fix(extensibility): added centralized session slash command discovery for extensions

- Added a shared session command helper that aggregates extension, prompt, and skill slash commands.
- Updated ACP, extension UI, runtime-init, and task executor extension contexts to return session command data instead of empty arrays.

packages/coding-agent/src/extensibility/extensions/get-commands-handler.ts

@@ -0,0 +1,77 @@
+/**
+ * Helper for wiring the `getCommands` action of {@link ExtensionAPI}.
+ *
+ * Centralizes the union over the three slash-command sources the runtime
+ * exposes so the five wiring sites (interactive UI, ACP, RPC, print, child
+ * task executor) cannot drift:
+ *   - extension-registered hook commands (`source: "extension"`)
+ *   - prompt commands loaded as `LoadedCustomCommand` — user/project/bundled
+ *     custom commands and MCP prompts (`source: "prompt"`)
+ *   - skill commands derived from `session.skills`, gated on
+ *     `skillsSettings.enableSkillCommands` (`source: "skill"`)
+ *
+ * Built-in slash commands are intentionally excluded; `getCommands()` is the
+ * surface extensions use to discover dynamic commands they did not register
+ * themselves. Each frontend (interactive-mode, ACP) prepends its own builtins.
+ */
+import type { SkillsSettings } from "../../config/settings";
+import type { CustomCommandSource, LoadedCustomCommand } from "../custom-commands";
+import { getSkillSlashCommandName, type Skill } from "../skills";
+import type { SlashCommandInfo, SlashCommandLocation } from "../slash-commands";
+import type { ExtensionRunner } from "./runner";
+
+interface CommandsCapableSession {
+	readonly extensionRunner?: ExtensionRunner;
+	readonly customCommands: ReadonlyArray<LoadedCustomCommand>;
+	readonly skills: ReadonlyArray<Skill>;
+	readonly skillsSettings?: SkillsSettings;
+}
+
+export function getSessionSlashCommands(session: CommandsCapableSession): SlashCommandInfo[] {
+	const out: SlashCommandInfo[] = [];
+
+	const runner = session.extensionRunner;
+	if (runner) {
+		for (const cmd of runner.getRegisteredCommands()) {
+			out.push({
+				name: cmd.name,
+				description: cmd.description,
+				source: "extension",
+			});
+		}
+	}
+
+	for (const cmd of session.customCommands) {
+		out.push({
+			name: cmd.command.name,
+			description: cmd.command.description,
+			source: "prompt",
+			location: customCommandLocation(cmd.source),
+			path: cmd.resolvedPath,
+		});
+	}
+
+	if (session.skillsSettings?.enableSkillCommands) {
+		for (const skill of session.skills) {
+			out.push({
+				name: getSkillSlashCommandName(skill),
+				description: skill.description || undefined,
+				source: "skill",
+				path: skill.filePath,
+			});
+		}
+	}
+
+	return out;
+}
+
+function customCommandLocation(source: CustomCommandSource): SlashCommandLocation | undefined {
+	switch (source) {
+		case "user":
+			return "user";
+		case "project":
+			return "project";
+		case "bundled":
+			return undefined;
+	}
+}

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

@@ -49,6 +49,7 @@ import { Settings } from "../../config/settings";
 import { clearPluginRootsAndCaches, resolveActiveProjectRegistryPath } from "../../discovery/helpers";
 import type { ExtensionUIContext, ExtensionUIDialogOptions } 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 { loadSlashCommands } from "../../extensibility/slash-commands";
 import { MCPManager } from "../../mcp/manager";
@@ -1741,7 +1742,7 @@ export class AcpAgent implements Agent {
 				getActiveTools: () => record.session.getActiveToolNames(),
 				getAllTools: () => record.session.getAllToolNames(),
 				setActiveTools: toolNames => record.session.setActiveToolsByName(toolNames),
-				getCommands: () => [],
+				getCommands: () => getSessionSlashCommands(record.session),
 				setModel: async model => {
 					const apiKey = await record.session.modelRegistry.getApiKey(model);
 					if (!apiKey) {

