diff --git a/crates/nu-engine/src/eval.rs b/crates/nu-engine/src/eval.rs
index 7bee98fbfc..322ea0cd74 100644
--- a/crates/nu-engine/src/eval.rs
+++ b/crates/nu-engine/src/eval.rs
@@ -62,6 +62,9 @@ fn eval_call(
             if let Some(arg) = call.positional.get(param_idx) {
                 let result = eval_expression(engine_state, caller_stack, arg)?;
                 callee_stack.add_var(var_id, result);
+            } else if let Some(arg) = &param.default_value {
+                let result = eval_expression(engine_state, caller_stack, arg)?;
+                callee_stack.add_var(var_id, result);
             } else {
                 callee_stack.add_var(var_id, Value::nothing(call.head));
             }
@@ -103,6 +106,10 @@ fn eval_call(
                         if let Some(arg) = &call_named.1 {
                             let result = eval_expression(engine_state, caller_stack, arg)?;
 
+                            callee_stack.add_var(var_id, result);
+                        } else if let Some(arg) = &named.default_value {
+                            let result = eval_expression(engine_state, caller_stack, arg)?;
+
                             callee_stack.add_var(var_id, result);
                         } else {
                             callee_stack.add_var(
@@ -126,6 +133,10 @@ fn eval_call(
                                 span: call.head,
                             },
                         )
+                    } else if let Some(arg) = &named.default_value {
+                        let result = eval_expression(engine_state, caller_stack, arg)?;
+
+                        callee_stack.add_var(var_id, result);
                     } else {
                         callee_stack.add_var(var_id, Value::Nothing { span: call.head })
                     }
diff --git a/crates/nu-parser/src/errors.rs b/crates/nu-parser/src/errors.rs
index dfd7734416..6e31ce7995 100644
--- a/crates/nu-parser/src/errors.rs
+++ b/crates/nu-parser/src/errors.rs
@@ -19,6 +19,13 @@ pub enum ParseError {
     #[diagnostic(code(nu::parser::extra_positional), url(docsrs), help("Usage: {0}"))]
     ExtraPositional(String, #[label = "extra positional argument"] Span),
 
+    #[error("Require positional parameter after optional parameter")]
+    #[diagnostic(code(nu::parser::required_after_optional), url(docsrs))]
+    RequiredAfterOptional(
+        String,
+        #[label = "required parameter {0} after optional parameter"] Span,
+    ),
+
     #[error("Unexpected end of code.")]
     #[diagnostic(code(nu::parser::unexpected_eof), url(docsrs))]
     UnexpectedEof(String, #[label("expected closing {0}")] Span),
@@ -246,6 +253,7 @@ impl ParseError {
             ParseError::UnknownCommand(s) => *s,
             ParseError::NonUtf8(s) => *s,
             ParseError::UnknownFlag(_, _, s) => *s,
+            ParseError::RequiredAfterOptional(_, s) => *s,
             ParseError::UnknownType(s) => *s,
             ParseError::MissingFlagParam(_, s) => *s,
             ParseError::ShortFlagBatchCantTakeArg(s) => *s,
diff --git a/crates/nu-parser/src/parse_keywords.rs b/crates/nu-parser/src/parse_keywords.rs
index a54e0daed6..18ca896fb3 100644
--- a/crates/nu-parser/src/parse_keywords.rs
+++ b/crates/nu-parser/src/parse_keywords.rs
@@ -176,6 +176,7 @@ pub fn parse_for(
                 desc: String::new(),
                 shape: SyntaxShape::Any,
                 var_id: Some(*var_id),
+                default_value: None,
             },
         );
     }
diff --git a/crates/nu-parser/src/parser.rs b/crates/nu-parser/src/parser.rs
index 8bfdb9efa1..607637cbb8 100644
--- a/crates/nu-parser/src/parser.rs
+++ b/crates/nu-parser/src/parser.rs
@@ -2677,6 +2677,7 @@ pub fn parse_row_condition(
                 desc: "row condition".into(),
                 shape: SyntaxShape::Any,
                 var_id: Some(var_id),
+                default_value: None,
             });
 
             working_set.add_block(block)
@@ -2742,11 +2743,14 @@ pub fn parse_signature_helper(
     working_set: &mut StateWorkingSet,
     span: Span,
 ) -> (Box<Signature>, Option<ParseError>) {
+    #[allow(clippy::enum_variant_names)]
     enum ParseMode {
         ArgMode,
         TypeMode,
+        DefaultValueMode,
     }
 
