vertical-slice

Resolve every role through ../../../.codex/docs/plugin-agent-delegation.md; do not require a repository-local .codex/agents/ or .codex/agent-packs/ tree. Before default delegation, run python3 ../../../tools/codex_studio/agent_delegation.py resolve --project-root <project-root> --role <role> and use only its returned role contract.

Codex-native operating rules

Ask at most one user question per turn and wait for the answer. Preserve incremental approval for material concept and design sections. Identify the intended artifact paths before authoring; approval of a section or bounded changeset authorizes writing that approved content, so do not ask again per line or per file. Pause again for a new design decision or scope expansion.

Purpose

The vertical slice answers a different question from the concept prototype: "Can we build this full game loop at production quality, on schedule?"

Default use — run late in Pre-Production, after GDDs, architecture, and UX specs are complete. It is a near-production-quality build demonstrating one complete [start → challenge → resolution] cycle.

Post-pivot? If a PIVOT verdict from an earlier vertical slice sent you back to revise GDDs and architecture, run this again after revisions to re-validate. It can be run as many times as needed until a PROCEED or KILL verdict is reached.

It validates:

1. The pipeline (can the team actually produce this quality of content?)
2. Execution feasibility (are the architecture decisions correct for this game?)
3. Fun survival (does the fun from the concept prototype survive full design?)
4. Velocity (how long did this take? That's your real production rate estimate.)

Earlier in the project? If you haven't written GDDs yet and want to validate whether the core idea is worth designing, run $prototype (concept prototype) instead.

Phase 1: Resolve Review Mode and Load Context

Resolve the review mode:

1. If --review [full|lean|solo] was passed → use that
2. Else read review_mode from .codex/studio.toml
3. Map review_mode = "phase-gated" to lean optional-review depth; mandatory director gates still run
4. If the config is unavailable or malformed, report it and use phase-gated behavior without writing configuration

See ../../../.codex/docs/director-gates.md for the full check pattern.

Read the following files to understand the full design intent:

• AGENTS.md — tech stack and engine
• design/gdd/game-concept.md — core fantasy and game pillars
• design/gdd/systems-index.md — MVP systems and their priorities
• docs/architecture/architecture.md — layer structure
• docs/architecture/control-manifest.md — technical rules for implementation
• Key GDDs for the systems being sliced

Phase 2: Define the Slice Scope and Validation Question

Before building, define the falsifiable validation question:

"Does a player, starting from nothing, experience [core fantasy from game-concept.md] within [N] minutes, without developer guidance — and can we build one such loop in [X] days at representative quality?"

Both parts matter: player experience AND build feasibility.

Scope discipline:

• Include ALL core loop systems (minimum). If a system is required to complete one [start → challenge → resolution] cycle, it must be in the slice.
• Target scope: 3–5 minutes of polished, continuous gameplay. This is the industry-standard vertical slice length — long enough to demonstrate mechanics and tone, short enough to build at representative quality. If your slice would take longer than 5 minutes to play through, cut content, not quality.
• Cut scope before cutting quality. A low-quality slice that looks nothing like the intended game cannot validate production feasibility.
• If the scope feels too large to build in 1–3 weeks, the slice scope is wrong — not too big to build, but the slice is trying to prove too much at once.

Scope creep warning: The vertical slice is the highest-risk moment for scope creep in the pre-production phase. Features feel "almost there" and it's tempting to add "just one more system." Resist this. Cut, do not extend.

Present scope to the user before building and get confirmation.

Phase 3: Plan the Build

Define in bullet points:

• Systems implemented (which GDD sections are being exercised)
• The complete game loop cycle ([start] → [challenge] → [resolution] exactly)
• Art and audio quality level (placeholder acceptable, representative preferred)
• Specific, measurable success criteria for the validation question
• Hard time limit: [X] days. If exceeded, scope was wrong — stop and reassess.

Ask the user to confirm scope before building.

Once confirmed, write a session checkpoint to production/session-state/active.md (create production/session-state/ if it does not exist). Include: concept name, validation question, systems in scope, art quality level, and current phase ("Phase 4 — Implement"). Update this file at the end of each build day with what was completed. This is the primary recovery mechanism if the session ends mid-slice — multi-week Engine builds will span many sessions.

Phase 4: Implement

Ask: "May I create the vertical slice directory at prototypes/[concept-name]-vertical-slice/ and begin implementation?"

