diff --git a/seq/src/model/forth.rs b/seq/src/model/forth.rs
index 7291d52..b268bf8 100644
--- a/seq/src/model/forth.rs
+++ b/seq/src/model/forth.rs
@@ -24,7 +24,7 @@ impl StepContext {
 pub type Variables = Arc<Mutex<HashMap<String, Value>>>;
 pub type Rng = Arc<Mutex<StdRng>>;
 
-#[derive(Clone, Debug)]
+#[derive(Clone, Debug, PartialEq)]
 pub enum Value {
     Int(i64),
     Float(f64),
@@ -1628,14 +1628,29 @@ fn compile_if(tokens: &[Token]) -> Result<(Vec<Op>, Vec<Op>, usize), String> {
     Ok((then_ops, else_ops, then_pos + 1))
 }
 
+pub type Stack = Arc<Mutex<Vec<Value>>>;
+
 pub struct Forth {
+    stack: Stack,
     vars: Variables,
     rng: Rng,
 }
 
 impl Forth {
     pub fn new(vars: Variables, rng: Rng) -> Self {
-        Self { vars, rng }
+        Self {
+            stack: Arc::new(Mutex::new(Vec::new())),
+            vars,
+            rng,
+        }
+    }
+
+    pub fn stack(&self) -> Vec<Value> {
+        self.stack.lock().unwrap().clone()
+    }
+
+    pub fn clear_stack(&self) {
+        self.stack.lock().unwrap().clear();
     }
 
     pub fn evaluate(&self, script: &str, ctx: &StepContext) -> Result<Vec<String>, String> {
@@ -1649,7 +1664,7 @@ impl Forth {
     }
 
     fn execute(&self, ops: &[Op], ctx: &StepContext) -> Result<Vec<String>, String> {
-        let mut stack: Vec<Value> = Vec::new();
+        let mut stack = self.stack.lock().unwrap();
         let mut outputs: Vec<String> = Vec::new();
         let mut time_stack: Vec<TimeContext> = vec![TimeContext {
             start: 0.0,
diff --git a/seq/src/model/forth_tests/arithmetic.rs b/seq/src/model/forth_tests/arithmetic.rs
new file mode 100644
index 0000000..a1b122c
--- /dev/null
+++ b/seq/src/model/forth_tests/arithmetic.rs
@@ -0,0 +1,152 @@
+use super::harness::*;
+
+#[test]
+fn add_integers() {
+    expect_int("2 3 +", 5);
+}
+
+#[test]
+fn add_floats() {
+    expect_int("2.5 3.5 +", 6);
+}
+
+#[test]
+fn add_mixed() {
+    expect_float("2 3.5 +", 5.5);
+}
+
+#[test]
+fn sub() {
+    expect_int("10 3 -", 7);
+}
+
+#[test]
+fn sub_negative() {
+    expect_int("3 10 -", -7);
+}
+
+#[test]
+fn mul() {
+    expect_int("4 5 *", 20);
+}
+
+#[test]
+fn mul_floats() {
+    expect_int("2.5 4 *", 10);
+}
+
+#[test]
+fn div() {
+    expect_int("10 2 /", 5);
+}
+
+#[test]
+fn div_float_result() {
+    expect_float("7 2 /", 3.5);
+}
+
+#[test]
+fn modulo() {
+    expect_int("7 3 mod", 1);
+}
+
+#[test]
+fn modulo_exact() {
+    expect_int("9 3 mod", 0);
+}
+
+#[test]
+fn neg_int() {
+    expect_int("5 neg", -5);
+}
+
+#[test]
+fn neg_float() {
+    expect_float("3.5 neg", -3.5);
+}
+
+#[test]
+fn neg_double() {
+    expect_int("-5 neg", 5);
+}
+
+#[test]
+fn abs_positive() {
+    expect_int("5 abs", 5);
+}
+
+#[test]
+fn abs_negative() {
+    expect_int("-5 abs", 5);
+}
+
+#[test]
+fn abs_float() {
+    expect_float("-3.5 abs", 3.5);
+}
+
+#[test]
+fn floor() {
+    expect_int("3.7 floor", 3);
+}
+
+#[test]
+fn floor_negative() {
