Add semantic machine testing and causal runtime probes

[SandroMaglione]

Summary

• add reusable state, step, and trace invariants for planner traces
• add bounded breadth-first exploration with shortest witnesses, explicit truncation frontiers, and fail-closed reachability assertions
• add testing-only causal runtime probes with acknowledged sendAndAwait delivery while keeping production MachineRef.send unchanged
• exercise the testing APIs in the media-player and platformer examples

Changeset

• Added or updated for a library or package-metadata change
• Not required because this PR does not change src/ or package.json

Validation

• pnpm check
• Relevant example checks, when examples changed
• Reviewed the automated type-performance report, when the public TypeScript API or inference changed
• Reviewed the automated runtime-performance report, when runtime behavior changed

Additional local validation:

• pnpm perf:types
• pnpm perf:runtime
• pnpm check in examples/playground
• pnpm check in examples/platformer

The runtime implementation has generic-versus-compiled differential coverage. Probe tests cover ignored and targetless events, value changes, reentry, raised-event macrosteps, invoke startup, interrupted waiters, stopped references, unsupported references, and processing failures.

[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.defineStates (3 states)	2,965	2,965	0 (0.0%)
Machine.make (3 states, 2 events)	8,715	8,715	0 (0.0%)
machine.handle (3 states, 2 transitions)	23,492	23,492	0 (0.0%)
machine.handle (depth 8)	117,075	117,075	0 (0.0%)
machine.handle (depth 12)	129,713	129,713	0 (0.0%)
machine.handle (depth 16)	145,055	145,055	0 (0.0%)
machine.handle (depth 24)	183,851	183,851	0 (0.0%)
machine.handle (wide depth 16)	214,189	214,189	0 (0.0%)
machine.handle (parallel/history/choice)	132,667	132,667	0 (0.0%)
machine.handle (4 successive calls)	128,674	128,674	0 (0.0%)
machine exact input/output/error/services	110,584	110,584	0 (0.0%)
execution adapter readiness	127,854	127,854	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.defineStates (3 states)	2,910	2,910	0 (0.0%)
Machine.make (3 states, 2 events)	5,742	5,742	0 (0.0%)
machine.handle (3 states, 2 transitions)	14,777	14,777	0 (0.0%)
machine.handle (depth 8)	109,601	109,601	0 (0.0%)
machine.handle (depth 12)	121,039	121,039	0 (0.0%)
machine.handle (depth 16)	135,181	135,181	0 (0.0%)
machine.handle (depth 24)	171,577	171,577	0 (0.0%)
machine.handle (wide depth 16)	202,224	202,224	0 (0.0%)
machine.handle (parallel/history/choice)	114,331	114,331	0 (0.0%)
machine.handle (4 successive calls)	113,703	113,703	0 (0.0%)
machine exact input/output/error/services	100,670	100,670	0 (0.0%)
execution adapter readiness	102,092	102,092	0 (0.0%)

Check times (informational)

Scenario	Base	PR
Effect only	0.03s	0.03s
Import effect-machine	0.04s	0.03s
Machine.defineStates (3 states)	0.11s	0.10s
Machine.make (3 states, 2 events)	0.17s	0.16s
machine.handle (3 states, 2 transitions)	0.23s	0.22s
machine.handle (depth 8)	0.51s	0.52s
machine.handle (depth 12)	0.57s	0.57s
machine.handle (depth 16)	0.63s	0.60s
machine.handle (depth 24)	0.72s	0.70s
machine.handle (wide depth 16)	0.78s	0.77s
machine.handle (parallel/history/choice)	0.60s	0.63s
machine.handle (4 successive calls)	0.60s	0.59s
machine exact input/output/error/services	0.51s	0.50s
execution adapter readiness	0.58s	0.62s

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 v24.18.0.

Pull request baseline

Scenario	Effect Machine	XState 5	XState 6 alpha
Plan counter transitions	131,858 transitions/s	129,858 transitions/s	12,987 transitions/s
Drain burst with terminal fence	503,917 increments/s	386,648 increments/s	187,202 increments/s
Drain burst with a change observer	465,160 increments/s	383,758 increments/s	186,985 increments/s
Lookup and send to one child	398,411 increments/s	382,107 increments/s	180,186 increments/s
Start and stop a machine	170,329 machines/s	181,455 machines/s	137,306 machines/s
Start and stop a parent with one child	31,687 families/s	67,627 families/s	40,117 families/s
Plan transitions through a compound state	117,707 transitions/s	—	—
Plan transitions through parallel regions	90,722 transitions/s	—	—
Drain burst through a compound state	495,108 events/s	341,647 events/s	185,189 events/s
Drain burst through two parallel regions	463,088 events/s	259,940 events/s	140,804 events/s
Drain a compound-state burst with a change observer	453,725 events/s	—	—

Effect runtime reference points

Scenario	Effect Machine	Effect runtime primitives
Start and stop a raw generic process	16,366 processes/s	—
Start and stop a raw compiled process	66,498 processes/s	—
Start and interrupt a suspended fiber	—	163,908 fibers/s
Start and stop a queue worker	—	115,260 workers/s
Start and stop an actor shell	—	106,078 actors/s
Start and stop two actor shells	—	64,562 families/s
Update an owner-only mutable snapshot	—	316,856,781 updates/s
Update a synchronized snapshot	—	1,524,228 updates/s
Create, resolve, and await a terminal latch	—	1,420,132 latches/s

