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23 July 2026

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!!BREAKING CHANGES!!

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Summary

An empty mcp_filter (mcp_filter = []) now grants no MCP tools instead of every MCP tool. This applies to orchestration workers ([orchestration.worker.<name>].mcp_filter) and to the single-agent [agent].mcp_filter. An omitted mcp_filter still grants every MCP tool, so existing configs that don’t write the field are unaffected.

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Why

Previously mcp_filter = [] and an omitted filter both meant “all tools”, which made two things impossible or dangerous:
  • “No MCP tools” was unexpressible. Tool-free workers (like a writer that only synthesizes text) had to resort to a non-matching dummy pattern such as ["__none__"].
  • Removing the last allowlist entry silently widened access. Editing mcp_filter = ["logs_*"] down to mcp_filter = [] expanded the worker from one tool family to every tool — the opposite of the intent.
See issue #378.

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The new rule

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Migration

  • If a config has mcp_filter = [] and relied on receiving all tools: delete the line (omit the field).
  • If a config used a non-matching dummy pattern (e.g. ["__none__"]) to keep a worker tool-free: replace it with mcp_filter = [].
client_tool_filter is unchanged: an empty client-tool filter still means all client tools.