diff --git a/.changeset/setup-url-guard.md b/.changeset/setup-url-guard.md
new file mode 100644
index 000000000..cb0542909
--- /dev/null
+++ b/.changeset/setup-url-guard.md
@@ -0,0 +1,5 @@
+---
+'@adcp/client': patch
+---
+
+Response builders now throw a descriptive error when `setup` is placed at the top level of a media buy response. The IO-signing setup URL belongs inside `account.setup` (a field on `Account`), not on the media buy itself. This was a silent trap because `DomainHandler` accepts `Record<string, unknown>` so the strict type wasn't catching it. Affects `mediaBuyResponse`, `updateMediaBuyResponse`, and `getMediaBuysResponse`.
diff --git a/src/lib/server/responses.ts b/src/lib/server/responses.ts
index 1a9e77b42..35e123337 100644
--- a/src/lib/server/responses.ts
+++ b/src/lib/server/responses.ts
@@ -68,6 +68,21 @@ export function toStructuredContent(data: object): Record<string, unknown> {
   return data as unknown as Record<string, unknown>;
 }
 
+// `setup` is only ever nested inside an `Account` (the IO-signing / pending_approval
+// path). A top-level `setup` on a media buy response means the builder read the
+// storyboard's "setup.url" shorthand as a top-level field. The strict handler types
+// would catch this, but `DomainHandler` accepts `Record<string, unknown>` for DX,
+// so the error has to move to runtime.
+function assertNoTopLevelSetup(data: unknown, builder: string): void {
+  if (data != null && typeof data === 'object' && 'setup' in data) {
+    throw new Error(
+      `${builder}: \`setup\` is not a field on the media buy — it belongs inside \`account.setup\`. ` +
+        `Move \`{ setup: { url, message } }\` to \`{ account: { ..., setup: { url, message } } }\`. ` +
+        `The setup URL is a property of the Account (returned alongside \`status: 'pending_approval'\`), not the MediaBuy.`
+    );
+  }
+}
+
 /**
  * Build a get_adcp_capabilities response.
  *
@@ -101,6 +116,7 @@ export function productsResponse(data: GetProductsResponse, summary?: string): M
  *   `status` is provided but `valid_actions` is not
  */
 export function mediaBuyResponse(data: CreateMediaBuySuccess, summary?: string): McpToolResponse {
+  assertNoTopLevelSetup(data, 'mediaBuyResponse');
   const withDefaults = { ...data };
   if (withDefaults.revision === undefined) {
     withDefaults.revision = 1;
@@ -161,6 +177,7 @@ export function listCreativeFormatsResponse(data: ListCreativeFormatsResponse, s
  * `valid_actions` from `validActionsForStatus()`.
  */
 export function updateMediaBuyResponse(data: UpdateMediaBuySuccess, summary?: string): McpToolResponse {
+  assertNoTopLevelSetup(data, 'updateMediaBuyResponse');
   const withDefaults = { ...data };
   if (withDefaults.valid_actions === undefined && withDefaults.status != null) {
     withDefaults.valid_actions = validActionsForStatus(withDefaults.status);
@@ -175,6 +192,11 @@ export function updateMediaBuyResponse(data: UpdateMediaBuySuccess, summary?: st
  * Build a get_media_buys response.
  */
 export function getMediaBuysResponse(data: GetMediaBuysResponse, summary?: string): McpToolResponse {
+  if (Array.isArray(data.media_buys)) {
+    for (const buy of data.media_buys) {
+      assertNoTopLevelSetup(buy, 'getMediaBuysResponse');
+    }
+  }
   return {
     content: [
       {
diff --git a/test/server-responses.test.js b/test/server-responses.test.js
index 2b42f98f9..931012a5e 100644
--- a/test/server-responses.test.js
+++ b/test/server-responses.test.js
@@ -107,6 +107,34 @@ describe('mediaBuyResponse', () => {
     });
     assert.deepStrictEqual(result.structuredContent.valid_actions, ['cancel']);
   });
+
+  it('throws when `setup` is placed at the top level instead of inside account', () => {
+    assert.throws(
+      () =>
+        mediaBuyResponse({
+          media_buy_id: 'mb_1',
+          packages: [],
+          status: 'pending_approval',
+          setup: { url: 'https://example.com/sign', message: 'Review IO' },
+        }),
+      /`setup` is not a field on the media buy.*belongs inside `account\.setup`/
+    );
+  });
+
+  it('accepts setup nested under account', () => {
+    const result = mediaBuyResponse({
+      media_buy_id: 'mb_1',
+      packages: [],
+      status: 'pending_approval',
+      account: {
+        account_id: 'acct_1',
+        name: 'Acme',
+        status: 'pending_approval',
+        setup: { url: 'https://example.com/sign', message: 'Review IO' },
+      },
+    });
+    assert.strictEqual(result.structuredContent.account.setup.url, 'https://example.com/sign');
+  });
 });
 
 describe('deliveryResponse', () => {
@@ -172,6 +200,18 @@ describe('updateMediaBuyResponse', () => {
     });
     assert.deepStrictEqual(result.structuredContent.valid_actions, ['cancel']);
   });
+
+  it('throws when `setup` is placed at the top level instead of inside account', () => {
+    assert.throws(
+      () =>
+        updateMediaBuyResponse({
+          media_buy_id: 'mb_1',
+          status: 'pending_approval',
+          setup: { url: 'https://example.com/sign', message: 'Review IO' },
+        }),
+      /`setup` is not a field on the media buy.*belongs inside `account\.setup`/
+    );
+  });
 });
 
 describe('getMediaBuysResponse', () => {
@@ -179,6 +219,23 @@ describe('getMediaBuysResponse', () => {
     const result = getMediaBuysResponse({ media_buys: [{ media_buy_id: 'mb_1' }] });
     assert.strictEqual(result.content[0].text, 'Found 1 media buy');
   });
+
+  it('throws when any media buy has `setup` at the top level', () => {
+    assert.throws(
+      () =>
+        getMediaBuysResponse({
+          media_buys: [
+            { media_buy_id: 'mb_1' },
+            {
+              media_buy_id: 'mb_2',
+              status: 'pending_approval',
+              setup: { url: 'https://example.com/sign', message: 'Review IO' },
+            },
+          ],
+        }),
+      /getMediaBuysResponse.*`setup` is not a field on the media buy/
+    );
+  });
 });
 
 describe('performanceFeedbackResponse', () => {