyflow's core edge and geometry utilities. 9 functions verified: addEdge (dedup — never loses edges, adds at most one), reconnectEdge (semantic: under a unique-id precondition, the result is in-place — |result| ≤ |edges|, no insertion — and when a matching edge existed with non-empty new endpoints, the output contains an edge with those endpoints. Uses //@ assume to characterize destructuring, find, and the constructed edge), connectionExists, getEdgeCenter (midpoint correctness), clamp (bounds), rectToBox/boxToRect (field arithmetic), getBoundsOfBoxes (enclosure), getOverlappingArea (non-negative), areSetsEqual (subset + same size). 14 Dafny proof obligations. Also adds a new verified feature — a DAG connection gate: canReach decides reachability soundly and completely, and wouldCreateCycle gates both new connections and edge reconnections so the graph is proven to stay acyclic, with isAcyclic (sound + complete) establishing the base case and a topological-rank witness giving a safe evaluation order (+29 obligations), shown live in a React Flow demo that refuses cycle-closing edges — extending the case study from verifying existing code to adding a verified feature. Dafny only.
• rallly-lemmascript — brownfield verification of rallly's meeting-poll Next.js app. 2 functions: validateRedirectUrl (in-place — open-redirect predicate; non-undefined outputs start with / but not //) and scorePoll (extracted ranking core — length preservation, score bounds, top-choice characterization, score-formula equality, within-poll monotonicity, tiebreaker injectivity). The injectivity proof surfaced a real spec-level constraint on the existing (yes + ifNeedBe) * 1000 + yes encoding: it overflows when an option has ≥ 1000 yes votes. 10 Dafny VCs, 0 errors. Drove four toolchain additions: s.startsWith(), T | null nullability, \result narrowing under ==>, Math.max(...arr) spread. Dafny only.
• opencode-lemmascript — brownfield verification of opencode's permission system and unified-diff patch parser. Highlights: (1) Patch.parsePatch carries conservation loop invariants over local ghost state — a parser bug here would silently corrupt user files when an AI applies a patch, and (2) the permission-engine work mechanically closes opencode bug #26514 (subagents bypassing Plan Mode's file-edit restrictions). 9 functions verified in-place, 0 errors. Dafny only.
• pi-lemmascript — brownfield, in-place verification of the context-compaction cut-point selector in pi (the earendil-works agent harness). When the context window fills, pi discards history before a chosen cut; a provider API rejects a retained prefix containing an orphaned toolResult (a tool result whose tool call was cut away). Both selector functions proven: the cut never lets the kept suffix start with — nor split a tool-use/tool-result run into — an orphaned tool result, even across the backward metadata snap. The no-orphan result forced the session tree's tool-pairing ordering into an explicit requires. 4 VCs, 0 errors. Drove five toolchain additions, headlined by an opaque fall-through type: a union LemmaScript can't discriminate (here an array-element union of unreachable imports) becomes a single opaque type — the field stays present so distinct values stay distinct, and with no constructor or tag predicate it can only be passed through, never unsoundly observed. Dafny + Lean: the no-orphan theorem, the changelog semver core, and both tool-output truncators also carry Lean 4 (Velvet/Loom) proofs from the same annotated source, zero sorry; the Lean port drove the backend's brownfield batch (cross-file externs, union destructor lowering, Bool-vs-Prop contexts, return-in-loop elimination).
• flue-lemmascript — brownfield, in-place verification of pure logic in Flue (withastro/flue), Astro's agent harness — crash-recovery and context-compaction functions proven byte-identical. What it drove into the toolchain: native Dafny continue, bounds-guarded noUncheckedIndexedAccess optional indexing, optional narrowing past an opt?.disc guard composing with discriminated-union matching, and object truthiness. Dafny only.
• balanced-match-lemmascript — brownfield, in-place verification of balanced-match, the ~70-line balanced-bracket finder pulled in by npm, webpack, and most of the JS tooling stack (1B+ downloads/month). The stack-based range core is verified by refinement: a pure recursive spec range_spec mirrors the loop one branch per recursive case, so the single equivalence range == range_spec transfers every property automatically — including an unconditional Dyck body-balance theorem for the interior of every returned pair. 2233 VCs, 0 errors under --isolate-assertions (registered on the dafny-slow track). Dafny only.
• guardians-lemmascript — greenfield verification of the core safety argument behind Guardians (Erik Meijer, "Guardians of the Agents", CACM Jan 2026), a generate-verify-execute checker for AI-agent workflows. Instead of verifying an app, it proves the agent guardrail itself sound — that a static taint/automaton check over the real recursive workflow AST can never admit an unsafe plan. Highlights: taint over nested conditionals as a sound branch-union over-approximation; per-source provenance with a join (a multi-input tool is tainted if any input was); unbounded loops discharged by a one-step pre-fixpoint (sat = t0 ‖ bodyTaint(t0)) that bounds taint over any iteration count without iterating to a fixpoint; and a unified capstone (verifyWfSound) — one clean verdict rules out, on every path, both a tainted-data-to-sink leak and a security-automaton error. 54 Dafny obligations, 0 errors. The verified cores are reached from a Guardians-style Workflow/Policy through a thin unverified adapter, differentially tested against the real Python Guardians (used as the oracle, not a porting target). Dafny only.
