diff --git a/frontend/editor/src/core/services/httpErrorHandler.test.ts b/frontend/editor/src/core/services/httpErrorHandler.test.ts
new file mode 100644
index 0000000000..8228cba831
--- /dev/null
+++ b/frontend/editor/src/core/services/httpErrorHandler.test.ts
@@ -0,0 +1,54 @@
+import { describe, expect, test, vi, beforeEach } from "vitest";
+import { handleHttpError } from "@app/services/httpErrorHandler";
+
+const alertMock = vi.hoisted(() => vi.fn());
+vi.mock("@app/components/toast", () => ({
+  alert: (options: unknown) => alertMock(options),
+}));
+
+// Reproduces a tool POST: responseType "blob" makes error.response.data a Blob.
+// jsdom's Blob has no .text(), so mirror the browser API the decode path uses.
+function blobAxiosError(status: number, body: string, contentType: string) {
+  return {
+    isAxiosError: true,
+    config: { url: "/api/v1/general/merge-pdfs" },
+    response: {
+      status,
+      statusText: "",
+      data: { type: contentType, text: () => Promise.resolve(body) },
+    },
+  };
+}
+
+describe("handleHttpError — blob error bodies", () => {
+  beforeEach(() => {
+    alertMock.mockClear();
+  });
+
+  test("shows the decoded server message from a JSON blob body", async () => {
+    await handleHttpError(
+      blobAxiosError(
+        500,
+        JSON.stringify({ message: "JPDFium merge failed" }),
+        "application/json",
+      ),
+    );
+
+    expect(alertMock).toHaveBeenCalledTimes(1);
+    expect(alertMock.mock.calls[0][0]).toMatchObject({
+      title: "Server error",
+      body: "JPDFium merge failed",
+    });
+  });
+
+  test("shows the decoded server message from a plain-text blob body", async () => {
+    await handleHttpError(
+      blobAxiosError(500, "JPDFium merge failed", "text/plain"),
+    );
+
+    expect(alertMock).toHaveBeenCalledTimes(1);
+    expect(alertMock.mock.calls[0][0]).toMatchObject({
+      body: "JPDFium merge failed",
+    });
+  });
+});
diff --git a/frontend/editor/src/core/services/httpErrorHandler.ts b/frontend/editor/src/core/services/httpErrorHandler.ts
index 55b0a425f9..81bdd1cbd6 100644
--- a/frontend/editor/src/core/services/httpErrorHandler.ts
+++ b/frontend/editor/src/core/services/httpErrorHandler.ts
@@ -147,10 +147,9 @@ export async function handleHttpError(error: any): Promise<boolean> {
 
   if (handleSaaSError(error)) return true;
 
-  // Compute title/body (friendly) from the error object
-  const { title, body } = extractAxiosErrorMessage(error);
-
-  // Normalize response data ONCE, reuse for both ID extraction and special-toast matching
+  // Decode response data ONCE. Tool POSTs use responseType "blob", so
+  // error.response.data is an unread Blob here. The decoded body drives the
+  // toast message, file-ID extraction, and special-toast matching alike.
   const raw = error?.response?.data as any;
   let normalized: unknown = raw;
   try {
@@ -159,6 +158,9 @@ export async function handleHttpError(error: any): Promise<boolean> {
     console.debug("normalizeAxiosErrorData", e);
   }
 
+  // Compute title/body (friendly) from the decoded body, not the raw Blob.
+  const { title, body } = extractAxiosErrorMessage(error, normalized);
+
   // 1) If server sends structured file IDs for failures, also mark them errored in UI
   try {
     const ids = extractErrorFileIds(normalized);
diff --git a/frontend/editor/src/core/services/httpErrorUtils.ts b/frontend/editor/src/core/services/httpErrorUtils.ts
index b3b54e2664..60ec75bb8f 100644
--- a/frontend/editor/src/core/services/httpErrorUtils.ts
+++ b/frontend/editor/src/core/services/httpErrorUtils.ts
@@ -25,7 +25,10 @@ function titleForStatus(status?: number): string {
   return "Request failed";
 }
 
-export function extractAxiosErrorMessage(error: any): {
+export function extractAxiosErrorMessage(
+  error: any,
+  decodedData?: unknown,
+): {
   title: string;
   body: string;
 } {
@@ -33,7 +36,10 @@ export function extractAxiosErrorMessage(error: any): {
     const status = error.response?.status;
     const _statusText = error.response?.statusText || "";
     let parsed: any = undefined;
-    const raw = error.response?.data;
+    // Tool POSTs use responseType "blob", so error.response.data is an unread
+    // Blob. Prefer the decoded body when the caller supplies it, otherwise the
+    // message falls back to the generic text and the server reason is lost.
+    const raw = decodedData !== undefined ? decodedData : error.response?.data;
     if (typeof raw === "string") {
       try {
         parsed = JSON.parse(raw);