Add Sonnet 5 prompt setting

extensions/copilot/package.json

@@ -4170,6 +4170,15 @@
               "onExp"
             ]
           },
+          "github.copilot.chat.claudeSonnet5Prompt.enabled": {
+            "type": "boolean",
+            "default": false,
+            "markdownDescription": "%github.copilot.config.claudeSonnet5Prompt.enabled%",
+            "tags": [
+              "experimental",
+              "onExp"
+            ]
+          },
           "github.copilot.chat.gpt55GetChangedFilesTool.enabled": {
             "type": "boolean",
             "default": true,

extensions/copilot/package.nls.json

@@ -352,6 +352,7 @@
 	"github.copilot.config.responsesApi.promptCacheBreakpoint.enabled": "Enables explicit prompt cache breakpoint markers for the Responses API.",
 	"github.copilot.config.updated53CodexPrompt.enabled": "Enables the updated prompt for gpt-5.3-codex model.",
 	"github.copilot.config.claudeOpus5Prompt.enabled": "Enables the updated system prompt tuned for the Claude Opus 5 model.",
+	"github.copilot.config.claudeSonnet5Prompt.enabled": "Enables the updated system prompt tuned for the Claude Sonnet 5 model.",
 	"github.copilot.config.gpt55GetChangedFilesTool.enabled": "Enables the Get Changed Files tool for gpt-5.5 models.",
 	"github.copilot.config.gpt56Verbosity.enabled": "Sets the response verbosity to low for gpt-5.6 models.",
 	"github.copilot.config.gemini3GetChangedFilesTool.enabled": "Enables the Get Changed Files tool for gemini-3 models.",

extensions/copilot/src/extension/prompts/node/agent/anthropicPrompts.tsx

@@ -422,6 +422,25 @@ class Claude46SonnetPrompt extends Claude46OptimizedBasePrompt {
 	}
 }
 
+/**
+ * Prompt for Claude Sonnet 5, extending the optimized Sonnet 4.6 prompt with a scope-discipline
+ * paragraph. Sonnet 5 explores and explains more than Sonnet 4.6 on the same task, which raises
+ * both its score and its token cost. Constraining scope alone regresses cases where a backwards
+ * compatibility shim looks like adjacent work, so the interface-preservation clause is required
+ * alongside it.
+ */
+class ClaudeSonnet5Prompt extends Claude46SonnetPrompt {
+	protected override renderAppendedInstructions() {
+		return <Tag name='scopeDiscipline'>
+			Do exactly what was asked - don't extend scope to new adjacent code, tests, examples, or unrelated files unless the task explicitly includes them.<br />
+			But always preserve existing public interfaces: never remove or change the signature of module-level functions, exported names, or class methods that other code might depend on, unless explicitly told to.<br />
+			If you think adjacent work is needed, mention it rather than doing it.<br />
+			Be direct and minimal on file reads and editor-tool calls; spend your reasoning on design decisions, debugging, and writing code.<br />
+			Keep your responses to the user concise - a few sentences explaining what you did and why is enough; the work happens in tool calls, not in prose.<br />
+		</Tag>;
+	}
+}
+
 /** Opus-specific optimized prompt for Claude 4.6. */
 class Claude46OpusPrompt extends Claude46OptimizedBasePrompt {
 	protected override renderExplorationGuidance(_tools: ReturnType<typeof detectToolCapabilities>) {
@@ -541,6 +560,10 @@ class AnthropicPromptResolver implements IAgentPrompt {
 		return endpoint.model.startsWith('claude-sonnet') || endpoint.family.startsWith('claude-sonnet');
 	}
 
+	private isSonnet5(endpoint: IChatEndpoint): boolean {
+		return endpoint.model.startsWith('claude-sonnet-5') || endpoint.family.startsWith('claude-sonnet-5');
+	}
+
 	private isHaiku(endpoint: IChatEndpoint): boolean {
 		return endpoint.model.startsWith('claude-haiku') || endpoint.family.startsWith('claude-haiku');
 	}
@@ -556,6 +579,9 @@ class AnthropicPromptResolver implements IAgentPrompt {
 		if (this.isClaude45(endpoint)) {
 			return Claude45DefaultPrompt;
 		}
+		if (this.isSonnet5(endpoint) && this.configurationService.getExperimentBasedConfig(ConfigKey.ClaudeSonnet5PromptEnabled, this.experimentationService)) {
+			return ClaudeSonnet5Prompt;
+		}
 		if (this.isSonnet(endpoint)) {
 			return Claude46SonnetPrompt;
 		}

extensions/copilot/src/platform/configuration/common/configurationService.ts

@@ -982,6 +982,8 @@ export namespace ConfigKey {
 	export const Updated53CodexPromptEnabled = defineSetting<boolean>('chat.updated53CodexPrompt.enabled', ConfigType.ExperimentBased, true);
 	/** Enable updated prompt for Claude Opus 5 model */
 	export const ClaudeOpus5PromptEnabled = defineSetting<boolean>('chat.claudeOpus5Prompt.enabled', ConfigType.ExperimentBased, false);
+	/** Enable updated prompt for Claude Sonnet 5 model */
+	export const ClaudeSonnet5PromptEnabled = defineSetting<boolean>('chat.claudeSonnet5Prompt.enabled', ConfigType.ExperimentBased, false);
 	/** Enable get_changed_files tool for GPT-5.5 models */
 	export const EnableGpt55GetChangedFilesTool = defineSetting<boolean>('chat.gpt55GetChangedFilesTool.enabled', ConfigType.ExperimentBased, true);
 	/** Enable low verbosity for GPT-5.6 models. */