diff --git a/.pi/extensions/pi-worktree-create.ts b/.pi/extensions/pi-worktree-create.ts
new file mode 100644
index 0000000..b081321
--- /dev/null
+++ b/.pi/extensions/pi-worktree-create.ts
@@ -0,0 +1,146 @@
+/**
+ * Git worktree create — pure validation and command-building functions.
+ * No pi SDK dependencies for testability.
+ */
+
+// ─── Type Definitions ──────────────────────────────────────────────
+
+/** Error returned when a worktree operation fails. */
+export interface WorktreeError {
+	/** Human-readable error message from git or validation. */
+	message: string;
+	/** Suggested fix for the user. */
+	suggestion?: string;
+}
+
+/** Parameters for creating a new worktree. */
+export interface CreateWorktreeParams {
+	/** Path for the new worktree (relative or absolute). */
+	path: string;
+	/** Branch name to check out. */
+	branch: string;
+	/** Whether to create the branch if it doesn't exist. Defaults to true. */
+	createBranch?: boolean;
+}
+
+// ─── Validation ────────────────────────────────────────────────────
+
+/**
+ * Validate input parameters for creating a worktree.
+ *
+ * Checks that path and branch name are non-empty and well-formed.
+ * Branch names follow git conventions: no spaces, no leading/trailing whitespace,
+ * no special characters like ~, ^, . at start or ?, *, [, ], ", ' in the middle (except / for nesting).
+ *
+ * @param worktreePath - Path for the new worktree
+ * @param branchName - Branch name to check out
+ * @returns null if valid, or a WorktreeError describing the problem
+ */
+export function validateCreateWorktreeInput(
+	worktreePath: string,
+	branchName: string,
+): WorktreeError | null {
+	// Validate path is non-empty
+	if (!worktreePath || !worktreePath.trim()) {
+		return {
+			message: "Path cannot be empty.",
+			suggestion: "Provide a valid path for the new worktree (e.g., '../worktrees/feature-x').",
+		};
+	}
+
+	// Validate branch name is non-empty
+	if (!branchName || !branchName.trim()) {
+		return {
+			message: "Branch name cannot be empty.",
+			suggestion: "Provide a valid branch name (e.g., 'feature-x' or 'bugfix/issue-42').",
+		};
+	}
+
+	// Validate branch name doesn't contain spaces
+	if (branchName.includes(" ")) {
+		return {
+			message: `Branch name '${branchName}' contains invalid characters (spaces).`,
+			suggestion: "Use hyphens or underscores instead of spaces (e.g., 'feature-x' not 'my feature').",
+		};
+	}
+
+	// Validate branch name doesn't start/end with special chars
+	if (/^[\/\-~^\.]/.test(branchName) || /[\/\-\.\s]$/.test(branchName)) {
+		return {
+			message: `Branch name '${branchName}' starts or ends with an invalid character.`,
+			suggestion: "Branch names should start with a letter and not end with '/', '-', '.', or whitespace.",
+		};
+	}
+
+	// Validate branch name doesn't contain dangerous characters
+	if (/[?*\[\]"'\\]/.test(branchName)) {
+		return {
+			message: `Branch name '${branchName}' contains invalid characters.`,
+			suggestion: "Avoid characters like ?, *, [, ], \", ' in branch names.",
+		};
+	}
+
+	return null;
+}
+
+// ─── Path Validation ──────────────────────────────────────────────
+
+/**
+ * Check if the target path already exists and return an error if so.
+ *
+ * @param worktreePath - Path to check
+ * @param checkExists - Function that checks if a path exists (injected for testability)
+ * @returns null if path is free, or a WorktreeError with the exact message format
+ */
+export function validatePathDoesNotExist(
+	worktreePath: string,
+	checkExists: (path: string) => boolean,
+): WorktreeError | null {
+	if (checkExists(worktreePath)) {
+		return {
+			message: `Path '${worktreePath}' already exists.`,
+			suggestion: "Choose a different location.",
+		};
+	}
+	return null;
+}
+
+// ─── Branch Validation ──────────────────────────────────────────────
+
+/**
+ * Check if the target branch is the same as the current branch.
+ *
+ * @param targetBranch - The branch we want to create the worktree on
+ * @param currentBranch - The currently checked out branch
+ * @returns true if they match (worktree creation should be refused)
+ */
+export function isCurrentBranch(targetBranch: string, currentBranch: string): boolean {
+	return targetBranch === currentBranch;
+}
+
+// ─── Command Building ──────────────────────────────────────────────
+
+/**
+ * Build the `git worktree add` command arguments.
+ *
+ * Uses `-b` flag when createBranch is true to create a new branch.
+ * Without `-b`, checks out an existing branch.
+ *
+ * @param params - Worktree creation parameters
+ * @returns Array of command arguments (excluding 'git' itself)
+ */
+export function buildWorktreeAddCommand(params: CreateWorktreeParams): string[] {
+	const { path: worktreePath, branch, createBranch = true } = params;
+
+	const args = ["worktree", "add"];
+
+	if (createBranch) {
+		args.push("-b", branch);
+	} else {
+		args.push(branch);
+	}
+
+	args.push(worktreePath);
+
+	return args;
+}
diff --git a/.pi/extensions/pi-worktree.test.ts b/.pi/extensions/pi-worktree.test.ts
index ff1a8fe..1217cd7 100644
--- a/.pi/extensions/pi-worktree.test.ts
+++ b/.pi/extensions/pi-worktree.test.ts
@@ -6,6 +6,12 @@
 
