Concurrency Slot Tracking for Subagents / Claude Code v2.1.217

Subagents now properly claim and release a concurrency slot via takeConcurrencySlot() / onRunSettled(). The store now tracks runningSubagents as a counter. This enables accurate display of running agent count and proper enforcement of concurrent subagent limits (controlled by CLAUDE_CODE_MAX_CONCURRENT_SUBAGENTS).

Evidence

Concurrency tracking (search for "Concurrent subagent limit reached")

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 a cap on concurrently-running subagents (default 20, override with CLAUDE_CODE_MAX_CONCURRENT_SUBAGENTS) so one message can't fan out…

See this entry in the whole of v2.1.217 →

The build it came inv2.1.217

21 Jul 2026 · 27 entries, this one is #13.

Either side of it

v2.1.216Older
v2.1.218Newer

Open the release →

Names in this entry

envCLAUDE_CODE_MAX_CONCURRENT_SUBAGENTS
How many subagents can be running in one session before the Agent tool refuses to spawn another (default: 20). Accepts a positive whole number in plain digits; anything else is ignored, so the variable can adjust the cap but can't disable it. Requires Claude Code v2.1.217 or later

Anthropic's note

Upstream's own release notes for v2.1.217 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 a cap on concurrently-running subagents (default 20, override with CLAUDE_CODE_MAX_CONCURRENT_SUBAGENTS) so one message can't fan out unbounded background agents

Is this right?