diff --git a/src/pages/api/transactions/[id]/index.ts b/src/pages/api/transactions/[id]/index.ts
index 9d50928..f84ea5e 100644
--- a/src/pages/api/transactions/[id]/index.ts
+++ b/src/pages/api/transactions/[id]/index.ts
@@ -40,13 +40,14 @@ export const PUT: APIRoute = async ({ request, params }) => {
     // If account is changing, validate new account exists
     let newAccount = oldAccount;
     if (updates.accountId && updates.accountId !== oldTransaction.accountId) {
-      newAccount = accounts.find((a) => a.id === updates.accountId);
-      if (!newAccount) {
+      const foundAccount = accounts.find((a) => a.id === updates.accountId);
+      if (!foundAccount) {
         return new Response(JSON.stringify({ error: "Account not found" }), {
           status: 404,
           headers: { "Content-Type": "application/json" },
         });
       }
+      newAccount = foundAccount;
     }
 
     // First, remove the old transaction's effect on the old account
diff --git a/src/test/accounts.test.ts b/src/test/accounts.test.ts
index 6d2fdbf..47eeed6 100644
--- a/src/test/accounts.test.ts
+++ b/src/test/accounts.test.ts
@@ -2,11 +2,12 @@ import { describe, it, expect } from "vitest";
 import { GET as listAccounts } from "../pages/api/accounts/index";
 import { GET as getAccount } from "../pages/api/accounts/[id]/index";
 import { GET as listTransactions } from "../pages/api/accounts/[id]/transactions/index";
+import { createMockAPIContext } from "./setup";
 
 describe("Accounts API", () => {
   describe("GET /api/accounts", () => {
     it("should return all accounts", async () => {
-      const response = await listAccounts();
+      const response = await listAccounts(createMockAPIContext() as any);
       const accounts = await response.json();
 
       expect(response.status).toBe(200);
@@ -18,7 +19,9 @@ describe("Accounts API", () => {
 
   describe("GET /api/accounts/:id", () => {
     it("should return a specific account", async () => {
-      const response = await getAccount({ params: { id: "1" } });
+      const response = await getAccount(
+        createMockAPIContext({ params: { id: "1" } }) as any
+      );
       const account = await response.json();
 
       expect(response.status).toBe(200);
@@ -27,7 +30,9 @@ describe("Accounts API", () => {
     });
 
     it("should return 404 for non-existent account", async () => {
-      const response = await getAccount({ params: { id: "999" } });
+      const response = await getAccount(
+        createMockAPIContext({ params: { id: "999" } }) as any
+      );
       const error = await response.json();
 
       expect(response.status).toBe(404);
@@ -36,8 +41,10 @@ describe("Accounts API", () => {
   });
 
   describe("GET /api/accounts/:id/transactions", () => {
-    it("should return transactions for a specific account", async () => {
-      const response = await listTransactions({ params: { id: "1" } });
+    it("should return transactions for an account", async () => {
+      const response = await listTransactions(
+        createMockAPIContext({ params: { id: "1" } }) as any
+      );
       const transactions = await response.json();
 
       expect(response.status).toBe(200);
@@ -46,7 +53,9 @@ describe("Accounts API", () => {
     });
 
     it("should return empty array for account with no transactions", async () => {
-      const response = await listTransactions({ params: { id: "999" } });
+      const response = await listTransactions(
+        createMockAPIContext({ params: { id: "999" } }) as any
+      );
       const transactions = await response.json();
 
       expect(response.status).toBe(200);
diff --git a/src/test/setup.ts b/src/test/setup.ts
index 2a40360..da93d5a 100644
--- a/src/test/setup.ts
+++ b/src/test/setup.ts
@@ -1,5 +1,31 @@
 import { beforeEach } from "vitest";
 import { accounts, transactions } from "../data/store";
+import type { APIContext } from "astro";
+
+// Create a mock APIContext factory
+export function createMockAPIContext<
+  T extends Record<string, string> = Record<string, string>
+>({ params = {} as T } = {}): Partial<APIContext> {
+  return {
+    params,
+    props: {},
+    request: new Request("http://localhost:4321"),
+    site: new URL("http://localhost:4321"),
+    generator: "test",
+    url: new URL("http://localhost:4321"),
+    clientAddress: "127.0.0.1",
+    cookies: new Headers() as any, // Cast Headers to cookies as we don't need cookie functionality in tests
+    redirect: () => new Response(),
+    locals: {},
+    preferredLocale: undefined,
+    preferredLocaleList: [],
+    currentLocale: undefined,
+    routePattern: "/api/[...path]",
+    originPathname: "/api",
+    getActionResult: () => undefined,
+    isPrerendered: false,
+  };
+}
 
