diff --git a/.github/workflows/ci.yml b/.github/workflows/ci.yml
index 054c3f9..8cfa4cd 100644
--- a/.github/workflows/ci.yml
+++ b/.github/workflows/ci.yml
@@ -84,6 +84,12 @@ jobs:
         echo "Testing numeric/gamma_fact..."
         cargo run --quiet --package goth-cli -- "$(pwd)/../examples/numeric/gamma_fact.goth" 5.0
 
+    - name: Run stdlib tests
+      run: GOTH=./crates/target/release/goth bash tests/stdlib_test.sh
+
+    - name: Run CLI smoke tests
+      run: GOTH=./crates/target/release/goth bash tests/cli_test.sh
+
     - name: Upload binary artifact
       uses: actions/upload-artifact@v4
       with:
diff --git a/crates/Cargo.lock b/crates/Cargo.lock
index 03fa664..62a5372 100644
--- a/crates/Cargo.lock
+++ b/crates/Cargo.lock
@@ -47,7 +47,7 @@ version = "1.1.5"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "40c48f72fd53cd289104fc64099abca73db4166ad86ea0b4341abe65af83dadc"
 dependencies = [
- "windows-sys 0.61.2",
+ "windows-sys 0.60.2",
 ]
 
 [[package]]
@@ -58,7 +58,7 @@ checksum = "291e6a250ff86cd4a820112fb8898808a366d8f9f58ce16d1f538353ad55747d"
 dependencies = [
  "anstyle",
  "once_cell_polyfill",
- "windows-sys 0.61.2",
+ "windows-sys 0.60.2",
 ]
 
 [[package]]
@@ -353,7 +353,7 @@ source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "39cab71617ae0d63f51a36d69f866391735b51691dbda63cf6f96d042b63efeb"
 dependencies = [
  "libc",
- "windows-sys 0.61.2",
+ "windows-sys 0.52.0",
 ]
 
 [[package]]
@@ -591,11 +591,11 @@ checksum = "2304e00983f87ffb38b55b444b5e3b60a884b5d30c0fca7d82fe33449bbe55ea"
 
 [[package]]
 name = "home"
-version = "0.5.12"
+version = "0.5.11"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "cc627f471c528ff0c4a49e1d5e60450c8f6461dd6d10ba9dcd3a61d3dff7728d"
+checksum = "589533453244b0995c858700322199b2becb13b627df2851f64a2775d024abcf"
 dependencies = [
- "windows-sys 0.61.2",
+ "windows-sys 0.59.0",
 ]
 
 [[package]]
@@ -1059,7 +1059,7 @@ version = "0.50.3"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "7957b9740744892f114936ab4a57b3f487491bbeafaf8083688b16841a4240e5"
 dependencies = [
- "windows-sys 0.61.2",
+ "windows-sys 0.59.0",
 ]
 
 [[package]]
@@ -1350,7 +1350,7 @@ dependencies = [
  "errno",
  "libc",
  "linux-raw-sys",
- "windows-sys 0.61.2",
+ "windows-sys 0.52.0",
 ]
 
 [[package]]
@@ -1634,7 +1634,7 @@ dependencies = [
  "getrandom 0.3.4",
  "once_cell",
  "rustix",
- "windows-sys 0.61.2",
+ "windows-sys 0.52.0",
 ]
 
 [[package]]
diff --git a/tests/cli_test.sh b/tests/cli_test.sh
new file mode 100644
index 0000000..7aa8abd
--- /dev/null
+++ b/tests/cli_test.sh
@@ -0,0 +1,206 @@
+#!/bin/bash
+# CLI smoke test suite for Goth
+# Tests all major CLI modes of the goth interpreter binary.
