DiffCI

Open-source test-impact analysis for faster CI. DiffCI finds tests affected by a code change and measures full versus selected runtime for teams evaluating CI savings.

npx "@diffci.com/diffci@latest" check

Requires Git and Node.js 22.5+. Runs tests locally and sends nothing by default; keep required CI authoritative.

Measured example: a controlled Cal.com sandbox replay measured 44.2% net reduction in an install + pretest + test workload, including analysis overhead. This is one job-equivalent comparison, not production savings or a forecast. Timings and limitations.

Star the open-source engine · Volunteer a pilot repository. Watch → Custom → Releases and Discussions on Core to follow support and benchmark updates.

The engine lives in DiffCI/core. This repository contains the CLI, integrations, website, and product surfaces.

Website · Test impact analysis guide · Open evidence study

The verification layer between AI-generated code and shipping. DiffCI snapshots an agent's staged, unstaged, and non-ignored untracked changes, determines the minimum executable test verification, and runs it. Uncertainty broadens to full verification; a failed, incomplete, or changed snapshot blocks. The check command remains the paired-runtime evaluation surface. observe and the Marketplace Action remain observation-only; DiffCI also generates a separate blocking verification workflow.

Try DiffCI

From a Git repository checkout, with Node.js 22.5+ and Git installed, run:

npx "@diffci.com/diffci@latest" verify --changed --json

verify --changed covers staged, unstaged, and non-ignored untracked content. It runs the selected command when DiffCI can prove one, otherwise the inferred full test command. Exit zero means that exact content-addressed snapshot passed and remained unchanged while tests ran. The JSON receipt includes verification, safe_to_continue, selection counts, the command result, and tree identities.

For paired full-versus-selected runtime measurement, use check. For analysis without test execution, use npx "@diffci.com/diffci@latest" observe --no-send.

To prepare a maintainer-reviewable pilot without changing the target repository or running its tests:

npx "@diffci.com/diffci@latest" pilot-packet --repo /path/to/checkout \
  --label owner/repository --repository-url https://github.com/owner/repository

The command writes pilot-packet.md, the raw observation, and a pinned non-blocking GitHub Actions workflow to ../diffci-output by default. The packet describes compatibility and planned selection; it does not present planned reduction as measured runtime savings.

To add instructions for coding agents, run:

npx "@diffci.com/diffci@latest" init

To also pin DiffCI as a development dependency and update the detected npm, pnpm, Yarn, or Bun lockfile, pass --install. Add --workflow for a non-blocking observation job and --verification-workflow for a blocking commit-range gate:

npx "@diffci.com/diffci@latest" init --install --workflow --verification-workflow

--install also adds diffci:verify, diffci:check, and diffci:observe package scripts. It preserves same-named scripts that the project already owns. After installation, CI or contributors can run npm run diffci:verify, npm run diffci:check, or npm run diffci:observe without knowing the package name or version. Generated agent rules use the detected package manager's local script when DiffCI is installed.

In CI, diffci verify --json resolves the pull-request or push range automatically. It requires a clean checkout, refuses when the analyzed head differs from executed HEAD, and binds the receipt to both the resolved range and checkout tree. Use --base and --head for an explicit range.

On Windows PowerShell, quote the package name:

npx "@diffci.com/diffci@latest" verify --changed --json

The copyable adoption kit includes an AGENTS.md instruction and maintainer PR text. AI-readable documentation is on Context7 CLI and Context7 Core. Use the GitHub Marketplace Action for a separate, non-blocking observation job. Required project CI remains authoritative.

One paired run is preliminary evidence. verify-savings can repeat comparisons, alternate arm order, record declared cache state, and run an explicit cache-preparation command before every arm. A result is labelled controlled only after at least three alternating, cache-prepared repetitions. On a full-validation fallback, check runs the full command once and reports 0% reduction.

Upgrade from 0.1.3: tests excluded by a source-only tsconfig.json could be discovered without their dependency edges, producing an incomplete selection. This is fixed in 0.1.4. Revalidate affected observations before using them as opportunity evidence; see the historical validation and release qualification.

