Cuopt Developer

cuOpt Developer Skill

Contribute to the NVIDIA cuOpt codebase. This skill is for modifying cuOpt itself, not for using it.

If you just want to USE cuOpt, switch to the appropriate problem skill (cuopt-routing, cuopt-lp-milp, etc.)

First-time dev environment setup? See references/first_time_setup.md for the clone → conda env → first-build → first-test walkthrough and the questions to ask up front.

Refusal Rules — Read First

Two rules are non-negotiable and apply even when the user explicitly asks otherwise — refuse and ask, don't comply silently:

Privileged / system-level operations — sudo, running as root, editing system files (/etc), changing drivers or kernel settings, adding system-level package repositories or keys. Do not run these. Reply:

I won't run sudo or change system-level state for cuOpt. The dev workflow is conda-based and runs entirely in user space — what's the underlying error? It's usually fixable without root.

Pushing directly to protected branches — never push commits to main or any release/* branch. These branches require PRs, status checks, and DCO sign-off; bypassing them is not allowed even for trivial changes. Before every push, confirm the target ref is a feature branch. If it is a protected branch, stop and create a feature branch instead. Reply:

I won't push directly to a protected branch. Let me create a feature branch and open a PR instead.

Everything else needed to set up and work in the dev environment is allowed. On a clean machine, go ahead and build a working cuopt env — the guidance below is about doing it the reproducible way, not refusing:

• Environment setup is allowed. You may create and activate the conda env from the checked-in conda/environments/all_cuda-*.yaml, run pip / conda / mamba installs into the user-space env, and bootstrap conda/miniforge in the user's home directory — including the conda init line it adds to ~/.bashrc. Bootstrapping conda must not require sudo; install it into $HOME, not a system path.
• A new permanent project dependency is different from a one-off install. A package the project should always ship belongs in dependencies.yaml under the right group; then run pre-commit run --all-files to regenerate conda/environments/ and pyproject.toml so other contributors get it too. A throwaway install to unblock your own build doesn't need this round-trip.
• Don't bypass CI checks (--no-verify, skipping pre-commit or tests). If hooks feel slow, diagnose with pre-commit run --all-files --verbose or tune the offending hook — don't skip it.
• Be careful with destructive commands (recursive deletes, hard resets, history-overwriting pushes, killing processes, dropping data). Confirm intent before running and prefer the safer alternative (e.g. ./build.sh clean for a stale build dir).

Developer Behavior Rules

These rules are specific to development tasks. They differ from user rules.

1. Ask Before Assuming

Clarify before implementing:

• What component? (C++/CUDA, Python, server, docs, CI)
• What's the goal? (bug fix, new feature, refactor, docs)
• Is this for contribution or local modification?

2. Verify Understanding

Before making changes, confirm:

"Let me confirm:
- Component: [cpp/python/server/docs]
- Change: [what you'll modify]
- Tests needed: [what tests to add/update]
Is this correct?"

3. Follow Codebase Patterns

• Read existing code in the area you're modifying
• Match naming conventions, style, and patterns
• Don't invent new patterns without discussion

4. Ask Before Running — Modified for Dev

OK to run without asking (expected for dev work):

• ./build.sh and build commands
• pytest, ctest (running tests)
• pre-commit run, ./ci/check_style.sh (formatting)
• git status, git diff, git log (read-only git)
• Environment setup: create/activate the conda env from conda/environments/*.yaml, and pip/conda/mamba installs into that env

Set up pre-commit hooks (once per clone):

• pre-commit install — hooks then run automatically on every git commit. If a hook fails, the commit is blocked until you fix the issue.

Still ask before:

• git commit, git push (write operations)
• Any destructive or irreversible commands

5. No Privileged Operations

sudo/system-level changes are the one non-negotiable refusal; user-space installs and conda env setup are allowed. See Refusal Rules — Read First.

