We believe great software
starts with great architecture.

Drag real infrastructure onto an infinite canvas and connect it with typed edges. Every node maps to a billable cloud service, so cost and health update as you draw.

• AWS, GCP, Azure and cloud-neutral components, all searchable by category
• Typed edges — sync, async, stream, bidirectional, dependency, data flow — each carrying auth, encryption, DLQ, retry, timeout and SLO metadata
• Instance picker and t-shirt sizing on 200+ components, feeding the cost estimate live
• Dockable, floatable, resizable panel rails for properties, AI, and health
• Select / pan / connect / text tools, multi-select, group actions, minimap
• Auto-save on a 2.5s debounce, then publish when you're ready to freeze a version

The workflows and lifecycles that boxes-and-arrows diagrams never capture. Draw the states, step through the runs, then export straight into XState.

• Initial, normal, final, choice, fork, join and history pseudostates with UML structural rules enforced
• Transitions carry a triggering event, a guard expression and an action, edited inline
• Entry / do / exit actions per state, fired visibly during simulation
• Simulator steps events one at a time, highlighting the active state and logging side effects
• Validate panel flags unreachable states, missing initial transitions, dangling forks, guardless choices
• AI panel can add retry loops, generate guards, or explain why a state is unreachable

A block editor for design docs, runbooks and research, organised in folders next to your diagrams. Slash commands, callouts, attachments, AI drafting.

• Folders with emoji, plus All Notes and Starred views across everything you've written
• Type / anywhere for headings, tip / warning / info callouts, code blocks, quotes, dividers
• Auto-generated table of contents that tracks the heading you're reading
• File attachments stored on S3 and listed per note
• Tags, sections, starring, and one-click Markdown export
• AI assist drafts or rewrites a section in place, and every new account starts with a guided welcome note

Not a chatbot bolted onto a diagram tool. Every message carries your full canvas state and your constraints, and the answers come back as changes you can apply.

• Constraint profile — cloud, budget, traffic, compliance, team size — injected into every request
• Eight callable tools: health score, anti-pattern check, place components, modify diagram, pattern suggest, ADR search, ADR draft, price lookup
• Structural suggestions arrive with an Apply button that mutates the canvas and snapshots a version
• Responses stream token by token, with tool calls narrated as they run
• Conversations are stored per diagram, so you can reopen a thread days later
• Every editor has its own AI panel grounded in that surface — canvas, sequence, state machine, data model, notes, patterns, pricing

A 0–100 composite across six weighted categories, computed by a deterministic rule engine — not a vibe. Each deduction names the components responsible and the fix.

• Reliability 25%, Security 25%, Scalability 20%, Observability 15%, Cost 10%, Maintainability 5%
• Critical violations cap a category outright — no auth on an external path can't be averaged away
• Readiness tiers gate on rule IDs: Prototype → Dev → Production → HA → Enterprise, with the exact blockers listed
• Topology analysis computes structural SPOFs, blast radius per node, critical sync-path length, and fan-in / fan-out
• Findings carry severity, penalty, affected nodes and edges, a written fix, and a link to the matching pattern
• AI observations run after publish to name structural risks the rules can't express

Choose the model tier behind your co-pilot globally or per task, and see exactly what each AI action costs before you spend it.

• Fast and Balanced run on Claude Haiku 4.5; Deep runs on Claude Sonnet 4.6
• Set a default tier, then override individual tasks — classification, summaries, architecture, health, ADRs, review
• Credit weights are published up front: chat 1, notes 2, generate 3, scan-import 5, deep review 8
• Usage table breaks the billing period down by action, call count and model
• Running out pauses AI only — canvas, health, pricing and diagrams keep working

Push traffic and failures through the diagram and watch it respond tick by tick — queues filling, autoscalers reacting, SLOs breaking. Then get the ranked list of what to fix first.

• Presets: steady state, Black Friday spike, viral launch ramp, daily cycle, database outage, region outage, cold cache
• Traffic dial for baseline RPS, peak multiplier, spike window, payload size, read/write split, cache hit rate
• Inject failures per node — down, degraded, slow, partition — with start time, duration and severity
• Model retries, backoff, timeouts and circuit breakers, plus single / active-passive / active-active regions
• Timeline scrubs the whole run with a p95 chart, SLO breach bands and autoscaling markers; particles and a load ribbon show flow on the canvas itself
• Results rail returns max sustainable RPS, SLO verdicts, ranked bottlenecks with one-click fix patches, a capacity runway table, and cost now / at peak / at horizon

A refreshed pricing table pulled straight from AWS, GCP and Azure, wired into every component on your canvas and browsable on its own.

• Fetched per provider on a weekly cron with atomic per-provider replacement — a failed fetch keeps the old rows rather than blanking them
• Freshness banner tells you exactly how current each provider's data is
• Filter by provider, service, region, instance type and pricing dimension; export the result as CSV
• Compare view maps nine service groups — compute, managed DB, cache, API gateway, object storage, serverless, Kubernetes, NoSQL, networking — across all three clouds
• Eleven geo-region groupings so you compare US East to US East, not to whatever the provider called it
• Per-component cost lands in the properties panel and rolls into a diagram total; the AI panel answers pricing questions against the same table

Systems and AI patterns written out properly — roles, tradeoffs, pitfalls, code in two languages — and every single one ships with an interactive demo.

• 22 systems patterns across behavioural, resilience, architectural, creational and infrastructure
• 10 AI patterns across prompting, agentic, reliability, evaluation and integration — RAG, ReAct, tool use, guardrails, semantic cache and more
• Every pattern: overview, plain-language analogy, participant roles, when to use, when to avoid
• Real-world examples naming the systems that use it, plus the pitfalls that bite people
• Runnable TypeScript and Python side by side, stored as content so they update without a deploy
• Beginner / intermediate / advanced difficulty, related-pattern links, and AI suggestions for what your diagram is missing

Books, articles, videos, courses and tools worth your time, organised by topic — plus your own private shelf.

• Topics: system design, backend engineering, DevOps & infra, microservices, databases, security, AI/ML infra, career
• Types: books, articles, videos, courses, tools, newsletters and sites, each badged and levelled
• Bookmark anything and track reading progress as a percentage per resource
• My Zone is your own library — add any URL with a title, type, topic and level
• Beginner / intermediate / advanced filters, plus a topic cloud for browsing sideways

Work freely in an auto-saved draft, then publish a labelled version that snapshots the topology, the cost and the health score together.

• Draft holds your in-progress state; publishing freezes it as a numbered version with a changelog message
• Every published version stores node and edge counts, estimated monthly cost, and the health result at that moment
• Switch to any previous version, or discard the draft and snap back to what was last published
• Version drawer lists the full history with who published it and when
• AI-generated Markdown design doc exports from any published version
• Free plans keep five versions per diagram; Pro keeps all of them

The landing surface: portfolio-level numbers up top, thumbnailed diagrams below, templates one click away.

• Totals for diagram count, estimated monthly cost across everything, and average health score
• Recent diagrams render as live thumbnails with their health colour and cost badge
• Star what matters, tag what needs grouping, and filter down from there
• Start-from-template row sits inline so a new diagram is two clicks away
• Soft delete means a wrong click is recoverable, not fatal
• Collapsible sidebar spanning all five editors, the library, and account settings