Kairos: Semantics-Aware Orchestration of Planning Engines across Classical, Numeric, Temporal and Hybrid PDDL

Francisco Martín Rico; Esther Aguado González; Francisco Miguel Moreno Olivo; Rodrigo Pérez Rodríguez; José Miguel Guerrero Hernández; Francisco J. Rodríguez Lera

cs.AIOct 29, 2026

Kairos: Semantics-Aware Orchestration of Planning Engines across Classical, Numeric, Temporal and Hybrid PDDL

Authors: Francisco Martín Rico, Esther Aguado González, Francisco Miguel Moreno Olivo, Rodrigo Pérez Rodríguez, José Miguel Guerrero Hernández, Francisco J. Rodríguez Lera

Organizations: Intelligent Robotics Lab, Universidad Rey Juan Carlos, Spain · Robotics Group, Universidad de León, Spain

Abstract

Planning at run time, as in robots that re-plan while they act, needs a planner that reads the PDDL its users write (durative actions, numeric resources, time windows, continuous change), answers quickly, and delivers only validated plans. Specialised planners are strong on individual fragments of PDDL, but they differ in coverage, latency and in what their failures mean. We study whether selecting heterogeneous engines from the semantics of each task can give broad coverage of PDDL, from STRIPS to PDDL+, while remaining competitive with specialised planners and making explicit what each answer guarantees. Kairos profiles each task to decide which of five engines apply (ground heuristic search, compressed and snap-action temporal search, time steps for hybrid dynamics, lifted search) and schedules them under one budget of time and memory. Every candidate plan is checked by a validator independent of the planner's transformations, and "unsolvable" is reported only when a search whose exhaustion is a proof exhausts its space. On 1,756 tasks from nine comparative suites, Kairos solves more tasks at the full limits than the strongest evaluated planner of each suite, with the largest differences on numeric, temporal, PDDL3 and PDDL+ planning, and it also solves the most tasks at 100 ms and 1 s. Resampling over domains, the gain is above zero in six of the nine suites, while on classical planning against LAMA it is within the noise. A further 450 feature-oriented tasks and 26 one-feature probes show that Kairos handles all seventeen features probed, where no other planner evaluated handles more than eight. Ablations identify compressed temporal search and the novelty fallback as major contributors, and a comparison with VAL traces every disagreement to a PDDL+ convention or to a documented VAL defect. Planner and benchmark are open source.

Figures & tables

Feature	LAMA	P.lifted	M-FF	ENHSP	POPF	OPTIC	TFD	LPG-td	NextFLAP	SMTPlan+	Kairos
conditional effects	✓	✗	✓	✓	✗	✗	$\LEFTcircle$	✗	$\LEFTcircle$	✗	✓
quantified preconditions	✓	✗	✓	✓	✓	✓	✓	✓	$\LEFTcircle$	$\LEFTcircle$	✓
disjunctive preconditions	✓	✓	✓	✓	$\LEFTcircle$	$\LEFTcircle$	✓	✓	$\LEFTcircle$	$\LEFTcircle$	✓
derived predicates	✓	✗	✗	✗	✗	✗	$\LEFTcircle$	✓	✗	✗	✓
numeric fluents	✗	✗	✓	✓	✓	✓	✓	✓	✓	✓	✓
non-linear numeric conditions	✗	✗	✗	✓	✗	✗	✓	✓	✓	✓	✓

Table 1. PDDL feature support, measured by execution rather than by declaration (Section 12.3 ). P.lifted: Powerlifted; M-FF: Metric-FF. Each planner ran, for 10 s, on 26 trivial tasks in the style of the IPC domains: one per feature, plus a variant with durative actions of each non-temporal one. ✓ : it solved the feature’s task with a plan that the validator accepts (for preferences, one that satisfies the preference).

\LEFTcircle

: partial or doubtful: it crashed or returned an invalid plan on the feature’s task, or solved only another task that uses the feature. ✗ : it rejected every task that uses the feature, or gave no answer. Symbols differ in shape as well as colour.

