diff --git a/fix-codegen/src/disassembler.rs b/fix-codegen/src/disassembler.rs
index 45d0b6a..3087ce1 100644
--- a/fix-codegen/src/disassembler.rs
+++ b/fix-codegen/src/disassembler.rs
@@ -291,7 +291,7 @@ impl<'a, Ctx: DisassemblerContext> Disassembler<'a, Ctx> {
             Op::Call => {
                 self.read_operand_data();
                 ("Call", "arg=?".into())
-            },
+            }
             Op::DispatchPrimOp => {
                 let id = BuiltinId::try_from_primitive(self.read_u8()).expect("invalid builtin id");
                 ("DispatchPrimOp", format!("id={id:?}"))
@@ -419,6 +419,7 @@ impl<'a, Ctx: DisassemblerContext> Disassembler<'a, Ctx> {
                 let force = self.read_u8();
                 ("ConcatStrings", format!("count={} force={}", count, force))
             }
+            Op::CoerceToString => ("CoerceToString", String::new()),
             Op::ResolvePath => ("ResolvePath", String::new()),
             Op::Assert => {
                 let raw_idx = self.read_u32();
diff --git a/fix-codegen/src/lib.rs b/fix-codegen/src/lib.rs
index 659d138..4e64238 100644
--- a/fix-codegen/src/lib.rs
+++ b/fix-codegen/src/lib.rs
@@ -77,6 +77,8 @@ pub enum Op {
     JumpIfTrue,
     Jump,
 
+    CoerceToString,
+
     ConcatStrings,
     ResolvePath,
 
@@ -250,6 +252,11 @@ impl<'a, Ctx: BytecodeContext> BytecodeEmitter<'a, Ctx> {
         self.ctx.get_code_mut().push(op as u8);
     }
 
+    #[inline]
+    fn emit_bool(&mut self, val: bool) {
+        self.emit_u8(u8::from(val));
+    }
+
     #[inline]
     fn emit_u8(&mut self, val: u8) {
         self.ctx.get_code_mut().push(val);
@@ -615,17 +622,16 @@ impl<'a, Ctx: BytecodeContext> BytecodeEmitter<'a, Ctx> {
                 );
             }
             &Ir::ConcatStrings {
-                parts: _,
-                force_string: _,
+                ref parts,
+                force_string,
             } => {
-                todo!("redesign ConcatStrings");
-                // self.emit_op(Op::ConcatStrings);
-                // self.emit_u16(parts.len() as u16);
-                // self.emit_u8(if force_string { 1 } else { 0 });
-                // for &part in parts.iter() {
-                //     let operand = self.inline_maybe_thunk(part);
-                //     self.emit_inline_operand(operand);
-                // }
+                for &part in parts.iter() {
+                    self.emit_expr(part);
+                    self.emit_op(Op::CoerceToString);
+                }
+                self.emit_op(Op::ConcatStrings);
+                self.emit_u16(parts.len() as u16);
+                self.emit_bool(force_string);
             }
             &Ir::HasAttr { lhs, ref rhs } => {
                 self.emit_has_attr(lhs, rhs);
@@ -842,7 +848,7 @@ impl<'a, Ctx: BytecodeContext> BytecodeEmitter<'a, Ctx> {
             self.emit_u32(total_slots as u32);
             self.emit_u16(required.len() as u16);
             self.emit_u16(optional.len() as u16);
-            self.emit_u8(if *ellipsis { 1 } else { 0 });
+            self.emit_bool(*ellipsis);
 
             for &(sym, _) in required.iter() {
                 self.emit_str_id(sym);
diff --git a/fix-vm/src/dispatch_tailcall.rs b/fix-vm/src/dispatch_tailcall.rs
index e6ebad1..5fb45b8 100644
--- a/fix-vm/src/dispatch_tailcall.rs
+++ b/fix-vm/src/dispatch_tailcall.rs
@@ -180,6 +180,8 @@ tail_fn!(op_jump_if_false, (reader, mc));
 tail_fn!(op_jump_if_true, (reader, mc));
 tail_fn!(op_jump, (reader));
 
+tail_fn!(op_coerce_to_string, (reader, mc));
+
 tail_fn!(op_concat_strings, (ctx, reader, mc));
 tail_fn!(op_resolve_path, (ctx));
 
@@ -272,6 +274,8 @@ table! {
     JumpIfTrue            => op_jump_if_true,
     Jump                  => op_jump,
 