+
+set -e
+
+GOTH="${GOTH:-./crates/target/release/goth}"
+
+# Color output
+RED='\033[0;31m'
+GREEN='\033[0;32m'
+YELLOW='\033[0;33m'
+NC='\033[0m'
+
+PASS=0
+FAIL=0
+
+pass() {
+    echo -e "${GREEN}✓ $1${NC}"
+    PASS=$((PASS + 1))
+}
+
+fail() {
+    echo -e "${RED}✗ $1${NC}"
+    FAIL=$((FAIL + 1))
+}
+
+# Test exact output match
+run_test() {
+    local name="$1"
+    local expected="$2"
+    shift 2
+
+    local actual
+    actual=$("$@" 2>&1) || true
+
+    if [ "$actual" = "$expected" ]; then
+        pass "$name"
+    else
+        fail "$name"
+        echo -e "${YELLOW}  Expected:${NC} $(echo "$expected" | head -1)"
+        echo -e "${YELLOW}  Got:${NC} $(echo "$actual" | head -1)"
+    fi
+}
+
+# Test that output contains a substring (case-sensitive)
+run_test_contains() {
+    local name="$1"
+    local substring="$2"
+    shift 2
+
+    local actual
+    actual=$("$@" 2>&1) || true
+
+    if echo "$actual" | grep -qF "$substring"; then
+        pass "$name"
+    else
+        fail "$name"
+        echo -e "${YELLOW}  Expected to contain:${NC} $substring"
+        echo -e "${YELLOW}  Got:${NC} $(echo "$actual" | head -3)"
+    fi
+}
+
+# Test that output contains a substring (case-insensitive)
+run_test_contains_i() {
+    local name="$1"
+    local substring="$2"
+    shift 2
+
+    local actual
+    actual=$("$@" 2>&1) || true
+
+    if echo "$actual" | grep -qiF "$substring"; then
+        pass "$name"
+    else
+        fail "$name"
+        echo -e "${YELLOW}  Expected to contain (case-insensitive):${NC} $substring"
+        echo -e "${YELLOW}  Got:${NC} $(echo "$actual" | head -3)"
+    fi
+}
+
+# Test that stdout is empty
+run_test_empty() {
+    local name="$1"
+    shift
+
+    local actual
+    actual=$("$@" 2>&1) || true
+
+    if [ -z "$actual" ]; then
+        pass "$name"
+    else
+        fail "$name"
+        echo -e "${YELLOW}  Expected empty output${NC}"
+        echo -e "${YELLOW}  Got:${NC} $(echo "$actual" | head -1)"
+    fi
+}
+
+echo "=== Goth CLI Smoke Tests ==="
+echo ""
+
+# --- Section 1: Expression evaluation (-e) ---
+echo "Section: Expression evaluation (-e)"
+run_test "integer arithmetic" "7" $GOTH -e "1 + 2 * 3"
+run_test "negative result" "-7" $GOTH -e "3 - 10"
+run_test "boolean true" "⊤" $GOTH -e "3 > 2"
+run_test "boolean false" "⊥" $GOTH -e "3 < 2"
+run_test "lambda application" "6" $GOTH -e "(λ→ ₀ + 1) 5"
+run_test "array sum" "15" $GOTH -e "Σ [1, 2, 3, 4, 5]"
+run_test "let binding" "15" $GOTH -e "let x ← 10 in x + 5"
+echo ""
+
+# --- Section 2: File execution ---
+echo "Section: File execution"
+run_test "identity" "42" $GOTH examples/basic/identity.goth 42
+run_test "add_one" "100" $GOTH examples/basic/add_one.goth 99
+run_test "square" "49" $GOTH examples/basic/square.goth 7
+run_test "factorial" "120" $GOTH examples/recursion/factorial.goth 5
+run_test "fibonacci" "55" $GOTH examples/recursion/fibonacci.goth 10
+run_test "isPrime" "⊤" $GOTH examples/algorithms/isPrime.goth 17
+echo ""
+
+# --- Section 3: Multi-argument programs ---
+echo "Section: Multi-argument programs"
+run_test "gcd" "4" $GOTH examples/recursion/gcd.goth 12 8
+run_test "compose" "36" $GOTH examples/higher-order/compose.goth 3
+echo ""
+
+# --- Section 4: Parse-only mode (-p) ---
+echo "Section: Parse-only mode (-p)"
+run_test_contains "parse file" "Parsed:" $GOTH -p examples/basic/identity.goth
+run_test_empty "parse expression" $GOTH -p -e "1 + 2"
+echo ""
+
+# --- Section 5: AST mode (-a) ---
+echo "Section: AST mode (-a)"
+run_test_contains "ast expression" "AST:" $GOTH -a -e "1 + 2"
+run_test_contains "ast file" "Parsed AST:" $GOTH -a examples/basic/identity.goth
+echo ""
+
+# --- Section 6: Type check mode (-c) ---
+echo "Section: Type check mode (-c)"
+run_test_contains "type check expression" "Type:" $GOTH -c -e "1 + 2"
+run_test "type check file with args" "100" $GOTH -c examples/basic/add_one.goth 99
+echo ""
+
+# --- Section 7: JSON workflow ---
+echo "Section: JSON workflow"
+TMPDIR=$(mktemp -d)
