# Step 5: implementation-plan

## Read These Files First

Read the following files before editing:

- `/AGENTS.md`
- `/docs/PRD.md`
- `/docs/ARCHITECTURE.md`
- `/docs/ADR.md`
- `/docs/SOLVER_AGENT_DESIGN.md`
- `/docs/implementation-plans/README.md`
- `/docs/requirements/euler-beam-3d.md`
- `/docs/research/euler-beam-3d-research.md`
- `/docs/formulations/euler-beam-3d-formulation.md`
- `/docs/numerical-reviews/euler-beam-3d-review.md`
- `/docs/io-definitions/euler-beam-3d-io.md`
- `/docs/reference-models/euler-beam-3d-reference-models.md`
- `/src/fesa/model/element.hpp`
- `/tests/unit/model_element_test.cpp`

## Task

Create `/docs/implementation-plans/euler-beam-3d-implementation-plan.md`.

The implementation plan must be ready for C++ TDD work and must include:

- readiness check for requirements, research, formulation, numerical review, I/O, reference model contract
- explicit kernel-first implementation scope
- tasks matching the remaining phase steps:
  - model beam topology
  - local stiffness kernel
  - global transform and end-force recovery
  - build/test report
  - release readiness note
- test IDs and RED/GREEN conditions for each production change
- candidate files:
  - `src/fesa/model/element.hpp`
  - `src/fesa/model/element.cpp`
  - `tests/unit/model_element_test.cpp`
  - `src/fesa/elements/euler_beam_3d.hpp`
  - `src/fesa/elements/euler_beam_3d.cpp`
  - `tests/unit/euler_beam_3d_local_stiffness_test.cpp`
  - `tests/unit/euler_beam_3d_transform_recovery_test.cpp`
- CTest commands:
  - `ctest --test-dir build/msvc-debug --output-on-failure -C Debug -R model_element_test`
  - `ctest --test-dir build/msvc-debug --output-on-failure -C Debug -R euler_beam_3d_local_stiffness_test`
  - `ctest --test-dir build/msvc-debug --output-on-failure -C Debug -R euler_beam_3d_transform_recovery_test`
- acceptance traceability from requirements to tests

Do not create C++ files in this step.

## Tests To Write First

- No C++ test is required in this documentation-only step.

## Acceptance Criteria

```powershell
python -m unittest discover -s scripts -p "test_*.py"
python scripts/validate_workspace.py
```

## Verification Notes

1. Confirm every production file named in the plan has a related test.
2. Confirm the plan does not ask Implementation Agent to change requirements, formulation, I/O contracts, reference artifacts, or tolerance policies.
3. Update `phases/euler-beam-3d/index.json` step 5:
   - success: `"status": "completed"`, `"summary": "3D Euler beam implementation plan added"`
   - failure after retries: `"status": "error"`, `"error_message": "<specific error>"`
   - blocked: `"status": "blocked"`, `"blocked_reason": "<specific reason>"`

## Forbidden

- Do not modify source, tests, reference artifacts, or CMake files.