diff --git a/tests/multi-model.test.ts b/tests/multi-model.test.ts
index 75c062b..959674e 100644
--- a/tests/multi-model.test.ts
+++ b/tests/multi-model.test.ts
@@ -14,8 +14,11 @@ import {
   sanitizeName,
   runCommand,
   getWorktreePath,
+  getSessionPath,
   undoWorktree,
   launchModelInWindow,
+  getWorktreesForSession,
+  getBinaryName,
 } from "../src/core/utils";
 
 let originalBun$: typeof Bun.$ = (globalThis as any).Bun?.$;
@@ -351,6 +354,19 @@ describe("multi-model launch", () => {
 });
 
 describe("core utils", () => {
+  let bunMock: ReturnType<typeof setupMockBun$>;
+
+  beforeEach(() => {
+    spyOn(os, "homedir").mockReturnValue("/mock/home");
+    bunMock = setupMockBun$();
+    executedCommands = [];
+    mockCommandResponses = {};
+  });
+
+  afterEach(() => {
+    restoreOriginalBun$();
+  });
+
   describe("shellQuote", () => {
     test("normal string without special characters", () => {
       expect(shellQuote("test")).toBe("'test'");
@@ -556,4 +572,138 @@ describe("core utils", () => {
       expect(sanitizeName("-test-session-")).toBe("test-session");
     });
   });
+
+  describe("runCommand", () => {
+    test("returns ok true for successful command", async () => {
+      mockCommandResponses["echo hello"] = { ok: true, stdout: "hello", stderr: "" };
+      const result = await runCommand(["echo", "hello"]);
+      expect(result.ok).toBe(true);
+      expect(result.stdout).toBe("hello");
+      expect(result.stderr).toBe("");
+    });
+
+    test("returns ok false for failed command", async () => {
+      mockCommandResponses["false"] = { ok: false, stdout: "", stderr: "error" };
+      const result = await runCommand(["false"]);
+      expect(result.ok).toBe(false);
+      expect(result.stderr).toBe("error");
+    });
+  });
+
+  describe("getWorktreePath", () => {
+    test("returns correct worktree path", () => {
+      spyOn(os, "homedir").mockReturnValue("/home/user");
+      const result = getWorktreePath("my-session", "gpt-4o");
+      expect(result).toBe("/home/user/.local/share/opencode/multi-model/my-session/gpt-4o");
+    });
+
+    test("handles nested session names", () => {
+      spyOn(os, "homedir").mockReturnValue("/home/user");
+      const result = getWorktreePath("parent/child", "window");
+      expect(result).toBe("/home/user/.local/share/opencode/multi-model/parent/child/window");
+    });
+  });
+
+  describe("getSessionPath", () => {
+    test("returns correct session path", () => {
+      spyOn(os, "homedir").mockReturnValue("/home/user");
+      const result = getSessionPath("my-session");
+      expect(result).toBe("/home/user/.local/share/opencode/multi-model/my-session");
+    });
+  });
+
+  describe("undoWorktree", () => {
+    test("returns empty array on success", async () => {
+      mockCommandResponses["git worktree remove -f /path/to/worktree"] = { ok: true, stdout: "", stderr: "" };
+      mockCommandResponses["git branch -D opencode/session/window"] = { ok: true, stdout: "", stderr: "" };
+
+      const errors = await undoWorktree("/path/to/worktree", "opencode/session/window");
+      expect(errors).toEqual([]);
+    });
+
+    test("returns error when worktree remove fails", async () => {
+      mockCommandResponses["git worktree remove -f /path/to/worktree"] = { ok: false, stdout: "", stderr: "remove failed" };
+      mockCommandResponses["git branch -D opencode/session/window"] = { ok: true, stdout: "", stderr: "" };
+
+      const errors = await undoWorktree("/path/to/worktree", "opencode/session/window");
+      expect(errors.length).toBe(1);
+      expect(errors[0]).toContain("Failed to remove worktree");
+    });
+