+trap "rm -rf $TMPDIR" EXIT
+
+run_test_contains "to-json produces JSON" '"decls"' $GOTH --to-json examples/basic/identity.goth
+
+# Compact JSON should be a single line
+COMPACT_OUTPUT=$($GOTH --compact --to-json examples/basic/identity.goth 2>&1) || true
+COMPACT_LINES=$(echo "$COMPACT_OUTPUT" | wc -l)
+if [ "$COMPACT_LINES" -eq 1 ] && echo "$COMPACT_OUTPUT" | grep -qF '"decls"'; then
+    pass "compact to-json is single line"
+else
+    fail "compact to-json is single line"
+    echo -e "${YELLOW}  Lines: $COMPACT_LINES${NC}"
+fi
+
+# JSON round-trip
+$GOTH --to-json examples/basic/add_one.goth > "$TMPDIR/add_one.json" 2>&1
+ROUNDTRIP=$($GOTH --from-json "$TMPDIR/add_one.json" 99 2>&1) || true
+if echo "$ROUNDTRIP" | grep -qF "100"; then
+    pass "json round-trip preserves semantics"
+else
+    fail "json round-trip preserves semantics"
+    echo -e "${YELLOW}  Expected output to contain: 100${NC}"
+    echo -e "${YELLOW}  Got:${NC} $ROUNDTRIP"
+fi
+echo ""
+
+# --- Section 8: No-main mode ---
+echo "Section: No-main mode (--no-main)"
+run_test_contains "no-main shows declarations" "fn main" $GOTH --no-main examples/basic/identity.goth
+echo ""
+
+# --- Section 9: Error handling ---
+echo "Section: Error handling"
+run_test_contains_i "nonexistent file" "error" $GOTH nonexistent_file.goth
+run_test_contains_i "syntax error" "error" $GOTH -e "1 + + 2"
+
+# Render from JSON
+$GOTH --to-json examples/basic/add_one.goth > "$TMPDIR/render_test.json" 2>&1
+RENDER_OUTPUT=$($GOTH --render --from-json "$TMPDIR/render_test.json" 2>&1) || true
+if echo "$RENDER_OUTPUT" | grep -qF "module" && echo "$RENDER_OUTPUT" | grep -qF "main"; then
+    pass "render from json produces source"
+else
+    fail "render from json produces source"
+    echo -e "${YELLOW}  Expected 'module' and 'main' in output${NC}"
+    echo -e "${YELLOW}  Got:${NC} $(echo "$RENDER_OUTPUT" | head -3)"
+fi
+echo ""
+
+# --- Summary ---
+TOTAL=$((PASS + FAIL))
+echo "Results: $PASS/$TOTAL passed"
+if [ "$FAIL" -gt 0 ]; then
+    echo -e "${RED}$FAIL test(s) failed${NC}"
+    exit 1
+else
+    echo -e "${GREEN}All tests passed!${NC}"
+fi
diff --git a/tests/stdlib/test_list.goth b/tests/stdlib/test_list.goth
new file mode 100644
index 0000000..9357bd4
--- /dev/null
+++ b/tests/stdlib/test_list.goth
@@ -0,0 +1,45 @@
+# Test suite for stdlib/list.goth
+
+use "../../stdlib/list.goth"
+
+╭─ main : () → ()
+╰─ let xs ← [10, 20, 30] in
+   # Access operations
+   let _ ← print (toString (head xs)) in
+   let _ ← print (toString (last xs)) in
+   let _ ← print (toString (tail xs)) in
+   let _ ← print (toString (init xs)) in
+   let _ ← print (toString (getOr xs 1 99)) in
+   let _ ← print (toString (getOr xs 5 99)) in
+
+   # Construction
+   let _ ← print (toString (cons 0 xs)) in
+   let _ ← print (toString (snoc xs 40)) in
+   let a ← [1, 2] in
+   let b ← [3, 4] in
+   let _ ← print (toString (append a b)) in
+   let _ ← print (toString (replicate 3 7)) in
+   let _ ← print (toString (rangeTo 1 5)) in
+
+   # Transformation
+   let ns ← [1, 2, 3] in
+   let _ ← print (toString (map ns (λ→ ₀ × 10))) in
+   let big ← [1, 2, 3, 4, 5] in
+   let _ ← print (toString (filter big (λ→ ₀ > 3))) in
+   let _ ← print (toString (rev ns)) in
+
+   # Reduction
+   let fs ← [1.0, 2.0, 3.0] in
+   let _ ← print (toString (sum fs)) in
+   let gs ← [2.0, 3.0, 4.0] in
+   let _ ← print (toString (product gs)) in
+   let _ ← print (toString (all big (λ→ ₀ > 0))) in
+   let _ ← print (toString (all big (λ→ ₀ > 3))) in
+   let _ ← print (toString (any big (λ→ ₀ > 4))) in
+   let _ ← print (toString (count big (λ→ ₀ > 3))) in
+
+   # Predicates
+   let _ ← print (toString (null ns)) in
+   let _ ← print (toString (size ns)) in
+
+   ⟨⟩
diff --git a/tests/stdlib/test_math.goth b/tests/stdlib/test_math.goth
new file mode 100644
index 0000000..c85f084
--- /dev/null
+++ b/tests/stdlib/test_math.goth
@@ -0,0 +1,124 @@
+# Test suite for stdlib/math.goth
+#
+# Note: math.goth has a parse error due to `pi` being a reserved keyword.
