Fix JSON error: use 1 worker, add error handlers, validate fetch responses

- Dockerfile: reduce gunicorn to --workers 1 (stateful in-memory app cannot
share state across multiple processes), increase --threads 4 for concurrency
- genetic_backend.py: add @app .errorhandler(404/500) returning JSON so API
routes always return JSON instead of HTML error pages; wrap all route
handlers in try/except for graceful error reporting
- templates/genetic_evolver.html: validate Content-Type before .json() to
give clear error messages instead of cryptic JSON parse errors; skip
backend call during slider init to avoid race condition on startup

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Dockerfile

@@ -18,4 +18,4 @@ COPY --chown=user . /app
 EXPOSE 7860
 
 # Run Flask app using gunicorn
-CMD ["gunicorn", "--bind", "0.0.0.0:7860", "--workers", "2", "--threads", "2", "--timeout", "120", "genetic_backend:app"]
+CMD ["gunicorn", "--bind", "0.0.0.0:7860", "--workers", "1", "--threads", "4", "--timeout", "120", "genetic_backend:app"]

genetic_backend.py

@@ -790,86 +790,115 @@ class GeneticEquationEvolver:
 # Global evolver instance
 evolver = GeneticEquationEvolver()
 
+@app.errorhandler(404)
+def not_found(e):
+    return jsonify({'error': 'Not found', 'message': str(e)}), 404
+
+@app.errorhandler(500)
+def server_error(e):
+    return jsonify({'error': 'Internal server error', 'message': str(e)}), 500
+
 @app.route('/')
 def index():
     return render_template('genetic_evolver.html')
 
 @app.route('/api/population')
 def get_population():
-    lite = request.args.get('lite', '0') in ('1', 'true', 'True')
-    return jsonify(evolver.get_population_data(lite=lite))
+    try:
+        lite = request.args.get('lite', '0') in ('1', 'true', 'True')
+        return jsonify(evolver.get_population_data(lite=lite))
+    except Exception as e:
+        return jsonify({'error': str(e)}), 500
 
 @app.route('/api/evolve', methods=['POST'])
 def evolve():
-    data = request.get_json()
-    feedback_ids = data.get('selected_ids', [])
-    # Always evolve: empty selection means "Select None" feedback
-    evolver.evolve_generation(feedback_ids)
-    return jsonify({'success': True, 'generation': evolver.generation})
+    try:
+        data = request.get_json()
+        feedback_ids = data.get('selected_ids', [])
+        # Always evolve: empty selection means "Select None" feedback
+        evolver.evolve_generation(feedback_ids)
+        return jsonify({'success': True, 'generation': evolver.generation})
+    except Exception as e:
+        return jsonify({'error': str(e)}), 500
 
 @app.route('/api/set_base_equation', methods=['POST'])
 def set_base_equation():
-    data = request.get_json()
-    set_name = data.get('set_name')
-    if evolver.set_base_equation_set(set_name):
-        return jsonify({'success': True, 'base_set': evolver.current_base_set})
-    return jsonify({'success': False, 'error': 'Invalid equation set name'}), 400
+    try:
+        data = request.get_json()
+        set_name = data.get('set_name')
+        if evolver.set_base_equation_set(set_name):
+            return jsonify({'success': True, 'base_set': evolver.current_base_set})
+        return jsonify({'success': False, 'error': 'Invalid equation set name'}), 400
+    except Exception as e:
+        return jsonify({'error': str(e)}), 500
 
 @app.route('/api/set_jump_size', methods=['POST'])
 def set_jump_size():
-    data = request.get_json()
-    jump_size = data.get('jump_size')
-    if jump_size is not None and isinstance(jump_size, (int, float)) and 0.01 <= jump_size <= 10:
-        evolver.jump_size_multiplier = float(jump_size)
-        return jsonify({'success': True, 'jump_size': evolver.jump_size_multiplier})
-    return jsonify({'success': False, 'error': 'Invalid jump size'}), 400
+    try:
+        data = request.get_json()
+        jump_size = data.get('jump_size')
+        if jump_size is not None and isinstance(jump_size, (int, float)) and 0.01 <= jump_size <= 10:
+            evolver.jump_size_multiplier = float(jump_size)
+            return jsonify({'success': True, 'jump_size': evolver.jump_size_multiplier})
+        return jsonify({'success': False, 'error': 'Invalid jump size'}), 400
+    except Exception as e:
+        return jsonify({'error': str(e)}), 500
 
 @app.route('/api/reset_preferences', methods=['POST'])
 def reset_preferences():
     """Reset all parameter preferences (color history)"""
-    evolver.parameter_preferences = {}
-    return jsonify({'success': True})
+    try:
+        evolver.parameter_preferences = {}
+        return jsonify({'success': True})
+    except Exception as e:
+        return jsonify({'error': str(e)}), 500
 
 @app.route('/api/reset_all', methods=['POST'])
 def reset_all():
     """Reset the entire evolution process from the start"""
-    # Reset all evolution state
-    evolver.generation = 0
-    evolver.parameter_preferences = {}
-    evolver.approved_trig_sites = set()
-    evolver.liked_variants = []
-    evolver.select_none_history = []
-    evolver.mutation_penalties = {}
-    evolver.feedback_history = []
-    evolver.elites = []
-    evolver.next_node_id = 1
-    evolver.graph = {'nodes': [], 'edges': [], 'feedback': []}
-    evolver.trig_mutation_prob_base = 1.0  # Reset base to 1.0
-    evolver.trig_mutation_prob_base_generation = 0  # Reset generation offset
-    evolver.jump_size_multiplier = 1.0  # Reset jump size to default
-    
-    # Reinitialize population from scratch
-    evolver.initialize_population()
-    
-    return jsonify({'success': True, 'generation': evolver.generation})
+    try:
+        # Reset all evolution state
+        evolver.generation = 0
+        evolver.parameter_preferences = {}
+        evolver.approved_trig_sites = set()
+        evolver.liked_variants = []
+        evolver.select_none_history = []
+        evolver.mutation_penalties = {}
+        evolver.feedback_history = []
+        evolver.elites = []
+        evolver.next_node_id = 1
+        evolver.graph = {'nodes': [], 'edges': [], 'feedback': []}
+        evolver.trig_mutation_prob_base = 1.0  # Reset base to 1.0
+        evolver.trig_mutation_prob_base_generation = 0  # Reset generation offset
+        evolver.jump_size_multiplier = 1.0  # Reset jump size to default
+
+        # Reinitialize population from scratch
+        evolver.initialize_population()
+
+        return jsonify({'success': True, 'generation': evolver.generation})
+    except Exception as e:
+        return jsonify({'error': str(e)}), 500
 
 @app.route('/api/set_trig_mutation_prob', methods=['POST'])
 def set_trig_mutation_prob():
     """Set trigonometric mutation probability. Sets base value and generation for auto-decay."""
-    data = request.get_json()
-    prob = data.get('trig_mutation_prob')
-    if prob is None:
-        # User wants to use current value as new base (resume auto-decay from current)
-        current_prob = evolver.get_trig_mutation_probability()
-        evolver.trig_mutation_prob_base = current_prob
-        evolver.trig_mutation_prob_base_generation = evolver.generation
-        return jsonify({'success': True, 'trig_mutation_prob': current_prob, 'auto': True})
-    elif isinstance(prob, (int, float)) and 0.0 <= prob <= 1.0:
-        # User sets a value - this becomes the new base for auto-decay starting from current generation
-        evolver.trig_mutation_prob_base = float(prob)
-        evolver.trig_mutation_prob_base_generation = evolver.generation
-        return jsonify({'success': True, 'trig_mutation_prob': float(prob), 'auto': False})
-    return jsonify({'success': False, 'error': 'Invalid probability (must be 0.0-1.0 or null for auto)'}), 400
+    try:
+        data = request.get_json()
+        prob = data.get('trig_mutation_prob')
+        if prob is None:
+            # User wants to use current value as new base (resume auto-decay from current)
+            current_prob = evolver.get_trig_mutation_probability()
+            evolver.trig_mutation_prob_base = current_prob
+            evolver.trig_mutation_prob_base_generation = evolver.generation
+            return jsonify({'success': True, 'trig_mutation_prob': current_prob, 'auto': True})
+        elif isinstance(prob, (int, float)) and 0.0 <= prob <= 1.0:
+            # User sets a value - this becomes the new base for auto-decay starting from current generation
+            evolver.trig_mutation_prob_base = float(prob)
+            evolver.trig_mutation_prob_base_generation = evolver.generation
+            return jsonify({'success': True, 'trig_mutation_prob': float(prob), 'auto': False})
+        return jsonify({'success': False, 'error': 'Invalid probability (must be 0.0-1.0 or null for auto)'}), 400
+    except Exception as e:
+        return jsonify({'error': str(e)}), 500
 
 if __name__ == '__main__':
     app.run(debug=True, host='0.0.0.0', port=5000)

templates/genetic_evolver.html

@@ -233,8 +233,15 @@
 
     function loadPopulation(){
       fetch('/api/population')
-        .then(r=>r.json())
+        .then(r=>{
+          const ct = r.headers.get('content-type')||'';
+          if(!ct.includes('application/json')){
+            return r.text().then(t=>{throw new Error('Server returned non-JSON (status '+r.status+'). Possible startup issue.');});
+          }
+          return r.json();
+        })
         .then(data=>{
+          if(data.error){alert('API error: '+data.error);return;}
           variants=data.variants;
           currentParams = data.base_params || currentParams;  // Update base parameters
           parameterPreferences = data.parameter_preferences || {};  // Update preferences
@@ -1450,7 +1457,10 @@
     const jumpSlider = document.getElementById('jump-size-slider');
     if(jumpSlider) {
       jumpSlider.value = 3; // Index 3 = 1.0
-      updateJumpSize(3);
+      // Set local multiplier without sending to backend during init (loadPopulation syncs state)
+      jumpSizeMultiplier = 1.0;
+      const valueDisplay = document.getElementById('jump-size-value');
+      if(valueDisplay) valueDisplay.textContent = '1.00';
     }
     loadPopulation();
   </script>