feat: add nix flake support (sorry for this duplicate)

.gitignore

@@ -148,3 +148,4 @@ CLAUDE.md
 
 # Pnpm
 .pnpm-store/
+result

README.md

@@ -135,6 +135,8 @@ These tools automatically read workflow instructions from `openspec/AGENTS.md`.
 
 #### Step 1: Install the CLI globally
 
+**Option A: Using npm**
+
 ```bash
 npm install -g @fission-ai/openspec@latest
 ```
@@ -144,6 +146,39 @@ Verify installation:
 openspec --version
 ```
 
+**Option B: Using Nix (NixOS and Nix package manager)**
+
+Run OpenSpec directly without installation:
+```bash
+nix run github:Fission-AI/OpenSpec -- init
+```
+
+Or install to your profile:
+```bash
+nix profile install github:Fission-AI/OpenSpec
+```
+
+Or add to your development environment in `flake.nix`:
+```nix
+{
+  inputs = {
+    nixpkgs.url = "github:NixOS/nixpkgs/nixos-unstable";
+    openspec.url = "github:Fission-AI/OpenSpec";
+  };
+
+  outputs = { nixpkgs, openspec, ... }: {
+    devShells.x86_64-linux.default = nixpkgs.legacyPackages.x86_64-linux.mkShell {
+      buildInputs = [ openspec.packages.x86_64-linux.default ];
+    };
+  };
+}
+```
+
+Verify installation:
+```bash
+openspec --version
+```
+
 #### Step 2: Initialize OpenSpec in your project
 
 Navigate to your project directory:

flake.nix

@@ -0,0 +1,87 @@
+{
+  description = "OpenSpec - AI-native system for spec-driven development";
+
+  inputs = {
+    nixpkgs.url = "github:NixOS/nixpkgs/nixos-unstable";
+  };
+
+  outputs = { self, nixpkgs }:
+    let
+      supportedSystems = [ "x86_64-linux" "aarch64-linux" "x86_64-darwin" "aarch64-darwin" ];
+
+      forAllSystems = f: nixpkgs.lib.genAttrs supportedSystems (system: f system);
+    in
+    {
+      packages = forAllSystems (system:
+        let
+          pkgs = nixpkgs.legacyPackages.${system};
+        in
+        {
+          default = pkgs.stdenv.mkDerivation (finalAttrs: {
+            pname = "openspec";
+            version = "0.18.0";
+
+            src = ./.;
+
+            pnpmDeps = pkgs.fetchPnpmDeps {
+              inherit (finalAttrs) pname version src;
+              pnpm = pkgs.pnpm_9;
+              fetcherVersion = 3;
+              hash = "sha256-ltG+wmr2aiRmghEMGOZVYifvrY8qdLDfizLq6a8Ds0s=";
+            };
+
+            nativeBuildInputs = with pkgs; [
+              nodejs_20
+              npmHooks.npmInstallHook
+              pnpmConfigHook
+              pnpm_9
+            ];
+
+            buildPhase = ''
+              runHook preBuild
+
+              pnpm run build
+
+              runHook postBuild
+            '';
+
+            dontNpmPrune = true;
+
+            meta = with pkgs.lib; {
+              description = "AI-native system for spec-driven development";
+              homepage = "https://github.com/Fission-AI/OpenSpec";
+              license = licenses.mit;
+              maintainers = [ ];
+              mainProgram = "openspec";
+            };
+          });
+        });
+
+      apps = forAllSystems (system: {
+        default = {
+          type = "app";
+          program = "${self.packages.${system}.default}/bin/openspec";
+        };
+      });
+
+      devShells = forAllSystems (system:
+        let
+          pkgs = nixpkgs.legacyPackages.${system};
+        in
+        {
+          default = pkgs.mkShell {
+            buildInputs = with pkgs; [
+              nodejs_20
+              pnpm_9
+            ];
+
+            shellHook = ''
+              echo "OpenSpec development environment"
+              echo "Node version: $(node --version)"
+              echo "pnpm version: $(pnpm --version)"
+              echo "Run 'pnpm install' to install dependencies"
+            '';
+          };
+        });
+    };
+}

openspec/changes/archive/2026-01-07-add-nix-flake-support/.openspec.yaml

@@ -0,0 +1,2 @@
+schema: spec-driven
+created: 2026-01-07

openspec/changes/archive/2026-01-07-add-nix-flake-support/design.md

@@ -0,0 +1,94 @@
+## Context
+
+OpenSpec is a TypeScript CLI tool using pnpm for dependency management. The project requires Node.js ≥20.19.0. Nix uses its own build system that needs to understand how to fetch dependencies and build the project reproducibly.
