opam "install ... failed" while building dependencies in CI

opam resolved a solution but one of the packages failed to build during install. The real cause, a compile error or a missing system dependency for C stubs, is in the captured build log above the summary.

What this error means

opam install ends with "[ERROR] The following actions failed" and a list of packages, each pointing to a build log under ~/.opam/log.

opam

[ERROR] The following actions failed
   - build conf-gmp 4
The following actions were aborted
   - build zarith 1.13
# fatal error: gmp.h: No such file or directory

Common causes

A missing system library for a C-stub package

Packages like zarith or ssl need system headers (gmp, openssl) that the runner image does not ship, so the native build fails.

A genuine build failure in a dependency

A package version incompatible with the compiler, or a broken release, fails to compile during the install action.

How to fix it

Install the system dependencies first

Let opam report external deps, then install them (setup-ocaml can do this) before building.

Terminal

opam install . --deps-only --yes --depext
# or install the OS package the log names, e.g.:
sudo apt-get install -y libgmp-dev

Read the package build log

Open the log path opam prints for the failed package.
Find the first error: or fatal error: line.
Install the missing header or pin a compatible package version.

How to prevent it

Use --depext (or setup-ocaml depext) to pull system deps for C stubs.
Bake common -dev libraries into a custom runner image.
Pin dependency versions known to build on the CI compiler.