• quorum-lemmascript — greenfield verified when2meet/Doodle-style group scheduler (React + Cloudflare Durable Objects), with one domain.ts running unchanged in the browser, the in-app query, and the server. The standout is that the proof licenses the architecture: countFree is a homomorphism from participant-list concatenation to integer addition (so the heatmap is order-independent) plus same-participant last-writer-wins convergence — which is exactly what makes the lock-free, no-login, optimistic multi-device backend safe, with the Durable Object and the browser applying the same verified applyOp (server-authoritatively, client-optimistically) with no rollback or operational transform. Also: heatmap is exactly the per-slot count and isBest exactly its argmax; monotonicity; invariant-preserving mutations + op-log replay; a sparse export codec round-trip; an in-app whoIsFree(e, s) whose length provably equals the cell's count; and a separate grid.ts proving the (day, time) → slot map in-range + injective — which makes specific-dates-vs-days-of-the-week pure shell labeling at zero proof cost; and full element-level permutation invariance (heatmapPermInvariant — the heatmap depends only on the multiset of participant rows), which drove the perm(...) spec predicate into LemmaScript itself. 100 Dafny VCs (90 + 10), 0 errors. The aggregate is proven; the React UI, WebSocket/DO I/O, and timezone labeling are the stated trust boundary. Dafny only.
• quota-lemmascript — greenfield verified booking app: providers publish a page of limited-capacity featured slots, signed-in users grab them (React + Cloudflare Durable Objects + D1). The deliberate inverse of quorum-lemmascript: bookings contend for shared inventory, so the invariant is a bound — for every slot j, confirmedCount(bookings, j) <= slots[j].capacity — and the same domain.ts runs in the browser and the Durable Object server-authoritatively, never overselling under contention (no optimistic client apply). Proven: no overbooking across tryBook/cancel, accept-iff-room, an idempotent three-way tryBook (a retry reads as success, not rejection), cancellation frees seats, replay determinism, and order-invariance — confirmedCountPerm / hasRoomPermInvariant show availability depends only on the multiset of the booking log, via the same perm(...) predicate Quorum drove into LemmaScript. 80 Dafny VCs, 0 errors. Trust boundary: auth, the React UI, WebSocket/DO/D1 I/O, email, slot date/time labeling, and abuse/rate-limiting. Dafny only.
• henri-lemmascript — a runnable TypeScript coding-agent CLI (a port of henri; multi-provider, Anthropic + AWS Bedrock) whose security- and protocol-critical core is verified and imported directly by the live agent — decide() gates every real tool call and the conversation invariant is asserted on every turn (it streams end-to-end against Bedrock). Three modules, 48 Dafny VCs, 0 errors. (1) Permission gate: soundness (decide == Allow ⟺ isAllowed), path-traversal containment — auto-allow-in-cwd can never resolve outside cwd, with ./.. normalization proved in-core so the shell is trusted only to resolve().split('/') — grant monotonicity, and rejectPrompts is deny-only. (2) Conversation protocol: tool-call/result pairing plus the pi-lemmascript-style no-orphaned-tool_result property, proved as an invariant preserved by the loop — wellFormed(msgs + [assistant(calls), tool(makeResults(calls))]) — not checked after the fact. (3) Hook/config merge: removal, tool-name uniqueness — a fix (henri concatenated hook tool lists with no dedup, so two hooks could shadow a name), order-independence, and additivity composed cross-module with the gate's monotonicity (merging only grows the allow-sets, which by P3 never revokes an Allow). The merge is verified in place via //@ declare-type Tool { name: string }, shadowing the real Tool's function-valued execute so the actual mergeTools(Tool[]) is the proof target rather than a parallel model. Dafny only.
• eslint-plugin-with-lemmascript — a greenfield verified feature shipped through a brownfield host's extension API: a real, npm-published ESLint flat-config plugin whose rule decision is a machine-checked theorem. no-forbidden-reach enforces architecture boundaries — "the UI must never reach the DB layer" — through any import chain, catching the laundered ui → service → db violation that every one-hop incumbent (import/no-restricted-paths, eslint-plugin-boundaries, Nx module boundaries) silently passes. The verified core decides reachability soundly and completely (reachesAny / violates) and constructs the offending chain in proven code (findReachPath — a path-carrying BFS proven sound + complete by mirroring the frontier's endpoints in a ghost seq, so completeness reduces to the same closure argument as the reachability search; the chain printed in the lint error is therefore itself a verified import path, not a heuristic guess). The headline is a meta-theorem — Domination + Strictness — proving the transitive check strictly dominates one-hop checking: every direct violation is caught, and there provably exist laundered violations that direct-edge checks miss. 30 Dafny VCs, 0 errors. The reachability decision and the witness are proven; the import-graph extraction (which edges exist) is the stated trust boundary, and dist/*.js is tsc's erasure of the verified source it ships alongside. Dafny only.
• infisical-lemmascript — brownfield, in-place verification of the permission-boundary glob set-containment check in Infisical — the privilege-escalation guard that stops a role from delegating broader secret-path access than it holds. The loop-bearing segmentMatch compiles to a Dafny method (unnameable in specs), so soundness is proven by refinement: the method certifies result == segMatchSpec (a pure mirror), and a standalone lemma proves that spec sound against a hand-written segment-glob semantics — if it returns true, every path the subset glob matches, the parent matches too. 12 Dafny VCs, 0 errors; drove a toolchain fix (recursive methods now carry a method-level //@ decreases). Dafny only.

