TypeScript
```typescript
import { CopilotClient } from "@github/copilot-sdk";
const client = new CopilotClient();
await client.start();
const session = await client.createSession({
model: "gpt-5.4",
customAgents: [
{
name: "researcher",
displayName: "Research Agent",
description: "Explores codebases and answers questions using read-only tools",
tools: ["grep", "glob", "view"],
prompt: "You are a research assistant. Analyze code and answer questions. Do not modify any files.",
},
{
name: "editor",
displayName: "Editor Agent",
description: "Makes targeted code changes",
tools: ["view", "edit", "bash"],
prompt: "You are a code editor. Make minimal, surgical changes to files as requested.",
},
],
onPermissionRequest: async () => ({ kind: "approve-once" }),
});
```
Python
```python
from copilot import CopilotClient, PermissionDecisionApproveOnce
client = CopilotClient()
await client.start()
session = await client.create_session(
on_permission_request=lambda req, inv: PermissionDecisionApproveOnce(),
model="gpt-5.4",
custom_agents=[
{
"name": "researcher",
"display_name": "Research Agent",
"description": "Explores codebases and answers questions using read-only tools",
"tools": ["grep", "glob", "view"],
"prompt": "You are a research assistant. Analyze code and answer questions. Do not modify any files.",
},
{
"name": "editor",
"display_name": "Editor Agent",
"description": "Makes targeted code changes",
"tools": ["view", "edit", "bash"],
"prompt": "You are a code editor. Make minimal, surgical changes to files as requested.",
},
],
)
```
Go
```golang
ctx := context.Background()
client := copilot.NewClient(nil)
client.Start(ctx)
session, _ := client.CreateSession(ctx, &copilot.SessionConfig{
Model: "gpt-5.4",
CustomAgents: []copilot.CustomAgentConfig{
{
Name: "researcher",
DisplayName: "Research Agent",
Description: "Explores codebases and answers questions using read-only tools",
Tools: []string{"grep", "glob", "view"},
Prompt: "You are a research assistant. Analyze code and answer questions. Do not modify any files.",
},
{
Name: "editor",
DisplayName: "Editor Agent",
Description: "Makes targeted code changes",
Tools: []string{"view", "edit", "bash"},
Prompt: "You are a code editor. Make minimal, surgical changes to files as requested.",
},
},
OnPermissionRequest: func(req copilot.PermissionRequest, inv copilot.PermissionInvocation) (rpc.PermissionDecision, error) {
return &rpc.PermissionDecisionApproveOnce{}, nil
},
})
```
.NET
```csharp
using GitHub.Copilot;
using GitHub.Copilot.Rpc;
await using var client = new CopilotClient();
await using var session = await client.CreateSessionAsync(new SessionConfig
{
Model = "gpt-5.4",
CustomAgents = new List
{
new()
{
Name = "researcher",
DisplayName = "Research Agent",
Description = "Explores codebases and answers questions using read-only tools",
Tools = new List { "grep", "glob", "view" },
Prompt = "You are a research assistant. Analyze code and answer questions. Do not modify any files.",
},
new()
{
Name = "editor",
DisplayName = "Editor Agent",
Description = "Makes targeted code changes",
Tools = new List { "view", "edit", "bash" },
Prompt = "You are a code editor. Make minimal, surgical changes to files as requested.",
},
},
OnPermissionRequest = (req, inv) =>
Task.FromResult(PermissionDecision.ApproveOnce()),
});
```
Java
```java
import com.github.copilot.CopilotClient;
import com.github.copilot.rpc.*;
import java.util.List;
try (var client = new CopilotClient()) {
client.start().get();
var session = client.createSession(
new SessionConfig()
.setModel("gpt-5.4")
.setCustomAgents(List.of(
new CustomAgentConfig()
.setName("researcher")
.setDisplayName("Research Agent")
.setDescription("Explores codebases and answers questions using read-only tools")
.setTools(List.of("grep", "glob", "view"))
.setPrompt("You are a research assistant. Analyze code and answer questions. Do not modify any files."),
new CustomAgentConfig()
.setName("editor")
.setDisplayName("Editor Agent")
.setDescription("Makes targeted code changes")
.setTools(List.of("view", "edit", "bash"))
.setPrompt("You are a code editor. Make minimal, surgical changes to files as requested.")
