feat(rpc): expose slash command metadata

packages/coding-agent/src/modes/acp/acp-agent.ts

@@ -56,7 +56,7 @@ import {
 } from "../../extensibility/extensions";
 import { runExtensionCompact } from "../../extensibility/extensions/compact-handler";
 import { getSessionSlashCommands } from "../../extensibility/extensions/get-commands-handler";
-import { buildSkillPromptMessage, getSkillSlashCommandName } from "../../extensibility/skills";
+import { buildSkillPromptMessage } from "../../extensibility/skills";
 import { loadSlashCommands } from "../../extensibility/slash-commands";
 import { resolveLocalUrlToPath } from "../../internal-urls";
 import { MCPManager } from "../../mcp/manager";
@@ -71,12 +71,8 @@ import {
 	type SessionInfo as StoredSessionInfo,
 	type UsageStatistics,
 } from "../../session/session-manager";
-import {
-	ACP_BUILTIN_RESERVED_NAMES,
-	ACP_BUILTIN_SLASH_COMMANDS,
-	executeAcpBuiltinSlashCommand,
-	isAcpBuiltinShadowedName,
-} from "../../slash-commands/acp-builtins";
+import { executeAcpBuiltinSlashCommand } from "../../slash-commands/acp-builtins";
+import { buildAvailableSlashCommands, toAcpAvailableCommands } from "../../slash-commands/available-commands";
 import { AUTO_THINKING, parseConfiguredThinkingLevel } from "../../thinking";
 import { normalizeLocalScheme } from "../../tools/path-utils";
 import { runResolveInvocation } from "../../tools/resolve";
@@ -1662,66 +1658,7 @@ export class AcpAgent implements Agent {
 	}
 
 	async #buildAvailableCommands(session: AgentSession): Promise<AvailableCommand[]> {
-		const commands: AvailableCommand[] = [];
-		const seenNames = new Set<string>();
-		const appendCommand = (command: AvailableCommand): void => {
-			if (seenNames.has(command.name)) {
-				return;
-			}
-			seenNames.add(command.name);
-			commands.push(command);
-		};
-
-		// Advertise in the order dispatch resolves them (mirrors AgentSession
-		// dispatch: builtins → skills → extensions → custom TS → file-based).
-		// `appendCommand` dedupes by name so earlier entries win; extension
-		// commands therefore correctly shadow custom TS commands of the same
-		// name, matching the runtime behaviour of #tryExecuteExtensionCommand
-		// running before #tryExecuteCustomCommand.
-		for (const command of ACP_BUILTIN_SLASH_COMMANDS) {
-			appendCommand(command);
-		}
-
-		if (session.skillsSettings?.enableSkillCommands) {
-			for (const skill of session.skills) {
-				appendCommand({
-					name: getSkillSlashCommandName(skill),
-					description: skill.description || `Run ${skill.name} skill`,
-					input: { hint: "arguments" },
-				});
-			}
-		}
-
-		for (const command of session.extensionRunner?.getRegisteredCommands(ACP_BUILTIN_RESERVED_NAMES) ?? []) {
-			// Reserved-set filtering in getRegisteredCommands only covers exact
-			// names; colon-namespaced names whose prefix is a builtin (e.g.
-			// `model:foo`) would still dispatch to the builtin in ACP.
-			if (isAcpBuiltinShadowedName(command.name)) {
-				continue;
-			}
-			appendCommand({
-				name: command.name,
-				description: command.description ?? "(extension command)",
-				input: { hint: "arguments" },
-			});
-		}
-
-		for (const command of session.customCommands) {
-			appendCommand({
-				name: command.command.name,
-				description: command.command.description,
-				input: { hint: "arguments" },
-			});
-		}
-
-		for (const command of await loadSlashCommands({ cwd: session.sessionManager.getCwd() })) {
-			appendCommand({
-				name: command.name,
-				description: command.description,
-			});
-		}
-
-		return commands;
+		return toAcpAvailableCommands(await buildAvailableSlashCommands(session));
 	}
 
 	#toSessionInfo(session: StoredSessionInfo): SessionInfo {

packages/coding-agent/src/modes/rpc/rpc-client.ts

@@ -13,6 +13,7 @@ import type { FileSink } from "bun";
 import type { BashResult } from "../../exec/bash-executor";
 import type { AgentSessionEvent, SessionStats } from "../../session/agent-session";
 import type {
+	RpcAvailableSlashCommand,
 	RpcCommand,
 	RpcExtensionUIRequest,
 	RpcHandoffResult,
@@ -511,6 +512,14 @@ export class RpcClient {
 		return this.#getData<{ models: ModelInfo[] }>(response).models;
 	}
 
+	/**
+	 * Get list of available slash commands.
+	 */
+	async getAvailableCommands(): Promise<RpcAvailableSlashCommand[]> {
+		const response = await this.#send({ type: "get_available_commands" });
+		return this.#getData<{ commands: RpcAvailableSlashCommand[] }>(response).commands;
+	}
+
 	/**
 	 * Set thinking level.
 	 */

