diff --git a/.gitignore b/.gitignore
index 358127e..f6af235 100644
--- a/.gitignore
+++ b/.gitignore
@@ -8,3 +8,4 @@ yarn-debug.log*
 yarn-error.log*
 pnpm-debug.log*
 reports/
+
diff --git a/package.json b/package.json
index 0e40b57..e721810 100644
--- a/package.json
+++ b/package.json
@@ -8,18 +8,18 @@
     "packages/*"
   ],
   "scripts": {
-    "build": "pnpm --filter @effect-template/app build",
-    "check": "pnpm --filter @effect-template/app check",
+    "build": "pnpm --filter @prover-coder-ai/openapi-effect build",
+    "check": "pnpm --filter @prover-coder-ai/openapi-effect check",
     "changeset": "changeset",
     "changeset-publish": "node -e \"if (!process.env.NPM_TOKEN) { console.log('Skipping publish: NPM_TOKEN is not set'); process.exit(0); }\" && changeset publish",
     "changeset-version": "changeset version",
-    "dev": "pnpm --filter @effect-template/app dev",
-    "lint": "pnpm --filter @effect-template/app lint",
-    "lint:tests": "pnpm --filter @effect-template/app lint:tests",
-    "lint:effect": "pnpm --filter @effect-template/app lint:effect",
-    "test": "pnpm --filter @effect-template/app test",
-    "typecheck": "pnpm --filter @effect-template/app typecheck",
-    "start": "pnpm --filter @effect-template/app start"
+    "dev": "pnpm --filter @prover-coder-ai/openapi-effect dev",
+    "lint": "pnpm --filter @prover-coder-ai/openapi-effect lint",
+    "lint:tests": "pnpm --filter @prover-coder-ai/openapi-effect lint:tests",
+    "lint:effect": "pnpm --filter @prover-coder-ai/openapi-effect lint:effect",
+    "test": "pnpm --filter @prover-coder-ai/openapi-effect test",
+    "typecheck": "pnpm --filter @prover-coder-ai/openapi-effect typecheck",
+    "start": "pnpm --filter @prover-coder-ai/openapi-effect start"
   },
   "devDependencies": {
     "@changesets/changelog-github": "^0.5.2",
diff --git a/packages/app/package.json b/packages/app/package.json
index 8c542ce..392febe 100644
--- a/packages/app/package.json
+++ b/packages/app/package.json
@@ -1,14 +1,23 @@
 {
-  "name": "@effect-template/app",
+  "name": "@prover-coder-ai/openapi-effect",
   "version": "1.0.16",
-  "description": "Minimal Vite-powered TypeScript console starter using Effect",
-  "main": "dist/main.js",
-  "directories": {
-    "doc": "doc"
+  "description": "Drop-in replacement for openapi-fetch with an opt-in Effect API",
+  "type": "module",
+  "main": "dist/index.js",
+  "exports": {
+    ".": {
+      "types": "./src/index.ts",
+      "import": "./dist/index.js",
+      "default": "./dist/index.js"
+    }
   },
+  "files": [
+    "dist",
+    "src"
+  ],
   "scripts": {
-    "build": "vite build --ssr src/app/main.ts",
-    "dev": "vite build --watch --ssr src/app/main.ts",
+    "build": "vite build",
+    "dev": "vite build --watch",
     "lint": "npx @ton-ai-core/vibecode-linter src/",
     "lint:tests": "npx @ton-ai-core/vibecode-linter tests/",
     "lint:effect": "npx eslint --config eslint.effect-ts-check.config.mjs .",
@@ -32,7 +41,6 @@
   ],
   "author": "",
   "license": "ISC",
-  "type": "module",
   "bugs": {
     "url": "https://github.com/ProverCoderAI/effect-template/issues"
   },
@@ -52,6 +60,7 @@
     "@effect/typeclass": "^0.38.0",
     "@effect/workflow": "^0.16.0",
     "effect": "^3.19.16",
+    "openapi-fetch": "^0.15.2",
     "openapi-typescript-helpers": "^0.1.0",
     "ts-morph": "^27.0.2"
   },
diff --git a/packages/app/src/index.ts b/packages/app/src/index.ts
index b07cca5..cec08a4 100644
--- a/packages/app/src/index.ts
+++ b/packages/app/src/index.ts
@@ -1,57 +1,39 @@
-// CHANGE: Main entry point for openapi-effect package with Effect-native error handling
-// WHY: Enable default import of createClient function with proper error channel design
-// QUOTE(ТЗ): "import createClient from \"openapi-effect\""
-// REF: PR#3 comment from skulidropek about Effect representation
+// CHANGE: Make openapi-effect a drop-in replacement for openapi-fetch (Promise API), with an opt-in Effect API.
+// WHY: Consumer projects must be able to swap openapi-fetch -> openapi-effect with near-zero code changes.
+// QUOTE(ТЗ): "openapi-effect должен почти 1 в 1 заменяться с openapi-fetch" / "Просто добавлять effect поведение"
 // SOURCE: n/a
 // PURITY: SHELL (re-exports)
 // COMPLEXITY: O(1)
 
