feat(pi): add Skill tool resolution (#1661)

* fix(gsd extension): detect initialized projects in health widget

Use .gsd presence plus project-state detection for the health widget so bootstrapped projects no longer appear as unloaded before metrics exist.

* fix(gsd extension): detect initialized projects in health widget

Use .gsd presence plus project-state detection for the health widget so bootstrapped projects no longer appear as unloaded before metrics exist.

* feat: add Skill tool resolution for Pi agent

Expose a built-in Skill tool so dispatched prompts can resolve skill names without guessing file paths. This aligns runtime behavior with skill activation prompts and adds coverage for exact activation and unknown-skill handling.

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

@@ -25,6 +25,7 @@ import type {
 } from "@gsd/pi-agent-core";
 import type { AssistantMessage, ImageContent, Message, Model, TextContent } from "@gsd/pi-ai";
 import { modelsAreEqual, resetApiProviders, supportsXhigh } from "@gsd/pi-ai";
+import { Type } from "@sinclair/typebox";
 import { getDocsPath } from "../config.js";
 import { getErrorMessage } from "../utils/error.js";
 import { theme } from "../modes/interactive/theme/theme.js";
@@ -732,9 +733,10 @@ export class AgentSession {
 	 * Changes take effect on the next agent turn.
 	 */
 	setActiveToolsByName(toolNames: string[]): void {
+		const requestedToolNames = [...new Set([...toolNames, ...this._getBuiltinToolNames()])];
 		const tools: AgentTool[] = [];
 		const validToolNames: string[] = [];
-		for (const name of toolNames) {
+		for (const name of requestedToolNames) {
 			const tool = this._toolRegistry.get(name);
 			if (tool) {
 				tools.push(tool);
@@ -743,6 +745,7 @@ export class AgentSession {
 		}
 		this.agent.setTools(tools);
 
+
 		// Rebuild base system prompt with new tool set
 		this._baseSystemPrompt = this._rebuildSystemPrompt(validToolNames);
 		this.agent.setSystemPrompt(this._baseSystemPrompt);
@@ -858,6 +861,48 @@ export class AgentSession {
 		return Array.from(unique);
 	}
 
+	private _findSkillByName(skillName: string) {
+		return this.resourceLoader.getSkills().skills.find((skill) => skill.name === skillName);
+	}
+
+	private _formatMissingSkillMessage(skillName: string): string {
+		const availableSkills = this.resourceLoader.getSkills().skills.map((skill) => skill.name).join(", ") || "(none)";
+		return `Skill "${skillName}" not found. Available skills: ${availableSkills}`;
+	}
+
+	private _emitSkillExpansionError(skillFilePath: string, err: unknown): void {
+		this._extensionRunner?.emitError({
+			extensionPath: skillFilePath,
+			event: "skill_expansion",
+			error: getErrorMessage(err),
+		});
+	}
+
+	private _renderSkillInvocation(skill: { name: string; filePath: string; baseDir: string }, args?: string): string {
+		const content = readFileSync(skill.filePath, "utf-8");
+		const body = stripFrontmatter(content).trim();
+		const skillBlock = `<skill name="${skill.name}" location="${skill.filePath}">\nReferences are relative to ${skill.baseDir}.\n\n${body}\n</skill>`;
+		return args && args.trim() ? `${skillBlock}\n\n${args.trim()}` : skillBlock;
+	}
+
+	private _expandSkillByName(skillName: string, args?: string): string {
+		const skill = this._findSkillByName(skillName);
+		if (!skill) {
+			throw new Error(this._formatMissingSkillMessage(skillName));
+		}
+
+		try {
+			return this._renderSkillInvocation(skill, args);
+		} catch (err) {
+			this._emitSkillExpansionError(skill.filePath, err);
+			throw err;
+		}
+	}
+
+	private _formatSkillInvocation(skillName: string, args?: string): string {
+		return this._expandSkillByName(skillName, args);
+	}
+
 	private _rebuildSystemPrompt(toolNames: string[]): string {
 		const validToolNames = toolNames.filter((name) => this._toolRegistry.has(name));
 		const toolSnippets: Record<string, string> = {};
@@ -1103,25 +1148,78 @@ export class AgentSession {
 		const skillName = spaceIndex === -1 ? text.slice(7) : text.slice(7, spaceIndex);
 		const args = spaceIndex === -1 ? "" : text.slice(spaceIndex + 1).trim();
 
-		const skill = this.resourceLoader.getSkills().skills.find((s) => s.name === skillName);
-		if (!skill) return text; // Unknown skill, pass through
+		if (!this._findSkillByName(skillName)) return text;
 
 		try {
-			const content = readFileSync(skill.filePath, "utf-8");
-			const body = stripFrontmatter(content).trim();
-			const skillBlock = `<skill name="${skill.name}" location="${skill.filePath}">\nReferences are relative to ${skill.baseDir}.\n\n${body}\n</skill>`;
-			return args ? `${skillBlock}\n\n${args}` : skillBlock;
-		} catch (err) {
-			// Emit error like extension commands do
-			this._extensionRunner?.emitError({
-				extensionPath: skill.filePath,
-				event: "skill_expansion",
-				error: getErrorMessage(err),
-			});
-			return text; // Return original on error
+			return this._formatSkillInvocation(skillName, args);
+		} catch {
+			return text;
 		}
 	}
 
