diff --git a/src/compression.ts b/src/compression.ts
index 4373003..f996a39 100644
--- a/src/compression.ts
+++ b/src/compression.ts
@@ -1,3 +1,4 @@
+import { SdkError, SdkErrorCode } from "./errors.js";
 import type { RequestInterceptor, ResponseInterceptor } from "./interceptors.js";
 
 export type CompressionAlgorithm = "gzip" | "deflate";
@@ -141,3 +142,159 @@ export function createCompressionResponseInterceptor(_config: CompressionConfig)
     };
   };
 }
+
+// ---------------------------------------------------------------------------
+// Invoice metadata encoding (#619)
+// ---------------------------------------------------------------------------
+
+/** Default byte ceiling for an encoded metadata string. */
+export const DEFAULT_METADATA_MAX_BYTES = 512;
+
+/**
+ * base64url alphabet. Trailing padding is tolerated on decode even though it
+ * is never produced, so a caller that padded the value elsewhere still round
+ * trips.
+ */
+const BASE64URL_PATTERN = /^[A-Za-z0-9_-]*={0,2}$/;
+
+/**
+ * base64url encode, without depending on the Node `Buffer` global.
+ *
+ * This module is isomorphic - it feature-detects `CompressionStream` and falls
+ * back to `node:zlib` - so the metadata helpers must not reach for a Node-only
+ * global either. `TextEncoder`/`TextDecoder` and `btoa`/`atob` exist in both
+ * browsers and Node >= 16.
+ */
+function toBase64Url(json: string): string {
+  const bytes = new TextEncoder().encode(json);
+
+  // Chunked rather than String.fromCharCode(...bytes): spreading a large array
+  // overflows the call stack, and the size limit is only checked after encoding.
+  const CHUNK_SIZE = 0x8000;
+  let binary = "";
+  for (let i = 0; i < bytes.length; i += CHUNK_SIZE) {
+    binary += String.fromCharCode(...bytes.subarray(i, i + CHUNK_SIZE));
+  }
+
+  return btoa(binary).replace(/\+/g, "-").replace(/\//g, "_").replace(/=+$/, "");
+}
+
+/** base64url decode, tolerating optional padding. Mirror of {@link toBase64Url}. */
+function fromBase64Url(encoded: string): string {
+  const base64 = encoded.replace(/-/g, "+").replace(/_/g, "/");
+  const padded = base64 + "=".repeat((4 - (base64.length % 4)) % 4);
+  const binary = atob(padded);
+  const bytes = Uint8Array.from(binary, (character) => character.charCodeAt(0));
+  return new TextDecoder().decode(bytes);
+}
+
+/**
+ * Encode an invoice metadata object as a compact base64url string.
+ *
+ * The value is JSON-serialised then base64url encoded without padding, so the
+ * result is safe to place in a Stellar transaction memo or an IPFS payload.
+ *
+ * @param metadata - Any JSON-serialisable plain object.
+ * @param maxBytes - Ceiling for the encoded string, in bytes.
+ * @returns The encoded metadata.
+ * @throws {SdkError} With {@link SdkErrorCode.CONTRACT_REJECTED} when the
+ *   input is not a serialisable plain object, or when the encoded result
+ *   exceeds `maxBytes`.
+ */
+export function compressMetadata(
+  metadata: Record<string, unknown>,
+  maxBytes: number = DEFAULT_METADATA_MAX_BYTES,
+): string {
+  if (typeof metadata !== "object" || metadata === null || Array.isArray(metadata)) {
+    throw new SdkError(
+      "Metadata must be a plain object",
+      SdkErrorCode.CONTRACT_REJECTED,
+      { received: metadata === null ? "null" : typeof metadata },
+    );
+  }
+
+  if (!Number.isFinite(maxBytes) || maxBytes <= 0) {
+    throw new SdkError(
+      "maxBytes must be a positive, finite number",
+      SdkErrorCode.CONTRACT_REJECTED,
+      { maxBytes },
+    );
+  }
+
+  let json: string;
+  try {
+    json = JSON.stringify(metadata);
+  } catch (err) {
+    // Circular references and BigInt values both reach here.
+    throw new SdkError(
+      "Metadata is not JSON-serialisable",
+      SdkErrorCode.CONTRACT_REJECTED,
+      { reason: err instanceof Error ? err.message : String(err) },
+    );
+  }
+
+  const encoded = toBase64Url(json);
+  // base64url is ASCII-only, so the character count is the byte count.
+  const bytes = encoded.length;
+
+  if (bytes > maxBytes) {
+    throw new SdkError(
+      `Encoded metadata is ${bytes} bytes, over the ${maxBytes} byte limit`,
+      SdkErrorCode.CONTRACT_REJECTED,
+      { bytes, maxBytes },
+    );
+  }
+
+  return encoded;
+}
+
+/**
+ * Decode a metadata string produced by {@link compressMetadata}.
+ *
+ * @param encoded - base64url-encoded metadata.
+ * @returns The decoded object.
