Sim

Sim is a Next.js and Bun monorepo backed by PostgreSQL, with Socket.io for realtime, Trigger.dev for background jobs and E2B or isolated-vm for code execution, per the README tech stack. It can run as the hosted service at sim.ai, as a macOS desktop app, or self-hosted with an npx wizard that starts published Docker Compose images.

Multi-agent work is expressed as a workflow graph. Several Agent blocks are wired together with Router and Condition blocks for branching, Parallel and Loop containers for fan-out, and Workflow blocks for nesting; each block's output is read by later blocks by name. A separate Chat surface delegates work to built-in specialised subagents that appear as collapsible sections. Deployed workflows can be exposed through API, chat or MCP endpoints.

State is the saved workflow graph with immutable deployment versions, run logs, and workspace resources such as tables, files and knowledge bases. Agent memory is optional and keyed to a conversation; without it each run starts fresh, per the docs.

The Human in the Loop block stops a run at a chosen point and shows the approver context in a portal, with notifications by Slack, Gmail, Microsoft Teams, SMS or webhook. The approver fills a resume form (for example approve or reject plus comments) that later blocks can read, and the run stays paused with no timeout. The same pause can be resumed through the API or a webhook. Pi Coding Agent blocks produce pull requests for a person to review and merge rather than committing to the default branch.

npx sim-setup

# Self-host (Node.js 20+ and Docker required)
npx sim-setup                    # wizard: database, secrets, .env, Chat API key, Docker Compose
# open http://localhost:3000 when it finishes

# manage the install from its directory
npx sim-setup status
npx sim-setup logs
npx sim-setup update

# add optional capabilities later
npx sim-setup add llm
npx sim-setup add integration slack

# let Claude Code use your Sim workspace through Sim's MCP server (Sim Cloud URL)
claude mcp add --transport http sim https://mcp.sim.ai/mcp

• Self-hosting needs Node.js 20+ and Docker; the wizard creates a sim/ deployment directory and does not clone the repository.
• Chat is a Sim-managed service: the wizard connects a Chat API key and opens your browser to sign in.
• Remote code execution in the tech stack uses E2B or isolated-vm; an open issue asks for a local Docker sandbox for self-hosted installs.
• A self-hosted MCP server URL is https://<your-sim-host>/api/mcp; Claude's hosted connectors need a self-hosted Sim to be reachable from the internet.
• MCP API keys do not expire until revoked; the docs prefer OAuth.
• Pi Coding Agent Create PR mode needs sandbox execution, your own provider API key and a GitHub token.

MCP yes in both directions. A2A is documented as a client block that calls external A2A agents; no server side for serving Sim workflows over A2A was found.

The code is Apache-2.0 and can be self-hosted with npx sim-setup. Sim Studio also sells hosted plans (Free, Pro, Max and Enterprise on its pricing page), so the pricing model is open-core. Model provider costs are separate.

By composition on a canvas: several Agent blocks connected with Router, Condition, Parallel, Loop and Workflow blocks, plus a Human in the Loop block for approvals. Chat separately delegates to built-in subagents.

Yes, as an MCP client: the docs show claude mcp add --transport http sim https://mcp.sim.ai/mcp and codex mcp add sim --url https://mcp.sim.ai/mcp. Sim does not launch those harnesses itself; its Pi Coding Agent block runs the Pi harness.