diff --git a/lib/dispatcher/env-http-proxy-agent.js b/lib/dispatcher/env-http-proxy-agent.js
index f88437f1936..df2e9fec9f4 100644
--- a/lib/dispatcher/env-http-proxy-agent.js
+++ b/lib/dispatcher/env-http-proxy-agent.js
@@ -10,6 +10,22 @@ const DEFAULT_PORTS = {
   'https:': 443
 }
 
+/**
+ * Normalizes a proxy URL by prepending a scheme if one is missing.
+ * This matches the behavior of curl and Go's httpproxy package, which
+ * assume http:// for scheme-less proxy values.
+ *
+ * @param {string} proxyUrl - The proxy URL to normalize
+ * @param {string} defaultScheme - The scheme to prepend if missing ('http' or 'https')
+ * @returns {string} The normalized proxy URL
+ */
+function normalizeProxyUrl (proxyUrl, defaultScheme) {
+  if (!proxyUrl) return proxyUrl
+  // If the value already contains a scheme (e.g. http://, https://, socks5://), return as-is
+  if (/^[a-z][a-z0-9+\-.]*:\/\//i.test(proxyUrl)) return proxyUrl
+  return `${defaultScheme}://${proxyUrl}`
+}
+
 class EnvHttpProxyAgent extends DispatcherBase {
   #noProxyValue = null
   #noProxyEntries = null
@@ -23,14 +39,20 @@ class EnvHttpProxyAgent extends DispatcherBase {
 
     this[kNoProxyAgent] = new Agent(agentOpts)
 
-    const HTTP_PROXY = httpProxy ?? process.env.http_proxy ?? process.env.HTTP_PROXY
+    const HTTP_PROXY = normalizeProxyUrl(
+      httpProxy ?? process.env.http_proxy ?? process.env.HTTP_PROXY,
+      'http'
+    )
     if (HTTP_PROXY) {
       this[kHttpProxyAgent] = new ProxyAgent({ ...agentOpts, uri: HTTP_PROXY })
     } else {
       this[kHttpProxyAgent] = this[kNoProxyAgent]
     }
 
-    const HTTPS_PROXY = httpsProxy ?? process.env.https_proxy ?? process.env.HTTPS_PROXY
+    const HTTPS_PROXY = normalizeProxyUrl(
+      httpsProxy ?? process.env.https_proxy ?? process.env.HTTPS_PROXY,
+      'https'
+    )
     if (HTTPS_PROXY) {
       this[kHttpsProxyAgent] = new ProxyAgent({ ...agentOpts, uri: HTTPS_PROXY })
     } else {
diff --git a/test/env-http-proxy-agent.js b/test/env-http-proxy-agent.js
index 97969cfaf20..bf061704906 100644
--- a/test/env-http-proxy-agent.js
+++ b/test/env-http-proxy-agent.js
@@ -124,6 +124,61 @@ test('prefers lowercase over uppercase env vars even when empty', async (t) => {
   return dispatcher.close()
 })
 
+test('handles scheme-less HTTP_PROXY by assuming http://', async (t) => {
+  t = tspl(t, { plan: 2 })
+  process.env.HTTP_PROXY = 'example.com:8080'
+  const dispatcher = new EnvHttpProxyAgent()
+  t.ok(dispatcher[kHttpProxyAgent] instanceof ProxyAgent)
+  t.equal(dispatcher[kHttpProxyAgent][kProxy].uri, 'http://example.com:8080/')
+  return dispatcher.close()
+})
+
+test('handles scheme-less HTTPS_PROXY by assuming https://', async (t) => {
+  t = tspl(t, { plan: 2 })
+  process.env.HTTPS_PROXY = 'example.com:8443'
+  const dispatcher = new EnvHttpProxyAgent()
+  t.ok(dispatcher[kHttpsProxyAgent] instanceof ProxyAgent)
+  t.equal(dispatcher[kHttpsProxyAgent][kProxy].uri, 'https://example.com:8443/')
+  return dispatcher.close()
+})
+
+test('handles scheme-less httpProxy option by assuming http://', async (t) => {
+  t = tspl(t, { plan: 2 })
+  const dispatcher = new EnvHttpProxyAgent({ httpProxy: 'myproxy:9000' })
+  t.ok(dispatcher[kHttpProxyAgent] instanceof ProxyAgent)
+  t.equal(dispatcher[kHttpProxyAgent][kProxy].uri, 'http://myproxy:9000/')
+  return dispatcher.close()
+})
+
+test('handles scheme-less httpsProxy option by assuming https://', async (t) => {
+  t = tspl(t, { plan: 2 })
+  const dispatcher = new EnvHttpProxyAgent({ httpsProxy: 'myproxy:9443' })
+  t.ok(dispatcher[kHttpsProxyAgent] instanceof ProxyAgent)
+  t.equal(dispatcher[kHttpsProxyAgent][kProxy].uri, 'https://myproxy:9443/')
+  return dispatcher.close()
+})
+
+test('does not modify proxy URLs that already have a scheme', async (t) => {
+  t = tspl(t, { plan: 4 })
+  process.env.HTTP_PROXY = 'http://example.com:8080'
+  process.env.HTTPS_PROXY = 'http://example.com:8443'
+  const dispatcher = new EnvHttpProxyAgent()
+  t.ok(dispatcher[kHttpProxyAgent] instanceof ProxyAgent)
+  t.equal(dispatcher[kHttpProxyAgent][kProxy].uri, 'http://example.com:8080/')
+  t.ok(dispatcher[kHttpsProxyAgent] instanceof ProxyAgent)
+  t.equal(dispatcher[kHttpsProxyAgent][kProxy].uri, 'http://example.com:8443/')
+  return dispatcher.close()
+})
+
+test('handles scheme-less hostname-only proxy values', async (t) => {
+  t = tspl(t, { plan: 2 })
+  process.env.HTTP_PROXY = 'myproxy'
+  const dispatcher = new EnvHttpProxyAgent()
+  t.ok(dispatcher[kHttpProxyAgent] instanceof ProxyAgent)
+  t.equal(dispatcher[kHttpProxyAgent][kProxy].uri, 'http://myproxy/')
+  return dispatcher.close()
+})
+
 test('creates a proxy agent only for https when only https_proxy is set', async (t) => {
   t = tspl(t, { plan: 5 })
   process.env.https_proxy = 'http://example.com:8443'