diff --git a/src/browser/history/CaptureFormatError.ts b/src/browser/history/CaptureFormatError.ts
index c3e4da9b3e..0fdc026909 100644
--- a/src/browser/history/CaptureFormatError.ts
+++ b/src/browser/history/CaptureFormatError.ts
@@ -9,7 +9,8 @@ export class CaptureFormatError extends Error {
       | "limit",
     message: string,
     readonly pointer = "",
+    cause?: unknown,
   ) {
-    super(message);
+    super(message, cause === undefined ? undefined : { cause });
   }
 }
diff --git a/src/browser/history/SazMetadata.test.ts b/src/browser/history/SazMetadata.test.ts
index 4aa482a7a4..54126937c1 100644
--- a/src/browser/history/SazMetadata.test.ts
+++ b/src/browser/history/SazMetadata.test.ts
@@ -2,6 +2,50 @@ import { expect, it } from "vitest";
 import { CaptureFormatError } from "./CaptureFormatError.js";
 import { decodeSazMetadata } from "./SazMetadata.js";
 
+it.each([
+  ["UTF-8", Buffer.from('<?xml version="1.0" encoding="UTF-8"?><Session/>')],
+  [
+    "BOM-marked UTF-16LE",
+    Buffer.concat([
+      Buffer.from([0xff, 0xfe]),
+      Buffer.from(
+        '<?xml version="1.0" encoding="UTF-16"?><Session/>',
+        "utf16le",
+      ),
+    ]),
+  ],
+] as const)("accepts matching %s XML declarations", (_label, bytes) => {
+  expect(() => decodeSazMetadata(bytes)).not.toThrow();
+});
+
+it.each([
+  [
+    "UTF-16 declaration on UTF-8 bytes",
+    Buffer.from('<?xml version="1.0" encoding="UTF-16"?><Session/>'),
+    "format",
+  ],
+  [
+    "UTF-8 declaration on UTF-16 bytes",
+    Buffer.concat([
+      Buffer.from([0xff, 0xfe]),
+      Buffer.from(
+        '<?xml version="1.0" encoding="UTF-8"?><Session/>',
+        "utf16le",
+      ),
+    ]),
+    "format",
+  ],
+  [
+    "unsupported declared encoding",
+    Buffer.from('<?xml version="1.0" encoding="ISO-8859-1"?><Session/>'),
+    "unsupported",
+  ],
+] as const)("rejects %s", (_label, bytes, reason) => {
+  expect(() => decodeSazMetadata(bytes)).toThrow(
+    expect.objectContaining({ reason }),
+  );
+});
+
 it("retains U+FFFD in well-formed metadata as an ordinary XML character", () => {
   const decoded = JSON.stringify(
     decodeSazMetadata(
diff --git a/src/browser/history/SazMetadata.ts b/src/browser/history/SazMetadata.ts
index a5dd239bd0..5ae8d950b7 100644
--- a/src/browser/history/SazMetadata.ts
+++ b/src/browser/history/SazMetadata.ts
@@ -2,23 +2,43 @@ import { DOMParser, ParseError, type Node } from "@xmldom/xmldom";
 import { SaxesParser } from "saxes";
 import type { JsonValue } from "../../domain/jsonValue.js";
 import { WEB_NETWORK_CAPTURE_LIMITS } from "../../domain/webNetworkCapture.js";
+import {
+  decodeXmlPlistText,
+  XmlPlistTextDecodeError,
+} from "../../domain/propertyListXmlText.js";
 import { CaptureFormatError } from "./CaptureFormatError.js";
 
