diff --git a/src/fetch-util.ts b/src/fetch-util.ts
index e39bd8a5..66a21f69 100644
--- a/src/fetch-util.ts
+++ b/src/fetch-util.ts
@@ -129,6 +129,13 @@ export async function fetchWithTimeout(
             ...opts,
             signal: controller.signal,
             dispatcher: opts.dispatcher ?? directDispatcher(effective),
+            // Forward-proxy correctness: never silently follow a redirect.
+            // undici's default (follow) downgrades POST→GET and drops the body
+            // on 301/302/303, so a redirecting upstream (CDN/WAF) turns a valid
+            // POST into a 405 at the redirect target. Pass the 3xx through to
+            // the client, which follows it with its own policy. Internal bili
+            // fetches that want to follow (registry, upstream test) opt in.
+            redirect: opts.redirect ?? "manual",
         };
         // `fetch` is undici's global; it accepts `dispatcher` at runtime. @types/node
         // types RequestInit.dispatcher as its internal `Dispatcher` interface,
diff --git a/src/registry.ts b/src/registry.ts
index ce819098..c9e7103d 100644
--- a/src/registry.ts
+++ b/src/registry.ts
@@ -109,6 +109,7 @@ async function fetchFresh(): Promise<RegistryShape | null> {
             const { response, clearTimer } = await fetchWithTimeout(REGISTRY_URL, {
                 ...opts,
                 headers: { Accept: "application/json" },
+                redirect: "follow",
             }, 15_000);
             const text = await response.text();
             clearTimer();
diff --git a/src/server.ts b/src/server.ts
index c594eef6..e93cabec 100644
--- a/src/server.ts
+++ b/src/server.ts
@@ -872,6 +872,7 @@ async function handle(
         try {
             const result = await fetchWithTimeout(targetUrl, {
                 method: "HEAD",
+                redirect: "follow",
                 ...(proxyUrl ? { dispatcher: proxyDispatcher(proxyUrl, 15_000) } : {}),
             }, 15_000);
             result.clearTimer();
diff --git a/tests/redirect-passthrough.test.ts b/tests/redirect-passthrough.test.ts
new file mode 100644
index 00000000..a5c99bf6
--- /dev/null
+++ b/tests/redirect-passthrough.test.ts
@@ -0,0 +1,188 @@
+import test from "node:test";
+import assert from "node:assert/strict";
+import http from "node:http";
+import { once } from "node:events";
+import { defaultConfig } from "acp-kernel";
+import { startServer } from "../src/server.ts";
+import { SessionStore, _setStoreForTest } from "../src/persist.ts";
+import type { ProxyOptions } from "../src/config.ts";
+import { _setForTest as setRegistryForTest } from "../src/registry.ts";
+
+// #661 follow-up: a forward proxy must not silently follow upstream redirects.
+// undici's default (redirect: "follow") downgrades a POST to a GET and drops
+// the body on 301/302/303, so a redirecting upstream (CDN/WAF) turns a valid
+// POST into a 405 at the redirect target. bili must pass the 3xx through to
+// the client, which follows it with its own policy. ACP compression stays on:
+// the kernel round-trip still rewrites the body before the (now non-followed)
+// forward.
+
+function listen(server: http.Server): Promise<void> {
+    if (server.listening) return Promise.resolve();
+    return once(server, "listening").then(() => undefined);
+}
+
+function close(server: http.Server): Promise<void> {
+    return new Promise((resolve, reject) => server.close((error) => (error ? reject(error) : resolve())));
+}
+
+const POST_BODY = JSON.stringify({
+    model: "claude-test",
+    max_tokens: 4096,
+    messages: [{ role: "user", content: [{ type: "text", text: "hi" }] }],
+});
+
+function makeOpts(upstream: string): ProxyOptions {
+    return {
+        port: 0,
+        host: "127.0.0.1",
+        upstream,
+        routes: {},
+        modelContextLimit: 400_000,
+        kernelConfig: defaultConfig(400_000),
+        compress: { injectTool: true, injectNudge: true },
+        promptCache: { routing: "auto" },
+        sessionHeader: "x-acp-session",
+        log: false,
+        debug: false,
+        passthrough: false,
+        autoUpdate: false,
+        mitm: { enabled: false, domains: [] },
+    };
+}
+
+interface Seen {
+    method: string | null;
+    body: string;
+}
+
+function makeRedirector(capture: Seen, targetUrl: string): http.Server {
+    return http.createServer((req, res) => {
+        const chunks: Buffer[] = [];
+        req.on("data", (c: Buffer) => chunks.push(c));
+        req.on("end", () => {
+            capture.method = req.method ?? null;
+            capture.body = Buffer.concat(chunks).toString("utf8");
+            res.writeHead(302, { location: targetUrl });
+            res.end();
+        });
+    });
+}
+
+function makeTarget(seen: { hit: boolean; method: string | null }): http.Server {
+    return http.createServer((req, res) => {
+        seen.hit = true;
+        seen.method = req.method ?? null;
+        if (req.method === "POST") {
+            res.writeHead(200, { "content-type": "application/json" });
