diff --git a/src/reporters/site-api.test.ts b/src/reporters/site-api.test.ts
index 19d0106..4776392 100644
--- a/src/reporters/site-api.test.ts
+++ b/src/reporters/site-api.test.ts
@@ -1,3 +1,4 @@
+import { gunzipSync } from "node:zlib";
 import { test, expect, describe, afterEach, vi } from "vitest";
 import {
   classifyIngestFailure,
@@ -280,6 +281,16 @@ describe("postToSiteApi authentication", () => {
 
     expect(headersFrom(fetchMock).has("authorization")).toBe(false);
   });
+
+  test("gzips the request body and advertises Content-Encoding: gzip", async () => {
+    const fetchMock = stubOkFetch();
+
+    await postToSiteApi("https://api.querydoctor.com", [makeQuery("hash-a")]);
+
+    expect(headersFrom(fetchMock).get("content-encoding")).toBe("gzip");
+    const body = fetchMock.mock.calls[0]![1]!.body as Buffer;
+    expect(JSON.parse(gunzipSync(body).toString("utf8")).repo).toBeDefined();
+  });
 });
 
 describe("postToSiteApi payload baseBranch", () => {
@@ -300,7 +311,8 @@ describe("postToSiteApi payload baseBranch", () => {
   }
 
   function bodyFrom(fetchMock: ReturnType<typeof vi.fn>): Record<string, unknown> {
-    return JSON.parse(fetchMock.mock.calls[0]![1]!.body as string);
+    const body = fetchMock.mock.calls[0]![1]!.body as Buffer;
+    return JSON.parse(gunzipSync(body).toString("utf8"));
   }
 
   test("forwards GITHUB_BASE_REF as baseBranch on a PR run", async () => {
diff --git a/src/reporters/site-api.ts b/src/reporters/site-api.ts
index 1ea86b4..c97a4b8 100644
--- a/src/reporters/site-api.ts
+++ b/src/reporters/site-api.ts
@@ -1,3 +1,5 @@
+import { gzip } from "node:zlib";
+import { promisify } from "node:util";
 import * as github from "@actions/github";
 import { isTestOriginQuery } from "@query-doctor/core";
 import type { ComputedStats, FullSchema, IndexRecommendation, Nudge, SQLCommenterTag, StatisticsMode, TableReference } from "@query-doctor/core";
@@ -5,6 +7,8 @@ import { DEFAULT_CONFIG, type AnalyzerConfig } from "../config.ts";
 import type { OptimizedQuery } from "../sql/recent-query.ts";
 import type { Op } from "jsondiffpatch/formatters/jsonpatch";
 
+const gzipAsync = promisify(gzip);
+
 interface CiRunPayload {
   repo: string;
   branch: string;
@@ -452,16 +456,26 @@ export async function postToSiteApi(
   }
 
   const url = `${endpoint.replace(/\/$/, "")}/ci/runs`;
-  console.log(`Posting CI run to ${url} (${queries.length} queries)`);
+
+  // Gzip the body: CI run payloads reach multiple MB (many queries + schema),
+  // and the Site API decompresses request bodies before enforcing its size
+  // limit. See Query-Doctor/Site raise-json-body-limit work.
+  const body = await gzipAsync(JSON.stringify(payload));
+
+  const sentKib = (body.byteLength / 1024).toFixed(1);
+  console.log(
+    `Posting CI run to ${url} (${queries.length} queries, ${sentKib} KiB gzipped)`,
+  );
 
   try {
     const response = await fetch(url, {
       method: "POST",
       headers: {
         "Content-Type": "application/json",
+        "Content-Encoding": "gzip",
         ...(token ? { Authorization: `Bearer ${token}` } : {}),
       },
-      body: JSON.stringify(payload),
+      body,
     });
     if (!response.ok) {
       const text = await response.text();
@@ -474,18 +488,18 @@ export async function postToSiteApi(
         },
       };
     }
-    const body = (await response.json()) as {
+    const responseBody = (await response.json()) as {
       id: string;
       url?: string | null;
       metadata?: CiRunMetadata | null;
     };
-    console.log(`Site API ingestion successful: ${JSON.stringify(body)}`);
+    console.log(`Site API ingestion successful: ${JSON.stringify(responseBody)}`);
     return {
       ok: true,
       result: {
-        id: body.id,
-        url: body.url ?? null,
-        metadata: body.metadata ?? null,
+        id: responseBody.id,
+        url: responseBody.url ?? null,
+        metadata: responseBody.metadata ?? null,
       },
     };
   } catch (err) {