 import { describe, it, expect } from "bun:test";
 import { parseWorktreeList } from "./pi-worktree-parser";
+import {
+	validateCreateWorktreeInput,
+	buildWorktreeAddCommand,
+	validatePathDoesNotExist,
+	isCurrentBranch,
+} from "./pi-worktree-create";
 
 describe("parseWorktreeList", () => {
 	describe("normal output", () => {
@@ -115,3 +121,324 @@ describe("parseWorktreeList", () => {
 		});
 	});
 });
+
+describe("validateCreateWorktreeInput (Issue #4)", () => {
+	it("returns null for valid path and branch", () => {
+		const result = validateCreateWorktreeInput("/tmp/new-worktree", "feature-x");
+		expect(result).toBeNull();
+	});
+
+	it("returns error for empty path", () => {
+		const result = validateCreateWorktreeInput("", "feature-x");
+		expect(result).not.toBeNull();
+		if (result) expect(result.message).toContain("cannot be empty");
+	});
+
+	it("returns error for empty branch name", () => {
+		const result = validateCreateWorktreeInput("/tmp/new-worktree", "");
+		expect(result).not.toBeNull();
+		if (result) expect(result.message).toContain("cannot be empty");
+	});
+
+	it("returns error for path with only whitespace", () => {
+		const result = validateCreateWorktreeInput("   ", "feature-x");
+		expect(result).not.toBeNull();
+	});
+
+	it("returns error for branch name with invalid characters (spaces)", () => {
+		const result = validateCreateWorktreeInput("/tmp/new-worktree", "my bad branch");
+		expect(result).not.toBeNull();
+		if (result) expect(result.message).toContain("invalid characters");
+	});
+
+	it("accepts branch names with hyphens and slashes", () => {
+		const result = validateCreateWorktreeInput("/tmp/new-worktree", "feature/my-branch-name");
+		expect(result).toBeNull();
+	});
+
+	it("accepts branch names with underscores", () => {
+		const result = validateCreateWorktreeInput("/tmp/new-worktree", "issue_42_fix");
+		expect(result).toBeNull();
+	});
+
+	// Task 007: Branch name start/end validation
+	it("rejects branch name starting with dash", () => {
+		const result = validateCreateWorktreeInput("/tmp/x", "-feature");
+		expect(result).not.toBeNull();
+		if (result) expect(result.message).toContain("invalid character");
+	});
+
+	it("rejects branch name starting with slash", () => {
+		const result = validateCreateWorktreeInput("/tmp/x", "/feature");
+		expect(result).not.toBeNull();
+	});
+
+	it("rejects branch name starting with tilde", () => {
+		const result = validateCreateWorktreeInput("/tmp/x", "~user");
+		expect(result).not.toBeNull();
+	});
+
+	it("rejects branch name starting with caret", () => {
+		const result = validateCreateWorktreeInput("/tmp/x", "^feature");
+		expect(result).not.toBeNull();
+	});
+
+	it("rejects branch name starting with dot", () => {
+		const result = validateCreateWorktreeInput("/tmp/x", ".hidden");
+		expect(result).not.toBeNull();
+	});
+
+	it("rejects branch name ending with slash", () => {
+		const result = validateCreateWorktreeInput("/tmp/x", "feature/");
+		expect(result).not.toBeNull();
+		if (result) expect(result.message).toContain("invalid character");
+	});
+
+	it("rejects branch name ending with dash", () => {
+		const result = validateCreateWorktreeInput("/tmp/x", "feature-");
+		expect(result).not.toBeNull();
+	});
+
+	it("rejects branch name ending with dot", () => {
+		const result = validateCreateWorktreeInput("/tmp/x", "feature.");
+		expect(result).not.toBeNull();
+	});
+
+	// Task 008: Dangerous character validation
+	it("rejects branch name with square brackets", () => {
+		const result = validateCreateWorktreeInput("/tmp/x", "feature[v1]");
+		expect(result).not.toBeNull();
+		if (result) expect(result.message).toContain("invalid characters");
+	});
+