-// High-level API (recommended for most users)
-export { createClient as default } from "./shell/api-client/create-client.js"
+// Promise-based client (openapi-fetch compatible)
+export { default } from "openapi-fetch"
+export { default as createClient } from "openapi-fetch"
+export * from "openapi-fetch"
+
+// Effect-based client (opt-in)
+export * as FetchHttpClient from "@effect/platform/FetchHttpClient"
+
+// Strict Effect client (advanced)
+export type * from "./core/api-client/index.js"
+export { assertNever } from "./core/api-client/index.js"
+
 export type {
-  ClientEffect,
-  ClientOptions,
   DispatchersFor,
   StrictApiClient,
   StrictApiClientWithDispatchers
 } from "./shell/api-client/create-client.js"
-export { createClientEffect, registerDefaultDispatchers } from "./shell/api-client/create-client.js"
-
-// Core types (for advanced type manipulation)
-// Effect Channel Design:
-// - ApiSuccess<Responses>: 2xx responses → success channel
-// - ApiFailure<Responses>: HttpError (4xx, 5xx) + BoundaryError → error channel
-export type {
-  ApiFailure,
-  ApiSuccess,
-  BodyFor,
-  BoundaryError,
-  ContentTypesFor,
-  DecodeError,
-  HttpError,
-  HttpErrorResponseVariant,
-  HttpErrorVariants,
-  OperationFor,
-  ParseError,
-  PathsForMethod,
-  ResponsesFor,
-  ResponseVariant,
-  StatusCodes,
-  SuccessVariants,
-  TransportError,
-  UnexpectedContentType,
-  UnexpectedStatus
-} from "./core/api-client/index.js"
 
-// Shell utilities (for custom implementations)
 export type { Decoder, Dispatcher, RawResponse, StrictClient, StrictRequestInit } from "./shell/api-client/index.js"
 
 export {
+  createClient as createClientStrict,
+  createClientEffect,
   createDispatcher,
   createStrictClient,
   createUniversalDispatcher,
   executeRequest,
   parseJSON,
+  registerDefaultDispatchers,
   unexpectedContentType,
   unexpectedStatus
 } from "./shell/api-client/index.js"
diff --git a/packages/app/src/shell/api-client/create-client-types.ts b/packages/app/src/shell/api-client/create-client-types.ts
index 762d90d..34970e7 100644
--- a/packages/app/src/shell/api-client/create-client-types.ts
+++ b/packages/app/src/shell/api-client/create-client-types.ts
@@ -10,6 +10,7 @@
 
 import type * as HttpClient from "@effect/platform/HttpClient"
 import type { Effect } from "effect"
+import type { ClientOptions as OpenapiFetchClientOptions } from "openapi-fetch"
 import type { HttpMethod } from "openapi-typescript-helpers"
 
 import type {
@@ -28,12 +29,7 @@ import type { Dispatcher } from "../../core/axioms.js"
  *
  * @pure - immutable configuration
  */
-export type ClientOptions = {
-  readonly baseUrl: string
-  readonly credentials?: RequestCredentials
-  readonly headers?: HeadersInit
-  readonly fetch?: typeof globalThis.fetch
-}
+export type ClientOptions = OpenapiFetchClientOptions
 
 // CHANGE: Add dispatcher map type for auto-dispatching clients
 // WHY: Enable creating clients that infer dispatcher from path+method without per-call parameter
diff --git a/packages/app/src/shell/api-client/create-client.ts b/packages/app/src/shell/api-client/create-client.ts
index 8344dc5..53571ac 100644
--- a/packages/app/src/shell/api-client/create-client.ts
+++ b/packages/app/src/shell/api-client/create-client.ts
@@ -83,6 +83,53 @@ const resolveDefaultDispatchers = <Paths extends object>(): DispatchersFor<Paths
   return asDispatchersFor<DispatchersFor<Paths>>(defaultDispatchers)
 }
 
