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Architecture Documentation: openproject-mcp

1. System Overview

openproject-mcp is a Model Context Protocol (MCP) server providing LLM applications (Claude Desktop, Cursor, Antigravity, etc.) with structured access to an OpenProject instance. The server interfaces with OpenProject's REST API v3 using personal API keys, exposing tools and resources for browsing and querying projects, work packages, custom queries, taxonomies, and users.

flowchart LR
    subgraph Clients ["MCP Clients (Multi-User)"]
        UserA["User A Client\n(Key A)"]
        UserB["User B Client\n(Key B)"]
    end

    subgraph Transports ["Transport Layer"]
        Stdio["Stdio Transport\n(Personal Process Isolation)"]
        SSE["SSE / HTTP Transport\n(Shared Central Daemon)"]
    end

    subgraph ContextLayer ["Session & Request Context Provider"]
        ALS["AsyncLocalStorage /\nSession Context Resolver"]
        CtxA["RequestContext A\n(Client A, Key A)"]
        CtxB["RequestContext B\n(Client B, Key B)"]
    end

    subgraph DomainServices ["Domain Services (Stateless)"]
        ProjSvc["Projects Service"]
        WpSvc["Work Packages Service"]
        QuerySvc["Queries Service"]
        MetaSvc["Metadata & Taxonomies"]
    end

    subgraph Remote ["OpenProject Instance"]
        ApiV3["OpenProject REST API v3\n(/api/v3 - RBAC Enforced)"]
    end

    UserA -->|stdio or HTTP headers| Stdio
    UserB -->|HTTP headers| SSE
    Stdio --> ALS
    SSE --> ALS
    ALS --> CtxA
    ALS --> CtxB
    CtxA --> DomainServices
    CtxB --> DomainServices
    DomainServices -->|Authenticated API Requests| ApiV3

2. Core Architectural Layers

The system is organized into four modular layers:

2.1. MCP Server & Transport Layer

  • Runtime: Bun (>= 1.2/1.3) providing native TypeScript execution without build steps, automatic .env loading, and fast startup for CLI/stdio subprocesses.
  • Responsibility: Manages the MCP connection lifecycle, tool definitions, input validation, and JSON-RPC dispatching.
  • Protocol: MCP specification implemented via @modelcontextprotocol/sdk.
  • Default Transport: StdioServerTransport for local integration with desktop clients and CLI tools.
  • Remote Transport: Native Bun.serve HTTP/SSE server (src/http-server.ts) supporting multi-tenant sessions over Server-Sent Events (text/event-stream).
  • Schema Validation: Each tool declares its arguments using Zod schemas, automatically generating standard JSON Schema definitions for the LLM.

2.2. Domain Services / Tool Providers

Domain services map MCP tool calls to concrete business logic and OpenProject API operations:

  • Projects Service: Listing accessible projects, retrieving project details, and fetching project-level schemas.
  • Work Packages Service: Listing work packages with filters, searching by text or ID, retrieving work package details, and inspecting relations.
  • Queries Service: Listing saved queries (views) configured in OpenProject and executing them.
  • Metadata & Taxonomies Service: Retrieving statuses, work package types, priorities, categories, versions, and users to enable LLMs to construct valid queries and interpret responses.

2.3. OpenProject Client Layer

  • HTTP Transport: Handles HTTPS communication against the configured OPENPROJECT_BASE_URL.
  • Authentication Handler: Enforces OpenProject API v3 authentication by injecting Authorization: Basic base64(apikey:<api_key>) headers.
  • HAL+JSON Parser: OpenProject API v3 represents entities using HAL+JSON format (_links, _embedded). The parser unwraps links, extracts embedded entities, and converts them into clean, token-efficient JSON structures for LLM consumption.
  • Query Filter Builder: Serializes user-friendly filtering parameters into OpenProject's JSON-based filter query syntax (e.g. filters=[{"status_id":{"operator":"o","values":[]}}]).
  • Error Normalizer: Intercepts HTTP 4xx/5xx responses and maps them to descriptive MCP tool errors without exposing sensitive credential data.

2.4. Configuration & Environment Layer

  • Parses and validates configuration at startup:
    • OPENPROJECT_BASE_URL: Base URL of the OpenProject instance (e.g., https://openproject.example.com).
    • OPENPROJECT_API_KEY: User API key generated in OpenProject under "My Account > Access tokens" (required in stdio mode; optional in HTTP/SSE mode).
    • OPENPROJECT_READ_ONLY: Boolean flag (true / false, default: false). Can also be set via --read-only CLI argument.
    • PORT: HTTP server listening port (e.g. 3000 or --port <num>). When set, activates HTTP/SSE remote transport. Supports HOST_PORT as an environment variable fallback.
    • HOST: Server bind address (or --host <addr>, default: 0.0.0.0).
  • Fails fast with actionable setup advice if required credentials are missing or invalid in stdio mode.

