Connect
Optimize
Secure
Fastbill MCP Server
for AI Agents
Production-ready Fastbill MCP server with 57 extensible actions — plus built-in authentication, security, and optimized execution.
Fastbill MCP Server
Built by 
StackOne
StackOne 
Coverage
57 Agent Actions
Create, read, update, and delete across Fastbill — and extend your agent's capabilities with custom actions.
Authentication
Agent Tool Authentication
Per-user OAuth in one call. Your Fastbill MCP server gets session-scoped tokens with zero credentials stored on your infra.
Agent Auth →
Security
Agent Protection
Every Fastbill tool response scanned for prompt injection in milliseconds — 88.7% accuracy, all running on CPU.
Prompt Injection Defense →
Performance
Max Agent Context. Min Cost.
Free up to 96% of your agent's context window to enhance reasoning and reduce cost, on every Fastbill call.
Tools Discovery →What is the Fastbill MCP Server?
A Fastbill MCP server lets AI agents read and write Fastbill data through the Model Context Protocol — Anthropic's open standard for connecting LLMs to external tools. StackOne's Fastbill MCP server ships with 57 pre-built actions, fully extensible via the Connector Builder — plus managed authentication, prompt injection defense, and optimized agent context. Connect it from MCP clients like Claude Desktop, Cursor, and VS Code, or from agent frameworks like OpenAI Agents SDK, LangChain, and Vercel AI SDK.
All Fastbill MCP Tools and Actions
Every action from Fastbill's API, ready for your agent. Create, read, update, and delete — scoped to exactly what you need.
Customers
- Create Customer
Create a new customer in Fastbill.
- List Customers
Retrieve a list of customers from Fastbill with optional filtering.
- Get Customer
Retrieve a specific customer by ID from Fastbill.
- Update Customer
Update an existing customer in Fastbill.
- Delete Customer
Delete a customer from Fastbill.
Contacts
- Create Contact
Create a new contact for a customer in Fastbill.
- List Contacts
Retrieve contacts for customers from Fastbill.
- Update Contact
Update an existing contact in Fastbill.
- Delete Contact
Delete a contact from Fastbill.
Invoices
- Create Invoice
Create a new invoice draft in Fastbill.
- List Invoices
Retrieve a list of invoices from Fastbill with optional filtering.
- Get Invoice
Retrieve a specific invoice by ID from Fastbill.
- Update Invoice
Update an existing invoice draft in Fastbill.
- Delete Invoice
Delete an invoice draft from Fastbill.
Estimates
- Create Estimate
Create a new estimate/quote in Fastbill.
- List Estimates
Retrieve a list of estimates/quotes from Fastbill with optional filtering.
- Get Estimate
Retrieve a specific estimate by ID from Fastbill.
- Delete Estimate
Delete an estimate from Fastbill.
Recurring Invoices
- Create Recurring Invoice
Create a new recurring invoice in Fastbill.
- List Recurring Invoices
Retrieve a list of recurring invoices from Fastbill.
- Get Recurring Invoice
Retrieve a specific recurring invoice by ID from Fastbill.
- Update Recurring Invoice
Update an existing recurring invoice in Fastbill.
- Delete Recurring Invoice
Delete a recurring invoice from Fastbill.
Revenues
- Create Revenue
Create a new revenue entry in Fastbill.
- List Revenues
Retrieve a list of revenue entries from Fastbill with optional filtering.
- Get Revenue
Retrieve a specific revenue entry by ID from Fastbill.
- Delete Revenue
Delete a revenue entry from Fastbill.
Expenses
- Create Expense
Create a new expense entry in Fastbill.
- List Expenses
Retrieve a list of expense entries from Fastbill with optional filtering.
- Get Expense
Retrieve a specific expense entry by ID from Fastbill.
Articles
- Create Article
Create a new product/article in Fastbill.
- List Articles
Retrieve a list of products/articles from Fastbill.
- Get Article
Retrieve a specific product/article by ID from Fastbill.
- Update Article
Update an existing product/article in Fastbill.
- Delete Article
Delete a product/article from Fastbill.
Projects
- Create Project
Create a new project in Fastbill.
- List Projects
Retrieve a list of projects from Fastbill with optional filtering.
- Get Project
Retrieve a specific project by ID from Fastbill.
- Update Project
Update an existing project in Fastbill.
- Delete Project
Delete a project from Fastbill.
Time Entrys
- Create Time Entry
Create a new time tracking entry in Fastbill.
- Get Time Entry
Retrieve a specific time entry by ID from Fastbill.
- Update Time Entry
Update an existing time entry in Fastbill.
- Delete Time Entry
Delete a time entry from Fastbill.
Documents
- Create Document
Create a document entry in the Fastbill inbox.
- List Documents
Retrieve a list of documents and folders from Fastbill.
Other (11)
- Send Invoice By Email
Send a completed invoice to a recipient via email with PDF attachment.
- Send Invoice By Post
Send a completed invoice via postal mail.
- Send Estimate By Email
Send an estimate to a recipient via email with PDF attachment.
