diff --git a/.changeset/fix-http-client-request-content-length.md b/.changeset/fix-http-client-request-content-length.md
new file mode 100644
index 00000000000..12f66418c14
--- /dev/null
+++ b/.changeset/fix-http-client-request-content-length.md
@@ -0,0 +1,5 @@
+---
+"effect": patch
+---
+
+Remove stale `content-length` headers when replacing an HTTP client request body with one of unknown length.
diff --git a/packages/effect/src/unstable/http/HttpClientRequest.ts b/packages/effect/src/unstable/http/HttpClientRequest.ts
index 2363374e1ef..9ac4077fe67 100644
--- a/packages/effect/src/unstable/http/HttpClientRequest.ts
+++ b/packages/effect/src/unstable/http/HttpClientRequest.ts
@@ -658,6 +658,8 @@ export const setBody: {
     }
     if (body.contentLength !== undefined) {
       headers = Headers.set(headers, "content-length", body.contentLength.toString())
+    } else {
+      headers = Headers.remove(headers, "content-length")
     }
   }
   return makeWith(
diff --git a/packages/effect/test/unstable/http/HttpClientRequest.test.ts b/packages/effect/test/unstable/http/HttpClientRequest.test.ts
index 85dc93564a6..ba09c404a9f 100644
--- a/packages/effect/test/unstable/http/HttpClientRequest.test.ts
+++ b/packages/effect/test/unstable/http/HttpClientRequest.test.ts
@@ -2,7 +2,7 @@ import { describe, it } from "@effect/vitest"
 import { assertNone, assertSome, assertTrue, deepStrictEqual, strictEqual } from "@effect/vitest/utils"
 import { Effect, Stream } from "effect"
 import * as Option from "effect/Option"
-import { Headers, HttpClientRequest } from "effect/unstable/http"
+import { Headers, HttpBody, HttpClientRequest } from "effect/unstable/http"
 
 describe("HttpClientRequest", () => {
   describe("appendUrl", () => {
@@ -82,6 +82,16 @@ describe("HttpClientRequest", () => {
     })
   })
 
+  describe("setBody", () => {
+    it("removes stale content length when the replacement body has no known length", () => {
+      const request = HttpClientRequest.bodyText(HttpClientRequest.post("https://example.com"), "abc").pipe(
+        HttpClientRequest.setBody(HttpBody.stream(Stream.empty))
+      )
+
+      strictEqual(request.headers["content-length"], undefined)
+    })
+  })
+
   describe("removeHeader", () => {
     it("removes an existing header", () => {
       const request = HttpClientRequest.get("/").pipe(