commit ccbe9cd7bf0922f69fa4b9816adb6ff16536eae4
Author: Evan Reichard <evan@reichard.io>
Date:   Tue Jan 27 09:55:09 2026 -0500

    initial commit

diff --git a/AGENTS.md b/AGENTS.md
new file mode 100644
index 0000000..d7dfd25
--- /dev/null
+++ b/AGENTS.md
@@ -0,0 +1,138 @@
+# Poiesis
+
+## Module Name
+
+`reichard.io/poiesis`
+
+## Overview
+
+Go tool that transpiles TypeScript to JavaScript using esbuild API and executes it with github.com/fastschema/qjs. Features a flexible builtin system for exposing Go functions to TypeScript with support for both synchronous and asynchronous (Promise-based) operations.
+
+## Build & Test
+
+```bash
+go build ./cmd/poiesis
+go test ./...
+golangci-lint run
+```
+
+## Project Structure
+
+```
+reichard.io/poiesis/
+├── cmd/poiesis/              # CLI entry point
+│   └── main.go
+├── internal/
+│   ├── runtime/              # Runtime management, transpilation, execution
+│   │   ├── runtime.go
+│   │   ├── runtime_test.go
+│   │   └── options.go
+│   ├── functions/            # Function registration framework
+│   │   ├── registry.go
+│   │   ├── types.go
+│   │   ├── typescript_test.go
+│   │   └── functions_test.go
+│   ├── tsconvert/            # Go-to-TypeScript type conversion utilities
+│   │   ├── convert.go
+│   │   ├── types.go
+│   │   └── convert_test.go
+│   └── stdlib/               # Standard library implementations
+│       ├── fetch.go
+│       └── fetch_test.go
+```
+
+## Key Packages
+
+- `reichard.io/poiesis/internal/runtime` - Runtime management, TypeScript transpilation, JavaScript execution, per-runtime type management
+- `reichard.io/poiesis/internal/functions` - Generic function registration framework (sync/async wrappers, automatic JS/Go conversion via JSON)
+- `reichard.io/poiesis/internal/tsconvert` - Go-to-TypeScript type conversion utilities and type declaration generation
+- `reichard.io/poiesis/internal/stdlib` - Standard library implementations (fetch)
+
+## Function System
+
+### Registration
+
+Two types of functions:
+
+- **Sync**: `RegisterFunction[T, R](name, fn)` - executes synchronously, returns value
+- **Async**: `RegisterAsyncFunction[T, R](name, fn)` - runs in goroutine, returns Promise
+
+### Requirements
+
+- Args must be a struct implementing `Args` interface with `Validate() error` method
+- Use JSON tags for TypeScript type definitions
+- Async functions automatically generate `Promise<R>` return types
+
+### Calling Convention
+
+**Important**: Functions have different calling conventions on Go vs JavaScript sides:
+
+- **Go side**: Function receives a **single argument struct** with all parameters
+- **JavaScript side**: Function is called with **individual arguments** matching the struct fields (in field order)
+
+Fields are ordered by their position in the struct, with the generated TypeScript signature using those field names as argument names.
+
+### Example
+
+```go
+type AddArgs struct {
+    A int `json:"a"`
+    B int `json:"b"`
+}
+
+func (a AddArgs) Validate() error { return nil }
+
+func Add(_ context.Context, args AddArgs) (int, error) {
+    return args.A + args.B, nil
+}
+
+// Register sync function
+functions.RegisterFunction[AddArgs, int]("add", Add)
+
+// Register async function
+functions.RegisterAsyncFunction[FetchArgs, *FetchResult]("fetch", Fetch)
+```
+
+## Testing Patterns
+
+- **Test framework**: Go's built-in `testing` package
+- **Assertions**: `github.com/stretchr/testify/assert` and `require`
+- **Linting**: `golangci-lint run` - must pass before committing
+- **Test organization**: Test files use `_test.go` suffix, test functions prefixed with `Test`
+- **TypeScript test files**: Tests that require TypeScript files should create them inline using `os.CreateTemp()` instead of relying on external test files
+
+## Dependencies
+
+- `github.com/evanw/esbuild/pkg/api` - TypeScript transpilation
+- `github.com/fastschema/qjs` - JavaScript execution (CGO-free QuickJS runtime)
+- `github.com/stretchr/testify/assert` - Test assertions
+
+## Code Conventions
+
+- Handle all return values from external functions (enforced by golangci-lint)
+- Use `os` package instead of deprecated `io/ioutil`
+- Error logging uses `_, _ = fmt.Fprintf(stderr, ...)` pattern
+- Package structure follows standard Go project layout with internal packages
+
+### Comment Style
+
+Code blocks (even within functions) should be separated with title-cased comments describing what the block does:
+
+```go
+// Create Runtime
+r := &Runtime{opts: qjs.Option{Context: ctx}}
+
+// Create QuickJS Context
+rt, err := qjs.New(r.opts)
+
+// Populate Globals
+if err := r.populateGlobals(); err != nil {
+    return nil, err
+}
+```
+
+For more complex blocks, use a hyphen to add elaboration:
+
+```go
+// Does Thing - We do this here because we need to do xyz...
+```
diff --git a/README.md b/README.md
new file mode 100644
index 0000000..ba5e5e8
--- /dev/null
+++ b/README.md
@@ -0,0 +1,158 @@
+# Poiesis
+
+A Go tool that transpiles TypeScript to JavaScript using esbuild and executes it with qjs, with an extensible function system.
+
+## Project Structure
+
+```
+reichard.io/poiesis/
+├── cmd/
+│   └── poiesis/                    # CLI application entry point
+│       └── main.go
+├── internal/
+│   ├── runtime/                    # Runtime management, transpilation, execution
+│   │   ├── runtime.go              # Core runtime, transpilation, execution
+│   │   └── runtime_test.go         # Runtime tests
+│   ├── functions/                  # Function registration framework
+│   │   ├── registry.go             # Registration system
+│   │   ├── types.go                # Core interfaces and types
+│   │   ├── typescript.go           # TypeScript definition generation
+│   │   ├── collector.go            # Type collection utilities
+│   │   └── typescript_test.go      # Type system tests
+│   └── stdlib/                     # Standard library implementations
+│       ├── fetch.go                # HTTP fetch implementation
+│       └── fetch_test.go           # Fetch tests
+```
+
+## Architecture
+
+The project is cleanly separated into three packages:
+
+1. **`internal/runtime`** - Runtime management
+   - TypeScript transpilation with esbuild
+   - JavaScript execution with qjs
+   - Automatic function registration and execution
+
+2. **`internal/functions`** - Generic function registration framework
+   - Type-safe registration with generics
+   - Bidirectional Go ↔ JavaScript type conversion
+   - Automatic TypeScript declaration generation
+
+3. **`internal/stdlib`** - Standard library implementations
+   - `fetch` - HTTP requests
+   - Extensible for additional standard functions
+
+## Installation & Build
+
+```bash
+go build ./cmd/poiesis
+```
+
+## Testing
+
+```bash
+go test ./...
+golangci-lint run
+```
+
+## Usage
+
+```bash
+poiesis <typescript-file>
+poiesis -print-types
+```
+
+## Function System
+
+The function system allows you to easily expose Go functions to TypeScript/JavaScript.
+
+### Adding a Function
+
+Just write a Go function and register it:
+
+```go
+package mystdlib
+
+import (
+    "context"
+    "reichard.io/poiesis/internal/functions"
+)
+
+type AddArgs struct {
+    A int `json:"a"`
+    B int `json:"b"`
+}
+
+func (a AddArgs) Validate() error {
+    return nil
+}
+
+func Add(_ context.Context, args AddArgs) (int, error) {
+    return args.A + args.B, nil
+}
+
+func init() {
+    functions.RegisterFunction[AddArgs, int]("add", Add)
+}
+```
+
+That's it! The framework automatically:
+- Converts JavaScript values to Go types
+- Handles errors (panics as JS errors)
+- Generates TypeScript definitions
+- Manages the qjs integration
+
+### Calling Convention
+
+**Important**: There's an important difference between how functions are defined in Go versus how they're called in JavaScript:
+
+- **Go side**: The function receives a **single argument struct** containing all parameters
+- **JavaScript side**: The function is called with the **struct fields as individual arguments** (in the order they appear in the struct)
+
+```go
+// Go: Single struct argument
+type AddArgs struct {
+    A int `json:"a"`
+    B int `json:"b"`
+}
+
+func Add(_ context.Context, args AddArgs) (int, error) {
+    return args.A + args.B, nil
+}
+```
+
+```typescript
+// JavaScript: Individual arguments (not an object!)
+const result = add(5, 10);  // NOT add({ a: 5, b: 10 })
+```
+
+The framework extracts the JSON tags from the struct fields and uses them to generate the correct TypeScript function signature.
