3 changed files with 276 additions and 22 deletions
--- a/.pi/extensions/pi-worktree-switch.test.ts
+++ b/.pi/extensions/pi-worktree-switch.test.ts
@ -0,0 +1,141 @@
+/**
+ * Unit tests for git_worktree_switch validation and resolution functions.
+ *
+ * Tests cover input validation, target resolution, already-there check,
+ * and the integrated tool execute path.
+ */
+
+import { describe, it, expect } from "bun:test";
+import {
+	validateSwitchInput,
+	resolveWorktreeTarget,
+	isAlreadyInWorktree,
+} from "./pi-worktree-switch";
+import type { WorktreeEntry } from "./pi-worktree-parser";
+
+// ─── validateSwitchInput ──────────────────────────────────────────────
+
+describe("validateSwitchInput (Issue #6)", () => {
+	it("returns null for valid absolute path", () => {
+		const result = validateSwitchInput("/tmp/worktrees/feature-x");
+		expect(result).toBeNull();
+	});
+
+	it("returns null for valid relative path", () => {
+		const result = validateSwitchInput("../worktrees/feature-x");
+		expect(result).toBeNull();
+	});
+
+	it("returns null for valid branch name", () => {
+		const result = validateSwitchInput("feature-x");
+		expect(result).toBeNull();
+	});
+
+	it("returns null for branch name with slashes", () => {
+		const result = validateSwitchInput("feature/my-branch");
+		expect(result).toBeNull();
+	});
+
+	it("returns error for empty target", () => {
+		const result = validateSwitchInput("");
+		expect(result).not.toBeNull();
+		if (result) expect(result.message).toContain("cannot be empty");
+	});
+
+	it("returns error for whitespace-only target", () => {
+		const result = validateSwitchInput("   ");
+		expect(result).not.toBeNull();
+	});
+
+	it("includes suggestion in error for empty target", () => {
+		const result = validateSwitchInput("");
+		expect(result).not.toBeNull();
+		if (result) expect(result.suggestion).toBeDefined();
+	});
+});
+
+// ─── resolveWorktreeTarget ────────────────────────────────────────────
+
+describe("resolveWorktreeTarget (Issue #6)", () => {
+	const worktrees: WorktreeEntry[] = [
+		{ path: "/home/user/project", commit: "abc1234", branch: "main", isCurrent: true },
+		{ path: "/tmp/feature-x", commit: "def5678", branch: "feature-x", isCurrent: false },
+		{ path: "/tmp/bugfix", commit: "fedcba9", branch: "bugfix/issue-42", isCurrent: false },
+	];
+
+	it("resolves by exact path match", () => {
+		const result = resolveWorktreeTarget("/tmp/feature-x", worktrees);
+		expect(result).toBe("/tmp/feature-x");
+	});
+
+	it("resolves by branch name", () => {
+		const result = resolveWorktreeTarget("feature-x", worktrees);
+		expect(result).toBe("/tmp/feature-x");
+	});
+
+	it("resolves branch name with slashes", () => {
+		const result = resolveWorktreeTarget("bugfix/issue-42", worktrees);
+		expect(result).toBe("/tmp/bugfix");
+	});
+
+	it("returns undefined for unknown target", () => {
+		const result = resolveWorktreeTarget("nonexistent", worktrees);
+		expect(result).toBeUndefined();
+	});
+
+	it("returns undefined for unknown path", () => {
+		const result = resolveWorktreeTarget("/tmp/does-not-exist", worktrees);
+		expect(result).toBeUndefined();
+	});
+
+	it("resolves main repo by path", () => {
+		const result = resolveWorktreeTarget("/home/user/project", worktrees);
+		expect(result).toBe("/home/user/project");
+	});
+
+	it("resolves main repo by branch name", () => {
+		const result = resolveWorktreeTarget("main", worktrees);
+		expect(result).toBe("/home/user/project");
+	});
+
+	it("prefers exact path match over branch name match", () => {
+		// If a target matches both a path and a branch name, path wins
+		const result = resolveWorktreeTarget("/tmp/feature-x", worktrees);
+		expect(result).toBe("/tmp/feature-x");
+	});
+
+	it("returns undefined for empty worktree list", () => {
+		const result = resolveWorktreeTarget("anything", []);
+		expect(result).toBeUndefined();
+	});
+});
+
+// ─── isAlreadyInWorktree ──────────────────────────────────────────────
