[SandroMaglione]

Summary

• Upgrade the pinned Effect ecosystem (effect, @effect/vitest, @effect/atom-react, @effect/platform-*) to 4.0.5 and raise published peer/dependency floors to ^4.0.5, keeping the pinned development baseline equal to the supported minimum. 4.0.5 includes Effect's fix for infinite derived-atom rebuilds.
• Replace the React guide's "until @effect/atom-react supports selection" note: upstream declined hook-level select/equality (Add selector and equality support to Atom React hooks Effect-TS/effect#7518) in favour of derived atoms. Document the atom-level pattern (Atom.family keyed by the machine atom + Atom.mapResult + Atom.withEquality).
• Add a React test proving a field projection is render-stable and suppresses rerenders when the projected value is unchanged (verified to fail without the custom equality).

Changeset

• Added or updated for a library or package-metadata change
• Not required because this PR does not change a publishable package

Minor: the supported Effect range narrows to ^4.0.5.

Validation

• pnpm check
• Automated type-performance measurement passed or was not required
• Automated runtime- and memory-performance measurement passed or was not required

🤖 Generated with Claude Code

[github-actions]

Type performance

Measured with TypeScript 6.0.3 and skipLibCheck=true.

Scenario	Base	PR	Difference
Effect only	55	55	0 (0.0%)
Import effect-machine	55	55	0 (0.0%)
Machine.state (3 children)	2,664	2,664	0 (0.0%)
Machine.make (3 states, 2 events)	8,932	8,932	0 (0.0%)
machine.handle (3 states, 2 transitions)	39,348	39,348	0 (0.0%)
declared transition (10 named branches)	60,165	60,165	0 (0.0%)
atomic construction, guards, and reentry	85,402	85,402	0 (0.0%)
registered invocation (input-taking Effect)	39,467	39,467	0 (0.0%)
machine.handle (depth 24)	194,526	194,526	0 (0.0%)
machine.handle (wide depth 16)	218,233	218,233	0 (0.0%)
machine.handle (parallel/history/choice)	91,633	91,633	0 (0.0%)
machine definition (3 independent implementations)	98,225	98,225	0 (0.0%)
machine exact input/output/error/services	82,791	82,791	0 (0.0%)
execution adapter readiness	71,416	71,416	0 (0.0%)

Marginal instantiations are measured against the matching setup without that API call:

Scenario	Base	PR	Difference
Import effect-machine	0	0	0
Machine.state (3 children)	2,609	2,609	0 (0.0%)
Machine.make (3 states, 2 events)	6,260	6,260	0 (0.0%)
machine.handle (3 states, 2 transitions)	30,416	30,416	0 (0.0%)
declared transition (10 named branches)	51,612	51,612	0 (0.0%)
atomic construction, guards, and reentry	70,062	70,062	0 (0.0%)
registered invocation (input-taking Effect)	32,330	32,330	0 (0.0%)
machine.handle (depth 24)	177,217	177,217	0 (0.0%)
machine.handle (wide depth 16)	201,702	201,702	0 (0.0%)
machine.handle (parallel/history/choice)	79,135	79,135	0 (0.0%)
machine definition (3 independent implementations)	87,544	87,544	0 (0.0%)
machine exact input/output/error/services	70,582	70,582	0 (0.0%)
execution adapter readiness	31,918	31,918	0 (0.0%)

Check times (informational)

Scenario	Base	PR
Effect only	0.03s	0.03s
Import effect-machine	0.03s	0.03s
Machine.state (3 children)	0.09s	0.09s
Machine.make (3 states, 2 events)	0.14s	0.14s
machine.handle (3 states, 2 transitions)	0.30s	0.31s
declared transition (10 named branches)	0.39s	0.40s
atomic construction, guards, and reentry	0.51s	0.51s
registered invocation (input-taking Effect)	0.31s	0.31s
machine.handle (depth 24)	0.64s	0.65s
machine.handle (wide depth 16)	0.72s	0.71s
machine.handle (parallel/history/choice)	0.52s	0.52s
machine definition (3 independent implementations)	0.55s	0.53s
machine exact input/output/error/services	0.49s	0.49s
execution adapter readiness	0.45s	0.44s

Type instantiations are the comparison metric. Check time varies with runner load and is informational only.

[github-actions]

Runtime performance

Median of 5 independent benchmark processes on AMD EPYC 7763 64-Core Processor with Node v24.21.0.

