Agent Architecture

Xum

Xum (Coder, formerly Mux; the adapter id stays mux) is a desktop/browser host for isolated, parallel agentic development. It provides isolated workspaces, multi-model support, central git status, MCP servers, policy, secrets, skills, CLI, ACP editor integration, and a mobile-responsive server UI. Its config home moved from .mux to .xum; a legacy .mux home keeps working.

Source reviewed #22Checked 2026-09-07

Setup guide Source

Run type

Desktop

Adapter

mcp-only

Source

OSS · coder/xum

Stars

★ 2k

Archive Context

This page is one host entry inside the full architecture archive and keeps multi-surface products visible instead of collapsing them into a single run type.

42 host archive

Host page

mux

Run cohort

Desktop: 9

Archive route

/agents

Host Surface Perspectives

The host is the primary architecture boundary. Each runnable surface below separates what the host owns from what agent-connector can safely install for Xum.

1 surface entry

Desktop

desktop

Host entry: A desktop app or standalone editor owns window state, sign-in, project indexing, embedded terminal behavior, and local product storage.
Host owns: The desktop host owns orchestration UI, model routing, approval UX, background jobs, and any cloud or app-store managed extension points.
Connector role: agent-connector can write documented workspace/global config and content files, but it does not replace the desktop runtime or UI shell. Current wired connector surfaces: MCP server, Skills, Memory.
Implementation consequence: Desktop surfaces need source-review separation because product pages often document capabilities while runtime state lives in app-managed storage. No visible host-native gap is currently tracked for this surface.

Verified Architecture

Source-checked on 2026-09-07. These notes are intentionally separate from the generated surface matrix below.

Sequence #22

Source pass

Current source review findings are kept as the first architecture section so readers can separate external evidence from the generated local coverage model.

Coder renamed Mux to Xum on 2026-09-02 (repo coder/mux to coder/xum, xum.coder.com) and moved the config home from .mux to .xum; legacy .mux files stay readable, and the adapter id stays mux.
Xum is a desktop and browser application for parallel agentic development, not a single terminal agent runtime.
Its features center on isolated workspaces, git divergence views, local/worktree/SSH execution modes, and multi-model agent execution.
The repository includes desktop app, browser, mobile, VS Code, and `.mux` artifacts, so the page should present Mux as an orchestration host.

Runtime boundary

The adapter target is the Xum desktop/browser orchestration host, not a single underlying agent. Xum owns isolated workspaces, git status overview, model selection, browser/desktop UI, server mode, and agent management.

Docs describe Xum as running parallel coding agents, each with its own isolated workspace.
The active home decides file locations: .xum is preferred, an existing .mux is still read, and new installs default to .xum.

MCP registration

MCP servers live in mcp.jsonc under the Xum home: ~/.xum/mcp.jsonc (user) and <project>/.xum/mcp.jsonc (project), with the legacy .mux paths read when the .xum file is absent. The root key is servers, and each entry is a single shell-command string rather than an object, so the adapter builds and upserts that string itself.

Xum documents no ${env:VAR} interpolation, so environment references resolve to literals at install time, and server env maps have no native equivalent.
Project secrets and policy files are adjacent configuration surfaces.

Hook bridge

Xum is mcp-only: it exposes no lifecycle hook system, and MCP is its extensibility mechanism, so the adapter installs the MCP server and reports hooks as unavailable, the same shape as the Warp reference adapter.

Downstream agents running inside a workspace can have their own hooks, but those belong to the downstream runtime.
There is no separate hook file; mcp.jsonc is the only runtime config the adapter touches.

Content surfaces

Skills are wired: <project>/.xum/skills and ~/.xum/skills (legacy .mux), with directory names matching ^[a-z0-9]+(?:-[a-z0-9]+)*$ (1 to 64 characters). Memory is wired as an instruction file. Workspaces, forking, compaction, plan mode, and Best-of-N orchestration are host features outside the file writer.

No command or subagent file surface is documented for this adapter.
Compaction and context boundaries are host-level context management features.