+    #[derive(Debug)]
     enum Arg {
         Positional(PositionalArg, bool), // bool - required
         RestPositional(PositionalArg),
@@ -2756,7 +2760,13 @@ pub fn parse_signature_helper(
     let mut error = None;
     let source = working_set.get_span_contents(span);
 
-    let (output, err) = lex(source, span.start, &[b'\n', b'\r', b','], &[b':'], false);
+    let (output, err) = lex(
+        source,
+        span.start,
+        &[b'\n', b'\r', b','],
+        &[b':', b'='],
+        false,
+    );
     error = error.or(err);
 
     let mut args: Vec<Arg> = vec![];
@@ -2776,12 +2786,24 @@ pub fn parse_signature_helper(
                         ParseMode::ArgMode => {
                             parse_mode = ParseMode::TypeMode;
                         }
-                        ParseMode::TypeMode => {
+                        ParseMode::TypeMode | ParseMode::DefaultValueMode => {
                             // We're seeing two types for the same thing for some reason, error
                             error =
                                 error.or_else(|| Some(ParseError::Expected("type".into(), span)));
                         }
                     }
+                } else if contents == b"=" {
+                    match parse_mode {
+                        ParseMode::ArgMode | ParseMode::TypeMode => {
+                            parse_mode = ParseMode::DefaultValueMode;
+                        }
+                        ParseMode::DefaultValueMode => {
+                            // We're seeing two default values for some reason, error
+                            error = error.or_else(|| {
+                                Some(ParseError::Expected("default value".into(), span))
+                            });
+                        }
+                    }
                 } else {
                     match parse_mode {
                         ParseMode::ArgMode => {
@@ -2802,6 +2824,7 @@ pub fn parse_signature_helper(
                                         short: None,
                                         required: false,
                                         var_id: Some(var_id),
+                                        default_value: None,
                                     }));
                                 } else {
                                     let short_flag = &flags[1];
@@ -2832,6 +2855,7 @@ pub fn parse_signature_helper(
                                             short: Some(chars[0]),
                                             required: false,
                                             var_id: Some(var_id),
+                                            default_value: None,
                                         }));
                                     } else {
                                         error = error.or_else(|| {
@@ -2864,6 +2888,7 @@ pub fn parse_signature_helper(
                                     short: Some(chars[0]),
                                     required: false,
                                     var_id: Some(var_id),
+                                    default_value: None,
                                 }));
                             } else if contents.starts_with(b"(-") {
                                 let short_flag = &contents[2..];
@@ -2921,6 +2946,7 @@ pub fn parse_signature_helper(
                                         name,
                                         shape: SyntaxShape::Any,
                                         var_id: Some(var_id),
+                                        default_value: None,
                                     },
                                     false,
                                 ))
@@ -2935,6 +2961,7 @@ pub fn parse_signature_helper(
                                     name,
                                     shape: SyntaxShape::Any,
                                     var_id: Some(var_id),
+                                    default_value: None,
                                 }));
                             } else {
                                 let name = String::from_utf8_lossy(contents).to_string();
@@ -2949,6 +2976,7 @@ pub fn parse_signature_helper(
                                         name,
                                         shape: SyntaxShape::Any,
                                         var_id: Some(var_id),
+                                        default_value: None,
                                     },
                                     true,
                                 ))
@@ -2982,6 +3010,97 @@ pub fn parse_signature_helper(
                             }
                             parse_mode = ParseMode::ArgMode;
                         }
+                        ParseMode::DefaultValueMode => {
+                            if let Some(last) = args.last_mut() {
+                                let (expression, err) =
+                                    parse_value(working_set, span, &SyntaxShape::Any);
+                                error = error.or(err);
+
+                                //TODO check if we're replacing a custom parameter already
+                                match last {
+                                    Arg::Positional(
+                                        PositionalArg {
+                                            shape,
+                                            var_id,
+                                            default_value,
+                                            ..
+                                        },
+                                        required,
+                                    ) => {
+                                        let var_id = var_id.expect("internal error: all custom parameters must have var_ids");
+                                        let var_type = working_set.get_variable(var_id);
+                                        match var_type {
+                                            Type::Unknown => {
+                                                working_set.set_variable_type(
+                                                    var_id,
