refactor: switch to jest-diff for better output and simpler code

- Replace custom diff implementation with jest-diff
- Reduce code from 229 to 178 lines (22% reduction)
- Get professional GitHub-style diff output
- Automatic word-level highlighting built-in
- Smaller bundle size (jest-diff: 85KB vs diff: 492KB)

openspec/README.md

@@ -10,6 +10,23 @@ OpenSpec is an AI-native system for change-driven development where:
 - **AI drives the process** - You generate proposals, humans review and approve
 - **Specs are living documentation** - Always kept in sync with deployed code
 
+## Start Simple
+
+**Default to minimal implementations:**
+- New features should be <100 lines of code initially
+- Use the simplest solution that works
+- Avoid premature optimization (no caching, parallelization, or complex patterns without proven need)
+- Choose boring technology over cutting-edge solutions
+
+**Complexity triggers** - Only add complexity when you have:
+- **Performance data** showing current solution is too slow
+- **Scale requirements** with specific numbers (>1000 users, >100MB data)
+- **Multiple use cases** requiring the same abstraction
+- **Regulatory compliance** mandating specific patterns
+- **Security threats** that simple solutions cannot address
+
+When triggered, document the specific justification in your change proposal.
+
 ## Directory Structure
 
 ```
@@ -72,6 +89,11 @@ Before any task:
 - Adding tests for existing behavior
 - Documentation fixes
 
+**Complexity assessment:**
+- If your solution requires >100 lines of new code, justify the complexity
+- If adding dependencies, frameworks, or architectural patterns, document why simpler alternatives won't work
+- Default to single-file implementations until proven insufficient
+
 ### 3. Creating a Change Proposal
 
 When a user requests a significant change:
@@ -383,10 +405,12 @@ Progress communication:
 - "Implementing approved changes..."
 
 ### For AI Assistants
+- **Bias toward simplicity** - Propose the minimal solution that works
 - Use your exploration tools liberally before proposing
 - Batch operations for efficiency
 - Communicate your progress
 - It's OK to revise proposals based on discoveries
+- **Question complexity** - If your solution feels complex, simplify first
 
 ## Edge Case Handling
 
@@ -442,6 +466,7 @@ Proposal REQUIRED if:
 - Specs must always reflect deployed reality
 - Changes are proposed, not imposed
 - Impact analysis prevents surprises
-- The simplicity is the power - just markdown files
+- Simplicity is the power - just markdown files, minimal solutions
+- Start simple, add complexity only when justified
 
 By following these conventions, you enable true spec-driven development where documentation stays current, changes are traceable, and evolution is intentional.

openspec/changes/add-complexity-guidelines/tasks.md

@@ -1,9 +0,0 @@
-# Implementation Tasks
-
-## 1. Update OpenSpec README
-- [ ] 1.1 Add "Start Simple" section after Core Principle
-- [ ] 1.2 Add complexity triggers to "When to Create Change Proposals" section
-- [ ] 1.3 Update AI workflow guidance to emphasize minimal implementations
-
-## 2. Update CLAUDE.md
-- [ ] 2.1 Add complexity management rules to project instructions

openspec/changes/add-diff-command/proposal.md

@@ -0,0 +1,19 @@
+# Add Diff Command to OpenSpec CLI
+
+## Why
+
+Developers need to easily view differences between proposed spec changes and current specs without manually comparing files.
+
+## What Changes
+
+- Add `openspec diff [change-name]` command that shows differences between change specs and current specs
+- Compare files in `changes/[change-name]/specs/` with corresponding files in `specs/`
+- Display unified diff output showing added/removed/modified lines
+- Support colored output for better readability
+
+## Impact
+
+- Affected specs: New capability `cli-diff` will be added
+- Affected code:
+  - `src/cli/index.ts` - Add diff command
+  - `src/core/diff.ts` - New file with diff logic (~80 lines)

openspec/changes/add-diff-command/specs/cli-diff/spec.md

@@ -0,0 +1,77 @@
+# CLI Diff Command Specification
+
+## Purpose
+
+The `openspec diff` command provides developers with a visual comparison between proposed spec changes and the current deployed specs.