Marketplace and host-only affordances

The host-only affordance is parallel orchestration: isolated workspaces, central git status, multi-model support, desktop/browser UI, mobile-responsive server mode, and ACP editor integrations. Xum also ships an experimental Agent Plugins reader (Settings, Experiments) that installs into ~/.xum/plugins; agent-connector does not route this host to the agent-plugin format because the reader is behind a flag, absent from the compatible-clients list, and installs servers disabled by default.

Xum is useful for studying multi-agent control planes rather than single-agent CLIs.
AGPL licensing and Coder ownership should be visible source facts.

Host-specific shape

Xum (Coder, formerly Mux) is a desktop/web-adjacent agent host with MCP, skills, and memory under the .xum home (legacy .mux); no command, hook, or subagent writable surface is documented for this adapter.

Form factor: Desktop.
Native surfaces: MCP server, Skills, Memory.
Wired surfaces: MCP server, Skills, Memory.
Visible gaps: none.

Hooks

No user-installable lifecycle hook boundary is confirmed for this host.

MCP

The adapter writes this host's native MCP registration dialect, including its root key, scope, transport fields, and environment syntax.

Memory

Managed memory text is written into the rules or memory file this host actually reads, with ownership markers for reversible uninstall.

Marketplace and affordances

Current install is a direct native-file write path; any package artifact is manual unless a driver is added.

Runtime handlers: none.
User-visible affordances: Users see managed skills + memory surfaces after install; runtime-only surfaces stay behind the host boundary..

Known Limits

Xum has no hook system; do not read downstream-agent hooks as host coverage.
Revisit the agent-plugin routing if Xum ships its plugin reader on by default or joins the compatible-clients list.
Downstream agent capabilities should remain separated from Xum orchestration.
Workspace and server-mode features may have account or deployment assumptions.

Evidence checked

Xum documentation Xum repository

Architecture Diagram

Host-specific flow for the Desktop surface.

mcp-only

1
Entry point: Desktop
Desktop or standalone editor app reads global/product storage plus workspace files before handing control to its embedded agent runtime.
2
Connector package
defineConnector package declares server, hooks, content, memory, and actions.
3
Adapter module: mux
mcp-only renderer at src/adapters/mux/index.ts.
4
Native host artifacts
Adapter writes MCP config + content files + memory/rules through this host's native file, package, or marketplace surface.
5
Runtime boundary
host calls MCP tools through its client
6
User-visible surface
Users see managed skills + memory surfaces after install; runtime-only surfaces stay behind the host boundary.
7
Coverage ceiling
3/3 native surfaces wired. No visible host-native gap in the current coverage matrix.

Component Diagram

Static parts of the Xum integration and which side owns each one.

Xum host
Desktop surface owns the model loop, UI shell, config discovery, and native lifecycle for this integration.
agent-connector package
Normalizes connector declarations into MCP, hook, memory, content, and action payloads before delegating to a host adapter.
mux adapter
Implements the mcp-only translation layer in src/adapters/mux/index.ts.
Native artifacts
Adapter writes MCP config + content files + memory/rules through this host's native file, package, or marketplace surface.
Runtime bridge
host calls MCP tools through its client
User-visible surface
Users see managed skills + memory surfaces after install; runtime-only surfaces stay behind the host boundary.

Runtime Sequence Flow

Message order from install to runtime for the Desktop surface.

1 · User / project
Choose host adapter
Install or update starts against Xum; the selected surface is Desktop.
2 · agent-connector
Resolve connector declaration
The package reads connector metadata and routes it to the mux adapter instead of exposing a generic host contract.
3 · mux adapter
Render host-native shape
Adapter writes MCP config + content files + memory/rules through this host's native file, package, or marketplace surface.
4 · Xum
Host loads artifacts
host calls MCP tools through its client
5 · Agent runtime
Run through host boundary
Users see managed skills + memory surfaces after install; runtime-only surfaces stay behind the host boundary.
6 · Coverage matrix
Expose remaining ceiling
3/3 native surfaces wired. No visible host-native gap in the current coverage matrix.

Architecture Archive

The same five study axes are rendered for every host: runtime shape, hooks, MCP, memory, and host-only affordances.

Host-specific shape

Form factor: Desktop.
Native surfaces: MCP server, Skills, Memory.
Wired surfaces: MCP server, Skills, Memory.
Visible gaps: none.

Hooks

No user-installable lifecycle hook boundary is confirmed for this host.

MCP

The adapter writes this host's native MCP registration dialect, including its root key, scope, transport fields, and environment syntax.

Memory

Managed memory text is written into the rules or memory file this host actually reads, with ownership markers for reversible uninstall.

Marketplace and affordances

Current install is a direct native-file write path; any package artifact is manual unless a driver is added.

Runtime handlers: none.
User-visible affordances: Users see managed skills + memory surfaces after install; runtime-only surfaces stay behind the host boundary..

Special Feature Inventory

Host-specific capabilities separated from the generic surface matrix: lifecycle hooks, MCP registration, memory/rules, marketplace delivery, and UI or content affordances owned by the host.

Lifecycle hooks

Not confirmed

No user-installable lifecycle hook boundary is confirmed for this host.

MCP registration

Wired

The adapter writes this host's native MCP registration dialect, including its root key, scope, transport fields, and environment syntax.

Memory and rules

Wired

Managed memory text is written into the rules or memory file this host actually reads, with ownership markers for reversible uninstall.

Marketplace delivery

Direct files

Current install is a direct native-file write path; any package artifact is manual unless a driver is added.

Host-only affordances

1/1 wired

Wired host affordances: Skills. Host-native gaps: none. Not confirmed: Commands, Subagents, Statusline handler, Action affordances.

Source Review Notes

Latest source pass for host-specific architecture details. This is separate from generated adapter coverage and local drift guards.

Checked 2026-09-07Open review source

coder/xum repository and xum.coder.com docs

Coder renamed Mux to Xum on 2026-09-02 (repo coder/mux to coder/xum, xum.coder.com) and moved the config home from .mux to .xum; legacy .mux files stay readable, and the adapter id stays mux.
Xum is a desktop and browser application for parallel agentic development, not a single terminal agent runtime.
Its features center on isolated workspaces, git divergence views, local/worktree/SSH execution modes, and multi-model agent execution.
The repository includes desktop app, browser, mobile, VS Code, and `.mux` artifacts, so the page should present Mux as an orchestration host.

Evidence Map

Each host page ties public source evidence to the local adapter and drift guards that keep the rendered archive aligned with code.

Public source: coder/xum

Primary public repository or product source used for current host status and source links.

external

Open source

Local adapter implementation

Defines the install/uninstall behavior, native config rendering, capability flags, and any runtime parse/format bridge for this host.

local

src/adapters/mux/index.ts

Platform metadata and host-native matrix

Records the form factor, public source target, ranking source, native host surfaces, and current agent-connector coverage cells.

local

site/src/platform-data.ts

Architecture drift guard

Asserts that every registered host has a page, diagram, host-specific brief, source link, form-factor band, and adapter-aligned surface data.

local

tests/docs/platform-drift.test.ts

Runtime Shape

The host accepts MCP registration, but has no confirmed hook bridge for agent-connector to call.

MCP registration: Wired through this adapter
Hook bridge: No confirmed hook surface

Connector Surfaces

Static content

MCP server, Skills, Memory

Runtime handlers

No runtime handler surfaces wired

Surface Matrix

Wired means agent-connector installs it now. Host native marks support we know about but have not wired yet.

SurfaceStatus

MCP server

Static content

agent-connector installs this surface today.

Wired

Hooks

Static content

No confirmed host-native surface.

N/A

Commands

Static content

No confirmed host-native surface.

N/A

Skills

Static content

agent-connector installs this surface today.

Wired

Subagents

Static content

No confirmed host-native surface.

N/A

Memory

Static content

agent-connector installs this surface today.

Wired

Statusline handler

Runtime handler

No confirmed host-native surface.

N/A

Action affordances

Runtime handler

No confirmed host-native surface.

N/A