+    test("returns error when branch delete fails", async () => {
+      mockCommandResponses["git worktree remove -f /path/to/worktree"] = { ok: true, stdout: "", stderr: "" };
+      mockCommandResponses["git branch -D opencode/session/window"] = { ok: false, stdout: "", stderr: "branch delete failed" };
+
+      const errors = await undoWorktree("/path/to/worktree", "opencode/session/window");
+      expect(errors.length).toBe(1);
+      expect(errors[0]).toContain("Failed to delete branch");
+    });
+
+    test("returns both errors when both operations fail", async () => {
+      mockCommandResponses["git worktree remove -f /path/to/worktree"] = { ok: false, stdout: "", stderr: "remove failed" };
+      mockCommandResponses["git branch -D opencode/session/window"] = { ok: false, stdout: "", stderr: "branch delete failed" };
+
+      const errors = await undoWorktree("/path/to/worktree", "opencode/session/window");
+      expect(errors.length).toBe(2);
+    });
+  });
+
+  describe("launchModelInWindow", () => {
+    test("sends correct tmux command", async () => {
+      mockCommandResponses["tmux send-keys -t session:window opencode --model 'openai/gpt-4o' C-m"] = { ok: true, stdout: "", stderr: "" };
+
+      const result = await launchModelInWindow("session", { model: "openai/gpt-4o", windowName: "window" });
+      expect(result.ok).toBe(true);
+      expect(executedCommands).toContain("tmux send-keys -t session:window opencode --model 'openai/gpt-4o' C-m");
+    });
+
+    test("uses custom binary name", async () => {
+      mockCommandResponses["tmux send-keys -t session:window kilo --model 'openai/gpt-4o' C-m"] = { ok: true, stdout: "", stderr: "" };
+
+      const result = await launchModelInWindow("session", { model: "openai/gpt-4o", windowName: "window" }, "kilo");
+      expect(result.ok).toBe(true);
+      expect(executedCommands).toContain("tmux send-keys -t session:window kilo --model 'openai/gpt-4o' C-m");
+    });
+  });
+
+  describe("getWorktreesForSession", () => {
+    test("returns session-specific worktrees", async () => {
+      spyOn(os, "homedir").mockReturnValue("/home/user");
+      mockCommandResponses["git worktree list --porcelain"] = {
+        ok: true,
+        stdout: "worktree /home/user/.local/share/opencode/multi-model/my-session/gpt-4o\nbranch refs/heads/opencode/my-session/gpt-4o\n\nworktree /home/user/other-repo\nbranch refs/heads/main",
+        stderr: "",
+      };
+
+      const worktrees = await getWorktreesForSession("my-session");
+      expect(worktrees.length).toBe(1);
+      expect(worktrees[0]?.path).toBe("/home/user/.local/share/opencode/multi-model/my-session/gpt-4o");
+      expect(worktrees[0]?.branch).toBe("opencode/my-session/gpt-4o");
+    });
+
+    test("returns empty array when no matching worktrees", async () => {
+      spyOn(os, "homedir").mockReturnValue("/home/user");
+      mockCommandResponses["git worktree list --porcelain"] = {
+        ok: true,
+        stdout: "worktree /home/user/other-repo\nbranch refs/heads/main",
+        stderr: "",
+      };
+
+      const worktrees = await getWorktreesForSession("my-session");
+      expect(worktrees).toEqual([]);
+    });
+  });
+
+  describe("getBinaryName", () => {
+    test("returns default 'opencode' when env var not set", () => {
+      delete process.env.OPENCODE_MULTI_MODEL_BINARY;
+      expect(getBinaryName()).toBe("opencode");
+    });
+
+    test("returns env var value when set", () => {
+      process.env.OPENCODE_MULTI_MODEL_BINARY = "kilo";
+      expect(getBinaryName()).toBe("kilo");
+      delete process.env.OPENCODE_MULTI_MODEL_BINARY;
+    });
+  });
 });