 // Reset test data before each test
 beforeEach(() => {
diff --git a/src/test/transactions.test.ts b/src/test/transactions.test.ts
index 1bbf9bc..f6b4dee 100644
--- a/src/test/transactions.test.ts
+++ b/src/test/transactions.test.ts
@@ -6,6 +6,7 @@ import {
 } from "../pages/api/transactions/[id]/index";
 import { accounts, transactions } from "../data/store";
 import type { Transaction } from "../types";
+import { createMockAPIContext } from "./setup";
 
 describe("Transactions API", () => {
   describe("POST /api/transactions", () => {
@@ -18,14 +19,14 @@ describe("Transactions API", () => {
         amount: -25.0,
       };
 
-      const response = await createTransaction({
-        request: new Request("http://localhost:4321/api/transactions", {
-          method: "POST",
-          headers: { "Content-Type": "application/json" },
-          body: JSON.stringify(newTransaction),
-        }),
+      const ctx = createMockAPIContext() as any;
+      ctx.request = new Request("http://localhost:4321/api/transactions", {
+        method: "POST",
+        headers: { "Content-Type": "application/json" },
+        body: JSON.stringify(newTransaction),
       });
 
+      const response = await createTransaction(ctx);
       const result = await response.json();
 
       expect(response.status).toBe(201);
@@ -40,14 +41,14 @@ describe("Transactions API", () => {
         // Missing required fields
       };
 
-      const response = await createTransaction({
-        request: new Request("http://localhost:4321/api/transactions", {
-          method: "POST",
-          headers: { "Content-Type": "application/json" },
-          body: JSON.stringify(invalidTransaction),
-        }),
+      const ctx = createMockAPIContext() as any;
+      ctx.request = new Request("http://localhost:4321/api/transactions", {
+        method: "POST",
+        headers: { "Content-Type": "application/json" },
+        body: JSON.stringify(invalidTransaction),
       });
 
+      const response = await createTransaction(ctx);
       const error = await response.json();
 
       expect(response.status).toBe(400);
@@ -56,20 +57,20 @@ describe("Transactions API", () => {
 
     it("should reject transaction with invalid account", async () => {
       const invalidTransaction = {
-        accountId: "999", // Non-existent account
+        accountId: "999",
         date: "2025-04-24",
         description: "Invalid Account Test",
         amount: 100,
       };
 
-      const response = await createTransaction({
-        request: new Request("http://localhost:4321/api/transactions", {
-          method: "POST",
-          headers: { "Content-Type": "application/json" },
-          body: JSON.stringify(invalidTransaction),
-        }),
+      const ctx = createMockAPIContext() as any;
+      ctx.request = new Request("http://localhost:4321/api/transactions", {
+        method: "POST",
+        headers: { "Content-Type": "application/json" },
+        body: JSON.stringify(invalidTransaction),
       });
 
+      const response = await createTransaction(ctx);
       const error = await response.json();
 
       expect(response.status).toBe(404);
@@ -77,14 +78,14 @@ describe("Transactions API", () => {
     });
 
     it("should reject invalid request body", async () => {
-      const response = await createTransaction({
-        request: new Request("http://localhost:4321/api/transactions", {
-          method: "POST",
-          headers: { "Content-Type": "application/json" },
-          body: "invalid json",
-        }),
+      const ctx = createMockAPIContext() as any;
+      ctx.request = new Request("http://localhost:4321/api/transactions", {
+        method: "POST",
+        headers: { "Content-Type": "application/json" },
+        body: "invalid json",
       });
 