+                                                    expression.ty.clone(),
+                                                );
+                                            }
+                                            t => {
+                                                if t != &expression.ty {
+                                                    error = error.or_else(|| {
+                                                        Some(ParseError::AssignmentMismatch(
+                                                            "Default value wrong type".into(),
+                                                            format!("default value not {}", t),
+                                                            expression.span,
+                                                        ))
+                                                    })
+                                                }
+                                            }
+                                        }
+                                        *shape = expression.ty.to_shape();
+                                        *default_value = Some(expression);
+                                        *required = false;
+                                    }
+                                    Arg::RestPositional(..) => {
+                                        error = error.or_else(|| {
+                                            Some(ParseError::AssignmentMismatch(
+                                                "Rest parameter given default value".into(),
+                                                "can't have default value".into(),
+                                                expression.span,
+                                            ))
+                                        })
+                                    }
+                                    Arg::Flag(Flag {
+                                        arg,
+                                        var_id,
+                                        default_value,
+                                        ..
+                                    }) => {
+                                        let var_id = var_id.expect("internal error: all custom parameters must have var_ids");
+                                        let var_type = working_set.get_variable(var_id);
+
+                                        let expression_ty = expression.ty.clone();
+                                        let expression_span = expression.span;
+
+                                        *default_value = Some(expression);
+
+                                        // Flags with a boolean type are just present/not-present switches
+                                        if var_type != &Type::Bool {
+                                            match var_type {
+                                                Type::Unknown => {
+                                                    *arg = Some(expression_ty.to_shape());
+                                                    working_set
+                                                        .set_variable_type(var_id, expression_ty);
+                                                }
+                                                t => {
+                                                    if t != &expression_ty {
+                                                        error = error.or_else(|| {
+                                                            Some(ParseError::AssignmentMismatch(
+                                                                "Default value wrong type".into(),
+                                                                format!("default value not {}", t),
+                                                                expression_span,
+                                                            ))
+                                                        })
+                                                    }
+                                                }
+                                            }
+                                        }
+                                    }
+                                }
+                            }
+                            parse_mode = ParseMode::ArgMode;
+                        }
                     }
                 }
             }
@@ -3030,6 +3149,14 @@ pub fn parse_signature_helper(
         match arg {
             Arg::Positional(positional, required) => {
                 if required {
+                    if !sig.optional_positional.is_empty() {
+                        error = error.or_else(|| {
+                            Some(ParseError::RequiredAfterOptional(
+                                positional.name.clone(),
+                                span,
+                            ))
+                        })
+                    }
                     sig.required_positional.push(positional)
                 } else {
                     sig.optional_positional.push(positional)
@@ -3379,6 +3506,7 @@ pub fn parse_block_expression(
                 name: "$it".into(),
                 desc: String::new(),
                 shape: SyntaxShape::Any,
+                default_value: None,
             });
             output.signature = Box::new(signature);
         }
@@ -4503,6 +4631,7 @@ fn wrap_expr_with_collect(working_set: &mut StateWorkingSet, expr: &Expression)
             name: "$in".into(),
             desc: String::new(),
             shape: SyntaxShape::Any,
+            default_value: None,
         });
 
         let mut expr = expr.clone();
diff --git a/crates/nu-plugin/src/serializers/capnp/signature.rs b/crates/nu-plugin/src/serializers/capnp/signature.rs
index 82be9eac0e..9d017f7d6a 100644
--- a/crates/nu-plugin/src/serializers/capnp/signature.rs
+++ b/crates/nu-plugin/src/serializers/capnp/signature.rs
@@ -218,6 +218,7 @@ fn deserialize_argument(reader: argument::Reader) -> Result<PositionalArg, Shell
         desc: desc.to_string(),
         shape,
         var_id: None,
+        default_value: None,
     })
 }
 