+ * @throws {SdkError} With {@link SdkErrorCode.CONTRACT_REJECTED} when the
+ *   input is not base64url, does not contain JSON, or does not decode to a
+ *   plain object.
+ */
+export function decompressMetadata(encoded: string): Record<string, unknown> {
+  if (typeof encoded !== "string" || !BASE64URL_PATTERN.test(encoded)) {
+    throw new SdkError(
+      "Encoded metadata is not a base64url string",
+      SdkErrorCode.CONTRACT_REJECTED,
+      { received: typeof encoded },
+    );
+  }
+
+  let json: string;
+  try {
+    json = fromBase64Url(encoded);
+  } catch (err) {
+    throw new SdkError(
+      "Encoded metadata is not valid base64url",
+      SdkErrorCode.CONTRACT_REJECTED,
+      { reason: err instanceof Error ? err.message : String(err) },
+    );
+  }
+
+  let parsed: unknown;
+  try {
+    parsed = JSON.parse(json);
+  } catch (err) {
+    throw new SdkError(
+      "Encoded metadata does not contain valid JSON",
+      SdkErrorCode.CONTRACT_REJECTED,
+      { reason: err instanceof Error ? err.message : String(err) },
+    );
+  }
+
+  if (typeof parsed !== "object" || parsed === null || Array.isArray(parsed)) {
+    throw new SdkError(
+      "Encoded metadata did not decode to an object",
+      SdkErrorCode.CONTRACT_REJECTED,
+      { decodedType: parsed === null ? "null" : Array.isArray(parsed) ? "array" : typeof parsed },
+    );
+  }
+
+  return parsed as Record<string, unknown>;
+}
diff --git a/src/index.ts b/src/index.ts
index f9cb56b..73e4775 100644
--- a/src/index.ts
+++ b/src/index.ts
@@ -1427,3 +1427,13 @@ export type {
   SubmitTransactionOptions,
   SubmitServer,
 } from "./transaction/submit.js";
+
+// ---------------------------------------------------------------------------
+// #619 - Invoice metadata encoding
+// ---------------------------------------------------------------------------
+
+export {
+  compressMetadata,
+  decompressMetadata,
+  DEFAULT_METADATA_MAX_BYTES,
+} from "./compression.js";
diff --git a/test/compression.metadata.invalid-base64url.test.ts b/test/compression.metadata.invalid-base64url.test.ts
new file mode 100644
index 0000000..9e97952
--- /dev/null
+++ b/test/compression.metadata.invalid-base64url.test.ts
@@ -0,0 +1,19 @@
+import { describe, expect, it } from "vitest";
+
+import { decompressMetadata } from "../src/compression.js";
+import { SdkError, SdkErrorCode } from "../src/errors.js";
+
+describe("decompressMetadata malformed base64url", () => {
+  it.each(["A", "A=", "=="])(
+    "normalizes decoder failure for %j to CONTRACT_REJECTED",
+    (encoded) => {
+      try {
+        decompressMetadata(encoded);
+        throw new Error("expected malformed base64url to be rejected");
+      } catch (error) {
+        expect(error).toBeInstanceOf(SdkError);
+        expect((error as SdkError).code).toBe(SdkErrorCode.CONTRACT_REJECTED);
+      }
+    },
+  );
+});
diff --git a/test/compression.metadata.test.ts b/test/compression.metadata.test.ts
new file mode 100644
index 0000000..11f9f35
--- /dev/null
+++ b/test/compression.metadata.test.ts
@@ -0,0 +1,175 @@
+/**
+ * Tests for invoice metadata encoding (Issue #619).
+ * Pure functions — no network, no filesystem.
+ */
+
+import { describe, it, expect } from "vitest";
+
+import {
+  DEFAULT_METADATA_MAX_BYTES,
+  compressMetadata,
+  decompressMetadata,
+} from "../src/compression.js";
+import { SdkError, SdkErrorCode } from "../src/errors.js";
+
+const expectRejected = (fn: () => unknown) => {
+  try {
+    fn();
+    throw new Error("expected the call to throw an SdkError");
+  } catch (error) {
+    expect(error).toBeInstanceOf(SdkError);
+    expect((error as SdkError).code).toBe(SdkErrorCode.CONTRACT_REJECTED);
+    return error as SdkError;
+  }
+};
+
+describe("DEFAULT_METADATA_MAX_BYTES", () => {
+  it("is 512", () => {
+    expect(DEFAULT_METADATA_MAX_BYTES).toBe(512);
+  });
+});
+
+describe("compressMetadata", () => {
+  it("encodes without base64 padding", () => {
+    // "{}" is 2 bytes, which is the case standard base64 would pad.
+    const encoded = compressMetadata({});
+
+    expect(encoded).not.toContain("=");
+    expect(encoded).toMatch(/^[A-Za-z0-9_-]+$/);
+  });
+
+  it("uses the base64url alphabet, never + or /", () => {
+    // Bytes that encode to '+' and '/' under standard base64.
