diff --git a/src/__tests__/proxy-csp-blob-preview.test.ts b/src/__tests__/proxy-csp-blob-preview.test.ts
new file mode 100644
index 000000000..06811925f
--- /dev/null
+++ b/src/__tests__/proxy-csp-blob-preview.test.ts
@@ -0,0 +1,100 @@
+import { afterEach, beforeEach, describe, expect, it, vi } from "vitest";
+import { NextRequest } from "next/server";
+
+/**
+ * Refs #787 — the record fence made every vault preview load through
+ * `useFencedObjectUrl`, i.e. `blob:` URLs. The app CSP had no `frame-src`, so
+ * `default-src 'self'` blocked the PDF preview iframe loudly, and
+ * `img-src 'self' data:` degraded every blob thumbnail to an icon silently.
+ *
+ * These tests pin the fix exactly:
+ *   - `frame-src` exists and admits precisely 'self' and blob: — nothing else.
+ *   - `img-src` admits blob: alongside 'self' and data:.
+ *   - The boundaries that make blob: safe stay untouched: `object-src 'none'`,
+ *     `frame-ancestors 'none'`, `default-src 'self'`, nonce-bound script-src.
+ *   - The vault SERVE route keeps its own stricter document CSP unchanged.
+ *
+ * Security rationale (mirrors the comment in proxy.ts): a `blob:` URL can only
+ * be minted by same-origin script, and script execution stays nonce-bound, so
+ * admitting blob: admits exactly the app's own decrypted documents.
+ */
+
+vi.mock("@/lib/process-type", () => ({
+  shouldRunWeb: () => true,
+}));
+
+import { proxy } from "../proxy";
+
+function setNodeEnv(value: "development" | "production") {
+  vi.stubEnv("NODE_ENV", value);
+}
+
+beforeEach(() => {
+  setNodeEnv("production");
+});
+
+afterEach(() => {
+  vi.unstubAllEnvs();
+});
+
+function makeRequest(pathname: string): NextRequest {
+  return new NextRequest(`http://localhost${pathname}`, {
+    headers: { cookie: "healthlog_session=sess-1" },
+  });
+}
+
+/** Parse one directive's source list out of a CSP header string. */
+function directive(csp: string, name: string): string[] | null {
+  const match = csp
+    .split(";")
+    .map((part) => part.trim())
+    .find((part) => part.startsWith(`${name} `) || part === name);
+  if (!match) return null;
+  return match.split(/\s+/).slice(1);
+}
+
+describe("proxy.ts blob preview CSP (#787)", () => {
+  it("production frame-src admits exactly 'self' and blob:", () => {
+    const res = proxy(makeRequest("/documents"));
+    const csp = res.headers.get("content-security-policy") ?? "";
+    expect(directive(csp, "frame-src")).toEqual(["'self'", "blob:"]);
+  });
+
+  it("production img-src admits blob: alongside 'self' and data:", () => {
+    const res = proxy(makeRequest("/documents"));
+    const csp = res.headers.get("content-security-policy") ?? "";
+    const sources = directive(csp, "img-src");
+    expect(sources).toContain("blob:");
+    expect(sources).toContain("'self'");
+    expect(sources).toContain("data:");
+  });
+
+  it("production keeps the boundaries that make blob: safe", () => {
+    const res = proxy(makeRequest("/documents"));
+    const csp = res.headers.get("content-security-policy") ?? "";
+    expect(directive(csp, "object-src")).toEqual(["'none'"]);
+    expect(directive(csp, "frame-ancestors")).toEqual(["'none'"]);
+    expect(directive(csp, "default-src")).toEqual(["'self'"]);
+    expect(csp).toMatch(/script-src 'self' 'nonce-/);
+  });
+
+  it("development frame-src admits exactly 'self' and blob:", () => {
+    setNodeEnv("development");
+    const res = proxy(makeRequest("/documents"));
+    const csp = res.headers.get("content-security-policy") ?? "";
+    expect(directive(csp, "frame-src")).toEqual(["'self'", "blob:"]);
+  });
+
+  it("development img-src admits blob:", () => {
+    setNodeEnv("development");
+    const res = proxy(makeRequest("/documents"));
+    const csp = res.headers.get("content-security-policy") ?? "";
+    expect(directive(csp, "img-src")).toContain("blob:");
+  });
+
+  it("leaves the vault serve route's document CSP untouched (no frame-src, no blob:)", () => {
+    const res = proxy(makeRequest("/api/documents/inbound/doc-1/original"));
+    const csp = res.headers.get("content-security-policy") ?? "";
+    expect(csp).toBe("default-src 'none'; frame-ancestors 'self';");
+  });
+});
diff --git a/src/proxy.ts b/src/proxy.ts
index 1a2b435f5..dd95fdf28 100644
--- a/src/proxy.ts
+++ b/src/proxy.ts
@@ -528,9 +528,21 @@ export function proxy(request: NextRequest) {
       // HTML/SVG denied entirely, and attachment-only posture for every
       // non-passive format.
       `default-src 'none'; frame-ancestors 'self';`
-    : isDev
-      ? `default-src 'self'; script-src 'self' 'unsafe-eval' 'unsafe-inline'; style-src 'self' 'unsafe-inline'; img-src 'self' data:; connect-src 'self'; font-src 'self';`
-      : `default-src 'self'; script-src 'self' 'nonce-${nonce}'; style-src 'self' 'unsafe-inline'; img-src 'self' data:; connect-src 'self'${aiConnectSrc}${withingsConnectSrc}${whoopConnectSrc}${stravaConnectSrc}; font-src 'self'; object-src 'none'; base-uri 'self'; form-action 'self'; frame-ancestors 'none'; worker-src 'self'; report-uri ${cspReportEndpoint}; report-to csp-endpoint;`;
+    : // v1.37.15 — `frame-src 'self' blob:` + `blob:` in `img-src` (both dev
+      // and prod). Since the record fence (v1.37.0) every vault preview loads
+      // through `useFencedObjectUrl`, i.e. a `blob:` object URL: without a
+      // `frame-src` the PDF preview iframe fell through to `default-src
+      // 'self'` and was blocked loudly, and `img-src 'self' data:` degraded
+      // every blob thumbnail to its fallback icon silently. Admitting `blob:`
+      // is same-origin-tight: a blob: URL can only be minted by script running
+      // on this origin (`URL.createObjectURL`), script execution stays
+      // nonce-bound (prod), and `object-src 'none'` / `frame-ancestors
+      // 'none'` / `default-src 'self'` are unchanged — no foreign origin
+      // gains anything. Reverting the previews to direct `/api` src instead
+      // is barred by the record-fence guard.
+      isDev
+      ? `default-src 'self'; script-src 'self' 'unsafe-eval' 'unsafe-inline'; style-src 'self' 'unsafe-inline'; img-src 'self' data: blob:; frame-src 'self' blob:; connect-src 'self'; font-src 'self';`
+      : `default-src 'self'; script-src 'self' 'nonce-${nonce}'; style-src 'self' 'unsafe-inline'; img-src 'self' data: blob:; frame-src 'self' blob:; connect-src 'self'${aiConnectSrc}${withingsConnectSrc}${whoopConnectSrc}${stravaConnectSrc}; font-src 'self'; object-src 'none'; base-uri 'self'; form-action 'self'; frame-ancestors 'none'; worker-src 'self'; report-uri ${cspReportEndpoint}; report-to csp-endpoint;`;
   response.headers.set("Content-Security-Policy", csp);
 
   // Production-only headers. HSTS carries `preload` so the domain stays