+      const response = await createTransaction(ctx);
       const error = await response.json();
 
       expect(response.status).toBe(400);
@@ -101,15 +102,14 @@ describe("Transactions API", () => {
         amount: -75.0,
       };
 
-      const response = await updateTransaction({
-        params: { id: "1" },
-        request: new Request("http://localhost:4321/api/transactions/1", {
-          method: "PUT",
-          headers: { "Content-Type": "application/json" },
-          body: JSON.stringify(updates),
-        }),
+      const ctx = createMockAPIContext({ params: { id: "1" } }) as any;
+      ctx.request = new Request("http://localhost:4321/api/transactions/1", {
+        method: "PUT",
+        headers: { "Content-Type": "application/json" },
+        body: JSON.stringify(updates),
       });
 
+      const response = await updateTransaction(ctx);
       const result = await response.json();
 
       expect(response.status).toBe(200);
@@ -121,15 +121,14 @@ describe("Transactions API", () => {
     });
 
     it("should reject update with invalid request body", async () => {
-      const response = await updateTransaction({
-        params: { id: "1" },
-        request: new Request("http://localhost:4321/api/transactions/1", {
-          method: "PUT",
-          headers: { "Content-Type": "application/json" },
-          body: "invalid json",
-        }),
+      const ctx = createMockAPIContext({ params: { id: "1" } }) as any;
+      ctx.request = new Request("http://localhost:4321/api/transactions/1", {
+        method: "PUT",
+        headers: { "Content-Type": "application/json" },
+        body: "invalid json",
       });
 
+      const response = await updateTransaction(ctx);
       const error = await response.json();
 
       expect(response.status).toBe(400);
@@ -137,15 +136,14 @@ describe("Transactions API", () => {
     });
 
     it("should reject update for non-existent transaction", async () => {
-      const response = await updateTransaction({
-        params: { id: "999" },
-        request: new Request("http://localhost:4321/api/transactions/999", {
-          method: "PUT",
-          headers: { "Content-Type": "application/json" },
-          body: JSON.stringify({ description: "Test" }),
-        }),
+      const ctx = createMockAPIContext({ params: { id: "999" } }) as any;
+      ctx.request = new Request("http://localhost:4321/api/transactions/999", {
+        method: "PUT",
+        headers: { "Content-Type": "application/json" },
+        body: JSON.stringify({ description: "Test" }),
       });
 
+      const response = await updateTransaction(ctx);
       const error = await response.json();
 
       expect(response.status).toBe(404);
@@ -156,15 +154,14 @@ describe("Transactions API", () => {
       // First update the transaction to point to a non-existent account
       transactions[0].accountId = "999";
 
-      const response = await updateTransaction({
-        params: { id: "1" },
-        request: new Request("http://localhost:4321/api/transactions/1", {
-          method: "PUT",
-          headers: { "Content-Type": "application/json" },
-          body: JSON.stringify({ amount: -100 }),
-        }),
+      const ctx = createMockAPIContext({ params: { id: "1" } }) as any;
+      ctx.request = new Request("http://localhost:4321/api/transactions/1", {
+        method: "PUT",
+        headers: { "Content-Type": "application/json" },
+        body: JSON.stringify({ amount: -100 }),
       });
 
+      const response = await updateTransaction(ctx);
       const error = await response.json();
 
       expect(response.status).toBe(404);
@@ -183,19 +180,17 @@ describe("Transactions API", () => {
       const oldTransaction = transactions.find((t) => t.id === "1");
       if (!oldTransaction) throw new Error("Test transaction not found");
 
-      // Update transaction to move it to a different account
-      const response = await updateTransaction({
-        params: { id: "1" },
-        request: new Request("http://localhost:4321/api/transactions/1", {
-          method: "PUT",
-          headers: { "Content-Type": "application/json" },
-          body: JSON.stringify({
-            accountId: newAccount.id,
-            amount: -100,
-          }),
+      const ctx = createMockAPIContext({ params: { id: "1" } }) as any;
+      ctx.request = new Request("http://localhost:4321/api/transactions/1", {
+        method: "PUT",
+        headers: { "Content-Type": "application/json" },
+        body: JSON.stringify({
+          accountId: newAccount.id,
+          amount: -100,
         }),
       });
 
