Address PR review feedback: fix docs, add examples and E2E tests

- Fix incorrect package name in nodejs/README.md (@anthropic-ai/sdk -> @github/copilot-sdk)
- Add standalone 'System Message Customization' sections with full
  code examples to Python and Go READMEs (matching TypeScript/.NET)
- Add E2E tests for customize mode to Python, Go, and .NET
  (matching existing Node.js E2E test coverage)
- Fix 'end of the prompt' wording in docs to 'additional instructions'

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

Author: MackinnonBuck

dotnet/test/SessionTests.cs

@@ -91,6 +91,37 @@ public async Task Should_Create_A_Session_With_Replaced_SystemMessage_Config()
         Assert.Equal(testSystemMessage, GetSystemMessage(traffic[0]));
     }
 
+    [Fact]
+    public async Task Should_Create_A_Session_With_Customized_SystemMessage_Config()
+    {
+        var customTone = "Respond in a warm, professional tone. Be thorough in explanations.";
+        var appendedContent = "Always mention quarterly earnings.";
+        var session = await CreateSessionAsync(new SessionConfig
+        {
+            SystemMessage = new SystemMessageConfig
+            {
+                Mode = SystemMessageMode.Customize,
+                Sections = new Dictionary<string, SectionOverride>
+                {
+                    [SystemPromptSections.Tone] = new() { Action = SectionOverrideAction.Replace, Content = customTone },
+                    [SystemPromptSections.CodeChangeRules] = new() { Action = SectionOverrideAction.Remove },
+                },
+                Content = appendedContent
+            }
+        });
+
+        await session.SendAsync(new MessageOptions { Prompt = "Who are you?" });
+        var assistantMessage = await TestHelper.GetFinalAssistantMessageAsync(session);
+        Assert.NotNull(assistantMessage);
+
+        var traffic = await Ctx.GetExchangesAsync();
+        Assert.NotEmpty(traffic);
+        var systemMessage = GetSystemMessage(traffic[0]);
+        Assert.Contains(customTone, systemMessage);
+        Assert.Contains(appendedContent, systemMessage);
+        Assert.DoesNotContain("<code_change_instructions>", systemMessage);
+    }
+
     [Fact]
     public async Task Should_Create_A_Session_With_AvailableTools()
     {

go/README.md

@@ -178,6 +178,52 @@ Event types: `SessionLifecycleCreated`, `SessionLifecycleDeleted`, `SessionLifec
 
 - `Bool(v bool) *bool` - Helper to create bool pointers for `AutoStart` option
 
+### System Message Customization
+
+Control the system prompt using `SystemMessage` in session config:
+
+```go
+session, err := client.CreateSession(ctx, &copilot.SessionConfig{
+    SystemMessage: &copilot.SystemMessageConfig{
+        Content: "Always check for security vulnerabilities before suggesting changes.",
+    },
+})
+```
+
+The SDK auto-injects environment context, tool instructions, and security guardrails. The default CLI persona is preserved, and your `Content` is appended after SDK-managed sections. To change the persona or fully redefine the prompt, use `Mode: "replace"` or `Mode: "customize"`.
+
+#### Customize Mode
+
+Use `Mode: "customize"` to selectively override individual sections of the prompt while preserving the rest:
+
+```go
+session, err := client.CreateSession(ctx, &copilot.SessionConfig{
+    SystemMessage: &copilot.SystemMessageConfig{
+        Mode: "customize",
+        Sections: map[string]copilot.SectionOverride{
+            // Replace the tone/style section
+            copilot.SectionTone: {Action: "replace", Content: "Respond in a warm, professional tone. Be thorough in explanations."},
+            // Remove coding-specific rules
+            copilot.SectionCodeChangeRules: {Action: "remove"},
+            // Append to existing guidelines
+            copilot.SectionGuidelines: {Action: "append", Content: "\n* Always cite data sources"},
+        },
+        // Additional instructions appended after all sections
+        Content: "Focus on financial analysis and reporting.",
+    },
+})
+```
+
+Available section constants: `SectionIdentity`, `SectionTone`, `SectionToolEfficiency`, `SectionEnvironmentContext`, `SectionCodeChangeRules`, `SectionGuidelines`, `SectionSafety`, `SectionToolInstructions`, `SectionCustomInstructions`.
+
+Each section override supports four actions:
+- **`replace`** — Replace the section content entirely
+- **`remove`** — Remove the section from the prompt
+- **`append`** — Add content after the existing section
+- **`prepend`** — Add content before the existing section
+
+Unknown section IDs are handled gracefully: content from `replace`/`append`/`prepend` overrides is appended to additional instructions, and `remove` overrides are silently ignored.
+
 ## Image Support
 
 The SDK supports image attachments via the `Attachments` field in `MessageOptions`. You can attach images by providing their file path:

go/internal/e2e/session_test.go

@@ -184,6 +184,51 @@ func TestSession(t *testing.T) {
 		}
 	})
 