+
+## Command Syntax
+
+```bash
+openspec diff [change-name]
+```
+
+## Behavior
+
+### Without Arguments
+
+WHEN running `openspec diff` without arguments
+THEN list all available changes in the `changes/` directory (excluding archive)
+AND prompt user to select a change
+
+### With Change Name
+
+WHEN running `openspec diff <change-name>`
+THEN compare all spec files in `changes/<change-name>/specs/` with corresponding files in `specs/`
+
+### Diff Output
+
+FOR each spec file in the change:
+- IF file exists in both locations THEN show unified diff
+- IF file only exists in change THEN show as new file (all lines with +)
+- IF file only exists in current specs THEN show as deleted (all lines with -)
+
+### Display Format
+
+The diff SHALL use standard unified diff format:
+- Lines prefixed with `-` for removed content
+- Lines prefixed with `+` for added content
+- Lines without prefix for unchanged context
+- File headers showing the paths being compared
+
+### Color Support
+
+WHEN terminal supports colors:
+- Removed lines displayed in red
+- Added lines displayed in green
+- File headers displayed in bold
+- Context lines in default color
+
+### Error Handling
+
+WHEN specified change doesn't exist THEN display error "Change '<name>' not found"
+WHEN no specs directory in change THEN display "No spec changes found for '<name>'"
+WHEN changes directory doesn't exist THEN display "No OpenSpec changes directory found"
+
+## Examples
+
+```bash
+# View diff for specific change
+$ openspec diff add-auth-feature
+
+--- specs/user-auth/spec.md
++++ changes/add-auth-feature/specs/user-auth/spec.md
+@@ -10,6 +10,8 @@
+ Users SHALL authenticate with email and password.
+ 
++Users MAY authenticate with OAuth providers.
++
+ WHEN credentials are valid THEN issue JWT token.
+
+# List all changes and select
+$ openspec diff
+Available changes:
+  1. add-auth-feature
+  2. update-payment-flow
+  3. add-status-command
+Select a change (1-3): 
+```

openspec/changes/add-diff-command/tasks.md

@@ -0,0 +1,23 @@
+# Implementation Tasks
+
+## 1. Core Implementation
+- [x] 1.1 Create `src/core/diff.ts` with diff logic
+- [x] 1.2 Implement change directory scanning
+- [x] 1.3 Implement file comparison using unified diff format
+- [x] 1.4 Add color support for terminal output
+
+## 2. CLI Integration
+- [x] 2.1 Add diff command to `src/cli/index.ts`
+- [x] 2.2 Implement interactive change selection when no argument provided
+- [x] 2.3 Add error handling for missing changes
+
+## 3. Enhancements
+- [x] 3.1 Replace with jest-diff for professional diff output
+- [x] 3.2 Improve file headers with status and statistics
+- [x] 3.3 Add summary view with file counts and line changes
+
+## 4. Testing
+- [ ] 4.1 Test diff generation for modified files
+- [ ] 4.2 Test handling of new files
+- [ ] 4.3 Test handling of deleted files
+- [ ] 4.4 Test interactive mode

openspec/changes/add-update-command/tasks.md

@@ -1,20 +0,0 @@
-# Implementation Tasks
-
-## 1. Update Command Implementation
-- [ ] 1.1 Create `src/core/update.ts` with `UpdateCommand` class
-- [ ] 1.2 Check if `openspec` directory exists (use `FileSystemUtils.directoryExists`)
-- [ ] 1.3 Write `readmeTemplate` to `openspec/README.md` using `FileSystemUtils.writeFile`
-- [ ] 1.4 Update `CLAUDE.md` using markers via `FileSystemUtils.updateFileWithMarkers` and `TemplateManager.getClaudeTemplate()`
-- [ ] 1.5 Display ASCII-safe success message: `Updated OpenSpec instructions`
-
-## 2. CLI Integration
-- [ ] 2.1 Register `update` command in `src/cli/index.ts`
-- [ ] 2.2 Add command description: `Update OpenSpec instruction files`
