Rendering
Offline, in-process, instant
Real PlantUML compiled to run in the app rather than shelled out to Java, so the preview keeps up with typing. No server round trip, and no JRE to install.
The architecture workbench
Write Code,
Visualise Architecture
A desktop workbench for software architects. Write C4 and PlantUML, turn trust boundaries into a STRIDE threat register, design agent systems, and let Claude Code validate and render diagrams through a local MCP server. Your diagrams stay on your machine.
One system, four views. Rendered by Gnomon, offline.
01 — Write it
Text in. Diagram out. No server in between.
Every example on this page uses the templates and the rendering engine that ship in the app.
@startuml
!include https://raw.githubusercontent.com/plantuml-stdlib/C4-PlantUML/master/C4_Context.puml
title System Context Diagram
Person(user, "User", "A user of the system")
System(system, "System", "The system being designed")
System_Ext(external, "External System", "An external dependency")
Rel(user, system, "Uses")
Rel(system, external, "Calls")
@enduml
Thirteen lines of C4, with the include path already right.
Open this template →109Templates
15Notations
30D3 charts
11Agent exit types
0Diagram bytes sent
02 — Read it
The threat model was in your C4 all along
Trust boundaries and data flows are the inputs STRIDE wants, and you drew them weeks ago. Gnomon reads them and writes the register: severity on the diagram, and a decision recorded against each threat.
- Spoofing, tampering, repudiation, disclosure, denial, elevation
- Severity badges drawn onto the diagram itself
- Your decisions kept with the file, not in someone's notes
03 — Design it
A design framework for agent systems
Agents, channels, tools, knowledge, state, routing, guardrails and exits, modelled from the whole estate down to a single turn. The model is built from how Claude Managed Agents, LangGraph, Google ADK, Strands and ElevenLabs actually work, so a design maps onto whichever one you build on.
4Zoom levels
5Aspects
7Viewpoints
Then Gnomon reads the design back: how much of each run is determined, which hook points have no control, which requirements nothing enforces, and the ways a run can end that you never modelled.
See the agentic templates
L1 · Landscape
L2 · Topology
L3 · Flow
L4 · Control plane04 — Keep it
Your diagrams stay local
Your diagrams stay on your machine. Every outbound connection the app makes is listed here.
Read the privacy policyOutbound, in full
- Rendering
- on your machine
- Template library
- bundled, offline
- Threat analysis
- on your machine
- Knowledge index
- on your machine
- Licence activation
- key and computer name
- Licence check
- key and activation ID, daily
- Update check
- our website, sees your IP
- Usage analytics
- Google Analytics, only if you opt in
- Crash reports
- off unless you turn them on
- Claude
- your own Claude Code, not ours
- Your diagrams
- never sent by Gnomon
05 — Start from one
109 templates, across 15 notations
C4, UML, ArchiMate, AWS, Azure, Google Cloud, Kubernetes, integration patterns, hexagonal, portfolio treemaps, threat models, agentic design, 30 D3 charts, and the Markdown documents that go around them.
06 — And underneath
And the editor it all runs on
Drawing
30 D3 charts, as real D3
Sankey, treemap, chord, calendar, force graphs: the views PlantUML cannot draw, as JavaScript you can edit, executed in a sandbox with no reach into your machine.
Planning
A board and a canvas beside it
Trello-style lanes and a typed idea graph in the same window as the architecture, so the work and the design are not in two tools that disagree.
Agents
Claude Code, in your own repositories
Gnomon registers an MCP server with Claude Code, so every session on your machine can validate, render and open diagrams with the bundled engine, in whichever repository it is working.
Safety net
Local history, no commit needed
Every version of a file, including the ones an agent wrote, recoverable without having committed anything anywhere.
Anywhere
The same editor in a browser
The web editor renders in the tab, opens folders on your own disk, needs no account, and is free. Same rendering engine as the desktop app: designing agent systems is the one thing reserved for it.
Point it at something you already drew
Open an architecture you finished months ago and see what it says about it. Thirty days, everything unlocked, no card and no account.
Pricing · Personal and Business · cancel whenever