fix(scripts): clear stale llama.cpp CMake cache on generator mismatch

[ndizazzo]

Native builds can fail with:

CMake Error: Error: generator : Ninja
Does not match the generator used previously: Unix Makefiles
Either remove the CMakeCache.txt file and CMakeFiles directory or choose a different binary directory.

This happens when ninja appears on (or leaves) PATH between builds. scripts/build-llama.sh picks the CMake generator from PATH at configure time, but the build directory under .deps/llama-build/ still holds a CMakeCache.txt configured with the previous generator, and CMake refuses to reuse a cache across generators. On a dev machine that installed Homebrew ninja, just release-build (and the other native build recipes) fail at the CMake configure step with no obvious remedy, because the stale cache lives outside the source tree.

Diagnostics

• just release-build on main (clean tree, macOS arm64): host build succeeded, release-runtime-build failed at the llama.cpp CMake configure with the generator-mismatch error above.
• Compared the cached generator against the one the script selects: CMAKE_GENERATOR:INTERNAL=Unix Makefiles in .deps/llama-build/build-stage-abi-dynamic-metal/CMakeCache.txt (configured before ninja was installed) vs Ninja selected now.
• Removing the stale build directory and re-running just release-build completes: metal runtime packaged, tarball sidecar checksum OK.

Fix

scripts/build-llama.sh now reads the generator recorded in the existing CMakeCache.txt right before the CMake configure. On a mismatch it clears the build directory (pure cache, regenerated on rebuild) and logs what it did. The guard sits after the stamp fast path and the --require-existing exit, so warm and pre-warmed caches that are only being verified are never touched, and already-valid outputs are still reused via the fast path.

scripts/tests/test_build_llama.py pins the behavior with stubbed cmake/ninja (self-contained, no toolchain required): stale cache cleared, matching cache kept, --require-existing untouched. AGENTS.md documents the failure mode in the Building section next to the npm exit-handler note.

Validation

• I ran the relevant local checks, or explained why they do not apply.
  • python3 -m unittest scripts/tests/test_build_llama.py (3/3 OK)
  • python3 -m unittest scripts/tests/test_static_abi_artifacts.py scripts/tests/test_llama_upstream_canary_contract.py scripts/tests/test_llama_canary_agent_repair_contract.py (60/60 OK, the contract tests that pin this script)
  • shellcheck scripts/build-llama.sh and python3 -m py_compile scripts/tests/test_build_llama.py: clean
  • End-to-end: after clearing the stale cache, just release-build completed locally (host + metal runtime + tarball, sidecar checksum OK)
• UI changes include screenshots or video, or explain why visual aids are not needed.
  • No UI changes.

Summary by CodeRabbit

• Bug Fixes

  • Native builds now automatically recover when an existing CMake cache was created with a different build generator, preventing “generator does not match” failures.
  • Existing compatible build caches continue to be reused, while required-existing builds retain their refusal behavior.

• Documentation

  • Added guidance for diagnosing and resolving CMake generator mismatch errors, including steps for older checkouts.

• Tests

  • Added coverage for generator selection, cache handling, dynamic builds, and required-existing behavior.

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• scripts/build-llama.sh
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📝 Walkthrough

Walkthrough

The native build script now selects a CMake generator explicitly and removes stale build caches when the cached generator differs. New tests cover generator selection, cache handling, successful builds, and --require-existing. Documentation describes the related CMake error.

Changes

CMake generator guard

Layer / File(s)	Summary
Generator detection and cache reset scripts/build-llama.sh, AGENTS.md	The script passes Ninja or Unix Makefiles explicitly to CMake and removes mismatched build directories. The documentation describes the error and recovery steps.
Generator guard validation scripts/tests/test_build_llama.py	Tests stub CMake and Ninja and verify generator selection, stale-cache removal, matching-cache preservation, successful builds, and --require-existing behavior.

Priority: ⬇️ Low

Estimated code review effort: 2 (Simple) | ~10 minutes

Change: Bug fix

Merge Risk: ⚪ Minimal · up to ed4bc

The build script now consistently selects and enforces its CMake generator while safely resetting incompatible caches. No actionable merge-blocking risk remains.

🚥 Pre-merge checks | ✅ 4 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Docstring Coverage	⚠️ Warning	Docstring coverage is 0.00% which is insufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Analyzed 7 functions across 2 files.	Write docstrings for the functions missing them to satisfy the coverage threshold.

✅ Passed checks (4 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title clearly and concisely describes the main change: clearing stale llama.cpp CMake caches when the generator does not match.
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.

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Actionable comments posted: 1

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Inline comments:
In `@scripts/build-llama.sh`:
- Line 235: Update the CMake configuration flow in the build script to always
pass -G using SELECTED_CMAKE_GENERATOR, including when ninja is unavailable, so
inherited CMAKE_GENERATOR values cannot override the selected generator. Add a
regression test covering the no-ninja environment and confirming the selected
generator is passed.

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ℹ️ Review info

⚙️ Run configuration

Configuration used: Organization UI

Review profile: CHILL

Plan: Advanced

Run ID: c2da559d-6cb4-421a-8762-addab0ddb3e1

📥 Commits

Reviewing files that changed from the base of the PR and between 0e84ac7 and 69de382.

📒 Files selected for processing (3)

• AGENTS.md
• scripts/build-llama.sh
• scripts/tests/test_build_llama.py

Included review availability: Your plan provides up to 8 included reviews per hour; 7 remain after this review.