diff --git a/dotnet/src/Microsoft.Agents.AI.Foundry.Hosting/ServiceCollectionExtensions.cs b/dotnet/src/Microsoft.Agents.AI.Foundry.Hosting/ServiceCollectionExtensions.cs
index 2735325864..a0f53b342e 100644
--- a/dotnet/src/Microsoft.Agents.AI.Foundry.Hosting/ServiceCollectionExtensions.cs
+++ b/dotnet/src/Microsoft.Agents.AI.Foundry.Hosting/ServiceCollectionExtensions.cs
@@ -3,6 +3,7 @@
 using System;
 using System.ClientModel.Primitives;
 using System.Diagnostics.CodeAnalysis;
+using System.Runtime.CompilerServices;
 using Azure.AI.AgentServer.Responses;
 using Azure.Core;
 using Azure.Identity;
@@ -230,12 +231,21 @@ public static class FoundryHostingExtensions
         var chatClient = agent.GetService<IChatClient>();
         if (chatClient?.GetService<OpenAIRequestPolicies>() is { } policies)
         {
-            // The HostedAgentUserAgentPolicy is idempotent on the wire (it skips when the
-            // supplement is already present in the User-Agent header), so we can register it
-            // unconditionally without dedup. MEAI's lock-free CAS-append on _entries is safe.
-            policies.AddPolicy(HostedAgentUserAgentPolicy.Instance, PipelinePosition.PerCall);
+            // Hosted agents are typically singletons resolved per request, so AddPolicy must be
+            // called at most once per OpenAIRequestPolicies instance to avoid unbounded growth of
+            // the policy list (each entry adds per-request CPU work even though the User-Agent
+            // value stays stable). Track which instances we have already wired with a
+            // ConditionalWeakTable keyed on the OpenAIRequestPolicies reference; the table holds
+            // weak references so it does not extend the lifetime of the chat client.
+            if (s_userAgentRegistrations.TryAdd(policies, s_boxedTrue))
+            {
+                policies.AddPolicy(HostedAgentUserAgentPolicy.Instance, PipelinePosition.PerCall);
+            }
         }
 
         return agent;
     }
+
+    private static readonly object s_boxedTrue = new();
+    private static readonly ConditionalWeakTable<OpenAIRequestPolicies, object> s_userAgentRegistrations = new();
 }
diff --git a/dotnet/src/Microsoft.Agents.AI.Foundry/FoundryAgent.cs b/dotnet/src/Microsoft.Agents.AI.Foundry/FoundryAgent.cs
index c2052cf75d..db66b30a88 100644
--- a/dotnet/src/Microsoft.Agents.AI.Foundry/FoundryAgent.cs
+++ b/dotnet/src/Microsoft.Agents.AI.Foundry/FoundryAgent.cs
@@ -102,7 +102,7 @@ public sealed class FoundryAgent : DelegatingAIAgent
     /// Internal constructor used by <c>AsAIAgent</c> extension methods that already have an <see cref="AIProjectClient"/> and a configured <see cref="ChatClientAgent"/>.
     /// </summary>
     internal FoundryAgent(AIProjectClient aiProjectClient, ChatClientAgent innerAgent)
-        : base(Throw.IfNull(innerAgent))
+        : base(WireClientHeaders(Throw.IfNull(innerAgent)))
     {
         this._aiProjectClient = Throw.IfNull(aiProjectClient);
     }
@@ -166,7 +166,7 @@ public sealed class FoundryAgent : DelegatingAIAgent
 
     #region Private helpers
 
-    private static ClientHeadersAgent CreateInnerAgent(
+    private static AIAgent CreateInnerAgent(
         AIProjectClient aiProjectClient,
         string model, string instructions,
         string? name, string? description,
@@ -196,7 +196,7 @@ public sealed class FoundryAgent : DelegatingAIAgent
         return CreateResponsesChatClientAgent(aiProjectClient, options, clientFactory, loggerFactory, services);
     }
 
-    private static ClientHeadersAgent CreateResponsesChatClientAgent(
+    private static AIAgent CreateResponsesChatClientAgent(
         AIProjectClient aiProjectClient,
         ChatClientAgentOptions agentOptions,
         Func<IChatClient, IChatClient>? clientFactory,
@@ -215,9 +215,25 @@ public sealed class FoundryAgent : DelegatingAIAgent
             chatClient = clientFactory(chatClient);
         }
 
-        // Register the ClientHeadersPolicy on the chat client's OpenAIRequestPolicies, if available.
-        // Silent no-op when the chat client is not OpenAI-backed.
-        if (chatClient.GetService<OpenAIRequestPolicies>() is { } policies)
+        return WireClientHeaders(new ChatClientAgent(chatClient, agentOptions, loggerFactory, services));
+    }
+
+    /// <summary>
+    /// Registers <see cref="ClientHeadersPolicy"/> on the agent's underlying chat client (if it
+    /// exposes <see cref="OpenAIRequestPolicies"/>) and wraps the agent in a
+    /// <see cref="ClientHeadersAgent"/> so per-call <c>x-client-*</c> headers stamped via
+    /// <see cref="ClientHeadersExtensions.WithClientHeader(ChatOptions, string, string)"/> reach
+    /// the wire. Idempotent: if the chain already contains a <see cref="ClientHeadersAgent"/>,
+    /// the original instance is returned unchanged.
+    /// </summary>
+    private static AIAgent WireClientHeaders(ChatClientAgent innerAgent)
+    {
+        if (innerAgent.GetService<ClientHeadersAgent>() is not null)
+        {
+            return innerAgent;
+        }
+
+        if (innerAgent.ChatClient.GetService<OpenAIRequestPolicies>() is { } policies)
         {
             OpenAIRequestPoliciesReflection.AddPolicyIfMissing(
                 policies,
@@ -225,11 +241,10 @@ public sealed class FoundryAgent : DelegatingAIAgent
                 System.ClientModel.Primitives.PipelinePosition.PerCall);
         }
 
-        var inner = new ChatClientAgent(chatClient, agentOptions, loggerFactory, services);
-        return new ClientHeadersAgent(inner);
+        return new ClientHeadersAgent(innerAgent);
     }
 
-    private static ClientHeadersAgent CreateInnerAgentFromEndpoint(
+    private static AIAgent CreateInnerAgentFromEndpoint(
         AIProjectClient aiProjectClient,
         Uri agentEndpoint,
         IList<AITool>? tools,
@@ -254,16 +269,7 @@ public sealed class FoundryAgent : DelegatingAIAgent
             chatClient = clientFactory(chatClient);
         }
 
-        if (chatClient.GetService<OpenAIRequestPolicies>() is { } policies)
-        {
-            OpenAIRequestPoliciesReflection.AddPolicyIfMissing(
-                policies,
-                ClientHeadersPolicy.Instance,
-                System.ClientModel.Primitives.PipelinePosition.PerCall);
-        }
-
-        var inner = new ChatClientAgent(chatClient, agentOptions, services: services);
-        return new ClientHeadersAgent(inner);
+        return WireClientHeaders(new ChatClientAgent(chatClient, agentOptions, services: services));
     }
 
     private static AIProjectClient CreateProjectClient(Uri endpoint, AuthenticationTokenProvider credential, AIProjectClientOptions? clientOptions = null)