feat: add model field to CustomAgentConfig across all SDKs (#1309)

* feat(nodejs): add model field to CustomAgentConfig

Add optional `model` property to the Node/TypeScript CustomAgentConfig
interface. When set, the runtime will attempt to use the specified model
for the agent, falling back to the parent session model if unavailable.

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

* feat(python): add model field to CustomAgentConfig

Add optional `model` key to the Python CustomAgentConfig TypedDict and
wire it through `_convert_custom_agent_to_wire_format` so the runtime
receives it in the session.create / session.resume payloads.

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

* feat(go): add Model field to CustomAgentConfig

Add optional `Model` field to the Go CustomAgentConfig struct. The field
serializes as `"model"` and is omitted when empty. When set, the runtime
will attempt to use the specified model for the agent.

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

* feat(dotnet): add Model property to CustomAgentConfig

Add optional `Model` property to the .NET CustomAgentConfig class. The
property serializes as `"model"` and is omitted when null. When set,
the runtime will attempt to use the specified model for the agent.

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

* feat(rust): add model field to CustomAgentConfig

Add optional `model` field to the Rust CustomAgentConfig struct with a
`with_model` builder method. Serializes as `"model"` (camelCase
rename is a no-op for single-word fields) and is skipped when None.

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

* test(nodejs): verify model field is forwarded in session.create payload

Add a test case that creates a custom agent with a model property and
asserts it appears in the session.create RPC payload.

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

* test(python): verify model field in CustomAgentConfig wire conversion

Add unit tests asserting that the model key is correctly forwarded
to the camelCase wire payload, and omitted when not set.

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

* test(go): verify CustomAgentConfig model JSON serialization

Add tests asserting that the model field round-trips through JSON when
set and is omitted from the payload when empty.

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

* test(dotnet): verify CustomAgentConfig.Model is preserved through Clone

Update the SessionConfig clone test to set Model on a CustomAgentConfig
and assert it survives the clone operation.

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

* test(rust): verify CustomAgentConfig model builder and serialization

Add unit tests for the with_model() builder, JSON serialization with
model set, and confirming model is omitted from wire when None.

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

---------

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

Author: patniko

dotnet/src/Types.cs

@@ -1933,6 +1933,14 @@ public class CustomAgentConfig
     /// </summary>
     [JsonPropertyName("skills")]
     public IList<string>? Skills { get; set; }
+
+    /// <summary>
+    /// Model identifier for this agent (e.g. "claude-haiku-4.5").
+    /// When set, the runtime will attempt to use this model for the agent,
+    /// falling back to the parent session model if unavailable.
+    /// </summary>
+    [JsonPropertyName("model")]
+    public string? Model { get; set; }
 }
 
 /// <summary>

dotnet/test/Unit/CloneTests.cs

@@ -95,7 +95,7 @@ public void SessionConfig_Clone_CopiesAllProperties()
             EnableSessionTelemetry = false,
             IncludeSubAgentStreamingEvents = false,
             McpServers = new Dictionary<string, McpServerConfig> { ["server1"] = new McpStdioServerConfig { Command = "echo" } },
-            CustomAgents = [new CustomAgentConfig { Name = "agent1" }],
+            CustomAgents = [new CustomAgentConfig { Name = "agent1", Model = "claude-haiku-4.5" }],
             Agent = "agent1",
             DefaultAgent = new DefaultAgentConfig { ExcludedTools = ["hidden-tool"] },
             SkillDirectories = ["/skills"],
@@ -120,6 +120,7 @@ public void SessionConfig_Clone_CopiesAllProperties()
         Assert.Equal(original.IncludeSubAgentStreamingEvents, clone.IncludeSubAgentStreamingEvents);
         Assert.Equal(original.McpServers.Count, clone.McpServers!.Count);
         Assert.Equal(original.CustomAgents.Count, clone.CustomAgents!.Count);
+        Assert.Equal(original.CustomAgents[0].Model, clone.CustomAgents[0].Model);
         Assert.Equal(original.Agent, clone.Agent);
         Assert.Equal(original.DefaultAgent!.ExcludedTools, clone.DefaultAgent!.ExcludedTools);
         Assert.Equal(original.SkillDirectories, clone.SkillDirectories);

go/types.go

@@ -532,6 +532,10 @@ type CustomAgentConfig struct {
 	Infer *bool `json:"infer,omitempty"`
 	// Skills is the list of skill names to preload into this agent's context at startup (opt-in; omit for none)
 	Skills []string `json:"skills,omitempty"`
+	// Model is the model identifier for this agent (e.g. "claude-haiku-4.5").
+	// When set, the runtime will attempt to use this model for the agent,
+	// falling back to the parent session model if unavailable.
+	Model string `json:"model,omitempty"`
 }
 