-- [ ] 2.3 Handle errors with `ora().fail(...)` and exit code 1 (missing `openspec` directory, file write errors)
-
-## 3. Testing
-- [ ] 3.1 Verify `openspec/README.md` is fully replaced with latest template
-- [ ] 3.2 Verify `CLAUDE.md` OpenSpec block updates without altering user content outside markers
-- [ ] 3.3 Verify idempotency (running twice yields identical files, no duplicate markers)
-- [ ] 3.4 Verify error when `openspec` directory is missing with friendly message
-- [ ] 3.5 Verify success message displays properly in ASCII-only terminals

openspec/changes/archive/2025-01-11-add-update-command/tasks.md

@@ -0,0 +1,20 @@
+# Implementation Tasks
+
+## 1. Update Command Implementation
+- [x] 1.1 Create `src/core/update.ts` with `UpdateCommand` class
+- [x] 1.2 Check if `openspec` directory exists (use `FileSystemUtils.directoryExists`)
+- [x] 1.3 Write `readmeTemplate` to `openspec/README.md` using `FileSystemUtils.writeFile`
+- [x] 1.4 Update `CLAUDE.md` using markers via `FileSystemUtils.updateFileWithMarkers` and `TemplateManager.getClaudeTemplate()`
+- [x] 1.5 Display ASCII-safe success message: `Updated OpenSpec instructions`
+
+## 2. CLI Integration
+- [x] 2.1 Register `update` command in `src/cli/index.ts`
+- [x] 2.2 Add command description: `Update OpenSpec instruction files`
+- [x] 2.3 Handle errors with `ora().fail(...)` and exit code 1 (missing `openspec` directory, file write errors)
+
+## 3. Testing
+- [x] 3.1 Verify `openspec/README.md` is fully replaced with latest template
+- [x] 3.2 Verify `CLAUDE.md` OpenSpec block updates without altering user content outside markers
+- [x] 3.3 Verify idempotency (running twice yields identical files, no duplicate markers)
+- [x] 3.4 Verify error when `openspec` directory is missing with friendly message
+- [x] 3.5 Verify success message displays properly in ASCII-only terminals

openspec/changes/archive/2025-08-11-add-complexity-guidelines/tasks.md

@@ -0,0 +1,9 @@
+# Implementation Tasks
+
+## 1. Update OpenSpec README
+- [x] 1.1 Add "Start Simple" section after Core Principle
+- [x] 1.2 Add complexity triggers to "When to Create Change Proposals" section
+- [x] 1.3 Update AI workflow guidance to emphasize minimal implementations
+
+## 2. Update CLAUDE.md
+- [x] 2.1 Add complexity management rules to project instructions

openspec/specs/cli-update/spec.md

@@ -0,0 +1,59 @@
+# Update Command Specification
+
+## Purpose
+
+As a developer using OpenSpec, I want to update the OpenSpec instructions in my project when new versions are released, so that I can benefit from improvements to AI agent instructions.
+
+## Core Requirements
+
+### Update Behavior
+
+The update command SHALL update OpenSpec instruction files to the latest templates.
+
+WHEN a user runs `openspec update` THEN the command SHALL:
+- Check if the `openspec` directory exists
+- Replace `openspec/README.md` with the latest template (complete replacement)
+- Update the OpenSpec-managed block in `CLAUDE.md` using markers
+  - Preserve user content outside markers
+  - Create `CLAUDE.md` if missing
+- Display ASCII-safe success message: "Updated OpenSpec instructions"
+
+### Prerequisites
+
+The command SHALL require:
+- An existing `openspec` directory (created by `openspec init`)
+
+IF the `openspec` directory does not exist THEN:
+- Display error: "No OpenSpec directory found. Run 'openspec init' first."
+- Exit with code 1
+
+### File Handling
+
+The update command SHALL:
+- Completely replace `openspec/README.md` with the latest template
+- Update only the OpenSpec-managed block in `CLAUDE.md` using markers
+- Use the default directory name `openspec`
+- Be idempotent (repeated runs have no additional effect)
+
+## Edge Cases
