Adaptive Agent Foundation Model

Technique: Adaptive Agent Foundation Model — Efficient Hybrid Reasoning Routing

Current agent systems face a fundamental efficiency problem: reasoning-centric LLMs excel at internal chain-of-thought but cannot invoke external tools, while agentic LLMs can call tools but often lack deep reasoning. Both architectures tend to over-apply their primary capability—reasoning models overthink simple queries, agentic models make unnecessary tool calls. A²FM solves this by dynamically routing to the right mode for each query.

Rather than forcing all queries through the same pipeline, A²FM identifies simple queries that need instant answers, moderate queries requiring reasoning, and complex queries demanding tool interaction. This three-mode approach prevents wasted computation while maintaining performance across diverse benchmarks.

Core Concept

A²FM operates on a route-then-align principle:

• Task-aware routing: Classify incoming queries into three modes
• Mode-specific trajectories: Maintain specialized reasoning paths for each mode
• Shared foundation backbone: All modes align under a single 32B model
• Adaptive Policy Optimization: Cost-regularized rewards enforce efficient mode selection

The innovation prevents over-specification: answering "What is the capital of France?" through multi-step tool calls is wasteful, as is reasoning deeply about straightforward requests.

Architecture Overview

• Input Router: Query embedding → classify as instant/reasoning/tool mode
• Instant Mode Handler: Direct answer generation for factual questions (no reasoning overhead)
• Reasoning Mode: Deep chain-of-thought without external tool calls
• Tool Mode: Sequential tool invocation with verification loops
• Unified Backbone: Shared 32B foundation LLM with mode-specific adapter layers
• APO Training: Reward shaping that favors cost-efficient paths

Implementation Steps

The routing decision happens once per query. This example shows how to implement query classification and mode-specific forward passes.

import torch
import torch.nn as nn

class AdaptiveRouterA2FM(nn.Module):
    """Route queries to instant/reasoning/tool modes based on complexity."""

    def __init__(self, embedding_dim=768, hidden_dim=512):
        super().__init__()
        self.query_encoder = nn.Linear(embedding_dim, hidden_dim)
        self.router = nn.Sequential(
            nn.ReLU(),
            nn.Linear(hidden_dim, 256),
            nn.ReLU(),
            nn.Linear(256, 3)  # 3 modes: instant, reasoning, tool
        )
        self.mode_names = ["instant", "reasoning", "tool"]

    def forward(self, query_embedding):
        """
        Args:
            query_embedding: shape (batch, embedding_dim)

        Returns:
            mode_logits: shape (batch, 3) - scores for each mode
            selected_mode: shape (batch,) - argmax mode indices
        """
        encoded = self.query_encoder(query_embedding)
        mode_logits = self.router(encoded)
        selected_mode = torch.argmax(mode_logits, dim=1)
        return mode_logits, selected_mode


def adaptive_forward_pass(
    query,
    router_model,
    instant_handler,
    reasoning_model,
    tool_model
):
    """
    Encode query, route to appropriate handler, return answer.
    """
    # Encode query
    query_emb = encode_query(query)  # placeholder

    # Route to mode
    mode_logits, selected_mode = router_model(query_emb)
    mode_idx = selected_mode.item()

    # Execute mode-specific handler
    if mode_idx == 0:  # Instant mode
        answer = instant_handler.answer(query)
        cost = 1  # Cheap

    elif mode_idx == 1:  # Reasoning mode
        answer = reasoning_model.chain_of_thought(query, max_steps=10)
        cost = 5  # Medium

    else:  # Tool mode
        answer = tool_model.invoke_tools_with_reasoning(
            query, available_tools=TOOLS, max_calls=3
        )
        cost = 15  # Expensive

    return answer, cost, mode_idx

Cost-regularized rewards: During RL training, reward = accuracy - 0.1 * cost. Higher lambda penalizes wasteful routing. Tune on development set to balance accuracy vs. efficiency.

def adaptive_policy_optimization_reward(
    answer_correct,
    mode_cost,
    lambda_cost=0.1
):
    """
    Reward function for APO training.
    Penalizes both incorrect answers AND expensive routing.
    """
    accuracy_reward = float(answer_correct)
    cost_penalty = -lambda_cost * mode_cost
    total_reward = accuracy_reward + cost_penalty
    return total_reward

Practical Guidance

When to Use:

• Serving diverse query types with variable complexity
• Cost-constrained inference (fewer tokens per query matters)
• Mix of factual, reasoning, and tool-dependent tasks
• You have labeled training data for query complexity classification

When NOT to Use:

• All queries require deep reasoning (routing overhead not justified)
• Tool availability is sparse or unreliable
• No budget for routing classification overhead

Common Pitfalls:

• Router biases toward most common mode (oversample hard examples during training)
• Cost weights not calibrated to actual inference costs
• Instant handler lacks sufficient context—adds errors despite apparent cheapness
• Over-penalizing tool mode when tool calls are actually necessary

Reference

A²FM: An Adaptive Agent Foundation Model for Tool-Aware Hybrid Reasoning