The local default compares HEAD with its first parent; both commits must be available. For a specific comparison, add --base <base-sha> --head <head-sha>. DiffCI prints the selection, fallback reasons, and the path to a JSON report outside your checkout. REFUSED or ERROR is not a successful analysis; check the reported status even when the command exits successfully. See the support matrix for setup requirements and supported workloads. check runs inferred full and selected commands in the checkout and sends nothing by default. The commands may write generated files. Use observe --no-send for analysis without execution. See docs/ai-agents.md for Claude Code, Codex, Cursor, GitHub Copilot, and similar tools.

Measured example: a controlled Cal.com replay showed 44.2% net reduction in a job-equivalent install + pretest + test workload, including analysis overhead. This is one sandbox comparison, not Cal.com's production savings or a prediction for your repository. Read the timings and method.

Selection counts alone do not establish runtime savings. check reports a measured percentage only when both commands pass and the checked-out commit and worktree remain identical across both arms. The savings artifact embeds the base/head SHAs, observation SHA-256, commands, timings, and checkout snapshots. Snapshots use byte-level fingerprints for dirty files, manifests, lockfiles, and available resolved-dependency markers; the report also records the runner identity. A mismatch invalidates the comparison. observe does not execute tests.

For an advanced paired runtime check, you can still run observe first and then run verify-savings against the observation report. It compares your normal full command with DiffCI's proposed selected command and writes JSON plus Markdown evidence; see docs/npm-adoption.md.

For stronger evidence, run repeated measurements with an explicit cache preparation command:

npx "@diffci.com/diffci@latest" verify-savings \
  --repo . --full "npm test" --selected-from-report ../diffci-observation.json \
  --out ../diffci-savings.json --repetitions 3 --cache-state warm \
  --cache-prepare "node scripts/prepare-ci-cache.mjs"

DiffCI alternates full/selected order, reports min/median/max timings, and conservatively classifies stable selection misses, likely flakes, shared/pre-existing failures, infrastructure failures, and inconclusive results. Cache preparation is user-supplied because DiffCI must not delete repository or tool caches on its own.

Validate same-turn change specifications

When an agent or build tool supplies explicit specification IDs and logical targets, DiffCI can block two specifications from silently changing the same target:

{
  "specifications": [
    { "id": "SPEC-API", "logicalTarget": "com.example.Widget#run" },
    { "id": "SPEC-BEHAVIOR", "logicalTarget": "com.example.Widget#run" }
  ]
}

npx "@diffci.com/diffci@latest" validate-specs --file change-specifications.json

The command exits non-zero and names every conflicting specification ID. This surface is opt-in and does not infer semantic targets from source code; callers must provide stable logical target names.

Block in GitHub Actions

Generate a dedicated verification check with:

npx "@diffci.com/diffci@latest" init --verification-workflow

The generated .github/workflows/diffci-verify.yml checks out the exact pull-request head with full history, runs diffci verify --json, and uploads the commit-bound receipt even when verification fails. It has no continue-on-error: configure its DiffCI verification job as a required check after a successful pilot. A zero exit means the selected verification (or conservative full fallback) passed for the exact analyzed commit and checkout tree.

Observe in GitHub Actions

Save this as .github/workflows/diffci.yml to add a dedicated, non-blocking observation job:

name: DiffCI observation
on: [push, pull_request]
permissions:
  contents: read
jobs:
  diffci:
    runs-on: ubuntu-24.04
    continue-on-error: true
    steps:
      - uses: actions/checkout@3d3c42e5aac5ba805825da76410c181273ba90b1 # immutable checkout pin
        with:
          fetch-depth: 0
      - uses: DiffCI/DiffCI.com@b35f55c1b6ba9c092f3ad623efd8baa0422c450a

Then check the workflow locally with npx "@diffci.com/diffci@latest" verify-workflow. Keep the observer out of required checks and other jobs' needs lists. The Action adds a job summary and a diffci-observation artifact to the run; it does not alter which tests your other jobs execute. The example pins release v0.3.10 to its qualified feature commit SHA for reproducibility.

