diff --git a/loom-core/tests/spec_features.rs b/loom-core/tests/spec_features.rs
new file mode 100644
index 0000000..abe3e29
--- /dev/null
+++ b/loom-core/tests/spec_features.rs
@@ -0,0 +1,155 @@
+//! Spec-feature coverage tests: post-MVP WebAssembly features.
+//!
+//! v0.4.0 audit found LOOM's test corpus had zero fixtures using post-MVP
+//! features (SIMD, ref types, GC, tail calls, EH, threads, multi-memory).
+//! The parser is expected to *reject* unsupported instructions cleanly
+//! (return `Err`, never panic). Where LOOM does support the feature, we
+//! exercise the full optimize + round-trip path.
+//!
+//! Per CLAUDE.md: rejection paths matter as much as happy paths. We assert
+//! every fixture (a) does not panic the parser, and (b) either succeeds
+//! end-to-end or fails with a recognizable diagnostic.
+
+use loom_core::{encode, optimize, parse};
+use std::panic;
+
+/// Outcome bucket for a single fixture run.
+#[derive(Debug)]
+enum Outcome {
+    /// Parser panicked — should never happen.
+    Panicked,
+    /// Parser returned a clean error.
+    Rejected(String),
+    /// Parser succeeded; full module is available.
+    Accepted(Box<loom_core::Module>),
+}
+
+fn classify(wat: &str) -> Outcome {
+    let result = panic::catch_unwind(panic::AssertUnwindSafe(|| parse::parse_wat(wat)));
+    match result {
+        Err(_) => Outcome::Panicked,
+        Ok(Ok(module)) => Outcome::Accepted(Box::new(module)),
+        Ok(Err(e)) => Outcome::Rejected(format!("{e:#}")),
+    }
+}
+
+/// For features LOOM does not support, we require:
+///   * no panic
+///   * a clean Err (or, if the parser unexpectedly accepts, a successful
+///     re-encode — anything except a panic is acceptable; we just want to
+///     pin the contract).
+fn assert_no_panic(name: &str, wat: &str) {
+    match classify(wat) {
+        Outcome::Panicked => panic!("parser panicked on {name}"),
+        Outcome::Rejected(msg) => {
+            // Sanity: error string should be non-empty.
+            assert!(
+                !msg.is_empty(),
+                "{name}: rejection produced empty error message"
+            );
+            eprintln!("{name}: rejected cleanly: {msg}");
+        }
+        Outcome::Accepted(module) => {
+            // Surprise — parser accepted. Make sure encode also doesn't panic.
+            let encode_result =
+                panic::catch_unwind(panic::AssertUnwindSafe(|| encode::encode_wasm(&module)));
+            assert!(
+                encode_result.is_ok(),
+                "{name}: parser accepted but encoder panicked"
+            );
+            eprintln!("{name}: parser accepted (likely partial support)");
+        }
+    }
+}
+
+/// For LOOM-supported features, we want full optimize + re-encode round-trip.
+fn assert_optimize_roundtrip(name: &str, wat: &str) {
+    let outcome = classify(wat);
+    let mut module = match outcome {
+        Outcome::Panicked => panic!("parser panicked on {name}"),
+        Outcome::Rejected(msg) => panic!("{name}: expected acceptance, got rejection: {msg}"),
+        Outcome::Accepted(m) => *m,
+    };
+
+    let opt_result = panic::catch_unwind(panic::AssertUnwindSafe(|| {
+        optimize::optimize_module(&mut module)
+    }));
+    assert!(opt_result.is_ok(), "{name}: optimizer panicked");
+    opt_result
+        .unwrap()
+        .unwrap_or_else(|e| panic!("{name}: optimize_module returned Err: {e:#}"));
+
+    let enc_result = panic::catch_unwind(panic::AssertUnwindSafe(|| encode::encode_wasm(&module)));
+    assert!(enc_result.is_ok(), "{name}: encoder panicked");
+    let bytes = enc_result
+        .unwrap()
+        .unwrap_or_else(|e| panic!("{name}: encode_wasm returned Err: {e:#}"));
+
+    // Re-parse the encoded bytes to confirm round-trip stability.