+
+The Nix ecosystem has specific patterns for packaging Node.js/pnpm projects that differ from the traditional npm ecosystem.
+
+## Goals
+
+- Enable OpenSpec to be run directly via `nix run github:Fission-AI/OpenSpec`
+- Support all major platforms (Linux x86/ARM, macOS x86/ARM)
+- Use existing pnpm-lock.yaml for reproducible builds
+- Provide development environment for Nix users
+
+## Non-Goals
+
+- Replace existing npm/pnpm publishing workflow
+- Publish to nixpkgs (can be done later as separate effort)
+- Support Windows (Nix doesn't run natively on Windows)
+
+## Decisions
+
+### Use stdenv.mkDerivation instead of buildNpmPackage
+
+**Decision**: Package OpenSpec using `stdenv.mkDerivation` with pnpm hooks.
+
+**Rationale**: The zigbee2mqtt package in nixpkgs demonstrates the current best practice for pnpm projects. Using `buildNpmPackage` with pnpm requires complex configuration, while `mkDerivation` with the right hooks is more straightforward and better supported.
+
+**Alternative considered**: Using `buildNpmPackage` with `npmConfigHook = pkgs.pnpmConfigHook` - this is the older pattern and causes issues with dependency fetching.
+
+### Use fetchPnpmDeps with explicit pnpm version
+
+**Decision**: Use `pkgs.fetchPnpmDeps` with `pnpm = pkgs.pnpm_9` and `fetcherVersion = 3`.
+
+**Rationale**:
+- pnpm lockfile version 9.0 requires fetcherVersion 3
+- Explicit pnpm_9 ensures consistency between fetch and build
+- This is the documented way to handle pnpm projects in nixpkgs
+
+### Multi-platform support without flake-utils
+
+**Decision**: Implement multi-platform support using plain Nix with `nixpkgs.lib.genAttrs`.
+
+**Rationale**: Per user request, avoid extra dependencies. The `genAttrs` pattern is simple and well-understood in the Nix community.
+
+### Node.js 20 instead of latest
+
+**Decision**: Pin to nodejs_20 to match package.json engines requirement.
+
+**Rationale**: Ensures consistency with development environment and npm package requirements. Avoids potential compatibility issues with newer Node versions.
+
+## Key Implementation Details
+
+### Dependency Hash Management
+
+The `pnpmDeps.hash` field must be updated whenever dependencies change. The workflow:
+1. Set hash to fake value (all zeros)
+2. Run `nix build`
+3. Nix fails with actual hash
+4. Update flake.nix with correct hash
+
+This is standard Nix workflow for fixed-output derivations.
+
+### Build Inputs
+
+Required nativeBuildInputs:
+- `nodejs_20` - runtime
+- `npmHooks.npmInstallHook` - handles installation phase
+- `pnpmConfigHook` - configures pnpm environment
+- `pnpm_9` - pnpm executable
+
+The `dontNpmPrune = true` is important to keep all dependencies after build.
+
+## Risks / Trade-offs
+
+**[Risk]** Hash needs updating when dependencies change → **Mitigation**: Document this clearly; error message from Nix provides correct hash
+
+**[Risk]** Nix builds might lag behind npm releases → **Mitigation**: This is fine; Nix users can still use npm if they need bleeding edge
+
+**[Trade-off]** Additional maintenance burden for hash updates → **Benefit**: Better experience for Nix ecosystem users
+
+## Migration Plan
+
+1. Add flake.nix to repository
+2. Test builds on multiple platforms (can use GitHub Actions with Nix)
+3. Update README with Nix installation instructions
+4. Optionally add to CI pipeline to catch hash mismatches early
+
+No breaking changes - this is purely additive.
+
+## Open Questions
+
+- Should we add automatic hash updating to CI? (Could use nix-update-script)
+- Should we submit to nixpkgs after validation? (Separate decision)
+- Do we want to support older Node versions in flake? (Probably no - stick to package.json requirement)

openspec/changes/archive/2026-01-07-add-nix-flake-support/proposal.md

@@ -0,0 +1,25 @@
+## Why
+
+OpenSpec users on NixOS or using the Nix package manager cannot easily install or run OpenSpec without going through npm. Adding a Nix flake makes OpenSpec a first-class citizen in the Nix ecosystem, enabling users to run `nix run github:Fission-AI/OpenSpec -- init` or include OpenSpec in their development environments declaratively.
