diff --git a/open-sse/handlers/chatCore.ts b/open-sse/handlers/chatCore.ts
index ab35f63294..3d3c00fd6b 100644
--- a/open-sse/handlers/chatCore.ts
+++ b/open-sse/handlers/chatCore.ts
@@ -1037,6 +1037,13 @@ export async function handleChatCore({
 
   log?.debug?.("FORMAT", `${sourceFormat} → ${targetFormat} | stream=${stream}`);
 
+  // Preserve original body for cache signature — the body variable is mutated
+  // multiple times below (sanitization, memory/skills injection) before the
+  // cache store path runs at Phase 9.1 (non-streaming) / Phase 9.2 (streaming).
+  // Without this snapshot, the write-time signature differs from the read-time
+  // one, producing 0% hit rate. (#cache-signature-asymmetry)
+  const bodyForCacheWrite = body;
+
   // ── Phase 9.1: Semantic cache check (temp=0, any streaming mode) ──
   const cacheHit = await checkSemanticCache({
     semanticCacheEnabled,
@@ -4568,7 +4575,7 @@ export async function handleChatCore({
     // ── Phase 9.1: Cache store (non-streaming, temp=0) ──
     storeSemanticCacheResponse({
       enabled: semanticCacheEnabled,
-      body,
+      body: bodyForCacheWrite,
       headers: clientRawRequest?.headers,
       translatedResponse,
       model,
@@ -4922,7 +4929,7 @@ export async function handleChatCore({
       enabled: semanticCacheEnabled,
       streamStatus,
       streamResponseBody,
-      body,
+      body: bodyForCacheWrite,
       headers: clientRawRequest?.headers,
       model,
       apiKeyId: apiKeyInfo?.id ?? undefined,
diff --git a/tests/unit/cache-signature-roundtrip.test.ts b/tests/unit/cache-signature-roundtrip.test.ts
new file mode 100644
index 0000000000..4848adc603
--- /dev/null
+++ b/tests/unit/cache-signature-roundtrip.test.ts
@@ -0,0 +1,100 @@
+// TDD regression guard: semantic cache read-time vs write-time signature must match.
+// Without bodyForCacheWrite, the body variable is mutated between the cache read
+// (Phase 9.1, chatCore.ts:1055) and the cache write (Phase 9.1 non-streaming:4540 /
+// Phase 9.2 streaming:4896) by sanitizeChatRequestBody() and injectMemoryAndSkills().
+// The digest includes messages, so mutations produce a different key → 0% hit rate.
+import test from "node:test";
+import assert from "node:assert/strict";
+
+const { generateSignature } = await import("../../src/lib/semanticCache.ts");
+
+const BASE_BODY = {
+  model: "gpt-4o",
+  messages: [{ role: "user", content: "What is the capital of France?" }],
+  temperature: 0,
+  top_p: 1,
+};
+
+// Mutations that handleChatCore applies between cache read and cache write.
+function simulateSanitizeChatRequestBody(body: Record<string, unknown>): Record<string, unknown> {
+  return { ...body, _sanitized: true };
+}
+
+function simulateInjectMemoryAndSkills(body: Record<string, unknown>): Record<string, unknown> {
+  return {
+    ...body,
+    messages: [
+      { role: "system", content: "[Memory: user likes programming]" },
+      ...(body.messages as Array<unknown>),
+    ],
+  };
+}
+
+test("cache signature must be identical before and after body mutations", async () => {
+  // Compute signature at read-time (original body, line 1055).
+  const readSignature = await generateSignature(
+    BASE_BODY.model,
+    BASE_BODY.messages,
+    BASE_BODY.temperature,
+    BASE_BODY.top_p
+  );
+
+  // Simulate the mutations that happen between cache read and cache write.
+  let mutatedBody = simulateSanitizeChatRequestBody(BASE_BODY);
+  mutatedBody = simulateInjectMemoryAndSkills(mutatedBody);
+
+  // Compute signature at write-time using the mutated body (the bug).
+  const writeSignatureMutated = await generateSignature(
+    mutatedBody.model,
+    mutatedBody.messages,
+    mutatedBody.temperature,
+    mutatedBody.top_p
+  );
+
+  // Compute signature at write-time using the preserved snapshot (the fix).
+  const writeSignaturePreserved = await generateSignature(
+    BASE_BODY.model,
+    BASE_BODY.messages,
+    BASE_BODY.temperature,
+    BASE_BODY.top_p
+  );
+
+  // The mutated signature MUST differ — this is the bug.
+  assert.notStrictEqual(
+    readSignature,
+    writeSignatureMutated,
+    "BUG: mutated body produces different signature — cache has 0% hit rate"
+  );
+
+  // The preserved snapshot MUST match — this is the fix.
+  assert.strictEqual(
+    readSignature,
+    writeSignaturePreserved,
+    "FIX: preserved snapshot must produce the same signature as read-time"
+  );
+});
+
+test("cache signature with memory injection changes messages", async () => {
+  const bodyWithMemory = {
+    ...BASE_BODY,
+    messages: [
+      { role: "system", content: "[Memory: context from previous conversation]" },
+      ...BASE_BODY.messages,
+    ],
+  };
+
+  const sigOriginal = await generateSignature(
+    BASE_BODY.model,
+    BASE_BODY.messages,
+    BASE_BODY.temperature,
+    BASE_BODY.top_p
+  );
+  const sigWithMemory = await generateSignature(
+    bodyWithMemory.model,
+    bodyWithMemory.messages,
+    bodyWithMemory.temperature,
+    bodyWithMemory.top_p
+  );
+
+  assert.notStrictEqual(sigOriginal, sigWithMemory);
+});