+    let reparse = panic::catch_unwind(panic::AssertUnwindSafe(|| parse::parse_wasm(&bytes)));
+    assert!(reparse.is_ok(), "{name}: re-parser panicked");
+    reparse
+        .unwrap()
+        .unwrap_or_else(|e| panic!("{name}: re-parse failed: {e:#}"));
+}
+
+// ---------------------------------------------------------------------------
+// Post-MVP features LOOM does NOT yet support — assert clean rejection only.
+// ---------------------------------------------------------------------------
+
+#[test]
+fn spec_feature_simd_v128_does_not_panic() {
+    let wat = include_str!("../../tests/fixtures/spec-features/simd_v128_minimal.wat");
+    assert_no_panic("simd_v128", wat);
+}
+
+#[test]
+fn spec_feature_ref_types_does_not_panic() {
+    let wat = include_str!("../../tests/fixtures/spec-features/ref_types_minimal.wat");
+    assert_no_panic("ref_types", wat);
+}
+
+#[test]
+fn spec_feature_tail_calls_does_not_panic() {
+    let wat = include_str!("../../tests/fixtures/spec-features/tail_calls_minimal.wat");
+    assert_no_panic("tail_calls", wat);
+}
+
+#[test]
+fn spec_feature_exception_handling_does_not_panic() {
+    let wat = include_str!("../../tests/fixtures/spec-features/exception_handling_minimal.wat");
+    assert_no_panic("exception_handling", wat);
+}
+
+// ---------------------------------------------------------------------------
+// Post-MVP features LOOM partially supports — accept either rejection or
+// successful round-trip; never panic.
+// ---------------------------------------------------------------------------
+
+#[test]
+fn spec_feature_bulk_memory_does_not_panic() {
+    let wat = include_str!("../../tests/fixtures/spec-features/bulk_memory_minimal.wat");
+    assert_no_panic("bulk_memory", wat);
+}
+
+#[test]
+fn spec_feature_multi_memory_does_not_panic() {
+    let wat = include_str!("../../tests/fixtures/spec-features/multi_memory_minimal.wat");
+    assert_no_panic("multi_memory", wat);
+}
+
+// ---------------------------------------------------------------------------
+// Standardized features LOOM fully supports — full optimize + round-trip.
+// ---------------------------------------------------------------------------
+
+#[test]
+fn spec_feature_sign_extension_ops_round_trip() {
+    let wat = include_str!("../../tests/fixtures/spec-features/sign_extension_ops.wat");
+    assert_optimize_roundtrip("sign_extension_ops", wat);
+}
+
+#[test]
+fn spec_feature_saturating_trunc_round_trip() {
+    let wat = include_str!("../../tests/fixtures/spec-features/saturating_trunc.wat");
+    assert_optimize_roundtrip("saturating_trunc", wat);
+}
diff --git a/tests/fixtures/spec-features/bulk_memory_minimal.wat b/tests/fixtures/spec-features/bulk_memory_minimal.wat
new file mode 100644
index 0000000..786d466
--- /dev/null
+++ b/tests/fixtures/spec-features/bulk_memory_minimal.wat
@@ -0,0 +1,25 @@
+;; Bulk memory ops minimal fixture (post-MVP wasm feature, partial LOOM support)
+;; Exercises memory.copy, memory.fill, memory.init, data.drop.
