Agent distillation is the process of transferring the full task-solving behavior of large LLM agent systems — including reasoning chains, tool usage patterns, and multi-step decision trajectories — into smaller, deployable models. Unlike general model distillation which uses flat token-level supervision to mimic outputs, agent distillation explicitly handles the compositional structure of agent trajectories, segmenting them into reasoning and action components for fine-grained alignment. This enables models as small as 0.5B-3.8B parameters to achieve performance competitive with models 4-10x larger.

The critical difference is that agent distillation models the causal dependency between reasoning and actions — a thought process leads to a specific tool call, which produces an observation that informs the next reasoning step. Flat distillation loses this structure.

Trajectory distillation trains small models to imitate complete agent trajectories from teacher agents:

• Data generation: Large teacher model (e.g., GPT-4, Qwen-32B) executes tasks while logging full trajectories — every reasoning step, tool invocation, observation, and decision
• Trajectory segmentation: Each trajectory is parsed into typed spans: [REASON], [ACT], [OBSERVE]
• Supervised training: Small student model is fine-tuned on segmented trajectories using span-specific loss functions
• Curriculum training: Tasks are ordered by complexity, starting with simple single-step actions and progressing to multi-step chains

Key approaches include:

• AgentTrek — Synthesizes web agent trajectories from publicly available tutorials through a three-stage pipeline: harvest tutorial texts, transform into task specs, execute in real environments with VLM verification. Achieves state-of-the-art on WebArena and ScreenSpot benchmarks.
• Agent Distillation (Kang et al. 2025) — Introduces “first-thought prefix” prompting to improve teacher trajectory quality, and self-consistent action generation for student robustness. Models at 0.5B achieve performance competitive with 1.5B CoT-distilled models.

Reasoning chain distillation specifically supervises the internal thought process:

• Uses separate loss functions for reasoning spans vs action spans
• KL divergence on [REASON] tokens aligns multi-step rationale between teacher and student
• Cross-entropy on [ACT] tokens ensures correct tool calls
• Masking prevents cross-span interference during gradient computation
• Gradient projection techniques keep reasoning and action learning in separate subspaces

SAD is the first span-level distillation framework specifically for ReAct-style agents:

• Segments ReAct trajectories into Thought/Action/Observation spans
• Applies segment-specific losses with learnable weights
• Uses curriculum training progressing from simple to complex trajectories
• Achieves strong results on ALFWorld, HotPotQA-ReAct, and WebShop benchmarks
• Outperforms token-level baselines by 8-15% on planning tasks

# Agent distillation: trajectory segmentation and span-level training
from dataclasses import dataclass
from typing import List
import torch
import torch.nn.functional as F
 
@dataclass
class TrajectorySpan:
    type: str        # "reason", "action", "observation"
    tokens: List[int]
    start_idx: int
    end_idx: int
 
def segment_trajectory(trajectory_tokens, span_markers):
    """Parse agent trajectory into typed spans."""
    spans = []
    for marker in span_markers:
        spans.append(TrajectorySpan(
            type=marker["type"],
            tokens=trajectory_tokens[marker["start"]:marker["end"]],
            start_idx=marker["start"],
            end_idx=marker["end"],
        ))
    return spans
 
def compute_span_loss(student_logits, teacher_logits, spans, labels):
    """Compute span-specific losses for agent distillation."""
    total_loss = 0.0
    for span in spans:
        s, e = span.start_idx, span.end_idx
        student_span = student_logits[:, s:e, :]
        if span.type == "reason":
            # KL divergence for reasoning alignment
            teacher_span = F.softmax(teacher_logits[:, s:e, :], dim=-1)
            total_loss += F.kl_div(
                F.log_softmax(student_span, dim=-1),
                teacher_span, reduction="batchmean"
            )
        elif span.type == "action":
            # Cross-entropy for action correctness
            total_loss += F.cross_entropy(
                student_span.view(-1, student_span.size(-1)),
                labels[:, s:e].view(-1)
            )
        # Observation spans are masked (not supervised)
    return total_loss

Microsoft's Phi-3 Mini demonstrates that aggressive distillation combined with high-quality data curation can achieve reasoning parity with much larger models:

• 3.8B parameters — fraction of the size of GPT-4 or Llama-65B
• Trained on curated synthetic data generated by larger teacher models
• Chain-of-thought distillation from teacher reasoning traces
• Achieves competitive scores on reasoning benchmarks (MMLU, GSM8K, HumanEval)
• Key insight: data quality matters more than model size — carefully distilled reasoning traces from strong teachers enable small models to punch above their weight

The consistent pattern shows agent distillation provides roughly a 4x model size efficiency gain over standard chain-of-thought distillation.

The combined agent distillation objective:

<latex>\mathcal{L}_{AD} = \alpha \mathcal{L}_{KL}^{reason} + \beta \mathcal{L}_{CE}^{action} + \gamma \mathcal{L}_{curriculum}</latex>

where <latex>\alpha, \beta</latex> are learnable span weights, <latex>\mathcal{L}_{KL}^{reason}</latex> is KL divergence on reasoning spans, <latex>\mathcal{L}_{CE}^{action}</latex> is cross-entropy on action spans, and <latex>\mathcal{L}_{curriculum}</latex> is a complexity-weighted scheduling term.

• "Structured Agent Distillation for ReAct Agents" (SAD)
• Kang et al. "Distilling LLM Agent into Small Models with Retrieval and Code Tools" (arXiv:2505.17612)
• Xu et al. "AgentTrek: Agent Trajectory Synthesis via Guiding Replay with Web Tutorials" (arXiv:2412.09605)
• Chen et al. "FireAct: Toward Language Agent Fine-tuning"
• Microsoft "Phi-3 Technical Report"
• Agent Distillation GitHub Repository

• SWE-agent — Agent system whose trajectories can be distilled
• Gorilla — Specialized model training for API calling
• Self-Play Agents — Training agents through self-generated data
• MetaGPT — Multi-agent framework producing distillable trajectories