fix(coding-agent): make prompt snippets opt in closes #2285

packages/coding-agent/CHANGELOG.md

@@ -2,6 +2,10 @@
 
 ## [Unreleased]
 
+### Breaking Changes
+
+- Changed custom tool system prompt behavior so extension and SDK tools are included in the default `Available tools` section only when they provide `promptSnippet`. Omitting `promptSnippet` now leaves the tool out of that section instead of falling back to `description` ([#2285](https://github.com/badlogic/pi-mono/issues/2285))
+
 ## [0.58.4] - 2026-03-16
 
 ### Fixed

packages/coding-agent/docs/extensions.md

@@ -934,7 +934,7 @@ Register a custom tool callable by the LLM. See [Custom Tools](#custom-tools) fo
 
 Use `pi.setActiveTools()` to enable or disable tools (including dynamically added tools) at runtime.
 
-Use `promptSnippet` to customize that tool's one-line entry in `Available tools`, and `promptGuidelines` to append tool-specific bullets to the default `Guidelines` section when the tool is active.
+Use `promptSnippet` to opt a custom tool into a one-line entry in `Available tools`, and `promptGuidelines` to append tool-specific bullets to the default `Guidelines` section when the tool is active.
 
 See [dynamic-tools.ts](../examples/extensions/dynamic-tools.ts) for a full example.
 
@@ -1336,7 +1336,7 @@ 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`.
+Use `promptSnippet` for a short one-line entry in the `Available tools` section in the default system prompt. If omitted, custom tools are left out of that section.
 
 Use `promptGuidelines` to add tool-specific bullets to the default system prompt `Guidelines` section. These bullets are included only while the tool is active (for example, after `pi.setActiveTools([...])`).
 

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

@@ -339,7 +339,7 @@ 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. */
+	/** Optional one-line snippet for the Available tools section in the default system prompt. Custom tools are omitted from that section when this is not provided. */
 	promptSnippet?: string;
 	/** Optional guideline bullets appended to the default system prompt Guidelines section when this tool is active. */
 	promptGuidelines?: string[];

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

@@ -94,9 +94,10 @@ export function buildSystemPrompt(options: BuildSystemPromptOptions = {}): strin
 	// 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 visibleTools = tools.filter((name) => name in toolDescriptions || toolSnippets?.[name]);
 	const toolsList =
-		tools.length > 0
+		visibleTools.length > 0
-			? tools
+			? visibleTools
 					.map((name) => {
 						const snippet = toolSnippets?.[name] ?? toolDescriptions[name] ?? name;
 						return `- ${name}: ${snippet}`;

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

@@ -74,4 +74,50 @@ describe("AgentSession dynamic tool registration", () => {
 
 		session.dispose();
 	});
+
+	it("keeps custom tools active but omits them from available tools when promptSnippet is not provided", 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: "hidden_tool",
+							label: "Hidden Tool",
+							description: "Description should not appear in available tools",
+							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,
+		});
+
+		await session.bindExtensions({});
+
+		expect(session.getAllTools().map((tool) => tool.name)).toContain("hidden_tool");
+		expect(session.getActiveToolNames()).toContain("hidden_tool");
+		expect(session.systemPrompt).not.toContain("hidden_tool");
+		expect(session.systemPrompt).not.toContain("Description should not appear in available tools");
+
+		session.dispose();
+	});
 });

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

@@ -39,7 +39,7 @@ describe("buildSystemPrompt", () => {
 	});
 
 	describe("custom tool snippets", () => {
-		test("includes custom tools in available tools section", () => {
+		test("includes custom tools in available tools section when promptSnippet is provided", () => {
 			const prompt = buildSystemPrompt({
 				selectedTools: ["read", "dynamic_tool"],
 				toolSnippets: {
@@ -51,6 +51,16 @@ describe("buildSystemPrompt", () => {
 
 			expect(prompt).toContain("- dynamic_tool: Run dynamic test behavior");
 		});
+
+		test("omits custom tools from available tools section when promptSnippet is not provided", () => {
+			const prompt = buildSystemPrompt({
+				selectedTools: ["read", "dynamic_tool"],
+				contextFiles: [],
+				skills: [],
+			});
+
+			expect(prompt).not.toContain("dynamic_tool");
+		});
 	});
 
 	describe("prompt guidelines", () => {