diff --git a/tests/interop.rs b/tests/interop.rs
new file mode 100644
index 0000000..bd230ad
--- /dev/null
+++ b/tests/interop.rs
@@ -0,0 +1,151 @@
+use zwus::Zwus;
+
+/// Torture-test corpus — every Unicode edge case.
+/// ><  on the last line is where npm-encoded "test" (base 6) gets pasted.
+const CORPUS: &str = "\
+ASCII: Hello, World! 0123456789 ~!@#$%^&*()_+-=[]{}|;:'\",.<>?/\n\
+CJK Unified: 你好世界测试汉字\n\
+CJK Extension B: 𠀀𠀁𠀂\n\
+Korean: 한국어 테스트\n\
+Japanese: こんにちは カタカナ テスト\n\
+Emoji single: ✅❌🔥💀🎉🦀⭐🌍\n\
+Surrogate-heavy: 𐐷𐑌𐑀 𝄞\n\
+Math: ∑∏∫∂∇ ℵ ∞ ≠ ≤ ≥\n\
+Newlines:\n\
+Line2\n\
+Line3\n\
+Tabs:\tcol1\tcol2\tcol3\n\
+RTL Arabic: مرحبا بالعالم\n\
+RTL Hebrew: שלום עולם\n\
+Mixed: Hello مرحبا World שלום Fin\n\
+Skin tones: 👋🏻👋🏼👋🏽👋🏾👋🏿\n\
+Flags: 🇺🇸🇯🇵🇩🇪\n\
+Cuneiform: 𒀀𒀁𒀂\n\
+Egyptian: 𓀀𓀁𓀂\n\
+Musical: 𝄞𝄢\n\
+Repeated: 😀😀😀😀😀\n\
+Single: x\n\
+Zero: 0\n\
+Spaces:   multiple   spaces   here\n\
+Nested ZW: >PASTE_ZW_HERE<";
+
+// ── Paste encoded output from npm here ─────────────────────────────
+// In Node:
+//   import zwus from 'zwus';
+//   console.log(JSON.stringify(zwus.encodeString(CORPUS, 3)));
+//   console.log(JSON.stringify(zwus.encodeString(CORPUS, 6)));
+//   console.log(JSON.stringify(zwus.encodeString(CORPUS, 8)));
+const NPM_BASE_3: &str = "PASTE_HERE";
+const NPM_BASE_6: &str = "PASTE_HERE";
+const NPM_BASE_8: &str = "PASTE_HERE";
+
+// ── Encoding match: Rust encode == npm encode ──────────────────────
+
+#[test]
+fn rust_encode_matches_npm_base3() {
+    assert_ne!(NPM_BASE_3, "PASTE_HERE", "Paste npm base-3 encoded output");
+    let rust_encoded = Zwus::encode_string_with_base(CORPUS, 3);
+    assert_eq!(rust_encoded, NPM_BASE_3, "Base-3 encoding differs from npm");
+}
+
+#[test]
+fn rust_encode_matches_npm_base6() {
+    assert_ne!(NPM_BASE_6, "PASTE_HERE", "Paste npm base-6 encoded output");
+    let rust_encoded = Zwus::encode_string_with_base(CORPUS, 6);
+    assert_eq!(rust_encoded, NPM_BASE_6, "Base-6 encoding differs from npm");
+}
+
+#[test]
+fn rust_encode_matches_npm_base8() {
+    assert_ne!(NPM_BASE_8, "PASTE_HERE", "Paste npm base-8 encoded output");
+    let rust_encoded = Zwus::encode_string_with_base(CORPUS, 8);
+    assert_eq!(rust_encoded, NPM_BASE_8, "Base-8 encoding differs from npm");
+}
+
+// ── Decoding match: Rust decodes npm output back to CORPUS ─────────
+
+#[test]
+fn rust_decodes_npm_base3() {
+    assert_ne!(NPM_BASE_3, "PASTE_HERE", "Paste npm base-3 encoded output");
+    let decoded = Zwus::decode_to_string_with_base(NPM_BASE_3, 3);
+    assert_eq!(decoded, CORPUS);
+}
+
+#[test]
+fn rust_decodes_npm_base6() {
+    assert_ne!(NPM_BASE_6, "PASTE_HERE", "Paste npm base-6 encoded output");
+    let decoded = Zwus::decode_to_string_with_base(NPM_BASE_6, 6);
+    assert_eq!(decoded, CORPUS);
+}
+