System	Fragments	Components	Budget	Validator	Unsolvable
FD Stone Soup, IBaCoP	classical	configurations	time	no	some
OPTIC, POPF	temporal, numeric	one planner	–	no	no
ENHSP	numeric, PDDL+	configurations	–	built in	some
Unified Planning	per engine	external planners	–	yes	per engine
Kairos	all of Table 1	five engines	time, memory	yes	per engine

Table 2. Design differences with the closest systems. Budget: what the components share; Validator: whether the system validates plans itself; Unsolvable: which answers of unsolvability are proofs (Kairos: Table 3 ).

Panel of Figure 1 — Figure 1. The pipeline of Kairos. The orchestrator runs phases over the engines that the task profile allows, all under one budget of time and memory; grounding hands over to the lifted engine when it does not fit. The validator reads the parsed PDDL, not the planner’s task.

Fragment	Engines	Unsolvable only when	Optimality
classical, numeric (no time)	M1, lifted	M1 or the lifted engine exhausts the reachable space (Prop. 8.1 )	proven when a restart pruning by cost exhausts (Prop. 8.2 )
temporal, simple by value, fixed durations	M2, M3	the task without times has no path to the goal (Prop. 5.6 ); never from M2 (Example 5.5 )	not pursued: search optimises makespan
temporal with required concurrency or timed literals	M3	the task without times has no path to the goal (Prop. 5.6 ); never from M3 (memoisation is incomplete)	not plan cost
duration inequalities, continuous effects, PDDL+	time steps	never: no plan found	not plan cost
any	any	or grounding proves a goal unreachable even ignoring deletes

Table 3. What unsolvable and optimal mean, per fragment. Every returned plan has passed the validator (Section 9 ), under the semantics and tolerances stated there.

Fragment	A validated plan is
classical, numeric (no time)	valid under the semantics of PDDL, as parsed, in floating point
temporal	valid under the semantics of PDDL 2.1, with $\epsilon=0.001$ and a duration tolerance of 0.001
PDDL+, linear change	valid under ENHSP’s convention for steps at the same instant, with exact integration up to floating point
non-linear dynamics	accepted by a numerical integration (RK4, steps of 0.01 s, bisection on watched conditions); not certified

Table 4. What validation guarantees for each fragment. Validity refers to the PDDL model; it does not imply that the plan can be executed by a physical system.

Suite	Tasks	Content
Classical	220	IPC 1998–2023, 10 per domain
Numeric	420	IPC 2002, 2023, 2026, 10 per domain
Temporal	225	IPC 2002–2014, 5 per domain
Concurrency	55	required concurrency; temporal numeric
PDDL3	125	IPC 2006 preferences and constraints
PDDL+	255	hybrid domains; SMTPlan benchmarks

Table 5. Benchmark suites: 2 232 tasks in three groups. The nine comparative suites (1 756 tasks, from Classical to PlanSys2 at scale) are run by every planner that reads them (Tables 6 and 7 ); the feature bank (450 tasks) checks Kairos’s answers per feature, including the tasks without a plan of Section 12.6 , and is not compared; the 26 probes serve only Table 1 .

Suite (tasks)	Planner	100 ms	1 s	limit
Classical (220)	Kairos	76	111	166
	LAMA (anytime)	–	–	160
	lama-first	0	75	159
	Dual-BFWS	57	88	133
Numeric (420)	Kairos	168	216	299
	ENHSP	0	128	208

Table 6. Tasks solved with a valid plan within 100 ms, 1 s and the limit, with uncorrected times; per suite, Kairos and the best other planners. Every planner, with the counts without the start-up of Apptainer, is in Table 12 .

Figure 3. Tasks solved against time to the first plan (log scale), per suite: Kairos and the two best other planners that report their first plan. Grey lines at 100 ms and 1 s.