+
+### Example
+
+```typescript
+// TypeScript code - call with individual arguments matching struct fields
+const response = fetch("https://httpbin.org/get");
+console.log("OK:", response.ok);
+console.log("Status:", response.status);
+console.log("Body:", response.body);
+```
+
+### Built-in Functions
+
+- `fetch(options)` - HTTP requests
+  - `options.input` (string) - URL to fetch
+  - `options.init` (object) - Optional init object with `method`, `headers`, `body`
+
+## Dependencies
+
+- `github.com/evanw/esbuild/pkg/api` - TypeScript transpilation
+- `github.com/fastschema/qjs` - JavaScript execution (QuickJS)
+- `github.com/stretchr/testify/assert` - Test assertions
+
+## Development
+
+- **Test framework**: Go's built-in `testing` package
+- **Code style**: Follow standard Go conventions
+- **Linting**: `golangci-lint run` - must pass before committing
+- **TypeScript test files**: Tests that require TypeScript files should create them inline using `os.CreateTemp()` instead of relying on external test files
diff --git a/cmd/poiesis/main.go b/cmd/poiesis/main.go
new file mode 100644
index 0000000..a4adca0
--- /dev/null
+++ b/cmd/poiesis/main.go
@@ -0,0 +1,40 @@
+package main
+
+import (
+	"context"
+	"fmt"
+	"os"
+
+	"reichard.io/poiesis/internal/runtime"
+)
+
+func main() {
+	// Validate Arguments
+	if len(os.Args) < 2 {
+		fmt.Fprintln(os.Stderr, "Usage: poiesis <typescript-file>")
+		fmt.Fprintln(os.Stderr, "       poiesis -print-types")
+		os.Exit(1)
+	}
+
+	// Print Types
+	if os.Args[1] == "-print-types" {
+		rt, err := runtime.New(context.Background())
+		if err != nil {
+			panic(err)
+		}
+		fmt.Println(rt.GetTypeDeclarations())
+		return
+	}
+
+	// Create Runtime
+	rt, err := runtime.New(context.Background())
+	if err != nil {
+		panic(err)
+	}
+
+	// Run File
+	filePath := os.Args[1]
+	if err := rt.RunFile(filePath); err != nil {
+		panic(err)
+	}
+}
diff --git a/flake.lock b/flake.lock
new file mode 100644
index 0000000..beb68bf
--- /dev/null
+++ b/flake.lock
@@ -0,0 +1,61 @@
+{
+  "nodes": {
+    "flake-utils": {
+      "inputs": {
+        "systems": "systems"
+      },
+      "locked": {
+        "lastModified": 1731533236,
+        "narHash": "sha256-l0KFg5HjrsfsO/JpG+r7fRrqm12kzFHyUHqHCVpMMbI=",
+        "owner": "numtide",
+        "repo": "flake-utils",
+        "rev": "11707dc2f618dd54ca8739b309ec4fc024de578b",
+        "type": "github"
+      },
+      "original": {
+        "owner": "numtide",
+        "repo": "flake-utils",
+        "type": "github"
+      }
+    },
+    "nixpkgs": {
+      "locked": {
+        "lastModified": 1769318308,
+        "narHash": "sha256-Mjx6p96Pkefks3+aA+72lu1xVehb6mv2yTUUqmSet6Q=",
+        "owner": "NixOS",
+        "repo": "nixpkgs",
+        "rev": "1cd347bf3355fce6c64ab37d3967b4a2cb4b878c",
+        "type": "github"
+      },
+      "original": {
+        "owner": "NixOS",
+        "ref": "nixos-25.11",
+        "repo": "nixpkgs",
+        "type": "github"
+      }
+    },
+    "root": {
+      "inputs": {
+        "flake-utils": "flake-utils",
+        "nixpkgs": "nixpkgs"
+      }
+    },
+    "systems": {
+      "locked": {
+        "lastModified": 1681028828,
+        "narHash": "sha256-Vy1rq5AaRuLzOxct8nz4T6wlgyUR7zLU309k9mBC768=",
+        "owner": "nix-systems",
+        "repo": "default",
+        "rev": "da67096a3b9bf56a91d16901293e51ba5b49a27e",
+        "type": "github"
+      },
+      "original": {
+        "owner": "nix-systems",
+        "repo": "default",
+        "type": "github"
+      }
+    }
+  },
+  "root": "root",
+  "version": 7
+}
diff --git a/flake.nix b/flake.nix
new file mode 100644
index 0000000..2105dc7
--- /dev/null
+++ b/flake.nix
@@ -0,0 +1,37 @@
+{
+  description = "Development Environment";
+
+  inputs = {
+    nixpkgs.url = "github:NixOS/nixpkgs/nixos-25.11";
+    flake-utils.url = "github:numtide/flake-utils";
+  };
+
+  outputs =
+    { self
+    , nixpkgs
+    , flake-utils
+    ,
+    }:
+    flake-utils.lib.eachDefaultSystem (
+      system:
+      let
+        pkgs = (
+          import nixpkgs {
+            system = system;
+          }
+        );
+      in
+      {
+        devShells.default = pkgs.mkShell {
+          packages = with pkgs; [
+            # Backend
+            go
+            gopls
+            golangci-lint
+
+            tree
+          ];
+        };
+      }
+    );
+}
diff --git a/go.mod b/go.mod
new file mode 100644
index 0000000..4b96c3e
--- /dev/null
+++ b/go.mod
@@ -0,0 +1,17 @@
+module reichard.io/poiesis
+
+go 1.25.5
+
+require (
+	github.com/evanw/esbuild v0.27.2
+	github.com/fastschema/qjs v0.0.6
+	github.com/stretchr/testify v1.11.1
+)
+
+require (
+	github.com/davecgh/go-spew v1.1.1 // indirect
+	github.com/pmezard/go-difflib v1.0.0 // indirect
+	github.com/tetratelabs/wazero v1.9.0 // indirect
+	golang.org/x/sys v0.37.0 // indirect
+	gopkg.in/yaml.v3 v3.0.1 // indirect
+)
diff --git a/go.sum b/go.sum
new file mode 100644
index 0000000..f1e663f
--- /dev/null
+++ b/go.sum
@@ -0,0 +1,19 @@
+github.com/davecgh/go-spew v1.1.1 h1:vj9j/u1bqnvCEfJOwUhtlOARqs3+rkHYY13jYWTU97c=
+github.com/davecgh/go-spew v1.1.1/go.mod h1:J7Y8YcW2NihsgmVo/mv3lAwl/skON4iLHjSsI+c5H38=
+github.com/evanw/esbuild v0.27.2 h1:3xBEws9y/JosfewXMM2qIyHAi+xRo8hVx475hVkJfNg=
+github.com/evanw/esbuild v0.27.2/go.mod h1:D2vIQZqV/vIf/VRHtViaUtViZmG7o+kKmlBfVQuRi48=
+github.com/fastschema/qjs v0.0.6 h1:C45KMmQMd21UwsUAmQHxUxiWOfzwTg1GJW0DA0AbFEE=
+github.com/fastschema/qjs v0.0.6/go.mod h1:bbg36wxXnx8g0FdKIe5+nCubrQvHa7XEVWqUptjHt/A=
+github.com/pmezard/go-difflib v1.0.0 h1:4DBwDE0NGyQoBHbLQYPwSUPoCMWR5BEzIk/f1lZbAQM=
+github.com/pmezard/go-difflib v1.0.0/go.mod h1:iKH77koFhYxTK1pcRnkKkqfTogsbg7gZNVY4sRDYZ/4=
+github.com/stretchr/testify v1.11.1 h1:7s2iGBzp5EwR7/aIZr8ao5+dra3wiQyKjjFuvgVKu7U=
+github.com/stretchr/testify v1.11.1/go.mod h1:wZwfW3scLgRK+23gO65QZefKpKQRnfz6sD981Nm4B6U=
+github.com/tetratelabs/wazero v1.9.0 h1:IcZ56OuxrtaEz8UYNRHBrUa9bYeX9oVY93KspZZBf/I=
+github.com/tetratelabs/wazero v1.9.0/go.mod h1:TSbcXCfFP0L2FGkRPxHphadXPjo1T6W+CseNNY7EkjM=
+golang.org/x/sys v0.0.0-20220715151400-c0bba94af5f8/go.mod h1:oPkhp1MJrh7nUepCBck5+mAzfO9JrbApNNgaTdGDITg=
+golang.org/x/sys v0.37.0 h1:fdNQudmxPjkdUTPnLn5mdQv7Zwvbvpaxqs831goi9kQ=
+golang.org/x/sys v0.37.0/go.mod h1:OgkHotnGiDImocRcuBABYBEXf8A9a87e/uXjp9XT3ks=
+gopkg.in/check.v1 v0.0.0-20161208181325-20d25e280405 h1:yhCVgyC4o1eVCa2tZl7eS0r+SDo693bJlVdllGtEeKM=
+gopkg.in/check.v1 v0.0.0-20161208181325-20d25e280405/go.mod h1:Co6ibVJAznAaIkqp8huTwlJQCZ016jof/cbN4VW5Yz0=
+gopkg.in/yaml.v3 v3.0.1 h1:fxVm/GzAzEWqLHuvctI91KS9hhNmmWOoWu0XTYJS7CA=
+gopkg.in/yaml.v3 v3.0.1/go.mod h1:K4uyk7z7BCEPqu6E+C64Yfv1cQ7kz7rIZviUmN+EgEM=
diff --git a/internal/functions/registry.go b/internal/functions/registry.go
new file mode 100644
index 0000000..21c77e4
--- /dev/null
+++ b/internal/functions/registry.go
@@ -0,0 +1,115 @@
+package functions
+
+import (
+	"fmt"
+	"reflect"
+	"strings"
+	"sync"
+
+	"reichard.io/poiesis/internal/tsconvert"
+)
+
+var (
+	functionRegistry = make(map[string]Function)
+	registryMutex    sync.RWMutex
+)
+
+func registerFunction[A Args, R any](name string, isAsync bool, fn GoFunc[A, R]) {
+	// Lock Registry
+	registryMutex.Lock()
+	defer registryMutex.Unlock()
+
+	// Validate Args Type
+	tType := reflect.TypeFor[A]()
+	if tType.Kind() != reflect.Struct {
+		panic(fmt.Sprintf("function %s: argument must be a struct type, got %v", name, tType))
+	}
+
+	// Collect Types and Generate Definition
+	fnType := reflect.TypeOf(fn)
+	types := tsconvert.CollectTypes(tType, fnType)
+	definition := tsconvert.GenerateFunctionDecl(name, tType, fnType, isAsync)
+
+	// Register Function
+	functionRegistry[name] = &functionImpl[A, R]{
+		name:       name,
+		fn:         fn,
+		types:      types.All(),
+		definition: definition,
+		isAsync:    isAsync,
+	}
+}
+
+func GetFunctionDeclarations() string {
+	// Lock Registry
+	registryMutex.RLock()
+	defer registryMutex.RUnlock()
+
+	// Collect Type Definitions
+	typeDefinitions := make(map[string]bool)
+	var typeDefs []string
+	var functionDecls []string
+
+	for _, fn := range functionRegistry {
+		for _, t := range fn.Types() {
+			if !typeDefinitions[t] {
+				typeDefinitions[t] = true
+				typeDefs = append(typeDefs, t)
+			}
+		}
+		functionDecls = append(functionDecls, fn.Definition())
+	}
+
+	// Build Result
+	result := strings.Join(typeDefs, "\n\n")
+	if len(result) > 0 && len(functionDecls) > 0 {
+		result += "\n\n"
+	}
+	result += strings.Join(functionDecls, "\n")
+
+	return result
+}
+
+// GetTypeDeclarations returns all type declarations from all registered functions.
