sequent-verify 0.2.0

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Neural Formal Verification Engine — GNN proposes, Z3 disposes.

Sequent is a neurosymbolic code verifier for Python and JavaScript/TypeScript that proves your code correct — or finds the exact counterexample that breaks it.

How it works

Python Source → AST Parser → GATv2 (10M params) → Bug Predictions
                          → Z3 SMT Solver       → Formal Proofs
                          → Consensus Vote       → Verdict + Counterexample
                          → Auto-Repair          → Re-Verify

1. GNN proposes: A GATv2 graph neural network reads your Python AST and predicts which nodes are likely bugs
2. Z3 disposes: The Z3 SMT solver formally verifies 8 property classes against your code
3. Consensus: Neural predictions are merged with formal proofs — both must agree
4. Auto-repair: If a bug is found, Sequent generates a fix and re-verifies it through Z3

Install

pip install sequent-verify

Usage

# Verify a file
sequent check main.py

# Verify a specific function
sequent check main.py -f binary_search

# Auto-repair detected bugs
sequent check main.py --repair -v

# Export proof certificate
sequent check main.py --cert report.json

# JSON output for CI
sequent check main.py --json

# Verify JavaScript/TypeScript
sequent check app.js
sequent check utils.ts -f calculateTotal

# Watch mode (re-verify on save)
sequent watch src/

# Generate verification badge
sequent badge main.py -o badge.svg

# Start LSP server (for Neovim, Emacs, Helix, etc.)
sequent lsp
sequent lsp --tcp --port 2087

Example

$ sequent check test_buggy.py

  binary_search  ✗ BUG DETECTED  (42ms)
  ────────────────────────────────────────────────────────
  GNN  Buggy (87.3%)  12ms
       ⚑ Suspect lines: [4]
  Z3   ✗ counterexample  28ms
       ✗ loop_invariant: Loop uses '<' instead of '<=': misses case when low == high
         ↳ counterexample: {"low": "n", "high": "n"}
  ────────────────────────────────────────────────────────
  Consensus: BUG CONFIRMED: GNN detected bug and Z3 produced a formal counterexample.

  safe_divide  ✓ VERIFIED  (18ms)
  ────────────────────────────────────────────────────────
  Consensus: VERIFIED: Both GNN and Z3 agree — no bugs detected.

Supported bug classes

Bug Type	Description
off_by_one	Loop bounds off by 1 (< vs <=)
boundary_error	Array/index out of bounds
wrong_operator	Incorrect comparison/arithmetic operators
none_deref	Missing null/None checks
integer_overflow	Unchecked arithmetic overflow
missing_return	Missing return in code path
wrong_init	Incorrect variable initialization

Benchmark

Sequent vs pylint vs pyflakes on 20 hand-crafted cases (14 buggy, 6 clean). Static analyzers focus on style and syntax — they cannot reason about semantics, so they miss every logic bug.

Bug Category	Case	Bug	Sequent	pylint	pyflakes
Off-by-one	binary_search_obo	< vs <= in loop	Detected	Missed	Missed
Off-by-one	bubble_sort_obo	Index out of bounds	Detected	Missed	Missed
None deref	find_max_none	No None check	Detected	Missed	Missed
None deref	reverse_string_none	No None check	Detected	Missed	Missed
None deref	sum_list_none	No None check	Missed	Missed	Missed
Div-by-zero	average_no_guard	Empty list division	Detected	Missed	Missed
Div-by-zero	normalize_no_guard	Zero divisor	Detected	Missed	Missed
Wrong operator	is_even_wrong_op	== 1 vs == 0	Detected	Missed	Missed
Wrong operator	min_of_two_wrong	Returns max	Detected	Missed	Missed
Unsafe arith	factorial_no_guard	Negative n silent	Detected	Missed	Missed
Boundary	second_largest_no_check	No length check	Detected	Missed	Missed
Boundary	pop_empty	No empty check	Detected	Missed	Missed
Mutation	remove_dupes_mutate	Mutate while iterating	Detected	Missed	Missed
Logic	swap_wrong	Overwrite before save	Missed	Missed	Missed
Clean	binary_search_correct	—	FP	OK	OK
Clean	find_max_correct	—	FP	OK	OK
Clean	safe_divide_correct	—	OK	OK	OK
Clean	fibonacci_correct	—	OK	OK	OK
Clean	is_palindrome_correct	—	FP	OK	OK
Clean	gcd_correct	—	FP	OK	OK

Summary (20 cases)

Tool	Correct	Bugs found (of 14)	False positives (of 6)	Accuracy
Sequent	14/20	12/14 (85.7%)	4/6	70.0%
pylint	6/20	0/14 (0%)	0/6	30.0%
pyflakes	6/20	0/14 (0%)	0/6	30.0%

Sequent catches semantic bugs (off-by-one, division by zero, wrong operators, missing guards) that pylint and pyflakes are structurally blind to. The tradeoff is a higher false-positive rate on clean code — Sequent's Z3 verifier is conservative and flags potential edge cases even in correct implementations. Average Sequent latency: 230ms per function.

Z3 property checks

• Comparison consistency (off-by-one in loops)
• Array index bounds (upper and lower)
• None/null safety
• Return completeness
• Division by zero / arithmetic safety
• Loop termination and invariants
• Dead code detection

Architecture

• Model: GATv2 with 8 attention heads, 256 hidden channels, 3 layers (~10M params)
• Graph: Code Property Graph (CPG) = AST + CFG + data flow
• Features: 91 AST node types + structural features (depth, subtree size, loop/conditional context)
• Edges: Parent↔child (AST) + control flow (CFG) + data flow (def → use)
• Training: Focal loss + NT-Xent contrastive loss with Z3 verification outcomes as supervision
• Dataset: 14,144 synthetic mutations from 164 seed functions, seed-level train/test split
• Metrics: 74.0% accuracy, 83.0% precision, 73.6% recall, 78.0% F1

Web playground

# Start the API server
pip install sequent[server]
python -m backend.server

# Start the frontend (development)
cd frontend && npm install && npm run dev

Git Hook

Sequent can verify staged Python files automatically before every commit.

Manual install (copies hook into .git/hooks/):

bash hooks/install.sh              # install
bash hooks/install.sh --uninstall  # remove

pre-commit framework (pre-commit.com):

Add to your .pre-commit-config.yaml:

repos:
  - repo: https://github.com/devangpratapsingh/sequent
    rev: main
    hooks:
      - id: sequent-verify

The hook skips files over 10 KB for speed. To bypass on a single commit, use git commit --no-verify.

LSP Server

Sequent ships an LSP server for editor-agnostic verification. Works with any LSP client.

Neovim (via lspconfig):

vim.lsp.start({
  name = "sequent",
  cmd = { "sequent-lsp" },
  filetypes = { "python", "javascript", "typescript" },
})

Helix (~/.config/helix/languages.toml):

[[language]]
name = "python"
language-servers = ["sequent-lsp"]

[language-server.sequent-lsp]
command = "sequent-lsp"

Emacs (via lsp-mode):

(lsp-register-client
  (make-lsp-client :new-connection (lsp-stdio-connection "sequent-lsp")
                   :major-modes '(python-mode js-mode typescript-mode)
                   :server-id 'sequent))

GitHub Action

- uses: actions/setup-python@v5
  with:
    python-version: '3.11'
- run: pip install sequent
- run: sequent check src/ --ci --cert sequent-report.json

License

MIT