[SandroMaglione]

Summary

• Restore the effect, @effect/atom-react, and @effect/platform-* requirements to ^4.0.0 in core, React, and devtools. 0.41.0 raised them to ^4.0.5, but nothing in the library depends on 4.0.5: the derived-atom field-selection test added in Upgrade to Effect 4.0.5 and document atom-level projections #230 passes on 4.0.0.
• Return the pinned development baseline to 4.0.0 so it remains the tested minimum, as the README states. The latest-effect CI job continues to cover the newest stable Effect 4 release.
• Keep the atom-level projection docs and test from Upgrade to Effect 4.0.5 and document atom-level projections #230.

Changeset

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

Patch: widening the supported range is compatible.

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	0.04s	0.03s
Import effect-machine	0.04s	0.03s
Machine.state (3 children)	0.10s	0.09s
Machine.make (3 states, 2 events)	0.15s	0.14s
machine.handle (3 states, 2 transitions)	0.32s	0.31s
declared transition (10 named branches)	0.39s	0.39s
atomic construction, guards, and reentry	0.52s	0.50s
registered invocation (input-taking Effect)	0.31s	0.32s
machine.handle (depth 24)	0.65s	0.65s
machine.handle (wide depth 16)	0.70s	0.73s
machine.handle (parallel/history/choice)	0.51s	0.54s
machine definition (3 independent implementations)	0.54s	0.56s
machine exact input/output/error/services	0.52s	0.51s
execution adapter readiness	0.44s	0.45s

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	115,576 transitions/s
Drain burst with terminal fence	406,594 increments/s
Drain burst with a change observer	377,642 increments/s
Lookup and send to one child	331,944 increments/s
Start and stop a machine	131,510 machines/s
Start and stop a parent with one child	26,115 families/s
Complete an Effect invocation with tracing enabled	7,266 invocations/s
Complete an Effect invocation with tracing disabled	8,935 invocations/s
Plan transitions through a compound state	102,866 transitions/s
Plan transitions through parallel regions	82,109 transitions/s
Drain burst through a compound state	347,952 events/s
Drain burst through two parallel regions	320,787 events/s
Drain a compound-state burst with a change observer	324,239 events/s

Process runtime reference points

Scenario	Effect Machine
Start and stop a raw generic process	21,387 processes/s
Start and stop a raw compiled process	73,605 processes/s

Memory profile	Effect Machine
Idle machine	1.8 KiB
Raw generic managed process	11.5 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	9.4 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	117,610 transitions/s	1.1% MAD	115,576 transitions/s	1.5% MAD	-1.7%
Drain burst with terminal fence	404,491 increments/s	1.2% MAD	406,594 increments/s	0.4% MAD	+0.5%
Drain burst with a change observer	380,342 increments/s	1.0% MAD	377,642 increments/s	0.1% MAD	-0.7%
Lookup and send to one child	332,719 increments/s	0.7% MAD	331,944 increments/s	0.6% MAD	-0.2%
Start and stop a machine	132,714 machines/s	0.6% MAD	131,510 machines/s	0.1% MAD	-0.9%
Start and stop a parent with one child	25,397 families/s	2.2% MAD	26,115 families/s	2.8% MAD	+2.8%
Complete an Effect invocation with tracing enabled	7,232 invocations/s	0.4% MAD	7,266 invocations/s	1.1% MAD	+0.5%
Complete an Effect invocation with tracing disabled	8,894 invocations/s	0.4% MAD	8,935 invocations/s	2.2% MAD	+0.5%
Plan transitions through a compound state	103,131 transitions/s	0.8% MAD	102,866 transitions/s	1.5% MAD	-0.3%
Plan transitions through parallel regions	80,376 transitions/s	0.9% MAD	82,109 transitions/s	1.9% MAD	+2.2%
Drain burst through a compound state	345,740 events/s	1.7% MAD	347,952 events/s	1.3% MAD	+0.6%
Drain burst through two parallel regions	321,138 events/s	1.0% MAD	320,787 events/s	1.4% MAD	-0.1%
Drain a compound-state burst with a change observer	324,327 events/s	1.5% MAD	324,239 events/s	0.3% MAD	-0.0%
Idle machine heap per unit	1.8 KiB	0.4% MAD	1.8 KiB	0.5% MAD	+0.5%
Raw generic managed process heap per unit	11.6 KiB	0.0% MAD	11.5 KiB	0.0% MAD	-0.7%
Raw compiled process heap per unit	3.0 KiB	0.4% MAD	3.0 KiB	0.4% MAD	-0.2%
Two independent idle machines heap per unit	3.4 KiB	0.9% MAD	3.4 KiB	0.2% MAD	-0.2%
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	9.5 KiB	0.0% MAD	9.4 KiB	0.0% MAD	-1.1%
Parent with observed invoked child snapshots heap per unit	6.4 KiB	0.2% MAD	6.4 KiB	0.0% MAD	-0.1%

Process runtime reference change from base

Metric	Base	Base variability	PR	PR variability	Difference
Start and stop a raw generic process	19,442 processes/s	3.6% MAD	21,387 processes/s	1.2% MAD	+10.0%
Start and stop a raw compiled process	72,223 processes/s	1.0% MAD	73,605 processes/s	0.2% MAD	+1.9%

Regression guard

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

Versions and interpretation

• Effect Machine: 0.41.0

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.