ReAct Agent Loop — APEX OS Standard

Overview

From HandsOnLLM Ch.7. ReAct = Reason + Act. The fundamental pattern for tool-using agents. All APEX OS agents that use external tools follow this.

The ReAct Pattern

┌─────────────────────────────────────────────────────────────────────────┐
│  ReAct Loop                                                             │
│                                                                         │
│  Input → [THOUGHT] → [ACTION] → [OBSERVATION] → [THOUGHT] → ...        │
│                                                          ↓              │
│                                                    [FINAL ANSWER]       │
└─────────────────────────────────────────────────────────────────────────┘

Thought:  "I need to find the company email for this lead"
Action:   search_web(query="Acme Corp contact email")
Observation: "Found: contact@acme.com on their website"
Thought:  "I have the email. I can now score this lead."
Action:   score_lead(email="contact@acme.com", company="Acme Corp")
Observation: {"score": 78, "tier": "warm"}
Final Answer: Lead scored 78/100, warm tier.

Minimal ReAct Implementation

def react_agent(task: str, tools: dict, max_iterations: int = 10) -> str:
    messages = [
        {"role": "system", "content": REACT_SYSTEM_PROMPT},
        {"role": "user", "content": task}
    ]

    for iteration in range(max_iterations):
        response = llm.complete(messages)

        # Parse thought + action
        if "Final Answer:" in response:
            return response.split("Final Answer:")[-1].strip()

        action_name, action_input = parse_action(response)

        # Execute tool
        if action_name not in tools:
            observation = f"Error: Tool '{action_name}' not found."
        else:
            observation = tools[action_name](action_input)

        # Feed observation back
        messages.append({"role": "assistant", "content": response})
        messages.append({"role": "user", "content": f"Observation: {observation}"})

    return "Max iterations reached. Last state: " + response

Memory Strategies

┌──────────────────────────────────────────────────────────────────────┐
│ Strategy      │ How it works          │ Best for                     │
├──────────────────────────────────────────────────────────────────────┤
│ Buffer        │ Keep all messages     │ Short conversations (<20 msgs)│
│ Window        │ Keep last N messages  │ Medium sessions, rolling ctx  │
│ Summary       │ Summarise old msgs    │ Long sessions, preserve meaning│
│ Vector Store  │ Embed + retrieve      │ Long-term cross-session memory │
└──────────────────────────────────────────────────────────────────────┘

Window Memory (APEX OS default for agents):

class WindowMemory:
    def __init__(self, window_size: int = 10):
        self.messages = []
        self.window_size = window_size

    def add(self, role: str, content: str):
        self.messages.append({"role": role, "content": content})
        # Keep system prompt + last N turns
        if len(self.messages) > self.window_size + 1:
            self.messages = [self.messages[0]] + self.messages[-(self.window_size):]

    def get(self) -> list:
        return self.messages

Summary Memory (for long lead-gen pipeline sessions):

def summarise_old_messages(messages: list, keep_last: int = 6) -> list:
    if len(messages) <= keep_last + 1:
        return messages

    to_summarise = messages[1:-keep_last]  # skip system prompt
    summary = llm.complete(
        f"Summarise these conversation turns in 3 bullet points:\n"
        + "\n".join(f"{m['role']}: {m['content']}" for m in to_summarise)
    )
    return [messages[0],
            {"role": "system", "content": f"[Previous context summary]\n{summary}"},
            *messages[-keep_last:]]

Tool Description Rules (CRITICAL)

Tools fire based on their description. Write for semantic matching:

tools = [
    {
        "name": "search_company_info",
        # ❌ Bad: "Searches for company information"
        # ✅ Good: describes WHEN to use it
        "description": "Use to find company website, contact email, industry, "
                       "headcount, and LinkedIn URL for a given company name. "
                       "Do NOT use for person-level searches.",
        "parameters": {
            "type": "object",
            "properties": {
                "company_name": {"type": "string", "description": "Legal company name"}
            },
            "required": ["company_name"]
        }
    }
]

Infinite Loop Prevention

# Detect repeated actions
action_history = []

def check_loop(action_name: str, action_input: str) -> bool:
    key = f"{action_name}:{action_input}"
    if action_history.count(key) >= 2:
        return True  # Loop detected
    action_history.append(key)
    return False

# In loop:
if check_loop(action_name, action_input):
    return "Agent stuck in loop. Last action: " + action_name

Common Mistakes

• No max_iterations guard — agent loops forever on LLM error
• Tool descriptions say WHAT not WHEN — model picks wrong tool
• Using Buffer memory for long sessions — context fills, quality degrades
• Not feeding the observation back — agent repeats the same action
• Allowing tool errors to crash the loop — catch and pass as observation