Implement automated semantic drift detection for golden corpus stability

[Copilot]

Semantic Drift Detection Implementation - COMPLETE ✅

Rebase Status

✅ Rebased on latest main - Branch now includes Golden Corpus Determinism test fix from PR #34

Implementation Complete

All 49 tests passing, including:

• 7 new drift detection tests
• 42 existing tests (corpus, contract, observability)

The branch has been successfully rebased to pick up the corrected governance-invariants.yml workflow that uses pytest-json-report for deterministic test output.

Phase 1: Historical Baseline System ✅

• Baseline snapshot capture with metadata
• CLI: python -m base120.drift.capture_baseline

Phase 2: Drift Detection Logic ✅

• Snapshot comparison with 4 drift types
• CLI: python -m base120.drift.compare

Phase 3: CI Workflow Integration ✅

• .github/workflows/drift-detection.yml
• PR validation, main branch capture, periodic monitoring

Phase 4: Alerting and Documentation ✅

• docs/drift-detection.md - comprehensive guide
• artifacts/README.md - snapshot specification

Phase 5: Testing and Validation ✅

• Full test coverage
• Security validation
• Rebase on latest main

Original prompt

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This section details on the original issue you should resolve

<issue_title>Introduce Automated Semantic Drift Detection for Golden Corpus Stability</issue_title>
<issue_description>Establish a new workstream for automated detection of semantic drift in Base120:

• Develop CI-ready scripts/workflows to periodically rerun golden corpus validations and compare outputs to historical snapshots
• Flag, document, and block (where appropriate) unintentional semantic/output drift, even within frozen schemas/releases (e.g., v1.x)
• Codify the drift detection protocol and how it is monitored/audited over time

Outcome: Golden corpus invariants are infrastructure-enforced, with explicit drift detection, alerting, and recovery protocols for high stability.</issue_description>

<agent_instructions>Implement semantic drift detection infrastructure:

Phase 1: Historical Baseline System

• Create artifacts/golden_corpus_snapshots/ directory structure
• Implement snapshot storage mechanism (versioned by commit SHA + date)
• Add script to capture baseline: python -m base120.drift.capture_baseline
• Store snapshots with metadata (schema version, Base120 version, timestamp)

Phase 2: Drift Detection Workflow

• Create .github/workflows/drift-detection.yml for periodic runs
• Implement comparison logic: python -m base120.drift.compare
• Generate diff reports highlighting:
  • Encoding changes (same input, different output)
  • Semantic changes (structural differences in output)
  • Performance drift (timing variations)
• Use BASE120_FIXED_TIMESTAMP for deterministic comparisons

Phase 3: Alerting and Documentation

• Add drift threshold configuration (when to flag vs. block)
• Implement PR comment integration for drift reports
• Create docs/drift-detection.md explaining:
  • How drift detection works
  • How to update baselines (intentional changes)
  • Recovery protocols for unintentional drift
• Wire to governance classifier from PR Formalize governance as CI-enforced contract with automated classification and invariant validation #26

Build on Golden Corpus Determinism check from PR #26. Focus on semantic changes, not just byte-level.</agent_instructions>

Comments on the Issue (you are @copilot in this section)

• Fixes Introduce Automated Semantic Drift Detection for Golden Corpus Stability #22

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[github-actions]

🔍 Governance Classification

Change Class: potential-breaking (Impact Level: 5)

Description: Changes that may break backward compatibility

Evidence Requirements

• ✅ Full test suite passes
• ✅ Migration guide for users
• ✅ Audit update with detailed rationale
• ✅ Version bump justification
• ⚠️ v1.0.x: Breaking changes PROHIBITED

Review Requirements

• Minimum Reviewers: 3 external + CODEOWNER approval
• Audit Update Required: ✅ Yes

Next Steps

1. ✅ Ensure audit update in GOVERNANCE.md or CHANGELOG
2. ✅ Provide impact analysis in PR description
3. ✅ Wait for required approvals (3 + CODEOWNER)
4. ✅ Address any CI check failures

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📚 See GOVERNANCE.md for detailed requirements.

[hummbl-dev]

@copilot The Golden Corpus Determinism test is failing on this PR because it's using the old workflow from before the fix was merged in PR #34, and main now has the corrected workflow file. Please rebase this PR on top of the latest main branch to pick up the fixed workflow, which will resolve the failing check.

[Copilot]

@copilot The Golden Corpus Determinism test is failing on this PR because it's using the old workflow from before the fix was merged in PR #34, and main now has the corrected workflow file. Please reb...

Rebased on latest main (commit 3fbe537) which includes the Golden Corpus Determinism fix from PR #34. All 49 tests passing. The branch is ready - commits 062b6a9 through 4a7f441 now include the corrected workflow file.