Suite	Best other	$\Delta$ solved	95% interval	domains $+$ / $-$	sign test $p$
Classical	LAMA	+6	[-7, +17]	10 / 3	0.092
Numeric	ENHSP	+91	[+40, +143]	24 / 6	0.001
Temporal	LPG-td	+55	[+31, +80]	24 / 6	0.001
Concurrency	POPF	+9	[-1, +22]	3 / 1	0.625
PDDL3	OPTIC	+41	[+15, +67]	15 / 6	0.078
PDDL+	SMTPlan+	+113	[+73, +155]	22 / 0	$<$ 0.001

Table 7. Difference in tasks solved at the limit between Kairos and the best other planner, with a bootstrap interval over domains, the domains where each solves more, and a two-sided sign test over those domains.

Suite	solved	p50 (s)	p90 (s)	p99 (s)
Classical	166	0.145	17.09	28.46
Numeric	299	0.053	9.94	25.52
Temporal	180	0.061	3.76	26.08
Concurrency	33	0.012	0.65	4.77
PDDL3	86	0.026	2.04	22.40
PDDL+	148	0.307	6.58	20.65

Table 8. Time to Kairos’s first plan over the tasks it solves: median and 90th and 99th percentiles.

Suite	Planner	IPC score	tasks with a cost
Classical	Kairos	154.5	166
Classical	LAMA	152.5	160
Classical	lama-first	127.4	159
Classical	Scorpion Maidu	148.1	154
Classical	Scorpion Maidu agile	113.4	142
Classical	Levitron	149.5	156

Table 9. Quality at the limit (IPC score) on the suites where all planners report the problem’s metric.

Suite	Variant	100 ms	1 s	limit
Temporal	full	102	147	180
	M2 only	101	146	179
	M3 only	49	86	124
Concurrency	full	25	32	33
	M2 only	5	5	5
	M3 only	25	32	33

Table 10. Ablation: Kairos against variants that run one engine or one search configuration alone ( --engine , --search ), same limits and protocol.

Task (family)	Kairos	lama-first	OPTIC	POPF	ENHSP
object-fluents/delivery	unsolvable	error	error	error	error
object-fluents/delivery-time	unsolvable	error	error	error	error
object-fluents/elevator	unsolvable	error	error	error	error
object-fluents/keys	unsolvable	error	error	error	error
timed-initial-fluents/charger	unsolvable	error	error	error	error
timed-initial-fluents/market	unsolvable	error	error	error	error

Table 11. Answers on the eleven generated tasks of the feature bank that have no plan. “error”: the planner rejected the task (none of them supports object fluents, timed initial fluents or the warehouse’s features).

Appendix figures & tables2 assets

Supplementary material from the paper’s appendix.

Appendix

Suite	Planner	100 ms	1 s	limit
Classical	Kairos	76	111	166
	LAMA	–	–	160
	lama-first	0	75	159
	Scorpion Maidu	–	–	154
	Scorpion Maidu agile	0	73	142
	Levitron	–	–	156

Appendix

Table 12. Tasks solved within 100 ms, 1 s and the limit, by every planner, with uncorrected times; in parentheses, the counts without the start-up of planutils’s Apptainer images. “of

n

”: a run on a subset of the suite.

Suite	Domain	Kairos	Better
Classical	ipc-2000/blocks-strips-typed	9/10	Levitron 10, Scorpion Maidu 10
Classical	ipc-2002/depots-strips-automatic	9/10	Levitron 10, Scorpion Maidu 10, Dual-BFWS 10
Classical	ipc-2004/pipesworld-no-tankage-nontemporal-strips	9/10	Dual-BFWS 10
Classical	ipc-2006/trucks-propositional-strips	5/10	FDSS 2023 6, Levitron 6, Scorpion Maidu 6, Scorpion Maidu agile 6, Levitron agile 6
Classical	ipc-2011/parking-sequential-satisficing	7/10	LAMA 10, lama-first 10, FDSS 2023 8, Levitron 9, Scorpion Maidu 9, Dual-BFWS 10
Classical	ipc2023/folding	2/10	LAMA 3, lama-first 3, Levitron 3, Scorpion Maidu 3

Appendix

Table 13. Domains where another planner solves more tasks than Kairos (at the limit).

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