))
.setOnPermissionRequest(PermissionHandler.APPROVE_ALL)
).get();
}
```
## Configuration reference
| Property | Type | Required | Description |
| ----------------- | -------------------- | -------- | ----------------------------------------------------------------------------------------------------------------------------------------------------- |
| `name` | `string` | ✅ | Unique identifier for the agent |
| `displayName` | `string` | | Human-readable name shown in events |
| `description` | `string` | | What the agent does—helps the runtime select it |
| `tools` | `string[]` or `null` | | Tool names the agent can use. `null` or omitted = all tools |
| `prompt` | `string` | ✅ | System prompt for the agent |
| `mcpServers` | `object` | | MCP server configurations specific to this agent |
| `infer` | `boolean` | | Whether the runtime can auto-select this agent (default: `true`) |
| `skills` | `string[]` | | Skill names to preload into the agent's context at startup |
| `model` | `string` | | Model identifier to use while this agent runs |
| `reasoningEffort` | `string` | | Reasoning effort to use while this agent runs. When omitted, the SDK sends no per-agent override and the runtime resolves the effort (see note below) |
> \[!TIP]
> A good `description` helps the runtime match user intent to the right agent. Be specific about the agent's expertise and capabilities.
Set `model` and `reasoningEffort` to override the parent session's model settings while a custom agent runs. When `reasoningEffort` is omitted, the SDK sends no per-agent override and the runtime resolves the effort from its own precedence: a per-call client option, the resolved model's default, or the agent definition all take priority; otherwise the runtime inherits the parent session's effort only when the subagent runs the same model as the parent. When the subagent resolves to a different model, it falls back to that model's default instead of inheriting the parent's effort. Python uses `reasoning_effort`, .NET uses `ReasoningEffort`, Go uses `ReasoningEffort`, Java uses `setReasoningEffort`, and Rust uses `with_reasoning_effort`.
In addition to per-agent configuration above, you can set `agent` on the **session config** itself to pre-select which custom agent is active when the session starts. See [Selecting an Agent at Session Creation](#selecting-an-agent-at-session-creation) below.
| Session Config Property | Type | Description |
| ----------------------- | -------- | -------------------------------------------------------------------------------------------------- |
| `agent` | `string` | Name of the custom agent to pre-select at session creation. Must match a `name` in `customAgents`. |
## Per-agent skills
You can preload skills into an agent's context using the `skills` property. When specified, the **full content** of each listed skill is eagerly injected into the agent's context at startup—the agent doesn't need to invoke a skill tool; the instructions are already present. Skills are **opt-in**: agents receive no skills by default, and sub-agents do not inherit skills from the parent. Skill names are resolved from the session-level `skillDirectories`.
```typescript
const session = await client.createSession({
skillDirectories: ["./skills"],
customAgents: [
{
name: "security-auditor",
description: "Security-focused code reviewer",
prompt: "Focus on OWASP Top 10 vulnerabilities",
skills: ["security-scan", "dependency-check"],
},
{
name: "docs-writer",
description: "Technical documentation writer",
prompt: "Write clear, concise documentation",
skills: ["markdown-lint"],
},
],
onPermissionRequest: async () => ({ kind: "approve-once" }),
});
```
In this example, `security-auditor` starts with `security-scan` and `dependency-check` already injected into its context, while `docs-writer` starts with `markdown-lint`. An agent without a `skills` field receives no skill content.
## Selecting an agent at session creation
You can pass `agent` in the session config to pre-select which custom agent should be active when the session starts. The value must match the `name` of one of the agents defined in `customAgents`.
This is equivalent to calling `session.rpc.agent.select()` after creation, but avoids the extra API call and ensures the agent is active from the very first prompt.
