<step name="regression_gate">
Run prior phases' test suites to catch cross-phase regressions BEFORE verification.

**Skip if:** This is the first phase (no prior phases), or no prior VERIFICATION.md files exist.

**Step 1: Discover prior phases' test files**
```bash
# Find all VERIFICATION.md files from prior phases in current milestone
PRIOR_VERIFICATIONS=$(find .planning/phases/ -name "*-VERIFICATION.md" ! -path "*${PHASE_NUMBER}*" 2>/dev/null)
```

**Step 2: Extract test file lists from prior verifications**

For each VERIFICATION.md found, look for test file references:
- Lines containing `test`, `spec`, or `__tests__` paths
- The "Test Suite" or "Automated Checks" section
- File patterns from `key-files.created` in corresponding SUMMARY.md files that match `*.test.*` or `*.spec.*`

Collect all unique test file paths into `REGRESSION_FILES`.

**Step 3: Run regression tests (if any found)** — Read and execute `gsd-core/workflows/execute-phase/steps/regression-gate-run.md`. It resolves the project test command, normalizes it to a one-shot form (defeating vitest/jest watch mode via the shared `normalize-test-command` helper), runs it under `workflow.test_gate_timeout`, and aborts on timeout with a watch-mode hint (#1857). On `REGRESSION GATE ABORTED` (exit 124), HALT — do not proceed to verification.

**Step 4: Report results**

If all tests pass:
```
✓ Regression gate: {N} prior-phase test files passed — no regressions detected
```
→ Proceed to verify_phase_goal

If any tests fail:
```
## ⚠ Cross-Phase Regression Detected

Phase {X} execution may have broken functionality from prior phases.

| Test File | Phase | Status | Detail |
|-----------|-------|--------|--------|
| {file} | {origin_phase} | FAILED | {first_failure_line} |

Options:
1. Fix regressions before verification (recommended)
2. Continue to verification anyway (regressions will compound)
3. Abort phase — roll back and re-plan
```

If `TEXT_MODE` is true, present as a plain-text numbered list and ask the user to type their choice number. Otherwise, use AskUserQuestion to present the options.
</step>