 // DefaultAgentConfig configures the default agent (the built-in agent that handles turns when no custom agent is selected).

go/types_test.go

@@ -216,3 +216,49 @@ func TestProviderConfig_JSONOmitsUnsetTokenFields(t *testing.T) {
 		}
 	}
 }
+
+func TestCustomAgentConfig_JSONIncludesModel(t *testing.T) {
+	cfg := CustomAgentConfig{
+		Name:   "model-agent",
+		Prompt: "You are a model agent.",
+		Model:  "claude-haiku-4.5",
+	}
+
+	data, err := json.Marshal(cfg)
+	if err != nil {
+		t.Fatalf("failed to marshal CustomAgentConfig: %v", err)
+	}
+
+	var decoded map[string]any
+	if err := json.Unmarshal(data, &decoded); err != nil {
+		t.Fatalf("failed to unmarshal CustomAgentConfig: %v", err)
+	}
+
+	if decoded["model"] != "claude-haiku-4.5" {
+		t.Errorf("expected model 'claude-haiku-4.5', got %v", decoded["model"])
+	}
+	if decoded["name"] != "model-agent" {
+		t.Errorf("expected name 'model-agent', got %v", decoded["name"])
+	}
+}
+
+func TestCustomAgentConfig_JSONOmitsModelWhenEmpty(t *testing.T) {
+	cfg := CustomAgentConfig{
+		Name:   "no-model-agent",
+		Prompt: "You are an agent without a model.",
+	}
+
+	data, err := json.Marshal(cfg)
+	if err != nil {
+		t.Fatalf("failed to marshal CustomAgentConfig: %v", err)
+	}
+
+	var decoded map[string]any
+	if err := json.Unmarshal(data, &decoded); err != nil {
+		t.Fatalf("failed to unmarshal CustomAgentConfig: %v", err)
+	}
+
+	if _, present := decoded["model"]; present {
+		t.Errorf("expected model to be omitted when empty, got %v", decoded["model"])
+	}
+}

nodejs/src/types.ts

@@ -1213,6 +1213,12 @@ export interface CustomAgentConfig {
      * When omitted, no skills are injected (opt-in model).
      */
     skills?: string[];
+    /**
+     * Model identifier for this agent (e.g. "claude-haiku-4.5").
+     * When set, the runtime will attempt to use this model for the agent,
+     * falling back to the parent session model if unavailable.
+     */
+    model?: string;
 }
 
 /**

nodejs/test/client.test.ts

@@ -868,6 +868,29 @@ describe("CopilotClient", () => {
             expect(payload.customAgents).toEqual([expect.objectContaining({ name: "test-agent" })]);
         });
 
+        it("forwards custom agent model in session.create request", async () => {
+            const client = new CopilotClient();
+            await client.start();
+            onTestFinished(() => client.forceStop());
+
+            const spy = vi.spyOn((client as any).connection!, "sendRequest");
+            await client.createSession({
+                onPermissionRequest: approveAll,
+                customAgents: [
+                    {
+                        name: "model-agent",
+                        prompt: "You are a model agent.",
+                        model: "claude-haiku-4.5",
+                    },
+                ],
+            });
+
+            const payload = spy.mock.calls.find((c) => c[0] === "session.create")![1] as any;
+            expect(payload.customAgents).toEqual([
+                expect.objectContaining({ name: "model-agent", model: "claude-haiku-4.5" }),
+            ]);
+        });
+
         it("forwards agent in session.resume request", async () => {
             const client = new CopilotClient();
             await client.start();

python/copilot/client.py

@@ -2512,6 +2512,8 @@ def _convert_custom_agent_to_wire_format(
             wire_agent["infer"] = agent["infer"]
         if "skills" in agent:
             wire_agent["skills"] = agent["skills"]
+        if "model" in agent:
+            wire_agent["model"] = agent["model"]
         return wire_agent
 