The CLI sends no report with --no-send. The Action uploads a GitHub artifact by default; sending to DiffCI's hosted service requires an explicitly configured endpoint and token. Installation details · Seven-day pilot

Use DiffCI

The CLI bundles a pinned revision of the Core engine from GitHub. Users install only @diffci.com/diffci; the check command above uses it directly. Core performs Git analysis, dependency graphs, impact, path baseline, and selected-command planning. The report format and non-interfering GitHub Action remain in this repository. See docs/package-relationship.md for the source relationship. For evaluation results and their limits, start with docs/adoption-evidence.md.

Agent Adoption

Add DiffCI instructions to a repository:

npx "@diffci.com/diffci@latest" init

Then ask your coding agent to run:

npx "@diffci.com/diffci@latest" verify --changed --json

Agent-specific docs: Codex · Claude Code · Cursor · GitHub Copilot · Grok.

For installable Codex and Claude Code plugins, see the DiffCI agent-plugin guide. Other MCP-compatible clients can use the local or hosted configurations in the MCP guide.

Live discovery files: llms.txt · AI agents.

Adoption materials: outreach copy · metrics · targets.

For native agent integrations, DiffCI provides a stateless, read-only HTTPS MCP endpoint for validation guidance:

https://diffci.com/mcp

Use the stdio MCP server when tools need to inspect the local checkout or run tests:

npx -p "@diffci.com/diffci@latest" diffci-mcp

See the MCP server page for Codex, Claude Code, Cursor, and VS Code setup, or docs/mcp.md for repository documentation.

Project background

Public Core and private Cloud

DiffCI's public-good analysis engine is released separately from its commercial hosted product. The project lives in the DiffCI GitHub organization. This repository was transferred to DiffCI/DiffCI.com on 2026-09-16.

This mixed repository is public. The reviewed Core extraction was published separately, with fresh Git history. This repository retains its existing engine snapshot while package integration is migrated separately. No blanket AGPL license applies to this repository. Public Core is advisory-only; energy/carbon/cost are modeled estimates, not verified environmental savings. The exact extraction is recorded in the Core release audit.

The funded Core should run independently of DiffCI Cloud. AGPL permits commercial use and competing hosting; it adds source-sharing obligations for covered modifications, including qualifying remote network use. Grant eligibility depends on each grant's agreement and funded deliverables.

See the licensing boundaries, organization and migration plan, and prepared organization profile.

This repository moved out of the DentalPresence.in monorepo (previously diffci/ there) into its own repo on 2026-08-21, once the project outgrew being a subfolder. DentalPresence.in remains DiffCI's original dogfooding target - some code (the planner DentalPresence-specific PATH baseline/task registry, a few fixture tests) still reflects that origin - but the research and shadow-validation pipelines are generic and have been exercised against dozens of real third-party repositories.

Current state

Language expansion: initial Vue SFC, Go package-level, and conventional Maven reactor analysis is implemented through repository adapters. See the support matrix and setup requirements for exact scope, fallback behavior, and validation boundaries.

Three completed research stages plus an in-progress prospective-validation stage, in order:

• Stage 0 - a 2,000-delta historical benchmark across 20 real repositories, run through a real Cloudflare orchestrator. Verdict: GO WITH CONDITIONS.
• Stage 1A - forensic root-cause investigation of every repository/delta where Stage 0's confidence model degraded to UNSAFE, and of every historical "unsafe miss" candidate. Identified the top 3 highest-leverage fixes.
• Stage 1B - implemented those 3 fixes (reachability-aware confidence narrowing, an improved historical safety-measurement methodology, a tsconfig-scope + package.json-diffing fix), validated them live against real repositories (coverage improved, zero contradicted safety cases), and ran a real wall-clock FULL/PATH/DiffCI runtime pilot.
• Stage 2 (current) - prospective shadow validation on real, currently-arriving CI events, not more historical benchmarking. A live pipeline (Cloudflare Sandbox Containers + Worker, D1 + R2) observes real repositories, predicts before their outcome is known, and later reconciles against the real CI result. Current verdict: EXTEND SHADOW VALIDATION - the pipeline is real and defect-free, and since 2026-08-21 it runs autonomously: a Cron Trigger polls enrolled repositories every 10 minutes (src/research/cloudflare/shadow-cron.ts), and the registered DiffCI Shadow GitHub App (read-only; see docs/github-app-registration.md) delivers push/workflow events to /v1/shadow/webhook for instant predictions and exactly-on-time reconciliation - this repository shadow-observes itself through that App. See docs/research/2026-08-21-stage2-final-report.md for the full picture; what's honestly still missing is real observation volume, working GitHub Actions on our own repositories (account billing), and real design-partner repositories.

Every dated report behind these stages lives in docs/research/ - start with 2026-08-21-stage2-architecture.md for the fullest current picture of what's built vs not, or the Stage 0/1A/1B reports for the historical-validation story.

For the next product milestone, see docs/alpha-readiness.md. It tracks the private-alpha bar: install DiffCI, keep CI unchanged, collect real shadow observations, and render a trustworthy potential-savings report.

Architecture

DiffCI is now framed as an open-core product:

DiffCI
|
├── Open-source core
|   ├── DiffCI engine
|   ├── CLI / npm package
|   ├── Local analysis
|   └── Basic GitHub Action
|       |
|       └── Tidelift package support
|
└── Commercial DiffCI
    ├── Hosted service / DiffCI Cloud
    ├── Organization dashboard
    ├── Historical analytics
    ├── Advanced CI/CD optimization
    ├── Enterprise policies
    ├── Managed runners
    ├── Team features
    └── Support / enterprise services

The open-source core is the trust and adoption surface. It runs locally or in the host repository's own CI, writes a report, and changes nothing about CI execution. Commercial DiffCI adds hosted history, organization views, policy, managed operations, runners, and support. Tidelift belongs to the supported open-source package path, not the hosted product feature boundary. See docs/open-core-packaging.md and docs/tidelift-package-support.md.

The source tree follows that split:

• src/git/, src/repo/, src/planner/, and src/client/ are the installable OSS observer path.
• action.yml wraps the observer as a basic non-blocking GitHub Action.
• src/research/ and src/shadow/ run validation, GitHub App shadow observation, and reconciliation.
• src/product/, src/auth/, src/billing/, src/ingest/, src/ledger/, src/runner/, and src/usage/ are the commercial/control-plane layer.
• docs/oss-boundary.md records what is allowed into the npm package.

Commands

# Type-check and run the full test suite
npm run check

# Generate an example delta / impact / plan for the current repo's latest commit
npm run diffci
npm run impact

# Run a real Stage 0-style historical benchmark locally
npm run research:stage0

# Deploy the Cloudflare research/shadow Worker (D1 + R2 + Sandbox Containers)
npm run research:sandbox:deploy

Install Surfaces

DiffCI is intended to be installable as infrastructure, not only as a hosted shadow experiment:

- uses: DiffCI/DiffCI.com@b35f55c1b6ba9c092f3ad623efd8baa0422c450a

npx "@diffci.com/diffci@latest" observe
npx "@diffci.com/diffci@latest" check
npx "@diffci.com/diffci@latest" init
npx "@diffci.com/diffci@latest" verify-workflow

The GitHub Action and npm CLI establish the OSS/package distribution path. The hosted GitHub App and DiffCI Cloud build on that trust boundary for teams that want shared reports and history. See docs/distribution.md for the package and Action positioning, docs/open-core-packaging.md for the commercial split, and docs/npm-adoption.md for copy-paste pilot material.

Useful? Star DiffCI/core, watch Releases and Discussions, or volunteer a pilot repository.