+// This is used for aggregating types across multiple functions.
+func GetTypeDeclarations() map[string]string {
+	// Lock Registry
+	registryMutex.RLock()
+	defer registryMutex.RUnlock()
+
+	// Collect All Types
+	allTypes := make(map[string]string)
+	for _, fn := range functionRegistry {
+		for name, def := range fn.Types() {
+			if existing, ok := allTypes[name]; ok && existing != def {
+				// Type Conflict Detected - Skip
+				continue
+			}
+			allTypes[name] = def
+		}
+	}
+	return allTypes
+}
+
+func GetRegisteredFunctions() map[string]Function {
+	// Lock Registry
+	registryMutex.RLock()
+	defer registryMutex.RUnlock()
+
+	// Copy Registry
+	result := make(map[string]Function, len(functionRegistry))
+	for k, v := range functionRegistry {
+		result[k] = v
+	}
+	return result
+}
+
+func RegisterFunction[T Args, R any](name string, fn GoFunc[T, R]) {
+	// Register Sync Function
+	registerFunction(name, false, fn)
+}
+
+func RegisterAsyncFunction[T Args, R any](name string, fn GoFunc[T, R]) {
+	// Register Async Function
+	registerFunction(name, true, fn)
+}
diff --git a/internal/functions/types.go b/internal/functions/types.go
new file mode 100644
index 0000000..040c295
--- /dev/null
+++ b/internal/functions/types.go
@@ -0,0 +1,92 @@
+package functions
+
+import (
+	"context"
+	"errors"
+	"reflect"
+)
+
+type Function interface {
+	Name() string
+	Types() map[string]string
+	Definition() string
+	IsAsync() bool
+	Arguments() []reflect.Type
+	Call(context.Context, []any) (any, error)
+}
+
+type GoFunc[A Args, R any] func(context.Context, A) (R, error)
+
+type Args interface {
+	Validate() error
+}
+
+type functionImpl[A Args, R any] struct {
+	name       string
+	fn         GoFunc[A, R]
+	definition string
+	types      map[string]string
+	isAsync    bool
+}
+
+func (b *functionImpl[A, R]) Name() string {
+	return b.name
+}
+
+func (b *functionImpl[A, R]) Types() map[string]string {
+	return b.types
+}
+
+func (b *functionImpl[A, R]) Definition() string {
+	return b.definition
+}
+
+func (b *functionImpl[A, R]) IsAsync() bool {
+	return b.isAsync
+}
+
+func (b *functionImpl[A, R]) Function() any {
+	return b.fn
+}
+
+func (b *functionImpl[A, R]) Arguments() []reflect.Type {
+	// Collect Argument Types
+	var allTypes []reflect.Type
+
+	rType := reflect.TypeFor[A]()
+	for i := range rType.NumField() {
+		allTypes = append(allTypes, rType.Field(i).Type)
+	}
+
+	return allTypes
+}
+
+func (b *functionImpl[A, R]) Call(ctx context.Context, allArgs []any) (any, error) {
+	return b.CallGeneric(ctx, allArgs)
+}
+
+func (b *functionImpl[A, R]) CallGeneric(ctx context.Context, allArgs []any) (zeroR R, err error) {
+	// Populate Arguments
+	var fnArgs A
+	aVal := reflect.ValueOf(&fnArgs).Elem()
+
+	// Populate Fields
+	for i := range min(aVal.NumField(), len(allArgs)) {
+		field := aVal.Field(i)
+
+		// Validate Field is Settable
+		if !field.CanSet() {
+			return zeroR, errors.New("cannot set field")
+		}
+
+		// Validate and Set Field Value
+		argVal := reflect.ValueOf(allArgs[i])
+		if !argVal.Type().AssignableTo(field.Type()) {
+			return zeroR, errors.New("cannot assign field")
+		}
+		field.Set(argVal)
+	}
+
+	// Execute Function
+	return b.fn(ctx, fnArgs)
+}
diff --git a/internal/functions/typescript_test.go b/internal/functions/typescript_test.go
new file mode 100644
index 0000000..491e47c
--- /dev/null
+++ b/internal/functions/typescript_test.go
@@ -0,0 +1,272 @@
+package functions
+
+import (
+	"context"
+	"sync"
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+)
+
+type TestBasicArgs struct {
+	Name string `json:"name"`
+	Age  int    `json:"age"`
+}
+
+func (t TestBasicArgs) Validate() error { return nil }
+
+func TestBasicType(t *testing.T) {
+	// Reset Registry
+	resetRegistry()
+
+	// Register Function
+	RegisterFunction("basic", func(ctx context.Context, args TestBasicArgs) (string, error) {
+		return args.Name, nil
+	})
+
+	// Verify Declarations
+	defs := GetFunctionDeclarations()
+	assert.Contains(t, defs, "declare function basic(name: string, age: number): string;")
+	assert.Contains(t, defs, "interface TestBasicArgs")
+}
+
+func resetRegistry() {
+	// Lock Registry
+	registryLock.Lock()
+	defer registryLock.Unlock()
+
+	// Clear Registry
+	functionRegistry = make(map[string]Function)
+}
+
+var (
+	registryLock sync.Mutex
+)
+
+type TestComplexArgs struct {
+	Items []int          `json:"items"`
+	Data  map[string]any `json:"data"`
+	Flag  bool           `json:"flag"`
+}
+
+func (t TestComplexArgs) Validate() error { return nil }
+
+func TestComplexTypes(t *testing.T) {
+	// Reset Registry
+	resetRegistry()
+
+	// Register Function
+	RegisterFunction("complex", func(ctx context.Context, args TestComplexArgs) (bool, error) {
+		return args.Flag, nil
+	})
+
+	// Verify Declarations
+	defs := GetFunctionDeclarations()
+	assert.Contains(t, defs, "declare function complex(items: number[], data: Record<string, any>, flag: boolean): boolean;")
+}
+
+type TestNestedArgs struct {
+	User struct {
+		FirstName string `json:"firstName"`
+		LastName  string `json:"lastName"`
+	} `json:"user"`
+}
+
+func (t TestNestedArgs) Validate() error { return nil }
+
+func TestNestedStruct(t *testing.T) {
+	// Reset Registry
+	resetRegistry()
+
+	// Register Function
+	RegisterFunction("nested", func(ctx context.Context, args TestNestedArgs) (string, error) {
+		return args.User.FirstName, nil
+	})
+
+	// Verify Declarations
+	defs := GetFunctionDeclarations()
+	assert.Contains(t, defs, "declare function nested(user: {}): string;")
+}
+
+type TestOptionalArgs struct {
+	Name  string `json:"name"`
+	Age   *int   `json:"age,omitempty"`
+	Score *int   `json:"score"`
+}
+
+func (t TestOptionalArgs) Validate() error { return nil }
+
+func TestOptionalFields(t *testing.T) {
+	// Reset Registry
+	resetRegistry()
+
+	// Register Function
+	RegisterFunction[TestOptionalArgs, string]("optional", func(ctx context.Context, args TestOptionalArgs) (string, error) {
+		return args.Name, nil
+	})
+
+	// Verify Declarations
+	defs := GetFunctionDeclarations()
+	assert.Contains(t, defs, "declare function optional(name: string, age?: number | null, score?: number | null): string;")
+}
+
+type TestResult struct {
+	ID   int    `json:"id"`
+	Data []byte `json:"data"`
+}
+
+type TestResultArgs struct {
+	Input string `json:"input"`
+}
+
+func (t TestResultArgs) Validate() error { return nil }
+
+func TestResultStruct(t *testing.T) {
+	// Reset Registry
+	resetRegistry()
+
+	// Register Function
+	RegisterFunction[TestResultArgs, TestResult]("result", func(ctx context.Context, args TestResultArgs) (TestResult, error) {
+		return TestResult{ID: 1}, nil
+	})
+
+	// Verify Declarations
+	defs := GetFunctionDeclarations()
+	assert.Contains(t, defs, "declare function result(input: string): TestResult;")
