Prevent repeated migration of programs between islands

Adds a check to avoid re-migrating already migrated programs in the ProgramDatabase migration logic. This prevents exponential duplication of identical programs, conserves computational resources, and maintains diversity in the MAP-Elites + Island hybrid architecture. Also updates config.yaml to adjust LLM temperature and selection ratios for improved optimization.

Author: codelion

examples/llm_prompt_optimization/config.yaml

@@ -16,7 +16,7 @@ llm:
     - name: "gemini-2.5-flash-lite"  # Using Gemini 2.5 Flash Lite
       weight: 1.0
 
-  temperature: 0.8  # Optimal from experiments
+  temperature: 0.4  # Optimal from experiments
   max_tokens: 16000  # Optimal context
   timeout: 150
   retries: 3
@@ -53,8 +53,8 @@ database:
 
   # Selection parameters - Optimal ratios from testing
   elite_selection_ratio: 0.1   # 10% elite selection
-  exploration_ratio: 0.5       # 30% exploration
-  exploitation_ratio: 0.4      # 60% exploitation
+  exploration_ratio: 0.3       # 30% exploration
+  exploitation_ratio: 0.6      # 60% exploitation
 
   # Migration parameters - Optimal settings
   migration_interval: 10

openevolve/database.py

@@ -1347,6 +1347,26 @@ def migrate_programs(self) -> None:
             target_islands = [(i + 1) % len(self.islands), (i - 1) % len(self.islands)]
 
             for migrant in migrants:
+                # Prevent re-migration of already migrated programs to avoid exponential duplication.
+                # Analysis of actual evolution runs shows this causes severe issues:
+                # - Program cb5d07f2 had 183 descendant copies by iteration 850
+                # - Program 5645fbd2 had 31 descendant copies
+                # - IDs grow exponentially: program_migrant_2_migrant_3_migrant_4_migrant_0...
+                #
+                # This is particularly problematic for OpenEvolve's MAP-Elites + Island hybrid architecture:
+                # 1. All copies have identical code → same complexity/diversity/performance scores
+                # 2. They all map to the SAME MAP-Elites cell → only 1 survives, rest discarded
+                # 3. Wastes computation evaluating hundreds of identical programs
+                # 4. Reduces actual diversity as islands fill with duplicates
+                #
+                # By preventing already-migrated programs from migrating again, we ensure:
+                # - Each program migrates at most once per lineage
+                # - True diversity is maintained between islands
+                # - Computational resources aren't wasted on duplicates
+                # - Aligns with MAP-Elites' one-program-per-cell principle
+                if migrant.metadata.get("migrant", False):
+                    continue
+
                 for target_island in target_islands:
                     # Create a copy for migration (to avoid removing from source)
                     migrant_copy = Program(