+
+## What Changes
+
+- Add `flake.nix` to repository root with multi-platform support (x86_64-linux, aarch64-linux, x86_64-darwin, aarch64-darwin)
+- Package uses pnpm for dependency management (matching existing development workflow)
+- Support both direct execution via `nix run` and installation via `nix profile install`
+- Provide dev shell for contributors using Nix
+
+## Capabilities
+
+### New Capabilities
+- `nix-flake-support`: Nix flake configuration for building and running OpenSpec
+
+### Modified Capabilities
+- None
+
+## Impact
+
+- **New files**: `flake.nix` in repository root
+- **Documentation**: Should add installation instructions for Nix users
+- **CI/CD**: Could add flake checking to CI pipeline (optional)
+- **Maintenance**: Requires updating pnpmDeps hash when dependencies change

openspec/changes/archive/2026-01-07-add-nix-flake-support/specs/nix-flake-support/spec.md

@@ -0,0 +1,79 @@
+## ADDED Requirements
+
+### Requirement: Multi-platform Nix flake
+The system SHALL provide a Nix flake that builds OpenSpec for multiple platforms.
+
+#### Scenario: Build on Linux x86_64
+- **WHEN** user runs `nix build` on x86_64-linux system
+- **THEN** system builds OpenSpec package successfully
+- **AND** package includes the `openspec` binary
+
+#### Scenario: Build on macOS ARM
+- **WHEN** user runs `nix build` on aarch64-darwin system
+- **THEN** system builds OpenSpec package successfully
+- **AND** package includes the `openspec` binary
+
+#### Scenario: Build on Linux ARM
+- **WHEN** user runs `nix build` on aarch64-linux system
+- **THEN** system builds OpenSpec package successfully
+
+#### Scenario: Build on macOS x86_64
+- **WHEN** user runs `nix build` on x86_64-darwin system
+- **THEN** system builds OpenSpec package successfully
+
+### Requirement: Direct execution via nix run
+The system SHALL allow users to run OpenSpec directly from GitHub without installing.
+
+#### Scenario: Run init command from GitHub
+- **WHEN** user runs `nix run github:Fission-AI/OpenSpec -- init`
+- **THEN** system downloads and builds OpenSpec
+- **AND** executes `openspec init` command
+
+#### Scenario: Run any OpenSpec command
+- **WHEN** user runs `nix run github:Fission-AI/OpenSpec -- <command> <args>`
+- **THEN** system executes `openspec <command> <args>`
+
+### Requirement: pnpm dependency management
+The system SHALL use pnpm for building OpenSpec in the Nix flake.
+
+#### Scenario: Fetch dependencies with pnpm
+- **WHEN** Nix builds the package
+- **THEN** system uses `fetchPnpmDeps` to download dependencies
+- **AND** uses pnpm-lock.yaml for reproducible builds
+- **AND** uses fetcherVersion 3 for lockfile version 9.0
+
+#### Scenario: Build with pnpm
+- **WHEN** Nix runs the build phase
+- **THEN** system executes `pnpm run build`
+- **AND** produces dist directory with compiled TypeScript
+
+### Requirement: Node.js version compatibility
+The system SHALL use Node.js 20 as specified in package.json engines field.
+
+#### Scenario: Build with correct Node version
+- **WHEN** Nix builds OpenSpec
+- **THEN** system uses nodejs_20 from nixpkgs
+- **AND** build succeeds without version compatibility errors
+
+### Requirement: Development shell
+The system SHALL provide a Nix development shell for contributors.
+
+#### Scenario: Enter dev shell
+- **WHEN** user runs `nix develop` in OpenSpec repository
+- **THEN** system provides shell with nodejs_20 and pnpm_9
+- **AND** displays welcome message with versions
+- **AND** provides instructions to run `pnpm install`
+
+### Requirement: Proper binary installation
+The system SHALL install the openspec binary correctly.
+
+#### Scenario: Binary in PATH
+- **WHEN** package is built or installed
+- **THEN** `openspec` binary is available in `$out/bin/openspec`
+- **AND** binary is executable
+- **AND** binary can be invoked without full path when installed
+
+#### Scenario: Binary executes correctly
+- **WHEN** user runs the installed `openspec` command
+- **THEN** system executes the CLI entry point
+- **AND** all subcommands work correctly