SLCA-GRPO: Resolving Cross-Segment Credit Misattribution in Tool-Calling RL

Yan Zhan; Shaobo Liu; Qiunan Liu; Yuanjun Shi; Siqi Xu; WeiYi Hou; Xiang Xu; Zekang Li

cs.AISep 24, 2026

SLCA-GRPO: Resolving Cross-Segment Credit Misattribution in Tool-Calling RL

Authors: Yan Zhan, Shaobo Liu, Qiunan Liu, Yuanjun Shi, Siqi Xu, WeiYi Hou, Xiang Xu, Zekang Li, +2 more

Organizations: Peking University · Shenzhen University · Tencent PCG QQ Team

Abstract

Tool-calling agents produce heterogeneous outputs, interleaving structured tool invocations with user-facing natural language summaries. This output heterogeneity presents a structural failure mode in standard on-policy Reinforcement Learning (RL): algorithms like GRPO indiscriminately broadcast a homogeneous trajectory-level scalar advantage to all tokens. Consequently, gradient noise from summary generation leaks into tool-decision tokens, causing cross-segment credit misattribution and brittle optimization. In this work, we propose SLCA-GRPO, a framework incorporating Segment-Locked Credit Assignment (SLCA). To enable scalable exploration without costly real APIs and stable training, we first construct the Schema-Guided LLM Simulator (SGLS) as foundational training infrastructure. Building on this, SLCA decouples advantage estimation at the structural segment level within a single group of rollouts, without requiring additional rollouts from intermediate states. Supported by Hierarchical Rewards (HierR), SLCA routes execution advantages to tool tokens and preference advantages to summary tokens, eliminating advantage contamination (the dominant cross-segment credit misattribution channel) within each policy update. On a 7B backbone, SLCA-GRPO accelerates convergence and outperforms standard GRPO, ToolPO, and RLTR by +2.53 pp on in-domain evaluation, +1.36 pp on the Berkeley Function-Calling Leaderboard (BFCL), and +9.15 pp on $τ^2$ -Bench under the same training budgets, achieving higher accuracy with reduced tool redundancy and costs.

Figures & tables

Diagnostic cases. Unified reward aggregation can misassign credit across tool and summary segments. Left: an unnecessary tool call is paired with a correct delivery summary. Right: correct tool evidence is paired with an incorrect refund summary. Under standard GRPO, the mixed advantage can reward bad tool use or penalize good tool use.

Figure 1 : Overview of the SLCA -GRPO Framework. (Top) Infrastructure: SGLS enables scalable exploration via schema-constrained simulation, generating interleaved trajectories. (Middle) Signal: HierR decouples feedback into dense execution rewards ( ${\color[rgb]{0.418,0.3711,0.8203}R^{\mathrm{tool}}}$ ) and terminal outcome rewards ( ${\color[rgb]{0.3711,0.6406,0.3047}R^{\mathrm{sum}}}$ ). (Bottom) Optimization: SLCA decouples segment-wise advantages. By normalizing and routing advantages independently ( ${\color[rgb]{0.418,0.3711,0.8203}\hat{A}^{\mathrm{tool}}}$ vs. ${\color[rgb]{0.3711,0.6406,0.3047}\hat{A}^{\mathrm{sum}}}$ ), it blocks the defined summary-to-tool support path within each policy update.

Method	Name F1	ArgMatch	Process	Success
Qwen2.5-3B-Instruct
Original	0.5207	0.5073	0.5174	0.3816
SFT	0.7685 $\pm$ .0069	0.7132 $\pm$ .0239	0.7401 $\pm$ .0050	0.6500 $\pm$ .0089
SFT+GRPO	0.8910 $\pm$ .0087	0.8072 $\pm$ .0154	0.8577 $\pm$ .0057	0.7412 $\pm$ .0115
RLTR	0.7598 $\pm$ .0149	0.6353 $\pm$ .0229	0.6924 $\pm$ .0162	0.6627 $\pm$ .0151
ToolPO	0.8050 $\pm$ .0142	0.7769 $\pm$ .0160	0.8368 $\pm$ .0141	0.5128 $\pm$ .0145

Table 1 : Toucan-Test Results. Metrics: Name F1: Multiset F1 score measuring precision and recall of predicted tool-name occurrences against gold names; ArgMatch: Arithmetic mean of argument key and value matching scores; Process: The dense tool-segment reward (

S_{\text{process}}

) defined in Eq. 22 (App. A.4 ); Success: Strict binary indicator (

\mathbb{I}[S_{\text{process}}\geq 0.9]

) obtained by thresholding the process score. All trained entries report mean

\pm

std over three runs; Original rows are point evaluations. The matched rows share the per-backbone SFT initialization, data split, SGLS endpoint, mocker configuration, decoding settings, evaluation protocol,

G=16

, one RL epoch, and run set. GRPO, SLCA -GRPO , w/o SLCA, and w/o SGLS also share the HierR definition and subweights; w/o HierR removes HierR. ToolPO and RLTR retain their method-specific protocols. The ToolPO rows use the LLM-judge outcome reward; App. C.4 reports how this baseline responds to a rule-based outcome reward under the same estimator.

Panel of Figure 2 — Figure 2 : Training Dynamics. (a) Baseline comparison across 3B/7B/8B for SLCA / ToolPO / RLTR (the ToolPO diagnostic is described in App. D.3 ), and (b) cross-scale dynamics of success rate and average tool turns over training steps.

Panel of Figure 3 — Figure 3 : OOD Generalization across Scales. (a) Atomic generalization on BFCL V3 and (b) Collaboration robustness on $\tau^{2}$ -Bench for the three backbones (3B, 7B, 8B), comparing SLCA -GRPO against standard GRPO, RLTR, and ToolPO. Per-domain $\tau^{2}$ -Bench details are in App. D.6 .

	Toucan-Test		BFCL	$\tau^{2}$ -Bench
Setting	Process	Success	Acc	Pass 1
SLCA -GRPO	0.8766 $\pm$ .0045	0.7913 $\pm$ .0105	0.6977 $\pm$ .0047	0.4102 $\pm$ .0101
w/o SLCA	0.8667 $\pm$ .0061	0.7660 $\pm$ .0127	0.6841 $\pm$ .0011	0.3187 $\pm$ .0297
w/o SGLS	0.8787 $\pm$ .0051	0.7802 $\pm$ .0112	0.6875 $\pm$ .0048	0.3619 $\pm$ .0195
w/o HierR	0.8521 $\pm$ .0083	0.7344 $\pm$ .0121	0.6907 $\pm$ .0046	0.3450 $\pm$ .0144

Table 2: Ablation Studies on Qwen2.5-7B-Instruct. Entries report mean

\pm

std over three independent runs. Full results are provided in App. D.10 . Process is an evaluation metric computed post hoc for all settings, including w/o HierR.

Appendix figures & tables30 assets

Supplementary material from the paper’s appendix.

Appendix

Figure 4 : Mechanism of Cross-Segment Credit Misattribution and segment-locked routing. Unified advantage broadcast leaks summary rewards into tool tokens, whereas SLCA blocks this path by routing segment-wise advantages only to their corresponding tokens.

Parameter	Value
Model Specification
Model	GPT-OSS-120B
Parameters	120B
Serving Framework	vLLM (OpenAI-compatible API)
Decoding
Temperature	0.0 (fully greedy / deterministic)

Appendix

Table 3: Summary Judge: Deployment and Decoding Parameters.

Rating	Raw Score ( $r$ )	Normalized $S_{{\color[rgb]{0.3711,0.6406,0.3047}\mathrm{summary}}}$
very poor	1	0.00
poor	2	0.25
acceptable	3	0.50
good	4	0.75
excellent	5	1.00

Appendix

Table 4: Summary Judge: Rating-to-Score Mapping.

Figure 5 : Gradient Norm Dynamics (

\|\nabla_{\theta}\|_{2}

) during Training. We compare the gradient norms of SFT+GRPO (w/o SLCA) (Orange) and SLCA -GRPO (Blue) across Qwen2.5-3B, 7B, and Qwen3-8B. These are representative single-run traces and provide a descriptive view of update variability under the two estimators.

Model Backbone	Ours Std ( $\sigma_{\text{SLCA}}$ )	Baseline Std ( $\sigma_{\text{Base}}$ )	Improvement ( $\downarrow$ )
Qwen2.5-3B-Instruct	0.156	0.209	25.0%
Qwen2.5-7B-Instruct	0.094	0.108	13.1%
Qwen3-8B-Base	0.312	0.320	2.5%

Appendix

Table 5: Quantification of Gradient Stability. We report the Standard Deviation (Std) of the gradient norm. SLCA -GRPO has lower observed volatility in these representative runs.

Figure 6 : Data Processing Pipeline. The workflow applies strict relevance filtering and format check before isolating the evaluation set. The remaining training pool is split into SFT and RL partitions; the RL set combines native single-turn and decomposed multi-turn trajectories formatted for the SGLS.

Fine-Tuning Hyperparameters		RL Hyperparameters
Parameter	Value	Parameter	Value
Hardware Resources	8 $\times$ H20 GPUs	Hardware Resources	32 $\times$ H20 (4 Nodes)
Optimizer	AdamW	Precision	bfloat16
Total Batch Size	32	Total Batch Size	128
Per-Device Batch	1	Mini-batch Size	32
Grad Accumulation	4	Rollout Group Size ( $G$ )	16

Appendix

Table 6: Detailed Hyperparameters and Environment Settings. We list the configurations for both the supervised fine-tuning (SFT) and the subsequent RL training stages.

Parameter	Value
vLLM Deployment
Tensor-Parallel Size (TP)	8
Max Model Length	32,768
GPU Memory Utilization	0.86
Max Concurrent Sequences	300
Precision (dtype)	bfloat16

Appendix

Table 7: SGLS Frozen LLM: Deployment and Decoding Parameters.

Figure 7 : SGLS mock responses align with real-API outputs across three representative tool domains. The Semantic Mapping $\phi(o)$ extracts schemas, facts, and logic, realizing three levels of alignment (Mapping Marks): Identical Value for deterministic tools ( calculator ), Identical Schema for query-style tools ( weather ; individual values need not match a specific real-API snapshot but remain within plausible ranges), and Semantic Equivalence for open-ended tools ( search ; factually consistent content differing only in phrasing).

Method	Toucan Succ. (%)	BFCL Acc. (%)	$\tau^{2}$ -Bench Pass 1 (%)
SFT+GRPO (w/o SLCA)	75.74 $\pm$ 1.47	68.08 $\pm$ 0.36	32.74 $\pm$ 2.66
SLCA -GRPO	78.57 $\pm$ 1.15	69.32 $\pm$ 0.56	39.90 $\pm$ 1.46
$\Delta$	+2.83	+1.24	+7.16

Appendix

Table 8: Response-mocker check on Qwen2.5-7B-Instruct. GPT-OSS-120B replaces the response mocker; entries are mean

\pm

standard deviation under the matched policy setup.

Condition	Toucan Succ. (%)	BFCL Acc. (%)	$\tau^{2}$ -Bench Pass 1 (%)
ToolPO, LLM-judge $R_{\text{succ}}^{\text{judge}}$	20.00 $\pm$ 1.36	24.59 $\pm$ 0.87	30.44 $\pm$ 2.15
ToolPO, reference rule $R_{\text{succ}}^{\text{rule}}$	77.35 $\pm$ 1.19	68.86 $\pm$ 0.43	34.18 $\pm$ 2.42
Unified GRPO	76.60 $\pm$ 1.27	68.41 $\pm$ 0.11	31.87 $\pm$ 2.97
SLCA -GRPO	79.13 $\pm$ 1.05	69.77 $\pm$ 0.47	41.02 $\pm$ 1.01

Appendix

Table 9: ToolPO outcome reward protocol comparison on Qwen2.5-7B-Instruct. Entries are mean

\pm

standard deviation over three matched runs. The reference rule row uses Eq. 28 ; Unified GRPO and SLCA -GRPO are shown as contextual matched baselines under their standard reward protocol.

Parameter	Value
tau2 package	v0.2.1.dev0 (from @v0.2.0 tag)
Evaluation framework	evalscope 1.3.0
Task split	airline, retail, telecom (all 3 official domains)
Dataset source	evalscope/tau2-bench-data (ModelScope)
Task filtering	LLMGTAgent.check_valid_task()
Aggregation	mean_and_pass_hat_k (built-in)

Appendix

Table 10:

\tau^{2}

-Bench evaluation protocol.

Parameter	Value
Evaluation framework	evalscope 1.3.0
Evaluation mode	is_fc_model=True
Temperature	0 (greedy)
Max tokens	8192
parallel_tool_calls	True
underscore_to_dot	True

Appendix

Table 11: BFCL V3 evaluation protocol.

Parameter	Value
Dataset	tool_call_rl_40k_v1.parquet (4,000 held-out)
Agent loop max turns	10
Temperature	0.0 (greedy)
Max tokens	12,288
Top- $p$	1.0
Concurrency	256

Appendix

Table 12: Toucan in-domain evaluation protocol.

Candidate Size ( $K$ )	Original	20	50	100	150
Valid Samples	3,991	3,980	3,882	2,607	443
Avg. Tokens	943	4,614	11,064	17,177	23,635
Std. Dev.	1,114	3,679	5,256	1,550	733
Skip Rate	0.2%	0.5%	2.9%	34.8%	88.9%

Appendix

Table 13: System prompt length statistics vs. candidate set size (

K

) for Random distractors (Qwen2.5 tokenizer).

Candidate Size ( $K$ )	Original	20	50	100	150
Valid Samples	3,991	3,959	3,793	3,023	2,034
Avg. Tokens	943	3,693	8,522	14,483	20,609
Std. Dev.	1,114	2,735	3,806	2,595	1,719
Skip Rate	0.2%	1.0%	5.2%	24.4%	49.2%

Appendix

Table 14: System prompt length statistics vs. candidate set size (

K

) for Hard distractors (Qwen2.5 tokenizer).

Method	Name F1	ArgMatch	Parallel	Process	Summary	Success
Qwen2.5-3B-Instruct
Original	0.5207	0.5073	0.5412	0.5174	0.3477	0.3816
SFT	0.7685 $\pm$ .0069	0.7132 $\pm$ .0239	0.7772 $\pm$ .0071	0.7401 $\pm$ .0050	0.5512 $\pm$ .0047	0.6500 $\pm$ .0089
SFT+GRPO	0.8910 $\pm$ .0087	0.8072 $\pm$ .0154	0.9176 $\pm$ .0080	0.8577 $\pm$ .0057	0.7842 $\pm$ .0122	0.7412 $\pm$ .0115
RLTR	0.7598 $\pm$ .0149	0.6353 $\pm$ .0229	0.6729 $\pm$ .0098	0.6924 $\pm$ .0162	–	0.6627 $\pm$ .0151
ToolPO	0.8050 $\pm$ .0142	0.7769 $\pm$ .0160	0.8946 $\pm$ .0137	0.8368 $\pm$ .0141	0.6189 $\pm$ .0117	0.5128 $\pm$ .0145

Appendix

Table 15: Comprehensive Results on Toucan-Test across Scales. This table expands upon the main results by including all sub-metrics. In addition to Name F1, ArgMatch, Process, and Success (defined in Table 1 ), we report: Parallel: The parallelism/cardinality constraint score defined in Eq. 27 ; Summary: The user-facing response quality score evaluated by the LLM-judge. SLCA -GRPO has the highest mean Success and Process values across scales. RLTR and ToolPO are included for context; their method-specific protocols are described in App. C.4 . RLTR’s Summary column is marked “–” as its frozen summarizer is not optimized during RL. Trained rows report mean

\pm

std over three runs; Original rows are point evaluations.

Condition	No-call (%)	Process	Toucan (%)	Summary	BFCL (%)	$\tau^{2}$ (%)
SFT	–	0.8094 $\pm$ 0.0082	72.14 $\pm$ 0.87	0.6346 $\pm$ 0.0054	67.89 $\pm$ 0.28	32.54 $\pm$ 1.92
Unified GRPO	0.6	0.8667 $\pm$ 0.0061	76.60 $\pm$ 1.27	0.8241 $\pm$ 0.0108	68.41 $\pm$ 0.11	31.87 $\pm$ 2.97
$\hat{A}^{\mathrm{sum}}=0$ (except omission penalty episodes)	0.5	0.8748 $\pm$ 0.0061	78.72 $\pm$ 1.22	0.6512 $\pm$ 0.0208	69.44 $\pm$ 0.55	36.24 $\pm$ 1.58
$\hat{A}^{\mathrm{sum}}=0$ (including omission penalty episodes)	14.2	0.7506 $\pm$ 0.0227	67.54 $\pm$ 1.94	0.5587 $\pm$ 0.0248	61.53 $\pm$ 1.74	29.44 $\pm$ 2.28
SLCA -GRPO	0.4	0.8766 $\pm$ 0.0045	79.13 $\pm$ 1.05	0.8450 $\pm$ 0.0087	69.77 $\pm$ 0.47	41.02 $\pm$ 1.01
RLTR	–	0.7022 $\pm$ 0.0143	65.45 $\pm$ 1.68	–	63.11 $\pm$ 0.62	33.72 $\pm$ 2.38

Appendix

Table 16 : Summary advantage ablation on Qwen2.5-7B-Instruct. Process and Summary are unit-interval scores; benchmark columns are percentages. The no-call column is the end-of-training rate. Entries in the score columns are mean

\pm

standard deviation over three matched runs.

Figure 8 : ToolPO training behavior under the LLM-judge diagnostic (representative single-run traces). (a) Success@0.9 across three scales: a decline on 3B (

0.65{\to}0.51

), a sharper decline on 7B (

0.72{\to}0.20

), and an overall decline with a brief mid-training rebound on 8B, ending near

0.32

. (b) 7B: tag_open_n diverges from tag_close_n after step 80, reducing the valid parsed-call count from 1.91 to 0.73. (c) 8B: pred_tool_count surges

1.71{\to}3.98

(gold

{\approx}2.0

), driving Name F1 and Parallel Score down. (d) 3B: Process Score plateaus at

{\sim}0.84

, below the

S_{\text{process}}\geq 0.9

threshold, explaining why proxy metrics improve while strict Success declines.

Figure 9 : Diagnostic Case Study: Resolving Cross-Segment Credit Misattribution on a parallel tool-use task. (Top) Reference trajectory and rewards. (Middle) Standard GRPO. The agent adopts a redundant, trial-and-error trajectory (first issuing an unnecessary search_company_info call, then retrieving the two stock prices through serial get_stock_price invocations in separate turns), yet still produces a fluent, factually correct comparison ( ${\color[rgb]{0.3711,0.6406,0.3047}R_{\mathrm{sum}}}{=}1.00$ ). The tool-level decomposition exposes where the trajectory fails: the format reward is unaffected ( $1.00$ ), the key and value rewards remain high on the individual calls that do match the schema ( $1.00$ each), the name reward is diluted by the redundant search_company_info call ( $0.80$ ), and the parallel reward is $\mathbf{0.00}$ because the two required stock-price queries are issued as separate sequential turns (further preceded by a redundant exploration call), rather than batched into a single parallel invocation. Aggregated under the fixed weights above, this yields ${\color[rgb]{0.418,0.3711,0.8203}R_{\mathrm{tool}}}{=}0.65$ . Because standard GRPO merges ${\color[rgb]{0.418,0.3711,0.8203}R_{\mathrm{tool}}}$ and ${\color[rgb]{0.3711,0.6406,0.3047}R_{\mathrm{sum}}}$ into a single trajectory-level advantage, the high summary reward can offset the structural tool penalty in the unified advantage, allowing the inefficient pattern to be reinforced. (Bottom) SLCA -GRPO (Ours). The same model, trained under our advantage-isolation objective, produces a trajectory in which both tickers are known a priori and issues a single parallel tool call covering AAPL and MSFT simultaneously. All five tool sub-rewards saturate at $1.00$ , giving ${\color[rgb]{0.418,0.3711,0.8203}R_{\mathrm{tool}}}{=}1.00$ while preserving ${\color[rgb]{0.3711,0.6406,0.3047}R_{\mathrm{sum}}}{=}1.00$ . By routing ${\color[rgb]{0.418,0.3711,0.8203}A_{\mathrm{tool}}}$ and ${\color[rgb]{0.3711,0.6406,0.3047}A_{\mathrm{sum}}}$ through separate optimization pathways, segment routing keeps the summary signal from entering the tool-token update in this example. The tool-execution score increases by $+0.35$ while the summary score remains $1.00$ . The trajectory is qualitatively similar to the representative single-run pred_tool_count increase observed under ToolPO on 8B ( Figure 8 c); the two diagnostics use different protocols.

Method	Overall Acc	AST Live	AST Non-Live	Multi-Turn
Qwen2.5-3B-Instruct
Original	0.4225	0.5174	0.5139	0.0271
SFT	0.6163 $\pm$ .0026	0.6862 $\pm$ .0022	0.8104 $\pm$ .0043	0.0867 $\pm$ .0100
SFT+GRPO	0.6321 $\pm$ .0032	0.6936 $\pm$ .0030	0.8583 $\pm$ .0087	0.0600 $\pm$ .0033
RLTR	0.5838 $\pm$ .0065	0.6136 $\pm$ .0074	0.8275 $\pm$ .0044	0.0494 $\pm$ .0100
ToolPO	0.6095 $\pm$ .0086	0.6477 $\pm$ .0108	0.8325 $\pm$ .0139	0.0961 $\pm$ .0067

Appendix

Table 17: Comprehensive Generalization Results on BFCL V3 across Scales. This table details BFCL generalization performance. In addition to Overall Acc, AST Live, and AST Non-Live (reported in Figure 3(a) ), we include Multi-Turn accuracy to assess state tracking under schema generalization. Trained rows report mean

\pm

std over three runs; Original rows are point evaluations. ToolPO and RLTR retain their method-specific protocols.

Method	Overall Pass 1	Airline	Retail	Telecom
Qwen2.5-3B-Instruct
Original	0.2324	0.3810	0.1481	0.2692
SFT	0.2705 $\pm$ .0236	0.1364 $\pm$ .0227	0.3966 $\pm$ .0259	0.1954 $\pm$ .0217
SFT+GRPO	0.3454 $\pm$ .0257	0.2576 $\pm$ .0131	0.4598 $\pm$ .0217	0.2644 $\pm$ .0348
RLTR	0.2850 $\pm$ .0291	0.2727 $\pm$ .0227	0.3649 $\pm$ .0303	0.2098 $\pm$ .0303
ToolPO	0.2355 $\pm$ .0226	0.1288 $\pm$ .0262	0.3678 $\pm$ .0263	0.1437 $\pm$ .0179

Appendix

Table 18: Comprehensive Collaboration Results on

\tau^{2}

-Bench across Scales. We report Pass 1 rates for dual-control scenarios across three distinct domains: Airline (User Retention), Retail (Order Modification), and Telecom (Plan Management). Pass 1 is the proportion of successful tasks in the finite official task set; because each run evaluates a finite set, the reported values are discrete proportions and their standard deviations reflect variation of those proportions across runs. The Qwen3-8B-Base Original row is included and is zero in all four reported metrics. Trained rows are mean

\pm

std over three runs. ToolPO and RLTR retain their method-specific protocols.

Config	$S_{\text{fmt}}$	$S_{\text{name}}$	$S_{\text{key}}$	$S_{\text{val}}$	$S_{\text{par}}$	BFCL Acc (%)	$\tau^{2}$ Pass 1 (%)
	$S_{\text{process}}$ Weights
Default (SLCA)	0.10	0.25	0.15	0.20	0.30	69.77 $\pm$ 0.47	41.02 $\pm$ 1.01
Uniform (SLCA)	0.20	0.20	0.20	0.20	0.20	69.18 $\pm$ 0.55	38.62 $\pm$ 1.86
Value-heavy (SLCA)	0.05	0.20	0.20	0.35	0.20	69.44 $\pm$ 0.48	39.35 $\pm$ 1.73
w/o HierR	—					69.07 $\pm$ 0.46	34.50 $\pm$ 1.44
GRPO (unified adv.)	same as Default					68.41 $\pm$ 0.11	31.87 $\pm$ 2.97

Appendix

Table 19: HierR weight sensitivity on Qwen2.5-7B-Instruct. Entries are mean

\pm

std over three runs.

Reward ratio	Toucan Success@0.9 (%)	BFCL Acc. (%)	$\tau^{2}$ -Bench Pass 1 (%)
Unified $1{:}1$	76.60 $\pm$ 1.27	68.41 $\pm$ 0.11	31.87 $\pm$ 2.97
Unified $2{:}1$	77.54 $\pm$ 1.24	68.83 $\pm$ 0.31	34.62 $\pm$ 2.71
Unified $3{:}1$	77.88 $\pm$ 1.30	68.95 $\pm$ 0.36	35.24 $\pm$ 2.58
Unified $5{:}1$	77.02 $\pm$ 1.41	68.60 $\pm$ 0.44	32.90 $\pm$ 2.88
SLCA -GRPO	79.13 $\pm$ 1.05	69.77 $\pm$ 0.47	41.02 $\pm$ 1.01

Appendix

Table 20: Unified reward ratio sensitivity on Qwen2.5-7B-Instruct. Entries are mean

\pm

standard deviation over three matched runs. The ratio is applied before the unified group normalization; the scale-invariance identity holds up to the numerical floor described in App. A.3 .

Condition	Tool tokens	Summary tokens	Toucan (%)	BFCL (%)	$\tau^{2}$ (%)
Both closed (SLCA)	$T$	$S$	79.13 $\pm$ 1.05	69.77 $\pm$ 0.47	41.02 $\pm$ 1.01
$S\mathbin{\to}T$ open	$T+S$	$S$	77.21 $\pm$ 1.24	68.72 $\pm$ 0.29	33.95 $\pm$ 2.71
$T\mathbin{\to}S$ open	$T$	$T+S$	79.02 $\pm$ 1.12	69.84 $\pm$ 0.44	41.36 $\pm$ 1.24
Both open	$T+S$	$T+S$	78.30 $\pm$ 1.09	69.30 $\pm$ 0.41	37.42 $\pm$ 2.10
Unified (joint normalization)	joint normalized reward	joint normalized reward	76.60 $\pm$ 1.27	68.41 $\pm$ 0.11	31.87 $\pm$ 2.97

Appendix

Table 21 : Support control on Qwen2.5-7B-Instruct. The four support cells use separately normalized components. Unified is a joint-normalization reference. Entries are mean

\pm

standard deviation over three matched runs.

Effect	Toucan	BFCL	$\tau^{2}$ -Bench
Close $S\mathbin{\to}T$	1.32	0.80	5.51
Open $T\mathbin{\to}S$	0.49	0.33	1.91
Interaction	1.20	0.51	3.13

Appendix

Table 22: Factor effects from the 7B support control. Values are computed from the cell means in Table 21 and are reported in percentage points. Main effects average the two simple effects over the other factor; interaction entries use the unhalved difference-in-differences contrast.

Method	Toucan-Test				BFCL	$\tau^{2}$ -Bench
Method	Name F1	ArgMatch	Process	Success	Acc	Pass 1
Qwen2.5-3B-Instruct
SLCA -GRPO	0.9006 $\pm$ .0087	0.8092 $\pm$ .0112	0.8625 $\pm$ .0049	0.7647 $\pm$ .0093	0.6361 $\pm$ .0052	0.3563 $\pm$ .0091
w/o SLCA	0.8910 $\pm$ .0087	0.8072 $\pm$ .0154	0.8577 $\pm$ .0057	0.7412 $\pm$ .0115	0.6321 $\pm$ .0032	0.3454 $\pm$ .0257
w/o SGLS	0.8927 $\pm$ .0075	0.7982 $\pm$ .0109	0.8559 $\pm$ .0050	0.7584 $\pm$ .0101	0.6304 $\pm$ .0048	0.2995 $\pm$ .0182
w/o HierR	0.8399 $\pm$ .0084	0.7850 $\pm$ .0161	0.8264 $\pm$ .0075	0.6552 $\pm$ .0107	0.5993 $\pm$ .0046	0.3152 $\pm$ .0145

Appendix

Table 23: Comprehensive Ablation Studies Across Model Families. Comparison of Full SLCA -GRPO against variants removing key components. Metrics include Toucan-Test (Name F1, ArgMatch, Process, Success), BFCL (Accuracy), and

\tau^{2}

-Bench (Pass 1 ). Process is computed as a post-hoc evaluation metric for all settings, including w/o HierR. Trained rows in all three blocks report mean

\pm

std over three runs; Original evaluations are point values where shown. The w/o SLCA condition is the SFT+GRPO estimator.

Hard-Negative Sampling Qwen2.5-3B-Instruct
	Candidate Tool Set Size ( $K$ )
Method	Original	20	50	100	150
SFT+GRPO	0.7412	0.3400	0.2921	0.2662	0.2285
Ours	0.7647	0.3584	0.3209	0.2865	0.2510
Random Sampling Qwen2.5-3B-Instruct
SFT+GRPO	0.7412	0.6399	0.5701	0.4950	0.4360

Appendix

Table 24: Comprehensive Scalability Results Across Model Families. We report Success

@

0.9 as the candidate set size

K

increases. Comparison is made between Hard-Negative (Semantic) and Random sampling to isolate the impact of semantic interference. The Original column reports the multi-run mean at

K=0

, matching the main results; the

K>0

columns retain the stress-test evaluations. For

K\geq 100

, especially

K=150

, these stress-test values are computed on context-valid survivors after skip filtering.

Method	Toucan Succ. (%)	BFCL Acc. (%)	$\tau^{2}$ -Bench Pass 1 (%)
SFT+GRPO (w/o SLCA)	73.51 $\pm$ 1.45	66.28 $\pm$ 0.32	25.54 $\pm$ 2.13
SLCA -GRPO	76.43 $\pm$ 1.21	67.61 $\pm$ 0.51	30.18 $\pm$ 1.76
$\Delta$	+2.92	+1.33	+4.64

Appendix

Table 25: Execution-based

R_{\text{succ}}

comparison on Qwen2.5-7B-Instruct. Entries are mean

\pm

standard deviation over three runs; data, initialization, SGLS, and compute are matched.

Figure 10 : Training dynamics under execution-based

R_{\text{succ}}

(7B). The curves are single-run checkpoint evaluations on the same fixed set of 3,991 valid Toucan validation examples. The plotted

R_{\text{succ}}

is distinct from Toucan Success@0.9. At step 100, SLCA -GRPO reaches

R_{\text{succ}}{=}0.593

, compared with

0.424

for SFT+GRPO (w/o SLCA).

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SHARPO: Segment-Level Credit Assignment for Agentic Reinforcement Learning

Xinchen Du, Zhengze Zhou, Wenhui Zhu, Han Yu+3

LinkedIn Corporation · Georgia Institute of Technology

Agentic Reinforcement Learning Credit Assignment

Open paper AskPDF arXiv

Agentic reinforcement learning (RL) trains a large language model (LLM) to act over long, multi-step interactions. However, a single localized error can cause task failure, while trajectory-level rewards provide limited guidance for assigning credit to individual decisions. To address this limitation, we introduce Segment-level Hindsight Advantage Reweighting for Policy Optimization (SHARPO), a credit-assignment mechanism that refines Group Relative Policy Optimization (GRPO) at the level of environment-facing segments. Inspired by the existing on-policy self-distillation (OPSD) method, SHARPO computes teacher-student log-probability gaps within each segment and uses the resulting signal to compute a bounded multiplier on the GRPO advantage. This multiplier is shared by all tokens within the segment, allowing credit to vary across different segments. With Qwen2.5-7B-Instruct, SHARPO outperforms existing baselines on the ALFWorld and WebShop benchmarks, including GRPO, SDAR, RLSD, and StepOPSD.

Abstract

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