#GitHub Spec Kit vs. phax

A side-by-side comparison, last checked on 2026-10-04 against Spec Kit v1.1.0 (2026-10-02) — its docs site (github.github.io/spec-kit), README and CHANGELOG (github/spec-kit) — and phax 0.17.0.

#The one-line difference

Spec Kit is a methodology + prompt/template layer, with a workflow engine on top, that runs your AI agent as the worker for each step. phax is an orchestration engine that runs your AI agent inside isolated, mechanically gated phases. Same spec → plan → implement philosophy; very different enforcement.

#What Spec Kit is, concretely

You run specify init, which installs commands or skills for whichever agent you use (41 named integrations plus a generic one: Copilot, Claude Code, Codex, Gemini, Cursor, Mistral Vibe, …). Then, inside your agent, you drive:

/speckit.constitution → /speckit.specify → /speckit.clarify → /speckit.plan
→ /speckit.checklist → /speckit.tasks → /speckit.analyze → /speckit.implement
→ /speckit.converge

(spelled /speckit-specify or $speckit-specify on agents that take skills). Only specify → plan is required; clarify, checklist and analyze are optional AI "quality gates", and converge is an append-only self-check — the agent assesses the code against spec, plan and tasks and either reports "Converged" or appends missing tasks, so you loop implement → converge. Each step produces a Markdown artifact in your repo (constitution, spec, plan, tasks, checklists).

Since 1.0 (2026-08-21) Spec Kit is also a platform. The site now pitches it as "Build with a spec, fix a bug, or assess an idea — with your coding agent":

• Extensions add commands and before/after hooks on each core command (bundled opt-in bug and assess flows; a community catalog of ~180 extensions the maintainers do not review). Presets override templates; bundles version a stack of both.
• Workflows (specify workflow run) are YAML pipelines of command, prompt, shell, gate, if/switch, loop and fan-out/fan-in steps, pausable and resumable. The built-in speckit workflow runs specify → review-spec gate → plan → review-plan gate → tasks → implement, where a gate is a human approve/reject.
• Git is opt-in. Core no longer creates a feature branch; the git extension (specify extension add git) does, and can auto-commit around commands. Worktree isolation exists only as community extensions.

The runtime is still your agent, working in your normal tree with its native permissions. The docs are explicit that there is "no capability sandbox": a workflow shell step "runs a local command with your privileges".

#What phax is, concretely

A compiled CLI (Node launcher + Deno-compiled binary, Effect / TypeScript) that drives an AI coding agent (Claude Code by default; Codex and Mistral Vibe once enabled) through isolated, gated phases. You write a spec and a plan.md — by hand with the bundled skills, or headlessly with phax artifact new spec|plan --headless --brief — and approve them. phax run refuses a plan that is not Approved or has gone Stale, extracts it to a validated phax-plan.json (cached; headless-authored plans are already structured), then executes each phase in its own git worktree, runs a mechanical gate profile after each phase with an automatic same-session fix loop, reconciles the planned files against the actual diff, and stops at review_open with a review handoff, an optional compliance review and an optional GitHub PR. With records turned on, every phase leaves its manifest, gate results, reconciliation, diff, handoff and transcript on a phax/records/v1 branch. phax is the runtime; it spawns the agent headlessly.

#Side-by-side

#The key distinction

The workflows rhyme — both are spec → plan → tasks/phases → implement with review gates, and Spec Kit's workflow engine now makes it an orchestrator too. The difference is still what a gate means:

• Spec Kit gate = "a human reads the artifact and approves," plus AI self-checks (analyze, checklist, converge). A workflow can add a shell step that runs your linter or tests, but nothing in the default workflow does, and it runs in your tree with your privileges.
• phax gate = "the phase's code typechecks and tests pass, or the phase fails and the agent loops to fix it." Enforcement is a real process exit code after every phase, in an isolated, sandboxed worktree, with deterministic reconciliation of plan-vs-actual.

So Spec Kit is closer to a shared SDD discipline + prompt library + process platform you layer onto any agent; phax is closer to a build system / CI harness for agent work. Spec Kit standardizes the conversation, artifacts and process; phax standardizes the execution and verification.

#When each fits

• Spec Kit if you want an agent-agnostic methodology that meets you where you already work, across many assistants, with humans in the review loop, a large extension ecosystem, and the option to script the process as a workflow.
• phax if you want mechanical gates, worktree isolation, security sandboxing, provider routing, and a deterministic, auditable trajectory (reconciliation + records + compliance report + PR) — i.e., you trade agent-breadth and ecosystem for enforcement and safety.

#Two things worth noting

1. They are composable, and phax already mirrors Spec Kit's front half. phax's phax-spec and phax-planning skills (or artifact new --headless) are direct analogs of /speckit.specify and /speckit.plan. You could author Spec-Kit-style specs and execute them through phax's gated harness — spec discipline on the front, mechanical execution on the back.
2. Relative to the "review-by-trajectory" desktop idea (see docs/ideas/desktop-app.md, not on the roadmap): Spec Kit lives at "structure the prompts, artifacts and process"; phax lives at "make execution isolated and gates mechanical." The idea's "approve the trajectory, not the diff" is a step beyond both — it presumes exactly the mechanical gates, reconciliation and records phax has (and Spec Kit does not by default), then makes them the primary review surface. You cannot review-by-trajectory credibly when your gates are human approve/reject steps and AI self-assessments; you need real green-or-red evidence, which is precisely what phax produces.