Train a single LLM to decompose complex queries into subquestions and integrate retrieved contexts through two-stage supervised and preference-based reinforcement fine-tuning, achieving 7.6% average improvement and matching 685B models with 32B parameters.
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Train a single LLM to decompose complex queries into subquestions and integrate retrieved contexts through two-stage supervised and preference-based reinforcement fine-tuning, achieving 7.6% average improvement and matching 685B models with 32B parameters.
AceSearcher: Bootstrapping Reasoning and Search for LLMs via Reinforced Self-Play
Outcome
Implement a cooperative self-play framework that trains a single LLM to alternate between decomposer and solver roles, enhancing multi-hop reasoning and retrieval-augmented generation without requiring intermediate answer annotations. Achieve parameter-efficient performance matching much larger models through iterative preference optimization.
Problem Context
Complex question answering demands multi-step reasoning and precise information retrieval. Traditional approaches struggle with:
Decomposing ambiguous queries into retrievable subquestions
Integrating multiple retrieved passages without losing coherence
Requiring expensive intermediate supervision for each reasoning step
Scaling linearly with model size for competitive performance
AceSearcher addresses these by training a single LLM to handle both question decomposition and answer synthesis through self-play, using only final-answer signals for optimization.
Core Concept
AceSearcher treats reasoning as a cooperative process between two roles played by the same model:
Decomposer (ρ): Transforms an input question into a sequence of subquestions designed to guide retrieval. Given question q, produces subquestions z = (z₁, z₂, ..., zₙ).
Solver (π): Generates intermediate answers for each subquestion by consulting retrieved documents, then synthesizes all intermediate answers into a final response. Takes subquestion zᵢ, prior answers w<ᵢ, and retrieved context Dᵢ to produce intermediate answer wᵢ, then generates final answer a' from all components.
The key insight is that better decompositions enable the solver to retrieve more relevant context and produce higher-quality answers. This mutual dependency justifies joint optimization without separate roles or models.
Architecture Overview
Unified Model: Single LLM controlled by templates and prompts, no separate role-specific parameters
Two-Stage Training: Supervised fine-tuning on diverse reasoning tasks, followed by preference-based reinforcement optimization
Preference Generation: Multipath rollouts sample multiple decompositions and solution paths, creating preference pairs from max/min trajectories
Iterative DPO: Direct Preference Optimization applied across three preference datasets (decomposition quality, intermediate answers, final answers) with convergence guarantees
Retrieval Integration: Sparse retrieval allocates documents per subquestion (typically 15 total documents distributed across decomposed steps)
Implementation
Stage 1: Supervised Fine-Tuning on Diverse Tasks
The foundation requires exposure to search, reasoning, and decomposition patterns across 180,600 examples:
Generate preference pairs via multipath rollouts and optimize with iterative DPO:
# preference_generation.pyimport numpy as np
from typing importTuple, ListclassPreferenceDataGenerator:
def__init__(self, model, retriever, num_decompositions=3, num_solutions=4):
self.model = model
self.retriever = retriever
self.m = num_decompositions # candidate decompositions per questionself.m_prime = num_solutions # candidate solutions per decompositiondefcompute_reward(self, question: str, answer: str, gold_answer: str) -> float:
"""
Reward function: exact match with format validation
r(q, a', a) = EM(a', a) × 𝕀(f(q, a') = 1)
"""
em_score = 1.0ifself._normalize_answer(answer) == self._normalize_answer(gold_answer) else0.0
format_valid = self._check_format(question, answer)
return em_score * float(format_valid)
def_normalize_answer(self, answer: str) -> str:
"""Normalize answers for EM computation"""
answer = answer.lower().strip()
answer = ' '.join(answer.split())
return answer
def_check_format(self, question: str, answer: str) -> bool:
"""Validate format: proper structure for subquestions, answers, reasoning"""
required_markers = ['###', 'Answer:'] if'##'in question else []
returnall(marker in answer for marker in required_markers) if required_markers elseTruedefgenerate_rollouts(self, question: str, gold_answer: str) -> Tuple[List, List]:
"""
Generate m decompositions × m' solutions per decomposition
"""
decompositions = []
rewards_per_decomp = []
# Rollout m candidate decompositionsfor i inrange(self.m):
z_i = self.model.generate_decomposition(question, temperature=1.0)
# Retrieve documents for this decomposition
subquestions = self._parse_subquestions(z_i)
all_docs = []
for sq in subquestions:
docs = self.retriever.retrieve(sq, k=10//len(subquestions))
all_docs.extend(docs)
# Rollout m' solutions for this decomposition
solutions = []
solution_rewards = []
for j inrange(self.m_prime):
a_j = self.model.generate_answer(
question, z_i, all_docs, temperature=1.0
)
reward = self.compute_reward(question, a_j, gold_answer)
solutions.append((z_i, a_j))
solution_rewards.append(reward)
decompositions.append((z_i, solutions, solution_rewards))
avg_reward = np.mean(solution_rewards)
rewards_per_decomp.append(avg_reward)
return decompositions, rewards_per_decomp
defcreate_preference_pairs(self,
question: str,
decompositions: List,
rewards_per_decomp: List) -> dict:
"""
Create three preference pair datasets: D_decompose, D_subq, D_final
"""
preferences = {"decompose": [], "subquestion": [], "final": []}
# D_decompose: compare decompositions by expected reward
best_decomp_idx = np.argmax(rewards_per_decomp)
worst_decomp_idx = np.argmin(rewards_per_decomp)
if rewards_per_decomp[best_decomp_idx] != rewards_per_decomp[worst_decomp_idx]:
z_plus = decompositions[best_decomp_idx][0]
z_minus = decompositions[worst_decomp_idx][0]
preferences["decompose"].append({
"question": question,
"preferred": z_plus,
"dispreferred": z_minus
})
# D_subq: compare intermediate answers
best_decomp_idx = np.argmax(rewards_per_decomp)
best_z, best_solutions, best_rewards = decompositions[best_decomp_idx]
subquestions = self._parse_subquestions(best_z)
for i, sq inenumerate(subquestions):
best_sol_idx = np.argmax(best_rewards)
worst_sol_idx = np.argmin(best_rewards)
if best_rewards[best_sol_idx] != best_rewards[worst_sol_idx]:
z_i, w_i_best = best_solutions[best_sol_idx]
z_i, w_i_worst = best_solutions[worst_sol_idx]
preferences["subquestion"].append({
"subquestion": sq,
"preferred_answer": w_i_best,
"dispreferred_answer": w_i_worst
})
# D_final: compare final answers using best trajectory
best_sol_idx = np.argmax(best_rewards)
_, a_best = best_solutions[best_sol_idx]
worst_sol_idx = np.argmin(best_rewards)
_, a_worst = best_solutions[worst_sol_idx]
if best_rewards[best_sol_idx] != best_rewards[worst_sol_idx]:
preferences["final"].append({
"question": question,
"context": best_z,
"preferred_answer": a_best,
"dispreferred_answer": a_worst
})
return preferences
def_parse_subquestions(self, decomposition: str) -> List[str]:
"""Extract subquestions from decomposition text"""
lines = decomposition.split('\n')
subqs = [line.strip() for line in lines if line.startswith('###')]
return [sq.replace('###', '').strip() for sq in subqs]
Stage 2 Continued: Direct Preference Optimization
Apply iterative DPO across the unified preference dataset:
# dpo_training.pyimport torch
import torch.nn.functional as F
from torch.utils.data import DataLoader
classAceSearcherDPO:
def__init__(self, model, reference_model, beta=0.5, learning_rate=5e-7):
self.model = model
self.reference_model = reference_model
self.beta = beta # KL penalty coefficientself.optimizer = torch.optim.AdamW(model.parameters(), lr=learning_rate)
defdpo_loss(self,
prompt: str,
preferred_completion: str,
dispreferred_completion: str,
batch_size: int = 16) -> torch.Tensor:
"""
Direct Preference Optimization loss
ℒ_DPO := -log σ(β[log(p_θ(g^+|x)/p_ref(g^+|x)) - log(p_θ(g^-|x)/p_ref(g^-|x))])
"""# Tokenize inputs
prompt_ids = self.model.tokenize(prompt)
preferred_ids = self.model.tokenize(preferred_completion)
dispreferred_ids = self.model.tokenize(dispreferred_completion)
# Compute log probabilities with current modelwith torch.no_grad():
ref_logits_preferred = self.reference_model(
torch.cat([prompt_ids, preferred_ids], dim=-1)