+	it("rejects branch name with double quotes", () => {
+		const result = validateCreateWorktreeInput("/tmp/x", 'my"branch');
+		expect(result).not.toBeNull();
+	});
+
+	it("rejects branch name with single quotes", () => {
+		const result = validateCreateWorktreeInput("/tmp/x", "my'branch");
+		expect(result).not.toBeNull();
+	});
+
+	it("rejects branch name with backslash", () => {
+		const result = validateCreateWorktreeInput("/tmp/x", "feature\\name");
+		expect(result).not.toBeNull();
+	});
+
+	it("rejects branch name with asterisk", () => {
+		const result = validateCreateWorktreeInput("/tmp/x", "feature*name");
+		expect(result).not.toBeNull();
+	});
+
+	it("rejects branch name with question mark", () => {
+		const result = validateCreateWorktreeInput("/tmp/x", "feature?name");
+		expect(result).not.toBeNull();
+	});
+});
+
+describe("buildWorktreeAddCommand (Issue #4)", () => {
+	it("returns command with -b flag when create_branch is true", () => {
+		const result = buildWorktreeAddCommand({
+			path: "/tmp/new-worktree",
+			branch: "feature-x",
+			createBranch: true,
+		});
+
+		expect(result).toEqual([
+			"worktree",
+			"add",
+			"-b",
+			"feature-x",
+			"/tmp/new-worktree",
+		]);
+	});
+
+	it("returns command without -b flag when create_branch is false", () => {
+		const result = buildWorktreeAddCommand({
+			path: "/tmp/new-worktree",
+			branch: "existing-branch",
+			createBranch: false,
+		});
+
+		expect(result).toEqual([
+			"worktree",
+			"add",
+			"existing-branch",
+			"/tmp/new-worktree",
+		]);
+	});
+
+	it("defaults createBranch to true when not provided", () => {
+		const result = buildWorktreeAddCommand({
+			path: "/tmp/new-worktree",
+			branch: "new-feature",
+		});
+
+		expect(result).toContain("-b");
+	});
+
+	it("places path as the last argument", () => {
+		const result = buildWorktreeAddCommand({
+			path: "/tmp/new-worktree",
+			branch: "feature-x",
+			createBranch: true,
+		});
+
+		expect(result[result.length - 1]).toBe("/tmp/new-worktree");
+	});
+});
+
+describe("validatePathDoesNotExist (Issue #4)", () => {
+	it("returns null when path does not exist", () => {
+		const result = validatePathDoesNotExist(
+			"/tmp/nonexistent-path-abc123",
+			(p) => p === "/existing/path",
+		);
+		expect(result).toBeNull();
+	});
+
+	it("returns error with path in message when path exists", () => {
+		const existingPath = "/tmp/existing-worktree";
+		const result = validatePathDoesNotExist(existingPath, (p) => p === existingPath);
+
+		expect(result).not.toBeNull();
+		if (result) expect(result.message).toContain("/tmp/existing-worktree");
+		if (result) expect(result.message.toLowerCase()).toContain("already exists");
+	});
+
+	it("suggests choosing a different location", () => {
+		const existingPath = "/tmp/existing-worktree";
+		const result = validatePathDoesNotExist(existingPath, (p) => p === existingPath);
+
+		expect(result).not.toBeNull();
+		if (result) expect(result.suggestion?.toLowerCase()).toContain("different");
+	});
+});
+
+describe("isCurrentBranch (Issue #4)", () => {
+	it("returns true when branch matches current branch", () => {
+		expect(isCurrentBranch("main", "main")).toBe(true);
+	});
+
+	it("returns false when branch differs from current branch", () => {
+		expect(isCurrentBranch("feature-x", "main")).toBe(false);
+	});
+
+	it("handles branch names with slashes", () => {
+		expect(isCurrentBranch("feature/my-branch", "main")).toBe(false);
+		expect(isCurrentBranch("feature/my-branch", "feature/my-branch")).toBe(true);
+	});
+});
+
+// ─── Integration Tests for git_worktree_create execute flow (Task 004) ────
+
+describe("git_worktree_create execute flow (Issue #4)", () => {
+	// Minimal mock types matching what the extension needs
