Nix's `github:` fetcher wants a tarball from
github.com/<owner>/<repo>/archive/<rev>.tar.gz, and that endpoint is not
served in a cloud session. Three things that look like fixes are not:
- Raising the environment's network access to Full, or allowlisting
codeload.github.com, changes nothing: GitHub traffic bypasses the
domain allowlist entirely and goes through a proxy that scopes API
and release-asset requests to repositories attached to the session.
An input from any other owner gets a 403.
- Attaching the dependency repos is refused -- cross-tier adds are not
supported once the session has sources from another owner.
- Nix has no setting that makes `github:` inputs use the git transport.
The lock's own integrity machinery is what works. flake.lock records a
narHash per input, and Nix looks a source up in the store and its
substituters before fetching anything. The git and tarball fetchers
produce byte-identical trees for a revision, so prefetching the locked
rev over git+https lands the input at exactly the store path the lock
names:
lock narHash: sha256-l0KFg5HjrsfsO/JpG+r7fRrqm12kzFHyUHqHCVpMMbI=
git prefetch: sha256-l0KFg5HjrsfsO/JpG+r7fRrqm12kzFHyUHqHCVpMMbI=
scripts/nix-seed-flake-inputs.sh does that for every locked input. It
reads flake.lock and never writes it. Nix still verifies each tree
against the recorded hash, so this satisfies the lock rather than working
around it -- a mismatch fails loudly. Rewriting the lock's nodes to `git`
type and carrying the old narHash across would have been the opposite:
that hash is the claim the lock exists to make.
Verified from a fresh clone with a cleared git cache: 15/15 inputs
seeded, then `nix develop` gives rustc 1.98.1, cargo and crate2nix, with
flake.nix and flake.lock unmodified.
Not wired into the session-start hook: it costs minutes and several GB
once the devShell builds, and most work only needs the preinstalled cargo
toolchain.
Corrects three places that claimed nix could not work here --
context/cloud-agents.md, the design-review skill's kb.pl check, and the
hook's environment notes -- all written before the git transport was
tested. The doc also records the two alternatives, a binary-cache
substituter and a self-hosted environment.
Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01DyHK7L8S1XDX3EihTnSKaX
Claude
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