+    expect_int("-3.2 floor", -4);
+}
+
+#[test]
+fn ceil() {
+    expect_int("3.2 ceil", 4);
+}
+
+#[test]
+fn ceil_negative() {
+    expect_int("-3.7 ceil", -3);
+}
+
+#[test]
+fn round_down() {
+    expect_int("3.4 round", 3);
+}
+
+#[test]
+fn round_up() {
+    expect_int("3.6 round", 4);
+}
+
+#[test]
+fn round_half() {
+    expect_int("3.5 round", 4);
+}
+
+#[test]
+fn min() {
+    expect_int("3 5 min", 3);
+}
+
+#[test]
+fn min_reverse() {
+    expect_int("5 3 min", 3);
+}
+
+#[test]
+fn max() {
+    expect_int("3 5 max", 5);
+}
+
+#[test]
+fn max_reverse() {
+    expect_int("5 3 max", 5);
+}
+
+#[test]
+fn chain() {
+    // (2 + 3) * 4 - 1 = 19
+    expect_int("2 3 + 4 * 1 -", 19);
+}
+
+#[test]
+fn underflow() {
+    expect_error("1 +", "stack underflow");
+}
diff --git a/seq/src/model/forth_tests/comparison.rs b/seq/src/model/forth_tests/comparison.rs
new file mode 100644
index 0000000..5926493
--- /dev/null
+++ b/seq/src/model/forth_tests/comparison.rs
@@ -0,0 +1,136 @@
+use super::harness::*;
+
+#[test]
+fn eq_true() {
+    expect_int("3 3 =", 1);
+}
+
+#[test]
+fn eq_false() {
+    expect_int("3 4 =", 0);
+}
+
+#[test]
+fn eq_mixed_types() {
+    expect_int("3.0 3 =", 1);
+}
+
+#[test]
+fn ne_true() {
+    expect_int("3 4 <>", 1);
+}
+
+#[test]
+fn ne_false() {
+    expect_int("3 3 <>", 0);
+}
+
+#[test]
+fn lt_true() {
+    expect_int("2 3 <", 1);
+}
+
+#[test]
+fn lt_equal() {
+    expect_int("3 3 <", 0);
+}
+
+#[test]
+fn lt_false() {
+    expect_int("4 3 <", 0);
+}
+
+#[test]
+fn gt_true() {
+    expect_int("4 3 >", 1);
+}
+
+#[test]
+fn gt_equal() {
+    expect_int("3 3 >", 0);
+}
+
+#[test]
+fn gt_false() {
+    expect_int("2 3 >", 0);
+}
+
+#[test]
+fn le_less() {
+    expect_int("2 3 <=", 1);
+}
+
+#[test]
+fn le_equal() {
+    expect_int("3 3 <=", 1);
+}
+
+#[test]
+fn le_greater() {
+    expect_int("4 3 <=", 0);
+}
+
+#[test]
+fn ge_greater() {
+    expect_int("4 3 >=", 1);
+}
+
+#[test]
+fn ge_equal() {
+    expect_int("3 3 >=", 1);
+}
+
+#[test]
+fn ge_less() {
+    expect_int("2 3 >=", 0);
+}
+
+#[test]
+fn and_tt() {
+    expect_int("1 1 and", 1);
+}
+
+#[test]
+fn and_tf() {
+    expect_int("1 0 and", 0);
+}
+
+#[test]
+fn and_ff() {
+    expect_int("0 0 and", 0);
+}
+
+#[test]
+fn or_tt() {
+    expect_int("1 1 or", 1);
+}
+
+#[test]
+fn or_tf() {
+    expect_int("1 0 or", 1);
+}
+
+#[test]
+fn or_ff() {
+    expect_int("0 0 or", 0);
+}
+
+#[test]
+fn not_true() {
+    expect_int("1 not", 0);
+}
+
+#[test]
+fn not_false() {
+    expect_int("0 not", 1);
+}
+
+#[test]
+fn truthy_nonzero() {
+    expect_int("5 not", 0);
+}
+
+#[test]
+fn truthy_negative() {
+    expect_int("-1 not", 0);
+}
diff --git a/seq/src/model/forth_tests/context.rs b/seq/src/model/forth_tests/context.rs
new file mode 100644
index 0000000..2ade530
--- /dev/null
+++ b/seq/src/model/forth_tests/context.rs
@@ -0,0 +1,71 @@
+use super::harness::*;
+
+#[test]
+fn step() {
+    let ctx = ctx_with(|c| c.step = 7);
+    let f = run_ctx("step", &ctx);