@@ -262,6 +263,7 @@ fn deserialize_flag(reader: flag::Reader) -> Result<Flag, ShellError> {
         required,
         desc: desc.to_string(),
         var_id: None,
+        default_value: None,
     })
 }
 
diff --git a/crates/nu-protocol/src/ast/call.rs b/crates/nu-protocol/src/ast/call.rs
index 116478860d..2d137e17c7 100644
--- a/crates/nu-protocol/src/ast/call.rs
+++ b/crates/nu-protocol/src/ast/call.rs
@@ -1,7 +1,9 @@
+use serde::{Deserialize, Serialize};
+
 use super::Expression;
 use crate::{DeclId, Span, Spanned};
 
-#[derive(Debug, Clone)]
+#[derive(Debug, Clone, PartialEq, Serialize, Deserialize)]
 pub struct Call {
     /// identifier of the declaration to call
     pub decl_id: DeclId,
diff --git a/crates/nu-protocol/src/ast/cell_path.rs b/crates/nu-protocol/src/ast/cell_path.rs
index 02f4310f56..8487abb800 100644
--- a/crates/nu-protocol/src/ast/cell_path.rs
+++ b/crates/nu-protocol/src/ast/cell_path.rs
@@ -41,7 +41,7 @@ impl CellPath {
     }
 }
 
-#[derive(Debug, Clone)]
+#[derive(Debug, Clone, PartialEq, Serialize, Deserialize)]
 pub struct FullCellPath {
     pub head: Expression,
     pub tail: Vec<PathMember>,
diff --git a/crates/nu-protocol/src/ast/expr.rs b/crates/nu-protocol/src/ast/expr.rs
index 941cb3eb2a..17656065f3 100644
--- a/crates/nu-protocol/src/ast/expr.rs
+++ b/crates/nu-protocol/src/ast/expr.rs
@@ -1,9 +1,10 @@
 use chrono::FixedOffset;
+use serde::{Deserialize, Serialize};
 
 use super::{Call, CellPath, Expression, FullCellPath, Operator, RangeOperator};
 use crate::{ast::ImportPattern, BlockId, Signature, Span, Spanned, Unit, VarId};
 
-#[derive(Debug, Clone)]
+#[derive(Debug, Clone, PartialEq, Serialize, Deserialize)]
 pub enum Expr {
     Bool(bool),
     Int(i64),
diff --git a/crates/nu-protocol/src/ast/expression.rs b/crates/nu-protocol/src/ast/expression.rs
index a343310aa5..2aaa247328 100644
--- a/crates/nu-protocol/src/ast/expression.rs
+++ b/crates/nu-protocol/src/ast/expression.rs
@@ -1,8 +1,10 @@
+use serde::{Deserialize, Serialize};
+
 use super::{Expr, Operator};
 use crate::ast::ImportPattern;
 use crate::{engine::StateWorkingSet, BlockId, Signature, Span, Type, VarId, IN_VARIABLE_ID};
 
-#[derive(Debug, Clone)]
+#[derive(Debug, Clone, PartialEq, Serialize, Deserialize)]
 pub struct Expression {
     pub expr: Expr,
     pub span: Span,
diff --git a/crates/nu-protocol/src/ast/import_pattern.rs b/crates/nu-protocol/src/ast/import_pattern.rs
index 6c361f666d..5b0568aa11 100644
--- a/crates/nu-protocol/src/ast/import_pattern.rs
+++ b/crates/nu-protocol/src/ast/import_pattern.rs
@@ -1,21 +1,23 @@
+use serde::{Deserialize, Serialize};
+
 use crate::{span, OverlayId, Span};
 use std::collections::HashSet;
 