+	interface MockExecResult {
+		exitCode: number;
+		stdout: string;
+		stderr: string;
+	}
+
+	interface MockContext {
+		cwd: string;
+	}
+
+	interface MockPI {
+		exec: (cmd: string, args: string[], opts?: { cwd: string }) => Promise<MockExecResult>;
+	}
+
+	describe("validation pipeline ordering", () => {
+		it("rejects empty path before checking filesystem", async () => {
+			const result = validateCreateWorktreeInput("", "feature-x");
+			expect(result).not.toBeNull();
+			expect(result?.message).toContain("cannot be empty");
+		});
+
+		it("rejects invalid branch name before checking filesystem", async () => {
+			const result = validateCreateWorktreeInput("/tmp/x", "-bad-name");
+			expect(result).not.toBeNull();
+			expect(result?.message).toContain("invalid character");
+		});
+	});
+
+	describe("path existence check", () => {
+		it("returns error when path exists", () => {
+			const existingPath = "/tmp/existing-worktree";
+			const result = validatePathDoesNotExist(existingPath, (p) => p === existingPath);
+			expect(result).not.toBeNull();
+			expect(result?.message).toContain("/tmp/existing-worktree");
+			expect(result?.suggestion).toContain("different");
+		});
+
+		it("returns null when path does not exist", () => {
+			const result = validatePathDoesNotExist(
+				"/tmp/nonexistent-path-abc123",
+				(p) => p === "/existing/path",
+			);
+			expect(result).toBeNull();
+		});
+	});
+
+	describe("current branch check", () => {
+		it("detects when target matches current branch", () => {
+			expect(isCurrentBranch("main", "main")).toBe(true);
+		});
+
+		it("allows different branch from current", () => {
+			expect(isCurrentBranch("feature-x", "main")).toBe(false);
+		});
+	});
+
+	describe("command building after all validations pass", () => {
+		it("builds correct command with createBranch=true", () => {
+			const args = buildWorktreeAddCommand({
+				path: "/tmp/new-worktree",
+				branch: "feature-x",
+				createBranch: true,
+			});
+			expect(args).toEqual(["worktree", "add", "-b", "feature-x", "/tmp/new-worktree"]);
+		});
+
+		it("builds correct command with createBranch=false", () => {
+			const args = buildWorktreeAddCommand({
+				path: "/tmp/new-worktree",
+				branch: "existing-branch",
+				createBranch: false,
+			});
+			expect(args).toEqual(["worktree", "add", "existing-branch", "/tmp/new-worktree"]);
+		});
+
+		it("defaults createBranch to true when omitted", () => {
+			const args = buildWorktreeAddCommand({
+				path: "/tmp/new-worktree",
+				branch: "new-feature",
+			});
+			expect(args).toContain("-b");
+		});
+	});
+
+	describe("error formatting consistency", () => {
+		it("formats input validation error with suggestion", () => {
+			const result = validateCreateWorktreeInput("", "feature-x");
+			expect(result).not.toBeNull();
+			expect(result?.suggestion).toBeDefined();
+			expect(result?.suggestion).toContain("Provide a valid path");
+		});
+
+		it("formats path exists error with suggestion", () => {
+			const result = validatePathDoesNotExist("/tmp/x", () => true);
+			expect(result).not.toBeNull();
+			expect(result?.suggestion).toBeDefined();
+			expect(result?.suggestion).toContain("different");
+		});
+
+		it("formats branch name error with suggestion", () => {
+			const result = validateCreateWorktreeInput("/tmp/x", "bad name here");
+			expect(result).not.toBeNull();
+			expect(result?.suggestion).toBeDefined();
+			expect(result?.suggestion).toContain("hyphens");
+		});
+	});
+});
diff --git a/.pi/extensions/pi-worktree.ts b/.pi/extensions/pi-worktree.ts
index 15dd068..0ad2099 100644
--- a/.pi/extensions/pi-worktree.ts
+++ b/.pi/extensions/pi-worktree.ts
@@ -14,9 +14,17 @@
  */
 
