ENABLE_TOOL_SEARCH Percentage Control / Claude Code v2.1.9

What

The ENABLE_TOOL_SEARCH environment variable now supports percentage-based control with the auto:N syntax.

Usage

ENABLE_TOOL_SEARCH=auto:50  # 50% tool search probability
ENABLE_TOOL_SEARCH=auto:0   # Equivalent to "tst" (always tool search)
ENABLE_TOOL_SEARCH=auto:100 # Equivalent to standard/mcp-cli modes

Details

Values are clamped between 0-100
Invalid values log a warning and fall back to defaults
Provides fine-grained control over tool search behavior

Evidence

DX0() at line 292496 (auto percentage parser, contains "Invalid ENABLE_TOOL_SEARCH value", "expected auto:N where N is a number")

Strings lifted out of the shipped bundle, so the claim above can be checked against them.

How sure we are

One source agreesOne thing we can check says the same as this entry.

Anthropic's release notes agreeAdded auto:N syntax for configuring the MCP tool search auto-enable threshold, where N is the context window percentage (0-100)

See this entry in the whole of v2.1.9 →

The build it came inv2.1.9

15 Jan 2026 · 11 entries, this one is #3.

Either side of it

v2.1.8Older
v2.1.10Newer

Open the release →

Names in this entry

envENABLE_TOOL_SEARCH
Tool search now runs on Vertex AI instead of being refused outright.
settingauto
You can set the auto-compact window at launch with --autocompact, using auto or 100k to 1M.

Anthropic's note

Upstream's own release notes for v2.1.9 name something this entry names, so the two are probably about the same change. Probably: the pairing is made on a shared name rather than on the words around it.

Added auto:N syntax for configuring the MCP tool search auto-enable threshold, where N is the context window percentage (0-100)

Is this right?