Fcp

File Context Protocol (FCP)

MCP servers that let LLMs interact with complex file formats through a verb-based DSL.

FCP is to MCP what React is to the DOM — the LLM thinks in domain operations, FCP renders them into the target format.

Installation

FCP is a Claude Code marketplace plugin. Add it to your settings.json:

{
  "marketplace_plugins": {
    "fcp@os-tack/fcp": true
  }
}

This installs all FCP servers automatically. Prerequisites by server:

| Server | Requires | |--------|----------| | fcp-drawio | Node.js | | fcp-midi | Python 3.13+ | | fcp-terraform | Go | | fcp-rust | rust-analyzer | | fcp-sheets | Python 3.13+ | | fcp-slides | Python 3.13+ |

Manual Installation (non-Claude Code)

Each server can be installed and configured independently. Add to your MCP client config:

{
  "mcpServers": {
    "fcp-drawio": {
      "command": "npx",
      "args": ["-y", "@ostk-ai/fcp-drawio"]
    },
    "fcp-terraform": {
      "command": "fcp-terraform"
    }
  }
}

See each server's README for full installation instructions: fcp-drawio · fcp-midi · fcp-terraform · fcp-rust · fcp-sheets · fcp-slides

Quick Start

Here's a complete workflow — create a diagram from scratch, inspect it, and save:

> drawio_session("new \"Auth Flow\"")

! session "Auth Flow" created
[0s 0e 0g p:1/1]

> drawio(["add svc AuthService theme:blue", "add db UserDB theme:green", "connect AuthService -> UserDB label:\"queries\" style:dashed"])

+ svc AuthService @(0,0 140x60) blue
+ db UserDB @(200,0 120x80) green
~ AuthService -> UserDB "queries" dashed
[2s 1e 0g 320x80 p:1/1]

> drawio_query("status")

status: "Auth Flow" (unsaved, 4 ops, 0 checkpoints)
  page: Page 1 (2 shapes, 1 edge, 0 groups)

> drawio_session("save as:./auth-flow.drawio")

! saved ./auth-flow.drawio
[2s 1e 0g 320x80 p:1/1]

Every FCP server follows this same pattern: session → mutations → query → save.

Servers

| Server | Format | Description | |--------|--------|-------------| | fcp-drawio | draw.io diagrams | Architecture diagrams, flowcharts, ERDs | | fcp-midi | MIDI music | Notes, chords, tracks, instruments | | fcp-terraform | Terraform HCL | AWS/GCP/Azure infrastructure | | fcp-rust | Rust codebases | Navigation, inspection, refactoring via rust-analyzer | | fcp-sheets | Excel spreadsheets | Tables, formulas, charts, formatting | | fcp-slides | PowerPoint presentations | Slides, shapes, tables, charts, layouts |

What the LLM Sees

Every mutation returns a prefixed response and a digest line summarizing the current state. This gives the LLM structured feedback without reading the whole file:

| Prefix | Meaning | |--------|---------| | + | Created | | ~ | Modified / connected | | * | Style / metadata changed | | - | Removed | | ! | Meta operation | | @ | Bulk / layout |

Examples

Diagrams — shapes, edges, and a bounding-box digest:

> drawio(["add svc API theme:blue", "connect API -> DB"])

+ svc API @(0,0 140x60) blue
~ API -> DB
[2s 1e 0g 320x80 p:1/1]

Music — notes, chords, and a song-state digest:

> midi(["note Piano C4 at:1.1 dur:quarter vel:mf", "chord Strings Am at:2.1 dur:whole"])

+ note C4 at:1.1 dur:quarter vel:mf
+ chord Am at:2.1 dur:whole
[1t 4e tempo:120 4/4 2bars]

Infrastructure — resources, variables, and a block-count digest:

> terraform(["add resource aws_instance web ami:\"ami-0c55b159\" instance_type:t2.micro", "add variable region default:\"us-east-1\""])

+ resource aws_instance.web ami:"ami-0c55b159" instance_type:t2.micro
+ variable region default:"us-east-1"
2 blocks, 1 resource, 1 variable

Rust — symbol navigation, diagnostics, and refactoring:

> rust_session("open /path/to/my-project")

! session opened: /path/to/my-project (42 .rs files)

> rust_query("find Config @kind:struct")

Config (struct) src/config.rs:12
  fields: name, port, database_url

> rust(["rename old_handler handle_request @file:routes.rs"])

~ renamed old_handler → handle_request (3 references updated)

Spreadsheets — data blocks and a sheet-state digest:

> sheets(["data A1", "Name,Age,City", "Alice,30,NYC", "Bob,25,LA", "data end"])

+ Wrote 3 rows at A1
1 sheets, ~6 cells

Presentations — slides, shapes, and a deck-state digest:

> slides(["slide add layout:title", "placeholder set title \"Q4 Review\"", "placeholder set subtitle \"Finance Team\""])

+ slide 1 (Title Slide)
~ placeholder title = "Q4 Review"
~ placeholder subtitle = "Finance Team"
[1 slide 3 shapes]

How It Works

Every FCP server exposes exactly 4 MCP tools:

| Tool | Purpose | |------|---------| | {domain}(ops) | Batch mutation operations | | {domain}_query(q) | Read-only state inspection | | {domain}_session(action) | Lifecycle: new, open, save, undo, redo | | {domain}_help() | Self-documenting reference card |

Most servers are built on fcp-core (TypeScript + Python). fcp-terraform (Go) and fcp-rust (Rust) are standalone implementations.

Links

• [Building an FCP server](docs/writing-an-fcp-server.md)
• [Marketplace guide](docs/marketplace-guide.md)
• FCP specification

License

MIT

Source & license

This open-source MCP server is cataloged on AgentStack and links to its original source — we do not rehost the code.

• Author: os-tack
• Source: os-tack/fcp
• License: MIT

Install and usage instructions live in the source repository linked above.