diff --git a/tests/e2e/src/cases/passthrough-e2e.test.ts b/tests/e2e/src/cases/passthrough-e2e.test.ts
new file mode 100644
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+++ b/tests/e2e/src/cases/passthrough-e2e.test.ts
@@ -0,0 +1,208 @@
+import { createHash } from "node:crypto";
+import { afterAll, beforeAll, describe, expect, test } from "vitest";
+import {
+  AdminClient,
+  EtcdClient,
+  spawnApp,
+  startOpenAiUpstream,
+  waitConfigPropagation,
+  type OpenAiUpstream,
+  type SpawnedApp,
+} from "../harness/index.js";
+
+// E2E: /passthrough/{provider}/*rest end-to-end. Per gateway docs
+// `docs/api-proxy.md` §4.10, this is the lowest-overhead escape
+// hatch for provider endpoints the gateway hasn't yet wrapped
+// natively (e.g. OpenAI batches, files, fine-tuning, Anthropic
+// message-batches). The gateway:
+//
+//   - strips `/passthrough/{provider}` from the request URL,
+//     appending the rest to the configured Model's api_base
+//   - picks the first Model with the matching `provider` prefix
+//     (openai, anthropic, gemini, deepseek) and uses its
+//     credentials
+//   - injects the configured provider API key — Bearer for
+//     OpenAI / Gemini / DeepSeek, `x-api-key` + `anthropic-version`
+//     for Anthropic
+//   - forwards the body verbatim
+//
+// Prior to this file, the gateway had **zero** e2e coverage on
+// /passthrough — meaning every customer using batches, files, or
+// fine-tuning APIs had no regression protection on the wire.
+//
+// One user journey pinned:
+//
+//   - Anthropic passthrough — caller hits a custom Anthropic
+//     endpoint (e.g. /v1/messages/batches). Gateway must:
+//       * strip the /passthrough/anthropic prefix and forward
+//         path + body + method verbatim
+//       * inject Anthropic's auth shape (`x-api-key` +
+//         `anthropic-version`), NOT `Authorization: Bearer`
+//         (a regression that forwarded Bearer to Anthropic would
+//         401 in production)
+//
+// (The "OpenAI passthrough" case is held back pending a product /
+// docs reconciliation — `docs/api-admin.md` §4.3 publishes
+// `api_base: "https://api.openai.com/v1"` (with /v1), and
+// `docs/api-proxy.md` §4.10 example uses
+// `/passthrough/openai/v1/batches` (also with /v1). Together
+// these produce a double-/v1 upstream URL. See follow-up issue.)
+//
+// References:
+// - Gateway's own /passthrough contract: `docs/api-proxy.md` §4.10
+// - Anthropic auth headers spec
+//   <https://docs.anthropic.com/en/api/getting-started>
+
+const CALLER_PLAINTEXT = "sk-pt-e2e-caller";
+const CALLER_KEY_HASH = createHash("sha256")
+  .update(CALLER_PLAINTEXT)
+  .digest("hex");
+
+describe("passthrough e2e: /passthrough/anthropic/*rest auth-shape switching + verbatim forward", () => {
+  let app: SpawnedApp | undefined;
+  let admin: AdminClient | undefined;
+  let etcdReachable = false;
+  const upstreams: OpenAiUpstream[] = [];
+
+  beforeAll(async () => {
+    etcdReachable = await new EtcdClient().ping();
+    if (!etcdReachable) return;
+
+    app = await spawnApp();
+    admin = new AdminClient(app.adminUrl, app.adminKey);
+    await admin.createApiKey({
+      key_hash: CALLER_KEY_HASH,
+      allowed_models: ["*"],
+    });
+  });
+
+  afterAll(async () => {
+    await app?.exit();
+    await Promise.all(upstreams.map((u) => u.close()));
+  });
+
+  test("Anthropic passthrough: gateway uses x-api-key + anthropic-version, NOT Bearer", async (ctx) => {
+    if (!etcdReachable || !app || !admin) {
+      ctx.skip();
+      return;
+    }
+
+    const upstream = await startOpenAiUpstream({
+      nonStreamBody: {
+        // Shaped like an Anthropic message-batches response per
+        // <https://docs.anthropic.com/en/api/creating-message-batches>.
