Build Images

Building VM Images

Overview

Capsem image builds are profile-led.

• config/profiles/<profile_id>/profile.toml is the profile ledger.
• Profile sibling files own packages, MCP declarations, rule files, detection

files, tips, build-time hooks, and packaged guest root seed files.

• capsem-admin validates profile-owned inputs and materializes the generated

backend build workspace.

• The Python builder backend renders Docker templates and emits assets, build

ledgers, and OBOMs only when invoked by the admin build rail. Do not add

product truth directly to the backend image-spec path.

Source Layout

Read config/README.md before changing this layout.

config/
  settings/               UI/application preferences and generated UI schema
  corp/                   Corporate source contracts and rule files
  docker/                 Dockerfile/build templates
  profiles/<profile_id>/
    profile.toml          Source ledger; no hash/size pins
    enforcement.toml      Profile enforcement rules
    detection.yaml        Profile Sigma detections
    mcp.json              Profile MCP declarations
    apt-packages.txt      Profile apt package input
    python-requirements.txt
    npm-packages.txt
    build.sh              Profile image build hook
    tips.txt              Profile guest tips
    root/                 Guest / seed, projected by capsem-init
target/config/            Generated runtime config with asset/file evidence
guest/artifacts/          Core guest payloads: init, doctor, diagnostics, bench
assets/                   Generated VM assets
packages/                 Generated native packages

The materialized backend workspace may contain generated package-set files and

profile build scripts. Treat those as implementation details, not authoring

surfaces. The workspace is never a config root and never a second profile

catalog.

capsem-admin is a tool, not a config authority. It validates, materializes,

builds, and checks the profile/corp/settings contracts; it must not grow

scaffolding commands that invent profile, MCP, AI provider, package, or rule

truth outside config/profiles, config/corp, and config/settings.

Do not add admin config roots, guest config roots, settings metadata, provider

registries, or backend-owned profile catalogs as product truth. schema

validates one contract, catalog lists materialized profile instances, and UI

metadata only helps render settings.

CLI commands

just _build-assets code [arch]                # Profile-derived asset rebuild
just _build-kernel arm64 code                 # Kernel slice
just _build-rootfs arm64 code                 # Rootfs slice
uv run capsem-builder audit                  # Parse trivy/grype vulnerability output

Use admin/just recipes for all product image work. capsem-builder is a

backend helper only; it must not expose or document public build, validate,

inspect, mcp, render-only, or dry-run rails for profile/image authoring.

capsem-admin image build may call private Python modules such as

capsem.builder.image_build_backend; agents must not make those modules public

CLI contracts.

Building assets

Full rebuild (kernel + rootfs):

just _build-assets    # Runs doctor + validate + build for host arch

Individual templates:

just _build-kernel arm64
just _build-rootfs arm64

Per-arch asset layout

assets/
  manifest.json          Version, checksums, asset list
  B3SUMS                 BLAKE3 checksums
  arm64/
    vmlinuz              Kernel
    rootfs.erofs         Root filesystem
    initrd.img           Initial ramdisk (repacked by just exec)

Rootfs EROFS settings are profile-derived. The approved release default

is EROFS with lz4hc compression level 12.

Build Ledger

Each per-arch build emits build-ledger.log JSONL. The

rootfs.config_inputs record captures declared profile package inputs,

rendered rootfs package lists, profile root/build-script inputs, EROFS config,

git revision, and project version. Installed-package/component truth belongs in

the CycloneDX OBOM, not the build ledger.

Profile Source And Generated Evidence

Profile sibling files are ledgered source inputs, but agents must not add or

hand-edit hash or size fields in profile.toml. If editing

apt-packages.txt, python-requirements.txt, npm-packages.txt, build.sh,

rules, MCP declarations, tips, or root seed files makes

capsem-admin profile check fail, fix the source contract or the

validation/materialization rail with tests. Do not "just fix the hash" in TOML.

Generated runtime asset URLs/hashes belong in target/config after

capsem-admin profile materialize, not in checked-in source TOML. Profile

materialization must recopy descriptor files and root/ payloads from source

on every run; stale generated roots are a release blocker, not a cache.

Adding packages to the VM

• Edit the profile-owned package file, for example

config/profiles/code/apt-packages.txt,

python-requirements.txt, or npm-packages.txt.

• Run the admin/profile validation path.
• Run just _build-assets code to rebuild the rootfs.
• Verify with capsem-doctor inside a booted VM.

Do not edit generated Dockerfiles. Docker templates live under config/docker/.

Adding a guest CLI/tool

There are no image-owned AI providers. A CLI/tool exists only if the active

profile declares the package/build hook and any required guest root seed files.

• Add package input to the profile package files, or add build-time shell work

to profile-owned build.sh.

• Add config files under config/profiles/<profile_id>/root/ so they project

into the VM at boot.

• Add MCP declarations to profile-owned mcp.json when relevant.
• Add network/model/security behavior through profile/corp rules, not builder

provider config.

