Claude Web & Harness Engineering
Browser-based coding with devcontainers and session teleportation
Prerequisites
- Basic Claude Code usage
- Understanding of development environments
What You'll Learn
- Understand devcontainer architecture
- Teleport sessions between web and CLI
- Grasp what a harness is and why it matters
- Know how harness engineering affects LLM performance
Web vs CLI: Same Power, Different Access
| Feature | Web | CLI |
|---|---|---|
| Setup Required | None (browser) | Install npm package |
| File Access | Devcontainer workspace | Full local filesystem |
| IDE Integration | Web-based editor | VS Code, JetBrains, etc. |
| Git Operations | Full support | Full support |
| Best For | Quick starts, demos, shared devices | Daily development, large projects |
Inside the Devcontainer
Each web session spins up an isolated Linux container with pre-installed development tools:
Runtimes
- •Node.js 20 LTS
- •Python 3.11
- •Go 1.21
- •Rust (rustup)
Package Managers
- •npm / pnpm / yarn
- •pip / poetry
- •cargo
Dev Tools
- •Git
- •Docker
- •Make / CMake
- •LSP servers
Databases
- •SQLite
- •PostgreSQL client
- •Redis CLI
Utilities
- •curl / wget
- •jq
- •ripgrep
- •fd
Networking
- •Port forwarding
- •Preview URLs
- •SSH client
Session Teleportation
Move your entire work session from browser to local CLI seamlessly:
# In the web interface, get your session ID
# Click "Export Session" or use /export
# On your local machine:
$ claude --teleport session_0116r15MFQNNgYpysPJfgzMQ
Downloading session state...
Restoring 23 modified files...
Restoring conversation context...
Session restored! You can continue where you left off.All modified files are downloaded to your local directory
Conversation history and memory are preserved
Environment variables and configuration transfer too
🔧 Understanding Harness Engineering
A harness is the complete system that wraps an LLM to create a useful product. The same base model can produce dramatically different results depending on its harness:
Components of a Harness
Instructions that define behavior, persona, and constraints
What actions the LLM can take (read files, run code, search)
How conversation history and relevant info is maintained
How LLM responses are interpreted and executed
Recovery strategies when things go wrong
Multi-step reasoning and self-correction patterns
Why Harness Engineering Matters
Same model, different outcomes: Two products using the same Claude model can have vastly different performance because of their harness design.
Compound effects: Small improvements in each harness component multiply together. Better prompts + better tools + better error handling = exponentially better results.
Real-world optimization: Claude Code's harness is continuously refined based on millions of actual coding sessions, catching edge cases and improving workflows.
Key insight: When comparing AI coding tools, you're comparing harnesses as much as models. A well-engineered harness on a worse model can outperform a poor harness on a better model.
Claude Code's Harness Engineering
What makes Claude Code's harness particularly effective for software development:
Codebase-Aware Context
Intelligent summaries of project structure, relevant files loaded on-demand, and persistent memory of decisions made
Tool Chain Integration
Native Git, LSP, testing frameworks, and build tools—Claude can verify its own work and fix issues
Agentic Error Recovery
When builds fail or tests break, Claude automatically diagnoses and attempts fixes instead of giving up
Progressive Refinement
Complex tasks are broken into steps with checkpoints, allowing course correction and branch exploration
Harness Impact: A Real Example
Consider a task: "Add user authentication to this Express app"
Basic Harness
- •Generates auth code in isolation
- •Doesn't check existing patterns
- •Can't run tests to verify
- •No error recovery if it fails
- •User must manually integrate
Claude Code Harness
- •Scans codebase for existing auth patterns
- •Installs dependencies and updates configs
- •Runs tests, fixes failures automatically
- •Verifies the server starts correctly
- •Creates a commit with clear message
💡 Pro Tips
- 1.Use the web version for quick experiments or when on a shared computer—all computation happens in the cloud.
- 2.Teleport sessions when you need to continue work with local tools or connect to local databases/services.
- 3.Understanding harness engineering helps you write better prompts—you're working with the system, not against it.
- 4.When evaluating AI coding tools, test real-world tasks end-to-end—harness quality matters as much as model size.
Master Claude Code faster
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