If yes, create the directory. Every file must begin with:

// VERTICAL SLICE - NOT FOR PRODUCTION
// Validation Question: [What this build is proving]
// Date: [Current date]

Quality standards — higher than concept prototype, not full production:

• Follow architecture layers from docs/architecture/control-manifest.md
• Naming conventions from .codex/docs/technical-preferences.md
• No hardcoded gameplay values — use constants or config files
• Basic error handling on critical paths
• Placeholder art acceptable; representative art preferred

Multi-turn loop: After writing the initial files, ask the user to run the build and report what they observe. Iterate until the complete game loop cycle is demonstrable. Each round:

1. User runs → reports errors or observations
2. Agent fixes errors or adjusts systems
3. Repeat until the full [start → challenge → resolution] cycle is playable

Sunk cost checkpoint (day 3 of planned timeline): If the full game loop cycle is not yet demonstrable, stop and reassess. Either the scope is too large or an architectural assumption is wrong. Surface the blocker explicitly rather than continuing to iterate.

Conduct at least 1 playtest session once the loop is demonstrable.

Playtesting tip: If you can get anyone who hasn't seen the game to play it — a friend, family member, online community — watch them silently without explaining anything. Don't guide them. Their confusion reveals what the game isn't communicating on its own. This gives much better signal than self-testing.

No external testers available? Use rotation within the team: Dev A built system X, so Dev A is a naive tester for system Y. Even a two-person team can rotate effectively. Solo? Step away for 2-3 days then play through as a new player — you won't have perfect first-impression signal but you'll catch the critical blockers. Also try a "silent walkthrough": play your own slice in one sitting without stopping to fix anything and log every moment you hesitate.

Want richer observation data? Ask the tester to think aloud as they play — narrate what they're doing and why in real time. "I'm trying to figure out how to attack... I pressed E... nothing... is it click?" This surfaces confusion the instant it occurs rather than in retrospect. Best for onboarding and UI clarity validation. Silent observation is still better for feel testing; think-aloud changes the experience slightly but produces far more granular UX data.

Async remote option: Record a Loom or OBS session — give someone the build, ask them to record their screen + audio, and send you the video. You get genuine first-impression data without synchronous scheduling. Works across timezones.

Testing AI, NPC, or complex system behavior before it's fully implemented? Use the Wizard of Oz technique: one person plays normally while a second person secretly controls the NPC or system behavior in real time. The player believes it's automated. This validates the design intent of an AI or economy system before the implementation is complete — and reveals exactly what behaviors the system must produce to feel correct. Particularly useful for vertical slices where an AI system is in scope but not yet polished enough for unguided testing.

Phase 5: Playtest Debrief

The loop is demonstrable. Before writing the report, collect structured observations from actually playing it. Do NOT skip to report generation — the report is only as good as the observations you capture here.

Say exactly this:

"Play through the complete [start → challenge → resolution] cycle from scratch, as if you're a new player with no knowledge of how it was built. Don't skip ahead or use developer shortcuts. Come back when you've completed the full loop — or when you've hit something that stopped you."

Once the user returns, ask these questions one at a time:

1. Loop completion:

   "Did you complete the full [start → challenge → resolution] cycle on your own, without needing any guidance from me or prior knowledge of the build?"

2. Time check:

   "How long did it take to reach the first meaningful action — the first moment where you felt like you were actually playing the game?"

3. Core fantasy:

   "The game is supposed to make you feel [core fantasy from game-concept.md]. Did it? Be honest — not 'kind of' but specifically what you felt and when."

4. Blockers:

   "What stopped you, confused you, or pulled you out of the experience? Any moment where you weren't sure what to do, or where something broke?"

5. Pipeline check:

   "As the developer — not the player — does this feel achievable at this quality for the full game? What surprised you about how long things took to build?"

6. Verdict:

   "PROCEED, PIVOT, or KILL — and the specific reason."

If any answer is vague, ask: "Can you give me the specific moment where that happened?" Precise observations populate the report. Vague ones produce a useless report.

Phase 6: Generate Vertical Slice Report

Track velocity throughout the build. Log:

• Day 1: what was built
• Day 2: what was built
• etc.

This is the most honest data you will ever have about your production rate. Do not skip it. It feeds directly into sprint planning.

Read ../../../.codex/docs/templates/vertical-slice-report.md to get the report structure. If the template file is not found, use this fallback structure:

- `## Vertical Slice Report — [Game Title] — [Date]`
- `### Executive Summary` (PROCEED / PIVOT / STOP verdict + 2-sentence rationale)
- `### Core Loop Validation` (what was tested, what passed, what failed)
- `### Feel Assessment` (animation, controls, feedback — subjective notes)
- `### Technical Findings` (performance, engine issues, architectural risks)
- `### Velocity Log` (day-by-day actual progress — do not skip)
- `### Recommended Next Steps`

Fill in every section based on what was observed and built during this session.
The velocity log must reflect actual day-by-day progress, not estimates — this is
the most honest production rate data you will ever have. Replace all placeholder
text with real observations.

### Lessons Learned
- What assumptions were broken by actually building to near-production quality?
- What surprised us about the pipeline or architecture?
- What would we change about the slice scope if we ran this again?