2.5. Read-Only Execution Mode

The server supports a dedicated read-only operating mode designed for auditing, reporting, and exploratory agent workflows:

  • Tool Filtering: When read-only mode is active, only read/browse/query tools are registered in the MCP tool registry. Mutating tools (creating work packages, updating attributes, logging time) are completely excluded from the tool manifest exposed to the LLM.
  • Defense-in-Depth Guard: In addition to tool manifest omission, the internal tool router validates whether an operation is a mutating action. Any write attempt is immediately rejected with a structured error (SERVER_READ_ONLY: Server is operating in read-only mode. Write operations are disabled.).
  • Safe Auditing: Guarantees zero unintentional data modification or state changes in the OpenProject instance.

3. Multi-User Concurrency & Request-Scoped Architecture

The server is architected from the ground up to support concurrent, multi-user operations safely without cross-user credential leakage, state bleed, or privilege escalation.

3.1. Core Concurrency Rules

  1. Zero Global Mutable State: No global singleton OpenProjectClient or static API key variable exists.
  2. Stateless Domain Services: All domain services (ProjectsService, WorkPackagesService, etc.) are stateless and operate exclusively on the RequestContext resolved for the active execution.
  3. RequestContext Interface: 
    export interface RequestContext {
  client: OpenProjectClient;
  isReadOnly: boolean;
  sessionId?: string;
  userId?: string;
}
  4. Context Propagation via AsyncLocalStorage: In Bun/TypeScript, request context is propagated down asynchronous call trees using AsyncLocalStorage<RequestContext>. Tool handlers access getRequestContext() to obtain their session's isolated client and read-only flags cleanly.

3.2. Supported Deployment Models

Model A: Local Personal Desktop (Stdio Transport - Default)

  • Deployment: The MCP client (Claude Desktop, Cursor, local CLI) spawns openproject-mcp as a dedicated child process communicating via standard input/output.
  • Isolation: OS-level process and memory boundary.
  • Credentials: Passed via process environment variables or local MCP client configuration.
  • Security: Complete isolation. Zero network ports exposed.

Model B: Centralized Shared Server (SSE / HTTP Transport)

  • Deployment: A single shared openproject-mcp daemon runs centrally for a team or cluster (via docker-compose.server.yml or PORT=3000).
  • Isolation: Connection and request-level isolation.
  • Endpoints:
    • GET /health: Health probe returning server status and mode.
    • GET /sse: Persistent SSE stream establishing an isolated session.
    • POST /messages?sessionId=<uuid>: JSON-RPC message ingestion routed to the active session.
  • Credential Delivery & Extraction Precedence:
    • Each connecting client supplies their OpenProject API token via:
      1. Authorization: Bearer <key>
      2. X-OpenProject-Api-Key: <key>
      3. ?apiKey=<key> query parameter
    • Unauthenticated connection requests are rejected immediately with HTTP 401 Unauthorized.
    • The transport layer instantiates an ephemeral, per-session OpenProjectClient and executes tool calls inside runWithContext({ client, isReadOnly }, ...).
  • RBAC Enforcement: All calls execute against OpenProject REST API v3 using that specific user's token. OpenProject's internal Role-Based Access Control guarantees users only retrieve projects and work packages they have permission to access.
  • Lifecycle & Memory Safety: Abort signal listeners (req.signal) and stream cancellation handlers evict closed sessions from active memory, guaranteeing zero memory leaks.

4. Tool Specifications (Browse & Query Scope)

MCP Tool Name Description Key Parameters
openproject_list_projects List projects accessible to the authenticated user pageSize, offset, sortBy
openproject_get_project Get details of a single project by ID or identifier projectId (number or string identifier)
openproject_list_work_packages Browse work packages with filtering and pagination projectId, status, type, pageSize, offset, filters
openproject_get_work_package Get full details of a specific work package workPackageId (number)
openproject_list_queries List saved project or global queries (views) projectId, pageSize, offset
openproject_get_query Retrieve details and results of a saved query queryId (number)
openproject_list_types List all available work package types (Task, Bug, Milestone, etc.) None
openproject_list_statuses List all available work package statuses (New, In Progress, Closed, etc.) None
openproject_list_priorities List all priority levels None
openproject_list_users List users in the OpenProject instance pageSize, offset
openproject_get_openapi_spec Retrieve OpenProject API v3 OpenAPI specification for schema introspection path

5. OpenProject API v3 Specifics

5.1. Authentication Details

OpenProject API v3 utilizes HTTP Basic Authentication for API tokens:

GET /api/v3/projects HTTP/1.1
Host: openproject.example.com
Authorization: Basic YXBpa2V5OnlvdXItb3BlbnByb2plY3QtYXBpLWtleQ==
Accept: application/hal+json

Where YXBpa2V5OnlvdXItb3BlbnByb2plY3QtYXBpLWtleQ== is the Base64 encoding of apikey:<your-api-key>.

5.2. HAL+JSON Handling

API responses contain _links linking to related resources (parent project, author, assignee, status, type). The client layer normalizes these into human-readable labels and numeric IDs so the LLM agent can reason over them easily.