• Let the credential broker plugin capture/materialize credentials at runtime;

do not add settings-owned boot secrets.

• Rebuild with just _build-assets code and verify with capsem-doctor.

build.sh is executed only while constructing the rootfs image. It is the

right place for official installer commands such as Claude, AGY, or Ollama

when they cannot be represented as apt/npm/Python package inputs. It must

install stable runtime binaries under system paths such as /usr/local/bin;

anything left only under /root can be hidden by the runtime overlay.

Profile build.sh contract

Remember this rail when touching profile image contents:

• config/profiles/<profile_id>/build.sh is a profile-owned build hook.
• It runs inside the rootfs Docker build, before the EROFS image is produced.
• It does not run during native glow-up, service startup, VM boot, or user

session creation.

• It is for image construction work that cannot be cleanly expressed through

apt-packages.txt, python-requirements.txt, or npm-packages.txt.

• It may install public runtime tools such as Claude, AGY, and Ollama into

stable system paths.

• It is not a second profile format, provider registry, runtime settings file,

credential injection path, or local developer repair script.

• It must not bake credentials, per-user state, corp policy, rules, MCP

decisions, or runtime settings.

• The owning profile.toml must reference it through [files.build]; the

descriptor hash/size is refreshed by the profile-derived build rail, never by hand.

• Changing build.sh changes future rootfs assets only. Rebuild assets through

the profile-derived just/admin-tool rail before claiming a VM contains the

change.

• The same profile materialization path must be used locally and in CI; no

one-off Docker or installer path is release proof.

• Verification must be black-box: boot the rebuilt profile image, run the tool

from the VM, and inspect the generated session evidence when the tool should

produce network, model, MCP, file, process, or credential events.

Decision rule:

• Normal Debian package: use apt-packages.txt.
• Normal Python package: use python-requirements.txt.
• Normal npm package: use npm-packages.txt.
• Vendor shell installer, binary tarball, wrapper creation, or cleanup that must

happen while baking the immutable rootfs: use build.sh.

• Anything that depends on user/corp/runtime state: do not use build.sh.

Dockerfile templates

Templates live in config/docker/:

• Dockerfile.rootfs.j2 -- rootfs image (apt packages, Python packages, optional npm/curl package sets, profile root/build hook, diagnostics)
• Dockerfile.kernel.j2 -- kernel build (defconfig, modules, vmlinuz extraction)

Templates use Jinja2 with variables from the admin-materialized profile image

workspace. Do not add a second preview rail for product truth; if a build input

needs validation, add it to the normal profile/admin validation path.

Builder Internals (for modifying the builder itself)

Architecture: Profile -> admin materialization -> Pydantic -> context dict -> Jinja2 -> Dockerfile

The data flows through four layers:

• Profile ledger (config/profiles/<id>/profile.toml) and profile-owned

sibling files.

• capsem-admin validates and materializes a backend build workspace.
• Pydantic models (src/capsem/builder/models.py) parse that workspace.
• Context dict (src/capsem/builder/docker.py) feeds Jinja2 templates.
• Jinja2 templates (config/docker/) produce Dockerfiles.

Key files

| File | Role |

|------|------|

| src/capsem/builder/models.py | All Pydantic models (enums, configs, top-level GuestImageConfig) |

| src/capsem/builder/config.py | Backend loader for admin-materialized build workspaces |

| src/capsem/builder/docker.py | Context builders (_rootfs_context, _kernel_context), rendering, build execution |

| src/capsem/builder/image_build_backend.py | Private admin-invoked image build backend; not a public CLI |

| config/docker/Dockerfile.rootfs.j2 | Rootfs Dockerfile template |

| config/docker/Dockerfile.kernel.j2 | Kernel Dockerfile template |

| src/capsem/builder/validate.py | Validation rules (E001-E302, W001-W012) |

| src/capsem/builder/cli.py | Click CLI entry points |

Context dict (rootfs template variables)

_rootfs_context() in docker.py builds the dict passed to Dockerfile.rootfs.j2:

{
    "arch": ArchConfig,           # Per-arch settings (docker_platform, rust_target, etc.)
    "arch_name": str,             # "arm64" or "x86_64"
    "apt_packages": list[str],    # Materialized from profile apt-packages.txt
    "python_packages": list[str], # Materialized from profile python-requirements.txt
    "python_install_cmd": str,    # e.g. "uv pip install --system --break-system-packages"
    "npm_packages": list[str],    # Materialized from profile npm-packages.txt
    "profile_root_seed": bool,    # Whether profile-root/ is copied into the image
    "profile_build_script": bool, # Whether profile-build.sh is executed
    "npm_prefix": str,            # e.g. "/opt/ai-clis"
    "guest_binaries": list[str],  # ["capsem-pty-agent", "capsem-net-proxy", "capsem-mcp-server"]
}

Kernel context dict

{
    "arch": ArchConfig,
    "arch_name": str,
    "kernel_version": str,  # e.g. "6.6.130"
}

How to: Change a shipped CLI

• Prefer a profile package file (apt-packages.txt, npm-packages.txt, or

python-requirements.txt) when the tool has a normal package manager.