- List Time Entries
Retrieve a list of time tracking entries from Fastbill with optional filtering.
- List Templates
Retrieve a list of invoice/estimate templates from Fastbill.
- Set Invoice Paid
Mark an invoice as paid in Fastbill.
- Set Revenue Paid
Mark a revenue entry as paid in Fastbill.
- Complete Invoice
Finalize an invoice draft and convert it to a completed invoice.
- Cancel Invoice
Cancel a completed invoice in Fastbill.
- Lock Invoice
Finalize an invoice in Fastbill.
- Convert Estimate To Invoice
Convert an existing estimate into an invoice.
Set Up Your Fastbill MCP Server in Minutes
One endpoint. Any framework. Your agent is talking to Fastbill in under 10 lines of code.
MCP Clients
Agent Frameworks
JSON{
"mcpServers": {
"stackone": {
"command": "npx",
"args": [
"-y",
"mcp-remote@latest",
"https://api.stackone.com/mcp?x-account-id=<account_id>",
"--header",
"Authorization: Basic <YOUR_BASE64_TOKEN>"
]
}
}
}More Accounting MCP Servers
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Fastbill MCP Server Resources
MCP Code Mode: Keeping Tool Responses Out of Agent Context
Anthropic's code_execution processes data already in context. Custom MCP code mode keeps raw tool responses in a sandbox. 14K tokens vs 500.
11 min
Comparing BM25, TF-IDF, and Hybrid Search for MCP Tool Discovery
Benchmarking BM25, TF-IDF, and hybrid search for MCP tool discovery across 916 tools. The 80/20 TF-IDF/BM25 hybrid hits 21% Top-1 accuracy in under 1ms.
10 min
Indirect Prompt Injection Defense for MCP Tools: A Technical Guide
MCP tools that read emails, CRM records, and tickets are indirect prompt injection vectors. Here's how we built a two-tier defense that scans tool results in ~11ms.
12 min
Fastbill MCP Server FAQ
Fastbill MCP server vs direct API integration — what's the difference?
A Fastbill MCP server and direct API integration serve different use cases. Direct API integration is for software-to-software — backend code calling Fastbill. A Fastbill MCP server is for AI agents — MCP clients like Claude and Cursor, plus framework agents built with OpenAI, LangChain, or Vercel AI — discovering and calling Fastbill at runtime. StackOne provides both.
How does Fastbill authentication work for AI agents?
Fastbill authentication for AI agents works through a StackOne Connect Session. Create one via the dashboard or the SDK — you get an auth link and ready-to-paste config for Claude Desktop, Cursor, and other MCP clients. Your user authenticates their own Fastbill account; StackOne handles token exchange, storage, and refresh. Credentials never reach the LLM, and each user is isolated via
origin_owner_id.Are Fastbill MCP tools vulnerable to prompt injection?
Yes — Fastbill MCP tools can be vulnerable to indirect prompt injection. Any tool that reads user-written content — documents, messages, tickets, records, or free-text fields — is a potential vector. StackOne Defender scans every tool response before it enters the agent's context — regex patterns in ~1ms, then a MiniLM classifier in ~4ms. 88.7% accuracy, CPU-only.
What is the context bloat of a Fastbill agent and how do I avoid it?
Context bloat happens when Fastbill tool schemas and API responses eat your Fastbill agent's memory, preventing it from reasoning effectively. A single Fastbill query can return a massive JSON response, and connecting multiple tools compounds the problem. Tools Discovery and Code Mode reduce context bloat — loading only relevant tools per query and keeping raw responses out of the agent's context.
Can I limit which actions my Fastbill agent can access?
Yes — you can limit which actions your Fastbill agent can access directly from the StackOne dashboard. Toggle actions on or off, or restrict them to specific accounts, with no code changes to your agent. Session tokens can be scoped to exact actions so if one leaks, exposure stays contained.
Can I create custom agent actions for my Fastbill MCP server?
Yes — you can create custom agent actions for your Fastbill MCP server using Connector Builder. It's an integration agent your coding assistant (Claude Code, Cursor, or Copilot) can invoke to research Fastbill's API, generate production-ready connector YAML, test against the live API, and validate before you ship.
When should I NOT use a Fastbill MCP server?
Skip a Fastbill MCP server if your integration is purely software-to-software — direct Fastbill API integration is simpler when no AI agent is involved. For deterministic, compliance-critical operations (financial transactions, regulatory reporting), direct API gives you predictable behavior without agent-driven decision-making. MCP shines when AI agents need to dynamically discover and call Fastbill actions at runtime.
What AI frameworks and AI clients does the StackOne Fastbill MCP server support?
The StackOne Fastbill MCP server supports both. MCP clients (paste-and-go apps): Claude Desktop, Claude Code, Cursor, VS Code, Goose. Agent frameworks (code SDKs you build with): OpenAI Agents SDK, Anthropic, Vercel AI, Google ADK, CrewAI, Pydantic AI, LangChain, LangGraph, Azure AI Foundry.
Put your AI agents to work
All the tools you need to build and scale AI agent integrations, with best-in-class connectivity, execution, and security.