5.3. Filter Syntax Translation

OpenProject filters are passed via a URL-encoded JSON string array. Example:

[
  { "status_id": { "operator": "o", "values": [] } },
  { "project": { "operator": "=", "values": ["12"] } }
]

The client layer provides helper builders so tools can accept friendly parameters (e.g. { onlyOpen: true, projectId: 12 }) and translate them automatically into the required format.

6. Security & Isolation

  1. Token Protection: Credentials are read from environment variables or per-connection HTTP headers. Tokens are never echoed in logs or tool payloads.
  2. Permission Boundary: The MCP server operates strictly within the permission scope of the provided API key. No elevated access is granted beyond what the user possesses in OpenProject.
  3. Input Sanitization: All arguments from the LLM are validated via Zod schemas prior to making network requests, preventing request-smuggling or path traversal.
  4. Transport Isolation: The default stdio transport communicates only through standard input/output with the host client process, with no exposed network ports.
  5. Read-Only Enforcement: Configurable read-only mode (OPENPROJECT_READ_ONLY=true) guarantees zero write side-effects on the OpenProject instance when operating in exploratory or untrusted agent sessions.
  6. Multi-Tenant Memory Isolation: Ephemeral RequestContext instances isolated via AsyncLocalStorage guarantee zero token bleed or cross-user contamination in concurrent shared server environments.

7. Project Directory Structure

openproject-mcp/
├── docs/
│   ├── ARCHITECTURE.md          # System architecture and specifications
│   ├── DECISIONS.md             # Architecture Decision Records (ADR)
│   ├── plans/                   # Implementation plans
│   ├── specs/                   # Technical designs and specifications
│   └── TODO.md                  # Task tracking and roadmap
├── docker-compose.yml           # Local OpenProject 17 testing stack
├── docker-compose.server.yml    # Production hosted remote MCP server stack
├── Dockerfile                   # Multi-stage container image definition
├── src/
│   ├── index.ts                 # Dual-transport CLI entry point (stdio & HTTP)
│   ├── server.ts                # Stdio MCP server factory & tool registration
│   ├── http-server.ts           # Hosted HTTP/SSE/OpenAPI multi-tenant server (Bun.serve)
│   ├── openapi-spec.ts          # OpenAPI 3.1.0 specification generator for MCP tools
│   ├── context.ts               # RequestContext & AsyncLocalStorage
│   ├── client/
│   │   ├── api-client.ts        # OpenProject REST API v3 HTTP client
│   │   ├── hal-parser.ts        # HAL+JSON parsing and transformation
│   │   ├── filter-builder.ts    # OpenProject JSON filter builder
│   │   └── types.ts             # OpenProject API type definitions
│   ├── config/
│   │   └── index.ts             # Configuration loader and Zod schema
│   ├── services/
│   │   ├── projects.ts          # Projects domain service
│   │   ├── work-packages.ts     # Work packages domain service
│   │   ├── queries.ts           # Queries domain service
│   │   ├── metadata.ts          # Types, statuses, priorities, users
│   │   └── openapi.ts           # OpenAPI specification discovery & caching
│   └── tools/
│       ├── common.ts            # Common schemas and error formatters
│       ├── index.ts             # Tool registration and execution wrapper
│       ├── metadata.ts          # MCP tool definitions for metadata
│       ├── openapi.ts           # MCP tool definition for OpenAPI introspection
│       ├── projects.ts          # MCP tool definitions for projects
│       ├── queries.ts           # MCP tool definitions for saved queries
│       └── work-packages.ts     # MCP tool definitions for work packages
├── tests/
│   ├── fixtures/                # HAL+JSON mock fixtures
│   ├── client.test.ts           # OpenProject client unit tests
│   ├── config.test.ts           # Configuration loader unit tests
│   ├── docker.test.ts           # Docker packaging and compose tests
│   ├── http-server.test.ts      # Hosted HTTP/SSE server and protocol tests
│   ├── mcp-server.test.ts       # MCP server stdio integration tests
│   ├── openapi.test.ts          # OpenAPI service and tool tests
│   ├── read-only.test.ts        # Read-only execution mode guard tests
│   ├── services.test.ts         # Domain services integration tests
│   ├── smoke.test.ts            # Metadata and smoke tests
│   └── tools.test.ts            # Tool registration unit tests
├── AGENTS.md                    # Operating guidelines for AI agents
├── package.json                 # Project dependencies and scripts
├── bun.lock                     # Bun dependency lockfile
└── tsconfig.json                # TypeScript compiler configuration (Bun bundler mode)

8. Future Roadmap

  • Phase 1 (Completed): Read/browse capability for projects, work packages, queries, taxonomies, and OpenAPI introspection.
  • Phase 4 (Completed): Hosted remote MCP server (HTTP/SSE transport via Bun.serve) with multi-tenant per-session credential scoping and Docker Compose deployment.
  • Phase 2 (Upcoming): Mutating operations (create/update work packages, add comments, log time).
  • Phase 3: Attachment inspection and download resources.
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