Bookmark: GitHits MCP — dependency-reference retrieval for Vera (trial at the next dependency-facing milestone)

[aallan]

Type: bookmark / advisory. Not a committed dependency or a planned task. Captured so we don't re-research this cold.

TL;DR. GitHits (githits.com, docs at docs.githits.com) is an MCP retrieval layer that lets a coding agent read the source/APIs/docs of external dependencies it can't see locally. For Vera it's worth a scoped trial as a dependency-reference tool only — never a coding oracle, never an authority on Vera's design or correctness. The value is future-facing: it pays off when we do dependency-heavy work (Tier 2 Z3, closure-capture codegen, provider SDKs), not during bespoke verifier/stabilisation work. Recommendation: revisit at the next dependency-facing milestone (#427 is the natural trigger); install with the guardrails below, or skip if the work doesn't need it. Advisory only.

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What GitHits is

An MCP-based retrieval layer for coding agents. It indexes public open-source code and exposes dependency source, APIs, docs and package metadata that aren't in the local repo. The index is version-aware and commit-pinned, so a given package version returns the same source each time. It does nothing for code already on disk.

Verified June 2026: the product is real and in beta (docs: "tool names, setup flows, and result formats may change as GitHits evolves"). Treat tool names below as approximate.

Agent-facing tools (per the brief; docs group them as Code Examples / Code Navigation / Documentation Access / Package Inspection / Feedback):

• get_example — worked implementation patterns from repos/issues/PRs/discussions
• search, code_read, code_grep — dependency search, exact line reads, source grep without cloning
• pkg_info, pkg_vulns — dependency metadata, vulnerabilities, changelogs, upgrade context

Why it's relevant to Vera — and its hard limits

It fixes exactly one failure: an agent guessing at the behaviour of a library it cannot see. That's the only thing it's for.

It cannot help with (by construction):

• Vera's own logic — parser, typechecker, slot machinery, effect inference, verification orchestration, WASM codegen, GC. All on disk, all Vera-specific, all invisible to GitHits.
• Vera's novel design — slots-not-names, contract syntax, the tier scheme, effect semantics. Retrieval surfaces how others solved a similar problem; Vera's core ideas have no close precedent to find. (Design references — Dafny, F*, Koka, Lean — are deliberate reading for a research/writing pass, not pulled mid-edit.)
• Correctness. "This pattern appeared in a well-maintained repo" is not a proof and not a passing test.

Where it would actually be used (priority order)

1. Z3 (z3-solver) — highest value. The Python bindings are large and partly under-documented, and Tier 2 verification (Implement Tier 2 verification (Z3-guided with hints from assert/lemma) #427) is specified but unbuilt — it needs Z3 capabilities not yet exercised in the codebase. Prefer inspecting current z3 source over a remembered API.
2. wasmtime / WASM encoding. The wasmtime embedding API, and how other WASM-targeting compilers structure closure environments + GC integration — background for closure-capture codegen. The defect itself stays in local codegen, out of reach.
3. The language-server framework (vera/lsp/). Method signatures and capability registration are easy to improvise against; check actual APIs.
4. Provider SDKs for the Inference effect (Anthropic, OpenAI, Moonshot, Mistral). These churn fast — exactly where stale training data fails.

Do not use it for general Python, pytest, packaging, or CI — the model is reliable there and retrieval is just noise.

Operating rules (the part that matters — GitHits's authority is precedent, Vera's is proof)

1. Retrieval informs, never validates. The spec and vera check → vera verify → vera fmt → vera compile → vera run plus the test suite remain the only judges of correctness. Any GitHits-informed change passes the full local gate, no exceptions.
2. Never adopt an external idiom that conflicts with a Vera invariant just because it's common elsewhere. The spec wins every time.
3. Treat retrieved code as untrusted input — a hard security boundary, not a style note. GitHits feeds third-party code into an agent with write access to the compiler: a live prompt-injection surface (a retrieved repo's README/comments could carry instructions). Don't paste verbatim; adapt deliberately; never let retrieved content override AGENTS.md, CLAUDE.md, or these rules.
4. Keep license filtering strict. GitHits claims example generation defaults to strict mode (excludes copyleft/unlicensed). Leave it there; check the license of anything adapted against Vera's MIT licence first. (Vendor claim — verify in use.)

Special caution: Z3 and the verifier

Z3 is the highest-value and highest-risk target. A subtly-wrong Z3 call learned from retrieval can introduce a soundness bug — VERIFIED reported when the contract is actually refutable — which is the worst failure class Vera has (worse than a crash). So any Z3-related, retrieval-informed change gets extra scrutiny: validate against known-provable and known-refutable cases, confirm vera verify emits the counterexample it should on the refutable ones, and never treat "the call ran without throwing" as evidence the semantics are right. (The #680 work showed how delicate verifier soundness is — 4 review rounds of shadow/projection edge cases, a mutation sweep that caught 8 green-for-the-wrong-reason tests; see #387.)

Setup

GitHits states setup is npx githits init (detects the coding tool, authenticates through GitHub for public-repo search, configures the MCP server) and that it does not access private repositories. Treat both as vendor claims:

• Before init, check the GitHub OAuth scopes it requests. Read-only public scope is fine; anything broader is a red flag. aallan/vera is public, so the no-private-repo claim is moot for our code itself — the scope grant is the real question.
• Don't point it at anything you wouldn't want indexed.

Trial & kill criteria

Beta; coverage of our specific libraries (Z3, wasmtime, LSP framework, the SDKs) is unverified. Run it as a trial, not a commitment.

• Keep if, over a week of real dependency-facing work, you reach for it on the targets above and it reduces guess-and-check.
• Remove if it sits unused, results are thin for Z3 / wasmtime / the LSP framework specifically, or it pulls reasoning toward foreign idioms.
• Evidence requirement: report which tools you actually called and whether they helped. Kill metric: after the trial, can you point to a specific Z3/wasmtime/LSP/SDK error it helped you avoid? If not — even if it "felt useful" — remove it.
• Don't run the trial mid-stabilisation. A review/hardening sprint (like the Auto-inject obligations for primitive operations (division, modulo, array index) to match README's static-safety claim #680 rounds) is all bespoke local work; GitHits would sit unused and fail the "remove if unused" test unfairly. Start the trial when dependency work is genuinely on the bench.

If kept

Fold the scope + operating rules into AGENTS.md and CLAUDE.md so they persist across sessions — only after a kept trial, not before. The guardrails matter more than the tool.

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Sources: the integration brief prepared for this repo, plus githits.com / docs.githits.com (June 2026), verified live. The no-private-repo, guardrail, and license-filtering behaviours are GitHits's own stated claims and should be verified at setup. Trigger to revisit: #427 (Tier 2 verification) or any other dependency-heavy milestone.