+	private _createBuiltInSkillTool(): AgentTool {
+		const skillSchema = Type.Object({
+			skill: Type.String({ description: "The skill name. E.g., 'commit', 'review-pr', or 'pdf'" }),
+			args: Type.Optional(Type.String({ description: "Optional arguments for the skill" })),
+		});
+
+		return {
+			name: "Skill",
+			label: "Skill",
+			description:
+				"Execute a skill within the main conversation. Use this tool when users ask for a slash command or reference a skill by name. Returns the expanded skill block and appends args after it.",
+			parameters: skillSchema,
+			execute: async (_toolCallId, params: unknown) => {
+				const input = params as { skill: string; args?: string };
+				try {
+					return {
+						content: [
+							{
+								type: "text",
+								text: this._expandSkillByName(input.skill, input.args),
+							},
+						],
+						details: undefined,
+					};
+				} catch (err) {
+					return {
+						content: [{ type: "text", text: getErrorMessage(err) }],
+						details: undefined,
+					};
+				}
+			},
+		};
+	}
+
+	private _getBuiltinToolNames(): string[] {
+		return this._getBuiltinTools().map((tool) => tool.name);
+	}
+
+	private _getBuiltinTools(): AgentTool[] {
+		return [this._createBuiltInSkillTool()];
+	}
+
+	private _getRegisteredToolDefinitions(): ToolDefinition[] {
+		const registeredTools = this._extensionRunner?.getAllRegisteredTools() ?? [];
+		return registeredTools.map((tool) => tool.definition);
+	}
+
+	private _getBuiltinToolDefinitions(): ToolDefinition[] {
+		return this._getBuiltinTools().map((tool) => ({
+			name: tool.name,
+			label: tool.label,
+			description: tool.description,
+			parameters: tool.parameters,
+			execute: async () => ({ content: [], details: undefined }),
+		}));
+	}
+
+	getRenderableToolDefinition(toolName: string): ToolDefinition | undefined {
+		return [...this._getBuiltinToolDefinitions(), ...this._getRegisteredToolDefinitions()].find(
+			(tool) => tool.name === toolName,
+		);
+	}
+
 	/**
 	 * Queue a steering message to interrupt the agent mid-run.
 	 * Delivered after current tool execution, skips remaining tools.
@@ -1967,8 +2065,12 @@ export class AgentSession {
 		const wrappedExtensionTools = this._extensionRunner
 			? wrapRegisteredTools(allCustomTools, this._extensionRunner)
 			: [];
+		const builtinTools = this._getBuiltinTools();
 
 		const toolRegistry = new Map(this._baseToolRegistry);
+		for (const tool of builtinTools) {
+			toolRegistry.set(tool.name, tool);
+		}
 		for (const tool of wrappedExtensionTools as AgentTool[]) {
 			toolRegistry.set(tool.name, tool);
 		}
@@ -2694,14 +2796,11 @@ export class AgentSession {
 	async exportToHtml(outputPath?: string): Promise<string> {
 		const themeName = this.settingsManager.getTheme();
 
-		// Create tool renderer if we have an extension runner (for custom tool HTML rendering)
-		let toolRenderer: ToolHtmlRenderer | undefined;
-		if (this._extensionRunner) {
-			toolRenderer = createToolHtmlRenderer({
-				getToolDefinition: (name) => this._extensionRunner!.getToolDefinition(name),
-				theme,
-			});
-		}
+		// Create tool renderer for extension and built-in tool HTML rendering
+		const toolRenderer = createToolHtmlRenderer({
+			getToolDefinition: (name) => this.getRenderableToolDefinition(name),
+			theme,
+		});
 
 		return await exportSessionToHtml(this.sessionManager, this.state, {
 			outputPath,