+
+### File Permissions
+IF file write fails THEN let the error bubble up naturally with file path.
+
+### Missing CLAUDE.md
+IF CLAUDE.md doesn't exist THEN create it with the template content.
+
+### Custom Directory Name
+Not supported in this change. The default directory name `openspec` SHALL be used.
+
+## Success Criteria
+
+Users SHALL be able to:
+- Update OpenSpec instructions with a single command
+- Get the latest AI agent instructions
+- See clear confirmation of the update
+
+The update process SHALL be:
+- Simple and fast (no version checking)
+- Predictable (same result every time)
+- Self-contained (no network required)

package.json

@@ -53,7 +53,9 @@
   },
   "dependencies": {
     "@inquirer/prompts": "^7.8.0",
+    "chalk": "^5.5.0",
     "commander": "^14.0.0",
+    "jest-diff": "^30.0.5",
     "ora": "^8.2.0"
   }
 }

pnpm-lock.yaml

@@ -11,9 +11,15 @@ importers:
       '@inquirer/prompts':
         specifier: ^7.8.0
         version: 7.8.0(@types/node@24.2.0)
+      chalk:
+        specifier: ^5.5.0
+        version: 5.5.0
       commander:
         specifier: ^14.0.0
         version: 14.0.0
+      jest-diff:
+        specifier: ^30.0.5
+        version: 30.0.5
       ora:
         specifier: ^8.2.0
         version: 8.2.0
@@ -310,6 +316,18 @@ packages:
       '@types/node':
         optional: true
 
+  '@jest/diff-sequences@30.0.1':
+    resolution: {integrity: sha512-n5H8QLDJ47QqbCNn5SuFjCRDrOLEZ0h8vAHCK5RL9Ls7Xa8AQLa/YxAc9UjFqoEDM48muwtBGjtMY5cr0PLDCw==}
+    engines: {node: ^18.14.0 || ^20.0.0 || ^22.0.0 || >=24.0.0}
+
+  '@jest/get-type@30.0.1':
+    resolution: {integrity: sha512-AyYdemXCptSRFirI5EPazNxyPwAL0jXt3zceFjaj8NFiKP9pOi0bfXonf6qkf82z2t3QWPeLCWWw4stPBzctLw==}
+    engines: {node: ^18.14.0 || ^20.0.0 || ^22.0.0 || >=24.0.0}
+
+  '@jest/schemas@30.0.5':
+    resolution: {integrity: sha512-DmdYgtezMkh3cpU8/1uyXakv3tJRcmcXxBOcO0tbaozPwpmh4YMsnWrQm9ZmZMfa5ocbxzbFk6O4bDPEc/iAnA==}
+    engines: {node: ^18.14.0 || ^20.0.0 || ^22.0.0 || >=24.0.0}
+
   '@jridgewell/sourcemap-codec@1.5.4':
     resolution: {integrity: sha512-VT2+G1VQs/9oz078bLrYbecdZKs912zQlkelYpuf+SXF+QvZDYJlbx/LSx+meSAwdDFnF8FVXW92AVjjkVmgFw==}
 
@@ -416,6 +434,9 @@ packages:
     cpu: [x64]
     os: [win32]
 
+  '@sinclair/typebox@0.34.38':
+    resolution: {integrity: sha512-HpkxMmc2XmZKhvaKIZZThlHmx1L0I/V1hWK1NubtlFnr6ZqdiOpV72TKudZUNQjZNsyDBay72qFEhEvb+bcwcA==}
+
   '@types/chai@5.2.2':
     resolution: {integrity: sha512-8kB30R7Hwqf40JPiKhVzodJs2Qc1ZJ5zuT3uzw5Hq/dhNCl3G3l83jfpdI1e20BP348+fV7VIL/+FxaXkqBmWg==}
 
@@ -478,6 +499,10 @@ packages:
     resolution: {integrity: sha512-zbB9rCJAT1rbjiVDb2hqKFHNYLxgtk8NURxZ3IZwD3F6NtxbXZQCnnSi1Lkx+IDohdPlFp222wVALIheZJQSEg==}
     engines: {node: '>=8'}
 
+  ansi-styles@5.2.0:
+    resolution: {integrity: sha512-Cxwpt2SfTzTtXcfOlzGEee8O+c+MmUgGrNiBcXnuWxuFJHe6a5Hz7qwhwe5OgaSYI0IJvkLqWX1ASG+cJOkEiA==}
+    engines: {node: '>=10'}
+
   assertion-error@2.0.1:
     resolution: {integrity: sha512-Izi8RQcffqCeNVgFigKli1ssklIbpHnCYc6AknXGYoB6grJqyeby7jv12JUQgmTAnIDnbck1uxksT4dzN3PWBA==}
     engines: {node: '>=12'}
@@ -490,6 +515,10 @@ packages:
     resolution: {integrity: sha512-5nFxhUrX0PqtyogoYOA8IPswy5sZFTOsBFl/9bNsmDLgsxYTzSZQJDPppDnZPTQbzSEm0hqGjWPzRemQCYbD6A==}
     engines: {node: '>=18'}
 
+  chalk@4.1.2:
+    resolution: {integrity: sha512-oKnbhFyRIXpUuez8iBMmyEa4nbj4IOQyuhc/wy9kY7/WVPcwIO9VA668Pu8RkO7+0G76SLROeyw9CpQ061i4mA==}
+    engines: {node: '>=10'}
+
   chalk@5.5.0:
     resolution: {integrity: sha512-1tm8DTaJhPBG3bIkVeZt1iZM9GfSX2lzOeDVZH9R9ffRHpmHvxZ/QhgQH/aDTkswQVt+YHdXAdS/In/30OjCbg==}
     engines: {node: ^12.17.0 || ^14.13 || >=16.0.0}