TypeScript
```typescript
const session = await client.createSession({
customAgents: [
{
name: "researcher",
prompt: "You are a research assistant. Analyze code and answer questions.",
},
{
name: "editor",
prompt: "You are a code editor. Make minimal, surgical changes.",
},
],
agent: "researcher", // Pre-select the researcher agent
});
```
Python
```python
session = await client.create_session(
on_permission_request=PermissionHandler.approve_all,
custom_agents=[
{
"name": "researcher",
"prompt": "You are a research assistant. Analyze code and answer questions.",
},
{
"name": "editor",
"prompt": "You are a code editor. Make minimal, surgical changes.",
},
],
agent="researcher", # Pre-select the researcher agent
)
```
Go
```golang
session, _ := client.CreateSession(ctx, &copilot.SessionConfig{
CustomAgents: []copilot.CustomAgentConfig{
{
Name: "researcher",
Prompt: "You are a research assistant. Analyze code and answer questions.",
},
{
Name: "editor",
Prompt: "You are a code editor. Make minimal, surgical changes.",
},
},
Agent: "researcher", // Pre-select the researcher agent
})
```
.NET
```csharp
var session = await client.CreateSessionAsync(new SessionConfig
{
CustomAgents = new List
{
new() { Name = "researcher", Prompt = "You are a research assistant. Analyze code and answer questions." },
new() { Name = "editor", Prompt = "You are a code editor. Make minimal, surgical changes." },
},
Agent = "researcher", // Pre-select the researcher agent
});
```
Java
```java
import com.github.copilot.rpc.*;
import java.util.List;
var session = client.createSession(
new SessionConfig()
.setCustomAgents(List.of(
new CustomAgentConfig()
.setName("researcher")
.setPrompt("You are a research assistant. Analyze code and answer questions."),
new CustomAgentConfig()
.setName("editor")
.setPrompt("You are a code editor. Make minimal, surgical changes.")
))
.setAgent("researcher") // Pre-select the researcher agent
.setOnPermissionRequest(PermissionHandler.APPROVE_ALL)
).get();
```
## How sub-agent delegation works
When you send a prompt to a session with custom agents, the runtime evaluates whether to delegate to a sub-agent:
1. **Intent matching**—The runtime analyzes the user's prompt against each agent's `name` and `description`
2. **Agent selection**—If a match is found and `infer` is not `false`, the runtime selects the agent
3. **Isolated execution**—The sub-agent runs with its own prompt and restricted tool set
4. **Event streaming**—Lifecycle events (`subagent.started`, `subagent.completed`, etc.) stream back to the parent session
5. **Result integration**—The sub-agent's output is incorporated into the parent agent's response
### Controlling inference
By default, all custom agents are available for automatic selection (`infer: true`). Set `infer: false` to prevent the runtime from auto-selecting an agent—useful for agents you only want invoked through explicit user requests:
```typescript
{
name: "dangerous-cleanup",
description: "Deletes unused files and dead code",
tools: ["bash", "edit", "view"],
prompt: "You clean up codebases by removing dead code and unused files.",
infer: false, // Only invoked when user explicitly asks for this agent
}
```
## Listening to sub-agent events
When a sub-agent runs, the parent session emits lifecycle events. Subscribe to these events to build UIs that visualize agent activity.
Sub-agent-originated session events share the parent session stream and include envelope-level `agentId`. Root/main agent events and session-level events omit `agentId`, so renderers can keep the parent response separate from sub-agent traces by checking the event envelope.
### Event types
| Event | Emitted when | Data |
| --------------------- | ---------------------------------------- | ------------------------------------------------------------------------------------------------------------------ |
| `subagent.selected` | Runtime selects an agent for the task | `agentName`, `agentDisplayName`, `tools` |
| `subagent.started` | Sub-agent begins execution | `toolCallId`, `agentName`, `agentDisplayName`, `agentDescription`, `model?` |
| `subagent.completed` | Sub-agent finishes successfully | `toolCallId`, `agentName`, `agentDisplayName`, `model?`, `durationMs?`, `totalTokens?`, `totalToolCalls?` |
| `subagent.failed` | Sub-agent encounters an error | `toolCallId`, `agentName`, `agentDisplayName`, `error`, `model?`, `durationMs?`, `totalTokens?`, `totalToolCalls?` |
| `subagent.deselected` | Runtime switches away from the sub-agent | — |
### Subscribing to events
TypeScript
```typescript
session.on((event) => {
switch (event.type) {
case "subagent.started":
console.log(`▶ Sub-agent started: ${event.data.agentDisplayName}`);
console.log(` Description: ${event.data.agentDescription}`);
