diff --git a/src/services/api/client.ts b/src/services/api/client.ts
index d0720df..bf4b262 100644
--- a/src/services/api/client.ts
+++ b/src/services/api/client.ts
@@ -1,5 +1,10 @@
 import Anthropic, { type ClientOptions } from '@anthropic-ai/sdk'
 import { randomUUID } from 'crypto'
+import {
+  computeCch,
+  hasCchPlaceholder,
+  replaceCchPlaceholder,
+} from 'src/utils/cch.js'
 import type { GoogleAuth } from 'google-auth-library'
 import {
   checkAndRefreshOAuthTokenIfNeeded,
@@ -383,15 +388,14 @@ function buildFetch(
   // and unknown headers risk rejection by strict proxies (inc-4029 class).
   const injectClientRequestId =
     getAPIProvider() === 'firstParty' && isFirstPartyAnthropicBaseUrl()
-  return (input, init) => {
+  return async (input, init) => {
     // eslint-disable-next-line eslint-plugin-n/no-unsupported-features/node-builtins
     const headers = new Headers(init?.headers)
-    // Generate a client-side request ID so timeouts (which return no server
-    // request ID) can still be correlated with server logs by the API team.
-    // Callers that want to track the ID themselves can pre-set the header.
     if (injectClientRequestId && !headers.has(CLIENT_REQUEST_ID_HEADER)) {
       headers.set(CLIENT_REQUEST_ID_HEADER, randomUUID())
     }
+
+    let body = init?.body
     try {
       // eslint-disable-next-line eslint-plugin-n/no-unsupported-features/node-builtins
       const url = input instanceof Request ? input.url : String(input)
@@ -399,9 +403,20 @@ function buildFetch(
       logForDebugging(
         `[API REQUEST] ${new URL(url).pathname}${id ? ` ${CLIENT_REQUEST_ID_HEADER}=${id}` : ''} source=${source ?? 'unknown'}`,
       )
+
+      if (
+        url.includes('/v1/messages') &&
+        headers.has('anthropic-version') &&
+        typeof body === 'string' &&
+        hasCchPlaceholder(body)
+      ) {
+        const cch = await computeCch(body)
+        body = replaceCchPlaceholder(body, cch)
+        logForDebugging(`[CCH] signed request cch=${cch}`)
+      }
     } catch {
       // never let logging crash the fetch
     }
-    return inner(input, { ...init, headers })
+    return inner(input, { ...init, headers, body })
   }
 }