 import type { ExtensionAPI, ExtensionContext } from "@earendil-works/pi-coding-agent";
+import { stat } from "node:fs/promises";
 import { defineTool } from "@earendil-works/pi-coding-agent";
 import { Type } from "@earendil-works/pi-ai";
 import { parseWorktreeList, type WorktreeEntry } from "./pi-worktree-parser";
+import {
+	validateCreateWorktreeInput,
+	buildWorktreeAddCommand,
+	validatePathDoesNotExist,
+	isCurrentBranch,
+	type WorktreeError,
+} from "./pi-worktree-create";
 
 // ─── Type Definitions ──────────────────────────────────────────────
 
@@ -30,14 +38,6 @@ interface WorktreeListResult {
 	currentPath: string;
 }
 
-/** Error returned when a worktree operation fails. */
-interface WorktreeError {
-	/** Human-readable error message from git or validation. */
-	message: string;
-	/** Suggested fix for the user. */
-	suggestion?: string;
-}
-
 // ─── Tool Parameter Schemas ────────────────────────────────────────
 
 const createWorktreeParams = Type.Object({
@@ -45,7 +45,7 @@ const createWorktreeParams = Type.Object({
 	branch: Type.Optional(Type.String({ description: "Branch name to check out. Defaults to 'main'." })),
 	create_branch: Type.Optional(
 		Type.Boolean({
-			description: "If true and branch doesn't exist, create it. Defaults to false.",
+			description: "If true and branch doesn't exist, create it. Defaults to true.",
 		}),
 	),
 });
@@ -60,26 +60,77 @@ const removeWorktreeParams = Type.Object({
 	path: Type.String({ description: "Path of the worktree to remove." }),
 });
 
-// ─── Helper Functions (stubs) ──────────────────────────────────────
+// ─── Helper Functions ──────────────────────────────────────────────
 
 /**
- * Check if the target path exists.
- * @param checkPath - Path to check
- * @param ctx - Extension context for file system access
+ * Check if the target path exists using Bun's fs module.
+ * @param checkPath - Path to check (resolved against cwd)
+ * @param ctx - Extension context for working directory
  * @returns True if the path exists
  */
 async function pathExists(checkPath: string, ctx: ExtensionContext): Promise<boolean> {
-	throw new Error("not implemented");
+	const resolvedPath = checkPath.startsWith("/") ? checkPath : `${ctx.cwd}/${checkPath}`;
+
+	try {
+		await stat(resolvedPath);
+		return true;
+	} catch {
+		return false;
+	}
+}
+
+/**
+ * Get the current branch name from git.
+ * @param pi - Extension API for executing commands
+ * @param ctx - Extension context for working directory
+ * @returns Current branch name, or "unknown" if detection fails
+ */
+async function getCurrentBranch(pi: ExtensionAPI, ctx: ExtensionContext): Promise<string> {
+	const result = await pi.exec("git", ["rev-parse", "--abbrev-ref", "HEAD"], { cwd: ctx.cwd });
+
+	if (result.exitCode !== 0) {
+		return "unknown";
+	}
+
+	return result.stdout.trim();
 }
 
