System Design Session Skill

[!IMPORTANT] Run an interactive system design session that turns a product goal into a sized, justified architecture with diagrams, ADRs, a scorecard, and a machine-readable handoff.

Optional args: slug=, ticket=<id/url>, mode=interactive|autonomous|channel, channel=, auto_continue=true|false, profile=business|hybrid|technical.

Instructions

When the user asks to perform this workflow, execute the following steps:

System Design Workflow (Architecture / How Big)

Goal: Produce a capacity-justified architecture baseline that design-solution can turn into contracts.

Steps

1. Load inputs:
   • Load system-design-methodology plus matched siblings (estimation, building-blocks, data-architecture, resilience-ops, review, principles, diagramming).
   • Load PRD or ticket, existing architecture docs, and current traffic/incident data when reviewing an existing system.
2. Classify and announce:
   • Mode: new design | review existing | interview practice.
   • Depth: quick sketch (defaults assumed, each labeled ASSUMED) or full session (every gate confirmed).
   • Escalate quick to full when an irreversible or cross-team choice appears.
3. Intake (gate):
   • Ask max 3 blocking questions per turn from the intake checklist; supply a recommended default for each.
   • Record functional requirements, NFR targets, out-of-scope fence, operating team, and every ASSUMED value.
   • Review-existing mode: map current state, measure real traffic and incidents, and name the binding constraint before proposing change.
4. Estimate (gate):
   • Compute average and peak QPS, storage over retention, bandwidth, working-set memory, and monthly cost at that scale.
   • Name the shaping quantity and confirm the order of magnitude before any component is drawn.
5. Design incrementally:
   • Price the null option first (do nothing, buy, or extend an existing service); rejecting it needs a stated reason.
   • Start from client, API, service, store; add one component at a time as constraint -> component -> cost.
   • Fix API surface, data ownership, and consistency class per flow.
   • Render diagrams only after the component set is agreed, per system-design-diagramming: an architecture view plus sequence or dataflow for the critical path.
6. Deep dive and decide:
   • Dispatch the 2-3 riskiest components to specialist-system-architect, one brief each with its numbers and consistency requirement.
   • Merge the returned options, failure modes, and irreversible decisions; state bottlenecks, SPOFs, and rejected alternatives with reasons.
   • Write one ADR per irreversible decision, each with its reversal trigger; stage the plan as build now, enabling seam, and the metric threshold that triggers the next step.
   • Save the design to docs/design/system-design-[slug].md when file writes are allowed.
7. Score and hand off:
   • Run the nine-axis scorecard (including cost proportionality), record the risk register, and emit the handoff payload.
   • Route to design-solution; return to plan-feature when product scope is still undefined.

Runtime Contract

• Use when architecture, scale, or store selection is unsettled and the design would otherwise be guessed.
• Required inputs: a product goal or existing system, plus scale parameters or explicit permission to assume defaults.
• Never emit a component set before capacity numbers exist or assumptions are labeled.
• Return BLOCKED for undecided cross-team ownership, compliance/residency constraints, or a budget ceiling that changes the topology.

Handoff Payload

• slug, operator_profile, design doc path, mode and depth, requirement table, capacity numbers, component list with justifications, data ownership map, NFR thresholds, ADR list, scorecard, risk register, next workflow.

Blocking Questions

• Ask max 3 at a time with a recommended default and 2-3 options.

Output Template

# System Design: [Name]
## Mode And Depth
## Requirements (Functional / NFR / Out Of Scope)
## Assumptions
## Capacity Estimation (incl. monthly cost)
## Null Option Considered
## Component Architecture (constraint -> component -> cost)
## Diagrams (Architecture / Sequence / Data Flow)
## Data Ownership And Consistency
## Deep Dives
## Trade-offs And Rejected Alternatives
## Staged Plan (Now / Seam / Trigger)
## ADRs (with reversal triggers)
## Design Scorecard (9 axes)
## Risk Register

## Outcome Report
feature_status: design_ready | blocked
requirement_trace: BRD-OBJ-* -> REQ-* -> AC-* -> SRS-*
completed_evidence: []; missing_evidence: []; decision_needed: []; recommended_next_workflow: design-solution

## Next Workflow
design-solution | plan-feature
## Cost Report
Call `get_session_cost(workflow="system-design-session")` before final handoff.