@@ -585,6 +614,10 @@ packages:
     resolution: {integrity: sha512-vpeMIQKxczTD/0s2CdEWHcb0eeJe6TFjxb+J5xgX7hScxqrGuyjmv4c1D4A/gelKfyox0gJJwIHF+fLjeaM8kQ==}
     engines: {node: '>=18'}
 
+  has-flag@4.0.0:
+    resolution: {integrity: sha512-EykJT/Q1KjTWctppgIAgfSO0tKVuZUjhgMr17kqTumMl6Afv3EISleU7qZUzoXDFTAHTDC4NOoG/ZxU3EvlMPQ==}
+    engines: {node: '>=8'}
+
   iconv-lite@0.4.24:
     resolution: {integrity: sha512-v3MXnZAcvnywkTUEZomIActle7RXXeedOR31wwl7VlyoXO4Qi9arvSenNQWne1TcRwhCL1HwLI21bEqdpj8/rA==}
     engines: {node: '>=0.10.0'}
@@ -605,6 +638,10 @@ packages:
     resolution: {integrity: sha512-mE00Gnza5EEB3Ds0HfMyllZzbBrmLOX3vfWoj9A9PEnTfratQ/BcaJOuMhnkhjXvb2+FkY3VuHqtAGpTPmglFQ==}
     engines: {node: '>=18'}
 
+  jest-diff@30.0.5:
+    resolution: {integrity: sha512-1UIqE9PoEKaHcIKvq2vbibrCog4Y8G0zmOxgQUVEiTqwR5hJVMCoDsN1vFvI5JvwD37hjueZ1C4l2FyGnfpE0A==}
+    engines: {node: ^18.14.0 || ^20.0.0 || ^22.0.0 || >=24.0.0}
+
   js-tokens@9.0.1:
     resolution: {integrity: sha512-mxa9E9ITFOt0ban3j6L5MpjwegGz6lBQmM1IJkWeBZGcMxto50+eWdjC/52xDbS2vy0k7vIMK0Fe2wfL9OQSpQ==}
 
@@ -668,6 +705,13 @@ packages:
     resolution: {integrity: sha512-3Ybi1tAuwAP9s0r1UQ2J4n5Y0G05bJkpUIO0/bI9MhwmD70S5aTWbXGBwxHrelT+XM1k6dM0pk+SwNkpTRN7Pg==}
     engines: {node: ^10 || ^12 || >=14}
 
+  pretty-format@30.0.5:
+    resolution: {integrity: sha512-D1tKtYvByrBkFLe2wHJl2bwMJIiT8rW+XA+TiataH79/FszLQMrpGEvzUVkzPau7OCO0Qnrhpe87PqtOAIB8Yw==}
+    engines: {node: ^18.14.0 || ^20.0.0 || ^22.0.0 || >=24.0.0}
+
+  react-is@18.3.1:
+    resolution: {integrity: sha512-/LLMVyas0ljjAtoYiPqYiL8VWXzUUdThrmU5+n20DZv+a+ClRoevUzw5JxU+Ieh5/c87ytoTBV9G1FiKfNJdmg==}
+
   restore-cursor@5.1.0:
     resolution: {integrity: sha512-oMA2dcrw6u0YfxJQXm342bFKX/E4sG9rbTzO9ptUcR/e8A33cHuvStiYOwH7fszkZlZ1z/ta9AAoPk2F4qIOHA==}
     engines: {node: '>=18'}
@@ -724,6 +768,10 @@ packages:
   strip-literal@3.0.0:
     resolution: {integrity: sha512-TcccoMhJOM3OebGhSBEmp3UZ2SfDMZUEBdRA/9ynfLi8yYajyWX3JiXArcJt4Umh4vISpspkQIY8ZZoCqjbviA==}
 
+  supports-color@7.2.0:
+    resolution: {integrity: sha512-qpCAvRl9stuOHveKsn7HncJRvv501qIacKzQlO/+Lwxc9+0q2wLyv4Dfvt80/DPn2pqOBsJdDiogXGR9+OvwRw==}
+    engines: {node: '>=8'}
+
   tinybench@2.9.0:
     resolution: {integrity: sha512-0+DUvqWMValLmha6lr4kD8iAMK1HzV0/aKnCtWb9v9641TnP/MFb7Pc2bxoxQjTXAErryXVgUOfv2YqNllqGeg==}
 
@@ -1048,6 +1096,14 @@ snapshots:
     optionalDependencies:
       '@types/node': 24.2.0
 
+  '@jest/diff-sequences@30.0.1': {}
+
+  '@jest/get-type@30.0.1': {}
+
+  '@jest/schemas@30.0.5':
+    dependencies:
+      '@sinclair/typebox': 0.34.38
+
   '@jridgewell/sourcemap-codec@1.5.4': {}
 
   '@polka/url@1.0.0-next.29': {}
@@ -1112,6 +1168,8 @@ snapshots:
   '@rollup/rollup-win32-x64-msvc@4.46.2':
     optional: true
 
+  '@sinclair/typebox@0.34.38': {}
+
   '@types/chai@5.2.2':
     dependencies:
       '@types/deep-eql': 4.0.2
@@ -1189,6 +1247,8 @@ snapshots:
     dependencies:
       color-convert: 2.0.1
 
+  ansi-styles@5.2.0: {}
+
   assertion-error@2.0.1: {}
 
   cac@6.7.14: {}
@@ -1201,6 +1261,11 @@ snapshots:
       loupe: 3.2.0
       pathval: 2.0.1
 
+  chalk@4.1.2:
+    dependencies:
+      ansi-styles: 4.3.0
+      supports-color: 7.2.0
+
   chalk@5.5.0: {}
 
   chardet@0.7.0: {}
@@ -1289,6 +1354,8 @@ snapshots:
 
   get-east-asian-width@1.3.0: {}
 
+  has-flag@4.0.0: {}
+
   iconv-lite@0.4.24:
     dependencies:
       safer-buffer: 2.1.2
@@ -1301,6 +1368,13 @@ snapshots:
 
   is-unicode-supported@2.1.0: {}
 
+  jest-diff@30.0.5:
+    dependencies:
+      '@jest/diff-sequences': 30.0.1
+      '@jest/get-type': 30.0.1
+      chalk: 4.1.2
+      pretty-format: 30.0.5
+
   js-tokens@9.0.1: {}
 
