diff --git a/CLAUDE.md b/CLAUDE.md
new file mode 100644
index 0000000..92950bf
--- /dev/null
+++ b/CLAUDE.md
@@ -0,0 +1,57 @@
+# CLAUDE.md
+
+This file provides guidance to Claude Code (claude.ai/code) when working with code in this repository.
+
+## Project Overview
+
+UCL is an embeddable command-based scripting language implemented in Go, with Tcl/shell-like syntax. It is a **library** — there is no root `main.go`. All executables live under `cmd/`.
+
+## Build & Test Commands
+
+- **Run all tests:** `make test` (or `go test ./ucl/...`)
+- **Run a single test:** `go test ./ucl/... -run TestName`
+- **Run builtin module tests:** `go test ./ucl/builtins/...`
+- **Build site + WASM playground:** `make site`
+- **Clean build artifacts:** `make clean`
+
+## Architecture
+
+### Core Package (`ucl/`)
+
+The language engine follows a **parse → tree-walk evaluate** pipeline:
+
+- **`ast.go`** — AST node types and parser (using `participle` library with struct-tag grammar)
+- **`eval.go`** — Tree-walk evaluator: scripts, statements, commands, pipelines, arguments, dot-access, string interpolation
+- **`objs.go`** — Object type system: `StringObject`, `IntObject`, `BoolObject`, `ListObject`, `HashObject`, `TimeObject`, plus proxy types for Go interop via reflection
+- **`inst.go`** — `Inst` (interpreter instance): the public API for creating and configuring the evaluator
+- **`builtins.go`** — Built-in functions and macros (control flow, math, collections)
+- **`env.go`** — `evalCtx`: scoped evaluation context (linked-list chain for variable/command lookup)
+- **`userbuiltin.go`** — Public embedding API (`BuiltinHandler`, `CallArgs` with reflection-based argument binding)
+
+### Two Command Dispatch Kinds
+
+- **Invokables** (`invokable` interface): receive fully-evaluated arguments. Used by all regular functions, Go builtins, user `proc`s, and blocks.
+- **Macros** (`macroable` interface): receive unevaluated AST + evaluator. Used by `if`, `for`, `while`, `proc`, `try` for lazy evaluation control.
+
+### Optional Builtin Modules (`ucl/builtins/`)
+
+Modules (`strs`, `lists`, `itrs`, `fns`, `os`, `fs`, `log`, `csv`, `time`, `urls`) are opt-in via `ucl.WithModule(builtins.Strs())` etc. Each is a self-contained file.
+
+### Other Packages
+
+- **`repl/`** — REPL wrapper with `EvalAndDisplay()`, help system, and `AddTypePrinter[T]` generic
+- **`cmd/cmsh/`** — Interactive shell using `readline`
+- **`cmd/playwasm/`** — WASM playground (builds with `GOOS=js GOARCH=wasm`)
+- **`cmd/gendocs/`** — Markdown-to-HTML doc site generator (goldmark + frontmatter)
+
+## Testing Conventions
+
+- Tests use `testify/assert` with **table-driven** patterns (`[]struct{ desc, expr string; want any }`)
+- Test packages use the `_test` suffix for black-box testing
+- Shared test helpers (e.g., `WithTestBuiltin()`) are in `ucl/builtins_test.go` and provide test-only commands (`firstarg`, `toUpper`, `sjoin`, etc.)
+
+## CI
+
+Forgejo workflows in `.forgejo/workflows/`:
+- `test.yaml` — runs `make test` on `feature/*` branches
+- `build.yaml` — runs tests + `make site-deploy` on `main`
diff --git a/repl/evaldisplay.go b/repl/evaldisplay.go
index 083e4e3..4fc841d 100644
--- a/repl/evaldisplay.go
+++ b/repl/evaldisplay.go
@@ -8,6 +8,7 @@ import (
 	"os"
 	"reflect"
 	"strings"
+
 	"ucl.lmika.dev/ucl"
 )
 
@@ -44,7 +45,7 @@ func (r *REPL) echoPrinter(ctx context.Context, w io.Writer, args []any) (err er
 	}
 
 	_, err = fmt.Fprintln(w, res)
-	return nil
+	return err
 }
 
 func (r *REPL) displayResult(ctx context.Context, w io.Writer, res any, concise bool) (err error) {
@@ -56,7 +57,7 @@ func (r *REPL) displayResult(ctx context.Context, w io.Writer, res any, concise
 
 	switch v := res.(type) {
 	case nil:
-		if _, err = fmt.Fprintln(os.Stdout, "(nil)"); err != nil {
+		if _, err = fmt.Fprintln(w, "(nil)"); err != nil {
 			return err
 		}
 	case ucl.Listable:
@@ -92,6 +93,10 @@ func (r *REPL) displayResult(ctx context.Context, w io.Writer, res any, concise
 			}
 			fmt.Fprintf(w, "]")
 		} else {
+			if v == nil {
+				fmt.Fprintf(w, "(nil)")
+				return nil
+			}
 			// In the off-chance that this is actually a slice of printables
 			vt := reflect.SliceOf(reflect.TypeOf(v[0]))
 			if tp, ok := r.typePrinters[vt]; ok {
diff --git a/ucl/ast.go b/ucl/ast.go
index 719f381..e27ec01 100644
--- a/ucl/ast.go
+++ b/ucl/ast.go
@@ -174,5 +174,5 @@ var parser = participle.MustBuild[astScript](participle.Lexer(scanner),
 	participle.Elide("Whitespace", "Comment"))
 