-#[derive(Debug, Clone)]
+#[derive(Debug, Clone, PartialEq, Serialize, Deserialize)]
 pub enum ImportPatternMember {
     Glob { span: Span },
     Name { name: Vec<u8>, span: Span },
     List { names: Vec<(Vec<u8>, Span)> },
 }
 
-#[derive(Debug, Clone)]
+#[derive(Debug, Clone, PartialEq, Serialize, Deserialize)]
 pub struct ImportPatternHead {
     pub name: Vec<u8>,
     pub id: Option<OverlayId>,
     pub span: Span,
 }
 
-#[derive(Debug, Clone)]
+#[derive(Debug, Clone, PartialEq, Serialize, Deserialize)]
 pub struct ImportPattern {
     pub head: ImportPatternHead,
     pub members: Vec<ImportPatternMember>,
diff --git a/crates/nu-protocol/src/ast/operator.rs b/crates/nu-protocol/src/ast/operator.rs
index 8291c61341..3c8e916dca 100644
--- a/crates/nu-protocol/src/ast/operator.rs
+++ b/crates/nu-protocol/src/ast/operator.rs
@@ -56,7 +56,7 @@ pub enum RangeInclusion {
     RightExclusive,
 }
 
-#[derive(Debug, Copy, Clone)]
+#[derive(Debug, Copy, Clone, PartialEq, Serialize, Deserialize)]
 pub struct RangeOperator {
     pub inclusion: RangeInclusion,
     pub span: Span,
diff --git a/crates/nu-protocol/src/signature.rs b/crates/nu-protocol/src/signature.rs
index bfbb438ae7..511e0534bb 100644
--- a/crates/nu-protocol/src/signature.rs
+++ b/crates/nu-protocol/src/signature.rs
@@ -2,6 +2,7 @@ use serde::Deserialize;
 use serde::Serialize;
 
 use crate::ast::Call;
+use crate::ast::Expression;
 use crate::engine::Command;
 use crate::engine::EngineState;
 use crate::engine::Stack;
@@ -10,24 +11,28 @@ use crate::PipelineData;
 use crate::SyntaxShape;
 use crate::VarId;
 
-#[derive(Debug, Clone, PartialEq, Eq, Serialize, Deserialize)]
+#[derive(Debug, Clone, PartialEq, Serialize, Deserialize)]
 pub struct Flag {
     pub long: String,
     pub short: Option<char>,
     pub arg: Option<SyntaxShape>,
     pub required: bool,
     pub desc: String,
+
     // For custom commands
     pub var_id: Option<VarId>,
+    pub default_value: Option<Expression>,
 }
 
-#[derive(Debug, Clone, PartialEq, Eq, Serialize, Deserialize)]
+#[derive(Debug, Clone, PartialEq, Serialize, Deserialize)]
 pub struct PositionalArg {
     pub name: String,
     pub desc: String,
     pub shape: SyntaxShape,
+
     // For custom commands
     pub var_id: Option<VarId>,
+    pub default_value: Option<Expression>,
 }
 
 #[derive(Debug, Clone, PartialEq, Serialize, Deserialize)]
@@ -123,6 +128,7 @@ impl Signature {
             desc: "Display this help message".into(),
             required: false,
             var_id: None,
+            default_value: None,
         };
 
