We recently added outputSchema support for our MCP tools (not yet for
worker routes yet). Today, we always return structuredContent. On tool
execution errors we return structuredContent: {"error": "..."} with
isError: True, even when that shape does not match the tool’s declared
outputSchema. Since the MCP spec says clients SHOULD validate
structuredContent against outputSchema, some clients reject these
responses.
Since structuredContent is optional, we’re going to omit it when
isError: true.
<!-- CURSOR_SUMMARY -->
---
> [!NOTE]
> **Medium Risk**
> Changes the shape of tool error responses across the MCP server, which
may break clients or tools that previously relied on
`structuredContent["error"]` for failures. Behavior is more
spec-compliant but touches core request/response paths and test
expectations.
>
> **Overview**
> Prevents MCP tool error responses from violating a tool’s declared
`outputSchema` by **always setting `structuredContent=None` when
`isError=True`** (server execution errors, unknown tools, middleware
exceptions, and `Context.tools.call_raw` JSON-RPC errors).
>
> Updates requirement-failure error formatting to put the human-friendly
message in `content[0]` and (when present) serialize extra
machine-readable fields (e.g. `authorization_url`, `llm_instructions`)
into an additional `content` item. Examples and integration/unit tests
are updated to read errors from `content[0].text`, and
`arcade-mcp-server` is bumped to `1.19.2`.
>
> <sup>Reviewed by [Cursor Bugbot](https://cursor.com/bugbot) for commit
4213bdd4aa44362de85c30f5f31c576243c132d5. Bugbot is set up for automated
code reviews on this repo. Configure
[here](https://www.cursor.com/dashboard/bugbot).</sup>
<!-- /CURSOR_SUMMARY -->
|
||
|---|---|---|
| .cursor/rules | ||
| .github | ||
| .vscode | ||
| examples | ||
| libs | ||
| schemas/preview | ||
| tests | ||
| .editorconfig | ||
| .gitignore | ||
| .pre-commit-config.yaml | ||
| .prettierignore | ||
| .prettierrc.toml | ||
| .ruff.toml | ||
| CLAUDE.md | ||
| CONTRIBUTING.md | ||
| cspell.config.yaml | ||
| LICENSE | ||
| Makefile | ||
| pyproject.toml | ||
| README.md | ||
| SECURITY.md | ||
| uv_setup.sh | ||
Arcade MCP Server Framework
-
To see example servers built with Arcade MCP Server Framework (this repo), check out our examples
-
To learn more about the Arcade MCP Server Framework (this repo), check out our Arcade MCP documentation
-
To learn more about other offerings from Arcade.dev, check out our documentation.
Pst. hey, you, give us a star if you like it!
Quick Start: Create a New Server
The fastest way to get started is with the arcade new CLI command, which creates a complete MCP server project:
# Install the CLI
uv tool install arcade-mcp
# Create a new server project
arcade new my_server
# Navigate to the project
cd my_server/src/my_server
This generates a project with:
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server.py - Main server file with MCPApp and example tools
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pyproject.toml - Dependencies and project configuration
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.env.example - Example
.envfile containing a secret required by one of the generated tools inserver.py
The generated server.py includes proper command-line argument handling and three example tools:
#!/usr/bin/env python3
"""simple_server MCP server"""
import sys
from typing import Annotated
import httpx
from arcade_mcp_server import Context, MCPApp
from arcade_mcp_server.auth import Reddit
app = MCPApp(name="simple_server", version="1.0.0", log_level="DEBUG")
@app.tool
def greet(name: Annotated[str, "The name of the person to greet"]) -> str:
"""Greet a person by name."""
return f"Hello, {name}!"
# To use this tool locally, you need to either set the secret in the .env file or as an environment variable
@app.tool(requires_secrets=["MY_SECRET_KEY"])
def whisper_secret(context: Context) -> Annotated[str, "The last 4 characters of the secret"]:
"""Reveal the last 4 characters of a secret"""
# Secrets are injected into the context at runtime.
# LLMs and MCP clients cannot see or access your secrets
# You can define secrets in a .env file.
try:
secret = context.get_secret("MY_SECRET_KEY")
except Exception as e:
return str(e)
return "The last 4 characters of the secret are: " + secret[-4:]
# To use this tool locally, you need to install the Arcade CLI (uv tool install arcade-mcp)
# and then run 'arcade login' to authenticate.
@app.tool(requires_auth=Reddit(scopes=["read"]))
async def get_posts_in_subreddit(
context: Context, subreddit: Annotated[str, "The name of the subreddit"]
) -> dict:
"""Get posts from a specific subreddit"""
# Normalize the subreddit name
subreddit = subreddit.lower().replace("r/", "").replace(" ", "")
# Prepare the httpx request
# OAuth token is injected into the context at runtime.
# LLMs and MCP clients cannot see or access your OAuth tokens.
oauth_token = context.get_auth_token_or_empty()
headers = {
"Authorization": f"Bearer {oauth_token}",
"User-Agent": "{{ toolkit_name }}-mcp-server",
}
params = {"limit": 5}
url = f"https://oauth.reddit.com/r/{subreddit}/hot"
# Make the request
async with httpx.AsyncClient() as client:
response = await client.get(url, headers=headers, params=params)
response.raise_for_status()
# Return the response
return response.json()
# Run with specific transport
if __name__ == "__main__":
# Get transport from command line argument, default to "stdio"
# - "stdio" (default): Standard I/O for Claude Desktop, CLI tools, etc.
# Supports tools that require_auth or require_secrets out-of-the-box
# - "http": HTTPS streaming for Cursor, VS Code, etc.
# Does not support tools that require_auth or require_secrets unless the server is deployed
# using 'arcade deploy' or added in the Arcade Developer Dashboard with 'Arcade' server type
transport = sys.argv[1] if len(sys.argv) > 1 else "stdio"
# Run the server
app.run(transport=transport, host="127.0.0.1", port=8000)
This approach gives you:
-
Complete Project Setup - Everything you need in one command
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Best Practices - Proper dependency management with pyproject.toml
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Example Code - Learn from working examples of common patterns
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Production Ready - Structured for growth and deployment
Running Your Server
Run your server directly with Python:
# Run with stdio transport (default)
uv run server.py
# Run with http transport via command line argument
uv run server.py http
# Or use python directly
python server.py http
python server.py stdio
Your server will start and listen for connections. With HTTP transport, you can access the API docs at http://127.0.0.1:8000/docs.
Configure MCP Clients
Once your server is running, connect it to your favorite AI assistant:
arcade configure claude # Configure Claude Desktop to connect to your stdio server in your current directory
arcade configure cursor --transport http --port 8080 # Configure Cursor to connect to your local HTTP server on port 8080
arcade configure vscode --entrypoint my_server.py # Configure VSCode to connect to your stdio server that will run when my_server.py is executed directly
Installing this Repo from Source
git clone https://github.com/ArcadeAI/arcade-mcp.git && cd arcade-mcp && make install
Support and Community
- Discord: Join our Discord community for real-time support and discussions.
- GitHub: Contribute or report issues on the Arcade GitHub repository.
- Documentation: Find in-depth guides and API references at Arcade Documentation.