implementation-strategy

name	implementation-strategy
description	Classify and place pyfixest estimator, inference, formula, API, and core changes before implementation.

Choose the implementation strategy

Use this skill before changing a public estimation surface or shared estimation code. Read AGENTS.md and the nearest implementation and tests first.

Classify the change

Choose exactly one primary placement:

• Estimator add-on: standalone API or domain module composing stable primitives. This is the default for a new estimator.
• Post-estimation: standalone numerical module with thin fitted-model wrappers.
• Shared primitive: generic internal operation with a real shared consumer.
• Backend kernel: measured, non-vectorizable hot loop with a readable reference implementation where feasible.
• Core change: modification to formula planning, model matrices, generic fit orchestration, inference contracts, or result interfaces.

Do not put estimator-specific switches into the generic runner or numerical logic into model classes. Stop for maintainer design approval before expanding core merely to accommodate one new estimator.

Produce the strategy

Before editing, record:

1. the primary classification and target module;
2. the nearest in-repo precedent;
3. public API and result-object impact;
4. reused primitives and any proposed shared primitive;
5. behavior for aweights, fweights, fixed effects, IV, multiple estimation, lean, store_data, and relevant backends;
6. explicit unsupported paths and their errors;
7. the external numerical reference and permanent-test location;
8. documentation, exports, and changelog wiring.

If the support matrix or external reference is unresolved, resolve it before implementation rather than allowing the code to choose policy implicitly.