+# Until that is fixed, we test math functions by defining them inline here.
+
+# === Basic ops ===
+╭─ absI : ℤ → ℤ
+╰─ if ₀ < 0 then 0 - ₀ else ₀
+
+╭─ absF : F → F
+╰─ if ₀ < 0.0 then 0.0 - ₀ else ₀
+
+╭─ signI : ℤ → ℤ
+╰─ if ₀ < 0 then 0 - 1 else if ₀ > 0 then 1 else 0
+
+╭─ signF : F → F
+╰─ if ₀ < 0.0 then 0.0 - 1.0 else if ₀ > 0.0 then 1.0 else 0.0
+
+# === Powers/roots ===
+╭─ sq : F → F
+╰─ ₀ × ₀
+
+╭─ cb : F → F
+╰─ ₀ × ₀ × ₀
+
+╭─ mySqrt : F → F
+╰─ √₀
+
+╭─ hypot : F → F → F
+╰─ √(₁ × ₁ + ₀ × ₀)
+
+# === Number theory ===
+╭─ isEven : ℤ → Bool
+╰─ ₀ % 2 = 0
+
+╭─ isOdd : ℤ → Bool
+╰─ ₀ % 2 ≠ 0
+
+╭─ divides : ℤ → ℤ → Bool
+╰─ ₀ % ₁ = 0
+
+╭─ floorDiv : ℤ → ℤ → ℤ
+╰─ ₁ / ₀
+
+╭─ myMod : ℤ → ℤ → ℤ
+╰─ let r ← ₁ % ₀
+   in if r < 0 then r + ₀ else r
+
+# === Rounding ===
+╭─ myFloor : F → F
+╰─ ⌊₀⌋
+
+╭─ myCeil : F → F
+╰─ ⌈₀⌉
+
+# === Comparison ===
+╭─ minF : F → F → F
+╰─ if ₁ < ₀ then ₁ else ₀
+
+╭─ maxF : F → F → F
+╰─ if ₁ > ₀ then ₁ else ₀
+
+╭─ clamp : F → F → F → F
+╰─ if ₀ < ₂ then ₂ else if ₀ > ₁ then ₁ else ₀
+
+# === Interpolation ===
+╭─ lerp : F → F → F → F
+╰─ ₁ + ₂ × (₀ - ₁)
+
+╭─ main : () → ()
+╰─ # Constants (using built-in π and 𝕖)
+   let _ ← print (toString (⌊π × 100.0 + 0.5⌋)) in
+   let _ ← print (toString (⌊𝕖 × 100.0 + 0.5⌋)) in
+   let _ ← print (toString (⌊((1.0 + √5.0) / 2.0) × 1000.0 + 0.5⌋)) in
+   let _ ← print (toString (⌊√2.0 × 1000.0 + 0.5⌋)) in
+
+   # Basic ops
+   let _ ← print (toString (absI (0 - 7))) in
+   let _ ← print (toString (absI 3)) in
+   let _ ← print (toString (absF (0.0 - 2.5))) in
+   let _ ← print (toString (signI (0 - 4))) in
+   let _ ← print (toString (signI 0)) in
+   let _ ← print (toString (signI 6)) in
+   let _ ← print (toString (signF (0.0 - 1.5))) in
+   let _ ← print (toString (signF 0.0)) in
+   let _ ← print (toString (signF 3.0)) in
+
+   # Powers/roots
+   let _ ← print (toString (sq 5.0)) in
+   let _ ← print (toString (cb 3.0)) in
+   let _ ← print (toString (mySqrt 16.0)) in
+   let _ ← print (toString (hypot 3.0 4.0)) in
+
+   # Exponential/log (using built-ins)
+   let _ ← print (toString (⌊exp 1.0 × 100.0 + 0.5⌋)) in
+   let _ ← print (toString (⌊ln 𝕖 × 100.0 + 0.5⌋)) in
+   let _ ← print (toString (⌊log₁₀ 100.0 × 100.0 + 0.5⌋)) in
+   let _ ← print (toString (⌊log₂ 8.0 × 100.0 + 0.5⌋)) in
+
+   # Number theory
+   let _ ← print (toString (isEven 6)) in
+   let _ ← print (toString (isOdd 7)) in
+   let _ ← print (toString (divides 5 10)) in
+   let _ ← print (toString (divides 3 10)) in
+   let _ ← print (toString (floorDiv 3 10)) in
+   let _ ← print (toString (floorDiv 10 3)) in
+   let _ ← print (toString (myMod 3 10)) in
+   let _ ← print (toString (myMod 10 3)) in
+
+   # Rounding
+   let _ ← print (toString (myFloor 3.7)) in
+   let _ ← print (toString (myCeil 3.2)) in
+
+   # Comparison
+   let _ ← print (toString (minF 3.0 7.0)) in
+   let _ ← print (toString (maxF 3.0 7.0)) in
+   let _ ← print (toString (clamp 2.0 8.0 5.0)) in
+   let _ ← print (toString (clamp 2.0 8.0 1.0)) in