+	assert.Contains(t, defs, "interface TestResult {id: number; data: number[]}")
+}
+
+type TestAsyncArgs struct {
+	URL string `json:"url"`
+}
+
+func (t TestAsyncArgs) Validate() error { return nil }
+
+type TestAsyncResult struct {
+	Status int `json:"status"`
+}
+
+func TestAsyncPromise(t *testing.T) {
+	// Reset Registry
+	resetRegistry()
+
+	// Register Async Function
+	RegisterAsyncFunction[TestAsyncArgs, *TestAsyncStatus]("async", func(ctx context.Context, args TestAsyncArgs) (*TestAsyncStatus, error) {
+		return &TestAsyncStatus{Code: 200}, nil
+	})
+
+	// Verify Declarations
+	defs := GetFunctionDeclarations()
+	assert.Contains(t, defs, "declare function async(url: string): Promise<TestAsyncStatus | null>;")
+	assert.Contains(t, defs, "interface TestAsyncStatus")
+}
+
+type TestAsyncStatus struct {
+	Code int `json:"code"`
+}
+
+type TestNestedPointerResult struct {
+	Value string `json:"value"`
+}
+
+type TestNestedPointerArgs struct {
+	ID int `json:"id"`
+}
+
+func (t TestNestedPointerArgs) Validate() error { return nil }
+
+func TestNestedPointerInResult(t *testing.T) {
+	// Reset Registry
+	resetRegistry()
+
+	// Register Function
+	RegisterFunction[TestNestedPointerArgs, *TestNestedPointerResult]("pointerResult", func(ctx context.Context, args TestNestedPointerArgs) (*TestNestedPointerResult, error) {
+		return &TestNestedPointerResult{Value: "test"}, nil
+	})
+
+	// Verify Declarations
+	defs := GetFunctionDeclarations()
+	assert.Contains(t, defs, "declare function pointerResult(id: number): TestNestedPointerResult | null;")
+}
+
+type TestUintArgs struct {
+	Value uint `json:"value"`
+}
+
+func (t TestUintArgs) Validate() error { return nil }
+
+func TestUintType(t *testing.T) {
+	// Reset Registry
+	resetRegistry()
+
+	// Register Function
+	RegisterFunction[TestUintArgs, uint]("uint", func(ctx context.Context, args TestUintArgs) (uint, error) {
+		return args.Value, nil
+	})
+
+	// Verify Declarations
+	defs := GetFunctionDeclarations()
+	assert.Contains(t, defs, "declare function uint(value: number): number;")
+}
+
+type TestFloatArgs struct {
+	Amount float64 `json:"amount"`
+}
+
+func (t TestFloatArgs) Validate() error { return nil }
+
+func TestFloatType(t *testing.T) {
+	// Reset Registry
+	resetRegistry()
+
+	// Register Function
+	RegisterFunction[TestFloatArgs, float32]("float", func(ctx context.Context, args TestFloatArgs) (float32, error) {
+		return float32(args.Amount), nil
+	})
+
+	// Verify Declarations
+	defs := GetFunctionDeclarations()
+	assert.Contains(t, defs, "declare function float(amount: number): number;")
+}
+
+type TestPointerInArgs struct {
+	User *struct {
+		Name string `json:"name"`
+	} `json:"user"`
+}
+
+func (t TestPointerInArgs) Validate() error { return nil }
+
+func TestNestedPointerStruct(t *testing.T) {
+	// Reset Registry
+	resetRegistry()
+
+	// Register Function
+	RegisterFunction[TestPointerInArgs, string]("nestedPointer", func(ctx context.Context, args TestPointerInArgs) (string, error) {
+		return "test", nil
+	})
+
+	// Verify Declarations
+	defs := GetFunctionDeclarations()
+	assert.Contains(t, defs, "declare function nestedPointer(user?: {} | null): string;")
+}
+
+type TestErrorOnlyArgs struct {
+	Input string `json:"input"`
+}
+
+func (t TestErrorOnlyArgs) Validate() error { return nil }
+
+func TestErrorOnlyReturn(t *testing.T) {
+	// Reset Registry
+	resetRegistry()
+
+	// Register Function
+	RegisterFunction[TestErrorOnlyArgs, struct{}]("errorOnly", func(ctx context.Context, args TestErrorOnlyArgs) (struct{}, error) {
+		return struct{}{}, nil
+	})
+
+	// Verify Declarations
+	defs := GetFunctionDeclarations()
+	assert.Contains(t, defs, "declare function errorOnly(input: string): {};")
+}
diff --git a/internal/runtime/options.go b/internal/runtime/options.go
new file mode 100644
index 0000000..9213669
--- /dev/null
+++ b/internal/runtime/options.go
@@ -0,0 +1,19 @@
+package runtime
+
+import "io"
+
+type RuntimeOption func(*Runtime)
+
+func WithStdout(stdout io.Writer) RuntimeOption {
+	return func(r *Runtime) {
+		// Set Stdout
+		r.opts.Stdout = stdout
+	}
+}
+
+func WithStderr(stderr io.Writer) RuntimeOption {
+	return func(r *Runtime) {
+		// Set Stderr
+		r.opts.Stderr = stderr
+	}
+}
diff --git a/internal/runtime/runtime.go b/internal/runtime/runtime.go
new file mode 100644
index 0000000..e068fb6
--- /dev/null
+++ b/internal/runtime/runtime.go
@@ -0,0 +1,180 @@
+package runtime
+
+import (
+	"context"
+	"fmt"
+	"os"
+	"strings"
+
+	"github.com/evanw/esbuild/pkg/api"
+	"github.com/fastschema/qjs"
+
+	"reichard.io/poiesis/internal/functions"
+	_ "reichard.io/poiesis/internal/stdlib"
+)
+
+type Runtime struct {
+	ctx       *qjs.Context
+	opts      qjs.Option
+	funcs     map[string]functions.Function
+	typeDecls map[string]string
+}
+
+func New(ctx context.Context, opts ...RuntimeOption) (*Runtime, error) {
+	// Create Runtime
+	r := &Runtime{opts: qjs.Option{Context: ctx}}
+	for _, opt := range opts {
+		opt(r)
+	}
+
+	// Create QuickJS Context
+	rt, err := qjs.New(r.opts)
+	if err != nil {
+		return nil, err
+	}
+	r.ctx = rt.Context()
+
+	// Populate Globals
+	if err := r.populateGlobals(); err != nil {
+		return nil, err
+	}
+
+	return r, nil
+}
+
+func (r *Runtime) populateGlobals() error {
+	// Initialize Maps
+	r.funcs = make(map[string]functions.Function)
+	r.typeDecls = make(map[string]string)
+
+	// Load Requested Functions
+	allFuncs := functions.GetRegisteredFunctions()
+	for name, fn := range allFuncs {
+		r.funcs[name] = fn
+		if err := r.addFunctionTypes(fn); err != nil {
+			return fmt.Errorf("failed to add types for function %s: %w", name, err)
+		}
+	}
+
+	// Register Functions with QuickJS
+	for name, fn := range r.funcs {
+		if fn.IsAsync() {
+			r.ctx.SetAsyncFunc(name, func(this *qjs.This) {
+				qjsVal, err := callFunc(this, fn)
+				if err != nil {
+					_ = this.Promise().Reject(this.Context().NewError(err))
+					return
+				}
+				_ = this.Promise().Resolve(qjsVal)
+			})
+		} else {
+			r.ctx.SetFunc(name, func(this *qjs.This) (*qjs.Value, error) {
+				return callFunc(this, fn)
+			})
+		}
+	}
+
+	return nil
+}
+
+// addFunctionTypes adds types from a function to the runtime's type declarations.
+// Returns an error if there's a type conflict (same name, different definition).
+func (r *Runtime) addFunctionTypes(fn functions.Function) error {
+	for name, def := range fn.Types() {
+		if existing, ok := r.typeDecls[name]; ok && existing != def {
+			return fmt.Errorf("type conflict: %s has conflicting definitions (existing: %s, new: %s)",
+				name, existing, def)
+		}
+		r.typeDecls[name] = def
+	}
+	return nil
+}
+
+// GetTypeDeclarations returns all TypeScript type declarations for this runtime.
+// Includes both type definitions and function declarations.