         Signature {
@@ -160,6 +166,7 @@ impl Signature {
             desc: desc.into(),
             shape: shape.into(),
             var_id: None,
+            default_value: None,
         });
 
         self
@@ -177,6 +184,7 @@ impl Signature {
             desc: desc.into(),
             shape: shape.into(),
             var_id: None,
+            default_value: None,
         });
 
         self
@@ -193,6 +201,7 @@ impl Signature {
             desc: desc.into(),
             shape: shape.into(),
             var_id: None,
+            default_value: None,
         });
 
         self
@@ -215,6 +224,7 @@ impl Signature {
             required: false,
             desc: desc.into(),
             var_id: None,
+            default_value: None,
         });
 
         self
@@ -237,6 +247,7 @@ impl Signature {
             required: true,
             desc: desc.into(),
             var_id: None,
+            default_value: None,
         });
 
         self
@@ -258,6 +269,7 @@ impl Signature {
             required: false,
             desc: desc.into(),
             var_id: None,
+            default_value: None,
         });
 
         self
diff --git a/crates/nu-protocol/src/span.rs b/crates/nu-protocol/src/span.rs
index 282c19d1ce..391b8d7531 100644
--- a/crates/nu-protocol/src/span.rs
+++ b/crates/nu-protocol/src/span.rs
@@ -2,7 +2,7 @@ use miette::SourceSpan;
 use serde::{Deserialize, Serialize};
 
 /// A spanned area of interest, generic over what kind of thing is of interest
-#[derive(Clone, Debug, Serialize, Deserialize)]
+#[derive(Clone, Debug, Serialize, Deserialize, PartialEq)]
 pub struct Spanned<T>
 where
     T: Clone + std::fmt::Debug,
diff --git a/crates/nu-protocol/src/ty.rs b/crates/nu-protocol/src/ty.rs
index a0b166c795..b9e8ecb557 100644
--- a/crates/nu-protocol/src/ty.rs
+++ b/crates/nu-protocol/src/ty.rs
@@ -2,6 +2,8 @@ use serde::{Deserialize, Serialize};
 
 use std::fmt::Display;
 
+use crate::SyntaxShape;
+
 #[derive(Clone, Debug, PartialEq, Eq, Serialize, Deserialize)]
 pub enum Type {
     Int,
@@ -27,6 +29,34 @@ pub enum Type {
     Signature,
 }
 
+impl Type {
+    pub fn to_shape(&self) -> SyntaxShape {
+        match self {
+            Type::Int => SyntaxShape::Int,
+            Type::Float => SyntaxShape::Number,
+            Type::Range => SyntaxShape::Range,
+            Type::Bool => SyntaxShape::Boolean,
+            Type::String => SyntaxShape::String,
+            Type::Block => SyntaxShape::Block(None), // FIXME needs more accuracy
+            Type::CellPath => SyntaxShape::CellPath,
+            Type::Duration => SyntaxShape::Duration,
+            Type::Date => SyntaxShape::DateTime,
+            Type::Filesize => SyntaxShape::Filesize,
+            Type::List(x) => SyntaxShape::List(Box::new(x.to_shape())),
+            Type::Number => SyntaxShape::Number,
+            Type::Nothing => SyntaxShape::Any,
+            Type::Record(_) => SyntaxShape::Record,
+            Type::Table => SyntaxShape::Table,
+            Type::ListStream => SyntaxShape::List(Box::new(SyntaxShape::Any)),
+            Type::Unknown => SyntaxShape::Any,
+            Type::Error => SyntaxShape::Any,
+            Type::Binary => SyntaxShape::Binary,
+            Type::Custom => SyntaxShape::Custom(Box::new(SyntaxShape::Any), String::new()),
+            Type::Signature => SyntaxShape::Signature,
+        }
+    }
+}
+
 impl Display for Type {
     fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
         match self {
diff --git a/crates/nu-protocol/src/value/unit.rs b/crates/nu-protocol/src/value/unit.rs
index 27fd893014..523798ed06 100644
--- a/crates/nu-protocol/src/value/unit.rs
+++ b/crates/nu-protocol/src/value/unit.rs
@@ -1,4 +1,6 @@
-#[derive(Debug, Clone, Copy)]
+use serde::{Deserialize, Serialize};
+
+#[derive(Debug, Clone, Copy, PartialEq, Serialize, Deserialize)]
 pub enum Unit {
     // Filesize units: metric
     Byte,
diff --git a/crates/nu-protocol/tests/test_signature.rs b/crates/nu-protocol/tests/test_signature.rs
index a2d1ce0f6e..99f50b60b4 100644
--- a/crates/nu-protocol/tests/test_signature.rs
+++ b/crates/nu-protocol/tests/test_signature.rs
@@ -46,7 +46,8 @@ fn test_signature_chained() {
             name: "required".to_string(),
             desc: "required description".to_string(),
             shape: SyntaxShape::String,
-            var_id: None
+            var_id: None,
+            default_value: None,
         })
     );
     assert_eq!(
@@ -55,7 +56,8 @@ fn test_signature_chained() {
             name: "optional".to_string(),
             desc: "optional description".to_string(),
             shape: SyntaxShape::String,
-            var_id: None
+            var_id: None,
+            default_value: None,
         })
     );
     assert_eq!(
@@ -64,7 +66,8 @@ fn test_signature_chained() {
             name: "rest".to_string(),
             desc: "rest description".to_string(),
             shape: SyntaxShape::String,
-            var_id: None
+            var_id: None,
+            default_value: None,
         })
     );
 