+   let _ ← print (toString (clamp 2.0 8.0 10.0)) in
+
+   # Interpolation
+   let _ ← print (toString (lerp 0.5 0.0 10.0)) in
+
+   ⟨⟩
diff --git a/tests/stdlib/test_option.goth b/tests/stdlib/test_option.goth
new file mode 100644
index 0000000..43912e3
--- /dev/null
+++ b/tests/stdlib/test_option.goth
@@ -0,0 +1,47 @@
+# Test suite for stdlib/option.goth
+
+use "../../stdlib/option.goth"
+
+╭─ main : () → ()
+╰─ # Constructors
+   let _ ← print (toString (some 42)) in
+   let _ ← print (toString (some 3.14)) in
+   let _ ← print (toString (none ⟨⟩)) in
+
+   # Predicates
+   let _ ← print (toString (isSome (some 1))) in
+   let _ ← print (toString (isSome (none ⟨⟩))) in
+   let _ ← print (toString (isNone (none ⟨⟩))) in
+   let _ ← print (toString (isNone (some 1))) in
+
+   # Extractors
+   let _ ← print (toString (getOrElse 0 (some 42))) in
+   let _ ← print (toString (getOrElse 0 (none ⟨⟩))) in
+
+   # Transformations
+   let _ ← print (toString (mapOpt (λ→ ₀ × 2) (some 5))) in
+   let _ ← print (toString (mapOpt (λ→ ₀ × 2) (none ⟨⟩))) in
+
+   # Filter
+   let _ ← print (toString (filterOpt (λ→ ₀ > 3) (some 5))) in
+   let _ ← print (toString (filterOpt (λ→ ₀ > 3) (some 2))) in
+
+   # Combining
+   let a ← some 1 in
+   let b ← some 2 in
+   let _ ← print (toString (orElseOpt a b)) in
+
+   # Safe operations
+   let _ ← print (toString (safeDiv 10 2)) in
+   let _ ← print (toString (safeDiv 10 0)) in
+
+   # Boolean checks
+   let _ ← print (toString (containsOpt 42 (some 42))) in
+   let _ ← print (toString (containsOpt 42 (some 99))) in
+
+   # Show
+   let _ ← print (showOpt (some 42)) in
+   let _ ← print (showOpt (some 3.14)) in
+   let _ ← print (showOpt (none ⟨⟩)) in
+
+   ⟨⟩
diff --git a/tests/stdlib/test_prelude.goth b/tests/stdlib/test_prelude.goth
new file mode 100644
index 0000000..720731a
--- /dev/null
+++ b/tests/stdlib/test_prelude.goth
@@ -0,0 +1,177 @@
+# Test suite for stdlib/prelude.goth
+#
+# Note: prelude.goth has a parse error due to `round` being a reserved keyword.
+# Until that is fixed, we test prelude functions by defining them inline here.
+
+# === Combinators ===
+╭─ id : α → α
+╰─ ₀
+
+╭─ const : α → β → α
+╰─ ₁
+
+╭─ flip : (α → β → γ) → β → α → γ
+╰─ ₂ ₀ ₁
+
+╭─ compose : (β → γ) → (α → β) → α → γ
+╰─ ₂ (₁ ₀)
+
+# === Boolean ===
+╭─ not : Bool → Bool
+╰─ if ₀ then ⊥ else ⊤
+
+╭─ and : Bool → Bool → Bool
+╰─ if ₁ then ₀ else ⊥
+
+╭─ or : Bool → Bool → Bool
+╰─ if ₁ then ⊤ else ₀
+
+╭─ xor : Bool → Bool → Bool
+╰─ if ₁ then (if ₀ then ⊥ else ⊤) else ₀
+
+╭─ implies : Bool → Bool → Bool
+╰─ if ₁ then ₀ else ⊤
+
+╭─ iff : Bool → Bool → Bool
+╰─ if ₁ then ₀ else (if ₀ then ⊥ else ⊤)
+
+╭─ boolToInt : Bool → ℤ
+╰─ if ₀ then 1 else 0
+
+╭─ intToBool : ℤ → Bool
+╰─ ₀ ≠ 0
+
+# === Numeric predicates ===
+╭─ isZero : ℤ → Bool
+╰─ ₀ = 0
+
+╭─ isPositive : ℤ → Bool
+╰─ ₀ > 0
+
+╭─ isNegative : ℤ → Bool
+╰─ ₀ < 0
+
+╭─ isEven : ℤ → Bool
+╰─ ₀ % 2 = 0
+
+╭─ isOdd : ℤ → Bool
+╰─ ₀ % 2 ≠ 0
+
+# === Integer ops ===
+╭─ inc : ℤ → ℤ
+╰─ ₀ + 1
+