+const applyPathParams = (path: string, params?: Record<string, ParamValue>): string => {
+  if (params === undefined) {
+    return path
+  }
+
+  let url = path
+  for (const [key, value] of Object.entries(params)) {
+    url = url.replace("{" + key + "}", encodeURIComponent(String(value)))
+  }
+  return url
+}
+
+const buildQueryString = (query?: Record<string, QueryValue>): string => {
+  if (query === undefined) {
+    return ""
+  }
+
+  const searchParams = new URLSearchParams()
+  for (const [key, value] of Object.entries(query)) {
+    if (Array.isArray(value)) {
+      for (const item of value) {
+        searchParams.append(key, String(item))
+      }
+      continue
+    }
+    searchParams.set(key, String(value))
+  }
+  return searchParams.toString()
+}
+
+const appendQueryString = (url: string, queryString: string): string => {
+  if (queryString.length === 0) {
+    return url
+  }
+  return url.includes("?") ? url + "&" + queryString : url + "?" + queryString
+}
+
+const withBaseUrl = (baseUrl: string | undefined, url: string): string => {
+  // If baseUrl is not provided, keep a relative URL (browser-friendly)
+  if (baseUrl === undefined || baseUrl === "") {
+    return url
+  }
+
+  // Construct full URL
+  return new URL(url, baseUrl).toString()
+}
+
 /**
  * Build URL with path parameters and query string
  *
@@ -96,36 +143,15 @@ const resolveDefaultDispatchers = <Paths extends object>(): DispatchersFor<Paths
  * @complexity O(n + m) where n = |params|, m = |query|
  */
 const buildUrl = (
-  baseUrl: string,
+  baseUrl: string | undefined,
   path: string,
   params?: Record<string, ParamValue>,
   query?: Record<string, QueryValue>
 ): string => {
-  // Replace path parameters
-  let url = path
-  if (params) {
-    for (const [key, value] of Object.entries(params)) {
-      url = url.replace(`{${key}}`, encodeURIComponent(String(value)))
-    }
-  }
-
-  // Construct full URL
-  const fullUrl = new URL(url, baseUrl)
-
-  // Add query parameters
-  if (query) {
-    for (const [key, value] of Object.entries(query)) {
-      if (Array.isArray(value)) {
-        for (const item of value) {
-          fullUrl.searchParams.append(key, String(item))
-        }
-      } else {
-        fullUrl.searchParams.set(key, String(value))
-      }
-    }
-  }
-
-  return fullUrl.toString()
+  const urlWithParams = applyPathParams(path, params)
+  const queryString = buildQueryString(query)
+  const urlWithQuery = appendQueryString(urlWithParams, queryString)
+  return withBaseUrl(baseUrl, urlWithQuery)
 }
 
 /**
@@ -172,22 +198,63 @@ const needsJsonContentType = (body: BodyInit | object | undefined): boolean =>
   && !(body instanceof Blob)
   && !(body instanceof FormData)
 
+const toHeadersFromRecord = (
+  headersInit: Record<
+    string,
+    | string
+    | number
+    | boolean
+    | ReadonlyArray<string | number | boolean>
+    | null
+    | undefined
+  >
+): Headers => {
+  const headers = new Headers()
+
+  for (const [key, value] of Object.entries(headersInit)) {
+    if (value === null || value === undefined) {
+      continue
+    }
+    if (Array.isArray(value)) {
+      headers.set(key, value.map(String).join(","))
+      continue
+    }
+    headers.set(key, String(value))
+  }
+
+  return headers
+}
+
 /**
  * Merge headers from client options and request options
  *
  * @pure true
  * @complexity O(n) where n = number of headers
  */
+const toHeaders = (headersInit: ClientOptions["headers"] | undefined): Headers => {
+  if (headersInit === undefined) {
+    return new Headers()
+  }
+
+  if (headersInit instanceof Headers) {
+    return new Headers(headersInit)
+  }
+
+  if (Array.isArray(headersInit)) {
+    return new Headers(headersInit)
+  }
+
+  return toHeadersFromRecord(headersInit)
+}
+
 const mergeHeaders = (
-  clientHeaders: HeadersInit | undefined,
-  requestHeaders: HeadersInit | undefined
+  clientHeaders: ClientOptions["headers"] | undefined,
+  requestHeaders: ClientOptions["headers"] | undefined
 ): Headers => {
-  const headers = new Headers(clientHeaders)
-  if (requestHeaders) {
-    const optHeaders = new Headers(requestHeaders)
-    for (const [key, value] of optHeaders.entries()) {
-      headers.set(key, value)
-    }
+  const headers = toHeaders(clientHeaders)
+  const optHeaders = toHeaders(requestHeaders)
+  for (const [key, value] of optHeaders.entries()) {
+    headers.set(key, value)
   }
   return headers
 }
@@ -201,7 +268,7 @@ type MethodHandlerOptions = {
   params?: Record<string, ParamValue> | undefined
   query?: Record<string, QueryValue> | undefined
   body?: BodyInit | object | undefined
-  headers?: HeadersInit | undefined
+  headers?: ClientOptions["headers"] | undefined
   signal?: AbortSignal | undefined
 }
 
