.NET: Fix OpenAIResponsesAgentClient to include agentName in endpoint path (#5748)

* Fix OpenAIResponsesAgentClient endpoint to include agentName in path (#5324)

The sample OpenAIResponsesAgentClient used '/v1/' as the endpoint, which
routes to the multi-agent endpoint requiring agent.name in the request body.
However, AsIChatClient(agentName) maps agentName to the model field, not
agent.name, causing HTTP 400 errors on OpenAI-compatible endpoints.

Changed the endpoint to '/{agentName}/v1/' to match the pattern used by
OpenAIChatCompletionsAgentClient, routing to the single-agent endpoint
where no agent.name body field is needed.

Added regression test verifying that the model field alone is insufficient
for agent resolution on the multi-agent endpoint.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

* Address review feedback for #5324

- URL-escape agentName in OpenAIResponsesAgentClient endpoint path to
  handle reserved characters safely
- Add per-agent MapOpenAIResponses() calls in AgentHost so the sample
  host serves the /{agentName}/v1/responses routes the client now targets
- Replace brittle Assert.Contains("agent.name") assertions with stable
  machine-readable error code assertion ("missing_required_parameter")

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

* Address additional review feedback for #5324

- Apply Uri.EscapeDataString to OpenAIChatCompletionsAgentClient endpoint
  for consistency with OpenAIResponsesAgentClient
- Map OpenAI Responses and ChatCompletions endpoints for all builder-based
  agents (chemist, mathematician, literator, science workflows) so every
  discoverable agent is reachable via the single-agent endpoint path

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

---------

Co-authored-by: Copilot <copilot@github.com>
Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

dotnet/samples/05-end-to-end/AgentWebChat/AgentWebChat.AgentHost/Program.cs

@@ -163,10 +163,22 @@ app.MapA2AHttpJson(knightsKnavesAgentBuilder, path: "/a2a/knights-and-knaves");
 app.MapDevUI();
 
 app.MapOpenAIResponses();
+app.MapOpenAIResponses(pirateAgentBuilder);
+app.MapOpenAIResponses(knightsKnavesAgentBuilder);
+app.MapOpenAIResponses(chemistryAgent);
+app.MapOpenAIResponses(mathsAgent);
+app.MapOpenAIResponses(literatureAgent);
+app.MapOpenAIResponses(scienceSequentialWorkflow);
+app.MapOpenAIResponses(scienceConcurrentWorkflow);
 app.MapOpenAIConversations();
 
 app.MapOpenAIChatCompletions(pirateAgentBuilder);
 app.MapOpenAIChatCompletions(knightsKnavesAgentBuilder);
+app.MapOpenAIChatCompletions(chemistryAgent);
+app.MapOpenAIChatCompletions(mathsAgent);
+app.MapOpenAIChatCompletions(literatureAgent);
+app.MapOpenAIChatCompletions(scienceSequentialWorkflow);
+app.MapOpenAIChatCompletions(scienceConcurrentWorkflow);
 
 // Map the agents HTTP endpoints
 app.MapAgentDiscovery("/agents");

dotnet/samples/05-end-to-end/AgentWebChat/AgentWebChat.Web/OpenAIChatCompletionsAgentClient.cs

@@ -24,7 +24,7 @@ internal sealed class OpenAIChatCompletionsAgentClient(HttpClient httpClient) :
     {
         OpenAIClientOptions options = new()
         {
-            Endpoint = new Uri(httpClient.BaseAddress!, $"/{agentName}/v1/"),
+            Endpoint = new Uri(httpClient.BaseAddress!, $"/{Uri.EscapeDataString(agentName)}/v1/"),
             Transport = new HttpClientPipelineTransport(httpClient)
         };
 

dotnet/samples/05-end-to-end/AgentWebChat/AgentWebChat.Web/OpenAIResponsesAgentClient.cs

@@ -23,7 +23,7 @@ internal sealed class OpenAIResponsesAgentClient(HttpClient httpClient) : AgentC
     {
         OpenAIClientOptions options = new()
         {
-            Endpoint = new Uri(httpClient.BaseAddress!, "/v1/"),
+            Endpoint = new Uri(httpClient.BaseAddress!, $"/{Uri.EscapeDataString(agentName)}/v1/"),
             Transport = new HttpClientPipelineTransport(httpClient)
         };
 

dotnet/tests/Microsoft.Agents.AI.Hosting.OpenAI.UnitTests/OpenAIResponsesAgentResolutionIntegrationTests.cs

@@ -267,7 +267,43 @@ public sealed class OpenAIResponsesAgentResolutionIntegrationTests : IAsyncDispo
         Assert.Equal(System.Net.HttpStatusCode.BadRequest, httpResponse.StatusCode);
 
         string responseJson = await httpResponse.Content.ReadAsStringAsync();
-        Assert.Contains("agent.name", responseJson, StringComparison.OrdinalIgnoreCase);
+        using JsonDocument errorDoc1 = JsonDocument.Parse(responseJson);
+        string? errorCode = errorDoc1.RootElement.GetProperty("error").GetProperty("code").GetString();
+        Assert.Equal("missing_required_parameter", errorCode);
+    }
+
+    /// <summary>
+    /// Verifies that the model field alone is not used for agent resolution.
+    /// The multi-agent endpoint requires agent.name or metadata.entity_id; setting only model returns 400.
+    /// </summary>
+    [Fact]
+    public async Task CreateResponse_WithModelOnly_ReturnsBadRequestAsync()
+    {
+        // Arrange
+        const string AgentName = "test-agent";
+
+        this._httpClient = await this.CreateTestServerWithAgentResolutionAsync(
+            (AgentName, "Instructions", "Response"));
+
+        // Act - Send request with model=agentName but no agent.name or metadata.entity_id
+        using StringContent requestContent = new(JsonSerializer.Serialize(new
+        {
+            model = AgentName,
+            input = new[]
+            {
+                new { type = "message", role = "user", content = "Test message" }
+            }
+        }), Encoding.UTF8, "application/json");
+
+        using HttpResponseMessage httpResponse = await this._httpClient!.PostAsync(new Uri("/v1/responses", UriKind.Relative), requestContent);
+
+        // Assert - model is not used for agent resolution
+        Assert.Equal(System.Net.HttpStatusCode.BadRequest, httpResponse.StatusCode);
+
+        string responseJson = await httpResponse.Content.ReadAsStringAsync();
+        using JsonDocument errorDoc2 = JsonDocument.Parse(responseJson);
+        string? errorCode = errorDoc2.RootElement.GetProperty("error").GetProperty("code").GetString();
+        Assert.Equal("missing_required_parameter", errorCode);
     }
 
     /// <summary>