Pull request baseline

Scenario	Effect Machine
Plan counter transitions	116,492 transitions/s
Drain burst with terminal fence	401,726 increments/s
Drain burst with a change observer	376,607 increments/s
Lookup and send to one child	329,819 increments/s
Start and stop a machine	126,662 machines/s
Start and stop a parent with one child	26,398 families/s
Complete an Effect invocation with tracing enabled	7,109 invocations/s
Complete an Effect invocation with tracing disabled	8,701 invocations/s
Plan transitions through a compound state	102,466 transitions/s
Plan transitions through parallel regions	80,887 transitions/s
Drain burst through a compound state	342,858 events/s
Drain burst through two parallel regions	316,903 events/s
Drain a compound-state burst with a change observer	319,140 events/s

Process runtime reference points

Scenario	Effect Machine
Start and stop a raw generic process	19,579 processes/s
Start and stop a raw compiled process	71,911 processes/s

Memory profile	Effect Machine
Idle machine	1.8 KiB
Raw generic managed process	11.6 KiB
Raw compiled process	3.0 KiB
Two independent idle machines	3.4 KiB
Idle parent with one child	5.9 KiB
Parent with observed child registry	9.5 KiB
Parent with observed invoked child snapshots	6.4 KiB

Effect Machine change from base

Metric	Base	Base variability	PR	PR variability	Difference
Plan counter transitions	115,252 transitions/s	0.9% MAD	116,492 transitions/s	1.4% MAD	+1.1%
Drain burst with terminal fence	402,907 increments/s	0.8% MAD	401,726 increments/s	1.0% MAD	-0.3%
Drain burst with a change observer	375,734 increments/s	0.3% MAD	376,607 increments/s	0.2% MAD	+0.2%
Lookup and send to one child	331,297 increments/s	0.2% MAD	329,819 increments/s	1.1% MAD	-0.4%
Start and stop a machine	129,282 machines/s	1.5% MAD	126,662 machines/s	1.0% MAD	-2.0%
Start and stop a parent with one child	24,761 families/s	0.2% MAD	26,398 families/s	2.1% MAD	+6.6%
Complete an Effect invocation with tracing enabled	7,209 invocations/s	0.9% MAD	7,109 invocations/s	0.3% MAD	-1.4%
Complete an Effect invocation with tracing disabled	8,808 invocations/s	1.0% MAD	8,701 invocations/s	1.3% MAD	-1.2%
Plan transitions through a compound state	100,605 transitions/s	1.3% MAD	102,466 transitions/s	0.4% MAD	+1.9%
Plan transitions through parallel regions	80,463 transitions/s	1.4% MAD	80,887 transitions/s	1.0% MAD	+0.5%
Drain burst through a compound state	342,623 events/s	0.2% MAD	342,858 events/s	0.3% MAD	+0.1%
Drain burst through two parallel regions	318,090 events/s	0.5% MAD	316,903 events/s	0.4% MAD	-0.4%
Drain a compound-state burst with a change observer	321,844 events/s	0.8% MAD	319,140 events/s	0.5% MAD	-0.8%
Idle machine heap per unit	1.8 KiB	0.2% MAD	1.8 KiB	0.1% MAD	-0.1%
Raw generic managed process heap per unit	11.5 KiB	1.0% MAD	11.6 KiB	0.0% MAD	+0.7%
Raw compiled process heap per unit	3.0 KiB	0.4% MAD	3.0 KiB	0.0% MAD	-0.1%
Two independent idle machines heap per unit	3.4 KiB	0.0% MAD	3.4 KiB	0.4% MAD	+0.3%
Idle parent with one child heap per unit	5.9 KiB	0.0% MAD	5.9 KiB	0.0% MAD	+0.0%
Parent with observed child registry heap per unit	9.4 KiB	0.0% MAD	9.5 KiB	0.0% MAD	+1.1%
Parent with observed invoked child snapshots heap per unit	6.4 KiB	0.1% MAD	6.4 KiB	0.0% MAD	+0.4%

Process runtime reference change from base

Metric	Base	Base variability	PR	PR variability	Difference
Start and stop a raw generic process	21,342 processes/s	0.8% MAD	19,579 processes/s	1.0% MAD	-8.3%
Start and stop a raw compiled process	72,854 processes/s	1.3% MAD	71,911 processes/s	0.3% MAD	-1.3%

Regression guard

No large, noise-adjusted throughput or heap regressions detected.

Versions and interpretation

• Effect Machine: 0.40.0

Higher throughput is better; lower heap is better. Variability is the median absolute deviation across independent processes, relative to their median. Small differences on shared GitHub-hosted hardware remain informational; the required guard rejects only large changes beyond the measured noise allowance.