packages/coding-agent/src/modes/controllers/extension-ui-controller.ts

@@ -16,6 +16,7 @@ import type {
 	SendUserMessageHandler,
 	TerminalInputHandler,
 } from "../../extensibility/extensions";
+import { getSessionSlashCommands } from "../../extensibility/extensions/get-commands-handler";
 import { HookEditorComponent } from "../../modes/components/hook-editor";
 import { HookInputComponent } from "../../modes/components/hook-input";
 import { HookSelectorComponent } from "../../modes/components/hook-selector";
@@ -109,7 +110,7 @@ export class ExtensionUiController {
 			},
 			getThinkingLevel: () => this.ctx.session.thinkingLevel,
 			setThinkingLevel: level => this.ctx.session.setThinkingLevel(level),
-			getCommands: () => [],
+			getCommands: () => getSessionSlashCommands(this.ctx.session),
 			getSessionName: () => this.ctx.sessionManager.getSessionName(),
 			setSessionName: name => this.#updateSessionName(name),
 		};
@@ -349,7 +350,7 @@ export class ExtensionUiController {
 			},
 			getThinkingLevel: () => this.ctx.session.thinkingLevel,
 			setThinkingLevel: (level, persist) => this.ctx.session.setThinkingLevel(level, persist),
-			getCommands: () => [],
+			getCommands: () => getSessionSlashCommands(this.ctx.session),
 			getSessionName: () => this.ctx.sessionManager.getSessionName(),
 			setSessionName: name => this.#updateSessionName(name),
 		};

packages/coding-agent/src/modes/runtime-init.ts

@@ -7,6 +7,7 @@
  * caller-supplied hooks.
  */
 import { runExtensionCompact, runExtensionSetModel } from "../extensibility/extensions/compact-handler";
+import { getSessionSlashCommands } from "../extensibility/extensions/get-commands-handler";
 import type { ExtensionError, ExtensionUIContext } from "../extensibility/extensions/types";
 import type { AgentSession } from "../session/agent-session";
 
@@ -59,7 +60,7 @@ export async function initializeExtensions(session: AgentSession, options: Initi
 			getActiveTools: () => session.getActiveToolNames(),
 			getAllTools: () => session.getAllToolNames(),
 			setActiveTools: (toolNames: string[]) => session.setActiveToolsByName(toolNames),
-			getCommands: () => [],
+			getCommands: () => getSessionSlashCommands(session),
 			setModel: model => runExtensionSetModel(session, model),
 			getThinkingLevel: () => session.thinkingLevel,
 			setThinkingLevel: level => session.setThinkingLevel(level),

packages/coding-agent/src/task/executor.ts

@@ -16,6 +16,7 @@ import { Settings } from "../config/settings";
 import { SETTINGS_SCHEMA, type SettingPath } from "../config/settings-schema";
 import type { CustomTool } from "../extensibility/custom-tools/types";
 import { runExtensionCompact, runExtensionSetModel } from "../extensibility/extensions/compact-handler";
+import { getSessionSlashCommands } from "../extensibility/extensions/get-commands-handler";
 import type { Skill } from "../extensibility/skills";
 import type { HindsightSessionState } from "../hindsight/state";
 import type { LocalProtocolOptions } from "../internal-urls";
@@ -1119,7 +1120,7 @@ export async function runSubprocess(options: ExecutorOptions): Promise<SingleRes
 						getAllTools: () => session.getAllToolNames(),
 						setActiveTools: (toolNames: string[]) =>
 							session.setActiveToolsByName(toolNames.filter(name => !parentOwnedToolNames.has(name))),
-						getCommands: () => [],
+						getCommands: () => getSessionSlashCommands(session),
 						setModel: model => runExtensionSetModel(session, model),
 						getThinkingLevel: () => session.thinkingLevel,
 						setThinkingLevel: level => session.setThinkingLevel(level),