).logits
ref_logits_dispreferred = self.reference_model(
torch.cat([prompt_ids, dispreferred_ids], dim=-1)
).logits
# Get current model logits
logits_preferred = self.model(
torch.cat([prompt_ids, preferred_ids], dim=-1)
).logits
logits_dispreferred = self.model(
torch.cat([prompt_ids, dispreferred_ids], dim=-1)
).logits
# Compute log-probabilitiesdefget_log_prob(logits, token_ids):
"""Extract log probability of each token"""
log_probs = F.log_softmax(logits, dim=-1)
# Gather log probs for each position
token_log_probs = torch.gather(
log_probs[:, :-1, :], 2, token_ids[:, 1:].unsqueeze(-1)
).squeeze(-1)
return token_log_probs.sum(dim=-1)
log_prob_theta_preferred = get_log_prob(logits_preferred, preferred_ids)
log_prob_theta_dispreferred = get_log_prob(logits_dispreferred, dispreferred_ids)
log_prob_ref_preferred = get_log_prob(ref_logits_preferred, preferred_ids)
log_prob_ref_dispreferred = get_log_prob(ref_logits_dispreferred, dispreferred_ids)
# DPO objective
logit_diff = (
(log_prob_theta_preferred - log_prob_ref_preferred) -
(log_prob_theta_dispreferred - log_prob_ref_dispreferred)
)
loss = -F.logsigmoid(self.beta * logit_diff).mean()
return loss
deftrain_iteration(self, preference_dataset, num_epochs=1):
"""One complete DPO training pass"""
dataloader = DataLoader(preference_dataset, batch_size=32, shuffle=True)
self.model.train()
total_loss = 0for epoch inrange(num_epochs):
for batch in dataloader:
self.optimizer.zero_grad()
# Compute loss across all preference types
losses = []
for pref_type in ["decompose", "subquestion", "final"]:
if batch[pref_type]:
for pair in batch[pref_type]:
loss = self.dpo_loss(
pair["prompt"],
pair["preferred"],
pair["dispreferred"]
)
losses.append(loss)
if losses:
batch_loss = torch.stack(losses).mean()
batch_loss.backward()
self.optimizer.step()
total_loss += batch_loss.item()
return total_loss / len(dataloader)
Multi-Turn DPO with Convergence
Run iterative DPO with model updates:
# iterative_training.pyclassIterativeDPOTrainer:
def__init__(self, model_path, num_iterations=2):
self.num_iterations = num_iterations
self.model_path = model_path
defrun_multi_turn_dpo(self, train_questions, gold_answers, retriever):
"""
Multi-turn DPO: θ^(0) → θ^(1) → θ^(2)
At each iteration, use current model to generate new preference data
"""# Load or initialize model
model = self._load_model()
preference_gen = PreferenceDataGenerator(model, retriever)
dpo_trainer = AceSearcherDPO(model, model.copy()) # ref model = initial modelfor iteration inrange(self.num_iterations):
print(f"Iteration {iteration + 1}/{self.num_iterations}")
# Generate preference pairs using current model θ^(t)
all_preferences = {"decompose": [], "subquestion": [], "final": []}
for question, gold_answer inzip(train_questions, gold_answers):
decompositions, rewards = preference_gen.generate_rollouts(
question, gold_answer
)
prefs = preference_gen.create_preference_pairs(
question, decompositions, rewards
)
# Accumulate preferencesfor pref_type in ["decompose", "subquestion", "final"]:
all_preferences[pref_type].extend(prefs[pref_type])
# Filter out equivalent pairs (reward difference = 0)
filtered_prefs = self._filter_equivalent_pairs(all_preferences)
# Train for one epoch with DPO
loss = dpo_trainer.train_iteration(filtered_prefs, num_epochs=1)
print(f" Iteration {iteration + 1} loss: {loss:.4f}")
# Save checkpointself._save_checkpoint(model, iteration)
# Update reference model for next iteration
dpo_trainer.reference_model = model.copy()
return model
def_filter_equivalent_pairs(self, preferences):
"""Remove pairs where preferred and dispreferred have equal rewards"""
filtered = {"decompose": [], "subquestion": [], "final": []}
for pref_type, pairs in preferences.items():
for pair in pairs:
# Pairs already filtered during generation, but validateif pair.get("preferred") != pair.get("dispreferred"):
filtered[pref_type].append(pair)
return filtered
def_load_model(self):
return torch.load(self.model_path)
def_save_checkpoint(self, model, iteration):
torch.save(model.state_dict(), f"{self.model_path}_iter{iteration}")
Prompt Templates for QA Tasks
# prompt_templates.py
DECOMPOSITION_TEMPLATE = """Please break down the question "{question}" into multiple specific
sub-questions that address individual components of the original question.