Ask: "May I write this report to prototypes/[concept-name]-vertical-slice/REPORT.md?"

If yes, write the file. Then update prototypes/index.md (create if it does not exist) — append one row to the vertical slice table: concept name, date, verdict, and a link to the REPORT.md. Note whether this was a first-run slice or a re-run after a PIVOT. The velocity log in this report is some of the most valuable data in the project — cross-reference it with sprint estimates.

Phase 7: Creative Director Review

Review mode check:

• solo → skip. Note: "CD-PLAYTEST skipped — Solo mode."
• lean → skip (not a PHASE-GATE). Note: "CD-PLAYTEST skipped — Lean mode."
• full → delegate to creative-director through Codex custom-agent delegation using gate CD-PLAYTEST (../../../.codex/docs/director-gates.md).

Pass: the full REPORT.md content, the validation question, game pillars and core fantasy from design/gdd/game-concept.md.

The creative director evaluates the vertical slice result against the game's creative vision and pillars, then confirms, modifies, or overrides the recommendation. Their verdict is final. Update REPORT.md if the verdict differs.

Phase 8: Summary and Next Steps

Output a summary: the validation question, velocity data, and final recommendation. Link to prototypes/[concept-name]-vertical-slice/REPORT.md.

If PROCEED: Your vertical slice validated the full game loop. The project is ready for Production.

Recommended next steps:

• $create-epics layer:foundation — plan Foundation layer epics
• $create-epics layer:core — plan Core layer epics
• $create-stories [epic-slug] — break each epic into implementable stories
• $sprint-plan — plan the first sprint using velocity data from the slice
• $gate-check pre-production — formally advance the stage to Production

Playtest note: $gate-check will look for documented playtest evidence. At minimum, 1 documented session with a REPORT.md showing PROCEED is required to pass the gate. More sessions give more reliable signal — 3+ is recommended before committing the full team to Production, but is not a hard gate.

If PIVOT:

Before routing back to GDD revision, capture the carry-forward note. Ask these two questions (plain text, one at a time):

1. "What systems or mechanics worked at this quality level and should be preserved in the revised design?"
2. "What specifically failed — the core loop, the architecture, the pipeline, or the fun?"

Ask: "May I write this to prototypes/[concept-name]-vertical-slice/PIVOT-NOTE.md?"

If yes, write the file with: what worked, what failed, the specific systems or architecture decisions that need revision, and what the next slice should prove differently. When $vertical-slice is next run after a PIVOT, check the prototypes/ directory for a PIVOT-NOTE.md — use it to frame the new validation question and inform scope decisions.

• Revise affected GDDs with $design-system [mechanic]
• Address architecture issues via $architecture-decision
• Then re-run $vertical-slice to validate the revised direction

If KILL:

Before abandoning the concept, confirm the verdict is sound:

• Full game loop takes >5 minutes even for an experienced player?
• No emotional high point (delight, surprise, satisfaction) observed in any playtest session?
• 50%+ of testers confused or stuck at the same point after 2+ slice attempts?
• Architecture issues would require rebuilding more than 50% of what was built?
• This is the 3rd vertical slice attempt on the same concept?

If 2+ boxes apply → KILL verdict is sound. If 0–1 apply → one targeted PIVOT may recover the concept.

Document the kill in prototypes/GRAVEYARD.md (create if it doesn't exist). Ask: "May I append this to prototypes/GRAVEYARD.md?" If yes, add one entry:

## [Concept Name] Vertical Slice — YYYY-MM-DD
- **Kill reason:** [what specifically prevented the player from experiencing the core fantasy]
- **What worked at slice quality:** [systems or mechanics that held up]
- **What failed:** [core loop issue, architecture decision, or pipeline blocker]
- **Next time:** [one specific change for the next time a similar concept is attempted]

• Return to $brainstorm with what you learned
• Or run $prototype [new-concept] to test a new direction cheaply first

Important Constraints

• Vertical slice code must NEVER be refactored into production — it is reference only
• Production code must NEVER import from prototypes/
• If recommendation is PROCEED, production implementation is written from scratch using the slice as a design reference only
• Scope cuts are acceptable; quality cuts are not — a low-quality slice proves nothing
• Total effort: 1–3 weeks. If longer, scope is too large — cut the slice, not the quality.
• Day 3 sunk cost rule: if the full game loop cycle is not demonstrable by then, stop and surface the blocker
• Networked/multiplayer games: A local vertical slice cannot validate the feel of a networked mechanic. Latency fundamentally changes how combat, movement, and prediction feel — testing locally at 0ms will feel entirely different at 80ms network delay. The slice can validate that the game loop is interesting and complete; it cannot validate that networked mechanics feel good under real conditions. Network feel requires real peers or simulated latency.