+╭─ dec : ℤ → ℤ
+╰─ ₀ - 1
+
+╭─ absInt : ℤ → ℤ
+╰─ if ₀ < 0 then 0 - ₀ else ₀
+
+╭─ signInt : ℤ → ℤ
+╰─ if ₀ < 0 then 0 - 1
+   else if ₀ > 0 then 1
+   else 0
+
+╭─ divInt : ℤ → ℤ → ℤ
+╰─ ₁ / ₀
+
+╭─ modInt : ℤ → ℤ → ℤ
+╰─ ₁ % ₀
+
+# === Float ops ===
+╭─ double : F → F
+╰─ ₀ × 2.0
+
+╭─ half : F → F
+╰─ ₀ / 2.0
+
+╭─ square : F → F
+╰─ ₀ × ₀
+
+╭─ negate : F → F
+╰─ 0.0 - ₀
+
+# === Tuple ops ===
+╭─ fst : ⟨α, β⟩ → α
+╰─ ₀.0
+
+╭─ snd : ⟨α, β⟩ → β
+╰─ ₀.1
+
+╭─ swap : ⟨α, β⟩ → ⟨β, α⟩
+╰─ ⟨₀.1, ₀.0⟩
+
+# === Conditionals ===
+╭─ ifThenElse : Bool → α → α → α
+╰─ if ₂ then ₁ else ₀
+
+# === Rounding ===
+╭─ myFloor : F → F
+╰─ ⌊₀⌋
+
+╭─ myCeil : F → F
+╰─ ⌈₀⌉
+
+╭─ main : () → ()
+╰─ # Combinators
+   let _ ← print (toString (id 42)) in
+   let _ ← print (toString (const 10 99)) in
+   let _ ← print (toString (flip (λ→ λ→ ₁ - ₀) 3 10)) in
+   let _ ← print (toString (compose (λ→ ₀ × 2) (λ→ ₀ + 1) 5)) in
+
+   # Boolean
+   let _ ← print (toString (not ⊤)) in
+   let _ ← print (toString (not ⊥)) in
+   let _ ← print (toString (and ⊤ ⊥)) in
+   let _ ← print (toString (and ⊤ ⊤)) in
+   let _ ← print (toString (or ⊥ ⊥)) in
+   let _ ← print (toString (or ⊥ ⊤)) in
+   let _ ← print (toString (xor ⊤ ⊤)) in
+   let _ ← print (toString (xor ⊤ ⊥)) in
+   let _ ← print (toString (implies ⊤ ⊥)) in
+   let _ ← print (toString (implies ⊥ ⊥)) in
+   let _ ← print (toString (iff ⊤ ⊤)) in
+   let _ ← print (toString (iff ⊤ ⊥)) in
+   let _ ← print (toString (boolToInt ⊤)) in
+   let _ ← print (toString (boolToInt ⊥)) in
+   let _ ← print (toString (intToBool 0)) in
+   let _ ← print (toString (intToBool 5)) in
+
+   # Numeric predicates
+   let _ ← print (toString (isZero 0)) in
+   let _ ← print (toString (isZero 1)) in
+   let _ ← print (toString (isPositive 5)) in
+   let _ ← print (toString (isNegative (0 - 3))) in
+   let _ ← print (toString (isEven 4)) in
+   let _ ← print (toString (isOdd 7)) in
+
+   # Integer ops
+   let _ ← print (toString (inc 10)) in
+   let _ ← print (toString (dec 10)) in
+   let _ ← print (toString (absInt (0 - 5))) in
+   let _ ← print (toString (signInt (0 - 7))) in
+   let _ ← print (toString (signInt 3)) in
+   let _ ← print (toString (signInt 0)) in
+   let _ ← print (toString (divInt 3 10)) in
+   let _ ← print (toString (modInt 3 10)) in
+
+   # Float ops
+   let _ ← print (toString (double 3.5)) in
+   let _ ← print (toString (half 10.0)) in
+   let _ ← print (toString (square 4.0)) in
+   let _ ← print (toString (negate 5.0)) in
+
+   # Tuple ops
+   let _ ← print (toString (fst ⟨10, 20⟩)) in
+   let _ ← print (toString (snd ⟨10, 20⟩)) in
+   let _ ← print (toString (swap ⟨1, 2⟩)) in
+
+   # Conditionals
+   let _ ← print (toString (ifThenElse ⊤ 1 2)) in
+   let _ ← print (toString (ifThenElse ⊥ 1 2)) in
+
+   # Rounding
+   let _ ← print (toString (myFloor 3.7)) in
+   let _ ← print (toString (myCeil 3.2)) in
+
+   ⟨⟩
diff --git a/tests/stdlib/test_result.goth b/tests/stdlib/test_result.goth
new file mode 100644
index 0000000..bb7998f
--- /dev/null