+            res.end(JSON.stringify({ ok: true }));
+        } else {
+            res.writeHead(405, { "content-type": "application/json" });
+            res.end(JSON.stringify({ error: "method not allowed" }));
+        }
+    });
+}
+
+function requestThroughProxy(
+    proxyPort: number,
+    redirectorHost: string,
+    method: "POST" | "GET",
+    path: string,
+    body: string | null,
+): Promise<{ status: number; location: string | null; body: string }> {
+    return new Promise((resolve, reject) => {
+        const req = http.request(
+            {
+                host: "127.0.0.1",
+                port: proxyPort,
+                method,
+                path: `/bili/http://${redirectorHost}${path}`,
+                headers: {
+                    host: redirectorHost,
+                    "x-acp-session": "redirect-test",
+                    ...(body !== null ? { "content-type": "application/json", "content-length": String(Buffer.byteLength(body)) } : {}),
+                },
+            },
+            (res) => {
+                const chunks: Buffer[] = [];
+                res.on("data", (c) => chunks.push(c));
+                res.on("end", () => resolve({
+                    status: res.statusCode ?? 0,
+                    location: res.headers.location ?? null,
+                    body: Buffer.concat(chunks).toString("utf8"),
+                }));
+            },
+        );
+        req.on("error", reject);
+        if (body !== null) req.write(body);
+        req.end();
+    });
+}
+
+test("POST: a 302 is passed through to the client, NOT followed (no POST→GET downgrade to 405); compression stays on", async () => {
+    _setStoreForTest(new SessionStore({ enabled: false }));
+    setRegistryForTest({});
+
+    const targetSeen = { hit: false, method: null as string | null };
+    const target = makeTarget(targetSeen);
+    target.listen(0, "127.0.0.1");
+    await listen(target);
+    const targetUrl = `http://127.0.0.1:${(target.address() as { port: number }).port}/target`;
+
+    const redirectorSeen: Seen = { method: null, body: "" };
+    const redirector = makeRedirector(redirectorSeen, targetUrl);
+    redirector.listen(0, "127.0.0.1");
+    await listen(redirector);
+    const redirectorHost = `127.0.0.1:${(redirector.address() as { port: number }).port}`;
+
+    const proxy = await startServer(makeOpts(`http://${redirectorHost}`));
+    await listen(proxy);
+    const proxyPort = (proxy.address() as { port: number }).port;
+
+    try {
+        const out = await requestThroughProxy(proxyPort, redirectorHost, "POST", "/v1/messages", POST_BODY);
+
+        assert.equal(out.status, 302, `client must see the upstream 302 passed through; got ${out.status}: ${out.body}`);
+        assert.equal(out.location, targetUrl, "Location must be preserved so the client follows the redirect itself");
+        assert.equal(targetSeen.hit, false, "bili must NOT follow the redirect — the target must never be contacted");
+        assert.equal(redirectorSeen.method, "POST", "bili must forward the original method (POST) to the initial upstream");
+        assert.ok(redirectorSeen.body.includes("\x3cacp "), "ACP compression must still be active: the kernel round-trip rewrites the body before the forward");
+    } finally {
+        await close(proxy);
+        await close(redirector);
+        await close(target);
+    }
+});
+
+test("GET: a 302 is passed through to the client, NOT followed", async () => {
+    _setStoreForTest(new SessionStore({ enabled: false }));
+    setRegistryForTest({});
+
+    const targetSeen = { hit: false, method: null as string | null };
+    const target = makeTarget(targetSeen);
+    target.listen(0, "127.0.0.1");
+    await listen(target);
+    const targetUrl = `http://127.0.0.1:${(target.address() as { port: number }).port}/target`;
+
+    const redirectorSeen: Seen = { method: null, body: "" };
+    const redirector = makeRedirector(redirectorSeen, targetUrl);
+    redirector.listen(0, "127.0.0.1");
+    await listen(redirector);
+    const redirectorHost = `127.0.0.1:${(redirector.address() as { port: number }).port}`;
+
+    const proxy = await startServer(makeOpts(`http://${redirectorHost}`));
+    await listen(proxy);
+    const proxyPort = (proxy.address() as { port: number }).port;
+
+    try {
+        const out = await requestThroughProxy(proxyPort, redirectorHost, "GET", "/v1/models", null);
+
+        assert.equal(out.status, 302, `client must see the upstream 302 passed through; got ${out.status}: ${out.body}`);
+        assert.equal(out.location, targetUrl, "Location must be preserved");
+        assert.equal(targetSeen.hit, false, "bili must NOT follow the redirect — the target must never be contacted");
+        assert.equal(redirectorSeen.method, "GET", "bili must forward the original method (GET)");
+    } finally {
+        await close(proxy);
+        await close(redirector);
+        await close(target);
+    }
+});