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

packages/coding-agent/CHANGELOG.md

@@ -2,8 +2,13 @@
 
 ## [Unreleased]
 
+### Added
+
+- Added optional `promptSnippet` to `ToolDefinition` for one-line entries in the default system prompt's `Available tools` section. Active extension tools now appear there, including tools registered dynamically after startup.
+
 ### Fixed
 
+- Fixed `pi.registerTool()` dynamic registration after session initialization. Tools registered in `session_start` and later handlers now refresh immediately, become active, and are visible to the LLM without `/reload` ([#1720](https://github.com/badlogic/pi-mono/issues/1720))
 - Fixed session message persistence ordering by serializing `AgentSession` event processing, preventing `toolResult` entries from being written before their corresponding assistant tool-call messages when extension handlers are asynchronous ([#1717](https://github.com/badlogic/pi-mono/issues/1717))
 
 ## [0.55.3] - 2026-02-27

packages/coding-agent/docs/extensions.md

@@ -880,6 +880,12 @@ Subscribe to events. See [Events](#events) for event types and return values.
 
 Register a custom tool callable by the LLM. See [Custom Tools](#custom-tools) for full details.
 
+`pi.registerTool()` works both during extension load and after startup. You can call it inside `session_start`, command handlers, or other event handlers. New tools are refreshed immediately in the same session, so they appear in `pi.getAllTools()` and are callable by the LLM without `/reload`.
+
+Use `pi.setActiveTools()` to enable or disable tools (including dynamically added tools) at runtime.
+
+See [dynamic-tools.ts](../examples/extensions/dynamic-tools.ts) for a full example.
+
 ```typescript
 import { Type } from "@sinclair/typebox";
 import { StringEnum } from "@mariozechner/pi-ai";
@@ -888,6 +894,7 @@ pi.registerTool({
   name: "my_tool",
   label: "My Tool",
   description: "What this tool does",
+  promptSnippet: "Summarize or transform text according to action",
   parameters: Type.Object({
     action: StringEnum(["list", "add"] as const),
     text: Type.Optional(Type.String()),
@@ -1116,7 +1123,7 @@ const result = await pi.exec("git", ["status"], { signal, timeout: 5000 });
 
 ### pi.getActiveTools() / pi.getAllTools() / pi.setActiveTools(names)
 
-Manage active tools.
+Manage active tools. This works for both built-in tools and dynamically registered tools.
 
 ```typescript
 const active = pi.getActiveTools();  // ["read", "bash", "edit", "write"]
@@ -1276,6 +1283,8 @@ export default function (pi: ExtensionAPI) {
 
 Register tools the LLM can call via `pi.registerTool()`. Tools appear in the system prompt and can have custom rendering.
 
+Use `promptSnippet` for a short one-line entry in the `Available tools` section in the default system prompt. If omitted, pi falls back to `description`.
+
 Note: Some models are idiots and include the @ prefix in tool path arguments. Built-in tools strip a leading @ before resolving paths. If your custom tool accepts a path, normalize a leading @ as well.
 
 ### Tool Definition
@@ -1289,6 +1298,7 @@ pi.registerTool({
   name: "my_tool",
   label: "My Tool",
   description: "What this tool does (shown to LLM)",
+  promptSnippet: "List or add items in the project todo list",
   parameters: Type.Object({
     action: StringEnum(["list", "add"] as const),  // Use StringEnum for Google compatibility
     text: Type.Optional(Type.String()),
@@ -1886,6 +1896,7 @@ All examples in [examples/extensions/](../examples/extensions/).
 | `question.ts` | Tool with user interaction | `registerTool`, `ui.select` |
 | `questionnaire.ts` | Multi-step wizard tool | `registerTool`, `ui.custom` |
 | `todo.ts` | Stateful tool with persistence | `registerTool`, `appendEntry`, `renderResult`, session events |
+| `dynamic-tools.ts` | Register tools after startup and during commands | `registerTool`, `session_start`, `registerCommand` |
 | `truncated-tool.ts` | Output truncation example | `registerTool`, `truncateHead` |
 | `tool-override.ts` | Override built-in read tool | `registerTool` (same name as built-in) |
 | **Commands** |||

