diff --git a/oqpy/base.py b/oqpy/base.py
index ac99c9e..d69383b 100644
--- a/oqpy/base.py
+++ b/oqpy/base.py
@@ -374,9 +374,11 @@ def detect_and_convert_constants(val: float | np.floating[Any], program: Program
     """Construct a float ast expression which is either a literal or an expression using constants."""
     if val == 0:
         return ast.FloatLiteral(val)
+    if val < 0.5 or val > 100:
+        return ast.FloatLiteral(val)
     x = val / (math.pi / 4.0)
     rx = round(x)
-    if rx > 100 or not math.isclose(x, rx, rel_tol=1e-12):
+    if not math.isclose(x, rx, rel_tol=1e-12):
         return ast.FloatLiteral(val)
     term: OQPyExpression
     if rx == 4:
diff --git a/oqpy/classical_types.py b/oqpy/classical_types.py
index e8dfc08..252b831 100644
--- a/oqpy/classical_types.py
+++ b/oqpy/classical_types.py
@@ -45,6 +45,8 @@
 from oqpy.timing import make_duration
 
 if TYPE_CHECKING:
+    from typing import Literal
+
     from oqpy.program import Program
 
 __all__ = [
@@ -177,7 +179,7 @@ class _ClassicalVar(Var, OQPyExpression):
 
     def __init__(
         self,
-        init_expression: AstConvertible | None = None,
+        init_expression: AstConvertible | Literal["input", "output"] | None = None,
         name: str | None = None,
         needs_declaration: bool = True,
         annotations: Sequence[str | tuple[str, str]] = (),
@@ -196,6 +198,10 @@ def to_ast(self, program: Program) -> ast.Identifier:
 
     def make_declaration_statement(self, program: Program) -> ast.Statement:
         """Make an ast statement that declares the OQpy variable."""
+        if isinstance(self.init_expression, str) and self.init_expression in ("input", "output"):
+            return ast.IODeclaration(
+                ast.IOKeyword[self.init_expression], self.type, self.to_ast(program)
+            )
         init_expression_ast = optional_ast(program, self.init_expression)
         stmt = ast.ClassicalDeclaration(self.type, self.to_ast(program), init_expression_ast)
         stmt.annotations = make_annotations(self.annotations)
@@ -295,7 +301,7 @@ def __class_getitem__(cls, item: ast.FloatType) -> Callable[..., ComplexVar]:
 
     def __init__(
         self,
-        init_expression: AstConvertible | None = None,
+        init_expression: AstConvertible | Literal["input", "output"] | None = None,
         *args: Any,
         base_type: ast.FloatType = float64,
         **kwargs: Any,
@@ -303,7 +309,7 @@ def __init__(
         assert isinstance(base_type, ast.FloatType)
         self.base_type = base_type
 
-        if not isinstance(init_expression, (complex, type(None), OQPyExpression)):
+        if not isinstance(init_expression, (complex, type(None), str, OQPyExpression)):
             init_expression = complex(init_expression)  # type: ignore[arg-type]
         super().__init__(init_expression, *args, **kwargs, base_type=base_type)
 
@@ -315,12 +321,12 @@ class DurationVar(_ClassicalVar):
 
     def __init__(
         self,
-        init_expression: AstConvertible | None = None,
+        init_expression: AstConvertible | Literal["input", "output"] | None = None,
         name: str | None = None,
         *args: Any,
         **type_kwargs: Any,
     ) -> None:
-        if init_expression is not None:
+        if init_expression is not None and not isinstance(init_expression, str):
             init_expression = make_duration(init_expression)
         super().__init__(init_expression, name, *args, **type_kwargs)
 
diff --git a/tests/test_directives.py b/tests/test_directives.py
index 0381a97..d0a78e3 100644
--- a/tests/test_directives.py
+++ b/tests/test_directives.py
@@ -1949,3 +1949,31 @@ def test_oqpy_range():
     ).strip()
     assert prog.to_qasm() == expected
     _check_respects_type_hints(prog)
+
+
+def test_io_declaration():
+    x = oqpy.DurationVar("input", name="x")
+    y = oqpy.FloatVar("output", name="y")
+    wf = oqpy.WaveformVar("input", name="wf")
+    port = oqpy.PortVar(name="my_port", init_expression="input")
+    frame = oqpy.FrameVar(port, 5e9, 0, name="my_frame")
+
+    prog = Program()
+    prog.declare(x)
+    prog.set(y, 1)
+    prog.play(frame, wf)
+
+    expected = textwrap.dedent(
+        """
+        OPENQASM 3.0;
+        input port my_port;
+        output float[64] y;
+        frame my_frame = newframe(my_port, 5000000000.0, 0);
+        input waveform wf;
+        input duration x;
+        y = 1;
+        play(my_frame, wf);
+        """
+    ).strip()
+    assert prog.to_qasm() == expected
+    _check_respects_type_hints(prog)