René's URL Explorer Experiment

{"@context":"https://schema.org","@type":"DiscussionForumPosting","headline":"Design: future of `dependencies` parameter on MCPServer","articleBody":"## Context\n\nPR #1877 removed the `dependencies` parameter from `FastMCP` (now `MCPServer`). After investigation, we're restoring it for now because removing it without a replacement breaks high-profile users and leaves a functionality gap. This issue tracks the design decision for what to do long-term.\n\n## What `dependencies` does\n\nIt's a list of pip/uv package names declared on the server instance:\n\n```python\nmcp = MCPServer(\"Screenshot Demo\", dependencies=[\"pyautogui\", \"Pillow\"])\n```\n\nThe **only consumer** is the `mcp` CLI — `mcp dev` and `mcp install` read `server.dependencies` via `hasattr`/`getattr` and convert them to `uv run --with \u003cpkg\u003e` flags when launching the server or writing Claude Desktop config. See https://github.com/modelcontextprotocol/python-sdk/blob/7ba4fb881d85406f44a5af8169fb7200fa7c8e49/src/mcp/cli/cli.py#L261-L262 and https://github.com/modelcontextprotocol/python-sdk/blob/7ba4fb881d85406f44a5af8169fb7200fa7c8e49/src/mcp/cli/cli.py#L451-L453.\n\nIt has **no relationship to the MCP protocol** or the low-level `Server` class — it's purely a Python/uv packaging convenience that originated in jlowin's original FastMCP.\n\n## Why we can't just remove it\n\n**Real-world usage is significant** (~772 files on GitHub, ~95+ repos):\n\n- **awslabs/mcp** (8.5k★) — 47 server files use it as their standard template\n- **mindsdb/mindsdb** (38.8k★) — uses it\n- **redis/mcp-redis** (461★), volcengine/mcp-server, camel-ai/camel, plus ~15–20 community servers\n\nWhen jlowin's `fastmcp` package removed the same parameter, it caused 8+ breakage issues across awslabs/mcp, zotero-mcp, panther-labs, ClickHouse — all `TypeError: unexpected keyword argument 'dependencies'`.\n\n## The replacement gap\n\nThe v2 examples were migrated to **PEP 723 inline script metadata**:\n\n```python\n# /// script\n# dependencies = [\"pyautogui\", \"Pillow\"]\n# ///\n```\n\nBut this **does not currently work with `mcp install` / `mcp dev`**. Those commands generate `uv run --with mcp[cli] mcp run server.py` — `uv` only parses PEP 723 when the script is the direct target, not when it's an argument to another command. So the feature was removed without a working replacement.\n\n## Options (Claude's ideas)\n\n### A. Keep `dependencies`, do nothing\nSimplest. Works. Mixes packaging metadata into server code, which is architecturally questionable.\n\n### B. Deprecate → PEP 723, teach the CLI to parse it\nAdd a `DeprecationWarning` to the parameter. Update `mcp dev`/`mcp install` to parse `# /// script` blocks from the target file and convert `dependencies = [...]` to `--with` flags. PEP 723 has a ~15-line reference parser in the spec. Remove the parameter in a later release.\n\n### C. Follow FastMCP 2.0's lead: config file\njlowin's FastMCP 2.0 replaced `dependencies=` with a `fastmcp.json` config file (source/environment/deployment sections), deprecated for ~3.5 months, then removed. They also expanded `install` to 7 targets (claude-desktop, claude-code, cursor, gemini-cli, goose, mcp-json, stdio). This is the richest solution but the most work, and overlaps with what FastMCP 2.0 already provides.\n\n### D. Deprecate → point users at FastMCP 2.0\nIf the official SDK's CLI is meant to stay minimal, deprecate `dependencies` and `mcp install`/`mcp dev` together, pointing users to `fastmcp` for deployment tooling.\n\n\u003csub\u003e[AI Disclaimer](https://gist.github.com/maxisbey/6123d132484e4c533eab519a2800693d)\u003c/sub\u003e","author":{"url":"https://github.com/maxisbey","@type":"Person","name":"maxisbey"},"datePublished":"2026-03-26T13:42:21.000Z","interactionStatistic":{"@type":"InteractionCounter","interactionType":"https://schema.org/CommentAction","userInteractionCount":2},"url":"https://github.com/2354/python-sdk/issues/2354"}