Memory profile	Effect Machine	XState 5	XState 6 alpha	Effect runtime primitives
Idle machine	1.5 KiB	3.8 KiB	2.2 KiB	—
Raw generic managed process	12.7 KiB	—	—	—
Raw compiled process	2.8 KiB	—	—	—
Two independent idle machines	3.0 KiB	7.3 KiB	3.9 KiB	—
Idle parent with one child	4.8 KiB	5.5 KiB	4.0 KiB	—
Parent with observed child registry	9.0 KiB	—	—	—
Parent with observed invoked child snapshots	5.0 KiB	—	—	—
Suspended Effect fiber	—	—	—	0.6 KiB
Effect queue with waiting fiber	—	—	—	2.8 KiB
Effect mailbox actor shell	—	—	—	3.2 KiB
Two Effect actor shells	—	—	—	6.8 KiB

Effect Machine change from base

Metric	Base	Base variability	PR	PR variability	Difference
Plan counter transitions	131,098 transitions/s	1.8% MAD	131,858 transitions/s	0.4% MAD	+0.6%
Drain burst with terminal fence	521,974 increments/s	4.3% MAD	503,917 increments/s	1.2% MAD	-3.5%
Drain burst with a change observer	481,440 increments/s	4.2% MAD	465,160 increments/s	1.4% MAD	-3.4%
Lookup and send to one child	408,722 increments/s	4.1% MAD	398,411 increments/s	0.0% MAD	-2.5%
Start and stop a machine	167,196 machines/s	0.3% MAD	170,329 machines/s	0.2% MAD	+1.9%
Start and stop a parent with one child	32,997 families/s	4.2% MAD	31,687 families/s	1.1% MAD	-4.0%
Plan transitions through a compound state	119,432 transitions/s	0.5% MAD	117,707 transitions/s	0.3% MAD	-1.4%
Plan transitions through parallel regions	91,553 transitions/s	1.0% MAD	90,722 transitions/s	0.8% MAD	-0.9%
Drain burst through a compound state	513,880 events/s	3.7% MAD	495,108 events/s	2.4% MAD	-3.7%
Drain burst through two parallel regions	475,440 events/s	1.9% MAD	463,088 events/s	1.3% MAD	-2.6%
Drain a compound-state burst with a change observer	464,787 events/s	2.9% MAD	453,725 events/s	2.3% MAD	-2.4%
Idle machine heap per unit	1.5 KiB	0.2% MAD	1.5 KiB	0.1% MAD	+0.5%
Raw generic managed process heap per unit	12.1 KiB	0.0% MAD	12.7 KiB	0.0% MAD	+4.7%
Raw compiled process heap per unit	2.6 KiB	0.0% MAD	2.8 KiB	0.0% MAD	+8.1%
Two independent idle machines heap per unit	2.9 KiB	0.0% MAD	3.0 KiB	0.0% MAD	+0.4%
Idle parent with one child heap per unit	4.8 KiB	0.0% MAD	4.8 KiB	0.0% MAD	+0.4%
Parent with observed child registry heap per unit	8.9 KiB	0.0% MAD	9.0 KiB	0.0% MAD	+0.2%
Parent with observed invoked child snapshots heap per unit	5.0 KiB	0.0% MAD	5.0 KiB	0.0% MAD	-0.0%

Effect runtime reference change from base

Metric	Base	Base variability	PR	PR variability	Difference
Start and stop a raw generic process	17,632 processes/s	2.1% MAD	16,366 processes/s	0.9% MAD	-7.2%
Start and stop a raw compiled process	68,269 processes/s	0.3% MAD	66,498 processes/s	1.6% MAD	-2.6%
Start and interrupt a suspended fiber	164,718 fibers/s	0.8% MAD	163,908 fibers/s	0.8% MAD	-0.5%
Start and stop a queue worker	114,338 workers/s	0.2% MAD	115,260 workers/s	0.9% MAD	+0.8%
Start and stop an actor shell	104,965 actors/s	0.2% MAD	106,078 actors/s	1.1% MAD	+1.1%
Start and stop two actor shells	65,364 families/s	1.4% MAD	64,562 families/s	3.1% MAD	-1.2%
Update an owner-only mutable snapshot	316,856,781 updates/s	0.0% MAD	316,856,781 updates/s	0.0% MAD	0.0%
Update a synchronized snapshot	1,543,436 updates/s	1.2% MAD	1,524,228 updates/s	1.6% MAD	-1.2%
Create, resolve, and await a terminal latch	1,442,044 latches/s	1.4% MAD	1,420,132 latches/s	2.2% MAD	-1.5%

Versions and interpretation

• Effect Machine: 0.4.0
• XState 5: 5.32.5
• XState 6 alpha: 6.0.0-alpha.31
• Effect runtime primitives: 4.0.0-beta.107

Higher throughput is better; lower heap is better. Variability is the median absolute deviation across independent processes, relative to their median. Runtime measurements on shared GitHub-hosted hardware remain informational, so small differences should be confirmed across multiple workflow runs.