   log-symbols@6.0.0:
@@ -1356,6 +1430,14 @@ snapshots:
       picocolors: 1.1.1
       source-map-js: 1.2.1
 
+  pretty-format@30.0.5:
+    dependencies:
+      '@jest/schemas': 30.0.5
+      ansi-styles: 5.2.0
+      react-is: 18.3.1
+
+  react-is@18.3.1: {}
+
   restore-cursor@5.1.0:
     dependencies:
       onetime: 7.0.0
@@ -1431,6 +1513,10 @@ snapshots:
     dependencies:
       js-tokens: 9.0.1
 
+  supports-color@7.2.0:
+    dependencies:
+      has-flag: 4.0.0
+
   tinybench@2.9.0: {}
 
   tinyexec@0.3.2: {}

src/cli/index.ts

@@ -3,6 +3,8 @@ import ora from 'ora';
 import path from 'path';
 import { promises as fs } from 'fs';
 import { InitCommand } from '../core/init.js';
+import { UpdateCommand } from '../core/update.js';
+import { DiffCommand } from '../core/diff.js';
 
 const program = new Command();
 
@@ -44,4 +46,33 @@ program
     }
   });
 
+program
+  .command('update [path]')
+  .description('Update OpenSpec instruction files')
+  .action(async (targetPath = '.') => {
+    try {
+      const resolvedPath = path.resolve(targetPath);
+      const updateCommand = new UpdateCommand();
+      await updateCommand.execute(resolvedPath);
+    } catch (error) {
+      console.log(); // Empty line for spacing
+      ora().fail(`Error: ${(error as Error).message}`);
+      process.exit(1);
+    }
+  });
+
+program
+  .command('diff [change-name]')
+  .description('Show differences between proposed spec changes and current specs')
+  .action(async (changeName?: string) => {
+    try {
+      const diffCommand = new DiffCommand();
+      await diffCommand.execute(changeName);
+    } catch (error) {
+      console.log(); // Empty line for spacing
+      ora().fail(`Error: ${(error as Error).message}`);
+      process.exit(1);
+    }
+  });
+
 program.parse();

src/core/diff.ts

@@ -0,0 +1,179 @@
+import { promises as fs } from 'fs';
+import path from 'path';
+import chalk from 'chalk';
+import { diffStringsUnified } from 'jest-diff';
+import { select } from '@inquirer/prompts';
+
+// Constants
+const ARCHIVE_DIR = 'archive';
+const MARKDOWN_EXT = '.md';
+const OPENSPEC_DIR = 'openspec';
+const CHANGES_DIR = 'changes';
+const SPECS_DIR = 'specs';
+
+export class DiffCommand {
+  private filesChanged: number = 0;
+  private linesAdded: number = 0;
+  private linesRemoved: number = 0;
+
+  async execute(changeName?: string): Promise<void> {
+    const changesDir = path.join(process.cwd(), OPENSPEC_DIR, CHANGES_DIR);
+    
+    try {
+      await fs.access(changesDir);
+    } catch {
+      throw new Error('No OpenSpec changes directory found');
+    }
+
+    if (!changeName) {
+      changeName = await this.selectChange(changesDir);
+      if (!changeName) return;
+    }
+
+    const changeDir = path.join(changesDir, changeName);
+    
+    try {
+      await fs.access(changeDir);
+    } catch {
+      throw new Error(`Change '${changeName}' not found`);
+    }
+
+    const changeSpecsDir = path.join(changeDir, SPECS_DIR);
+    
+    try {
+      await fs.access(changeSpecsDir);
+    } catch {
+      console.log(`No spec changes found for '${changeName}'`);
+      return;
+    }
+
+    // Reset counters
+    this.filesChanged = 0;
+    this.linesAdded = 0;
+    this.linesRemoved = 0;
+
+    await this.showDiffs(changeSpecsDir);
+
+    // Show summary
+    if (this.filesChanged > 0) {
+      console.log(chalk.bold(`\n📊 Summary: ${this.filesChanged} file(s) changed, ${chalk.green(`+${this.linesAdded}`)} ${chalk.red(`-${this.linesRemoved}`)}`));
+    }
+  }
+
+  private async selectChange(changesDir: string): Promise<string | undefined> {
+    const entries = await fs.readdir(changesDir, { withFileTypes: true });