+#[test]
+fn rust_decodes_npm_base8() {
+    assert_ne!(NPM_BASE_8, "PASTE_HERE", "Paste npm base-8 encoded output");
+    let decoded = Zwus::decode_to_string_with_base(NPM_BASE_8, 8);
+    assert_eq!(decoded, CORPUS);
+}
+
+// ── Rust self-roundtrip ────────────────────────────────────────────
+
+#[test]
+fn rust_roundtrip_corpus_all_bases() {
+    for base in [3u8, 6, 8] {
+        let encoded = Zwus::encode_string_with_base(CORPUS, base);
+        let decoded = Zwus::decode_to_string_with_base(&encoded, base);
+        assert_eq!(decoded, CORPUS, "Roundtrip failed for base {base}");
+    }
+}
+
+// ── Nested zero-width: encode the whole corpus, then double-decode ─
+// The CORPUS itself contains base-6 encoded "test" between > and <.
+// First decode peels the outer layer → reveals visible text + inner ZW.
+// Second decode (base 6) on that inner ZW region → "test".
+
+#[test]
+fn nested_zero_width_double_decode() {
+    assert_ne!(NPM_BASE_3, "PASTE_HERE", "Paste npm base-3 encoded output");
+
+    // Outer layer: decode the full corpus from base 3
+    let outer_decoded = Zwus::decode_to_string_with_base(NPM_BASE_3, 3);
+    assert_eq!(outer_decoded, CORPUS);
+
+    // Find the nested ZW payload between > and <
+    let start = outer_decoded.find('>').expect("missing >") + 1;
+    let end = outer_decoded.rfind('<').expect("missing <");
+    let inner_payload = &outer_decoded[start..end];
+
+    // Inner layer: that region contains base-6 encoded "test"
+    // (pasted by Master into CORPUS between > and <)
+    assert!(
+        !inner_payload.is_empty() && inner_payload != "PASTE_ZW_HERE",
+        "Paste base-6 encoded 'test' between >< in CORPUS"
+    );
+    let inner_decoded = Zwus::decode_to_string_with_base(inner_payload, 6);
+    assert_eq!(inner_decoded, "test", "Inner nested ZW did not decode to 'test'");
+}
+
+// ── Surrogate safety ──────────────────────────────────────────────
+
+#[test]
+fn no_surrogates_in_codepoint_iteration() {
+    let codepoints: Vec<u32> = CORPUS.chars().map(|c| c as u32).collect();
+    for &cp in &codepoints {
+        assert!(
+            !(0xD800..=0xDFFF).contains(&cp),
+            "Surrogate in Rust char iteration: U+{cp:04X}"
+        );
+        assert!(cp <= 0x10FFFF, "Out of Unicode range: U+{cp:04X}");
+    }
+}
+
+// ── Number array roundtrip ────────────────────────────────────────
+
+#[test]
+fn number_array_roundtrip_all_bases() {
+    let codepoints: Vec<u32> = CORPUS.chars().map(|c| c as u32).collect();
+    for base in [3u8, 6, 8] {
+        let encoded = Zwus::encode_number_array_with_base(&codepoints, base);
+        let decoded = Zwus::decode_to_number_array_with_base(&encoded, base);
+        assert_eq!(decoded, codepoints, "Number array roundtrip failed base {base}");
+    }
+}