Mark each sub-question with ### at the beginning. If you need to refer to
answers from earlier sub-questions, use #1, #2, etc. to reference them.
Provide the sub-questions only, without any additional explanation."""
ANSWER_GENERATION_TEMPLATE = """You have the following passages:
{passages}
You are also given some subquestions and their answers:
{subquestion_answers}
Please answer the question "{original_question}" with a short span
using the documents and subquestions as reference. Be concise and direct."""# Example output
decomposition_example = """### What is the capital of France?
### What is the population of Paris?
### When did Paris become the capital?"""
answer_example = """The capital of France is Paris. Paris has a metropolitan population
of approximately 2.1 million people. Paris became the capital in 987 AD."""
Practical Guidance
Hyperparameter Selection
Parameter
Recommended Range
Impact
Notes
m (decompositions)
2-5
Quality vs. speed
3 offers good balance; diminishing returns beyond 4
m' (solutions per decomposition)
3-8
Preference diversity
4 typical; increase if reward signal is noisy
beta (KL penalty)
0.5-1.0
Regularization strength
Higher beta = stronger ref model adherence
k (retrieved passages)
8-15
Coverage vs. noise
Gains plateau at k=10; allocate per subquestion
num_iterations
1-3
Convergence
2 iterations balances performance and training cost
learning_rate SFT
5e-7 to 5e-6
Stability
Scale with model size; smaller LR for larger models
Closed-domain QA where a single passage answers the question
Real-time inference with strict latency constraints (2-3x longer than single-turn RAG)
Tasks requiring real-time knowledge updates (training cycle is multiday)
Scenarios where explainability of decomposition is not valued
Common Pitfalls
Insufficient SFT diversity: Training only on search or only on reasoning hurts both components. Ensure balanced exposure to all three task categories.
Subquestion ambiguity: Decompositions that are too similar or overlapping waste retrieval budget. Add examples of diverse decomposition patterns to SFT data.
Reward signal weakness: If EM-only rewards are sparse (e.g., <10% positive examples), add format validation or intermediate correctness checks.
Document allocation mismatches: Distributing more documents to some subquestions than others can create learning bias. Use uniform or learned allocation.
Convergence instability in RFT: Starting with too high a learning rate causes preference divergence. Use 0.5-1.0x the SFT learning rate for RFT.
Preference pair bottleneck: Generating m × m' rollouts per question scales poorly. Use sampling or importance weighting for large-scale training.
Efficiency Considerations
SFT on 180K examples at batch size 64 takes ~25 GPU hours (A100)
RFT with 2 iterations on 49K QA examples: ~12 GPU hours per iteration
Inference is 2-3× slower than single-turn retrieval due to decomposition
Parameter efficiency: AceSearcher-32B matches DeepSeek-V3 (685B) on document reasoning using <5% parameters
Reference
For full technical details including theoretical convergence proofs, ablation studies, and complete experimental results, see the paper:
