feat(config): auto-tune pool size for async workers

lib/pgbus/configuration.rb

@@ -485,6 +485,11 @@ def recurring_execution_retention=(value)
     # default formula provides.
     POOL_SIZE_WARN_THRESHOLD = 50
 
+    # Connections needed per async worker: one for the reactor's serial
+    # execution, one for polling, one for headroom. Fibers share connections
+    # because only one runs at a time per reactor thread.
+    ASYNC_POOL_CONNECTIONS = 3
+
     def resolved_pool_size
       return pool_size if pool_size
 
@@ -624,10 +629,19 @@ def sum_thread_counts(entries, default_threads:, group:)
           raise ArgumentError,
                 "#{group} threads must be a positive integer, got #{threads.inspect}"
         end
-        threads
+
+        if async_execution_mode?(entry)
+          ASYNC_POOL_CONNECTIONS
+        else
+          threads
+        end
       end
     end
 
+    def async_execution_mode?(entry)
+      execution_mode_for(entry) == :async
+    end
+
     def warn_if_oversized(size)
       return unless size > POOL_SIZE_WARN_THRESHOLD
 

spec/pgbus/configuration_spec.rb

@@ -259,6 +259,39 @@
         expect(warned_message).to match(/pool_size .* 62/)
       end
 
+      it "uses fewer connections for async workers (fibers share connections)" do
+        config.pool_size = nil
+        config.workers = [{ queues: %w[webhooks], threads: 100, execution_mode: :async }]
+        # Async workers need ~3 connections (reactor + polling + headroom),
+        # not 100 (one per fiber). Total: 3 + 1 dispatcher + 1 scheduler = 5
+        expect(config.resolved_pool_size).to eq(5)
+      end
+
+      it "mixes async and thread workers correctly" do
+        config.pool_size = nil
+        config.workers = [
+          { queues: %w[webhooks], threads: 50, execution_mode: :async },
+          { queues: %w[default], threads: 5 }
+        ]
+        # 3 (async) + 5 (threads) + 1 dispatcher + 1 scheduler = 10
+        expect(config.resolved_pool_size).to eq(10)
+      end
+
+      it "uses fewer connections for fiber mode (alias for async)" do
+        config.pool_size = nil
+        config.workers = [{ queues: %w[llm], threads: 200, execution_mode: :fiber }]
+        # 3 + 1 + 1 = 5
+        expect(config.resolved_pool_size).to eq(5)
+      end
+
+      it "honors global execution_mode when workers have no per-entry override" do
+        config.pool_size = nil
+        config.execution_mode = :async
+        config.workers = [{ queues: %w[default], threads: 50 }]
+        # Global async: 3 + 1 dispatcher + 1 scheduler = 5
+        expect(config.resolved_pool_size).to eq(5)
+      end
+
       it "does not warn for normal sizes" do
         config.pool_size = nil
         config.workers = [{ queues: %w[default], threads: 5 }]