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chore(deps): update fastmcp requirement from <4.0.0,>=3.0.0 to >=3.0.0,<5.0.0 #1222

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chore(deps): update fastmcp requirement from <4.0.0,>=3.0.0 to >=3.0.0,<5.0.0 #1222

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2 changes: 1 addition & 1 deletion pyproject.toml
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Expand Up @@ -30,7 +30,7 @@ dependencies = [
"tomli-w>=1.2.0",
"websockets>=15.0.1",
# MCP support
"fastmcp>=3.0.0,<4.0.0",
"fastmcp>=3.0.0,<5.0.0",

@cursor cursor Bot Sep 22, 2026

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FastMCP 4 drops tool schemas

Medium Severity

Widening fastmcp to &lt;5.0.0 installs FastMCP 4, which uses MCP SDK v2 snake_case models. _tool_from_server_tool reads inputSchema and falls back to {} when that attribute is absent, and WebSocket tools/list dumps models without aliases, so agents and MCP clients receive empty or non-spec tool schemas.

Fix in Cursor Fix in Web

Reviewed by Cursor Bugbot for commit 63de2da. Configure here.

@cursor cursor Bot Sep 22, 2026

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Tier 1 — FastMCP 4 breaks tool schemas. Widening to <5 installs FastMCP 4 (MCP SDK v2 snake_case). src/openenv/core/env_server/mcp_environment.py _tool_from_server_tool only reads camelCase inputSchema and falls back to {}, so agents/MCP clients get empty schemas. Keep <4.0.0 (or add FastMCP 4-compatible schema extraction + regressions) before merging; this is package cargo and must not land in 0.6.0 without that adapter work.

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This widens the core fastmcp bound to allow the 4.x major line. I verified uv sync --all-extras resolves fastmcp==4.0.5 and the MCP suite passes (116 tests), and the risky 4.x removals (ctx.elicit/sample/list_roots) aren't used here — so it's functional.

Two things worth a look before merge:

  1. fastmcp 4.x is an MCP-SDK-v2 rebuild (new sessionless protocol era) governed by RFC 003 – MCP Support (In Review).
  2. It surfaces a FastMCPDeprecationWarning for tool.inputSchema at src/openenv/core/env_server/mcp_environment.py:111 (bridged/works today; migrating to .input_schema future-proofs it).

Since the root uv.lock is git-ignored, there's no committed lockfile floor pinning a known-good 4.x for the core package.

# Web UI dependencies
"gradio>=4.0.0",
"httpx>=0.28.1",
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