+    const changes = entries
+      .filter(entry => entry.isDirectory() && entry.name !== ARCHIVE_DIR)
+      .map(entry => entry.name);
+
+    if (changes.length === 0) {
+      console.log('No changes found');
+      return undefined;
+    }
+
+    console.log('Available changes:');
+    const choices = changes.map((name) => ({
+      name: name,
+      value: name
+    }));
+
+    const answer = await select({
+      message: 'Select a change',
+      choices
+    });
+
+    return answer;
+  }
+
+  private async showDiffs(changeSpecsDir: string): Promise<void> {
+    const currentSpecsDir = path.join(process.cwd(), OPENSPEC_DIR, SPECS_DIR);
+    await this.walkAndDiff(changeSpecsDir, currentSpecsDir, '');
+  }
+
+  private async walkAndDiff(changeDir: string, currentDir: string, relativePath: string): Promise<void> {
+    const entries = await fs.readdir(path.join(changeDir, relativePath), { withFileTypes: true });
+    
+    for (const entry of entries) {
+      const entryPath = path.join(relativePath, entry.name);
+      
+      if (entry.isDirectory()) {
+        await this.walkAndDiff(changeDir, currentDir, entryPath);
+      } else if (entry.isFile() && entry.name.endsWith(MARKDOWN_EXT)) {
+        await this.diffFile(
+          path.join(changeDir, entryPath),
+          path.join(currentDir, entryPath),
+          entryPath
+        );
+      }
+    }
+  }
+
+  private async diffFile(changePath: string, currentPath: string, displayPath: string): Promise<void> {
+    let changeContent = '';
+    let currentContent = '';
+    let isNewFile = false;
+    let isDeleted = false;
+    
+    try {
+      changeContent = await fs.readFile(changePath, 'utf-8');
+    } catch {
+      changeContent = '';
+    }
+    
+    try {
+      currentContent = await fs.readFile(currentPath, 'utf-8');
+    } catch {
+      currentContent = '';
+      isNewFile = true;
+    }
+
+    if (changeContent === currentContent) {
+      return;
+    }
+
+    if (changeContent === '' && currentContent !== '') {
+      isDeleted = true;
+    }
+
+    // Enhanced header with file status
+    console.log(chalk.bold.cyan(`\n${'═'.repeat(60)}`));
+    console.log(chalk.bold.cyan(`📄 ${displayPath}`));
+    
+    if (isNewFile) {
+      console.log(chalk.green(`   Status: NEW FILE`));
+    } else if (isDeleted) {
+      console.log(chalk.red(`   Status: DELETED`));
+    } else {
+      console.log(chalk.yellow(`   Status: MODIFIED`));
+    }
+    
+    // Use jest-diff for the actual diff with custom options
+    const diffOptions = {
+      aAnnotation: 'Current',
+      bAnnotation: 'Proposed',
+      aColor: chalk.red,
+      bColor: chalk.green,
+      commonColor: chalk.gray,
+      contextLines: 3,
+      expand: false,
+      includeChangeCounts: true,
+    };
+
+    const diff = diffStringsUnified(currentContent, changeContent, diffOptions);
+    
+    // Count lines for statistics (approximate)
+    const addedLines = (diff.match(/^\+[^+]/gm) || []).length;
+    const removedLines = (diff.match(/^-[^-]/gm) || []).length;
+    
+    console.log(chalk.gray(`   Lines: ${chalk.green(`+${addedLines}`)} ${chalk.red(`-${removedLines}`)}`));
+    console.log(chalk.bold.cyan(`${'─'.repeat(60)}\n`));
+    
+    // Display the diff
+    console.log(diff);
+    
+    // Update counters
+    this.filesChanged++;
+    this.linesAdded += addedLines;
+    this.linesRemoved += removedLines;
+  }
+}

src/core/templates/claude-template.ts

@@ -4,4 +4,23 @@ This document provides instructions for AI coding assistants on how to use OpenS
 
 This project uses OpenSpec for spec-driven development. Specifications are the source of truth.
