diff --git a/.changeset/busy-clouds-walk.md b/.changeset/busy-clouds-walk.md
new file mode 100644
index 00000000..3a306b94
--- /dev/null
+++ b/.changeset/busy-clouds-walk.md
@@ -0,0 +1,5 @@
+---
+'@hono/prometheus': patch
+---
+
+Add explicit return types
diff --git a/packages/prometheus/src/index.ts b/packages/prometheus/src/index.ts
index 578431b9..c6941346 100644
--- a/packages/prometheus/src/index.ts
+++ b/packages/prometheus/src/index.ts
@@ -1,5 +1,6 @@
-import type { Context } from 'hono'
+import type { Context, MiddlewareHandler, TypedResponse } from 'hono'
 import { createMiddleware } from 'hono/factory'
+import type { ContentfulStatusCode } from 'hono/utils/http-status'
 import type { DefaultMetricsCollectorConfiguration, RegistryContentType } from 'prom-client'
 import { Registry, collectDefaultMetrics as promCollectDefaultMetrics } from 'prom-client'
 import { createStandardMetrics } from './standardMetrics'
@@ -22,7 +23,14 @@ const evaluateCustomLabels = (customLabels: MetricOptions['customLabels'], conte
   return labels
 }
 
-export const prometheus = (options?: PrometheusOptions) => {
+export const prometheus = (
+  options?: PrometheusOptions
+): {
+  printMetrics: (
+    c: Context
+  ) => Promise<Response & TypedResponse<string, ContentfulStatusCode, 'text'>>
+  registerMetrics: MiddlewareHandler
+} => {
   const {
     registry = new Registry(),
     collectDefaultMetrics = false,
diff --git a/packages/prometheus/src/standardMetrics.ts b/packages/prometheus/src/standardMetrics.ts
index 97b392c7..8c035118 100644
--- a/packages/prometheus/src/standardMetrics.ts
+++ b/packages/prometheus/src/standardMetrics.ts
@@ -2,15 +2,24 @@ import type { Context } from 'hono'
 import { Counter, Histogram } from 'prom-client'
 import type { CounterConfiguration, HistogramConfiguration, Metric, Registry } from 'prom-client'
 
-export type MetricOptions = {
+interface CounterOptions<T extends string> extends CounterConfiguration<T> {
+  type: 'counter'
   disabled?: boolean
   customLabels?: Record<string, (c: Context) => string>
-} & (
-  | ({ type: 'counter' } & CounterConfiguration<string>)
-  | ({ type: 'histogram' } & HistogramConfiguration<string>)
-)
+}
 
-const standardMetrics = {
+interface HistogramOptions<T extends string> extends HistogramConfiguration<T> {
+  type: 'histogram'
+  disabled?: boolean
+  customLabels?: Record<string, (c: Context) => string>
+}
+
+export type MetricOptions<T extends string = string> = CounterOptions<T> | HistogramOptions<T>
+
+const standardMetrics: {
+  requestDuration: HistogramOptions<string>
+  requestsTotal: CounterOptions<string>
+} = {
   requestDuration: {
     type: 'histogram',
     name: 'http_request_duration_seconds',
@@ -26,7 +35,7 @@ const standardMetrics = {
     help: 'Total number of HTTP requests',
     labelNames: ['method', 'status', 'ok', 'route'],
   },
-} satisfies Record<string, MetricOptions>
+}
 
 export type MetricName = keyof typeof standardMetrics
 
diff --git a/packages/prometheus/tsconfig.build.json b/packages/prometheus/tsconfig.build.json
index ccc2f65a..a82a1266 100644
--- a/packages/prometheus/tsconfig.build.json
+++ b/packages/prometheus/tsconfig.build.json
@@ -4,7 +4,8 @@
     "rootDir": "src",
     "outDir": "dist",
     "tsBuildInfoFile": "dist/tsconfig.build.tsbuildinfo",
-    "emitDeclarationOnly": false
+    "emitDeclarationOnly": false,
+    "isolatedDeclarations": true
   },
   "include": ["src/**/*.ts"],
   "exclude": ["**/*.test.ts"],