 /**
  * Resolve a worktree target (path or branch name) to its full path.
+ *
+ * Used by git_worktree_switch and git_worktree_remove — dead code until those tools ship.
+ *
  * @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 {
-	throw new Error("not implemented");
+	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
+ * @returns Formatted error string with suggestion if available
+ */
+function formatValidationError(error: { message: string; suggestion?: string }): string {
+	const base = `Error: ${error.message}`;
+	return error.suggestion ? `${base} Suggestion: ${error.suggestion}` : base;
+}
+
+/**
+ * Format a success response for worktree operations.
+ * @param action - The action performed (e.g., "Created")
+ * @param details - Key-value pairs to include in the message
+ * @returns Formatted success string
+ */
+function formatSuccessResponse(action: string, details: Record<string, string>): string {
+	const parts = Object.entries(details).map(([key, value]) => `${key} '${value}'`);
+	return `${action} worktree ${parts.join(" on ")}.`;
 }
 
 // ─── Tool Factories ────────────────────────────────────────────────
@@ -137,19 +188,76 @@ function createListWorktreeTool(pi: ExtensionAPI) {
 
 /**
  * Create the git_worktree_create tool definition.
- * @param _pi - Extension API (unused in stub)
+ * @param pi - Extension API for registering and executing tools
  * @returns The configured tool definition
  */
-function createCreateWorktreeTool(_pi: ExtensionAPI) {
+function createCreateWorktreeTool(pi: ExtensionAPI) {
 	return defineTool({
 		name: "git_worktree_create",
 		label: "Git Worktree Create",
 		description: "Create a linked worktree for a branch in the current repo.",
 		parameters: createWorktreeParams,
-		async execute(_toolCallId, _params, _signal, _onUpdate, _ctx) {
+		async execute(_toolCallId, params, _signal, _onUpdate, ctx) {
+			const worktreePath = params.path;
+			const branchName = params.branch || "main";
+			const createBranch = params.create_branch ?? true;
+
+			// 1. Validate input parameters
+			const inputError = validateCreateWorktreeInput(worktreePath, branchName);
+			if (inputError) {
+				return {
+					content: [{ type: "text", text: formatValidationError(inputError) }],
+					isError: true,
+				};
+			}
+
+			// 2. Check if path already exists — refuse if so
+			const pathExistsResult = await pathExists(worktreePath, ctx);
+			const pathError = validatePathDoesNotExist(worktreePath, () => pathExistsResult);
+			if (pathError) {
+				return {
+					content: [{ type: "text", text: formatValidationError(pathError) }],
+					isError: true,
+				};
+			}
+
+			// 3. Check if creating on current branch — refuse if so
+			const currentBranch = await getCurrentBranch(pi, ctx);
+			if (isCurrentBranch(branchName, currentBranch)) {
+				const currentBranchError: WorktreeError = {
+					message: "Can't create worktree on the current branch — switch first.",
+					suggestion: "Choose a different branch name or switch branches with 'git switch'.",
+				};
+				return {
+					content: [{ type: "text", text: formatValidationError(currentBranchError) }],
+					isError: true,
+				};
+			}
+
+			// 4. Build and run git worktree add command
+			const commandArgs = buildWorktreeAddCommand({
+				path: worktreePath,
+				branch: branchName,
+				createBranch,
+			});
+
+			const result = await pi.exec("git", commandArgs, { cwd: ctx.cwd });
+
+			if (result.exitCode !== 0) {
+				return {
+					content: [{
+						type: "text",
+						text: `Error creating worktree: ${result.stderr}. Suggestion: Check that the branch exists (or use create_branch=true) and that the parent directory is writable.`,
+					}],
+					isError: true,
+				};
+			}
+
 			return {
-				content: [{ type: "text", text: "Error: git_worktree_create is not yet implemented. Suggestion: Use `git worktree add` manually until this tool is complete." }],
-				isError: true,
+				content: [{
+					type: "text",
+					text: formatSuccessResponse("Created", { at: worktreePath, branch: branchName }),
+				}],
 			};
 		},
 	});