Prevent agents from calling duplicate tools with same inputs

[Copilot]

This PR implements duplicate tool call prevention to address a common failure mode where frontier models (GPT-5, Gemini-Flash-2.5) get stuck calling the same tool repeatedly with identical inputs.

Problem

Agents would sometimes enter infinite loops calling tools like:

<tool name="lookup_memory">{"query": "project timeline"}</tool>
<tool name="lookup_memory">{"query": "project timeline"}</tool>
<tool name="lookup_memory">{"query": "project timeline"}</tool>
<!-- ... continues indefinitely -->

Solution

The implementation tracks tool calls within each AgentRun execution and blocks duplicates based on tool name + normalized arguments. However, agents can override this by providing unique reasoning:

<!-- First call - allowed -->
<tool name="lookup_memory">{"query": "project timeline"}</tool>

<!-- Exact duplicate - blocked -->
<tool name="lookup_memory">{"query": "project timeline"}</tool>

<!-- Duplicate with unique reasoning - allowed -->
<tool name="lookup_memory">{"query": "project timeline", "_duplicate_reasoning": "searching with broader context"}</tool>

Key Features

• Argument normalization: Excludes _nonce and _duplicate_reasoning from duplicate detection
• Reasoning validation: Ensures _duplicate_reasoning is unique within the current session
• Format support: Works with both JSON and XML argument formats
• Clear error messages: Guides agents when duplicates are blocked
• Whitespace handling: Normalizes reasoning to prevent trivial bypasses
• Stress level adjustment: Increases when duplicates are blocked to encourage different behavior

Components Affected

• AgentRun: Primary implementation with tool call history tracking
• AgentLearn: Extended to support duplicate prevention
• Both components now initialize self.tool_call_history = [] and reset it on each execution

Technical Implementation

Added helper functions:

• normalize_tool_args(): Removes special parameters for comparison
• extract_nonce_and_reasoning(): Extracts special parameters from parsed args
• is_duplicate_tool_call(): Checks if tool call matches previous calls
• is_reasoning_unique(): Validates reasoning hasn't been used before

The duplicate check is integrated into handle_tool_use() before tool execution, providing immediate feedback when duplicates are detected.

Fixes #8.

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