Spec-first agentic development · spec-first.dev

What changes when agents work from specs This is how Agent Smith is built every day, across more than a thousand specs. VerifiedA fresh reviewer checks the spec against the code before the build. A gate runs your checks before the commit. ParallelOne branch and worktree per spec. Ids come from the clock, so two agents never take the same one. UninterruptedA reviewed spec has already answered the open questions, so the agent builds to its done criteria instead of coming back to ask. StructuredOne spec, one branch, one commit. A pull request carries one spec or a whole series, and every change has the same shape. ResumableState lives in context.yaml. Restart the machine or the agent and the work picks up where it stopped. Explained, as a side effectEach choice is logged with what it beat and why, so the reasoning stays next to the code. A whole series, side by side Cut a bigger change into a series of specs. /spec-first:deliver-series reviews all of them, builds the groups that share no file at the same time, verifies the merged tree and opens one pull request. deliver-series 2026-10-27-8a3f5 specs · 3 groupsbuilding atickets-provider-contractrequires nothing1 btickets-github-providerrequires a1 ctickets-jira-providerrequires a1 dsettings-provider-pagerequires nothing2 eci-restore-cacherequires nothing3 3 worktrees building in parallel · every spec commit through the gatePR #42 An example run of /spec-first:deliver-series. Specs that share no file are built at the same time; specs that depend on each other share a group. One spec, from idea to pull request Four skills take a piece of work through its own branch and worktree. Each one leaves a file behind for the next step and for whoever reads the history later. create-specGoal, steps, facts with file and line, what done means. No code yet.specs/planned/{id}.yaml review-specEvery citation resolved, then a reviewer who never saw the chat attacks it.BUILD · CUTS · REFUSE apply-specContracts, code, wiring, tests, following the project's own rules.decisions/{id}.yaml deliver-specOne commit through the gate, pushed, pull request opened.feat: … ({id}) The recordgrows with every spec, read before every step specs/Every spec, from planned to done, with its facts and done criteria. decisions/What each spec chose, what it chose over, and why. memory/What you told the agent once, so you don't say it twice. /spec-first:bootstrap-project writes the contexts once from your code: stack, rules and the checks a commit must pass. After that, each spec adds one line of state. A spec is reviewed before anyone builds it A REFUSE costs you one spec. Without the review, the same mistake would have cost you the branch. Spec2026-10-27-8a3f Applies toserver Branchspec/8a3f GoalTicket providers so pipelines can pull work from GitHub and Jira. Facts, read in the codePipelines take work only from the CLI todayProgram.cs:40-58 Config already carries a tracker sectionAppConfig.cs:12-19 DecisionAdapter over repository: read-mostly data, no unit of work needed. Done whenGitHub provider returns tickets end to endJira provider behind the same interfaceverify: stages green REVIEWED · BUILDfresh reviewer · 27 OCT review-spec · 2026-10-27-8a3f2 cuts ✓ src/Host/Program.cs:40-58 resolves ✓ src/Config/AppConfig.cs:12-19 resolves evidence resolves: 1 spec BUILD WITH CUTSthe spec holds once two steps change CUT 1Step jira needs a second auth mode the config can't hold yet. AppConfig.cs:15 Move it to its own spec. CUT 2Paging is missing: GitHub returns 30 issues per page. GitHubClient.cs:88 Add a done criterion. Every citation is checkedA script opens each file and line the spec cites. A claim nobody checked in the code has to be listed as an assumption. The reviewer starts freshA new agent with no conversation history reads the code as it is, not as the spec says it is. One verdict, with cuts namedBUILD, BUILD WITH CUTS or REFUSE, and every finding names a file and a line. The reasoning comes along for free Nobody has to write documentation afterwards. While it builds, the agent logs each choice that wasn't obvious: what it chose, what it chose over, and why. One file per spec, next to the code. decisions/2026-10-27-8a3f.yaml spec: 2026-10-27-8a3f decisions: - category: TradeOff chose: "Raw HttpClient for the GitHub API" over: "Octokit" reason: "Three megabytes of dependencies for four calls." - category: Architecture chose: "One ITicketProvider per tracker" over: "A generic REST mapper" reason: "Each tracker pages and authenticates differently; the mapper would leak it." No wiki to keep alivecat decisions/*.yaml is the project's reasoning, spec by spec, in the same history as the code it explains. Memory for what git can't tellHow the operator wants things done, a constraint outside the repo, a pointer elsewhere. One fact per file, one line in an index. Back in thirty secondsRestart the machine, restart the agent, open context.yaml. It knows what is done, what is active and what comes next. Nothing lands past a red stage The plugin's commit gate runs every verify stage your contexts declare when a commit names its spec. All green, it commits. The first red stage blocks it, and its output goes back to the agent. the commit git commit -m "feat: ticket providers (2026-10-27-8a3f)" The id in parentheses triggers it. -F <file>, -C and --amend --no-edit are gated too. verify: stages ✓builddotnet build ✓testsdotnet test ✕webcd web && npm test PASSEDCommittedevery stage green BLOCKEDNot committedthe first red stage stops it, and its output goes back to the agent # spec-first-gate.log · one line per spec commit 2026-10-27T09:14:02Z blocked 2026-10-27-8a3f /work/repo a1b2c3d web 2026-10-27T09:31:47Z passed 2026-10-27-8a3f /work/repo a1b2c3d build,tests,web A spec commit with no line in the ledger never met the gate. A project with its own gate says gate: project, and the plugin steps aside. Built this way Agent Smith The open source agentic software factory: a ticket goes in, a verified pull request comes out. Agent Smith is entirely built with this method, and the method grew out of building it. Everything in the plugin ran there first. Agent Smith runs the loop for you → 1,300+specs done 1,190+decision files: chose, over, why 650+specs through the gate Counted in the Agent Smith repository, October 2026. Install In Claude Code the plugin does the bookkeeping and runs the gate. Other agents follow the same files through AGENTS.md, and a git hook runs the same gate.