• Use profile-owned build.sh when the vendor ships an official shell

installer. The build hook runs during rootfs construction only.

• Make sure binaries end up in stable system paths such as /usr/local/bin.
• Validate and materialize through capsem-admin; if the rail cannot express

the change, implement it with tests first.

• Add or update capsem-admin materialization tests and Docker context tests.
• Rebuild: just _build-assets code and verify with capsem-doctor.

Ollama is intentionally installed by config/profiles/<id>/build.sh, not by a

VM one-off command. That keeps Codex, Claude, AGY, and OpenAI-compatible local

testing available in every shipped profile image that declares the hook.

How to: Add a new package to an existing set

• Edit config/profiles/<profile_id>/apt-packages.txt,

python-requirements.txt, or npm-packages.txt.

• Validate and materialize through capsem-admin.
• Keep the checked-in profile source free of generated hashes or sizes.
• Rebuild: just _build-assets <profile_id>.

How to: Add a new guest binary

Guest binaries are compiled from crates/capsem-agent/. On macOS, cross_compile_agent() delegates to container_compile_agent() which builds inside a Linux container (docker). On Linux (CI), cargo builds natively.

• Add the binary target in crates/capsem-agent/Cargo.toml
• Add the binary name to GUEST_BINARIES list in docker.py
• The template already loops {% for binary in guest_binaries %} to COPY + chmod 555

Verifying Linux builds locally

just _cross-compile [arch] builds everything in a container: agent binaries,

frontend, and the full Linux .deb package. Useful for catching system

dependency issues before CI.

just _cross-compile           # Build for host arch (arm64 on Apple Silicon)
just _cross-compile x86_64    # Build x86_64 deb

Backend Workspace Schema

The backend workspace is generated by capsem-admin; do not author it by

hand for product behavior. Its install inputs are package-set TOML files:

[npm]
name = "Node Packages"
manager = "npm"
install_cmd = "npm install -g --prefix /opt/ai-clis"
packages = ["@scope/package"]

Profiles own CLI/tool selection. If an installer cannot be represented as a

package set, put it in config/profiles/<profile_id>/build.sh, reference it

from [files.build] in profile.toml, refresh pins with capsem-admin, and

rebuild through the admin/just rail. Do not add a provider registry under

backend-generated image workspaces.

Build pipeline (what build_image() does)

For rootfs:

• Build guest agent binaries (cross_compile_agent -- on macOS delegates to container_compile_agent which builds inside a Linux container; on Linux compiles natively)
• Assemble build context (prepare_build_context) -- copies CA cert, shell configs, diagnostics, agent binaries
• Render Dockerfile from template
• docker build
• Export container filesystem as tar
• Create EROFS from tar (create_erofs -- runs mkfs.erofs in a container)
• Extract tool versions (extract_tool_versions)
• Clean up container image

For kernel:

• Resolve latest kernel version from kernel.org
• Assemble build context (defconfig, capsem-init)
• Render Dockerfile from template
• docker build
• Extract vmlinuz + initrd.img from image
• Clean up

Container runtime requirements

On macOS, Docker runs inside a Colima VM with limited resources.

The rootfs build runs apt, npm, and curl-based CLI installers concurrently --

the default RAM allocation may cause OOM kills (exit code 137).

Minimum: 12GB RAM. Recommended: 16GB RAM, 8 CPUs.

# Colima (macOS)
colima stop && colima start --vm-type vz --vz-rosetta --memory 16 --cpu 8

# Linux: Docker runs natively, no memory tuning needed
# sudo apt install docker.io

just doctor owns the product readiness gate. capsem-builder doctor is a

backend helper used by the build rail to check container/runtime prerequisites.

The resource check lives in src/capsem/builder/doctor.py:

• check_container_resources() -- checks docker info
• Thresholds: DOCKER_MIN_MEMORY_MB = 4096, DOCKER_RECOMMENDED_MEMORY_MB = 8192

Container image compatibility

The container builds use rust:slim-bookworm -- a minimal Debian image. Many common utilities (file, less, vim, etc.) are NOT available. Any shell commands run inside the container must use only coreutils (ls, cp, cat, test, etc.) or tools explicitly installed via apt-get in the same RUN step.

Lesson learned: using file /output/binary to verify compiled binaries failed because file is not in slim images. Replaced with ls -l which is always available and still confirms the copy succeeded. The real validation (existence + non-zero size) is done in Python after the container exits.

Rule: never assume a command exists in a slim container image. Stick to coreutils or install what you need explicitly.

Clock skew workaround

All apt-get update calls use -o Acquire::Check-Valid-Until=false to handle container VM clock drift.

Without this, apt rejects Release files whose timestamp is in the future relative to the VM's clock.

This can occur with any container VM backend on macOS.

Files affected:

• Dockerfile.kernel.j2 (line 11)
• Dockerfile.rootfs.j2 (line 11)
• docker.py create_erofs() function