+func (r *Runtime) GetTypeDeclarations() string {
+	var decls []string
+
+	// Add Type Definitions
+	for _, def := range r.typeDecls {
+		decls = append(decls, def)
+	}
+
+	// Add Function Declarations
+	for _, fn := range r.funcs {
+		decls = append(decls, fn.Definition())
+	}
+
+	return strings.Join(decls, "\n\n")
+}
+
+func (r *Runtime) RunFile(filePath string) error {
+	tsFileContent, err := os.ReadFile(filePath)
+	if err != nil {
+		return fmt.Errorf("error reading file: %w", err)
+	}
+
+	return r.RunCode(string(tsFileContent))
+}
+
+func (r *Runtime) RunCode(tsCode string) error {
+	transformedCode, err := r.transformCode(tsCode)
+	if err != nil {
+		return err
+	}
+
+	result, err := r.ctx.Eval("code.ts", qjs.Code(string(transformedCode)))
+	if result != nil {
+		result.Free()
+	}
+	return err
+}
+
+func (r *Runtime) transformCode(tsCode string) ([]byte, error) {
+	result := api.Transform(tsCode, api.TransformOptions{
+		Loader:      api.LoaderTS,
+		Target:      api.ES2022,
+		Format:      api.FormatIIFE,
+		Sourcemap:   api.SourceMapNone,
+		TreeShaking: api.TreeShakingFalse,
+	})
+
+	if len(result.Errors) > 0 {
+		var b strings.Builder
+		for i, e := range result.Errors {
+			if i > 0 {
+				b.WriteString(", ")
+			}
+			b.WriteString(e.Text)
+		}
+		return nil, fmt.Errorf("transpilation failed: %s", b.String())
+	}
+
+	return result.Code, nil
+}
+
+func callFunc(this *qjs.This, fn functions.Function) (*qjs.Value, error) {
+	qjsArgs := this.Args()
+	fnArgs := fn.Arguments()
+
+	var allArgs []any
+	for i := range min(len(fnArgs), len(qjsArgs)) {
+		rVal, err := qjs.JsArgToGo(qjsArgs[i], fnArgs[i])
+		if err != nil {
+			return nil, fmt.Errorf("argument conversion failed: %w", err)
+		}
+		allArgs = append(allArgs, rVal.Interface())
+	}
+
+	result, err := fn.Call(this.Context(), allArgs)
+	if err != nil {
+		return nil, err
+	}
+
+	val, err := qjs.ToJsValue(this.Context(), result)
+	if err != nil {
+		return nil, err
+	}
+
+	return val, nil
+}
diff --git a/internal/runtime/runtime_test.go b/internal/runtime/runtime_test.go
new file mode 100644
index 0000000..7e79576
--- /dev/null
+++ b/internal/runtime/runtime_test.go
@@ -0,0 +1,163 @@
+package runtime
+
+import (
+	"bytes"
+	"context"
+	"os"
+	"strings"
+	"testing"
+
+	"github.com/fastschema/qjs"
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
+
+	"reichard.io/poiesis/internal/functions"
+)
+
+type TestArgs struct {
+	Field1 string `json:"field1"`
+}
+
+func (t TestArgs) Validate() error {
+	return nil
+}
+
+func TestExecuteTypeScript(t *testing.T) {
+	// Create Buffers
+	var stdout, stderr bytes.Buffer
+
+	// Create Runtime
+	rt, err := New(context.Background(), WithStderr(&stderr), WithStdout(&stdout))
+	assert.NoError(t, err, "Expected no error")
+
+	// Create TypeScript Code
+	tsCode := `interface Person {
+  name: string;
+  age: number;
+  email: string;
+}
+
+function greet(person: Person): string {
+  return "Hello, " + person.name + "! You are " + person.age + " years old.";
+}
+
+const user: Person = {
+  name: "Alice",
+  age: 30,
+  email: "alice@example.com",
+};
+
+console.log(greet(user));
+console.log("Email: " + user.email);
+
+function calculateSum(a: number, b: number): number {
+  return a + b;
+}
+
+console.log("Sum of 5 and 10 is: " + calculateSum(5, 10));
+`
+
+	// Create Temp File
+	tmpFile, err := os.CreateTemp("", "*.ts")
+	assert.NoError(t, err, "Failed to create temp file")
+	t.Cleanup(func() {
+		_ = os.Remove(tmpFile.Name())
+	})
+
+	// Write Code to File
+	_, err = tmpFile.WriteString(tsCode)
+	assert.NoError(t, err, "Failed to write to temp file")
+	err = tmpFile.Close()
+	assert.NoError(t, err, "Failed to close temp file")
+
+	// Run File
+	err = rt.RunFile(tmpFile.Name())
+
+	// Verify Execution
+	assert.NoError(t, err, "Expected no error")
+	assert.Empty(t, stderr.String(), "Expected no error output")
+
+	// Verify Output
+	output := stdout.String()
+	assert.Contains(t, output, "Hello, Alice!", "Should greet Alice")
+	assert.Contains(t, output, "You are 30 years old", "Should show age")
+	assert.Contains(t, output, "Email: alice@example.com", "Should show email")
+	assert.Contains(t, output, "Sum of 5 and 10 is: 15", "Should calculate sum correctly")
+
+	// Verify Line Count
+	lines := strings.Split(strings.TrimSpace(output), "\n")
+	assert.GreaterOrEqual(t, len(lines), 3, "Should have at least 3 output lines")
+}
+
+func TestAsyncFunctionResolution(t *testing.T) {
+	// Register Async Function
+	functions.RegisterAsyncFunction("resolveTest", func(_ context.Context, args TestArgs) (string, error) {
+		return "test-result", nil
+	})
+
+	// Create Runtime
+	r, err := New(context.Background())
+	require.NoError(t, err)
+
+	// Execute Async Function - Must be awaited in async context
+	result, err := r.ctx.Eval("test.js", qjs.Code(`async function run() { return await resolveTest("hello"); }; run()`))
+	require.NoError(t, err)
+	require.NotNil(t, result)
+	defer result.Free()
+
+	// Verify Result
+	assert.Equal(t, "test-result", result.String())
+}
+
+func TestAsyncFunctionRejection(t *testing.T) {
+	// Register Async Function that Returns Error
+	functions.RegisterAsyncFunction("rejectTest", func(_ context.Context, args TestArgs) (string, error) {
+		return "", assert.AnError
+	})
+
+	// Create Runtime
+	r, err := New(context.Background())
+	require.NoError(t, err)
+
+	// Execute Async Function - Rejected Promises Throw When Awaited
+	_, err = r.ctx.Eval("test.js", qjs.Code(`async function run() { return await rejectTest("hello"); }; run()`))
+	assert.Error(t, err)
+}
+
+func TestNonPromise(t *testing.T) {
+	// Register Sync Function
+	functions.RegisterFunction("nonPromiseTest", func(_ context.Context, args TestArgs) (string, error) {
+		return "sync-result", nil
+	})
+
+	// Create Runtime
+	r, err := New(context.Background())
+	assert.NoError(t, err)
+
+	// Execute Sync Function
+	result, err := r.ctx.Eval("test.js", qjs.Code(`nonPromiseTest("hello")`))
+	assert.NoError(t, err)
+	defer result.Free()
+
+	// Verify Result
+	assert.Equal(t, "sync-result", result.String())
+}
+
+func TestGetTypeDeclarations(t *testing.T) {
+	// Register Function
+	functions.RegisterFunction("testFunc", func(_ context.Context, args TestArgs) (string, error) {
+		return "result", nil
+	})
+
+	// Create Runtime
+	r, err := New(context.Background())
+	require.NoError(t, err)
+
+	// Get Type Declarations
+	decls := r.GetTypeDeclarations()
+
+	// Verify Declarations
+	assert.Contains(t, decls, "interface TestArgs")
+	assert.Contains(t, decls, "declare function testFunc")
+	assert.Contains(t, decls, "field1: string")
+}
diff --git a/internal/stdlib/fetch.go b/internal/stdlib/fetch.go
new file mode 100644
index 0000000..74b6a4a
--- /dev/null
+++ b/internal/stdlib/fetch.go
@@ -0,0 +1,102 @@
+package stdlib
+
+import (
+	"context"
+	"fmt"
+	"io"
+	"maps"
+	"net/http"
+	"strings"
+
+	"reichard.io/poiesis/internal/functions"
+)
+
+func init() {
+	functions.RegisterAsyncFunction("fetch", Fetch)
+}
+
+type FetchArgs struct {
+	Input string       `json:"input"`
+	Init  *RequestInit `json:"init,omitempty"`
+}
+
+func (f FetchArgs) Validate() error {
+	return nil
+}
+
+type RequestInit struct {
+	Method  string            `json:"method,omitempty"`
+	Headers map[string]string `json:"headers,omitempty"`
+	Body    *string           `json:"body,omitempty"`
+}
+
+func (o *RequestInit) Validate() error {
+	return nil
+}
+
+type Response struct {
+	OK      bool              `json:"ok"`
+	Status  int               `json:"status"`
+	Body    string            `json:"body"`
+	Headers map[string]string `json:"headers"`
+}
+
+func Fetch(ctx context.Context, args FetchArgs) (Response, error) {
+	// Set Default Method and Headers
+	method := "GET"
+	headers := make(map[string]string)
+
+	// Apply Init Options
+	if args.Init != nil {
+		if args.Init.Method != "" {