packages/pi-coding-agent/src/core/skill-tool.test.ts

@@ -0,0 +1,89 @@
+import assert from "node:assert/strict";
+import { mkdirSync, mkdtempSync, rmSync, writeFileSync } from "node:fs";
+import { tmpdir } from "node:os";
+import { join } from "node:path";
+import { afterEach, beforeEach, describe, it } from "node:test";
+
+import { Agent } from "@gsd/pi-agent-core";
+import { AuthStorage } from "./auth-storage.js";
+import { AgentSession } from "./agent-session.js";
+import { ModelRegistry } from "./model-registry.js";
+import { DefaultResourceLoader } from "./resource-loader.js";
+import { SessionManager } from "./session-manager.js";
+import { SettingsManager } from "./settings-manager.js";
+
+let testDir: string;
+
+function writeSkill(cwd: string, name: string, description: string, body = `# ${name}\n`): string {
+	const skillDir = join(cwd, ".pi", "skills", name);
+	mkdirSync(skillDir, { recursive: true });
+	const skillPath = join(skillDir, "SKILL.md");
+	writeFileSync(skillPath, `---\nname: ${name}\ndescription: ${description}\n---\n\n${body}`);
+	return skillPath;
+}
+
+describe("Skill tool", () => {
+	beforeEach(() => {
+		testDir = mkdtempSync(join(tmpdir(), "skill-tool-test-"));
+	});
+
+	afterEach(() => {
+		rmSync(testDir, { recursive: true, force: true });
+	});
+
+	async function createSession() {
+		const agentDir = join(testDir, "agent-home");
+		const authStorage = AuthStorage.inMemory({});
+		const modelRegistry = new ModelRegistry(authStorage, join(agentDir, "models.json"));
+		const settingsManager = SettingsManager.inMemory();
+		const resourceLoader = new DefaultResourceLoader({
+			cwd: testDir,
+			agentDir,
+			settingsManager,
+			noExtensions: true,
+			noPromptTemplates: true,
+			noThemes: true,
+		});
+		await resourceLoader.reload();
+
+		return new AgentSession({
+			agent: new Agent(),
+			sessionManager: SessionManager.inMemory(testDir),
+			settingsManager,
+			cwd: testDir,
+			resourceLoader,
+			modelRegistry,
+		});
+	}
+
+	it("resolves a project-level skill to the exact skill block format", async () => {
+		const skillPath = writeSkill(
+			testDir,
+			"swift-testing",
+			"Use for Swift Testing assertions and verification patterns.",
+			"# Swift Testing\nUse this skill.\n",
+		);
+		const session = await createSession();
+
+		const tool = session.state.tools.find((entry) => entry.name === "Skill");
+		assert.ok(tool, "Skill tool should be registered");
+
+		const result = await tool.execute("call-1", { skill: "swift-testing" });
+		assert.equal(
+			result.content[0]?.type === "text" ? result.content[0].text : "",
+			`<skill name="swift-testing" location="${skillPath}">\nReferences are relative to ${join(testDir, ".pi", "skills", "swift-testing")}.\n\n# Swift Testing\nUse this skill.\n</skill>`,
+		);
+	});
+
+	it("returns a helpful error for unknown skills", async () => {
+		writeSkill(testDir, "swift-testing", "Use for Swift Testing assertions and verification patterns.");
+		const session = await createSession();
+		const tool = session.state.tools.find((entry) => entry.name === "Skill");
+		assert.ok(tool, "Skill tool should be registered");
+
+		const result = await tool.execute("call-2", { skill: "nonexistent" });
+		const message = result.content[0]?.type === "text" ? result.content[0].text : "";
+		assert.match(message, /^Skill "nonexistent" not found\. Available skills: /);
+		assert.match(message, /swift-testing/);
+	});
+});

packages/pi-coding-agent/src/core/skills.ts

@@ -299,7 +299,8 @@ export function formatSkillsForPrompt(skills: Skill[]): string {
 
 	const lines = [
 		"\n\nThe following skills provide specialized instructions for specific tasks.",
-		"Use the read tool to load a skill's file when the task matches its description.",
+		"Use the Skill tool with the exact skill name from <available_skills> when the task matches its description.",
+		"If the Skill tool reports an unknown skill, do not guess: use an exact name from <available_skills> or tell the user the skill is unavailable.",
 		"When a skill file references a relative path, resolve it against the skill directory (parent of SKILL.md / dirname of the path) and use that absolute path in tool commands.",
 		"",
 		"<available_skills>",

packages/pi-coding-agent/src/modes/interactive/interactive-mode.ts

@@ -1184,12 +1184,10 @@ export class InteractiveMode {
 	}
 
 	/**
-	 * Get a registered tool definition by name (for custom rendering).
+	 * Get a tool definition by name (for custom rendering).
 	 */
 	private getRegisteredToolDefinition(toolName: string) {
-		const tools = this.session.extensionRunner?.getAllRegisteredTools() ?? [];
-		const registeredTool = tools.find((t) => t.definition.name === toolName);
-		return registeredTool?.definition;
+		return this.session.getRenderableToolDefinition(toolName);
 	}
 
 	/**

src/resources/extensions/gsd/tests/skill-activation.test.ts

@@ -60,17 +60,17 @@ test("buildSkillActivationBlock matches installed skills from task context", ()
   }
 });
 
-test("buildSkillActivationBlock includes always_use_skills from preferences", () => {
+test("buildSkillActivationBlock includes always_use_skills from preferences using exact Skill tool format", () => {
   const base = makeTempBase();
   try {
-    writeSkill(base, "testing", "Use for test setup, assertions, and verification patterns.");
+    writeSkill(base, "swift-testing", "Use for Swift Testing assertions and verification patterns.");
     loadOnlyTestSkills(base);
 
     const result = buildBlock(base, { taskTitle: "Unrelated task title" }, {
-      always_use_skills: ["testing"],
+      always_use_skills: ["swift-testing"],
     });
 
-    assert.match(result, /Call Skill\('testing'\)/);
+    assert.equal(result, "<skill_activation>Call Skill('swift-testing').</skill_activation>");
   } finally {
     cleanup(base);
   }