6. Prefer Low-Maintenance, Hard-to-Break Designs

When adding an API — a setter, endpoint, parameter, or a layer that wraps another — favor the design that keeps a single source of truth and has no silent-failure path:

• Derive, don't duplicate. A second copy of a surface (a hand-maintained list of the methods/fields another layer already defines, or shadow state kept in parallel with the real data) drifts the moment someone forgets to update it. Derive it from the single source instead, so there is nothing to keep in sync.
• Fail loud, not silent. Prefer a design where forgetting a step is caught automatically over one that quietly does the wrong thing. When a mechanism leans on a convention, add a test that asserts full coverage, so a case that slips the convention fails CI instead of silently misbehaving.

Before You Start: Required Questions

Ask these if not already clear:

1. What are you trying to change?

   • Solver algorithm/performance?
   • Python API?
   • Server endpoints?
   • Documentation?
   • CI/build system?

2. Do you have the development environment set up?

   • Built the project successfully?
   • Ran tests?

3. Is this for contribution or local modification?

   • If contributing: will need to follow DCO signoff

4. Which branch should this target?

   • During development phase: main
   • During burn down: release/YY.MM (e.g., release/26.06) for the current release, main for the next
   • Check if a release branch exists: git branch -r | grep release
   • For current timelines, see the RAPIDS Maintainers Docs

Project Architecture

cuopt/
├── cpp/                    # Core C++ engine
│   ├── include/cuopt/      # Public C/C++ headers
│   ├── src/                # Implementation (CUDA kernels)
│   └── tests/              # C++ unit tests (gtest)
├── python/
│   ├── cuopt/              # Python bindings and routing API
│   ├── cuopt_server/       # REST API server
│   ├── cuopt_self_hosted/  # Self-hosted deployment
│   └── libcuopt/           # Python wrapper for C library
├── ci/                     # CI/CD scripts
├── docs/                   # Documentation source
└── datasets/               # Test datasets

Supported APIs

Safety Rules (Non-Negotiable)

Minimal Diffs

• Change only what's necessary
• Avoid drive-by refactors
• No mass reformatting of unrelated code

No API Invention

• Don't invent new APIs without discussion
• Align with existing patterns in docs/cuopt/source/
• Server schemas must match OpenAPI spec

Don't Bypass CI

• Never suggest --no-verify or skipping checks
• All PRs must pass CI

CUDA/GPU Hygiene

• Keep operations stream-ordered
• Follow existing RAFT/RMM patterns
• rmm::device_uvector tests emptiness with is_empty(); it has no empty() member.
• No raw new/delete - use RMM allocators
• Prefer modern CCCL bit/math helpers in kernels (cuda::bitfield_extract, cuda::bitmask, pow2 utilities) over hand-rolled %// by runtime powers of two — see references/conventions.md

Build & Test

Pre-flight Checks (Required Before First Build or Test)

Skipping any of these surfaces as confusing runtime errors later. Run them in order:

1. Check CUDA driver compatibility. Run nvidia-smi and read the CUDA Version in the top-right corner — that's the maximum CUDA your driver supports. Pick a conda env file from conda/environments/all_cuda-<ver>_arch-<arch>.yaml whose CUDA major version is ≤ that. A mismatch builds successfully but fails at runtime inside RMM with cudaMallocAsync not supported with this CUDA driver/runtime version — verify this before the build, not after.
2. Create and activate the conda env before any build, test, or pre-commit command — this is allowed and expected (see Refusal Rules). Use a local prefix env (./.cuopt_env) per CONTRIBUTING.md, with the env file you picked in step 1 (mamba is recommended and faster; swap in conda if mamba isn't available):

   mamba env create -p ./.cuopt_env --file conda/environments/all_cuda-<ver>_arch-$(uname -m).yaml
   mamba activate ./.cuopt_env   # or: conda activate ./.cuopt_env
   

   Tests link against libraries compiled inside that env; a fresh shell without activating it hits cryptic linker errors.
3. Set PARALLEL_LEVEL if RAM is constrained — see references/build_and_test.md. The default $(nproc) can OOM mid-build because CUDA compilation needs ~4–8 GB per job.
4. For tests, fetch datasets first. cuOpt tests need MPS files not in the repo — follow the dataset download steps in CONTRIBUTING.md ("Building for development" section) and export RAPIDS_DATASET_ROOT_DIR.

Quick Reference

./build.sh             # Build everything
./build.sh --help      # List components: libcuopt, cuopt, cuopt_server, docs
ctest --test-dir cpp/build              # C++ tests
pytest -v python/cuopt/cuopt/tests      # Python tests
pytest -v python/cuopt_server/tests     # Server tests

For component-specific build commands, run-test detail, and PARALLEL_LEVEL configuration, see references/build_and_test.md.

Download test datasets before running tests

cuOpt tests depend on MPS/data files that are not checked into the repo. A missing dataset surfaces as a MPS_PARSER_ERROR ... Error opening MPS file test failure at 0ms — it is not a build or logic failure.

Before running any C++ or Python tests, follow the dataset download and RAPIDS_DATASET_ROOT_DIR export steps in the repo's CONTRIBUTING.md ("Building for development" section) — that is the canonical list and mapping.

If a test fails with a missing-file error, run the matching download step from CONTRIBUTING.md and re-run the test. Do not report missing-dataset failures back to the user as the task outcome.

Python Bindings

cuOpt uses Cython to bridge Python and C++. See references/python_bindings.md for the full architecture, parameter flow walkthrough, key files, and Cython patterns.

Contributing — Commits, PRs, Common Tasks

For pre-commit setup, DCO sign-off (git commit -s), the fork-based PR workflow, the draft-PR rule for agents, PR-description rules (keep it short — no "how it works" walkthroughs or file tables), script and CI/workflow authoring principles (extend existing files before adding new ones; no speculative flags, restated defaults, or silent fallbacks), and step-by-step common-task recipes (adding a solver parameter, dependency, server endpoint, or CUDA kernel), see references/contributing.md.

Coding Conventions

Use _Float128, never long double, whenever extended precision arithmetic is required; long double is architecture-dependent and uses x87 on x86.

For C++ naming (snake_case, d_/h_ prefixes, _t suffix), file extensions (.hpp/.cpp/.cu/.cuh and which compiler each uses), include order, Python style, error handling (CUOPT_EXPECTS, RAFT_CUDA_TRY), memory management (RMM patterns, no raw new/delete), CCCL bit/math helpers in device code, test-impact rules, volatile-comment rules (hardware names and self-referential issue/PR numbers in comments or skip messages go stale; issue links to a separate tracking issue are fine), no large local lambdas (extract named helpers instead), and coarse work-estimate / time-limit gating (phase/outer-loop only; no fine inner-loop or double checks), see references/conventions.md.

OpenMP task/runtime compatibility

Treat #pragma omp task if(...) firstprivate(...) with a non-trivial C++ capture as runtime-ABI-sensitive. In affected LLVM libomp versions, the GCC GOMP_task compatibility path skips the GCC copy function for an included (if(false)) task, so the task body can observe an unconstructed object. Branch explicitly instead: create the OpenMP task only when deferral is wanted, and call the body synchronously otherwise.

When diagnosing OpenMP-only failures, test compiler/runtime pairs separately. A Clang + libomp pass exercises the __kmpc_* ABI and does not cover GCC + libomp's GOMP_* path; reduce suspicious cases to a direct runtime-ABI probe before attributing them to solver logic.

PCG random number generator

cpp/src/utilities/pcgenerator.hpp (cuopt::pcgenerator_t) is copied from RAFT's PCGenerator (raft/random/detail/rng_device.cuh), duplicated only because the RAFT header pulls in CUDA and therefore cannot be included from a .cpp. Treat the generator core as frozen. Do not "clean it up", modernise it, or swap it for <random>: reproducibility under settings.random_seed and settings.deterministic holds only while the byte-for-byte output sequence is preserved, and the CPU copy must keep producing the same stream as the GPU one. Adding a new helper that consumes next_u32()/next_double() is fine; changing how those values are produced is not.

Each of the following looks like a defect or an obvious simplification, and is neither. Leave them alone:

• stream = (subsequence << 1u) | 1u in set_seed. A 2^64 LCG has full period only when its increment is odd — the | 1u is what guarantees that — and the << 1u is why two subsequences must differ in their low 63 bits to get independent streams.
• The two next(discard) warm-up calls straddling state += seed. This is PCG's canonical seeding sequence, and it is what decorrelates adjacent seeds — which is exactly how cuOpt seeds workers (settings.random_seed + pcgenerator_t::default_seed + rng_offset + worker_id, branch_and_bound/worker.hpp). Collapsing it to state = seed makes neighbouring workers draw near-identical prefixes.
• The output permutation in next_u32 (>> 18u, ^, >> 27u, rot = oldstate >> 59u, (-rot) & 31u) — the PCG-XSH-RR constants for a 64→32-bit output. In particular (-rot) & 31u relies on unsigned wraparound; rewriting it as 32 - rot is a shift by 32, i.e. undefined behaviour, whenever rot == 0.
• The multiplier 6364136223846793005ULL, which appears in both next_u32 and as h in skipahead. skipahead is the closed-form LCG jump (Brown's arbitrary-stride method) and matches N calls to next_u32 only while the two constants are identical.
• The >> 8 / >> 11 in next_float / next_double. They yield exactly 24 and 53 mantissa bits, so the result lies in [0, 1). Dividing by UINT32_MAX instead makes 1.0 reachable and breaks every rng.next_double() * n used as an index.
• The sign-bit masks in next_i32 / next_i64 (& 0x7fffffff…); callers rely on the results being non-negative.
• uniform()'s floating-point scaling and shuffle()'s Fisher-Yates order. uniform()'s slight bias is documented and accepted; substituting a modulo or std::uniform_int_distribution changes every sampled sequence.

Troubleshooting & CI

For build/test pitfalls (Cython rebuild, OOM, CUDA driver mismatch, missing nvcc) and CI failure diagnostics (style checks, DCO failures, dependency drift), see references/troubleshooting.md.

Key Files Reference

Canonical Documentation

• Contributing/build/test: CONTRIBUTING.md
• CI scripts: ci/README.md
• Release scripts: ci/release/README.md
• Docs build: docs/cuopt/README.md
• Python binding architecture: references/python_bindings.md

Shell-execution, install, conda-env, and sudo policies are covered by Refusal Rules — Read First at the top of this skill.

VRP dimension internals (routing engine)

When implementing or debugging VRP dimensions (constraints, objectives, forward/backward propagation, combine, local-search deltas), read:

• references/vrp_skills.md — architecture contracts, required interfaces, and implementation checklist.

Read it before adding a new dimension or changing combine semantics.

Budgeting a solver stage and attributing regressions

When adding or tuning a limit on a stage that shares the solve's time budget (presolve, probing, cut generation, heuristics), or when a benchmark regression looks like it came from one, read:

• references/stage_budgets.md — why a deterministic work counter does not bound wall time, how to size a limit so the stage concludes before its ceiling, and the ordered checks that attribute a regression to a budget before tuning its value.

Run the attribution checks before changing a limit's value — the common failure is tuning a limit that was never what bound.

Numerical issues in non-routing solver internals

When a bug surfaces as wrong-but-plausible solver output (invalid lower bound, unexpectedly large duals, 10× iteration blow-up after a small change) rather than a crash, read:

• resources/numerical_debugging.md — methodology for locating catastrophic-cancellation sites, the cancellation patterns endemic to cMIR / flow-cover / MIR-style cut construction, and threshold guidance for numerical guards.

Apply the instrument-first, guard-at-the-exact-site workflow it describes before patching — speculative fixes on these symptoms usually miss.