+			method = args.Init.Method
+		}
+		if args.Init.Headers != nil {
+			maps.Copy(headers, args.Init.Headers)
+		}
+	}
+
+	// Create Request
+	req, err := http.NewRequestWithContext(ctx, method, args.Input, nil)
+	if err != nil {
+		return Response{}, fmt.Errorf("failed to create request: %w", err)
+	}
+
+	// Set Request Headers
+	for k, v := range headers {
+		req.Header.Set(k, v)
+	}
+
+	// Execute Request
+	resp, err := http.DefaultClient.Do(req)
+	if err != nil {
+		return Response{}, fmt.Errorf("failed to fetch: %w", err)
+	}
+	defer func() {
+		_ = resp.Body.Close()
+	}()
+
+	// Read Response Body
+	body, err := io.ReadAll(resp.Body)
+	if err != nil {
+		return Response{}, fmt.Errorf("failed to read body: %w", err)
+	}
+
+	// Collect Response Headers
+	resultHeaders := make(map[string]string)
+	for key, values := range resp.Header {
+		if len(values) > 0 {
+			val := values[0]
+			resultHeaders[key] = val
+			resultHeaders[strings.ToLower(key)] = val
+		}
+	}
+
+	// Return Response
+	return Response{
+		OK:      resp.StatusCode >= 200 && resp.StatusCode < 300,
+		Status:  resp.StatusCode,
+		Body:    string(body),
+		Headers: resultHeaders,
+	}, nil
+}
diff --git a/internal/stdlib/fetch_test.go b/internal/stdlib/fetch_test.go
new file mode 100644
index 0000000..4946670
--- /dev/null
+++ b/internal/stdlib/fetch_test.go
@@ -0,0 +1,130 @@
+package stdlib
+
+import (
+	"context"
+	"net/http"
+	"net/http/httptest"
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
+)
+
+func TestFetch(t *testing.T) {
+	// Create Context
+	ctx := context.Background()
+
+	// Create Test Server
+	server := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		w.Header().Set("Content-Type", "application/json")
+		w.Header().Set("X-Custom-Header", "test-value")
+		w.WriteHeader(http.StatusOK)
+		_, _ = w.Write([]byte(`{"status":"ok","message":"Hello from httptest"}`))
+	}))
+	defer server.Close()
+
+	// Execute Fetch
+	result, err := Fetch(ctx, FetchArgs{Input: server.URL})
+	require.NoError(t, err)
+
+	// Verify Response
+	assert.True(t, result.OK)
+	assert.Equal(t, http.StatusOK, result.Status)
+	assert.Contains(t, result.Body, "Hello from httptest")
+	assert.Contains(t, result.Body, `"status":"ok"`)
+	assert.Equal(t, "application/json", result.Headers["Content-Type"])
+	assert.Equal(t, "test-value", result.Headers["X-Custom-Header"])
+}
+
+func TestFetchHTTPBin(t *testing.T) {
+	// Create Context
+	ctx := context.Background()
+
+	// Create Test Server
+	server := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		w.Header().Set("Content-Type", "application/json")
+		w.WriteHeader(http.StatusOK)
+		_, _ = w.Write([]byte(`{"args":{}}`))
+	}))
+	defer server.Close()
+
+	// Execute Fetch
+	result, err := Fetch(ctx, FetchArgs{Input: server.URL})
+	require.NoError(t, err)
+
+	// Verify Response
+	assert.True(t, result.OK)
+	assert.Equal(t, http.StatusOK, result.Status)
+	assert.Contains(t, result.Body, `"args"`)
+	assert.Equal(t, "application/json", result.Headers["Content-Type"])
+}
+
+func TestFetchWith404(t *testing.T) {
+	// Create Context
+	ctx := context.Background()
+
+	// Execute Fetch
+	result, err := Fetch(ctx, FetchArgs{Input: "https://httpbin.org/status/404"})
+	require.NoError(t, err)
+
+	// Verify Response
+	assert.False(t, result.OK)
+	assert.Equal(t, http.StatusNotFound, result.Status)
+}
+
+func TestFetchWithInvalidURL(t *testing.T) {
+	// Create Context
+	ctx := context.Background()
+
+	// Execute Fetch - Should Fail
+	_, err := Fetch(ctx, FetchArgs{Input: "http://this-domain-does-not-exist-12345.com"})
+	assert.Error(t, err)
+	assert.Contains(t, err.Error(), "failed to fetch")
+}
+
+func TestFetchWithHeaders(t *testing.T) {
+	// Create Context
+	ctx := context.Background()
+
+	// Create Test Server
+	server := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		assert.Equal(t, "Bearer test-token", r.Header.Get("Authorization"))
+		assert.Equal(t, "GET", r.Method)
+		w.WriteHeader(http.StatusOK)
+		_, _ = w.Write([]byte(`ok`))
+	}))
+	defer server.Close()
+
+	// Configure Request Options
+	headers := map[string]string{
+		"Authorization": "Bearer test-token",
+	}
+	options := &RequestInit{
+		Method:  "GET",
+		Headers: headers,
+	}
+
+	// Execute Fetch with Headers
+	result, err := Fetch(ctx, FetchArgs{Input: server.URL, Init: options})
+	require.NoError(t, err)
+	assert.True(t, result.OK)
+}
+
+func TestFetchDefaults(t *testing.T) {
+	// Create Context
+	ctx := context.Background()
+
+	// Create Test Server
+	server := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		assert.Equal(t, "GET", r.Method, "default method should be GET")
+		w.WriteHeader(http.StatusOK)
+		_, _ = w.Write([]byte(`ok`))
+	}))
+	defer server.Close()
+
+	// Execute Fetch with Empty Options
+	options := &RequestInit{}
+	result, err := Fetch(ctx, FetchArgs{Input: server.URL, Init: options})
+	require.NoError(t, err)
+	assert.True(t, result.OK)
+}
diff --git a/internal/tsconvert/convert.go b/internal/tsconvert/convert.go
new file mode 100644
index 0000000..423fcd1
--- /dev/null
+++ b/internal/tsconvert/convert.go
@@ -0,0 +1,242 @@
+package tsconvert
+
+import (
+	"fmt"
+	"reflect"
+	"strings"
+)
+
+// ConvertType converts a Go reflect.Type to a TypeScript type string.
+func ConvertType(t reflect.Type) string {
+	return goTypeToTSType(t, false)
+}
+
+// goTypeToTSType converts a Go type to TypeScript type string.
+// isPointer tracks if we're inside a pointer chain.
+func goTypeToTSType(t reflect.Type, isPointer bool) string {
+	// Handle Pointer Types
+	if t.Kind() == reflect.Pointer {
+		return goTypeToTSType(t.Elem(), true)
+	}
+
+	// Determine Base Type
+	baseType := ""
+	switch t.Kind() {
+	case reflect.String:
+		baseType = "string"
+	case reflect.Int, reflect.Int8, reflect.Int16, reflect.Int32, reflect.Int64:
+		baseType = "number"
+	case reflect.Uint, reflect.Uint8, reflect.Uint16, reflect.Uint32, reflect.Uint64:
+		baseType = "number"
+	case reflect.Float32, reflect.Float64:
+		baseType = "number"
+	case reflect.Bool:
+		baseType = "boolean"
+	case reflect.Interface:
+		baseType = "any"
+	case reflect.Slice:
+		baseType = fmt.Sprintf("%s[]", goTypeToTSType(t.Elem(), false))
+	case reflect.Map:
+		if t.Key().Kind() == reflect.String && t.Elem().Kind() == reflect.Interface {
+			baseType = "Record<string, any>"
+		} else {
+			baseType = "Record<string, any>"
+		}
+	case reflect.Struct:
+		name := t.Name()
+		if name == "" {
+			baseType = "{}"
+		} else {
+			baseType = name
+		}
+	default:
+		baseType = "any"
+	}
+
+	// Add Null for Pointer Types
+	if isPointer {
+		baseType += " | null"
+	}
+	return baseType
+}
+
+// CollectTypes extracts all type declarations from a function signature.
+// It analyzes the args type and return type to find all struct types.
+func CollectTypes(argsType, fnType reflect.Type) *TypeSet {
+	// Create TypeSet
+	ts := NewTypeSet()
+
+	// Collect Types from Args Struct
+	collectStructTypes(argsType, ts)
+
+	// Collect Types from Return Type
+	if fnType.Kind() == reflect.Func && fnType.NumOut() > 0 {
+		lastIndex := fnType.NumOut() - 1
+		lastType := fnType.Out(lastIndex)
+
+		if lastType.Implements(reflect.TypeOf((*error)(nil)).Elem()) {
+			if fnType.NumOut() > 1 {
+				collectType(fnType.Out(0), ts)
+			}
+		} else {
+			collectType(lastType, ts)
+		}
+	}
+
+	return ts
+}
+
+// collectType recursively collects struct types.
+func collectType(t reflect.Type, ts *TypeSet) {
+	// Handle Pointer Types
+	if t.Kind() == reflect.Pointer {
+		collectType(t.Elem(), ts)
+		return
+	}
+
+	// Collect Struct Types
+	if t.Kind() == reflect.Struct {
+		name := t.Name()
+		if name != "" {
+			// Only Process if Not Already Processed
+			if _, exists := ts.Get(name); !exists {
+				collectStructTypes(t, ts)
+			}
+		}
+	}
+}
+
+// collectStructTypes converts a Go struct to TypeScript interface and adds to TypeSet.
