from typing import Dict

def compute_reward(grader_score: float, previous_score: float, step_number: int, max_steps: int, is_done: bool, action_valid: bool) -> float:
    """
    Computes a shaped reward based on the problem statement requirements:
    - Partial credit per step: +0.0-0.5 for incremental improvement
    - Completion bonus: +0.5 if grader score >= 0.8 when is_done=True
    - Step penalty: -0.02 per unnecessary step (> task's min required steps)
    - Invalid action penalty: -0.1 for empty/unparseable queries
    - Total clamped to [0.0, 1.0]
    """
    reward = 0.0
    
    if not action_valid:
        return -0.1
        
    improvement = max(0.0, grader_score - previous_score)
    # Give partial credit up to 0.5 based on improvement
    reward += improvement * 0.5
    
    if is_done and grader_score >= 0.8:
        reward += 0.5
        
    if step_number > max_steps:
        reward -= 0.02 * (step_number - max_steps)
        
    return max(0.0, min(1.0, reward))