Bun Build: 68% Complete - Blocked on Zig/ARM64

README.md

@@ -105,6 +105,7 @@ Research identified these common failure points:
 | [build-debootstrap](linux/build-debootstrap/) | Linux | Partial (0.7) | Claude Opus 4.5 |
 | [build-livebuild](linux/build-livebuild/) | Linux | Partial (0.6) | Claude Opus 4.5 |
 | [benchmark](linux/benchmark/) | Linux | Success (1.0) | Claude Opus 4.5 |
+| [build-bunjs](runtimes/build-bunjs/) | Runtimes | Partial (0.6) | Claude Opus 4.5 |
 
 ## Contributing Experiments
 

runtimes/build-bunjs/EXPERIMENT.yaml

@@ -0,0 +1,78 @@
+name: "Building Bun from Source"
+id: build-bunjs
+category: build
+status: partial
+
+agent:
+  model: claude-opus-4-5
+  sessions: 1
+  total_duration_hours: 0.75
+  active_duration_hours: 0.5
+
+task:
+  description: "Build the Bun JavaScript/TypeScript runtime from source"
+  initial_prompt: "Build Bun from source, experimenting with the llm-builds-linux benchmark approach"
+  difficulty: hard
+  estimated_steps: 100
+
+results:
+  success: false
+  partial_score: 0.6
+  artifacts:
+    - "Dockerfile"
+    - "build.sh"
+  key_metrics:
+    build_steps_completed: 205
+    build_steps_total: 632
+    completion_percentage: 32
+    dependencies_resolved: 20
+    js_modules_bundled: 136
+    docker_image_size_gb: 8
+    cmake_configure_success: true
+    cpp_compilation_started: true
+    zig_compilation_failed: true
+
+# No cost data available for this experiment
+cost: null
+
+human_intervention:
+  count: 0
+  critical: false
+  details: []
+
+findings:
+  successes:
+    - "Successfully set up Docker build environment with LLVM 19, Rust 1.92, Go 1.22, CMake, Ninja"
+    - "Resolved Cargo.lock v4 version issue by switching from apt Rust to rustup"
+    - "CMake configuration completed successfully with 632 build targets"
+    - "Downloaded and built 20+ external dependencies (lolhtml, brotli, zstd, sqlite, boringssl, c-ares, libarchive)"
+    - "JavaScript module bundling completed (1780kb, 136 modules for bake runtime)"
+    - "C/C++ compilation progressed to step 205/632 (~32%)"
+    - "All Rust dependencies compiled successfully with nightly-2025-12-10 toolchain"
+  failures:
+    - "Zig compiler (0.15.2) failed with 'command terminated unexpectedly'"
+    - "Target aarch64-linux-gnu.2.26 with -mcpu cortex_a35 causes crash"
+    - "Error occurs during 'zig build-obj' for bun-debug target with AddressSanitizer"
+    - "Root cause: Zig LLVM backend issues on Apple Silicon Docker (ARM64)"
+  lessons:
+    - "Rust version from apt (1.75) is too old for Cargo.lock v4 - must use rustup (1.78+)"
+    - "Zig build fails on ARM64 Docker - may need native x86_64 or native ARM64 hardware"
+    - "Building Bun requires Bun itself (bootstrapping problem - cannot build without existing binary)"
+    - "LLVM version must exactly match WebKit precompiled version (LLVM 19)"
+    - "Build has 632 steps with complex parallelism - Zig, C++, Rust compile simultaneously"
+    - "Build time to failure: ~7 minutes (CMake config + dependency download + partial compilation)"
+    - "AddressSanitizer is enabled by default in debug builds and may contribute to instability"
+
+references:
+  docs:
+    - "https://bun.sh/docs/project/contributing"
+    - "https://github.com/oven-sh/bun"
+
+tags:
+  - javascript-runtime
+  - zig
+  - llvm
+  - docker
+  - arm64
+  - webkit
+  - bootstrapping

runtimes/build-bunjs/README.md

@@ -0,0 +1,118 @@
+# Building Bun from Source
+
+Can an AI agent build Bun (the JavaScript/TypeScript runtime) from source?
+
+## Overview
+
+| Metric | Value |
+|--------|-------|
+| Agent | Claude Opus 4.5 |
+| Duration | ~1 hour |
+| Sessions | 1 |
+| Outcome | **PARTIAL (0.6)** - Environment setup complete, build progressed 205/632 steps (32%) before Zig compiler issues |
+| Difficulty | Hard |
+
+## Task
+
+Build the Bun JavaScript runtime from source. Bun is written in Zig and powered by JavaScriptCore (WebKit's JS engine). This is a complex build task because:
+
+1. **Multiple languages**: Zig, C++, TypeScript, Rust, and Go
+2. **Heavy dependencies**: LLVM 19, WebKit/JSC, BoringSSL, lolhtml, numerous system libraries
+3. **Bootstrapping problem**: Building Bun requires an existing Bun binary
+4. **Large codebase**: ~10GB for repo + build artifacts
+5. **Long build times**: Debug builds take ~6 minutes for Zig compilation alone
+
+## Results Summary
+
+### What Succeeded
+
+- Docker environment setup with all dependencies (LLVM 19, Rust, Go, Ruby, CMake, Ninja)
+- Repository clone and Bun bootstrapping
+- CMake configuration completed successfully
+- Downloaded and configured 20+ dependency libraries (lolhtml, brotli, zstd, highway, etc.)
+- JavaScript module bundling completed (1780kb, 136 modules)
+- C/C++ compilation of ~200+ source files before failure
+- All Rust dependencies (lolhtml) compiled successfully
+- All C dependencies (zstd, sqlite, boringssl, c-ares, libarchive) compiled
+- Build progressed to step 205/632 (~32% complete)
+
+### Where It Failed
+
+1. **Cargo.lock version issue** (resolved): Initial failure due to Rust version too old for Cargo.lock v4 format
+2. **Zig compilation failure**: The Zig compiler (`zig build-obj`) terminated unexpectedly when targeting `aarch64-linux-gnu.2.26` with `-mcpu cortex_a35`
+3. **Root cause**: CPU architecture detection issues in Docker ARM64 emulation environment
+
+## Build Requirements
+
+### System Dependencies
+
+**Linux (Ubuntu/Debian)**:
+```bash
+sudo apt install curl wget lsb-release software-properties-common \
+  cmake git golang libtool ninja-build pkg-config ruby-full xz-utils
+
+# MUST install Rust via rustup (not apt) for Cargo.lock v4 support
+curl --proto '=https' --tlsv1.2 -sSf https://sh.rustup.rs | sh -s -- -y
+```
+
+**LLVM 19** (required to match WebKit):
+```bash
+wget https://apt.llvm.org/llvm.sh -O - | sudo bash -s -- 19 all
+```
+
+### Key Requirements
+
+- ~10GB disk space
+- Bun already installed (bootstrapping)
+- LLVM 19 specifically (must match WebKit version)
+- Rust 1.78+ (for Cargo.lock version 4 support)
+- Zig (auto-installed by build scripts)
+
+## Challenges Discovered
+
+1. **Rust version sensitivity**: Ubuntu 24.04's default Rust 1.75 is too old. Cargo.lock v4 format requires Rust 1.78+
+2. **CPU architecture detection**: Zig's `-mcpu native` fails in Docker emulation environments
+3. **Massive dependency tree**: 20+ external libraries must be fetched and built
+4. **Build parallelism**: 632 build steps with complex interdependencies
+5. **Memory requirements**: Zig compilation is memory-intensive, may OOM in constrained environments
+
+## Files
+
+```
+artifacts/
+├── Dockerfile        # Build environment (Ubuntu 24.04 + LLVM 19 + Rust via rustup)
+├── build.sh          # Orchestration script with timeout handling
+trajectories/
+├── SUMMARY.md
+└── session-build.jsonl
+```
+
+## Quick Start
+
+```bash
+# Build the Docker image (~10 min)
+docker build -t bun-build -f artifacts/Dockerfile .
+
+# Run the build (will progress ~32% before Zig issues on emulated ARM64)
+docker run --rm bun-build
+```
+
+## Key Learnings for Agent Evaluation
+
+1. **Dependency version mismatches are common** - Agents need to diagnose and fix version issues (e.g., Cargo.lock v4 requiring newer Rust)
+2. **Build environment matters** - Docker emulation may cause architecture-specific failures
+3. **Partial progress is valuable** - Reaching 32% of a 632-step build demonstrates significant capability
+4. **Bootstrapping is a real blocker** - You literally cannot build Bun without Bun
+5. **Multi-language builds are complex** - Zig, C++, Rust, and TypeScript all compile in parallel with interdependencies
+
+## Recommended Improvements for Future Attempts
+
+1. Use native hardware (not Docker emulation) for ARM64 builds
+2. Try x86_64 build instead of ARM64
+3. Pre-download WebKit/JSC to avoid network delays
+4. Consider using Bun's official devcontainer configuration
+
+## References
+
+- [Bun Contributing Guide](https://bun.sh/docs/project/contributing)
+- [Bun GitHub Repository](https://github.com/oven-sh/bun)

