add /system-prompt command to show the exact prompt sent to the model

packages/coding-agent/src/core/agent-session.ts

@@ -733,6 +733,9 @@ export class AgentSession {
 	// Base system prompt (without extension appends) - used to apply fresh appends each turn
 	private _baseSystemPrompt = "";
 	private _baseSystemPromptOptions!: BuildSystemPromptOptions;
+	// Exact prompt last sent to the model. Distinct from state.systemPrompt, which is rebuilt
+	// early by tool/resource/refine changes before any request.
+	private _lastSentSystemPrompt: string | undefined;
 
 	constructor(config: AgentSessionConfig) {
 		this.agent = config.agent;
@@ -1870,6 +1873,17 @@ export class AgentSession {
 		return this.agent.state.systemPrompt;
 	}
 
+	/**
+	 * Returns the exact prompt last sent to the model. Before the first turn, falls back to a
+	 * freshly built base prompt (what would be sent next, before any per-turn extension changes).
+	 */
+	getSystemPromptForDisplay(): { prompt: string; sent: boolean } {
+		if (this._lastSentSystemPrompt !== undefined) {
+			return { prompt: this._lastSentSystemPrompt, sent: true };
+		}
+		return { prompt: this._rebuildSystemPrompt(this.getActiveToolNames()), sent: false };
+	}
+
 	/** Current retry attempt (0 if not retrying) */
 	get retryAttempt(): number {
 		return this._retryAttempt;
@@ -2197,6 +2211,7 @@ export class AgentSession {
 				// Ensure we're using the base prompt (in case previous turn had modifications)
 				this.agent.state.systemPrompt = this._baseSystemPrompt;
 			}
+			this._lastSentSystemPrompt = this.agent.state.systemPrompt;
 		} catch (error) {
 			preflightResult?.(false);
 			throw error;

packages/coding-agent/src/core/slash-commands.ts

@@ -45,6 +45,7 @@ const CANONICAL_BUILTIN_SLASH_COMMANDS: ReadonlyArray<BuiltinSlashCommand> = [
 	{ name: "copy", description: "Copy last agent message to clipboard" },
 	{ name: "name", description: "Set session display name" },
 	{ name: "session", description: "Show session info" },
+	{ name: "system-prompt", description: "Show the exact system prompt sent to the model" },
 	{ name: "logs", description: "Show where daemon and client logs are saved" },
 	{
 		name: "traces",

packages/coding-agent/src/modes/agent-connection/daemon-agent-connection.ts

@@ -366,6 +366,14 @@ export class DaemonAgentConnection implements AgentConnection {
 		return data.text ?? undefined;
 	}
 
+	async getSystemPrompt(): Promise<{ prompt: string; sent: boolean }> {
+		const data = await this.requestData<{ prompt: string; sent: boolean }>({
+			type: "get_system_prompt",
+			activeSessionId: this.activeSessionId,
+		});
+		return { prompt: data.prompt, sent: data.sent };
+	}
+
 	async getToolDefinition(name: string): Promise<AgentConnectionToolDefinition | undefined> {
 		const data = await this.requestData<{ toolDefinition?: AgentConnectionToolDefinition }>({
 			type: "get_tool_definition",
@@ -832,6 +840,7 @@ function invalidatesCachedSnapshot(commandType: DaemonCommandBody["type"]): bool
 		case "get_session_tree":
 		case "get_user_messages_for_forking":
 		case "get_last_assistant_text":
+		case "get_system_prompt":
 		case "get_tool_definition":
 		case "export_html":
 		case "export_jsonl":

packages/coding-agent/src/modes/agent-connection/deferred-agent-connection.ts

@@ -325,6 +325,10 @@ export class DeferredAgentConnection implements AgentConnection {
 		return this.real ? this.real.getLastAssistantText() : undefined;
 	}
 
+	async getSystemPrompt(): Promise<{ prompt: string; sent: boolean }> {
+		return this.real ? this.real.getSystemPrompt() : { prompt: "", sent: false };
+	}
+
 	async getToolDefinition(name: string): Promise<AgentConnectionToolDefinition | undefined> {
 		return this.real ? this.real.getToolDefinition(name) : undefined;
 	}

packages/coding-agent/src/modes/agent-connection/in-process-agent-connection.ts

@@ -184,6 +184,10 @@ export class InProcessAgentConnection implements AgentConnection {
 		return this.session.getLastAssistantText();
 	}
 
+	async getSystemPrompt(): Promise<{ prompt: string; sent: boolean }> {
+		return this.session.getSystemPromptForDisplay();
+	}
+
 	async getToolDefinition(name: string): Promise<AgentConnectionToolDefinition | undefined> {
 		return createAgentConnectionToolDefinition(this.session.getToolDefinition(name));
 	}

packages/coding-agent/src/modes/agent-connection/types.ts

@@ -554,6 +554,11 @@ export interface AgentConnection {
 	updateHeartbeat(action: AgentHeartbeatUpdateAction): Promise<AgentCronJob | undefined>;
 	getUserMessagesForForking(): Promise<AgentConnectionUserMessage[]>;
 	getLastAssistantText(): Promise<string | undefined>;
+	/**
+	 * The system prompt for display. `prompt` is the exact string last sent to the model;
+	 * if no turn has run yet, `sent` is false and `prompt` is the freshly built base prompt.
+	 */
+	getSystemPrompt(): Promise<{ prompt: string; sent: boolean }>;
 	getToolDefinition(name: string): Promise<AgentConnectionToolDefinition | undefined>;
 	setSessionEntryLabel(entryId: string, label: string | undefined): Promise<void>;
 	respondToExtensionUiRequest(requestId: string, response: AgentConnectionExtensionUiResponse): Promise<void>;

packages/coding-agent/src/modes/daemon/daemon-mode.ts

@@ -150,6 +150,7 @@ const DAEMON_COMMAND_TYPES: ReadonlySet<string> = new Set([
 	"get_session_tree",
 	"get_user_messages_for_forking",
 	"get_last_assistant_text",
+	"get_system_prompt",
 	"get_tool_definition",
 	"set_session_entry_label",
 	"extension_ui_response",
@@ -1379,6 +1380,11 @@ export class AgentDaemon {
 				});
 			}
 
+			case "get_system_prompt": {
+				const state = this.getSessionState(command.activeSessionId);
+				return success(command.id, "get_system_prompt", state.runtime.session.getSystemPromptForDisplay());
+			}
+
 			case "get_tool_definition": {
 				const state = this.getSessionState(command.activeSessionId);
 				return success(command.id, "get_tool_definition", {

packages/coding-agent/src/modes/daemon/daemon-protocol.ts

@@ -238,6 +238,7 @@ export type DaemonCommand =
 	| { id?: string; type: "get_session_tree"; activeSessionId: string }
 	| { id?: string; type: "get_user_messages_for_forking"; activeSessionId: string }
 	| { id?: string; type: "get_last_assistant_text"; activeSessionId: string }
+	| { id?: string; type: "get_system_prompt"; activeSessionId: string }
 	| { id?: string; type: "get_tool_definition"; activeSessionId: string; name: string }
 	| { id?: string; type: "set_session_entry_label"; activeSessionId: string; entryId: string; label?: string }
 	| {

packages/coding-agent/src/modes/interactive/interactive-mode.ts

@@ -3334,6 +3334,11 @@ export class InteractiveMode {
 				this.editor.setText("");
 				return;
 			}
+			if (commandName === "system-prompt" && !commandArgs) {
+				await this.handleSystemPromptCommand();
+				this.editor.setText("");
+				return;
+			}
 			if (commandName === "traces") {
 				await this.handleTracesCommand(canonicalCommandText);
 				this.editor.setText("");
@@ -6715,6 +6720,18 @@ export class InteractiveMode {
 		this.ui.requestRender();
 	}
 
+	private async handleSystemPromptCommand(): Promise<void> {
+		const { prompt, sent } = await this.agentConnection.getSystemPrompt();
+		const status = sent ? "exact, last sent" : "next turn, not yet sent";
+		const header = `${theme.bold("System Prompt")} ${theme.fg("dim", `(${prompt.length} chars, ${status})`)}`;
+
+		this.chatContainer.addChild(new Spacer(1));
+		this.chatContainer.addChild(new Text(header, 1, 0));
+		this.chatContainer.addChild(new Spacer(1));
+		this.chatContainer.addChild(new Text(prompt, 1, 0));
+		this.ui.requestRender();
+	}
+
 	private formatTraceUploadResult(result: AgentTraceUploadResult): string {
 		switch (result.status) {
 			case "uploaded":