Facet Standard
The Deterministic Contract Layer for AI. Defines strict tool-calling protocols, the Token Box Model for context allocation, and type-safe execution. Treats AI behavior as compiled software, not probabilistic improvisation. Eliminating runtime failure
What is Facet Standard?
Facet Standard is a Model Context Protocol (MCP) server that allows AI assistants like Claude, Cursor, and VS Code to deterministic contract layer for ai. defines strict tool-calling protocols, the token box model for context allocation, and type-safe execution. treats ai behavior as compiled software, not probabilis...
The Deterministic Contract Layer for AI. Defines strict tool-calling protocols, the Token Box Model for context allocation, and type-safe execution. Treats AI behavior as compiled software, not probabilistic improvisation. Eliminating runtime failure
This server falls under the Developer Tools category on MCPgee, the world's largest MCP server directory with 33,000+ servers.
Features
- The Deterministic Contract Layer for AI. Defines strict tool
Use Cases
Maintainer
Works with
Installation
Manual Installation
npx facet-standardConfiguration
Configuration Details
claude_desktop_config.json
Performance
Response Metrics
Resource Usage
How to Set Up and Use Facet Standard
Facet Standard is a deterministic contract layer and specification for AI tool-calling that treats AI behavior as compiled, type-safe software rather than probabilistic output. It defines strict protocols for how AI agents should call tools, allocate context via the Token Box Model, and execute operations—eliminating the ambiguity and runtime failures common in prompt-driven workflows. Developers use it to build reliable, auditable AI pipelines where every tool invocation follows an explicit, machine-verifiable contract written in the .facet domain-specific language.
Prerequisites
- Node.js 18 or later (for the npx-based runtime)
- An MCP-compatible client such as Claude Desktop or Claude Code
- Familiarity with the .facet contract syntax (see the SPECIFICATION.md in the repository)
- Optional: Rust toolchain if you want to build the reference compiler from source
Review the Facet specification
Before running the server, read SPECIFICATION.md in the repository to understand how .facet contracts, tool declarations, and the Token Box Model work. This context is needed to write valid contracts.
Run the server with npx
Start the Facet Standard MCP server using npx. No global install is required.
npx facet-standardAuthor a .facet contract
Write a contract file that declares your AI system's description, available tools, input variables, and deterministic execution steps. Below is the minimal contract structure from the official specification.
@system
description: "Deterministic payment orchestration"
tools: [$Payments]
@vars
amount: @input(type="float", min=0.01)
currency: @input(type="string", enum=["USD","EUR","GBP"])
result: $Payments.charge(amount=amount, currency=currency)Add the server to your MCP client config
Register facet-standard in your Claude Desktop configuration file so the client can launch and communicate with it.
Pass contracts to the server
Once connected, provide your .facet contract to the server through your MCP client. The server compiles and validates the contract before executing any tool calls.
Facet Standard Examples
Client configuration
Register the facet-standard server in Claude Desktop's config file.
{
"mcpServers": {
"facet-standard": {
"command": "npx",
"args": ["facet-standard"]
}
}
}Prompts to try
Use the server to validate and execute deterministic AI contracts.
- "Validate this .facet contract and report any schema errors"
- "Execute the payment contract with amount=99.99 and currency=USD"
- "What tools does this .facet system declaration expose?"
- "Show me how the Token Box Model allocates context for this contract"Troubleshooting Facet Standard
npx facet-standard exits immediately with no output
Check that the package exists on npm with 'npm view facet-standard'. If it is not yet published, clone the repository and follow the build-from-source instructions in the README using the Rust compiler reference at github.com/rokoss21/facet-compiler.
Contract validation fails with type mismatch errors
Consult SPECIFICATION.md v2.1.3 for the canonical type system. Ensure @input declarations use supported types (float, string, int, bool) and that enum values match exactly the strings listed in the contract.
Frequently Asked Questions about Facet Standard
What is Facet Standard?
Facet Standard is a Model Context Protocol (MCP) server that deterministic contract layer for ai. defines strict tool-calling protocols, the token box model for context allocation, and type-safe execution. treats ai behavior as compiled software, not probabilistic improvisation. eliminating runtime failure It connects AI assistants to external tools and data sources through a standardized interface.
How do I install Facet Standard?
Follow the installation instructions on the Facet Standard GitHub repository. Clone the repo, install dependencies, and add the server config to your AI client.
Which AI clients work with Facet Standard?
Facet Standard works with all major MCP-compatible AI clients including Claude Desktop, Claude Code, Cursor, VS Code (GitHub Copilot), Windsurf, and Cline.
Is Facet Standard free to use?
Yes, Facet Standard is open source and available under the MIT license. You can use it freely in both personal and commercial projects.
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