+    assert_eq!(stack_int(&f), 7);
+}
+
+#[test]
+fn beat() {
+    let ctx = ctx_with(|c| c.beat = 4.5);
+    let f = run_ctx("beat", &ctx);
+    assert!((stack_float(&f) - 4.5).abs() < 1e-9);
+}
+
+#[test]
+fn bank() {
+    let ctx = ctx_with(|c| c.bank = 2);
+    let f = run_ctx("bank", &ctx);
+    assert_eq!(stack_int(&f), 2);
+}
+
+#[test]
+fn pattern() {
+    let ctx = ctx_with(|c| c.pattern = 3);
+    let f = run_ctx("pattern", &ctx);
+    assert_eq!(stack_int(&f), 3);
+}
+
+#[test]
+fn tempo() {
+    let ctx = ctx_with(|c| c.tempo = 140.0);
+    let f = run_ctx("tempo", &ctx);
+    assert!((stack_float(&f) - 140.0).abs() < 1e-9);
+}
+
+#[test]
+fn phase() {
+    let ctx = ctx_with(|c| c.phase = 0.25);
+    let f = run_ctx("phase", &ctx);
+    assert!((stack_float(&f) - 0.25).abs() < 1e-9);
+}
+
+#[test]
+fn slot() {
+    let ctx = ctx_with(|c| c.slot = 5);
+    let f = run_ctx("slot", &ctx);
+    assert_eq!(stack_int(&f), 5);
+}
+
+#[test]
+fn runs() {
+    let ctx = ctx_with(|c| c.runs = 10);
+    let f = run_ctx("runs", &ctx);
+    assert_eq!(stack_int(&f), 10);
+}
+
+#[test]
+fn stepdur() {
+    // stepdur = 60.0 / tempo / 4.0 / speed = 60 / 120 / 4 / 1 = 0.125
+    let f = run("stepdur");
+    assert!((stack_float(&f) - 0.125).abs() < 1e-9);
+}
+
+#[test]
+fn context_in_computation() {
+    let ctx = ctx_with(|c| c.step = 3);
+    let f = run_ctx("60 step +", &ctx);
+    assert_eq!(stack_int(&f), 63);
+}
diff --git a/seq/src/model/forth_tests/control_flow.rs b/seq/src/model/forth_tests/control_flow.rs
new file mode 100644
index 0000000..8650d2a
--- /dev/null
+++ b/seq/src/model/forth_tests/control_flow.rs
@@ -0,0 +1,64 @@
+use super::harness::*;
+
+#[test]
+fn if_then_true() {
+    expect_int("1 if 42 then", 42);
+}
+
+#[test]
+fn if_then_false() {
+    let f = run("0 if 42 then");
+    assert!(f.stack().is_empty());
+}
+
+#[test]
+fn if_then_with_base() {
+    expect_int("100 0 if 50 + then", 100);
+}
+
+#[test]
+fn if_else_true() {
+    expect_int("1 if 42 else 99 then", 42);
+}
+
+#[test]
+fn if_else_false() {
+    expect_int("0 if 42 else 99 then", 99);
+}
+
+#[test]
+fn nested_tt() {
+    expect_int("1 if 1 if 100 else 200 then else 300 then", 100);
+}
+
+#[test]
+fn nested_tf() {
+    expect_int("1 if 0 if 100 else 200 then else 300 then", 200);
+}
+
+#[test]
+fn nested_f() {
+    expect_int("0 if 1 if 100 else 200 then else 300 then", 300);
+}
+
+#[test]
+fn if_with_computation() {
+    expect_int("3 2 > if 42 else 99 then", 42);
+}
+
+#[test]
+fn missing_then() {
+    expect_error("1 if 42", "missing 'then'");
+}
+
+#[test]
+fn deeply_nested() {
+    expect_int("1 if 1 if 1 if 42 then then then", 42);
+}
+
+#[test]
+fn chained_conditionals() {
+    // First if leaves nothing, second if runs
+    let f = run("0 if 1 then 1 if 42 then");
+    assert_eq!(stack_int(&f), 42);
+}
diff --git a/seq/src/model/forth_tests/errors.rs b/seq/src/model/forth_tests/errors.rs
new file mode 100644
index 0000000..0edae38
