fix(coding-agent): support dynamic tool registration and tool prompt snippets closes #1720

packages/coding-agent/examples/extensions/README.md

@@ -33,6 +33,7 @@ cp permission-gate.ts ~/.pi/agent/extensions/
 | `question.ts` | Demonstrates `ctx.ui.select()` for asking the user questions with custom UI |
 | `questionnaire.ts` | Multi-question input with tab bar navigation between questions |
 | `tool-override.ts` | Override built-in tools (e.g., add logging/access control to `read`) |
+| `dynamic-tools.ts` | Register tools after startup (`session_start`) and at runtime via command |
 | `built-in-tool-renderer.ts` | Custom compact rendering for built-in tools (read, bash, edit, write) while keeping original behavior |
 | `minimal-mode.ts` | Override built-in tool rendering for minimal display (only tool calls, no output in collapsed mode) |
 | `truncated-tool.ts` | Wraps ripgrep with proper output truncation (50KB/2000 lines) |

packages/coding-agent/examples/extensions/dynamic-tools.ts

@@ -0,0 +1,73 @@
+/**
+ * Dynamic Tools Extension
+ *
+ * Demonstrates registering tools after session initialization.
+ *
+ * - Registers one tool during session_start
+ * - Registers additional tools at runtime via /add-echo-tool <name>
+ */
+
+import type { ExtensionAPI } from "@mariozechner/pi-coding-agent";
+import { Type } from "@sinclair/typebox";
+
+const ECHO_PARAMS = Type.Object({
+	message: Type.String({ description: "Message to echo" }),
+});
+
+function normalizeToolName(input: string): string | undefined {
+	const trimmed = input.trim().toLowerCase();
+	if (!trimmed) return undefined;
+	if (!/^[a-z0-9_]+$/.test(trimmed)) return undefined;
+	return trimmed;
+}
+
+export default function dynamicToolsExtension(pi: ExtensionAPI) {
+	const registeredToolNames = new Set<string>();
+
+	const registerEchoTool = (name: string, label: string, prefix: string): boolean => {
+		if (registeredToolNames.has(name)) {
+			return false;
+		}
+
+		registeredToolNames.add(name);
+		pi.registerTool({
+			name,
+			label,
+			description: `Echo a message with prefix: ${prefix}`,
+			promptSnippet: `Echo back user-provided text with ${prefix.trim()} prefix`,
+			parameters: ECHO_PARAMS,
+			async execute(_toolCallId, params) {
+				return {
+					content: [{ type: "text", text: `${prefix}${params.message}` }],
+					details: { tool: name, prefix },
+				};
+			},
+		});
+
+		return true;
+	};
+
+	pi.on("session_start", (_event, ctx) => {
+		registerEchoTool("echo_session", "Echo Session", "[session] ");
+		ctx.ui.notify("Registered dynamic tool: echo_session", "info");
+	});
+
+	pi.registerCommand("add-echo-tool", {
+		description: "Register a new echo tool dynamically: /add-echo-tool <tool_name>",
+		handler: async (args, ctx) => {
+			const toolName = normalizeToolName(args);
+			if (!toolName) {
+				ctx.ui.notify("Usage: /add-echo-tool <tool_name> (lowercase, numbers, underscores)", "warning");
+				return;
+			}
+
+			const created = registerEchoTool(toolName, `Echo ${toolName}`, `[${toolName}] `);
+			if (!created) {
+				ctx.ui.notify(`Tool already registered: ${toolName}`, "warning");
+				return;
+			}
+
+			ctx.ui.notify(`Registered dynamic tool: ${toolName}`, "info");
+		},
+	});
+}