How Do Agentic LLMs Decide to Call Tools? A Tool-Call Vector Shaped by Suppression

Xijie Gong; Tingxu Han; Jiahao Zhang; Wei Song; Ziqi Ding; Hanqi Yan; Youcheng Sun; Lijie Hu

cs.AIOct 7, 2026

How Do Agentic LLMs Decide to Call Tools? A Tool-Call Vector Shaped by Suppression

Authors: Xijie Gong, Tingxu Han, Jiahao Zhang, Wei Song, Ziqi Ding, Hanqi Yan, Youcheng Sun, Lijie Hu

Organizations: Mohamed bin Zayed University of Artificial Intelligence · University of Electronic Science and Technology of China · Nanjing University · Griffith University · University of New South Wales · King’s College London

Abstract

Tool calling, invoking external tools on demand, is central to agentic LLMs, yet the mechanism that decides whether a model calls a tool or responds directly remains poorly understood. Agentic prompts are long and heavily scaffolded, combining role instructions, tool schemas, format templates, and the user's request across hundreds of tokens, creating a noisy, highly entangled context in which no single controllable variable for mechanistic analysis is obvious. To obtain such a variable, we propose a method that converts complex agentic prompts into minimal contrastive pairs in which a single request verb determines the tool-call decision: replacing an execution-verb (e.g., \textit{write}) with an analysis-verb (e.g., \textit{discuss}) reliably flips the decision, suggesting it is mediated by a compact internal state. We construct 500 such paired prompts across Python, Java, and C++ (300 for mechanistic analysis, 200 held out for evaluation). We trace the decision to a vector, $μ_Δ$ , that is both causally necessary and sufficient and generalizes beyond the discovery prompts to native multi-turn $τ^2$ -Bench trajectories and verb-free requests. Behavioral ablations show that the scaffold establishes a tool-call prior; Transcoder decomposition then reveals that analysis verbs suppress this prior through features signaling that tool use is unnecessary, whereas execution verbs largely leave it intact. Downstream scaffold-reading attention heads and MLP features read out the resulting state, and the same mechanism recurs across seven models from the Qwen, Mistral, and Granite families. Our code is available at https://github.com/XijieGo/MI4ToolCalling.

Figures & tables

Figure 1: Roadmap of the paper’s mechanistic analysis. The three numbered blocks summarize the stages of our analysis: (1) verb substitution turns long agentic prompts into a controlled causal interface; (2) the call-or-no-call decision passes through a tool-call vector, $\mu_{\Delta}$ , that is causally sufficient and necessary; and (3) this vector reflects suppression of a scaffold-induced tool-call default and generalizes across the model families. The six lettered steps show how the evidence is built.

Figure 2: Request verbs change tool-call rates across models. Bars show first-token call rates on HumanEval before behavioral filtering, by request type.

Figure 3: The call signal shifts from the verb to the prediction position. Layerwise patching finds the L24 residual state that determines when to call a tool or to give a text reply.

Intervention	Top-1 before	Top-1 after	$\Delta$ top-1	Logit before	Logit after	$\Delta$ logit	Suff.	Necc.
Add $\mu_{\Delta}$	0.00%	100.00%	${\color[rgb]{1,0,0}\uparrow}\,100.00$	25.11	32.50	${\color[rgb]{1,0,0}\uparrow}\,7.39$	1.03	–
Remove $\mu_{\Delta}$	100.00%	0.00%	${\color[rgb]{0,0.6,0}\downarrow}\,100.00$	32.28	24.80	${\color[rgb]{0,0.6,0}\downarrow}\,7.48$	–	1.04

Table 1: The tool-call vector causally controls the call-or-no-call decision. Adding

\mu_{\Delta}

to analysis prompts induces tool calls, while removing it from execution prompts suppresses them.

			Add $\mu_{\Delta}$		Remove $\mu_{\Delta}$
Target	Tool	$N$	Induction (%)	Norm. shift	Suppression (%)	Norm. shift
Web retrieval	web_search	100	100.0	0.93	100.0	0.75
SQL execution	run_sql	100	100.0	0.79	100.0	0.77
Email dispatch	send_email	100	100.0	0.91	100.0	1.30
Mean	—	300	100.0	0.88	100.0	0.94

Table 2: The coding-derived vector controls tool calling across web retrieval, SQL execution, and email dispatch. Qwen3-8B interventions keep the coding-derived direction fixed at L24 and match the vector norm to each target domain.

N

counts held-out pairs. Induction and suppression rates measure first-token switches from no-call to call under addition and from call to no-call under removal, respectively. Norm. shift is the mean <tool_call> logit increase for Add or decrease for Remove, divided by the baseline call–no-call logit gap. Means weight domains equally.

Setting	Intervention	$N$	Top-1 flip (%)	Mean $\Delta z_{\mathrm{call}}$	Top-1 rate (before $\to$ after)
$\tau^{2}$ -Bench, native call	Remove	200	36.7	$-5.55$	99.5% $\to$ 63.0%
$\tau^{2}$ -Bench, native text	Add	200	70.0	$+19.42$	0.0% $\to$ 70.0%
Verb-free, native call	Remove	160	93.1	$-9.95$	100.0% $\to$ 6.9%

Table 3: The coding-derived Qwen3-8B vector transfers beyond the discovery prompts. All entries use

1\times

gain at L24.

N

counts evaluated decision points. Top-1 flip is the fraction of baseline calls suppressed by removal or baseline text responses changed to calls by addition.

\Delta z_{\mathrm{call}}

is the mean paired after-minus-before change in <tool_call> logit over all

N

points. Top-1 rate gives the call rate before and after intervention over the same

N

points. Appendix C gives random controls and shows how the verb-free results vary with the chosen intervention gain.

Scaffold	Neutral request	Analysis request
$R+T+F$ (full)	0.8486	0.0038
$R+T$ (no format template)	$6.32\times 10^{-7}$	$8.31\times 10^{-10}$
$R+F$ (no tool schema)	0.9521	0.4108
$F$ only	1.0000	0.9952

Table 4: The scaffold creates a request-sensitive call prior. Entries are mean first-token call probabilities on 200 held-out Qwen3-8B tasks. Appendix D.1 gives all component combinations and a control that replaces the tool schema with text of the same length.

Figure 4: MLPs dominate formation-window writes. Left, Qwen3-8B component outputs projected onto $\hat{\mu}_{\Delta}$ . Right, the Transcoder architecture used to decompose MLP writes.

Layer	Dominant	$K_{\mathrm{corrupt}}$	$K_{\mathrm{clean}}$	$K_{\mathrm{corrupt}}/K_{\mathrm{clean}}$	Share (%)	Semantic label
L20	Clean	0.24	0.93	0.26	4.6	Execution requests
L21	Corrupt	1.47	1.10	1.34	16.7	Non-necessity
L22	Corrupt	1.85	1.33	1.39	22.8	Analysis-task contexts
L23	Corrupt	5.98	1.43	4.18	55.9	Analysis-verbs

Table 5: Features more active on analysis prompts dominate the formation window.

K_{\mathrm{corrupt}}

and

K_{\mathrm{clean}}

sum

|\kappa|

over all features with higher analysis and execution activations, respectively, on 200 held-out pairs. Share is each layer’s fraction of the net Transcoder feature write over L20–L23. Appendix D.4 gives the statistics for the labelled families of Transcoder features.

Figure 5: Scaffold-reading heads and a late structural feature read out the tool-call state. (A) Execution-minus-analysis DLA for the <tool_call> logit across Qwen3-8B attention heads in L25–L35. (B) L29H9, L33H11, and L33H29 share a shift toward the tool-format template and away from role instructions on execution prompts (blue) relative to analysis prompts (red). (C) Max-activating contexts for L34/F109925 highlight tool-schema boundaries, supporting its interpretation as a structural readout feature for the opening of a tool call.

	Localization		Intervention		Generalization (%)			Formation		Readout
Model	$l$	$r(l,p)$ (%)	Suff.	Necc.	Domains	$\tau^{2}$	Verb-free	MLP/Attn	$K_{\mathrm{corrupt}}/K_{\mathrm{clean}}$	Max Attn (pp)
Qwen3-4B	26	100.0	0.97	0.71	87.7	72.2	100.0	2.08	3.26	82.6
Qwen3-8B	24	100.0	1.03	1.04	100.0	53.3	93.1	3.48	1.99	72.6
Qwen3-14B	34	100.0	0.97	0.84	100.0	85.0	100.0	6.87	2.41	87.3
Qwen3.5-4B	31	99.0	0.84	0.61	91.1	47.9	100.0	–	1.58	50.3
Qwen3.5-9B	31	100.0	0.92	0.88	100.0	80.2	100.0	–	1.30	52.2

Table 6: The tool-call mechanism recurs across seven models and generalizes beyond the discovery prompts. Domains and

\tau^{2}

average conditional intervention rates; Verb-free reports suppression at unit gain. Domains uses norm calibration (Appendix I.1 ). Max Attn summarizes clean-minus-corrupt scaffold-attention shifts (Appendix F.3 ). For hybrid Qwen3.5, MLP/Attn is omitted and attention is summarized only for layers with full attention.

Appendix figures & tables30 assets

Supplementary material from the paper’s appendix.

Appendix

Source family	Models	Task content
APPS, CodeContests, HumanEval, MBPP	Qwen3-4B, Qwen3-8B, Qwen3-14B, Qwen3.5-9B, Granite-3.3-8B	Programming tasks with statements, function signatures, examples, tests, or reference solutions.
$\tau^{2}$ -Bench Telecom	Qwen3.5-4B, Mistral-Small-3.2-24B	Conversation histories with a controlled request-verb contrast.

Appendix

Table 7: Source families represented in the model-specific paired corpora.

Model	Source family	Train	Held-out
Qwen3-4B	Coding tasks	300	200
Qwen3-8B	Coding tasks	300	200
Qwen3-14B	Coding tasks	300	200
Qwen3.5-4B	$\tau^{2}$ -Bench Telecom	300	200
Qwen3.5-9B	Coding tasks	300	200
Mistral-Small-3.2-24B	$\tau^{2}$ -Bench Telecom	300	200

Appendix

Table 8: Per-model paired-prompt corpus sizes and splits.

Split	Execution verbs	Analysis verbs
Train (300)	add 60, build 60, complete 60, save 60, write 60	discuss 75, explore 75, review 75, study 75
Held-out (200)	add 53, build 20, complete 20, save 53, write 54	discuss 40, explore 40, inspect 40, review 40, study 40

Appendix

Table 9: Qwen3-8B execution- and analysis-verb counts by split.

Source	Train	Held-out
APPS	188	51
CodeContests	63	135
HumanEval	17	4
MBPP	32	10
Total	300	200

Appendix

Table 10: Source composition of the Qwen3-8B paired-prompt corpus.

Intervention	L22	L23	L24	L25
Analysis $+\mu_{\Delta}$	82.0%	100.0%	100.0%	100.0%
Execution $-\mu_{\Delta}$	45.7%	12.0%	0.0%	0.0%
Analysis $+$ random	0.0%	0.3%	18.7%	0.0%
Execution $-$ random	100.0%	99.7%	97.7%	100.0%

Appendix

Table 11: Neighboring-layer control with independently fitted directions. Entries are held-out post-intervention call rates. Random controls match the learned direction’s norm at each layer.

Model	$\mu_{\Delta}$ , $1\times$	Random, $1\times$
Qwen3-4B	61.3%	0.5%
Qwen3-8B	36.7%	1.5%
Qwen3-14B	97.0%	8.0%
Qwen3.5-4B	88.4%	0.0%
Qwen3.5-9B	91.3%	17.3%
Mistral-Small-3.2-24B	75.0%	0.5%

Appendix

Table 12:

\tau^{2}

-Bench removal: strict first-token flip rate among native tool-call decisions (

N_{\mathrm{call}}

baseline tool calls per model;

N=199

for Qwen3-8B).

Model	$\mu_{\Delta}$ , $1\times$	Random, $1\times$
Qwen3-4B	83.0%	0.0%
Qwen3-8B	70.0%	0.0%
Qwen3-14B	73.0%	0.0%
Qwen3.5-4B	7.5%	0.0%
Qwen3.5-9B	69.0%	0.0%
Mistral-Small-3.2-24B	92.0%	0.0%

Appendix

Table 13:

\tau^{2}

-Bench induction: strict first-token flip rate among native text decisions.

Model	$N$	$(-\mu_{\Delta})$ , $\alpha=1$	$(-\mu_{\Delta})$ , $\alpha=1.5$	$(-r)$ , $\alpha=1.5$
Qwen3-4B	117	100.0%	100.0%	34.2%
Qwen3-8B	160	93.1%	100.0%	16.2%
Qwen3-14B	161	100.0%	100.0%	44.7%
Qwen3.5-4B	186	100.0%	100.0%	33.9%
Qwen3.5-9B	187	100.0%	100.0%	73.8%
Mistral-Small-3.2-24B	147	100.0%	100.0%	5.4%

Appendix

Table 14: Implicit, verb-free generalization across all seven models.

N

is the evaluated baseline-positive subset;

r

is a norm-matched random direction.

Scaffold components	Neutral $p_{\mathrm{call}}$	Analysis $p_{\mathrm{call}}$	$P$
$R+T+F$ (full scaffold)	0.8486	0.0038	0.845
$R+F$ (no tool schema)	0.9521	0.4108	0.541
$R+T$ (no format template)	$6.32\times 10^{-7}$	$8.31\times 10^{-10}$	$6.3\times 10^{-7}$
$F$ only	1.0000	0.9952	0.005
$T$ only	$1.87\times 10^{-8}$	$2.05\times 10^{-11}$	$1.9\times 10^{-8}$
$R$ only	$8.77\times 10^{-9}$	$7.14\times 10^{-11}$	$8.7\times 10^{-9}$

Appendix

Table 15: Scaffold-component ablation, Qwen3-8B, 200 held-out prompts per condition.

p_{\mathrm{call}}

is the mean first-token <tool_call> probability;

P

is the neutral-minus-analysis gap.

Request	Call top-1	Mean call probability
Empty turn, greeting, or unrelated question	0.0%	$\approx 5.0\times 10^{-4}$ (max.)
Task body only	32.3%	0.3310
Neutral verb and task body	86.7%	0.8547
Analysis verb and task body	0.0%	0.0080
Execution verb and task body	100.0%	0.9993
Execution verb and task body, no scaffold	0.0%	$6.56\times 10^{-17}$

Appendix

Table 16: Qwen3-8B request baselines on a 300-task evaluation cohort. Empty, greeting, and unrelated turns are evaluated once each. The full scaffold is present except in the final row.

Request verb	write_file	submit_review
Write	100.0%	34.0%
Review	4.0%	100.0%

Appendix

Table 17: Request–tool crossover on Qwen3-8B. Entries are first-token call rates, with 100 requests per condition.

Diagnostic	Result
Probe AUC $>0.9$ first layer	L1
Logit-lens gap $>1$ first layer	L27
DLA top-3 layers (MLP write)	L35, L34, L33
Final gap mean (single RMSNorm, 95% CI)	7.093 [6.687, 7.498]
First 10% / 25% / 50% of final gap	L25 / L29 / L29
Largest increments	L29, L33, L24, L27, L32

Appendix

Table 18: Held-out early-signal diagnostics and layer-wise gap accumulation on the 200 held-out pairs (single-sample TransformerLens protocol with standard single RMSNorm).

Family	Side	$N_{f}$	Mean	Sum	Interpretation
analysis_non_execution	corrupt	115	0.0633	7.2787	Analysis requests and non-necessity semantics.
execution_request	clean	292	0.0156	4.5634	Execution requests and action verbs.
analysis_plain_discourse	corrupt	15	0.0020	0.0294	General analysis discourse.
schema_boundary_formatting	corrupt	4	0.0466	0.1862	Code and format boundaries.

Appendix

Table 19: Formation-window feature families selected on training pairs. Side is the majority activation side on training pairs. Counts cover 426 of 640 candidates; 214 remain unlabelled. Mean and sum use held-out

|\kappa|

Layer	$K_{\mathrm{corrupt}}$	$K_{\mathrm{clean}}$	Ratio	Net-write share
L20	0.24	0.93	0.26	4.6%
L21	1.47	1.10	1.34	16.7%
L22	1.85	1.33	1.39	22.8%
L23	5.98	1.43	4.18	55.9%

Appendix

Table 20: Layer-wise formation on the 200 Qwen3-8B feature-evaluation held-out pairs.

K_{\mathrm{corrupt}}

and

K_{\mathrm{clean}}

sum

|\kappa|

over features with higher activation on the corresponding side. Net-write share is the layer’s sum of

\kappa

divided by the total over L20–L23.

	Execution prompts		Analysis prompts
Condition	$g_{l}$	Call (%)	$g_{l}$	Call (%)
Baseline	60.06	100.0	60.06	0.0
MLP-output swap	15.78	93.0	15.09	95.0
Swap + $\hat{\mu}_{\Delta}$ restoration	60.06	100.0	60.06	0.0
Swap + orthogonal restoration	15.78	58.0	15.09	99.5
Swap + full-state restoration	60.06	100.0	60.06	0.0

Appendix

Table 21: Formation-window mediation on the 200 feature-evaluation pairs. Each arm computes

g_{l}

against the untouched paired opposite-side state. Call rate is the post-intervention top-1 rate.

Head	stage	DLA $\Delta$	recover shift	corrupt top-1	strict flip
L25H6	earlier	0.06	1.28	40.4%	36.9%
L28H3	earlier	0.26	1.38	36.1%	32.5%
L26H14	earlier	0.04	0.91	24.3%	20.7%
L29H9	earlier	4.15	1.34	21.3%	17.8%
L33H29	late	7.91	0.62	8.5%	4.9%
L33H11	late	3.32	-0.39	2.4%	0.0%

Appendix

Table 22: Selected downstream attention heads in the L25–L35 component sweep (

N=300

candidate pairs; baseline corrupt tool-call rate is 3.6%). DLA delta is clean minus corrupt for the <tool_call> logit. Figure 5A’s heatmap displays this component sweep cohort (with L33H29 DLA

\Delta=7.91

). In contrast, Section 6.1 and Table 24 report on the dedicated 200 balanced held-out pairs (where L33H29 exhibits Clean DLA 24.58 and Corrupt DLA 11.13, giving

\Delta=13.45

). Recovery shift and strict flip are measured by replacing the corrupt-side head output with the clean-side output.

MLP	recover shift	drop shift	corrupt top-1	strict flip
L34	1.74	2.32	53.3%	49.7%
L27	1.32	0.82	52.3%	48.7%
L25	1.70	0.96	44.8%	41.2%
L35	0.84	1.27	31.8%	28.2%
L31	0.90	0.90	28.6%	25.0%
L29	0.38	1.58	26.6%	23.1%

Appendix

Table 23: Top downstream MLP blocks in the L25–L35 component sweep (

N=300

, baseline corrupt top-1 call rate 3.6%), separate from the 200-pair prediction-position patching subset (42.5% top-1 / 39.0% strict recovery) and the 200-pair feature-evaluation cohort (41.0% top-1). Each row patches only one MLP output from clean into corrupt, or from corrupt into clean for the reverse drop measurement.

Head	Clean	Corrupt	Corrupt $+\mu_{\Delta}$
L29H9	6.99	1.43	6.91
L33H11	5.60	1.46	5.30
L33H29	24.58	11.13	24.43

Appendix

Table 24: Attention-head response to L24 vector addition on the 200 balanced Qwen3-8B held-out pairs. Entries are mean DLA for the call-opening token.

Measurement	Clean	Corrupt	Corrupt $+\mu_{\Delta}$
Mean activation	124.43	67.63	126.00
Projected call-token write	6.17	3.35	6.25

Appendix

Table 25: L34 F109925 on the 200 Qwen3-8B feature-evaluation held-out pairs. Projected write is activation multiplied by the feature decoder’s projection onto the call-token unembedding direction.

Condition	Call (%)
Corrupt baseline	0.0
Corrupt $+\mu_{\Delta}$ at L24	100.0
Replace F109925	37.0
Replace F91365	6.5
Patch full MLP34	41.0

Appendix

Table 26: L34 feature replacement and controls on the same 200 held-out pairs. All interventions act at the prediction position.

Model	$L_{\mathrm{tot}}$	$L^{*}$	$r(l,p)$ (%)	Suff.	Necc.
Qwen3-4B	36	L26	100.0	0.97	0.71
Qwen3-8B	36	L24	100.0	1.03	1.04
Qwen3-14B	40	L34	100.0	0.97	0.84
Qwen3.5-4B	32	L31	99.0	0.84	0.61
Qwen3.5-9B	32	L31	100.0	0.92	0.88
Mistral-Small-3.2-24B	40	L25	100.0	0.90	0.76

Appendix

Table 27: Per-model localization and shared-direction results.

L^{*}

is the commitment layer;

r(l,p)

is the top-1 native call-token recovery rate at the prediction position. Suff. and Necc. are the causal sufficiency and necessity scores for the shared direction

\mu_{\Delta}

Model	$L_{\mathrm{tot}}$	$L^{*}$	$N$	Probe AUC $>0.9$	Gap $>1.0$	Top-3 DLA layers
Qwen3-4B	36	L26	300	L0	L26	L34, L33, L32
Qwen3-8B	36	L24	200	L1	L27	L34, L33, L29
Qwen3-14B	40	L34	300	L1	L25	L34, L37, L38
Qwen3.5-4B	32	L31	300	L0	L14	L31, L30, L27
Qwen3.5-9B	32	L31	300	L0	L21	L27, L30, L23
Mistral-Small-3.2-24B	40	L25	300	L0	L17	L39, L37, L34

Appendix

Table 28: Early-signal and timing diagnostics across all seven models on held-out prompt pairs.

L^{*}

is the causal commitment layer from Table 6 ; Probe AUC

>0.9

is the first layer where the linear probe achieves

>0.9

AUC; Gap

>1.0

is the first layer where the logit-lens clean-minus-corrupt gap exceeds 1.0; Top-3 DLA reports the layers with the largest signed positive net direct logit attribution differences (

\Delta\mathrm{DLA}>0

L^{*}

denotes a block input; probe and lens layer labels denote block-output residuals, and DLA labels denote block writes.

Figure 6: Triplet diagnostics across all seven models, arranged with one model per row and three diagnostics per row. From left to right, each row shows the linear probe AUC, the logit-lens clean-minus-corrupt gap, and the layer-wise DLA sweep. Evaluation cohorts contain 200 pairs for Qwen3-8B, 240 for Granite-3.3-8B, and 300 for each of the other five models. Across all seven models, probe separability appears at L0–L1, while the largest DLA writes occur in upper layers.

Model	MLP/Attn	$K_{\mathrm{corrupt}}/K_{\mathrm{clean}}$
Qwen3-4B	2.08	3.26
Qwen3-8B	3.48	1.99
Qwen3-14B	6.87	2.41
Qwen3.5-4B	–	1.58
Qwen3.5-9B	–	1.30
Mistral-Small-3.2-24B	3.93	1.61

Appendix

Table 29: Formation-window summary across models.

K_{\mathrm{corrupt}}/K_{\mathrm{clean}}

is the ratio of corrupt-higher to clean-higher feature mass. Qwen3.5-4B feature mass is evaluated over L28–L30.

Model	Max Attn (pp)
Qwen3-4B	82.6
Qwen3-8B	72.6
Qwen3-14B	87.3
Qwen3.5-4B	50.3
Qwen3.5-9B	52.2
Mistral-Small-3.2-24B	40.5

Appendix

Table 30: Cross-model scaffold-attention summary on native held-out pairs. Max Attn reproduces Table 6 , in percentage points. Attention shifts are averaged over held-out pairs, then maximized over measured heads. The shift is clean minus corrupt in the target scaffold region. Qwen3-8B uses its three reported scaffold readers; Qwen3.5 uses full-attention layers only.

Model	Core nodes	Core edges	Suff.	Removal	Rand. Suff.	Gap
Qwen3-1.7B	18	54	0.9891	0.9824	0.8812	0.1078
Qwen3-4B	18	69	0.9979	0.8319	0.8712	0.1266
Qwen3-8B	18	63	1.0000	0.5975	0.9768	0.0232
Qwen3-14B	20	86	0.9999	0.4077	0.9554	0.0446

Appendix

Table 31: Sparse forward-core replay across Qwen3 scales. Core nodes and edges are taken from the aggregated forward sparse graph. Suff. is the mean replay ratio of the discovered core relative to the clean baseline, Removal is the mean ratio under clean-run removal, Rand. Suff. is the mean replay ratio of size-matched random cores, and Gap is the mean advantage of the discovered core over the random baseline.

Figure 7: EAP-IG top-

k

faithfulness curve for the tool-call task, normalized between the corrupt and clean baselines. The curve relates retained edge budget to recovery of the <tool_call> signal.

Retained edges	Faith.	Suff. logit diff	Nodes	Core overlap
1,000	-0.005	-13.12	139	17/18
2,000	-0.014	-13.38	229	18/18
5,000	0.043	-11.81	417	18/18
10,000	0.284	-5.19	553	18/18
20,000	0.451	-0.59	708	18/18
30,000	0.470	-0.08	805	18/18

Appendix

Table 32: EAP-IG top-

k

sweep on Qwen3-8B using 100 equal-length clean/corrupt prompt pairs. Faith. normalizes sufficiency between the corrupt and clean baselines. Suff. is the mean logit-difference score under corrupt-side replay with only the retained edges active. Nodes counts the retained component set induced by the edges; core overlap counts shared nodes with the 18-node forward core.

Figure 8: Prompt-specific feature graph from Circuit Tracing [ 28 ] . Nodes represent features or residual-stream components, and edges represent direct-effect attribution weights. The graph describes local contributors to the tool-call computation.

Source benchmark	License
APPS	MIT
HumanEval	MIT
MBPP	CC BY 4.0
CodeContests	CC BY 4.0 (data); Apache 2.0 (code)
$\tau^{2}$ -Bench	MIT
FEVER	CC BY-SA 3.0 (data)

Appendix

Table 33: Licenses of the source benchmarks. Links identify the original resources; the original papers are cited in the dataset and transfer sections.

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Tool Calling is Linearly Readable and Steerable in Language Models
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University College London · Holistic AI · Imperial College London

May 10, 2026, cs.CLJ/K move · Enter open · S save

LLM Agents Already Know When to Call Tools -- Even Without Reasoning

Chung-En Sun, Linbo Liu, Ge Yan, Zimo Wang+1

University of California, San Diego

Large Language Model Tool Use Large Language Model Agents

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Tool-augmented LLM agents tend to call tools indiscriminately, even when the model can answer directly. Each unnecessary call wastes API fees and latency, yet no existing benchmark systematically studies when a tool call is actually needed. We propose When2Tool, a benchmark of 18 environments (15 single-hop, 3 multi-hop) spanning three categories of tool necessity -- computational scale, knowledge boundaries, and execution reliability -- each with controlled difficulty levels that create a clear decision boundary between tool-necessary and tool-unnecessary tasks. We evaluate two families of training-free baselines: Prompt-only (varying the prompt to discourage unnecessary calls) and Reason-then-Act (requiring the model to reason about tool necessity before acting). Both provide limited control: Prompt-only suppresses necessary calls alongside unnecessary ones, and Reason-then-Act still incurs a disproportionate accuracy cost on hard tasks. To understand why these baselines fail, we probe the models' hidden states and find that tool necessity is linearly decodable from the pre-generation representation with AUROC 0.89--0.96 across six models, substantially exceeding the model's own verbalized reasoning. This reveals that models already know when tools are needed, but fail to act on this knowledge during generation. Building on this finding, we propose Probe&Prefill, which uses a lightweight linear probe to read the hidden-state signal and prefills the model's response with a steering sentence. Across all models tested, Probe&Prefill reduces tool calls by 48% with only 1.7% accuracy loss, while the best baseline at comparable accuracy only reduces 6% of tool calls, or achieves a similar tool call reduction but incurs a 5

\times

higher accuracy loss. Our code is available at https://github.com/Trustworthy-ML-Lab/when2tool

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