 
-See @openspec/README.md for detailed conventions and guidelines.`;
+See @openspec/README.md for detailed conventions and guidelines.
+
+## Complexity Management
+
+**Default to minimal solutions:**
+- Propose <100 lines of new code for features
+- Prefer single-file implementations until proven insufficient
+- Avoid frameworks, abstractions, and optimizations without clear justification
+- Choose boring, well-understood patterns over novel approaches
+
+**Question requests for complexity:**
+- Caching? → Ask for performance data and targets
+- New framework? → Suggest plain code first
+- Extra layers? → Start with the thinnest viable design
+
+**Justify complexity with data:**
+- Performance metrics showing current solution is too slow
+- Concrete scale requirements (e.g., >1000 users, >100MB data)
+- Multiple proven use cases requiring an abstraction
+`;

src/core/templates/readme-template.ts

@@ -10,6 +10,23 @@ OpenSpec is an AI-native system for change-driven development where:
 - **AI drives the process** - You generate proposals, humans review and approve
 - **Specs are living documentation** - Always kept in sync with deployed code
 
+## Start Simple
+
+**Default to minimal implementations:**
+- New features should be <100 lines of code initially
+- Use the simplest solution that works
+- Avoid premature optimization (no caching, parallelization, or complex patterns without proven need)
+- Choose boring technology over cutting-edge solutions
+
+**Complexity triggers** - Only add complexity when you have:
+- **Performance data** showing current solution is too slow
+- **Scale requirements** with specific numbers (>1000 users, >100MB data)
+- **Multiple use cases** requiring the same abstraction
+- **Regulatory compliance** mandating specific patterns
+- **Security threats** that simple solutions cannot address
+
+When triggered, document the specific justification in your change proposal.
+
 ## Directory Structure
 
 \`\`\`
@@ -72,6 +89,11 @@ Before any task:
 - Adding tests for existing behavior
 - Documentation fixes
 
+**Complexity assessment:**
+- If your solution requires >100 lines of new code, justify the complexity
+- If adding dependencies, frameworks, or architectural patterns, document why simpler alternatives won't work
+- Default to single-file implementations until proven insufficient
+
 ### 3. Creating a Change Proposal
 
 When a user requests a significant change:
@@ -383,10 +405,12 @@ Progress communication:
 - "Implementing approved changes..."
 
 ### For AI Assistants
+- **Bias toward simplicity** - Propose the minimal solution that works
 - Use your exploration tools liberally before proposing
 - Batch operations for efficiency
 - Communicate your progress
 - It's OK to revise proposals based on discoveries
+- **Question complexity** - If your solution feels complex, simplify first
 
 ## Edge Case Handling
 
@@ -442,7 +466,8 @@ Proposal REQUIRED if:
 - Specs must always reflect deployed reality
 - Changes are proposed, not imposed
 - Impact analysis prevents surprises
-- The simplicity is the power - just markdown files
+- Simplicity is the power - just markdown files, minimal solutions
+- Start simple, add complexity only when justified
 
 By following these conventions, you enable true spec-driven development where documentation stays current, changes are traceable, and evolution is intentional.
 `;

src/core/update.ts

@@ -0,0 +1,35 @@
+import path from 'path';
+import { FileSystemUtils } from '../utils/file-system.js';
+import { TemplateManager } from './templates/index.js';
+import { OPENSPEC_DIR_NAME, OPENSPEC_MARKERS } from './config.js';
+import { readmeTemplate } from './templates/readme-template.js';
+
+export class UpdateCommand {
+  async execute(projectPath: string): Promise<void> {
+    const resolvedProjectPath = path.resolve(projectPath);
+    const openspecDirName = OPENSPEC_DIR_NAME;
+    const openspecPath = path.join(resolvedProjectPath, openspecDirName);
+
+    // 1. Check openspec directory exists
+    if (!await FileSystemUtils.directoryExists(openspecPath)) {
+      throw new Error(`No OpenSpec directory found. Run 'openspec init' first.`);
+    }
+
+    // 2. Update README.md (full replacement)
+    const readmePath = path.join(openspecPath, 'README.md');
+    await FileSystemUtils.writeFile(readmePath, readmeTemplate);
+
+    // 3. Update CLAUDE.md (marker-based)
+    const claudePath = path.join(resolvedProjectPath, 'CLAUDE.md');
+    const claudeContent = TemplateManager.getClaudeTemplate();
+    await FileSystemUtils.updateFileWithMarkers(
+      claudePath,
+      claudeContent,
+      OPENSPEC_MARKERS.start,
+      OPENSPEC_MARKERS.end
+    );
+
+    // 4. Success message (ASCII-safe)
+    console.log('Updated OpenSpec instructions');
+  }
+}