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

@@ -21,6 +21,8 @@ import {
 } from "../../extensibility/extensions";
 import { type Theme, theme } from "../../modes/theme/theme";
 import type { AgentSession } from "../../session/agent-session";
+import { executeAcpBuiltinSlashCommand } from "../../slash-commands/acp-builtins";
+import { buildAvailableSlashCommands } from "../../slash-commands/available-commands";
 import type { EventBus } from "../../utils/event-bus";
 import { initializeExtensions } from "../runtime-init";
 import { isRpcHostToolResult, isRpcHostToolUpdate, RpcHostToolBridge } from "./host-tools";
@@ -511,6 +513,12 @@ export async function runRpcMode(
 		output(event);
 	});
 
+	const getAvailableCommands = async () => buildAvailableSlashCommands(session);
+	const emitAvailableCommandsUpdate = async () => {
+		output({ type: "available_commands_update", commands: await getAvailableCommands() });
+	};
+	await emitAvailableCommandsUpdate();
+
 	// Handle a single command
 	const handleCommand = async (command: RpcCommand): Promise<RpcResponse> => {
 		const id = command.id;
@@ -521,6 +529,26 @@ export async function runRpcMode(
 			// =================================================================
 
 			case "prompt": {
+				const builtinResult = await executeAcpBuiltinSlashCommand(command.message, {
+					session,
+					sessionManager: session.sessionManager,
+					settings: session.settings,
+					cwd: session.sessionManager.getCwd(),
+					output: text => output({ type: "command_output", text }),
+					refreshCommands: emitAvailableCommandsUpdate,
+					reloadPlugins: async () => {},
+					notifyTitleChanged: async () => {},
+					notifyConfigChanged: async () => {},
+				});
+				if (builtinResult !== false) {
+					if ("prompt" in builtinResult) {
+						session
+							.prompt(builtinResult.prompt, { images: command.images })
+							.catch(e => output(error(id, "prompt", e.message)));
+					}
+					return success(id, "prompt");
+				}
+
 				// Don't await - events will stream
 				// Extension commands are executed immediately, file prompt templates are expanded
 				// If streaming and streamingBehavior specified, queues via steer/followUp
@@ -592,6 +620,10 @@ export async function runRpcMode(
 				return success(id, "get_state", state);
 			}
 
+			case "get_available_commands": {
+				return success(id, "get_available_commands", { commands: await getAvailableCommands() });
+			}
+
 			case "set_todos": {
 				session.setTodoPhases(command.phases);
 				return success(id, "set_todos", { todoPhases: session.getTodoPhases() });

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

@@ -11,6 +11,7 @@ import type { BashResult } from "../../exec/bash-executor";
 import type { ContextUsage } from "../../extensibility/extensions/types";
 import type { AgentSessionEvent, SessionStats } from "../../session/agent-session";
 import type { FileEntry } from "../../session/session-manager";
+import type { AvailableSlashCommandSource } from "../../slash-commands/available-commands";
 import type {
 	AgentProgress,
 	SubagentEventPayload,
@@ -34,6 +35,7 @@ export type RpcCommand =
 
 	// State
 	| { id?: string; type: "get_state" }
+	| { id?: string; type: "get_available_commands" }
 	| { id?: string; type: "set_todos"; phases: TodoPhase[] }
 	| { id?: string; type: "set_host_tools"; tools: RpcHostToolDefinition[] }
 	| { id?: string; type: "set_host_uri_schemes"; schemes: RpcHostUriSchemeDefinition[] }
@@ -110,6 +112,15 @@ export interface RpcSessionState {
 	contextUsage?: ContextUsage;
 }
 
+export interface RpcAvailableSlashCommand {
+	name: string;
+	aliases?: string[];
+	description?: string;
+	input?: { hint?: string };
+	subcommands?: Array<{ name: string; description?: string; usage?: string }>;
+	source: AvailableSlashCommandSource;
+}
+
 export interface RpcHandoffResult {
 	savedPath?: string;
 }
@@ -156,6 +167,13 @@ export type RpcResponse =
 
 	// State
 	| { id?: string; type: "response"; command: "get_state"; success: true; data: RpcSessionState }
+	| {
+			id?: string;
+			type: "response";
+			command: "get_available_commands";
+			success: true;
+			data: { commands: RpcAvailableSlashCommand[] };
+	  }
 	| { id?: string; type: "response"; command: "set_todos"; success: true; data: { todoPhases: TodoPhase[] } }
 	| { id?: string; type: "response"; command: "set_host_tools"; success: true; data: { toolNames: string[] } }
 	| { id?: string; type: "response"; command: "set_host_uri_schemes"; success: true; data: { schemes: string[] } }