Holdout Validation Skill - Structural Test Separation

Version: 9.0 | Updated: 01-July-2026 | Architect: Karim Bhalwani | Deps: architect, guardian, verification-before-completion

Dependencies

Load the following via read_file before using this skill. Skills marked ★ have disable-model-invocation: true and cannot self-invoke - they must be loaded explicitly.

• ~/.copilot/skills/architect/SKILL.md - spec and holdout scenario authoring patterns (required for the Architect role in this workflow)
• ~/.copilot/skills/guardian/SKILL.md - holdout evaluation is a Guardian-owned phase; QA patterns apply
• ~/.copilot/skills/verification-before-completion/SKILL.md ★ - completion gate; confirm all holdout scenarios are evaluated before declaring review complete

When to Load This Skill

Load this skill when:

• Architect is producing a specification and needs to write holdout acceptance scenarios
• Guardian is reviewing an implementation that has associated holdout files in .copilot/holdout/
• Validating that structural blindness between implementation and evaluation is maintained
• Setting up a new project's holdout directory structure

Overview

When the entity writing the code can also read the tests, the separation between implementation and validation is no longer optional. This skill enforces holdout-set discipline: behavioral acceptance scenarios are authored during design, stored separately from the codebase, and evaluated by a structurally independent reviewer (Guardian).

This addresses the documented pattern where reasoning models engage in test gaming - hardcoding return values, rewriting tests to match buggy code, and optimizing for "tests pass" rather than "software works."

Core Principle

A model that can see the answer key will use it. Design the system so it cannot.

Agent Access Rules

These rules are structural constraints, not behavioral suggestions. Reasoning models will use available information regardless of instructions.

Quick Reference

Detailed Rules by Role

Architect – WRITE only

• Authors holdout scenarios during specification
• Spec references the holdout file but does NOT include scenarios inline

Implementation agents (senior-developer, data-engineer, data-scientist, ai-engineer) – NONE

• MUST NOT read files in .copilot/holdout/
• If an implementation agent is given access to or asked to read a file in .copilot/holdout/, the agent MUST respond: "Access denied: holdout files are reserved for Guardian evaluation. Proceeding with spec-based testing only." and continue implementation using only the specification.

Guardian – READ only

• Loads holdout scenarios during review
• Evaluates implementation against them

Context-engineer – READ only

• Tracks holdout pass rates in retrospective documents

Exception: Missing Holdouts

If holdout scenarios don't exist:

• Implementation agents proceed normally with spec-based testing
• Guardian notes "No holdout scenarios found" in the review
• Guardian recommends Architect provide them for future iterations

Holdout Scenario Structure

What Is a Holdout Scenario?

A holdout scenario is an intent-level validation statement, not a unit test. It describes what must be true from the perspective of a real user, not what a function should return.

Scenario Format

## Holdout Scenario: [ID]

**Actor:** [Who is performing the action]
**Intent:** [What they are trying to accomplish]
**Preconditions:** [What must be true before the scenario starts]
**Action:** [What the actor does]
**Success Criteria:**

- [Observable outcome 1]
- [Observable outcome 2]
  **Failure Modes:**
- [What should NOT happen]
  **Priority:** [Critical | High | Medium]

Workflow

During Design (Architect Produces)

1. After completing the specification, the Architect writes 3–10 holdout scenarios per specification document (i.e., per discrete unit of work that has its own spec file)
2. Scenarios focus on user-observable outcomes, not implementation details
3. Scenarios are written to .copilot/holdout/ (or a user-specified directory)
4. Each scenario file is named HOLDOUT-<feature>.md, matching the naming convention in the File Layout section
5. The spec references the holdout file but does NOT include the scenarios inline

During Implementation (Developer Is Blind)

1. Implementation agents receive the spec but never receive the holdout scenarios
2. Implementation agents write their own unit and integration tests based on the spec
3. The structural separation ensures tests written by developers validate the spec, not the holdouts
4. If an implementation agent requests access to holdout files, the request is denied

During Review (Guardian Evaluates)

1. Guardian loads the holdout scenarios from .copilot/holdout/
2. For each scenario, Guardian evaluates whether the implementation satisfies the intent
3. Guardian produces a Holdout Evaluation Report as part of its review
4. Holdout failures are rated as High severity (they indicate spec-to-intent gaps)
5. When one or more holdout scenarios fail, Guardian MUST block the review from passing and raise a finding requiring the implementation agent to address the gap before completion is declared. Guardian should reference the verification-before-completion skill to confirm all failures are resolved.

Holdout Evaluation Report Format

### Holdout Evaluation

| ID    | Actor    | Intent                            | Status | Evidence                                    |
| ----- | -------- | --------------------------------- | ------ | ------------------------------------------- |
| H-001 | Customer | Add item to cart with correct tax | PASS   | Tax calculation verified in checkout flow   |
| H-002 | Admin    | Export audit log for compliance   | FAIL   | Export function exists but omits timestamps |

**Holdout Pass Rate:** X/Y scenarios passed
**Findings:** [Specific gaps between implementation and user intent]

File Layout

.copilot/holdout/
  HOLDOUT-<feature-1>.md    -- Scenarios for feature 1
  HOLDOUT-<feature-2>.md    -- Scenarios for feature 2
  README.md                 -- Explains holdout directory purpose and access rules

The holdout README.md should contain:

# Holdout Validation Scenarios

**Access Rule:** Implementation agents (senior-developer, data-engineer, data-scientist, ai-engineer)
MUST NOT read files in this directory. These scenarios are exclusively for
Guardian evaluation during review.

**Authored by:** Architect agent during specification phase.
**Evaluated by:** Guardian agent during review phase.

When to Use

• Architect is producing a specification for a non-trivial feature
• Guardian is reviewing an implementation that has an associated spec
• Any time the same agent (or agent pipeline) is writing both code and tests
• When validating that software works for real users, not just for the test suite

Definition of Done

• Holdout scenarios written for every feature with a specification
• Scenarios stored in .copilot/holdout/, not in the main codebase
• Implementation agents confirmed to have no access to holdout files
• Guardian evaluation report produced with pass/fail per scenario
• Holdout failures tracked as High severity findings

Constraints

• Does NOT replace unit or integration tests (those are still written by implementation agents)
• Does NOT contain implementation details or code snippets
• Does NOT get loaded by implementation agents (structural blindness is the mechanism)
• Does NOT override Guardian's existing review process (it extends it)

Common Pitfalls

• Writing unit tests as holdouts: Holdouts are intent-level, not instruction-level. "Function returns 200" is a unit test. "User can log in and see their dashboard" is a holdout.
• Leaking holdouts into specs: The spec should reference that holdouts exist, but never include them inline. If the developer can read the spec and infer the holdouts, the separation is weakened.
• Skipping holdouts for "simple" features: Simple features still have user intent. Even a config change has a holdout: "The system behaves differently after this config is changed."
• Guardian skipping holdout evaluation: If holdout files exist in .copilot/holdout/, Guardian MUST evaluate them. Skipping is a review gap.

Integration Points

References

• Example Holdout Scenario - Complete worked example showing 3 holdout scenarios for a user authentication feature