console.log(` Tool call ID: ${event.data.toolCallId}`);
break;
case "subagent.completed":
console.log(`✅ Sub-agent completed: ${event.data.agentDisplayName}`);
if (event.data.durationMs !== undefined) console.log(` Duration: ${event.data.durationMs}ms`);
if (event.data.totalTokens !== undefined) console.log(` Tokens: ${event.data.totalTokens}`);
if (event.data.totalToolCalls !== undefined) console.log(` Tool calls: ${event.data.totalToolCalls}`);
break;
case "subagent.failed":
console.log(`❌ Sub-agent failed: ${event.data.agentDisplayName}`);
console.log(` Error: ${event.data.error}`);
if (event.data.durationMs !== undefined) console.log(` Duration: ${event.data.durationMs}ms`);
break;
case "subagent.selected":
console.log(`🎯 Agent selected: ${event.data.agentDisplayName}`);
console.log(` Tools: ${event.data.tools?.join(", ") ?? "all"}`);
break;
case "subagent.deselected":
console.log("↩ Agent deselected, returning to parent");
break;
}
});
const response = await session.sendAndWait({
prompt: "Research how authentication works in this codebase",
});
```
Python
```python
def handle_event(event):
if event.type == "subagent.started":
print(f"▶ Sub-agent started: {event.data.agent_display_name}")
print(f" Description: {event.data.agent_description}")
elif event.type == "subagent.completed":
print(f"✅ Sub-agent completed: {event.data.agent_display_name}")
elif event.type == "subagent.failed":
print(f"❌ Sub-agent failed: {event.data.agent_display_name}")
print(f" Error: {event.data.error}")
elif event.type == "subagent.selected":
tools = event.data.tools or "all"
print(f"🎯 Agent selected: {event.data.agent_display_name} (tools: {tools})")
unsubscribe = session.on(handle_event)
response = await session.send_and_wait("Research how authentication works in this codebase")
```
Go
```golang
session.On(func(event copilot.SessionEvent) {
switch d := event.Data.(type) {
case *copilot.SubagentStartedData:
fmt.Printf("▶ Sub-agent started: %s\n", d.AgentDisplayName)
fmt.Printf(" Description: %s\n", d.AgentDescription)
fmt.Printf(" Tool call ID: %s\n", d.ToolCallID)
case *copilot.SubagentCompletedData:
fmt.Printf("✅ Sub-agent completed: %s\n", d.AgentDisplayName)
case *copilot.SubagentFailedData:
fmt.Printf("❌ Sub-agent failed: %s — %v\n", d.AgentDisplayName, d.Error)
case *copilot.SubagentSelectedData:
fmt.Printf("🎯 Agent selected: %s\n", d.AgentDisplayName)
}
})
_, err := session.SendAndWait(ctx, copilot.MessageOptions{
Prompt: "Research how authentication works in this codebase",
})
```
.NET
```csharp
using var subscription = session.On
(evt =>
{
switch (evt)
{
case SubagentStartedEvent started:
Console.WriteLine($"▶ Sub-agent started: {started.Data.AgentDisplayName}");
Console.WriteLine($" Description: {started.Data.AgentDescription}");
Console.WriteLine($" Tool call ID: {started.Data.ToolCallId}");
break;
case SubagentCompletedEvent completed:
Console.WriteLine($"✅ Sub-agent completed: {completed.Data.AgentDisplayName}");
break;
case SubagentFailedEvent failed:
Console.WriteLine($"❌ Sub-agent failed: {failed.Data.AgentDisplayName} — {failed.Data.Error}");
break;
case SubagentSelectedEvent selected:
Console.WriteLine($"🎯 Agent selected: {selected.Data.AgentDisplayName}");
break;
}
});
await session.SendAndWaitAsync(new MessageOptions
{
Prompt = "Research how authentication works in this codebase"
});
```
Java
```java
session.on(event -> {
if (event instanceof SubagentStartedEvent e) {
System.out.println("▶ Sub-agent started: " + e.getData().agentDisplayName());
System.out.println(" Description: " + e.getData().agentDescription());
System.out.println(" Tool call ID: " + e.getData().toolCallId());
} else if (event instanceof SubagentCompletedEvent e) {
System.out.println("✅ Sub-agent completed: " + e.getData().agentName());
} else if (event instanceof SubagentFailedEvent e) {
System.out.println("❌ Sub-agent failed: " + e.getData().agentName());
System.out.println(" Error: " + e.getData().error());
} else if (event instanceof SubagentSelectedEvent e) {
System.out.println("🎯 Agent selected: " + e.getData().agentDisplayName());
} else if (event instanceof SubagentDeselectedEvent e) {
System.out.println("↩ Agent deselected, returning to parent");
}
});
var response = session.sendAndWait(
new MessageOptions().setPrompt("Research how authentication works in this codebase")
).get();
```
## Building an agent tree UI
Sub-agent events include `toolCallId` fields that let you reconstruct the execution tree. Here's a pattern for tracking agent activity:
```typescript
interface AgentNode {
toolCallId: string;
name: string;
displayName: string;
status: "running" | "completed" | "failed";
error?: string;
startedAt: Date;
completedAt?: Date;
}
const agentTree = new Map