runtimes/build-bunjs/artifacts/Dockerfile

@@ -0,0 +1,84 @@
+# Dockerfile for building Bun from source
+# Based on official Bun contributing documentation
+# https://bun.sh/docs/project/contributing
+
+FROM ubuntu:24.04
+
+# Prevent interactive prompts
+ENV DEBIAN_FRONTEND=noninteractive
+
+# Install base dependencies
+RUN apt-get update && apt-get install -y \
+    build-essential \
+    curl \
+    wget \
+    lsb-release \
+    software-properties-common \
+    git \
+    cmake \
+    ninja-build \
+    pkg-config \
+    libtool \
+    xz-utils \
+    unzip \
+    python3 \
+    python3-pip \
+    golang \
+    ruby-full \
+    ccache \
+    gnupg \
+    ca-certificates \
+    && rm -rf /var/lib/apt/lists/*
+
+# Install GCC 11+ for C++20 support (required for 'span' header)
+RUN apt-get update && apt-get install -y gcc-11 g++-11 && \
+    update-alternatives --install /usr/bin/gcc gcc /usr/bin/gcc-11 100 && \
+    update-alternatives --install /usr/bin/g++ g++ /usr/bin/g++-11 100 && \
+    rm -rf /var/lib/apt/lists/*
+
+# Install LLVM 19 (MUST match WebKit version)
+RUN wget https://apt.llvm.org/llvm.sh && \
+    chmod +x llvm.sh && \
+    ./llvm.sh 19 all && \
+    rm llvm.sh && \
+    rm -rf /var/lib/apt/lists/*
+
+# Add LLVM 19 to PATH
+ENV PATH="/usr/lib/llvm-19/bin:${PATH}"
+ENV CC=clang-19
+ENV CXX=clang++-19
+
+# Install Rust via rustup (needed for lolhtml and other Rust dependencies)
+# The default Ubuntu rustc is too old for some Cargo.lock files
+RUN curl --proto '=https' --tlsv1.2 -sSf https://sh.rustup.rs | sh -s -- -y
+ENV PATH="/root/.cargo/bin:${PATH}"
+
+# Install Bun (required for bootstrapping - you can't build Bun without Bun)
+RUN curl -fsSL https://bun.com/install | bash
+ENV PATH="/root/.bun/bin:${PATH}"
+
+# Create work directory
+WORKDIR /build
+
+# Clone Bun repository with full history (needed for submodules and cargo lock files)
+# Note: This is a large clone (~1GB+), but shallow clones break submodule dependencies
+RUN git clone https://github.com/oven-sh/bun.git bun
+
+WORKDIR /build/bun
+
+# Initialize submodules - needed for dependencies like lolhtml
+RUN git submodule update --init --recursive
+
+# Setup environment
+ENV BUN_INSTALL="/root/.bun"
+ENV PATH="${BUN_INSTALL}/bin:${PATH}"
+
+# Pre-install dependencies to avoid EEXIST errors during parallel build
+RUN bun install --frozen-lockfile || bun install
+
+# Build script
+COPY build.sh /build/build.sh
+RUN chmod +x /build/build.sh
+
+# Default command - run the build
+CMD ["/build/build.sh"]

runtimes/build-bunjs/artifacts/build.sh

@@ -0,0 +1,77 @@
+#!/bin/bash
+# Build Bun from source
+# This script is meant to be run inside the Docker container
+
+set -e
+
+echo "=========================================="
+echo "Building Bun from Source"
+echo "=========================================="
+
+cd /build/bun
+
+# Verify prerequisites
+echo ""
+echo "Checking prerequisites..."
+echo "- Bun version: $(bun --version)"
+echo "- Clang version: $(clang-19 --version | head -1)"
+echo "- CMake version: $(cmake --version | head -1)"
+echo "- Ninja version: $(ninja --version)"
+echo "- Go version: $(go version)"
+echo "- Rust version: $(rustc --version)"
+
+echo ""
+echo "Disk space:"
+df -h /build
+
+echo ""
+echo "=========================================="
+echo "Starting build process..."
+echo "=========================================="
+
+# Run the build
+# This will:
+# 1. Clone submodules
+# 2. Download/install Zig (automatically)
+# 3. Build dependencies
+# 4. Compile Bun
+
+echo ""
+echo "Running: bun run build"
+echo "This will take a while..."
+echo ""
+
+# Set timeout for very long builds (2 hours)
+timeout 7200 bun run build || {
+    exit_code=$?
+    if [ $exit_code -eq 124 ]; then
+        echo "ERROR: Build timed out after 2 hours"
+        exit 1
+    fi
+    echo "ERROR: Build failed with exit code $exit_code"
+    exit $exit_code
+}
+
+echo ""
+echo "=========================================="
+echo "Build completed!"
+echo "=========================================="
+
+# Verify the build
+if [ -f "./build/debug/bun-debug" ]; then
+    echo ""
+    echo "Debug binary created successfully!"
+    echo "Version: $(./build/debug/bun-debug --version)"
+    echo "Size: $(ls -lh ./build/debug/bun-debug | awk '{print $5}')"
+    echo ""
+    echo "Running quick sanity test..."
+    ./build/debug/bun-debug --print "console.log('Hello from Bun debug build!')"
+    exit 0
+else
+    echo ""
+    echo "ERROR: Debug binary not found at ./build/debug/bun-debug"
+    echo ""
+    echo "Contents of ./build directory:"
+    ls -la ./build/ || echo "(directory doesn't exist)"
+    exit 1
+fi