Setup

Prerequisites: Node.js >= 18. For the Lean backend: elan. For the Dafny backend: Dafny >= 4.x.

Install from npm:

npm install -g lemmascript

Or from source:

git clone https://github.com/midspiral/LemmaScript.git
cd LemmaScript && npm install && npm run build

Lean backend additionally requires the Loom and Velvet forks:

git clone https://github.com/namin/loom.git -b lemma ../loom
git clone https://github.com/namin/velvet.git -b lemma ../velvet

Usage

Dafny backend

lsc gen --backend=dafny src/myModule.ts
lsc check --backend=dafny src/myModule.ts
lsc regen --backend=dafny src/myModule.ts

With no file argument, lsc check batches over LemmaScript-files.txt (one filepath [timeout] [extra dafny flags…] per line — the list tools/check.sh runs in CI).

From a sibling source checkout, the equivalent of lsc is npx tsx ../LemmaScript/tools/src/lsc.ts — no build step, toolchain edits apply immediately.

The Dafny backend generates two files per TS source: foo.dfy.gen (always regeneratable) and foo.dfy (source of truth, with LLM/user proof additions). The diff between them must be additions-only.

Lean backend

lsc gen --backend=lean src/myModule.ts
lake build

Continuous Integration

LemmaScript ships a reusable GitHub Actions workflow that regenerates your artifacts, verifies them, and fails the build if any committed generated file is out of date. Call it from your own repo's workflow:

# .github/workflows/lemmascript.yml
name: LemmaScript

on:
  push:
    branches: [main]
  pull_request:
    branches: [main]

jobs:
  verify:
    uses: midspiral/LemmaScript/.github/workflows/verify.yml@main
    with:
      backend: dafny      # dafny | lean | dafny-slow

The workflow installs the toolchain and (per backend) the Dafny or Lean stack, then runs tools/check.sh, which batches over a LemmaScript-files.txt at your repo root. This file is the list of sources CI verifies — you create and maintain it. One entry per line, filepath [timeout] [extra dafny flags…]:

src/domain.ts
src/patch.ts 120
src/heavy.ts 300 --isolate-assertions

The optional second column is a per-file timeout in seconds; anything after it is passed verbatim to Dafny. The same list drives lsc check locally when you run it with no file argument, so CI and your local runs verify exactly the same set. To verify additional Dafny files outside that list, add an executable check-extra.sh at the root and it runs automatically (Dafny backends only).

Inputs (all optional):

To verify against both backends, add a second job with backend: lean.

Examples:

• talktimer-lemmascript — Dafny-only.
• pi-lemmascript — Dafny + Lean (two jobs).

Annotations

//@ requires arr.length > 0
//@ ensures \result >= -1 && \result < arr.length
//@ invariant 0 <= i && i <= arr.length
//@ decreases arr.length - i
//@ type i nat

For the full surface, see SPEC.md.

File Structure

Dafny backend

Lean backend