+        id: "msgbatch_pt_anthropic_01",
+        type: "message_batch",
+        processing_status: "in_progress",
+        request_counts: { processing: 1, succeeded: 0, errored: 0, canceled: 0, expired: 0 },
+        ended_at: null,
+        created_at: new Date().toISOString(),
+        expires_at: new Date(Date.now() + 24 * 3600 * 1000).toISOString(),
+      },
+    });
+    upstreams.push(upstream);
+
+    // Anthropic api_base is the bare host; bridge composes the
+    // rest of the path. For passthrough, the gateway appends
+    // `/*rest` directly, so we pass the bare host.
+    const pk = await admin.createProviderKey({
+      display_name: "pt-anthropic-pk",
+      secret: "sk-ant-mock",
+      api_base: upstream.baseUrl,
+    });
+    await admin.createModel({
+      display_name: "pt-anthropic-model",
+      provider: "anthropic",
+      model_name: "claude-3-5-haiku-20241022",
+      provider_key_id: pk.id,
+    });
+
+    const headers = {
+      authorization: `Bearer ${CALLER_PLAINTEXT}`,
+      "content-type": "application/json",
+    };
+
+    await waitConfigPropagation(async () => {
+      try {
+        const r = await fetch(
+          `${app!.proxyUrl}/passthrough/anthropic/v1/messages/batches`,
+          {
+            method: "POST",
+            headers,
+            body: JSON.stringify({ requests: [] }),
+          },
+        );
+        if (r.status !== 200) {
+          await r.text();
+          return false;
+        }
+        const j = (await r.json()) as { type?: unknown };
+        return j.type === "message_batch";
+      } catch {
+        return false;
+      }
+    });
+
+    const baseline = upstream.receivedRequests.length;
+    const requestBody = JSON.stringify({
+      requests: [
+        {
+          custom_id: "batch-req-1",
+          params: { model: "claude-3-5-haiku-20241022", max_tokens: 100 },
+        },
+      ],
+    });
+    const res = await fetch(
+      `${app.proxyUrl}/passthrough/anthropic/v1/messages/batches`,
+      {
+        method: "POST",
+        headers,
+        body: requestBody,
+      },
+    );
+
+    expect(res.status).toBe(200);
+    const body = (await res.json()) as { id?: unknown; type?: unknown };
+    expect(body.id).toBe("msgbatch_pt_anthropic_01");
+    expect(body.type).toBe("message_batch");
+
+    const testCalls = upstream.receivedRequests
+      .slice(baseline)
+      .filter((r) => r.path === "/v1/messages/batches");
+    expect(testCalls).toHaveLength(1);
+    expect(testCalls[0]?.method).toBe("POST");
+
+    // Auth injection per docs §4.10: Anthropic uses `x-api-key`
+    // + `anthropic-version` headers, NOT `Authorization: Bearer`.
+    // A regression that forwarded Bearer to Anthropic upstream
+    // would 401 in production but pass against the permissive
+    // mock — pinning the exact header set is the only line of
+    // defence.
+    expect(testCalls[0]?.headers["x-api-key"]).toBe("sk-ant-mock");
+    // Anthropic's documented current API version is `2023-06-01`
+    // per <https://docs.anthropic.com/en/api/getting-started>. A
+    // regression that injected a malformed-but-non-empty version
+    // (e.g. "v1", "latest") would 400 against real Anthropic but
+    // pass against the permissive mock without this exact pin.
+    expect(testCalls[0]?.headers["anthropic-version"]).toBe("2023-06-01");
+
+    // Caller's `Authorization: Bearer sk-pt-e2e-caller` is
+    // gateway-internal (validates against the ApiKey table) and
+    // MUST NOT leak upstream — that would put a gateway-side
+    // ApiKey credential into upstream provider logs. Pin: if the
+    // upstream's Authorization header is present at all, its value
+    // must NOT contain the caller's plaintext bearer.
+    //
+    // (Whether the gateway should inject ANY Authorization header
+    // when forwarding to Anthropic is a separate question — docs
+    // §4.10 implies only x-api-key + anthropic-version for the
+    // Anthropic auth shape. Tracked as follow-up.)
+    const upstreamAuth = testCalls[0]?.headers["authorization"];
+    if (upstreamAuth !== undefined) {
+      expect(upstreamAuth as string).not.toContain(CALLER_PLAINTEXT);
+    }
+
+    // Body verbatim — same contract as the OpenAI case.
+    expect(testCalls[0]?.body).toBe(requestBody);
+  });
+});