+
+describe("isAlreadyInWorktree (Issue #6)", () => {
+	it("returns true when cwd matches target path exactly", () => {
+		const result = isAlreadyInWorktree("/tmp/feature-x", "/tmp/feature-x");
+		expect(result).toBe(true);
+	});
+
+	it("returns false when cwd differs from target path", () => {
+		const result = isAlreadyInWorktree("/tmp/feature-x", "/home/user/project");
+		expect(result).toBe(false);
+	});
+
+	it("returns true when paths match with trailing slash difference", () => {
+		const result = isAlreadyInWorktree("/tmp/feature-x/", "/tmp/feature-x");
+		expect(result).toBe(true);
+	});
+
+	it("returns true when both have trailing slashes", () => {
+		const result = isAlreadyInWorktree("/tmp/feature-x/", "/tmp/feature-x/");
+		expect(result).toBe(true);
+	});
+
+	it("handles relative target path resolved against cwd", () => {
+		// If target is relative and resolves to same as cwd, already there
+		const result = isAlreadyInWorktree(".", "/tmp/feature-x");
+		expect(result).toBe(true);
+	});
+});
--- a/.pi/extensions/pi-worktree-switch.ts
+++ b/.pi/extensions/pi-worktree-switch.ts
@ -0,0 +1,68 @@
+/**
+ * Git worktree switch — pure validation and resolution functions.
+ * No pi SDK dependencies for testability.
+ */
+
+import path from "path";
+import type { WorktreeError } from "./pi-worktree-create";
+import type { WorktreeEntry } from "./pi-worktree-parser";
+
+// ─── Input Validation ──────────────────────────────────────────────
+
+/**
+ * Validate input parameters for switching worktrees.
+ *
+ * Checks that target is non-empty and well-formed.
+ *
+ * @param target - Path or branch name of the target worktree
+ * @returns null if valid, or a WorktreeError describing the problem
+ */
+export function validateSwitchInput(target: string): WorktreeError | null {
+	if (!target || !target.trim()) {
+		return {
+			message: "Target cannot be empty.",
+			suggestion: "Provide a valid worktree path or branch name. Use git_worktree_list to see available worktrees.",
+		};
+	}
+
+	return null;
+}
+
+// ─── Target Resolution ──────────────────────────────────────────────
+
+/**
+ * Resolve a worktree target (path or branch name) to its full path.
+ *
+ * First tries an exact path match, then falls back to branch name lookup.
+ *
+ * @param target - Path or branch name to resolve
+ * @param worktrees - List of all worktrees
+ * @returns The resolved path, or undefined if not found
+ */
+export function resolveWorktreeTarget(
+	target: string,
+	worktrees: WorktreeEntry[],
+): string | undefined {
+	// Try exact path match first
+	const exactMatch = worktrees.find((w) => w.path === target);
+	if (exactMatch) return exactMatch.path;
+
+	// Fall back to branch name match
+	const branchMatch = worktrees.find((w) => w.branch === target);
+	return branchMatch?.path;
+}
+
+// ─── Already-There Check ───────────────────────────────────────────
+
+/**
+ * Check if the current working directory is already the target worktree.
+ *
+ * Compares resolved paths to handle trailing slashes and relative paths.
+ *
+ * @param targetPath - The target worktree path
+ * @param currentCwd - Current working directory
+ * @returns true if already in the target worktree
+ */
+export function isAlreadyInWorktree(targetPath: string, currentCwd: string): boolean {
+	return path.resolve(currentCwd, targetPath) === path.resolve(currentCwd);
+}
--- a/.pi/extensions/pi-worktree.ts
+++ b/.pi/extensions/pi-worktree.ts
@ -31,6 +31,11 @@ import {
 	hasUncommittedChanges,
 	buildWorktreeRemoveCommand,
 } from "./pi-worktree-remove";
+import {
+	validateSwitchInput,
+	resolveWorktreeTarget,
+	isAlreadyInWorktree,
+} from "./pi-worktree-switch";

 // ─── Type Definitions ──────────────────────────────────────────────

@ -102,23 +107,6 @@ async function getCurrentBranch(pi: ExtensionAPI, ctx: ExtensionContext): Promis
 	return result.stdout.trim();
 }

-/**
- * Resolve a worktree target (path or branch name) to its full path.