diff --git a/packages/app/vite.config.ts b/packages/app/vite.config.ts
index cfd0bb5..249b519 100644
--- a/packages/app/vite.config.ts
+++ b/packages/app/vite.config.ts
@@ -1,3 +1,4 @@
+import fs from "node:fs"
 import path from "node:path"
 import { fileURLToPath } from "node:url"
 import { defineConfig } from "vite"
@@ -6,6 +7,21 @@ import tsconfigPaths from "vite-tsconfig-paths"
 const __filename = fileURLToPath(import.meta.url)
 const __dirname = path.dirname(__filename)
 
+type Pkg = {
+  dependencies?: Record<string, string> | undefined
+  peerDependencies?: Record<string, string> | undefined
+}
+
+// CHANGE: Build both the library entry (src/index.ts) and the CLI entry (src/app/main.ts).
+// WHY: Consumers need a JS entrypoint in dist for `import "openapi-effect"`, while we keep the template CLI working.
+// SOURCE: n/a
+const pkgPath = path.resolve(__dirname, "package.json")
+const pkg = JSON.parse(fs.readFileSync(pkgPath, "utf8")) as Pkg
+const dependencies = [...Object.keys(pkg.dependencies ?? {}), ...Object.keys(pkg.peerDependencies ?? {})]
+
+const isExternal = (id: string): boolean =>
+  dependencies.some((dep) => id === dep || id.startsWith(`${dep}/`))
+
 export default defineConfig({
   plugins: [tsconfigPaths()],
   publicDir: false,
@@ -18,15 +34,17 @@ export default defineConfig({
     target: "node20",
     outDir: "dist",
     sourcemap: true,
-    ssr: "src/app/main.ts",
     rollupOptions: {
+      preserveEntrySignatures: "exports-only",
+      input: {
+        index: path.resolve(__dirname, "src/index.ts"),
+        main: path.resolve(__dirname, "src/app/main.ts")
+      },
+      external: isExternal,
       output: {
         format: "es",
-        entryFileNames: "main.js"
+        entryFileNames: "[name].js"
       }
     }
-  },
-  ssr: {
-    target: "node"
   }
 })
diff --git a/pnpm-lock.yaml b/pnpm-lock.yaml
index 238f79d..c1ecbb7 100644
--- a/pnpm-lock.yaml
+++ b/pnpm-lock.yaml
@@ -61,6 +61,9 @@ importers:
       effect:
         specifier: ^3.19.16
         version: 3.19.16
+      openapi-fetch:
+        specifier: ^0.15.2
+        version: 0.15.2
       openapi-typescript-helpers:
         specifier: ^0.1.0
         version: 0.1.0
@@ -2729,6 +2732,12 @@ packages:
     resolution: {integrity: sha512-kbpaSSGJTWdAY5KPVeMOKXSrPtr8C8C7wodJbcsd51jRnmD+GZu8Y0VoU6Dm5Z4vWr0Ig/1NKuWRKf7j5aaYSg==}
     engines: {node: '>=6'}
 
+  openapi-fetch@0.15.2:
+    resolution: {integrity: sha512-rdYTzUmSsJevmNqg7fwUVGuKc2Gfb9h6ph74EVPkPfIGJaZTfqdIbJahtbJ3qg1LKinln30hqZniLnKpH0RJBg==}
+
+  openapi-typescript-helpers@0.0.15:
+    resolution: {integrity: sha512-opyTPaunsklCBpTK8JGef6mfPhLSnyy5a0IN9vKtx3+4aExf+KxEqYwIy3hqkedXIB97u357uLMJsOnm3GVjsw==}
+
   openapi-typescript-helpers@0.1.0:
     resolution: {integrity: sha512-OKTGPthhivLw/fHz6c3OPtg72vi86qaMlqbJuVJ23qOvQ+53uw1n7HdmkJFibloF7QEjDrDkzJiOJuockM/ljw==}
 
@@ -6284,6 +6293,12 @@ snapshots:
     dependencies:
       mimic-fn: 2.1.0
 
+  openapi-fetch@0.15.2:
+    dependencies:
+      openapi-typescript-helpers: 0.0.15
+
+  openapi-typescript-helpers@0.0.15: {}
+
   openapi-typescript-helpers@0.1.0: {}
 
   optionator@0.9.4: