Federated Fine-Tuning Approach

• Federated fine-tuning is an advanced machine learning paradigm that personalizes pre-trained models by collaboratively updating only lightweight parameters across distributed data owners.
• It leverages methods such as prompt tuning, adapter tuning, LoRA, bias tuning, and representation fine-tuning to efficiently adapt to resource, data heterogeneity, and privacy constraints.
• Innovative aggregation strategies like FedAvg, stacking-based, and expert selection optimize convergence speed and communication efficiency for practical deployments across various domains.

Federated fine-tuning is an advanced machine learning paradigm that combines federated learning (FL) with parameter-efficient adaptation techniques to personalize large, pre-trained models—such as Transformers—across distributed data owners, while maintaining privacy and reducing communication and computational costs. Rather than sharing raw data, federated fine-tuning orchestrates collaborative model optimization, typically updating only lightweight modules or parameters, to address the resource, data heterogeneity, and privacy constraints inherent to decentralized learning environments.

1. Parameter-Efficient Fine-Tuning Techniques in Federated Learning

Parameter-efficient fine-tuning (PEFT) is foundational to federated fine-tuning of large models. The predominant methodologies include:

• Prompt Tuning: Introduces trainable tokens at model input; underlying backbone remains frozen. While extremely lightweight (e.g., 17.3 KB for CLIP prompt tuning (Chen et al., 2022)), prompt tuning is susceptible to overfitting in few-shot or high non-IID settings (Chen et al., 2022, Weng et al., 27 Feb 2025).
• Adapter Tuning: Inserts learnable bottleneck modules between network layers; moderate footprint but can impact convergence under severe data heterogeneity (Chen et al., 2022, Babakniya et al., 2023).
• Low-Rank Adaptation (LoRA): Injects learnable rank-constrained matrices into dense layers, optimizing ΔW = BA with B ∈ ℝ^d×r, A ∈ ℝ^r×k, r ≪ min(d, k). LoRA is communication- and memory-efficient, but aggregation becomes mathematically problematic as clients use heterogeneous ranks (Babakniya et al., 2023, Wang et al., 9 Sep 2024, Liu et al., 2 Mar 2025, Wang et al., 18 Sep 2025).
• Bias Tuning (BitFit): Fine-tunes only bias terms—empirically, this often yields strong results in both vision and LLMs under FL, especially with strong pretraining (Chen et al., 2022).
• Representation Fine-Tuning (ReFT): Directly intervenes on hidden representations rather than weights, using sparse, low-dimensional intervention modules; more robust under extreme heterogeneity and achieves parameter efficiency (requiring <0.1% of parameters to be updated) (Siddika et al., 27 Aug 2025).

The table below summarizes these core PEFT methods and their characteristics in federated fine-tuning:

Method	Typical Memory Footprint	Robustness to Non-IID	Communication Efficiency
Prompt Tuning	Extremely low	Vulnerable	High
Adapter	Moderate	Limited (in some FL)	Moderate
LoRA	Low	Good (w/ aggregation)	High
Bias Tuning	Lowest	Strong (if backbone)	Highest
ReFT	Lowest	Strongest	Highest

A nuanced finding is that bias tuning achieves superior accuracy and F1 scores in both IID and non-IID scenarios, outperforming prompt and adapter tuning in the presence of strong pretraining (Chen et al., 2022).

2. Federated Aggregation and Communication Strategies

The aggregation step in federated fine-tuning is nontrivial, especially for modular or low-rank updates:

• Classic FedAvg: Aggregates local updates via weighted averages. For models with heterogeneous LoRA ranks, this leads to "aggregation noise" due to cross-term artifacts arising from naive averaging of A and B matrices rather than their product—which is mathematically incorrect (Wang et al., 9 Sep 2024, Liu et al., 2 Mar 2025).
• Stacking-Based Aggregation (FLoRA): Stacks each client’s local LoRA modules to expand the global low-rank adapters, avoiding cross-term noise and permitting heterogeneous LoRA ranks. This guarantees lossless and mathematically correct aggregation, supporting mixed client capabilities (Wang et al., 9 Sep 2024).
• Expert/Cluster Aggregation (FedLEASE, FedAMoLE): Clients are clustered according to data/task similarity; domain-specific LoRA experts are allocated and either aggregated at the cluster level or as a mixture-of-experts (MoE) (Zhang et al., 28 Nov 2024, Wang et al., 18 Sep 2025).
• All-But-Me Aggregation (FedReFT): Each client incorporates the geometric median of all other clients' parameter updates, preserving personalized adaptation and robustness to outliers (Siddika et al., 27 Aug 2025).

In wireless and resource-constrained settings, split architectures—where client devices update only lightweight modules (e.g., prompt or adapter) while heavy backbones reside on the server—significantly reduce uplink bandwidth and local computation costs (Wang et al., 3 Jul 2024, Cao et al., 24 Jul 2024, Asokan et al., 31 Jul 2024).

3. Personalization, Heterogeneity, and Adaptivity

Federated fine-tuning frameworks address personalization and heterogeneity through several adaptive mechanisms:

• Automatic Rank and Depth Learning: Adaptive algorithms (LEGEND, PF2LoRA, HierFedLoRA) dynamically assign LoRA depth (number of fine-tuning layers) and per-layer ranks, optimizing for each device’s computational and bandwidth profile (Liu et al., 28 Dec 2024, Hao et al., 5 Mar 2025, Liu et al., 27 Mar 2025).
• Two-Level/Hierarchical Adapters: Separating a common global adapter and lightweight client-specific adapters enables on-device personalization while retaining effective global knowledge (Hao et al., 5 Mar 2025).
• Expert Selection and Assignment: Mixture-of-experts modules and expert selection routers allow clients to leverage specialized knowledge—and decide per-input or per-client which subset of experts to invoke (Zhang et al., 28 Nov 2024, Wang et al., 18 Sep 2025).
• Rest-of-World LoRA (FedALT): Clients combine their own LoRA module with a "rest-of-world" LoRA built from other clients' updates, using a learned dynamic mixer for each input, thus achieving input-level personalization (Bian et al., 14 Mar 2025).

Aggregation bias due to heterogeneous LoRA ranks is specifically mitigated in HLoRA by reconstructing the global update as a full parameter matrix and then re-decomposing it for each client's resource constraints (Liu et al., 2 Mar 2025). Stacking-based and all-but-me strategies also address aggregation bias and personalized learning.

4. Practical Performance: Communication, Computation, and Convergence

Federated fine-tuning approaches yield substantial benefits over centralized or full-model FL:

• Reduction in Communication: LoRA-based FL can reduce communication costs by an order of magnitude (e.g., ~48× less than full model transmission (Asokan et al., 31 Jul 2024), 10.67× less with developmental tuning (Wu et al., 31 Jul 2025)).
• Computation Savings: Typically, only ~0.05%–1% of the parameters are updated per round; split and prompt-based methods further reduce local computational cost, enabling deployment on edge and mobile devices (Cao et al., 24 Jul 2024, Siddika et al., 27 Aug 2025).
• Convergence Speed: Adaptive resource allocation and developmental staged approaches (DevFT) achieve up to 4.59× faster convergence than E2E fine-tuning (Wu et al., 31 Jul 2025). Parameter-efficient methods (e.g., SLoRA) yield up to 90% training time reduction under strong non-IID conditions (Babakniya et al., 2023).
• Accuracy and Overfitting: Pre-trained vision-LLMs (e.g., CLIP) are notably robust in few-shot FL; aggregated fine-tuning relieves overfitting, especially in the non-IID and limited-data regimes, outperforming local-only and CNN approaches by up to 50% under Dirichlet splits (Chen et al., 2022).
• Resource Adaptivity: LEGEND and HierFedLoRA dynamically adapt assignment of fine-tuning layers and aggregation frequency to minimize computational bottleneck and maximize group learning efficiency, reporting up to 2.8× coarse-to-fine speedup (Liu et al., 28 Dec 2024, Liu et al., 27 Mar 2025).

5. Advanced Paradigms: Staged, Proxy, and One-Shot Federated Fine-Tuning

Recent advances introduce new paradigms and theoretical guarantees:

• Developmental Federated Tuning (DevFT): Fine-tuning proceeds through progressive cognitive "stages," growing from compact submodels to full models, transferring knowledge via deconfliction-guided layer grouping and differential layer fusion. This enables efficient optimization in non-convex landscapes (Wu et al., 31 Jul 2025).
• Proxy Fine-Tuning (FedPFT): Clients fine-tune compressed, layer-pruned sub-models (sub-FMs) aligned to the global FM via two-stage knowledge distillation (layer- and neuron-level). The framework achieves comprehensive knowledge transfer despite sub-model mismatch, with theoretical convergence guarantees under gradient alignment bounds (Peng et al., 17 Apr 2024).
• One-Shot Federated Fine-Tuning: For sufficiently large foundation models (≥1B params), a single round of communication achieves performance nearly indistinguishable from multi-round FL, due to smooth loss landscapes and low update magnitudes. This drastically reduces communication, supports asynchronous aggregation, and enhances privacy (Wang et al., 5 Dec 2024).
• Switching-Based LoRA in Wireless Networks: Devices dynamically select the most suitable LoRA module per round based on local and transmission conditions; selection, power control, and bandwidth allocation are optimized online under theoretical risk gap bounds (Wang et al., 5 Sep 2025).

6. Theoretical Analysis and Aggregation Bias

Federated fine-tuning research rigorously analyzes convergence properties and aggregation bias:

• Aggregation Bias: Classic FedAvg aggregation of LoRA adapters introduces cross-terms, leading to mathematical inconsistency and suboptimal convergence. Correct aggregation requires either stacking (FLoRA) or reconstructing the full update prior to per-client SVD-based decomposition (HLoRA) (Wang et al., 9 Sep 2024, Liu et al., 2 Mar 2025).
• Stability Under Heterogeneity: Bilevel and hierarchical optimization (PF2LoRA, FedAMoLE, HierFedLoRA) enables fast, scalable personalization without manual rank or depth tuning—even as client data distributions and capacities vary (Hao et al., 5 Mar 2025, Zhang et al., 28 Nov 2024, Liu et al., 27 Mar 2025).
• Convergence Bounds: For proxy/sub-model approaches, convergence toward full-model optima is established under bounded gradient differences and Lipschitz continuity assumptions (FedPFT Theorems 1–2) (Peng et al., 17 Apr 2024). For one-shot FL, the fine-tuning error is upper-bounded by the product of model smoothness (L), update magnitude (τ), and number of local steps, which is minimal for large foundation models (Wang et al., 5 Dec 2024).
• Robustness: Robust geometric median aggregation (ABM) guards against outlying or adversarial updates in highly heterogeneous FL settings (Siddika et al., 27 Aug 2025).

7. Applications, Practical Deployment, and Future Directions

Federated fine-tuning is broadly applicable in domains requiring privacy-preserving adaptation of large models:

• Healthcare: FL fine-tunes foundation models on siloed medical data (e.g., 3D segmentation via FLAP-SAM), reducing communication by up to 48× while outperforming full fine-tuning in accuracy (Asokan et al., 31 Jul 2024).
• Finance and Cross-Organization Collaboration: Parameter-efficient and proxy methods adapt models without exposing sensitive data or proprietary model weights (Peng et al., 17 Apr 2024).
• Edge and Mobile Intelligence: Federated split/prompt strategies allow large models (LLMs, vision models) to be adapted by resource-constrained IoT and mobile devices (Wang et al., 3 Jul 2024, Cao et al., 24 Jul 2024).
• Multi-Modal and Cross-Device Reasoning: Clustered, hierarchical, and asynchronous FL enables robust adaptation and fast convergence under heterogeneity (Ni et al., 27 Mar 2025).
• Emergent Directions: Adaptive expert allocation and automatic per-client personalization (FedLEASE, FedAMoLE), direct representation intervention (FedReFT), and staged learning (DevFT) provide modular building blocks for future scalable, robust, and interpretable federated systems.

Open research challenges include fully dynamic expert routing, adaptive aggregation for non-stationary client populations, integration of differential privacy guarantees, exploitation of federated unlearning, and multi-modal extensions (Ni et al., 27 Mar 2025, Zhang et al., 28 Nov 2024, Wang et al., 18 Sep 2025).

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In sum, federated fine-tuning enables scalable, privacy-preserving, and resource-efficient adaptation of large pre-trained models across heterogeneous, distributed environments, leveraging advanced aggregation, adaptation, and compression strategies grounded in rigorous empirical and theoretical foundations.