     def _convert_default_agent_to_wire_format(

python/copilot/session.py

@@ -818,6 +818,8 @@ class CustomAgentConfig(TypedDict, total=False):
     infer: NotRequired[bool]  # Whether agent is available for model inference
     # Skill names to preload into this agent's context at startup (opt-in; omit for none)
     skills: NotRequired[list[str]]
+    # Model identifier (e.g. "claude-haiku-4.5"); runtime falls back to parent model if unavailable
+    model: NotRequired[str]
 
 
 class DefaultAgentConfig(TypedDict, total=False):

python/test_client.py

@@ -982,3 +982,34 @@ async def test_aexit_calls_disconnect(self):
         with patch.object(session, "disconnect", new_callable=AsyncMock) as mock_disconnect:
             await session.__aexit__(None, None, None)
             mock_disconnect.assert_awaited_once()
+
+
+class TestCustomAgentWireFormat:
+    def test_model_field_is_forwarded_in_wire_format(self):
+        """The model key in CustomAgentConfig should appear as 'model' in the wire payload."""
+        from copilot.client import CopilotClient
+        from copilot.session import CustomAgentConfig
+
+        client = CopilotClient.__new__(CopilotClient)
+        agent: CustomAgentConfig = {
+            "name": "model-agent",
+            "prompt": "You are a model agent.",
+            "model": "claude-haiku-4.5",
+        }
+        wire = client._convert_custom_agent_to_wire_format(agent)
+        assert wire["model"] == "claude-haiku-4.5"
+        assert wire["name"] == "model-agent"
+        assert wire["prompt"] == "You are a model agent."
+
+    def test_model_field_is_omitted_when_absent(self):
+        """When model is not set, it should not appear in the wire payload."""
+        from copilot.client import CopilotClient
+        from copilot.session import CustomAgentConfig
+
+        client = CopilotClient.__new__(CopilotClient)
+        agent: CustomAgentConfig = {
+            "name": "no-model-agent",
+            "prompt": "You are an agent without a model.",
+        }
+        wire = client._convert_custom_agent_to_wire_format(agent)
+        assert "model" not in wire

rust/src/types.rs

@@ -526,6 +526,12 @@ pub struct CustomAgentConfig {
     /// Skill names to preload into this agent's context at startup.
     #[serde(default, skip_serializing_if = "Option::is_none")]
     pub skills: Option<Vec<String>>,
+    /// Model identifier for this agent (e.g. `"claude-haiku-4.5"`).
+    ///
+    /// When set, the runtime will attempt to use this model for the agent,
+    /// falling back to the parent session model if unavailable.
+    #[serde(default, skip_serializing_if = "Option::is_none")]
+    pub model: Option<String>,
 }
 
 impl CustomAgentConfig {
@@ -587,6 +593,12 @@ impl CustomAgentConfig {
         self.skills = Some(skills.into_iter().map(Into::into).collect());
         self
     }
+
+    /// Set the model identifier for this agent.
+    pub fn with_model(mut self, model: impl Into<String>) -> Self {
+        self.model = Some(model.into());
+        self
+    }
 }
 
 /// Configures the default (built-in) agent that handles turns when no
@@ -3196,6 +3208,31 @@ mod tests {
         assert!(tool.skip_permission);
     }
 
+    #[test]
+    fn custom_agent_config_builder_with_model() {
+        let agent = CustomAgentConfig::new("my-agent", "You are helpful.")
+            .with_model("claude-haiku-4.5")
+            .with_display_name("My Agent");
+        assert_eq!(agent.name, "my-agent");
+        assert_eq!(agent.model.as_deref(), Some("claude-haiku-4.5"));
+        assert_eq!(agent.display_name.as_deref(), Some("My Agent"));
+    }
+
+    #[test]
+    fn custom_agent_config_serializes_model() {
+        let agent = CustomAgentConfig::new("model-agent", "prompt").with_model("claude-haiku-4.5");
+        let wire = serde_json::to_value(&agent).unwrap();
+        assert_eq!(wire["model"], "claude-haiku-4.5");
+        assert_eq!(wire["name"], "model-agent");
+    }
+
+    #[test]
+    fn custom_agent_config_omits_model_when_none() {
+        let agent = CustomAgentConfig::new("no-model-agent", "prompt");
+        let wire = serde_json::to_value(&agent).unwrap();
+        assert!(wire.get("model").is_none());
+    }
+
     #[test]
     fn tool_with_parameters_handles_non_object_value() {
         let tool = Tool::new("noop").with_parameters(json!(null));