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The problem

An agent that is confidently wrong does not fix itself by looking again. The model wrote the change, so it agrees with the change. Reading the diff a second time produces the same answer with more words.

The failures that actually cost turns are also the ones that are mechanically decidable: a file that no longer parses, a shell construct the harness shell cannot run, a sorry that makes a proof vacuous, a type error three files away. None of those need judgement. They need a checker.

What it does

One tool, four actions, cheapest first.

Action	What it runs	What a pass means
status	Probes every checker with --version	What can be checked on this machine, and what cannot
lean	lean --json with warnings promoted to errors	The Lean kernel accepted the proof
batch	py_compile, node --check, tsc --noEmit, the PowerShell parser	The language's own compiler accepted the files
commands	Static rules over shell text, never executed	No known harness-shell hazard matched

The Lean tier is the strict one

A Lean check is worth having only because it can say no. Two things make that true here, and both are the opposite of the obvious implementation:

A sorry must fail the check. lean file.lean exits 0 on a file full of sorry, because an unfinished proof is a warning in Lean. A checker that ran Lean and read the exit code would report a vacuous theorem as verified. So the exact run is:

lean --json -DwarningAsError=true -DmaxErrors=0 -DautoImplicit=false -E hasSorry spec.lean

• -E hasSorry promotes only the missing-proof kind, so "no unfinished goals" is demanded without demanding "no warnings at all". The kind really is hasSorry, not sorry.
• -DmaxErrors=0 removes the default cap of 100, past which Lean stops reporting and exits — which would hide every error after the hundredth.
• -DautoImplicit=false closes a footgun that is on by default in Lean: with it enabled, a misspelled hypothesis name silently becomes a fresh universally quantified variable, and the theorem states something weaker than it reads.

The checker itself must be trustworthy. In 2026, eight soundness bugs were found in the Lean kernel and runtime; each was exploited to make the official kernel accept a proof of False, and they were fixed in 4.33.1. A proof accepted by an older toolchain is not evidence of anything, so the version is checked against that floor, is printed with every result, and an older toolchain turns a pass into unsafe-toolchain.

Everything is honest about not running

A verdict of unavailable means no checker ran and nothing was proved. It is never returned as a pass. If tsc is missing, the result says so with the path it looked for; if the resolved lean is the elan shim with no configured default toolchain, the result says that too instead of reporting a compiler error.

Install

The plugin is installed as a package into a dsh profile. dsh plugin forwards to pnpm inside the profile directory, so any spec pnpm accepts works.

# from GitHub (the published form)
dsh plugin --profile web add github:catsenior507/dsh-tool-strict-check

# a local checkout, while developing
dsh plugin --profile web add /absolute/path/to/dsh-tool-strict-check

web is the shipped GUI profile; substitute headless, sdk, acp, or your own profile name. On Windows, use forward slashes in a path.

Then restart the host and confirm with:

strict_check action=status

Lean is optional, and here is how to add it

The other three tiers work without Lean. status reports Lean as missing, and the lean action returns unavailable — never a pass.

# 1. elan, the Lean version manager
#    Windows (the installer writes to %USERPROFILE%\.elan):
Invoke-WebRequest https://elan.lean-lang.org/elan-init.ps1 -OutFile "$env:TEMP\elan-init.ps1"
& "$env:TEMP\elan-init.ps1" -NoPrompt 1 -DefaultToolchain none

# 2. the toolchain. 4.33.1 is the *minimum*, not a suggestion: see above.
& "$env:USERPROFILE\.elan\bin\elan.exe" toolchain install leanprover/lean4:v4.33.1

The plugin finds Lean without any configuration: it looks for a real compiler under <ELAN_HOME>/toolchains/*/bin first and only then falls back to PATH. It does this deliberately — the elan shim on PATH refuses to run until elan default has been executed once, and that refusal is easy to misread as "Lean is broken". To pin a specific toolchain, set leanPath in the plugin row.

Behind a proxy? elan downloads through curl, which reads ~/.curlrc. If the toolchain download stalls at 0 bytes while a browser works, write proxy = "http://127.0.0.1:<port>" into %USERPROFILE%\.curlrc and retry.

What install does not do

• No build step — the published JavaScript is the source; no prepare script runs, no bundler is needed.
• No dependencies — dependencies and peerDependencies are empty. Cordis is provided by the host at runtime.
• No mathlib — see below for what that costs.

Node.js 20 or newer.

What it does not do

Stated plainly, because a verification tool that oversells itself is worse than none:

• No mathlib. Core Init and Std ship with the toolchain, so import Std and by decide work with no project, no network, and no ~10 GB download. What you lose is the tactic library: no ring, norm_num, push_neg, field_simp. Substitutes are grind, decide, by_cases, simp, omega.
• No sandboxed re-check. lake check, comparator, and nanoda need Linux namespaces (bwrap) and cannot run on Windows at all. leanchecker re-checks what the kernel already checked and does not catch sorry, so wrapping it would be cost without coverage. Lean 4.35 is expected to ship lake check; that is the right moment to adopt it, not now.
• No statement-vs-intent check. The kernel verifies that the proof proves the theorem. Whether the theorem is the one you meant is a specification question, and this tool will not pretend otherwise — which is exactly why the commands action prints "a clean scan is weak evidence" rather than "OK".

Configure

dsh plugin add already inserted the plugin row. To change the defaults, edit that row's config in the profile's cordis.patch.yml:

- insert:
    - id: tool-strict-check
      name: '@dsh-external/dsh-tool-strict-check'
      config:
        leanPath: '<ELAN_HOME>/toolchains/<toolchain>/bin/lean.exe'
        projectDir: '<path to a Lake project, if your specs import one>'
        timeoutMs: 120000

leanPath wins over discovery, which is what pins one toolchain.

Development

npm test        # 61 tests; the Lean ones execute the real compiler

The Lean integration tests are skipped, not faked, when no safe toolchain exists. Parsing tests replay recorded lean --json output from Lean 4.33.1, so they fail if the wire format changes — including the detail that positions on the wire are 0-based and are converted to 1-based.

File	Role
lib/runner.js	Executable discovery, bounded process execution, spawn-failure handling
lib/parse.js	Checker output to normalized diagnostics
lib/static.js	Defects and policy violations decidable by reading
lib/language.js	Tier 0: the language's own compiler
lib/lean.js	The strict tier: flags, version floor, verdict
lib/tool.js	The strict_check tool
lib/policy-surface.js	The contract dsh-policy-strict-gate consumes

License

MIT