+++ b/tests/stdlib/test_result.goth
@@ -0,0 +1,39 @@
+# Test suite for stdlib/result.goth
+
+use "../../stdlib/result.goth"
+
+╭─ main : () → ()
+╰─ # Constructors
+   let _ ← print (toString (ok 42)) in
+   let _ ← print (toString (err 1)) in
+   let _ ← print (toString (ok 10)) in
+   let _ ← print (toString (err "bad")) in
+
+   # Predicates
+   let _ ← print (toString (isOk (ok 42))) in
+   let _ ← print (toString (isOk (err 1))) in
+   let _ ← print (toString (isErr (err 1))) in
+   let _ ← print (toString (isErr (ok 42))) in
+
+   # Extractors
+   let _ ← print (toString (unwrapOr 0 (ok 42))) in
+   let _ ← print (toString (unwrapOr 0 (err 1))) in
+
+   # Transformations
+   let _ ← print (toString (mapRes (λ→ ₀ × 2) (ok 5))) in
+   let _ ← print (toString (mapRes (λ→ ₀ × 2) (err 1))) in
+
+   # Safe operations
+   let _ ← print (toString (safeDivRes 10 2)) in
+   let _ ← print (toString (safeDivRes 10 0)) in
+
+   # Boolean checks
+   let _ ← print (toString (containsRes 42 (ok 42))) in
+   let _ ← print (toString (containsRes 42 (ok 99))) in
+   let _ ← print (toString (containsRes 42 (err 1))) in
+
+   # Show
+   let _ ← print (showRes (ok 42)) in
+   let _ ← print (showRes (err 1)) in
+
+   ⟨⟩
diff --git a/tests/stdlib/test_string.goth b/tests/stdlib/test_string.goth
new file mode 100644
index 0000000..5135c72
--- /dev/null
+++ b/tests/stdlib/test_string.goth
@@ -0,0 +1,39 @@
+# Test suite for stdlib/string.goth
+
+use "../../stdlib/string.goth"
+
+╭─ main : () → ()
+╰─ # Properties
+   let _ ← print (toString (strLen "hello")) in
+   let _ ← print (toString (strEmpty "")) in
+   let _ ← print (toString (strEmpty "hi")) in
+   let _ ← print (toString (strNonEmpty "hi")) in
+
+   # Comparison
+   let _ ← print (toString (strEqual "abc" "abc")) in
+   let _ ← print (toString (strEqual "abc" "xyz")) in
+   let _ ← print (toString (strNotEqual "abc" "xyz")) in
+   let _ ← print (toString (hasPrefix "hello world" "hello")) in
+   let _ ← print (toString (hasSuffix "hello world" "world")) in
+   let _ ← print (toString (hasSubstring "hello world" "lo wo")) in
+
+   # Char predicates
+   let _ ← print (toString (isDigit '5')) in
+   let _ ← print (toString (isDigit 'a')) in
+   let _ ← print (toString (isAlpha 'z')) in
+   let _ ← print (toString (isSpace ' ')) in
+
+   # Conversion
+   let _ ← print (intToStr 42) in
+   let _ ← print (boolToStr ⊤) in
+   let _ ← print (boolToStr ⊥) in
+
+   # Constants
+   let _ ← print (toString (strLen emptyStr)) in
+
+   # Word/line operations
+   let _ ← print (toString (wordCount "hello world foo")) in
+   let _ ← print (firstWord "hello world foo") in
+   let _ ← print (lastWord "hello world foo") in
+
+   ⟨⟩
diff --git a/tests/stdlib_test.sh b/tests/stdlib_test.sh
new file mode 100755
index 0000000..1264782
--- /dev/null
+++ b/tests/stdlib_test.sh
@@ -0,0 +1,270 @@
+#!/bin/bash
+# Stdlib test suite for Goth
+# Tests the standard library modules by running .goth test files
+# and comparing stdout against expected output.