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");
+		},
+	});
+}

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

@@ -268,6 +268,7 @@ export class AgentSession {
 
 	// Tool registry for extension getTools/setTools
 	private _toolRegistry: Map<string, AgentTool> = new Map();
+	private _toolPromptSnippets: Map<string, string> = new Map();
 
 	// Base system prompt (without extension appends) - used to apply fresh appends each turn
 	private _baseSystemPrompt = "";
@@ -682,8 +683,25 @@ export class AgentSession {
 		return this._resourceLoader.getPrompts().prompts;
 	}
 
+	private _normalizePromptSnippet(text: string | undefined): string | undefined {
+		if (!text) return undefined;
+		const oneLine = text
+			.replace(/[\r\n]+/g, " ")
+			.replace(/\s+/g, " ")
+			.trim();
+		return oneLine.length > 0 ? oneLine : undefined;
+	}
+
 	private _rebuildSystemPrompt(toolNames: string[]): string {
-		const validToolNames = toolNames.filter((name) => this._baseToolRegistry.has(name));
+		const validToolNames = toolNames.filter((name) => this._toolRegistry.has(name));
+		const toolSnippets: Record<string, string> = {};
+		for (const name of validToolNames) {
+			const snippet = this._toolPromptSnippets.get(name);
+			if (snippet) {
+				toolSnippets[name] = snippet;
+			}
+		}
+
 		const loaderSystemPrompt = this._resourceLoader.getSystemPrompt();
 		const loaderAppendSystemPrompt = this._resourceLoader.getAppendSystemPrompt();
 		const appendSystemPrompt =
@@ -698,6 +716,7 @@ export class AgentSession {
 			customPrompt: loaderSystemPrompt,
 			appendSystemPrompt,
 			selectedTools: validToolNames,
+			toolSnippets,
 		});
 	}
 
@@ -1934,6 +1953,7 @@ export class AgentSession {
 				getActiveTools: () => this.getActiveToolNames(),
 				getAllTools: () => this.getAllTools(),
 				setActiveTools: (toolNames) => this.setActiveToolsByName(toolNames),
+				refreshTools: () => this._refreshToolRegistry(),
 				getCommands,
 				setModel: async (model) => {
 					const key = await this.modelRegistry.getApiKey(model);
@@ -1969,6 +1989,60 @@ export class AgentSession {
 		);
 	}
 
+	private _refreshToolRegistry(options?: { activeToolNames?: string[]; includeAllExtensionTools?: boolean }): void {
+		const previousRegistryNames = new Set(this._toolRegistry.keys());
+		const previousActiveToolNames = this.getActiveToolNames();
+
+		const registeredTools = this._extensionRunner?.getAllRegisteredTools() ?? [];
+		const allCustomTools = [
+			...registeredTools,
+			...this._customTools.map((def) => ({ definition: def, extensionPath: "<sdk>" })),
+		];
+		this._toolPromptSnippets = new Map(
+			allCustomTools
+				.map((registeredTool) => {
+					const snippet = this._normalizePromptSnippet(
+						registeredTool.definition.promptSnippet ?? registeredTool.definition.description,
+					);
+					return snippet ? ([registeredTool.definition.name, snippet] as const) : undefined;
+				})
+				.filter((entry): entry is readonly [string, string] => entry !== undefined),
+		);
+		const wrappedExtensionTools = this._extensionRunner
+			? wrapRegisteredTools(allCustomTools, this._extensionRunner)
+			: [];
+
+		const toolRegistry = new Map(this._baseToolRegistry);
+		for (const tool of wrappedExtensionTools as AgentTool[]) {
+			toolRegistry.set(tool.name, tool);
+		}
+
+		if (this._extensionRunner) {
+			const wrappedAllTools = wrapToolsWithExtensions(Array.from(toolRegistry.values()), this._extensionRunner);
+			this._toolRegistry = new Map(wrappedAllTools.map((tool) => [tool.name, tool]));
+		} else {
+			this._toolRegistry = toolRegistry;
+		}
+
+		const nextActiveToolNames = options?.activeToolNames
+			? [...options.activeToolNames]
+			: [...previousActiveToolNames];
+
+		if (options?.includeAllExtensionTools) {
+			for (const tool of wrappedExtensionTools) {
+				nextActiveToolNames.push(tool.name);
+			}
+		} else if (!options?.activeToolNames) {
+			for (const toolName of this._toolRegistry.keys()) {
+				if (!previousRegistryNames.has(toolName)) {
+					nextActiveToolNames.push(toolName);
+				}
+			}
+		}
+
+		this.setActiveToolsByName([...new Set(nextActiveToolNames)]);
+	}
+
 	private _buildRuntime(options: {
 		activeToolNames?: string[];
 		flagValues?: Map<string, boolean | string>;
@@ -2012,52 +2086,14 @@ export class AgentSession {
 			this._applyExtensionBindings(this._extensionRunner);
 		}
 