/plugin marketplace add holgerleichsenring/specification-first-agentic-development /plugin install spec-first@specification-first-agentic-development Then run /spec-first:bootstrap-project.

It reads your code and writes the contexts and rules from what it finds. create-specWrites the spec on its own branch and worktree review-specEvidence check, then a fresh reviewer's verdict apply-specBuilds the spec up to its done criteria deliver-specCommits through the gate and opens the pull request deliver-seriesTakes a whole series of specs to one pull request log-decision · update-project · bootstrap-projectDecisions, upgrades, setup Questions about spec-first agentic development Specification-driven agentic development, in short answers.

What is spec-first agentic development?

A way of working with AI coding agents in which every change starts as a written spec in your repository. A fresh reviewer checks the spec against your code before anything is built, the agent builds it on its own branch and worktree, and the commit lands only when your checks pass.

What does specification-driven agentic development mean here?

The spec drives the work from start to finish. It names the goal, the facts it rests on with file and line, the steps and the done criteria, and the agent builds to exactly that. Spec-first agentic development is specification-driven agentic development in which the spec is reviewed against the code before the first line is written.

Do I need Claude Code?

No.

The Claude Code plugin runs the steps as commands. Codex, Cursor, Copilot or a terminal follow the same files through AGENTS.md, and a git commit-msg hook runs the same commit gate in every clone where it is installed.

How does it differ from vibe coding?

You still describe the feature in your own words and talk it through. After that, the agent works from a reviewed spec instead of from the last prompt, logs each decision with its reason, and a commit lands only when your build and tests pass.

What does a spec contain?

A one-sentence goal, the facts it rests on with file and line, the steps, what an upgrade costs an operator, and the done criteria. The decisions made while building go into a separate YAML file next to it. What does it cost?

Nothing.

The method, the templates and the Claude Code plugin are MIT licensed.

Where it comes from