+func collectStructTypes(t reflect.Type, ts *TypeSet) {
+	// Validate Struct Type
+	if t.Kind() != reflect.Struct {
+		return
+	}
+
+	// Get Struct Name
+	name := t.Name()
+	if name == "" {
+		return // Skip Anonymous Structs
+	}
+
+	// Check if Already Processed
+	if _, exists := ts.Get(name); exists {
+		return
+	}
+
+	// Collect Fields
+	var fields []string
+	for i := 0; i < t.NumField(); i++ {
+		field := t.Field(i)
+
+		// Skip Anonymous and Unexported Fields
+		if field.Anonymous || !field.IsExported() {
+			continue
+		}
+
+		// Get Field Name
+		fieldName := getFieldName(field)
+
+		// Determine Type and Optionality
+		var tsType string
+		var isOptional bool
+		isPointer := field.Type.Kind() == reflect.Pointer
+
+		if isPointer {
+			isOptional = true
+			tsType = goTypeToTSType(field.Type, true)
+		} else {
+			isOptional = strings.Contains(field.Tag.Get("json"), ",omitempty")
+			tsType = goTypeToTSType(field.Type, false)
+		}
+
+		// Mark Optional Fields
+		if isOptional {
+			fieldName += "?"
+		}
+
+		fields = append(fields, fmt.Sprintf("%s: %s", fieldName, tsType))
+
+		// Recursively Collect Nested Types
+		collectType(field.Type, ts)
+	}
+
+	// Add Type Definition
+	definition := fmt.Sprintf("interface %s {%s}", name, strings.Join(fields, "; "))
+	_ = ts.Add(name, definition)
+}
+
+// getFieldName extracts the field name from json tag or uses the Go field name.
+func getFieldName(field reflect.StructField) string {
+	// Get JSON Tag
+	jsonTag := field.Tag.Get("json")
+	if jsonTag != "" && jsonTag != "-" {
+		name, _, _ := strings.Cut(jsonTag, ",")
+		return name
+	}
+
+	// Use Go Field Name
+	return field.Name
+}
+
+// GenerateFunctionDecl creates a TypeScript function declaration.
+func GenerateFunctionDecl(name string, argsType, fnType reflect.Type, isAsync bool) string {
+	// Validate Args Type
+	if argsType.Kind() != reflect.Struct {
+		return ""
+	}
+
+	// Collect Parameters
+	var params []string
+	for i := 0; i < argsType.NumField(); i++ {
+		field := argsType.Field(i)
+		fieldName := getFieldName(field)
+		goType := field.Type
+
+		// Determine Type and Optionality
+		var tsType string
+		var isOptional bool
+		isPointer := goType.Kind() == reflect.Pointer
+
+		if isPointer {
+			isOptional = true
+			tsType = goTypeToTSType(goType, true)
+			if !strings.Contains(tsType, " | null") {
+				tsType += " | null"
+			}
+		} else {
+			isOptional = strings.Contains(field.Tag.Get("json"), ",omitempty")
+			tsType = goTypeToTSType(goType, false)
+		}
+
+		// Mark Optional Fields
+		if isOptional {
+			fieldName += "?"
+		}
+		params = append(params, fmt.Sprintf("%s: %s", fieldName, tsType))
+	}
+
+	// Determine Return Type
+	returnSignature := "any"
+	if fnType.Kind() == reflect.Func && fnType.NumOut() > 0 {
+		lastIndex := fnType.NumOut() - 1
+		lastType := fnType.Out(lastIndex)
+
+		if lastType.Implements(reflect.TypeOf((*error)(nil)).Elem()) {
+			if fnType.NumOut() > 1 {
+				returnType := fnType.Out(0)
+				returnSignature = goTypeToTSType(returnType, returnType.Kind() == reflect.Pointer)
+			}
+		} else {
+			returnSignature = goTypeToTSType(lastType, lastType.Kind() == reflect.Pointer)
+		}
+	}
+
+	// Wrap in Promise for Async Functions
+	if isAsync {
+		returnSignature = fmt.Sprintf("Promise<%s>", returnSignature)
+	}
+
+	// Generate Declaration
+	return fmt.Sprintf("declare function %s(%s): %s;", name, strings.Join(params, ", "), returnSignature)
+}
diff --git a/internal/tsconvert/convert_test.go b/internal/tsconvert/convert_test.go
new file mode 100644
index 0000000..87b7e23
--- /dev/null
+++ b/internal/tsconvert/convert_test.go
@@ -0,0 +1,333 @@
+package tsconvert
+
+import (
+	"reflect"
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
+)
+
+// Test types for conversion
+type SimpleStruct struct {
+	Name  string `json:"name"`
+	Count int    `json:"count"`
+}
+
+type NestedStruct struct {
+	ID      int           `json:"id"`
+	Simple  SimpleStruct  `json:"simple"`
+	Pointer *SimpleStruct `json:"pointer,omitempty"`
+}
+
+type OptionalFields struct {
+	Required string  `json:"required"`
+	Optional *string `json:"optional,omitempty"`
+	Number   int     `json:"number,omitempty"`
+}
+
+type ComplexTypes struct {
+	Strings []string       `json:"strings"`
+	Numbers []int          `json:"numbers"`
+	Mapping map[string]any `json:"mapping"`
+	Nested  []SimpleStruct `json:"nested"`
+}
+
+func TestConvertType(t *testing.T) {
+	tests := []struct {
+		name     string
+		input    any
+		expected string
+	}{
+		{"string", "", "string"},
+		{"int", int(0), "number"},
+		{"int8", int8(0), "number"},
+		{"int16", int16(0), "number"},
+		{"int32", int32(0), "number"},
+		{"int64", int64(0), "number"},
+		{"uint", uint(0), "number"},
+		{"float32", float32(0), "number"},
+		{"float64", float64(0), "number"},
+		{"bool", true, "boolean"},
+		{"interface", (*any)(nil), "any | null"},
+		{"slice of strings", []string{}, "string[]"},
+		{"slice of ints", []int{}, "number[]"},
+		{"map", map[string]any{}, "Record<string, any>"},
+		{"struct", SimpleStruct{}, "SimpleStruct"},
+		{"pointer to string", (*string)(nil), "string | null"},
+		{"pointer to struct", (*SimpleStruct)(nil), "SimpleStruct | null"},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			// Get Input Type
+			var inputType reflect.Type
+			if tt.input == nil {
+				inputType = reflect.TypeOf((*any)(nil)).Elem()
+			} else {
+				inputType = reflect.TypeOf(tt.input)
+			}
+
+			// Convert Type
+			result := ConvertType(inputType)
+
+			// Verify Result
+			assert.Equal(t, tt.expected, result)
+		})
+	}
+}
+
+func TestCollectTypes(t *testing.T) {
+	t.Run("simple struct", func(t *testing.T) {
+		// Collect Types
+		argsType := reflect.TypeOf(SimpleStruct{})
+		fnType := reflect.TypeOf(func() (SimpleStruct, error) { return SimpleStruct{}, nil })
+		ts := CollectTypes(argsType, fnType)
+
+		// Verify TypeSet
+		require.NotNil(t, ts)
+		assert.Len(t, ts.All(), 1)
+
+		// Verify SimpleStruct Definition
+		def, ok := ts.Get("SimpleStruct")
+		assert.True(t, ok)
+		assert.Contains(t, def, "interface SimpleStruct")
+		assert.Contains(t, def, "name: string")
+		assert.Contains(t, def, "count: number")
+	})
+
+	t.Run("nested struct", func(t *testing.T) {
+		// Collect Types
+		argsType := reflect.TypeOf(NestedStruct{})
+		fnType := reflect.TypeOf(func() (NestedStruct, error) { return NestedStruct{}, nil })
+		ts := CollectTypes(argsType, fnType)
+
+		// Verify TypeSet
+		require.NotNil(t, ts)
+		all := ts.All()
+		assert.Len(t, all, 2) // NestedStruct and SimpleStruct
+
+		// Verify NestedStruct Definition
+		def, ok := ts.Get("NestedStruct")
+		assert.True(t, ok)
+		assert.Contains(t, def, "interface NestedStruct")
+		assert.Contains(t, def, "id: number")
+		assert.Contains(t, def, "simple: SimpleStruct")
+		assert.Contains(t, def, "pointer?: SimpleStruct | null")
+
+		// Verify SimpleStruct is Also Included
+		_, ok = ts.Get("SimpleStruct")
+		assert.True(t, ok)
+	})
+
+	t.Run("optional fields", func(t *testing.T) {
+		// Collect Types
+		argsType := reflect.TypeOf(OptionalFields{})
+		fnType := reflect.TypeOf(func() (OptionalFields, error) { return OptionalFields{}, nil })
+		ts := CollectTypes(argsType, fnType)
+
+		// Verify TypeSet
+		require.NotNil(t, ts)
+
+		// Verify OptionalFields Definition
+		def, ok := ts.Get("OptionalFields")
+		assert.True(t, ok)
+		assert.Contains(t, def, "required: string")
+		assert.Contains(t, def, "optional?: string | null")
+		assert.Contains(t, def, "number?: number")
+	})
+
+	t.Run("complex types", func(t *testing.T) {
+		// Collect Types
+		argsType := reflect.TypeOf(ComplexTypes{})
+		fnType := reflect.TypeOf(func() (ComplexTypes, error) { return ComplexTypes{}, nil })
+		ts := CollectTypes(argsType, fnType)
+
+		// Verify TypeSet
+		require.NotNil(t, ts)
+
+		// Verify ComplexTypes Definition
+		def, ok := ts.Get("ComplexTypes")
+		assert.True(t, ok)
+		assert.Contains(t, def, "strings: string[]")
+		assert.Contains(t, def, "numbers: number[]")
+		assert.Contains(t, def, "mapping: Record<string, any>")