 /** Preserve XML node order, attribute names and text without interpreting producer-specific flags. */
 export const decodeSazMetadata = (bytes: Buffer): JsonValue => {
   let xml: string;
   try {
-    const encoding =
-      bytes[0] === 0xff && bytes[1] === 0xfe
-        ? "utf-16le"
-        : bytes[0] === 0xfe && bytes[1] === 0xff
-          ? "utf-16be"
-          : "utf-8";
-    xml = new TextDecoder(encoding, { fatal: true }).decode(bytes);
-  } catch {
+    xml = decodeXmlPlistText(bytes);
+  } catch (cause: unknown) {
+    if (
+      cause instanceof XmlPlistTextDecodeError &&
+      cause.reason === "unsupported-encoding"
+    )
+      throw new CaptureFormatError(
+        "unsupported",
+        `SAZ metadata declares unsupported XML encoding ${cause.declaredEncoding}.`,
+        "",
+        cause,
+      );
+    if (
+      cause instanceof XmlPlistTextDecodeError &&
+      cause.reason === "encoding-mismatch"
+    )
+      throw new CaptureFormatError(
+        "format",
+        "SAZ metadata encoding declaration disagrees with its bytes.",
+        "",
+        cause,
+      );
     throw new CaptureFormatError(
       "unsupported",
       "SAZ metadata must use UTF-8 or BOM-marked UTF-16.",
+      "",
+      cause,
     );
   }
   validateSazXml(xml);
diff --git a/src/domain/propertyListXmlText.test.ts b/src/domain/propertyListXmlText.test.ts
new file mode 100644
index 0000000000..8cae86b390
--- /dev/null
+++ b/src/domain/propertyListXmlText.test.ts
@@ -0,0 +1,25 @@
+import { expect, it } from "vitest";
+
+import {
+  decodeXmlPlistText,
+  XmlPlistTextDecodeError,
+} from "./propertyListXmlText.js";
+
+it.each([
+  [
+    Buffer.from('<?xml version="1.0" encoding="UTF-16"?><plist/>'),
+    "encoding-mismatch",
+  ],
+  [
+    Buffer.from('<?xml version="1.0" encoding="ISO-8859-1"?><plist/>'),
+    "unsupported-encoding",
+  ],
+  [Buffer.from([0xff, 0xfe, 0x3c]), "invalid-bytes"],
+] as const)("tags XML decoding failures as %s", (bytes, reason) => {
+  expect(() => decodeXmlPlistText(bytes)).toThrow(
+    expect.objectContaining({
+      name: "XmlPlistTextDecodeError",
+      reason,
+    } satisfies Partial<XmlPlistTextDecodeError>),
+  );
+});
diff --git a/src/domain/propertyListXmlText.ts b/src/domain/propertyListXmlText.ts
index 2d6820120f..8cf52d772d 100644
--- a/src/domain/propertyListXmlText.ts
+++ b/src/domain/propertyListXmlText.ts
@@ -1,4 +1,19 @@
-/** Decode BOM-aware XML plist text with fatal byte validation. */
+export class XmlPlistTextDecodeError extends TypeError {
+  constructor(
+    readonly reason:
+      | "invalid-bytes"
+      | "unsupported-encoding"
+      | "encoding-mismatch",
+    message: string,
+    readonly declaredEncoding?: string,
+    options?: ErrorOptions,
+  ) {
+    super(message, options);
+    this.name = "XmlPlistTextDecodeError";
+  }
+}
+
+/** Decode BOM-aware XML plist text and verify any declaration against its bytes. */
 export const decodeXmlPlistText = (bytes: Uint8Array): string => {
   const encoding =
     bytes[0] === 0xff && bytes[1] === 0xfe
@@ -6,7 +21,17 @@ export const decodeXmlPlistText = (bytes: Uint8Array): string => {
       : bytes[0] === 0xfe && bytes[1] === 0xff
         ? "utf-16be"
         : "utf-8";
-  const text = new TextDecoder(encoding, { fatal: true }).decode(bytes);
+  let text: string;
+  try {
+    text = new TextDecoder(encoding, { fatal: true }).decode(bytes);
+  } catch (cause: unknown) {
+    throw new XmlPlistTextDecodeError(
+      "invalid-bytes",
+      "XML text bytes are invalid for the detected encoding",
+      undefined,
+      { cause },
+    );
+  }
   const declaration = /^<\?xml\s[^?]*\bencoding\s*=\s*["']([^"']+)["']/u.exec(
     text,
   )?.[1];
@@ -15,14 +40,20 @@ export const decodeXmlPlistText = (bytes: Uint8Array): string => {
     declared !== undefined &&
     !["utf-8", "utf-16", "utf-16le", "utf-16be"].includes(declared)
   )
-    throw new TypeError(
+    throw new XmlPlistTextDecodeError(
+      "unsupported-encoding",
       `Unsupported XML encoding declaration: ${declaration}. Supported encodings are UTF-8 and BOM-marked UTF-16 in either byte order.`,
+      declaration,
     );
   if (
     declared !== undefined &&
     declared !== encoding &&
     !(encoding !== "utf-8" && declared === "utf-16")
   )
-    throw new TypeError("XML encoding declaration disagrees with its bytes");
+    throw new XmlPlistTextDecodeError(
+      "encoding-mismatch",
+      "XML encoding declaration disagrees with its bytes",
+      declaration,
+    );
   return text;
 };
diff --git a/tests/boundary/browser/sazWebCapture.test.ts b/tests/boundary/browser/sazWebCapture.test.ts
index d6192e2e69..3e16fac4c3 100644
--- a/tests/boundary/browser/sazWebCapture.test.ts
+++ b/tests/boundary/browser/sazWebCapture.test.ts
@@ -342,6 +342,51 @@ it("keeps CRLF separators inside a body after LF-only HTTP headers", async () =>
   expect(await readFile(path)).toEqual(bytes);
 });
 
+it("validates SAZ XML declarations against archive member bytes at the CLI boundary", async () => {
+  const withMetadata = (metadata: Buffer): [string, Buffer][] =>
+    members().map(([name, bytes]) =>
+      name === "raw/0042_m.xml" ? [name, metadata] : [name, bytes],
+    );
+  const utf8 = Buffer.from(
+    '<?xml version="1.0" encoding="UTF-8"?><Session SID="0042"/>',
+  );
+  const utf16le = Buffer.concat([
+    Buffer.from([0xff, 0xfe]),
+    Buffer.from(
+      '<?xml version="1.0" encoding="UTF-16"?><Session SID="0042"/>',
+      "utf16le",
+    ),
+  ]);
+  for (const metadata of [utf8, utf16le]) {
+    const result = await cli(
+      await fixture(await archive(withMetadata(metadata))),
+    );
+    expect(
+      webNetworkCaptureSchema.parse(result.normalized_result).records,
+    ).toHaveLength(2);
+  }
+  const mismatch = Buffer.from(
+    '<?xml version="1.0" encoding="UTF-16"?><Session SID="0042"/>',
+  );
+  const rejected = await cliResponse(
+    await fixture(await archive(withMetadata(mismatch))),
+    ["--record", "1"],
+  );
+  expect(rejected.ok).toBe(false);
+  expect(JSON.parse(rejected.stdout)).toMatchObject({
+    category: "invalid_input",
+    details: {
+      issues: [
+        expect.objectContaining({
+          message: expect.stringContaining(
+            "SAZ metadata encoding declaration disagrees with its bytes",
+          ),
+        }),
+      ],
+    },
+  });
+});
+
 it("excludes explicit literals from XML, HTTP byte views, member coordinates and binary sidecars", async () => {
   const path = await fixture(await archive());
   const result = await cli(path, [