+(module
+  (memory 1)
+  (data $d "hello world")
+  (func (export "bulk_test")
+    ;; memory.fill: dst=0, val=0, len=4
+    i32.const 0
+    i32.const 0
+    i32.const 4
+    memory.fill
+    ;; memory.copy: dst=8, src=0, len=4
+    i32.const 8
+    i32.const 0
+    i32.const 4
+    memory.copy
+    ;; memory.init from data segment $d: dst=16, src=0, len=11
+    i32.const 16
+    i32.const 0
+    i32.const 11
+    memory.init $d
+    ;; data.drop: drop the data segment
+    data.drop $d
+  )
+)
diff --git a/tests/fixtures/spec-features/exception_handling_minimal.wat b/tests/fixtures/spec-features/exception_handling_minimal.wat
new file mode 100644
index 0000000..ed7cc03
--- /dev/null
+++ b/tests/fixtures/spec-features/exception_handling_minimal.wat
@@ -0,0 +1,21 @@
+;; Exception handling minimal fixture (post-MVP wasm feature)
+;; LOOM does not currently support EH; parser must reject cleanly (no panic).
+;; Uses the legacy try/catch encoding plus throw/throw_ref for breadth.
+(module
+  (tag $exn (param i32))
+  (func (export "eh_test") (result i32)
+    (try (result i32)
+      (do
+        i32.const 1
+        throw $exn
+        i32.const 0
+      )
+      (catch $exn
+        ;; on catch the i32 payload is on the stack
+      )
+      (catch_all
+        i32.const -1
+      )
+    )
+  )
+)
diff --git a/tests/fixtures/spec-features/multi_memory_minimal.wat b/tests/fixtures/spec-features/multi_memory_minimal.wat
new file mode 100644
index 0000000..1503086
--- /dev/null
+++ b/tests/fixtures/spec-features/multi_memory_minimal.wat
@@ -0,0 +1,23 @@
+;; Multi-memory minimal fixture (post-MVP wasm feature, partial LOOM support)
+;; Two memories with explicit memarg `mem` index on loads/stores.
+(module
+  (memory $m0 1)
+  (memory $m1 1)
+  (func (export "mm_test") (result i32)
+    ;; store 42 at offset 0 of memory $m1
+    i32.const 0
+    i32.const 42
+    i32.store (memory $m1)
+    ;; load from memory $m1
+    i32.const 0
+    i32.load (memory $m1)
+    ;; store 7 at offset 0 of memory $m0
+    i32.const 0
+    i32.const 7
+    i32.store (memory $m0)
+    ;; load from memory $m0 and add
+    i32.const 0
+    i32.load (memory $m0)
+    i32.add
+  )
+)
diff --git a/tests/fixtures/spec-features/ref_types_minimal.wat b/tests/fixtures/spec-features/ref_types_minimal.wat
new file mode 100644
index 0000000..c8aec29
--- /dev/null
+++ b/tests/fixtures/spec-features/ref_types_minimal.wat
@@ -0,0 +1,20 @@
+;; Reference types minimal fixture (post-MVP wasm feature)
+;; Exercises ref.null, ref.func, table.get on a funcref table.
+(module
+  (table $t 2 funcref)
+  (func $callee (result i32)
+    i32.const 7
+  )
+  (elem (i32.const 0) $callee)
+  (func (export "ref_test") (result funcref)
+    ;; ref.null funcref
+    ref.null func
+    drop
+    ;; table.get
+    i32.const 0
+    table.get $t
+    drop
+    ;; ref.func returning a funcref
+    ref.func $callee
+  )
+)
diff --git a/tests/fixtures/spec-features/saturating_trunc.wat b/tests/fixtures/spec-features/saturating_trunc.wat
new file mode 100644
index 0000000..c6b387b
--- /dev/null
+++ b/tests/fixtures/spec-features/saturating_trunc.wat
@@ -0,0 +1,28 @@
+;; Non-trapping float-to-int (saturating truncation), LOOM-supported.
+;; Should parse, optimize, and round-trip cleanly.