+    CoerceToString                 => op_coerce_to_string,
+
     ConcatStrings         => op_concat_strings,
     ResolvePath           => op_resolve_path,
 
diff --git a/fix-vm/src/instructions/arithmetic.rs b/fix-vm/src/instructions/arithmetic.rs
index cb735de..9c719a3 100644
--- a/fix-vm/src/instructions/arithmetic.rs
+++ b/fix-vm/src/instructions/arithmetic.rs
@@ -326,6 +326,9 @@ fn numeric_binop<'gc>(
         (Some(NixNum::Float(a)), Some(NixNum::Int(b))) => {
             Ok(Value::new_float(float_op(a, b as f64)))
         }
-        _ => Err(crate::vm_err("cannot perform arithmetic on non-numbers")),
+        _ => Err(crate::vm_err(format!(
+            "cannot perform arithmetic on non-numbers: {:?}",
+            (lhs.ty(), rhs.ty())
+        ))),
     }
 }
diff --git a/fix-vm/src/instructions/misc.rs b/fix-vm/src/instructions/misc.rs
index 61377b2..258c58c 100644
--- a/fix-vm/src/instructions/misc.rs
+++ b/fix-vm/src/instructions/misc.rs
@@ -1,6 +1,8 @@
 use fix_builtins::BuiltinId;
+use fix_common::StringId;
 use num_enum::TryFromPrimitive;
 
+use crate::value::{NixString, StrictValue};
 use crate::{BytecodeReader, PrimOp, Step, Value, VmRuntimeCtx};
 
 impl<'gc> crate::Vm<'gc> {
@@ -34,6 +36,21 @@ impl<'gc> crate::Vm<'gc> {
         todo!("LoadScopedBinding");
     }
 
+    #[inline(always)]
+    pub(crate) fn op_coerce_to_string(
+        &mut self,
+        reader: &mut BytecodeReader<'_>,
+        mc: &gc_arena::Mutation<'gc>,
+    ) -> Step {
+        let val = self.try_force::<StrictValue>(reader, mc)?;
+        if val.is::<StringId>() || val.is::<NixString>() {
+            self.push(val.relax());
+        } else {
+            todo!("coerce other types to string: {:?}", val.ty());
+        }
+        Step::Continue(())
+    }
+
     #[inline(always)]
     pub(crate) fn op_concat_strings(
         &mut self,
@@ -41,14 +58,30 @@ impl<'gc> crate::Vm<'gc> {
         reader: &mut BytecodeReader<'_>,
         _mc: &gc_arena::Mutation<'gc>,
     ) -> Step {
-        let _parts_count = reader.read_u16() as usize;
+        use crate::VmRuntimeCtxExt;
+
+        let count = reader.read_u16() as usize;
         let _force_string = reader.read_u8() != 0;
-        let mut _operands: smallvec::SmallVec<[crate::OperandData; 4]> =
-            smallvec::SmallVec::with_capacity(_parts_count);
-        for _ in 0.._parts_count {
-            _operands.push(reader.read_operand_data(ctx));
+
+        let mut total_len = 0;
+        for i in 0..count {
+            let val = self.peek_forced(count - 1 - i);
+            let s = ctx.get_string(val).expect("coerced");
+            total_len += s.len();
         }
-        todo!("implement ConcatStrings (force parts, coerce to string, concatenate)");
+
+        let mut result = String::with_capacity(total_len);
+        for i in 0..count {
+            let val = self.peek_forced(count - 1 - i);
+            let s = ctx.get_string(val).expect("coerced");
+            result.push_str(s);
+        }
+
+        self.stack.truncate(self.stack.len() - count);
+
+        let sid = ctx.intern_string(result);
+        self.push(Value::new_inline(sid));
+        Step::Continue(())
     }
 
     #[inline(always)]
diff --git a/fix-vm/src/lib.rs b/fix-vm/src/lib.rs
index d6c1ee7..5ba903a 100644
--- a/fix-vm/src/lib.rs
+++ b/fix-vm/src/lib.rs
@@ -492,7 +492,7 @@ impl<'gc> Vm<'gc> {
                 });
                 self.push(func);
                 self.call(reader, mc, arg, resume_pc)
-            },
+            }
             ThunkState::Blackhole => {
                 self.finish_err(Error::eval_error("infinite recursion encountered"))
             }
@@ -666,6 +666,7 @@ impl<'gc> Vm<'gc> {
                 Jump => self.op_jump(&mut reader),
 
                 ConcatStrings => self.op_concat_strings(ctx, &mut reader, mc),
+                CoerceToString => self.op_coerce_to_string(&mut reader, mc),
                 ResolvePath => self.op_resolve_path(ctx),
 
                 Assert => self.op_assert(&mut reader),