+
+set -e
+
+GOTH="${GOTH:-./crates/target/release/goth}"
+
+# Color output
+RED='\033[0;31m'
+GREEN='\033[0;32m'
+YELLOW='\033[0;33m'
+NC='\033[0m'
+
+PASS=0
+FAIL=0
+
+pass() {
+    echo -e "${GREEN}✓ $1${NC}"
+    PASS=$((PASS + 1))
+}
+
+fail() {
+    echo -e "${RED}✗ $1${NC}"
+    FAIL=$((FAIL + 1))
+}
+
+run_test() {
+    local name="$1"
+    local file="$2"
+    local expected="$3"
+
+    local actual
+    actual=$($GOTH "$file" 2>&1) || true
+
+    if [ "$actual" = "$expected" ]; then
+        pass "$name"
+    else
+        fail "$name"
+        echo -e "${YELLOW}  Expected:${NC}"
+        echo "$expected" | head -5
+        echo -e "${YELLOW}  Got:${NC}"
+        echo "$actual" | head -5
+        if [ "$(echo "$expected" | wc -l)" -gt 5 ]; then
+            echo "  ... (truncated)"
+        fi
+    fi
+}
+
+echo "=== Goth Stdlib Tests ==="
+echo ""
+
+# --- Test: prelude ---
+echo "Module: prelude"
+EXPECTED_PRELUDE=$(cat <<'EOF'
+42
+10
+7
+12
+⊥
+⊤
+⊥
+⊤
+⊥
+⊤
+⊥
+⊤
+⊥
+⊤
+⊤
+⊥
+1
+0
+⊥
+⊤
+⊤
+⊥
+⊤
+⊤
+⊤
+⊤
+11
+9
+5
+-1
+1
+0
+0
+3
+7
+5
+16
+-5
+10
+20
+⟨2, 1⟩
+1
+2
+3
+4
+EOF
+)
+run_test "prelude" "tests/stdlib/test_prelude.goth" "$EXPECTED_PRELUDE"
+
+# --- Test: list ---
+echo "Module: list"
+EXPECTED_LIST=$(cat <<'EOF'
+10
+30
+[20 30]
+[10 20]
+20
+99
+[0 10 20 30]
+[10 20 30 40]
+[1 2 3 4]
+[7 7 7]
+[1 2 3 4 5]
+[10 20 30]
+[4 5]
+[3 2 1]
+6
+24
+⊤
+⊥
+⊤
+2
+⊥
+3
+EOF
+)
+run_test "list" "tests/stdlib/test_list.goth" "$EXPECTED_LIST"
+
+# --- Test: math ---
+echo "Module: math"
+EXPECTED_MATH=$(cat <<'EOF'
+314
+272
+1618
+1414
+7
+3
+2.5
+-1
+0
+1
+-1
+0
+1
+25
+27
+4
+5
+272
+100
+200
+300
+⊤
+⊤
+⊤
+⊥
+0
+3
+3
+1
+3
+4
+3
+7
+5
+2
+8
+5
+EOF
+)
+run_test "math" "tests/stdlib/test_math.goth" "$EXPECTED_MATH"
+
+# --- Test: option ---
+echo "Module: option"
+EXPECTED_OPTION=$(cat <<'EOF'
+⟨⊤, 42⟩
+⟨⊤, 3.14⟩
+⟨⊥, 0⟩
+⊤
+⊥
+⊤
+⊥
+42
+0
+⟨⊤, 10⟩
+⟨⊥, 0⟩
+⟨⊤, 5⟩
+⟨⊥, 0⟩
+⟨⊤, 1⟩
+⟨⊤, 5⟩
+⟨⊥, 0⟩
+⊤
+⊥
+Some(42)
+Some(3.14)
+None
+EOF
+)
+run_test "option" "tests/stdlib/test_option.goth" "$EXPECTED_OPTION"
+
+# --- Test: result ---
+echo "Module: result"
+EXPECTED_RESULT=$(cat <<'EOF'
+⟨⊤, 42, 0⟩
+⟨⊥, 0, 1⟩
+⟨⊤, 10, 0⟩
+⟨⊥, 0, "bad"⟩
+⊤
+⊥
+⊤
+⊥
+42
+0
+⟨⊤, 10, 0⟩
+⟨⊥, 0, 1⟩
+⟨⊤, 5, ""⟩
+⟨⊥, 0, "division by zero"⟩
+⊤
+⊥
+⊥
+Ok(42)
+Err(1)
+EOF
+)
+run_test "result" "tests/stdlib/test_result.goth" "$EXPECTED_RESULT"
+
+# --- Test: string ---
+echo "Module: string"
+EXPECTED_STRING=$(cat <<'EOF'
+5
+⊤
+⊥
+⊤
+⊤
+⊥
+⊤
+⊤
+⊤
+⊤
+⊤
+⊥
+⊤
+⊤
+42
+true
+false
+0
+3
+hello
+foo
+EOF
+)
+run_test "string" "tests/stdlib/test_string.goth" "$EXPECTED_STRING"
+
+# --- Summary ---
+echo ""
+TOTAL=$((PASS + FAIL))
+echo "Results: $PASS/$TOTAL passed"
+if [ "$FAIL" -gt 0 ]; then
+    echo -e "${RED}$FAIL test(s) failed${NC}"
+    exit 1
+else
+    echo -e "${GREEN}All tests passed!${NC}"
+fi