-		const registeredTools = this._extensionRunner?.getAllRegisteredTools() ?? [];
-		const allCustomTools = [
-			...registeredTools,
-			...this._customTools.map((def) => ({ definition: def, extensionPath: "<sdk>" })),
-		];
-		const wrappedExtensionTools = this._extensionRunner
-			? wrapRegisteredTools(allCustomTools, this._extensionRunner)
-			: [];
-
-		const toolRegistry = new Map(this._baseToolRegistry);
-		for (const tool of wrappedExtensionTools as AgentTool[]) {
-			toolRegistry.set(tool.name, tool);
-		}
-
 		const defaultActiveToolNames = this._baseToolsOverride
 			? Object.keys(this._baseToolsOverride)
 			: ["read", "bash", "edit", "write"];
 		const baseActiveToolNames = options.activeToolNames ?? defaultActiveToolNames;
-		const activeToolNameSet = new Set<string>(baseActiveToolNames);
-		if (options.includeAllExtensionTools) {
-			for (const tool of wrappedExtensionTools as AgentTool[]) {
-				activeToolNameSet.add(tool.name);
-			}
-		}
-
-		const extensionToolNames = new Set(wrappedExtensionTools.map((tool) => tool.name));
-		const activeBaseTools = Array.from(activeToolNameSet)
-			.filter((name) => this._baseToolRegistry.has(name) && !extensionToolNames.has(name))
-			.map((name) => this._baseToolRegistry.get(name) as AgentTool);
-		const activeExtensionTools = wrappedExtensionTools.filter((tool) => activeToolNameSet.has(tool.name));
-		const activeToolsArray: AgentTool[] = [...activeBaseTools, ...activeExtensionTools];
-
-		if (this._extensionRunner) {
-			const wrappedActiveTools = wrapToolsWithExtensions(activeToolsArray, this._extensionRunner);
-			this.agent.setTools(wrappedActiveTools as AgentTool[]);
-
-			const wrappedAllTools = wrapToolsWithExtensions(Array.from(toolRegistry.values()), this._extensionRunner);
-			this._toolRegistry = new Map(wrappedAllTools.map((tool) => [tool.name, tool]));
-		} else {
-			this.agent.setTools(activeToolsArray);
-			this._toolRegistry = toolRegistry;
-		}
-
-		const systemPromptToolNames = Array.from(activeToolNameSet).filter((name) => this._baseToolRegistry.has(name));
-		this._baseSystemPrompt = this._rebuildSystemPrompt(systemPromptToolNames);
-		this.agent.setSystemPrompt(this._baseSystemPrompt);
+		this._refreshToolRegistry({
+			activeToolNames: baseActiveToolNames,
+			includeAllExtensionTools: options.includeAllExtensionTools,
+		});
 	}
 
 	async reload(): Promise<void> {

packages/coding-agent/src/core/extensions/loader.ts

@@ -119,6 +119,8 @@ export function createExtensionRuntime(): ExtensionRuntime {
 		getActiveTools: notInitialized,
 		getAllTools: notInitialized,
 		setActiveTools: notInitialized,
+		// registerTool() is valid during extension load; refresh is only needed post-bind.
+		refreshTools: () => {},
 		getCommands: notInitialized,
 		setModel: () => Promise.reject(new Error("Extension runtime not initialized")),
 		getThinkingLevel: notInitialized,
@@ -162,6 +164,7 @@ function createExtensionAPI(
 				definition: tool,
 				extensionPath: extension.path,
 			});
+			runtime.refreshTools();
 		},
 
 		registerCommand(name: string, options: Omit<RegisteredCommand, "name">): void {

packages/coding-agent/src/core/extensions/runner.ts

@@ -244,6 +244,7 @@ export class ExtensionRunner {
 		this.runtime.getActiveTools = actions.getActiveTools;
 		this.runtime.getAllTools = actions.getAllTools;
 		this.runtime.setActiveTools = actions.setActiveTools;
+		this.runtime.refreshTools = actions.refreshTools;
 		this.runtime.getCommands = actions.getCommands;
 		this.runtime.setModel = actions.setModel;
 		this.runtime.getThinkingLevel = actions.getThinkingLevel;

packages/coding-agent/src/core/extensions/types.ts

@@ -339,6 +339,8 @@ export interface ToolDefinition<TParams extends TSchema = TSchema, TDetails = un
 	label: string;
 	/** Description for LLM */
 	description: string;
+	/** Optional one-line snippet for the Available tools section in the default system prompt. Falls back to description when omitted. */
+	promptSnippet?: string;
 	/** Parameter schema (TypeBox) */
 	parameters: TParams;
 
@@ -1251,6 +1253,8 @@ export type GetCommandsHandler = () => SlashCommandInfo[];
 
 export type SetActiveToolsHandler = (toolNames: string[]) => void;
 
+export type RefreshToolsHandler = () => void;
+
 export type SetModelHandler = (model: Model<any>) => Promise<boolean>;
 
 export type GetThinkingLevelHandler = () => ThinkingLevel;
@@ -1291,6 +1295,7 @@ export interface ExtensionActions {
 	getActiveTools: GetActiveToolsHandler;
 	getAllTools: GetAllToolsHandler;
 	setActiveTools: SetActiveToolsHandler;
+	refreshTools: RefreshToolsHandler;
 	getCommands: GetCommandsHandler;
 	setModel: SetModelHandler;
 	getThinkingLevel: GetThinkingLevelHandler;

packages/coding-agent/src/core/system-prompt.ts

@@ -21,6 +21,8 @@ export interface BuildSystemPromptOptions {
 	customPrompt?: string;
 	/** Tools to include in prompt. Default: [read, bash, edit, write] */
 	selectedTools?: string[];
+	/** Optional one-line tool snippets keyed by tool name. */
+	toolSnippets?: Record<string, string>;
 	/** Text to append to system prompt. */
 	appendSystemPrompt?: string;
 	/** Working directory. Default: process.cwd() */
@@ -36,6 +38,7 @@ export function buildSystemPrompt(options: BuildSystemPromptOptions = {}): strin
 	const {
 		customPrompt,
 		selectedTools,
+		toolSnippets,
 		appendSystemPrompt,
 		cwd,
 		contextFiles: providedContextFiles,
@@ -94,9 +97,18 @@ export function buildSystemPrompt(options: BuildSystemPromptOptions = {}): strin
 	const docsPath = getDocsPath();
 	const examplesPath = getExamplesPath();
 
-	// Build tools list based on selected tools (only built-in tools with known descriptions)
-	const tools = (selectedTools || ["read", "bash", "edit", "write"]).filter((t) => t in toolDescriptions);
-	const toolsList = tools.length > 0 ? tools.map((t) => `- ${t}: ${toolDescriptions[t]}`).join("\n") : "(none)";
+	// Build tools list based on selected tools.
+	// Built-ins use toolDescriptions. Custom tools can provide one-line snippets.
+	const tools = selectedTools || ["read", "bash", "edit", "write"];
+	const toolsList =
+		tools.length > 0
+			? tools
+					.map((name) => {
+						const snippet = toolSnippets?.[name] ?? toolDescriptions[name] ?? name;
+						return `- ${name}: ${snippet}`;
+					})
+					.join("\n")
+			: "(none)";
 
 	// Build guidelines based on which tools are actually available
 	const guidelinesList: string[] = [];

packages/coding-agent/test/agent-session-dynamic-tools.test.ts

@@ -0,0 +1,75 @@
+import { existsSync, mkdirSync, rmSync } from "node:fs";
+import { tmpdir } from "node:os";
+import { join } from "node:path";
+import { getModel } from "@mariozechner/pi-ai";
+import { Type } from "@sinclair/typebox";
+import { afterEach, beforeEach, describe, expect, it } from "vitest";
+import { DefaultResourceLoader } from "../src/core/resource-loader.js";
+import { createAgentSession } from "../src/core/sdk.js";
+import { SessionManager } from "../src/core/session-manager.js";
+import { SettingsManager } from "../src/core/settings-manager.js";
+
+describe("AgentSession dynamic tool registration", () => {
+	let tempDir: string;
+	let agentDir: string;
+
+	beforeEach(() => {
+		tempDir = join(tmpdir(), `pi-dynamic-tool-test-${Date.now()}-${Math.random().toString(36).slice(2)}`);
+		agentDir = join(tempDir, "agent");
+		mkdirSync(agentDir, { recursive: true });
+	});
+
+	afterEach(() => {
+		if (tempDir && existsSync(tempDir)) {
+			rmSync(tempDir, { recursive: true, force: true });
+		}
+	});
+
+	it("refreshes tool registry when tools are registered after initialization", async () => {
+		const settingsManager = SettingsManager.create(tempDir, agentDir);
+		const sessionManager = SessionManager.inMemory();
+
+		const resourceLoader = new DefaultResourceLoader({
+			cwd: tempDir,
+			agentDir,
+			settingsManager,
+			extensionFactories: [
+				(pi) => {
+					pi.on("session_start", () => {
+						pi.registerTool({
+							name: "dynamic_tool",
+							label: "Dynamic Tool",
+							description: "Tool registered from session_start",
+							promptSnippet: "Run dynamic test behavior",
+							parameters: Type.Object({}),
+							execute: async () => ({
+								content: [{ type: "text", text: "ok" }],
+								details: {},
+							}),
+						});
+					});
+				},
+			],
+		});
+		await resourceLoader.reload();
+
+		const { session } = await createAgentSession({
+			cwd: tempDir,
+			agentDir,
+			model: getModel("anthropic", "claude-sonnet-4-5")!,
+			settingsManager,
+			sessionManager,
+			resourceLoader,
+		});
+
+		expect(session.getAllTools().map((tool) => tool.name)).not.toContain("dynamic_tool");
+
+		await session.bindExtensions({});
+
+		expect(session.getAllTools().map((tool) => tool.name)).toContain("dynamic_tool");
+		expect(session.getActiveToolNames()).toContain("dynamic_tool");
+		expect(session.systemPrompt).toContain("- dynamic_tool: Run dynamic test behavior");
+
+		session.dispose();
+	});
+});

packages/coding-agent/test/extensions-runner.test.ts

@@ -60,6 +60,7 @@ describe("ExtensionRunner", () => {
 		getActiveTools: () => [],
 		getAllTools: () => [],
 		setActiveTools: () => {},
+		refreshTools: () => {},
 		getCommands: () => [],
 		setModel: async () => false,
 		getThinkingLevel: () => "off",

packages/coding-agent/test/system-prompt.test.ts

@@ -37,4 +37,19 @@ describe("buildSystemPrompt", () => {
 			expect(prompt).toContain("- write:");
 		});
 	});
+
+	describe("custom tool snippets", () => {
+		test("includes custom tools in available tools section", () => {
+			const prompt = buildSystemPrompt({
+				selectedTools: ["read", "dynamic_tool"],
+				toolSnippets: {
+					dynamic_tool: "Run dynamic test behavior",
+				},
+				contextFiles: [],
+				skills: [],
+			});
+
+			expect(prompt).toContain("- dynamic_tool: Run dynamic test behavior");
+		});
+	});
 });