+		assert.Contains(t, def, "nested: SimpleStruct[]")
+	})
+
+	t.Run("no return type", func(t *testing.T) {
+		// Collect Types
+		argsType := reflect.TypeOf(SimpleStruct{})
+		fnType := reflect.TypeOf(func() error { return nil })
+		ts := CollectTypes(argsType, fnType)
+
+		// Verify TypeSet - Only SimpleStruct from args
+		require.NotNil(t, ts)
+		assert.Len(t, ts.All(), 1)
+	})
+}
+
+func TestTypeSet(t *testing.T) {
+	t.Run("add and get", func(t *testing.T) {
+		// Create TypeSet
+		ts := NewTypeSet()
+
+		// Add Type
+		err := ts.Add("User", "interface User { name: string }")
+		require.NoError(t, err)
+
+		// Verify Type
+		def, ok := ts.Get("User")
+		assert.True(t, ok)
+		assert.Equal(t, "interface User { name: string }", def)
+	})
+
+	t.Run("duplicate same definition", func(t *testing.T) {
+		// Create TypeSet
+		ts := NewTypeSet()
+
+		// Add Type
+		err := ts.Add("User", "interface User { name: string }")
+		require.NoError(t, err)
+
+		// Add Same Type with Same Definition - Should Not Error
+		err = ts.Add("User", "interface User { name: string }")
+		require.NoError(t, err)
+
+		// Verify Count
+		assert.Len(t, ts.All(), 1)
+	})
+
+	t.Run("conflicting definitions", func(t *testing.T) {
+		// Create TypeSet
+		ts := NewTypeSet()
+
+		// Add Type
+		err := ts.Add("User", "interface User { name: string }")
+		require.NoError(t, err)
+
+		// Add Same Type with Different Definition - Should Error
+		err = ts.Add("User", "interface User { id: number }")
+		require.Error(t, err)
+		assert.Contains(t, err.Error(), "type conflict")
+		assert.Contains(t, err.Error(), "User")
+	})
+
+	t.Run("merge type sets", func(t *testing.T) {
+		// Create First TypeSet
+		ts1 := NewTypeSet()
+		err := ts1.Add("User", "interface User { name: string }")
+		require.NoError(t, err)
+
+		// Create Second TypeSet
+		ts2 := NewTypeSet()
+		err = ts2.Add("Post", "interface Post { title: string }")
+		require.NoError(t, err)
+
+		// Merge TypeSets
+		err = ts1.Merge(ts2)
+		require.NoError(t, err)
+
+		// Verify Merged Types
+		assert.Len(t, ts1.All(), 2)
+		_, ok := ts1.Get("User")
+		assert.True(t, ok)
+		_, ok = ts1.Get("Post")
+		assert.True(t, ok)
+	})
+
+	t.Run("merge with conflict", func(t *testing.T) {
+		// Create First TypeSet
+		ts1 := NewTypeSet()
+		err := ts1.Add("User", "interface User { name: string }")
+		require.NoError(t, err)
+
+		// Create Second TypeSet with Conflicting Type
+		ts2 := NewTypeSet()
+		err = ts2.Add("User", "interface User { id: number }")
+		require.NoError(t, err)
+
+		// Merge Should Fail Due to Conflict
+		err = ts1.Merge(ts2)
+		require.Error(t, err)
+		assert.Contains(t, err.Error(), "type conflict")
+	})
+}
+
+func TestExtractName(t *testing.T) {
+	tests := []struct {
+		definition string
+		expected   string
+	}{
+		{"interface User { name: string }", "User"},
+		{"interface MyType { }", "MyType"},
+		{"type MyAlias = string", "MyAlias"},
+		{"type ComplexType = { a: number }", "ComplexType"},
+		{"invalid syntax here", ""},
+		{"", ""},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.definition, func(t *testing.T) {
+			// Extract Name
+			result := ExtractName(tt.definition)
+
+			// Verify Result
+			assert.Equal(t, tt.expected, result)
+		})
+	}
+}
+
+func TestGenerateFunctionDecl(t *testing.T) {
+	t.Run("simple function", func(t *testing.T) {
+		// Generate Declaration
+		argsType := reflect.TypeOf(SimpleStruct{})
+		fnType := reflect.TypeOf(func() (SimpleStruct, error) { return SimpleStruct{}, nil })
+		decl := GenerateFunctionDecl("myFunc", argsType, fnType, false)
+
+		// Verify Declaration
+		assert.Equal(t, "declare function myFunc(name: string, count: number): SimpleStruct;", decl)
+	})
+
+	t.Run("async function", func(t *testing.T) {
+		// Generate Declaration
+		argsType := reflect.TypeOf(SimpleStruct{})
+		fnType := reflect.TypeOf(func() (SimpleStruct, error) { return SimpleStruct{}, nil })
+		decl := GenerateFunctionDecl("myAsyncFunc", argsType, fnType, true)
+
+		// Verify Declaration
+		assert.Equal(t, "declare function myAsyncFunc(name: string, count: number): Promise<SimpleStruct>;", decl)
+	})
+
+	t.Run("function with optional fields", func(t *testing.T) {
+		// Generate Declaration
+		argsType := reflect.TypeOf(OptionalFields{})
+		fnType := reflect.TypeOf(func() (OptionalFields, error) { return OptionalFields{}, nil })
+		decl := GenerateFunctionDecl("optionalFunc", argsType, fnType, false)
+
+		// Verify Declaration
+		assert.Contains(t, decl, "required: string")
+		assert.Contains(t, decl, "optional?: string | null")
+		assert.Contains(t, decl, "number?: number")
+	})
+
+	t.Run("function with no return", func(t *testing.T) {
+		// Generate Declaration
+		argsType := reflect.TypeOf(SimpleStruct{})
+		fnType := reflect.TypeOf(func() error { return nil })
+		decl := GenerateFunctionDecl("noReturn", argsType, fnType, false)
+
+		// Verify Declaration
+		assert.Equal(t, "declare function noReturn(name: string, count: number): any;", decl)
+	})
+
+	t.Run("non-struct args returns empty", func(t *testing.T) {
+		// Generate Declaration
+		argsType := reflect.TypeOf("")
+		fnType := reflect.TypeOf(func() error { return nil })
+		decl := GenerateFunctionDecl("invalid", argsType, fnType, false)
+
+		// Verify Declaration
+		assert.Equal(t, "", decl)
+	})
+}
diff --git a/internal/tsconvert/types.go b/internal/tsconvert/types.go
new file mode 100644
index 0000000..71efb21
--- /dev/null
+++ b/internal/tsconvert/types.go
@@ -0,0 +1,92 @@
+// Package tsconvert provides utilities for converting Go types to TypeScript definitions.
+package tsconvert
+
+import (
+	"fmt"
+	"regexp"
+)
+
+// TypeDecl represents a TypeScript type declaration.
+type TypeDecl struct {
+	Name       string // e.g., "UserConfig"
+	Definition string // e.g., "interface UserConfig { name: string }"
+}
+
+// TypeSet manages a collection of type declarations with deduplication support.
+type TypeSet struct {
+	types map[string]string // name -> definition
+}
+
+// NewTypeSet creates a new empty TypeSet.
+func NewTypeSet() *TypeSet {
+	return &TypeSet{
+		types: make(map[string]string),
+	}
+}
+
+// Add adds a type declaration to the set.
+// Returns an error if a type with the same name but different definition already exists.
+func (ts *TypeSet) Add(name, definition string) error {
+	if existing, ok := ts.types[name]; ok {
+		if existing != definition {
+			return fmt.Errorf("type conflict: %s has conflicting definitions", name)
+		}
+		// Same name and definition, no conflict
+		return nil
+	}
+	ts.types[name] = definition
+	return nil
+}
+
+// Get retrieves a type definition by name.
+func (ts *TypeSet) Get(name string) (string, bool) {
+	def, ok := ts.types[name]
+	return def, ok
+}
+
+// All returns all type declarations as a map.
+func (ts *TypeSet) All() map[string]string {
+	result := make(map[string]string, len(ts.types))
+	for k, v := range ts.types {
+		result[k] = v
+	}
+	return result
+}
+
+// Names returns all type names in the set.
+func (ts *TypeSet) Names() []string {
+	names := make([]string, 0, len(ts.types))
+	for name := range ts.types {
+		names = append(names, name)
+	}
+	return names
+}
+
+// Merge merges another TypeSet into this one.
+// Returns an error if there are conflicting definitions.
+func (ts *TypeSet) Merge(other *TypeSet) error {
+	for name, def := range other.types {
+		if err := ts.Add(name, def); err != nil {
+			return err
+		}
+	}
+	return nil
+}
+
+// ExtractName extracts the type name from a TypeScript declaration.
+// Supports "interface Name {...}" and "type Name = ..." patterns.
+func ExtractName(definition string) string {
+	// Try interface pattern: "interface Name { ... }"
+	interfaceRe := regexp.MustCompile(`^interface\s+(\w+)`)
+	if matches := interfaceRe.FindStringSubmatch(definition); len(matches) > 1 {
+		return matches[1]
+	}
+
+	// Try type alias pattern: "type Name = ..."
+	typeRe := regexp.MustCompile(`^type\s+(\w+)`)
+	if matches := typeRe.FindStringSubmatch(definition); len(matches) > 1 {
+		return matches[1]
+	}
+
+	return ""
+}