Swift "ld: symbol(s) not found for architecture arm64" in CI

The compile step succeeded but the linker could not find one or more symbols for arm64. A dependency library was not built or linked, an object targets a different architecture, or a system framework is not linked.

What this error means

The build ends at link time with "ld: symbol(s) not found for architecture arm64" and "clang: error: linker command failed with exit code 1".

swift

Undefined symbols for architecture arm64:
  "_$s10Networking6ClientCACycfC", referenced from:
      _$s3App0A0V6clientAA10Networking6ClientCvpfi in App.o
ld: symbol(s) not found for architecture arm64
clang: error: linker command failed with exit code 1 (use -v to see invocation)

Common causes

A dependency was not built or linked

The module compiled against a dependency whose object code was not built or linked for arm64, leaving its symbols undefined.

An architecture or SDK mismatch

Objects built for a different architecture (or against a different SDK) mix with the arm64 build, so symbols do not line up at link time.

How to fix it

Clean and rebuild the full dependency graph

Remove .build so no stale objects are reused.
Rebuild for a single architecture so all objects match.
Confirm every linked dependency built successfully first.

Terminal

rm -rf .build
swift build

Link the missing framework or library

If the undefined symbol belongs to a system framework, declare it as a linked framework on the target.

Package.swift

.target(
  name: "App",
  linkerSettings: [.linkedFramework("Security")]
)

How to prevent it

Key build caches on architecture so arm64 and x86_64 objects never mix.
Declare required system frameworks in linkerSettings.
Rebuild dependencies cleanly when switching runner architectures.