@@ -76,7 +79,8 @@ fn test_signature_chained() {
             arg: Some(SyntaxShape::String),
             required: true,
             desc: "required named description".to_string(),
-            var_id: None
+            var_id: None,
+            default_value: None,
         })
     );
 
@@ -88,7 +92,8 @@ fn test_signature_chained() {
             arg: Some(SyntaxShape::String),
             required: true,
             desc: "required named description".to_string(),
-            var_id: None
+            var_id: None,
+            default_value: None,
         })
     );
 }
diff --git a/src/tests/test_engine.rs b/src/tests/test_engine.rs
index ac0bc828a2..db9ca46bd8 100644
--- a/src/tests/test_engine.rs
+++ b/src/tests/test_engine.rs
@@ -287,3 +287,66 @@ fn bool_variable() -> TestResult {
 fn bool_variable2() -> TestResult {
     run_test(r#"$false"#, "false")
 }
+
+#[test]
+fn default_value1() -> TestResult {
+    run_test(r#"def foo [x = 3] { $x }; foo"#, "3")
+}
+
+#[test]
+fn default_value2() -> TestResult {
+    run_test(r#"def foo [x: int = 3] { $x }; foo"#, "3")
+}
+
+#[test]
+fn default_value3() -> TestResult {
+    run_test(r#"def foo [--x = 3] { $x }; foo"#, "3")
+}
+
+#[test]
+fn default_value4() -> TestResult {
+    run_test(r#"def foo [--x: int = 3] { $x }; foo"#, "3")
+}
+
+#[test]
+fn default_value5() -> TestResult {
+    run_test(r#"def foo [x = 3] { $x }; foo 10"#, "10")
+}
+
+#[test]
+fn default_value6() -> TestResult {
+    run_test(r#"def foo [x: int = 3] { $x }; foo 10"#, "10")
+}
+
+#[test]
+fn default_value7() -> TestResult {
+    run_test(r#"def foo [--x = 3] { $x }; foo --x 10"#, "10")
+}
+
+#[test]
+fn default_value8() -> TestResult {
+    run_test(r#"def foo [--x: int = 3] { $x }; foo --x 10"#, "10")
+}
+
+#[test]
+fn default_value9() -> TestResult {
+    fail_test(r#"def foo [--x = 3] { $x }; foo --x a"#, "expected int")
+}
+
+#[test]
+fn default_value10() -> TestResult {
+    fail_test(r#"def foo [x = 3] { $x }; foo a"#, "expected int")
+}
+
+#[test]
+fn default_value11() -> TestResult {
+    fail_test(
+        r#"def foo [x = 3, y] { $x }; foo a"#,
+        "after optional parameter",
+    )
+}
+
+#[test]
+fn default_value12() -> TestResult {
+    fail_test(r#"def foo [--x:int = "a"] { $x }"#, "default value not int")
+}