+(module
+  (func (export "i32_trunc_sat_f32_s") (param f32) (result i32)
+    local.get 0
+    i32.trunc_sat_f32_s
+  )
+  (func (export "i32_trunc_sat_f32_u") (param f32) (result i32)
+    local.get 0
+    i32.trunc_sat_f32_u
+  )
+  (func (export "i32_trunc_sat_f64_s") (param f64) (result i32)
+    local.get 0
+    i32.trunc_sat_f64_s
+  )
+  (func (export "i32_trunc_sat_f64_u") (param f64) (result i32)
+    local.get 0
+    i32.trunc_sat_f64_u
+  )
+  (func (export "i64_trunc_sat_f32_s") (param f32) (result i64)
+    local.get 0
+    i64.trunc_sat_f32_s
+  )
+  (func (export "i64_trunc_sat_f64_u") (param f64) (result i64)
+    local.get 0
+    i64.trunc_sat_f64_u
+  )
+)
diff --git a/tests/fixtures/spec-features/sign_extension_ops.wat b/tests/fixtures/spec-features/sign_extension_ops.wat
new file mode 100644
index 0000000..662aebd
--- /dev/null
+++ b/tests/fixtures/spec-features/sign_extension_ops.wat
@@ -0,0 +1,24 @@
+;; Sign-extension operators (wasm 1.1 / standardized feature, LOOM-supported).
+;; Should parse, optimize, and round-trip cleanly.
+(module
+  (func (export "ext8_s") (param i32) (result i32)
+    local.get 0
+    i32.extend8_s
+  )
+  (func (export "ext16_s") (param i32) (result i32)
+    local.get 0
+    i32.extend16_s
+  )
+  (func (export "i64_ext8_s") (param i64) (result i64)
+    local.get 0
+    i64.extend8_s
+  )
+  (func (export "i64_ext16_s") (param i64) (result i64)
+    local.get 0
+    i64.extend16_s
+  )
+  (func (export "i64_ext32_s") (param i64) (result i64)
+    local.get 0
+    i64.extend32_s
+  )
+)
diff --git a/tests/fixtures/spec-features/simd_v128_minimal.wat b/tests/fixtures/spec-features/simd_v128_minimal.wat
new file mode 100644
index 0000000..baa35e3
--- /dev/null
+++ b/tests/fixtures/spec-features/simd_v128_minimal.wat
@@ -0,0 +1,11 @@
+;; SIMD/v128 minimal fixture (post-MVP wasm feature)
+;; LOOM does not currently support SIMD; parser must reject cleanly (no panic).
+(module
+  (func (export "simd_test") (result v128)
+    (v128.const i8x16 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15)
+    (v128.const i8x16 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1)
+    i8x16.add
+    (v128.const i8x16 1 0 1 0 1 0 1 0 1 0 1 0 1 0 1 0)
+    v128.bitselect
+  )
+)
diff --git a/tests/fixtures/spec-features/tail_calls_minimal.wat b/tests/fixtures/spec-features/tail_calls_minimal.wat
new file mode 100644
index 0000000..528a7c6
--- /dev/null
+++ b/tests/fixtures/spec-features/tail_calls_minimal.wat
@@ -0,0 +1,23 @@
+;; Tail calls minimal fixture (post-MVP wasm feature)
+;; LOOM does not currently support tail calls; parser must reject cleanly.
+(module
+  (type $sig (func (param i32) (result i32)))
+  (table $t 1 funcref)
+  (elem (i32.const 0) $direct)
+  (func $direct (param i32) (result i32)
+    local.get 0
+    i32.const 1
+    i32.add
+  )
+  (func $tail_direct (param i32) (result i32)
+    local.get 0
+    return_call $direct
+  )
+  (func $tail_indirect (param i32) (result i32)
+    local.get 0
+    i32.const 0
+    return_call_indirect (type $sig)
+  )
+  (export "tail_direct" (func $tail_direct))
+  (export "tail_indirect" (func $tail_indirect))
+)