[juaristi22]

Fixes #2137

Saved reforms and Scenarios that set the removed gov.dwp.carer_premium.couple parameter now receive migration directions naming gov.dwp.carer_premium.single, the per-person weekly amount. They previously failed with a bare lookup error.

Register the removed path with the existing generic alias installer. Valid per-person reforms and the State Pension/OBR migration aliases retain their behavior; no benefit formula, policy value or alias-installation function changes.

Validation: 179 focused tests passed (6 new regressions and 173 existing migration, reform, Scenario and carer controls). Before the change, the new regressions produced 5 failures and 1 passing control. make format, Ruff and git diff --check passed. The tests cover the public entry points, correct alias parent, hidden removed-parameter metadata and valid replacement reforms.

Local Python 3.13.14 / Core 3.32.27 differs from locked Core 3.33.2. Full make test, documentation builds and private-data simulations were not run for this diagnostic-only registry change; hosted full CI remains required.

Reproducer: policyengine_uk/tests/test_removed_carer_premium_parameter.py::test_removed_carer_premium_reforms_give_migration_directions

axiom: n/a: migration diagnostics only; no policy provision, parameter value or benefit calculation changes.

[vahid-ahmadi]

Looks right, fine to approve.

@juaristi22, this is a small, correct change. Main already resolves each removed path's parent from the path itself, so the one new REMOVED_PARAMETERS entry is all the fix needs. The message points to the right replacement: carer_premium.single is the per-person amount, and carer_premium.py:73 pays it once for each qualifying claimant or partner.

1. nit, policyengine_uk/utils/parameters.py:163: #2490 also edits this file. I merged the two branches locally and they merge cleanly. Whichever lands second should still rerun this test file.

What I checked

• Read the issue, the PR body and the full diff. Read add_removed_parameter_aliases on main and confirmed it already walks the parent path.
• Ran pytest policyengine_uk/tests/test_removed_carer_premium_parameter.py: 6 passed. With utils/parameters.py reverted to main: 5 failed, 1 passed (the valid-reform control).
• ruff check and ruff format --check on both changed Python files: clean.
• Hosted CI on 413de23 is green.
• Process: the changelog fragment is .fixed.md; the Reproducer and axiom lines are present; the Fix evidence check is green.
• Not checked: the policyengine.py or API layers that read the parameter tree.

🤖 Generated with Claude Code

[vahid-ahmadi]

Approving, per my review above. The shoulds and nits can follow.