 func parse(fname string, r io.Reader) (*astScript, error) {
-	return parser.Parse("test", r)
+	return parser.Parse(fname, r)
 }
diff --git a/ucl/builtins.go b/ucl/builtins.go
index 59edecd..c984394 100644
--- a/ucl/builtins.go
+++ b/ucl/builtins.go
@@ -455,7 +455,6 @@ func callBuiltin(ctx context.Context, args invocationArgs) (Object, error) {
 		for i := 0; i < argList.Len(); i++ {
 			calledArgs[i] = argList.Index(i)
 		}
-		args.shift(1)
 	}
 
 	invArgs := args.fork(calledArgs)
@@ -932,7 +931,7 @@ func (s seqObject) Len() int {
 
 func (s seqObject) Index(i int) Object {
 	l := s.Len()
-	if i < 0 || i > l {
+	if i < 0 || i >= l {
 		return nil
 	}
 	if s.from > s.to {
@@ -1028,6 +1027,9 @@ func tryBuiltin(ctx context.Context, args macroArgs) (res Object, err error) {
 			finallyBlocks = append(finallyBlocks, i+1)
 		}
 	}
+	if len(finallyBlocks) > 1 {
+		return nil, tooManyFinallyBlocksError(args.ast.Pos)
+	}
 
 	defer func() {
 		if isBreakErr(err) {
@@ -1122,7 +1124,13 @@ func foreachBuiltin(ctx context.Context, args macroArgs) (Object, error) {
 	case Hashable:
 		err := t.Each(func(k string, v Object) error {
 			last, err = args.evalBlock(ctx, blockIdx, []Object{StringObject(k), v}, false)
-			return err
+			if err != nil {
+				if errors.As(err, &breakErr) && breakErr.isCont {
+					return nil
+				}
+				return err
+			}
+			return nil
 		})
 		if errors.As(err, &breakErr) {
 			if !breakErr.isCont {
diff --git a/ucl/builtins/lists.go b/ucl/builtins/lists.go
index e62ed77..0aa8264 100644
--- a/ucl/builtins/lists.go
+++ b/ucl/builtins/lists.go
@@ -116,6 +116,7 @@ func eachListOrIterItem(ctx context.Context, o ucl.Object, f func(int, ucl.Objec
 			}
 			idx++
 		}
+		return nil
 	}
 	return errors.New("expected listable")
 }
diff --git a/ucl/builtins/log.go b/ucl/builtins/log.go
index c336833..0f2458f 100644
--- a/ucl/builtins/log.go
+++ b/ucl/builtins/log.go
@@ -4,6 +4,7 @@ import (
 	"context"
 	"log"
 	"strings"
+
 	"ucl.lmika.dev/ucl"
 )
 
@@ -36,9 +37,10 @@ func (lh logHandler) logMessage(ctx context.Context, args ucl.CallArgs) (any, er
 	var s ucl.Object
 
 	for args.NArgs() > 0 {
-		if err := args.Bind(&s); err == nil {
-			sb.WriteString(s.String())
+		if err := args.Bind(&s); err != nil {
+			return nil, err
 		}
+		sb.WriteString(s.String())
 	}
 	return nil, lh.handler(sb.String())
 }
diff --git a/ucl/env.go b/ucl/env.go
index 7a58cc1..307a372 100644
--- a/ucl/env.go
+++ b/ucl/env.go
@@ -113,7 +113,7 @@ func (ec *evalCtx) lookupInvokable(name string) invokable {
 		}
 	}
 
-	return ec.parent.lookupInvokable(name)
+	return nil
 }
 
 func (ec *evalCtx) lookupMacro(name string) macroable {
@@ -127,5 +127,5 @@ func (ec *evalCtx) lookupMacro(name string) macroable {
 		}
 	}
 
-	return ec.parent.lookupMacro(name)
+	return nil
 }
diff --git a/ucl/eval.go b/ucl/eval.go
index c6469ae..3d8c14e 100644
--- a/ucl/eval.go
+++ b/ucl/eval.go
@@ -326,7 +326,7 @@ func (e evaluator) assignArg(ctx context.Context, ec *evalCtx, n astCmdArg, toVa
 			}
 			return toVal, nil
 		}
-		return nil, errors.New("unknown pseudo-variable: " + *n.Var)
+		return nil, fmt.Errorf("unknown pseudo-variable: %v", n.PseudoVar)
 	case n.MaybeSub != nil:
 		return nil, errors.New("cannot assign to a subexpression")
 	case n.ListOrHash != nil:
diff --git a/ucl/objs.go b/ucl/objs.go
index fedb25f..70f7cd8 100644
--- a/ucl/objs.go
+++ b/ucl/objs.go
@@ -444,17 +444,6 @@ func (ia invocationArgs) expectArgn(x int) error {
 	return nil
 }
 
-func (ia invocationArgs) stringArg(i int) (string, error) {
-	if len(ia.args) < i {
-		return "", errors.New("expected at least " + strconv.Itoa(i) + " args")
-	}
-	s, ok := ia.args[i].(fmt.Stringer)
-	if !ok {
-		return "", errors.New("expected a string arg")
-	}
-	return s.String(), nil
-}
-
 func (ia invocationArgs) intArg(i int) (int, error) {
 	if len(ia.args) < i {
 		return 0, errors.New("expected at least " + strconv.Itoa(i) + " args")
diff --git a/ucl/userbuiltin.go b/ucl/userbuiltin.go
index 87046f3..1222d17 100644
--- a/ucl/userbuiltin.go
+++ b/ucl/userbuiltin.go
@@ -239,7 +239,6 @@ func canBindProxyObject(v interface{}, r reflect.Value) bool {
 
 	for {
 		if r.Type().AssignableTo(argValue.Elem().Type()) {
-			argValue.Elem().Set(r)
 			return true
 		}
 		if r.Type().Kind() != reflect.Pointer {