+    const encoded = compressMetadata({ v: "ÿÿÿ????>>>" });
+
+    expect(encoded).not.toContain("+");
+    expect(encoded).not.toContain("/");
+  });
+
+  it("rejects a payload over the default limit", () => {
+    const error = expectRejected(() =>
+      compressMetadata({ blob: "x".repeat(1000) }),
+    );
+
+    expect(error.message).toContain(String(DEFAULT_METADATA_MAX_BYTES));
+  });
+
+  it("honours a custom maxBytes", () => {
+    expect(() => compressMetadata({ a: 1 }, 4)).toThrow(SdkError);
+    expect(() => compressMetadata({ a: 1 }, 64)).not.toThrow();
+  });
+
+  it("reports the actual size alongside the limit", () => {
+    const error = expectRejected(() => compressMetadata({ a: 1 }, 4));
+    const details = error.details as { bytes: number; maxBytes: number };
+
+    expect(details.maxBytes).toBe(4);
+    expect(details.bytes).toBeGreaterThan(4);
+  });
+
+  it.each([
+    ["null", null],
+    ["an array", [1, 2, 3]],
+    ["a string", "not an object"],
+    ["a number", 42],
+  ])("rejects %s", (_label, value) => {
+    expectRejected(() => compressMetadata(value as never));
+  });
+
+  it("rejects a circular structure rather than throwing a raw TypeError", () => {
+    const circular: Record<string, unknown> = {};
+    circular["self"] = circular;
+
+    expectRejected(() => compressMetadata(circular));
+  });
+
+  it("rejects a BigInt value, which JSON cannot serialise", () => {
+    expectRejected(() => compressMetadata({ amount: 1n as unknown }));
+  });
+
+  it.each([0, -1, Number.NaN, Number.POSITIVE_INFINITY])(
+    "rejects the invalid maxBytes %s",
+    (maxBytes) => {
+      expectRejected(() => compressMetadata({ a: 1 }, maxBytes));
+    },
+  );
+});
+
+describe("decompressMetadata", () => {
+  it("rejects a string outside the base64url alphabet", () => {
+    expectRejected(() => decompressMetadata("not base64!"));
+    expectRejected(() => decompressMetadata("has+plus/slash"));
+  });
+
+  it("rejects base64url that does not contain JSON", () => {
+    // "Hello" — valid base64url, not JSON.
+    expectRejected(() => decompressMetadata("SGVsbG8"));
+  });
+
+  it("rejects a non-string input", () => {
+    expectRejected(() => decompressMetadata(undefined as never));
+    expectRejected(() => decompressMetadata(123 as never));
+  });
+
+  it.each([
+    ["an array", "[1,2,3]"],
+    ["a number", "42"],
+    ["a string", '"hello"'],
+    ["null", "null"],
+  ])("rejects encoded JSON that decodes to %s", (_label, json) => {
+    const encoded = Buffer.from(json, "utf8").toString("base64url");
+
+    expectRejected(() => decompressMetadata(encoded));
+  });
+
+  it("accepts padded input even though it never emits padding", () => {
+    // A caller may have padded the value elsewhere; decoding should still work.
+    const padded = Buffer.from(JSON.stringify({ p: 1 }), "utf8").toString("base64");
+
+    expect(decompressMetadata(padded)).toEqual({ p: 1 });
+  });
+});
+
+describe("browser safety", () => {
+  it("encodes and decodes with no Node Buffer global present", () => {
+    // compression.ts is isomorphic - it feature-detects CompressionStream and
+    // falls back to node:zlib - so these helpers must not need a Node global.
+    // A browser bundle without a Buffer polyfill is exactly this shape.
+    const originalBuffer = globalThis.Buffer;
+
+    try {
+      // @ts-expect-error deliberately simulating an environment with no Buffer
+      delete globalThis.Buffer;
+
+      const value = { id: "evt_1", note: "héllo ✓ 日本語" };
+      const encoded = compressMetadata(value);
+
+      expect(encoded).not.toContain("=");
+      expect(decompressMetadata(encoded)).toEqual(value);
+    } finally {
+      globalThis.Buffer = originalBuffer;
+    }
+  });
+});
+
+describe("round trip", () => {
+  const cases: Array<[string, Record<string, unknown>]> = [
+    ["an empty object", {}],
+    ["a flat object", { invoiceId: "inv_1", amount: 1000 }],
+    ["nested objects and arrays", { a: { b: { c: [1, 2, { d: true }] } } }],
+    ["null and boolean values", { n: null, t: true, f: false }],
+    ["unicode", { note: "héllo ✓ 日本語" }],
+    ["keys needing escaping", { 'quote"key': 'value with "quotes"' }],
+  ];
+
+  it.each(cases)("round-trips %s", (_label, value) => {
+    expect(decompressMetadata(compressMetadata(value))).toEqual(value);
+  });
+
+  it("survives a second round trip unchanged", () => {
+    const value = { invoiceId: "inv_2", tags: ["a", "b"] };
+    const once = compressMetadata(value);
+
+    expect(compressMetadata(decompressMetadata(once))).toBe(once);
+  });
+});