+	t.Run("should create a session with customized systemMessage config", func(t *testing.T) {
+		ctx.ConfigureForTest(t)
+
+		customTone := "Respond in a warm, professional tone. Be thorough in explanations."
+		appendedContent := "Always mention quarterly earnings."
+		session, err := client.CreateSession(t.Context(), &copilot.SessionConfig{
+			OnPermissionRequest: copilot.PermissionHandler.ApproveAll,
+			SystemMessage: &copilot.SystemMessageConfig{
+				Mode: "customize",
+				Sections: map[string]copilot.SectionOverride{
+					copilot.SectionTone:            {Action: "replace", Content: customTone},
+					copilot.SectionCodeChangeRules: {Action: "remove"},
+				},
+				Content: appendedContent,
+			},
+		})
+		if err != nil {
+			t.Fatalf("Failed to create session: %v", err)
+		}
+
+		_, err = session.SendAndWait(t.Context(), copilot.MessageOptions{Prompt: "Who are you?"})
+		if err != nil {
+			t.Fatalf("Failed to send message: %v", err)
+		}
+
+		// Validate the system message sent to the model
+		traffic, err := ctx.GetExchanges()
+		if err != nil {
+			t.Fatalf("Failed to get exchanges: %v", err)
+		}
+		if len(traffic) == 0 {
+			t.Fatal("Expected at least one exchange")
+		}
+		systemMessage := getSystemMessage(traffic[0])
+		if !strings.Contains(systemMessage, customTone) {
+			t.Errorf("Expected system message to contain custom tone, got %q", systemMessage)
+		}
+		if !strings.Contains(systemMessage, appendedContent) {
+			t.Errorf("Expected system message to contain appended content, got %q", systemMessage)
+		}
+		if strings.Contains(systemMessage, "<code_change_instructions>") {
+			t.Error("Expected system message to NOT contain code_change_instructions (it was removed)")
+		}
+	})
+
 	t.Run("should create a session with availableTools", func(t *testing.T) {
 		ctx.ConfigureForTest(t)
 

nodejs/README.md

@@ -450,8 +450,8 @@ The SDK auto-injects environment context, tool instructions, and security guardr
 Use `mode: "customize"` to selectively override individual sections of the prompt while preserving the rest:
 
 ```typescript
-import { SYSTEM_PROMPT_SECTIONS } from "@anthropic-ai/sdk";
-import type { SectionOverride, SystemPromptSection } from "@anthropic-ai/sdk";
+import { SYSTEM_PROMPT_SECTIONS } from "@github/copilot-sdk";
+import type { SectionOverride, SystemPromptSection } from "@github/copilot-sdk";
 
 const session = await client.createSession({
     model: "gpt-5",

python/README.md

@@ -158,6 +158,54 @@ unsubscribe()
 - `session.foreground` - A session became the foreground session in TUI
 - `session.background` - A session is no longer the foreground session
 
+### System Message Customization
+
+Control the system prompt using `system_message` in session config:
+
+```python
+session = await client.create_session(
+    system_message={
+        "content": "Always check for security vulnerabilities before suggesting changes."
+    }
+)
+```
+
+The SDK auto-injects environment context, tool instructions, and security guardrails. The default CLI persona is preserved, and your `content` is appended after SDK-managed sections. To change the persona or fully redefine the prompt, use `mode: "replace"` or `mode: "customize"`.
+
+#### Customize Mode
+
+Use `mode: "customize"` to selectively override individual sections of the prompt while preserving the rest:
+
+```python
+from copilot import SYSTEM_PROMPT_SECTIONS
+
+session = await client.create_session(
+    system_message={
+        "mode": "customize",
+        "sections": {
+            # Replace the tone/style section
+            "tone": {"action": "replace", "content": "Respond in a warm, professional tone. Be thorough in explanations."},
+            # Remove coding-specific rules
+            "code_change_rules": {"action": "remove"},
+            # Append to existing guidelines
+            "guidelines": {"action": "append", "content": "\n* Always cite data sources"},
+        },
+        # Additional instructions appended after all sections
+        "content": "Focus on financial analysis and reporting.",
+    }
+)
+```
+
+Available section IDs: `"identity"`, `"tone"`, `"tool_efficiency"`, `"environment_context"`, `"code_change_rules"`, `"guidelines"`, `"safety"`, `"tool_instructions"`, `"custom_instructions"`. Use the `SYSTEM_PROMPT_SECTIONS` dict for descriptions of each section.
+
+Each section override supports four actions:
+- **`replace`** — Replace the section content entirely
+- **`remove`** — Remove the section from the prompt
+- **`append`** — Add content after the existing section
+- **`prepend`** — Add content before the existing section
+
+Unknown section IDs are handled gracefully: content from `replace`/`append`/`prepend` overrides is appended to additional instructions, and `remove` overrides are silently ignored.
+
 ### Tools
 
 Define tools with automatic JSON schema generation using the `@define_tool` decorator and Pydantic models:

python/e2e/test_session.py

@@ -88,6 +88,35 @@ async def test_should_create_a_session_with_replaced_systemMessage_config(
         system_message = _get_system_message(traffic[0])
         assert system_message == test_system_message  # Exact match
 
+    async def test_should_create_a_session_with_customized_systemMessage_config(
+        self, ctx: E2ETestContext
+    ):
+        custom_tone = "Respond in a warm, professional tone. Be thorough in explanations."
+        appended_content = "Always mention quarterly earnings."
+        session = await ctx.client.create_session(
+            {
+                "system_message": {
+                    "mode": "customize",
+                    "sections": {
+                        "tone": {"action": "replace", "content": custom_tone},
+                        "code_change_rules": {"action": "remove"},
+                    },
+                    "content": appended_content,
+                },
+                "on_permission_request": PermissionHandler.approve_all,
+            }
+        )
+
+        assistant_message = await session.send_and_wait({"prompt": "Who are you?"})
+        assert assistant_message is not None
+
+        # Validate the system message sent to the model
+        traffic = await ctx.get_exchanges()
+        system_message = _get_system_message(traffic[0])
+        assert custom_tone in system_message
+        assert appended_content in system_message
+        assert "<code_change_instructions>" not in system_message
+
     async def test_should_create_a_session_with_availableTools(self, ctx: E2ETestContext):
         session = await ctx.client.create_session(
             {