+      const response = await updateTransaction(ctx);
       const result = await response.json();
 
       expect(response.status).toBe(200);
@@ -211,15 +206,17 @@ describe("Transactions API", () => {
     });
 
     it("should reject update without transaction ID", async () => {
-      const response = await updateTransaction({
-        params: {},
-        request: new Request("http://localhost:4321/api/transactions/undefined", {
+      const ctx = createMockAPIContext() as any;
+      ctx.request = new Request(
+        "http://localhost:4321/api/transactions/undefined",
+        {
           method: "PUT",
           headers: { "Content-Type": "application/json" },
           body: JSON.stringify({ description: "Test" }),
-        }),
-      });
+        }
+      );
 
+      const response = await updateTransaction(ctx);
       const error = await response.json();
 
       expect(response.status).toBe(400);
@@ -231,15 +228,14 @@ describe("Transactions API", () => {
       const savedAccounts = [...accounts];
       accounts.length = 0;
 
-      const response = await updateTransaction({
-        params: { id: "1" },
-        request: new Request("http://localhost:4321/api/transactions/1", {
-          method: "PUT",
-          headers: { "Content-Type": "application/json" },
-          body: JSON.stringify({ amount: -100 }),
-        }),
+      const ctx = createMockAPIContext({ params: { id: "1" } }) as any;
+      ctx.request = new Request("http://localhost:4321/api/transactions/1", {
+        method: "PUT",
+        headers: { "Content-Type": "application/json" },
+        body: JSON.stringify({ amount: -100 }),
       });
 
+      const response = await updateTransaction(ctx);
       const error = await response.json();
 
       expect(response.status).toBe(404);
@@ -250,18 +246,17 @@ describe("Transactions API", () => {
     });
 
     it("should reject update when new account doesn't exist", async () => {
-      const response = await updateTransaction({
-        params: { id: "1" },
-        request: new Request("http://localhost:4321/api/transactions/1", {
-          method: "PUT",
-          headers: { "Content-Type": "application/json" },
-          body: JSON.stringify({
-            accountId: "999", // Non-existent account
-            amount: -100,
-          }),
+      const ctx = createMockAPIContext({ params: { id: "1" } }) as any;
+      ctx.request = new Request("http://localhost:4321/api/transactions/1", {
+        method: "PUT",
+        headers: { "Content-Type": "application/json" },
+        body: JSON.stringify({
+          accountId: "999", // Non-existent account
+          amount: -100,
         }),
       });
 
+      const response = await updateTransaction(ctx);
       const error = await response.json();
 
       expect(response.status).toBe(404);
@@ -275,9 +270,9 @@ describe("Transactions API", () => {
       const transactionAmount = transactions[0].amount;
       const initialCount = transactions.length;
 
-      const response = await deleteTransaction({
-        params: { id: "1" },
-      });
+      const response = await deleteTransaction(
+        createMockAPIContext({ params: { id: "1" } }) as any
+      );
 
       expect(response.status).toBe(204);
       expect(transactions).toHaveLength(initialCount - 1);
@@ -285,9 +280,7 @@ describe("Transactions API", () => {
     });
 
     it("should reject delete without transaction ID", async () => {
-      const response = await deleteTransaction({
-        params: {},
-      });
+      const response = await deleteTransaction(createMockAPIContext() as any);
 
       const error = await response.json();
 
@@ -296,9 +289,9 @@ describe("Transactions API", () => {
     });
 
     it("should return 404 for non-existent transaction", async () => {
-      const response = await deleteTransaction({
-        params: { id: "999" },
-      });
+      const response = await deleteTransaction(
+        createMockAPIContext({ params: { id: "999" } }) as any
+      );
 
       const error = await response.json();
 
@@ -317,9 +310,9 @@ describe("Transactions API", () => {
       };
       transactions.push(testTransaction);
 
-      const response = await deleteTransaction({
-        params: { id: "test-delete" },
-      });
+      const response = await deleteTransaction(
+        createMockAPIContext({ params: { id: "test-delete" } }) as any
+      );
 
       const error = await response.json();