--- /dev/null
+++ b/seq/src/model/forth_tests/errors.rs
@@ -0,0 +1,106 @@
+use super::harness::*;
+
+#[test]
+fn empty_script() {
+    expect_error("", "empty script");
+}
+
+#[test]
+fn whitespace_only() {
+    expect_error("   \n\t  ", "empty script");
+}
+
+#[test]
+fn unknown_word() {
+    expect_error("foobar", "unknown word");
+}
+
+#[test]
+fn string_not_number() {
+    expect_error(r#""hello" neg"#, "expected number");
+}
+
+#[test]
+fn get_expects_string() {
+    expect_error("42 get", "expected string");
+}
+
+#[test]
+fn set_expects_string() {
+    expect_error("1 2 set", "expected string");
+}
+
+#[test]
+fn comment_ignored() {
+    expect_int("1 (this is a comment) 2 +", 3);
+}
+
+#[test]
+fn multiline_comment() {
+    expect_int("1 (multi\nline\ncomment) 2 +", 3);
+}
+
+#[test]
+fn negative_literal() {
+    expect_int("-5", -5);
+}
+
+#[test]
+fn float_literal() {
+    let f = run("3.14159");
+    let val = stack_float(&f);
+    assert!((val - 3.14159).abs() < 1e-9);
+}
+
+#[test]
+fn string_with_spaces() {
+    let f = run(r#""hello world" "x" set "x" get"#);
+    match stack_top(&f) {
+        crate::model::forth::Value::Str(s) => assert_eq!(s, "hello world"),
+        other => panic!("expected string, got {:?}", other),
+    }
+}
+
+#[test]
+fn list_count() {
+    // [ 1 2 3 ] => stack: 1 2 3 3 (items + count on top)
+    let f = run("[ 1 2 3 ]");
+    assert_eq!(stack_int(&f), 3);
+}
+
+#[test]
+fn list_empty() {
+    expect_int("[ ]", 0);
+}
+
+#[test]
+fn list_preserves_values() {
+    // [ 10 20 ] => stack: 10 20 2
+    // drop => 10 20
+    // + => 30
+    expect_int("[ 10 20 ] drop +", 30);
+}
+
+#[test]
+fn conditional_based_on_step() {
+    let ctx0 = ctx_with(|c| c.step = 0);
+    let ctx1 = ctx_with(|c| c.step = 1);
+
+    let f0 = run_ctx("step 2 mod 0 = if 100 else 200 then", &ctx0);
+    let f1 = run_ctx("step 2 mod 0 = if 100 else 200 then", &ctx1);
+
+    assert_eq!(stack_int(&f0), 100);
+    assert_eq!(stack_int(&f1), 200);
+}
+
+#[test]
+fn accumulator() {
+    let f = forth();
+    let ctx = default_ctx();
+    f.evaluate(r#"0 "acc" set"#, &ctx).unwrap();
+    for _ in 0..5 {
+        f.clear_stack();
+        f.evaluate(r#""acc" get 1 + dup "acc" set"#, &ctx).unwrap();
+    }
+    assert_eq!(stack_int(&f), 5);
+}
diff --git a/seq/src/model/forth_tests/harness.rs b/seq/src/model/forth_tests/harness.rs
new file mode 100644
index 0000000..0e64318
--- /dev/null
+++ b/seq/src/model/forth_tests/harness.rs
@@ -0,0 +1,136 @@
+use crate::model::forth::{Forth, Rng, StepContext, Value, Variables};
+use rand::rngs::StdRng;
+use rand::SeedableRng;
+use std::collections::HashMap;
+use std::sync::{Arc, Mutex};
+
+pub fn default_ctx() -> StepContext {
+    StepContext {
+        step: 0,
+        beat: 0.0,
+        bank: 0,
+        pattern: 0,
+        tempo: 120.0,
+        phase: 0.0,
+        slot: 0,
+        runs: 0,
+        speed: 1.0,
+    }
+}
+
+pub fn ctx_with(f: impl FnOnce(&mut StepContext)) -> StepContext {
+    let mut ctx = default_ctx();
+    f(&mut ctx);
+    ctx
+}
+
+pub fn new_vars() -> Variables {
+    Arc::new(Mutex::new(HashMap::new()))
+}
+
+pub fn seeded_rng(seed: u64) -> Rng {
+    Arc::new(Mutex::new(StdRng::seed_from_u64(seed)))
+}
+
+pub fn forth() -> Forth {
+    Forth::new(new_vars(), seeded_rng(42))
+}
+
+pub fn forth_seeded(seed: u64) -> Forth {
+    Forth::new(new_vars(), seeded_rng(seed))
+}
+
+pub fn run(script: &str) -> Forth {
+    let f = forth();
+    f.evaluate(script, &default_ctx()).unwrap();
+    f
+}
+
+pub fn run_ctx(script: &str, ctx: &StepContext) -> Forth {
+    let f = forth();
+    f.evaluate(script, ctx).unwrap();
+    f
+}
+
+pub fn stack_top(f: &Forth) -> Value {
+    f.stack().pop().expect("stack empty")
+}
+
+pub fn stack_int(f: &Forth) -> i64 {
+    match stack_top(f) {
+        Value::Int(i) => i,
+        other => panic!("expected Int, got {:?}", other),
+    }
+}
+
+pub fn stack_float(f: &Forth) -> f64 {
+    match stack_top(f) {
+        Value::Float(x) => x,
+        Value::Int(i) => i as f64,
+        other => panic!("expected number, got {:?}", other),
+    }
+}
+
+pub fn expect_stack(script: &str, expected: &[Value]) {
+    let f = run(script);
+    let stack = f.stack();
+    assert_eq!(stack, expected, "script: {}", script);
+}
+
+pub fn expect_int(script: &str, expected: i64) {
+    expect_stack(script, &[Value::Int(expected)]);
+}
+
+pub fn expect_float(script: &str, expected: f64) {
+    let f = run(script);
+    let stack = f.stack();
+    assert_eq!(stack.len(), 1, "expected single value on stack");
+    let val = stack_float(&f);
+    assert!(
+        (val - expected).abs() < 1e-9,
+        "expected {}, got {}",
+        expected,
+        val
+    );
+}
+
+pub fn expect_floats_close(script: &str, expected: f64, epsilon: f64) {
+    let f = run(script);
+    let val = stack_float(&f);
+    assert!(
+        (val - expected).abs() < epsilon,
+        "expected ~{}, got {}",
+        expected,
+        val
+    );
+}
+
+pub fn expect_error(script: &str, expected_substr: &str) {
+    let f = forth();
+    let result = f.evaluate(script, &default_ctx());
+    assert!(result.is_err(), "expected error for '{}'", script);
+    let err = result.unwrap_err();
+    assert!(
+        err.contains(expected_substr),
+        "error '{}' does not contain '{}'",
+        err,
+        expected_substr
+    );
+}
+
+pub fn expect_outputs(script: &str, count: usize) -> Vec<String> {
+    let f = forth();
+    let outputs = f.evaluate(script, &default_ctx()).unwrap();
+    assert_eq!(outputs.len(), count, "expected {} outputs", count);
+    outputs
+}
+
+pub fn expect_output_contains(script: &str, substr: &str) {
+    let outputs = expect_outputs(script, 1);
+    assert!(
+        outputs[0].contains(substr),
+        "output '{}' does not contain '{}'",
+        outputs[0],
+        substr
+    );
+}
diff --git a/seq/src/model/forth_tests/mod.rs b/seq/src/model/forth_tests/mod.rs
new file mode 100644
index 0000000..5534465
--- /dev/null
+++ b/seq/src/model/forth_tests/mod.rs
@@ -0,0 +1,10 @@
+mod arithmetic;
+mod comparison;
+mod context;
+mod control_flow;
+mod errors;
+mod harness;
+mod randomness;
+mod sound;
+mod stack;
+mod variables;
diff --git a/seq/src/model/forth_tests/randomness.rs b/seq/src/model/forth_tests/randomness.rs
new file mode 100644
index 0000000..011d037
--- /dev/null
+++ b/seq/src/model/forth_tests/randomness.rs
@@ -0,0 +1,114 @@
+use super::harness::*;
+
+#[test]
+fn rand_in_range() {
+    let f = forth_seeded(12345);
+    f.evaluate("0 10 rand", &default_ctx()).unwrap();
+    let val = stack_float(&f);
+    assert!(val >= 0.0 && val < 10.0, "rand {} not in [0, 10)", val);
+}
+
+#[test]
+fn rand_deterministic() {
+    let f1 = forth_seeded(99);
+    let f2 = forth_seeded(99);
+    f1.evaluate("0 100 rand", &default_ctx()).unwrap();
+    f2.evaluate("0 100 rand", &default_ctx()).unwrap();
+    assert_eq!(f1.stack(), f2.stack());
+}
+
+#[test]
+fn rrand_inclusive() {
+    let f = forth_seeded(42);
+    for _ in 0..20 {
+        f.clear_stack();
+        f.evaluate("1 3 rrand", &default_ctx()).unwrap();
+        let val = stack_int(&f);
+        assert!(val >= 1 && val <= 3, "rrand {} not in [1, 3]", val);
+    }
+}
+
+#[test]
+fn seed_resets() {
+    let f1 = forth_seeded(1);
+    f1.evaluate("42 seed 0 100 rand", &default_ctx()).unwrap();
+    let f2 = forth_seeded(999);
+    f2.evaluate("42 seed 0 100 rand", &default_ctx()).unwrap();
+    assert_eq!(f1.stack(), f2.stack());
+}
+
+#[test]
+fn coin_binary() {
+    let f = forth_seeded(42);
+    f.evaluate("coin", &default_ctx()).unwrap();
+    let val = stack_int(&f);
+    assert!(val == 0 || val == 1);
+}
+
+#[test]
+fn chance_zero() {
+    expect_int("0.0 chance", 0);
+}
+
+#[test]
+fn chance_one() {
+    expect_int("1.0 chance", 1);
+}
+
+#[test]
+fn choose_from_list() {
+    let f = forth_seeded(42);
+    f.evaluate("10 20 30 3 choose", &default_ctx()).unwrap();
+    let val = stack_int(&f);
+    assert!(val == 10 || val == 20 || val == 30);
+}
+
+#[test]
+fn choose_underflow() {
+    expect_error("1 2 5 choose", "stack underflow");
+}
+
+#[test]
+fn cycle_deterministic() {
+    for runs in 0..6 {
+        let ctx = ctx_with(|c| c.runs = runs);
+        let f = run_ctx("10 20 30 3 cycle", &ctx);
+        let expected = [10, 20, 30][runs % 3];
+        assert_eq!(stack_int(&f), expected, "cycle at runs={}", runs);
+    }
+}
+
+#[test]
+fn cycle_zero_count() {
+    expect_error("1 2 3 0 cycle", "cycle count must be > 0");
+}
+
+#[test]
+fn mtof_a4() {
+    expect_float("69 mtof", 440.0);
+}
+
+#[test]
+fn mtof_octave() {
+    expect_float("81 mtof", 880.0);
+}
+
+#[test]
+fn mtof_c4() {
+    expect_floats_close("60 mtof", 261.6255653, 0.001);
+}
+
+#[test]
+fn ftom_440() {
+    expect_float("440 ftom", 69.0);
+}
+
+#[test]
+fn ftom_880() {
+    expect_float("880 ftom", 81.0);
+}
+
+#[test]
+fn mtof_ftom_roundtrip() {
+    expect_float("60 mtof ftom", 60.0);
+}
diff --git a/seq/src/model/forth_tests/sound.rs b/seq/src/model/forth_tests/sound.rs
new file mode 100644
index 0000000..f6beaf8
--- /dev/null
+++ b/seq/src/model/forth_tests/sound.rs
@@ -0,0 +1,101 @@
+use super::harness::*;
+
+#[test]
+fn basic_emit() {
+    let outputs = expect_outputs(r#""kick" sound emit"#, 1);
+    assert!(outputs[0].contains("sound/kick"));
+}
+
+#[test]
+fn alias_s() {
+    let outputs = expect_outputs(r#""snare" s emit"#, 1);
+    assert!(outputs[0].contains("sound/snare"));
+}
+
+#[test]
+fn with_params() {
+    let outputs = expect_outputs(r#""kick" s 440 freq 0.5 gain emit"#, 1);
+    assert!(outputs[0].contains("sound/kick"));
+    assert!(outputs[0].contains("freq/440"));
+    assert!(outputs[0].contains("gain/0.5"));
+}
+
+#[test]
+fn auto_dur() {
+    let outputs = expect_outputs(r#""kick" s emit"#, 1);
+    assert!(outputs[0].contains("dur/"));
+}
+
+#[test]
+fn auto_delaytime() {
+    let outputs = expect_outputs(r#""kick" s emit"#, 1);
+    assert!(outputs[0].contains("delaytime/"));
+}
+
+#[test]
+fn emit_no_sound() {
+    expect_error("emit", "no sound set");
+}
+
+#[test]
+fn implicit_emit() {
+    let outputs = expect_outputs(r#""kick" s 440 freq"#, 1);
+    assert!(outputs[0].contains("sound/kick"));
+    assert!(outputs[0].contains("freq/440"));
+}
+
+#[test]
+fn multiple_emits() {
+    let outputs = expect_outputs(r#""kick" s emit "snare" s emit"#, 2);
+    assert!(outputs[0].contains("sound/kick"));
+    assert!(outputs[1].contains("sound/snare"));
+}
+
+#[test]
+fn subdivide_each() {
+    let outputs = expect_outputs(r#""kick" s 4 div each"#, 4);
+}
+
+#[test]
+fn window_pop() {
+    let outputs = expect_outputs(
+        r#"0.0 0.5 window "kick" s emit pop 0.5 1.0 window "snare" s emit"#,
+        2,
+    );
+    assert!(outputs[0].contains("sound/kick"));
+    assert!(outputs[1].contains("sound/snare"));
+}
+
+#[test]
+fn pop_root_fails() {
+    expect_error("pop", "cannot pop root time context");
+}
+
+#[test]
+fn subdivide_zero() {
+    expect_error(r#""kick" s 0 div each"#, "subdivide count must be > 0");
+}
+
+#[test]
+fn each_without_div() {
+    expect_error(r#""kick" s each"#, "each requires subdivide first");
+}
+
+#[test]
+fn envelope_params() {
+    let outputs = expect_outputs(
+        r#""synth" s 0.01 attack 0.1 decay 0.7 sustain 0.3 release emit"#,
+        1,
+    );
+    assert!(outputs[0].contains("attack/0.01"));
+    assert!(outputs[0].contains("decay/0.1"));
+    assert!(outputs[0].contains("sustain/0.7"));
+    assert!(outputs[0].contains("release/0.3"));
+}
+
+#[test]
+fn filter_params() {
+    let outputs = expect_outputs(r#""synth" s 2000 lpf 0.5 lpq emit"#, 1);
+    assert!(outputs[0].contains("lpf/2000"));
+    assert!(outputs[0].contains("lpq/0.5"));
+}
diff --git a/seq/src/model/forth_tests/stack.rs b/seq/src/model/forth_tests/stack.rs
new file mode 100644
index 0000000..0c2a134
--- /dev/null
+++ b/seq/src/model/forth_tests/stack.rs
@@ -0,0 +1,90 @@
+use super::harness::*;
+use crate::model::forth::Value::Int;
+
+#[test]
+fn dup() {
+    expect_stack("3 dup", &[Int(3), Int(3)]);
+}
+
+#[test]
+fn dup_underflow() {
+    expect_error("dup", "stack underflow");
+}
+
+#[test]
+fn drop() {
+    expect_stack("1 2 drop", &[Int(1)]);
+}
+
+#[test]
+fn drop_underflow() {
+    expect_error("drop", "stack underflow");
+}
+
+#[test]
+fn swap() {
+    expect_stack("1 2 swap", &[Int(2), Int(1)]);
+}
+
+#[test]
+fn swap_underflow() {
+    expect_error("1 swap", "stack underflow");
+}
+
+#[test]
+fn over() {
+    expect_stack("1 2 over", &[Int(1), Int(2), Int(1)]);
+}
+
+#[test]
+fn over_underflow() {
+    expect_error("1 over", "stack underflow");
+}
+
+#[test]
+fn rot() {
+    expect_stack("1 2 3 rot", &[Int(2), Int(3), Int(1)]);
+}
+
+#[test]
+fn rot_underflow() {
+    expect_error("1 2 rot", "stack underflow");
+}
+
+#[test]
+fn nip() {
+    expect_stack("1 2 nip", &[Int(2)]);
+}
+
+#[test]
+fn nip_underflow() {
+    expect_error("1 nip", "stack underflow");
+}
+
+#[test]
+fn tuck() {
+    expect_stack("1 2 tuck", &[Int(2), Int(1), Int(2)]);
+}
+
+#[test]
+fn tuck_underflow() {
+    expect_error("1 tuck", "stack underflow");
+}
+
+#[test]
+fn stack_persists() {
+    let f = forth();
+    let ctx = default_ctx();
+    f.evaluate("1 2 3", &ctx).unwrap();
+    assert_eq!(f.stack(), vec![Int(1), Int(2), Int(3)]);
+    f.evaluate("4 5", &ctx).unwrap();
+    assert_eq!(f.stack(), vec![Int(1), Int(2), Int(3), Int(4), Int(5)]);
+}
+
+#[test]
+fn clear_stack() {
+    let f = forth();
+    f.evaluate("1 2 3", &default_ctx()).unwrap();
+    f.clear_stack();
+    assert!(f.stack().is_empty());
+}
diff --git a/seq/src/model/forth_tests/variables.rs b/seq/src/model/forth_tests/variables.rs
new file mode 100644
index 0000000..3c26f1e
--- /dev/null
+++ b/seq/src/model/forth_tests/variables.rs
@@ -0,0 +1,39 @@
+use super::harness::*;
+
+#[test]
+fn set_get() {
+    expect_int(r#"42 "x" set "x" get"#, 42);
+}
+
+#[test]
+fn get_nonexistent() {
+    expect_int(r#""novar" get"#, 0);
+}
+
+#[test]
+fn persistence_across_evals() {
+    let f = forth();
+    let ctx = default_ctx();
+    f.evaluate(r#"10 "counter" set"#, &ctx).unwrap();
+    f.clear_stack();
+    f.evaluate(r#""counter" get 1 +"#, &ctx).unwrap();
+    assert_eq!(stack_int(&f), 11);
+}
+
+#[test]
+fn overwrite() {
+    expect_int(r#"1 "x" set 99 "x" set "x" get"#, 99);
+}
+
+#[test]
+fn multiple_vars() {
+    let f = run(r#"10 "a" set 20 "b" set "a" get "b" get +"#);
+    assert_eq!(stack_int(&f), 30);
+}
+
+#[test]
+fn float_var() {
+    let f = run(r#"3.14 "pi" set "pi" get"#);
+    let val = stack_float(&f);
+    assert!((val - 3.14).abs() < 1e-9);
+}
diff --git a/seq/src/model/mod.rs b/seq/src/model/mod.rs
index bc10df8..83452c5 100644
--- a/seq/src/model/mod.rs
+++ b/seq/src/model/mod.rs
@@ -1,5 +1,7 @@
 mod file;
 pub mod forth;
+#[cfg(test)]
+mod forth_tests;
 mod project;
 mod script;