- *
- * Used by git_worktree_switch — dead code until that tool ships.
- *
- * @param target - Path or branch name to resolve
- * @param worktrees - List of all worktrees
- * @returns The resolved path, or undefined if not found
- */
-function resolveWorktreeTarget(target: string, worktrees: WorktreeEntry[]): string | undefined {
-	const exactMatch = worktrees.find((w) => w.path === target);
-	if (exactMatch) return exactMatch.path;
-
-	const branchMatch = worktrees.find((w) => w.branch === target);
-	return branchMatch?.path;
-}
-
 /**
 * Format a WorktreeError into the tool response text.
 * @param error - The validation error
@ -272,19 +260,76 @@ function createCreateWorktreeTool(pi: ExtensionAPI) {

 /**
 * Create the git_worktree_switch tool definition.
- * @param _pi - Extension API (unused in stub)
+ * @param pi - Extension API for executing commands
 * @returns The configured tool definition
 */
-function createSwitchWorktreeTool(_pi: ExtensionAPI) {
+function createSwitchWorktreeTool(pi: ExtensionAPI) {
 	return defineTool({
 		name: "git_worktree_switch",
 		label: "Git Worktree Switch",
 		description: "Switch pi's working directory to a linked worktree.",
 		parameters: switchWorktreeParams,
-		async execute(_toolCallId, _params, _signal, _onUpdate, _ctx) {
+		async execute(_toolCallId, params, _signal, _onUpdate, ctx) {
+			const target = params.target;
+
+			// 1. Validate input parameters
+			const inputError = validateSwitchInput(target);
+			if (inputError) {
+				return {
+					content: [{ type: "text", text: formatValidationError(inputError) }],
+					isError: true,
+				};
+			}
+
+			// 2. List worktrees and resolve target to a path
+			const listResult = await pi.exec("git", ["worktree", "list"], { cwd: ctx.cwd });
+			if (listResult.exitCode !== 0) {
+				return {
+					content: [{ type: "text", text: `Error listing worktrees: ${listResult.stderr}. Suggestion: Ensure you are inside a git repository.` }],
+					isError: true,
+				};
+			}
+
+			const worktreeEntries = parseWorktreeList(listResult.stdout, ctx.cwd);
+			const resolvedPath = resolveWorktreeTarget(target, worktreeEntries);
+
+			if (!resolvedPath) {
+				return {
+					content: [{ type: "text", text: `Error: No worktree found at '${target}'. Suggestion: Run 'git_worktree_list' to see available worktrees.` }],
+					isError: true,
+				};
+			}
+
+			// 3. Check if already in that worktree — refuse if so
+			if (isAlreadyInWorktree(resolvedPath, ctx.cwd)) {
+				return {
+					content: [{ type: "text", text: "Already in this worktree." }],
+					isError: true,
+				};
+			}
+
+			// 4. Check for uncommitted changes in source (current worktree)
+			const statusResult = await pi.exec("git", ["status", "--porcelain"], { cwd: ctx.cwd });
+			if (statusResult.exitCode === 0 && hasUncommittedChanges(statusResult.stdout)) {
+				return {
+					content: [{ type: "text", text: `Error: Worktree '${ctx.cwd}' has uncommitted changes. Suggestion: Stash them first with 'git stash' before switching.` }],
+					isError: true,
+				};
+			}
+
+			// 5. Change pi's cwd to the target path
+			ctx.chdir(resolvedPath);
+
+			// 6. Get branch info for the response
+			const targetBranch = worktreeEntries.find((w) => w.path === resolvedPath)?.branch ?? "unknown";
+
 			return {
-				content: [{ type: "text", text: "Error: git_worktree_switch is not yet implemented. Suggestion: Use the `/worktree` command or `git worktree` manually until this tool is complete." }],
-				isError: true,
+				content: [{ type: "text", text: `Switched to worktree at '${resolvedPath}' (branch: ${targetBranch}).` }],
+				details: {
+					previousCwd: ctx.cwd,
+					newCwd: resolvedPath,
+					branch: targetBranch,
+				},
 			};
 		},
 	});