diff --git a/open-sse/services/claudeCodeConstraints.ts b/open-sse/services/claudeCodeConstraints.ts
index 42ed52f996..094727e11b 100644
--- a/open-sse/services/claudeCodeConstraints.ts
+++ b/open-sse/services/claudeCodeConstraints.ts
@@ -130,27 +130,34 @@ export function ensureCacheControlOnLastUserMessage(body: Record<string, unknown
   if (!Array.isArray(messages) || messages.length === 0) return;
 
   const system = body.system as Array<Record<string, unknown>> | undefined;
-  const systemCacheControlCount = Array.isArray(system)
+  let cacheControlCount = Array.isArray(system)
     ? system.filter((block) => block.cache_control).length
     : 0;
+  let hasFiveMinuteCacheControl = Array.isArray(system)
+    ? system.some(
+        (block) => (block.cache_control as Record<string, unknown> | undefined)?.ttl === "5m"
+      )
+    : false;
 
   for (const message of messages) {
     const content = message.content as Array<Record<string, unknown>> | undefined;
-    if (Array.isArray(content) && content.some((block) => block.cache_control)) {
-      return;
-    }
+    if (!Array.isArray(content)) continue;
+    cacheControlCount += content.filter((block) => block.cache_control).length;
+    hasFiveMinuteCacheControl ||= content.some(
+      (block) => (block.cache_control as Record<string, unknown> | undefined)?.ttl === "5m"
+    );
   }
 
-  if (systemCacheControlCount >= MAX_CACHE_CONTROL_BLOCKS) return;
-
   // Find the last user message
   for (let i = messages.length - 1; i >= 0; i--) {
     if (String(messages[i].role) === "user") {
       const content = messages[i].content;
       if (Array.isArray(content) && content.length > 0) {
         const lastBlock = content[content.length - 1] as Record<string, unknown>;
-        if (!lastBlock.cache_control) {
-          lastBlock.cache_control = { type: "ephemeral" };
+        if (!lastBlock.cache_control && cacheControlCount < MAX_CACHE_CONTROL_BLOCKS) {
+          lastBlock.cache_control = hasFiveMinuteCacheControl
+            ? { type: "ephemeral", ttl: "5m" }
+            : { type: "ephemeral" };
         }
       }
       break;
@@ -158,38 +165,31 @@ export function ensureCacheControlOnLastUserMessage(body: Record<string, unknown
   }
 }
 
-/**
- * Real Claude Code (and CC-protocol-compatible clients) commonly send
- * `cache_control: { type: "ephemeral" }` with no `ttl`. On the native Claude
- * OAuth path the outbound anthropic-beta set always includes
- * extended-cache-ttl-2025-04-11 (see ANTHROPIC_BETA_BASE /
- * ANTHROPIC_BETA_CLAUDE_OAUTH in anthropicHeaders.ts), so requesting the 1h
- * TTL is always valid here — but Anthropic only honors it when `ttl` is
- * explicitly set; an absent `ttl` silently falls back to the platform
- * default of 5 minutes even though the 1h beta was negotiated. Any pause
- * longer than 5 minutes between turns then forces a full prefix rewrite
- * instead of a cache hit. Default the ttl to "1h" wherever it's missing;
- * never touch a cache_control that already specifies one (explicit client
- * choice is preserved).
- */
+/** Defaults missing TTLs to 1h until a 5m breakpoint; later defaults stay at 5m. */
 export function normalizeCacheControlTtl(body: Record<string, unknown>): void {
+  let hasFiveMinuteCacheControl = false;
+
   const defaultMissingTtl = (block: Record<string, unknown> | null | undefined) => {
     const cc = block?.cache_control as Record<string, unknown> | undefined;
-    if (cc && cc.type === "ephemeral" && cc.ttl === undefined) {
-      cc.ttl = "1h";
+    if (!cc || cc.type !== "ephemeral") return;
+
+    if (cc.ttl === "5m") {
+      hasFiveMinuteCacheControl = true;
+    } else if (cc.ttl === undefined) {
+      cc.ttl = hasFiveMinuteCacheControl ? "5m" : "1h";
     }
   };
 
-  const system = body.system as Array<Record<string, unknown>> | undefined;
-  if (Array.isArray(system)) {
-    for (const block of system) defaultMissingTtl(block);
-  }
-
   const tools = body.tools as Array<Record<string, unknown>> | undefined;
   if (Array.isArray(tools)) {
     for (const tool of tools) defaultMissingTtl(tool);
   }
 
+  const system = body.system as Array<Record<string, unknown>> | undefined;
+  if (Array.isArray(system)) {
+    for (const block of system) defaultMissingTtl(block);
+  }
+
   const messages = body.messages as Array<Record<string, unknown>> | undefined;
   if (Array.isArray(messages)) {
     for (const message of messages) {
diff --git a/tests/unit/chatcore-translation-paths.test.ts b/tests/unit/chatcore-translation-paths.test.ts
index db081c9345..8ae3d4d36d 100644
--- a/tests/unit/chatcore-translation-paths.test.ts
+++ b/tests/unit/chatcore-translation-paths.test.ts
@@ -1085,13 +1085,16 @@ test("chatCore preserves Opus 5 mid-conversation system cache breakpoints", asyn
   );
   assert.deepEqual(call.body.messages[2].content[0].cache_control, {
     type: "ephemeral",
-    ttl: "1h",
+    ttl: "5m",
   });
   assert.equal(
     call.body.system.some((block: { text?: string }) => block.text === "compact continuation"),
     false
   );
-  assert.equal(call.body.messages[3].content[0].cache_control, undefined);
+  assert.deepEqual(call.body.messages[3].content[0].cache_control, {
+    type: "ephemeral",
+    ttl: "5m",
+  });
 });
 test("chatCore keeps Claude normalization for non-Claude-Code Claude passthrough", async () => {
   const { call, result } = await invokeChatCore({
@@ -1264,12 +1267,12 @@ test("chatCore preserves cache_control automatically for Claude Code single-mode
   assert.deepEqual(call.body.system[2].cache_control, { type: "ephemeral", ttl: "5m" });
   assert.deepEqual(call.body.messages[0].content[0].cache_control, {
     type: "ephemeral",
-    ttl: "1h",
+    ttl: "5m",
   });
   // base.ts executor explicitly strips cache_control from tools for Claude Code clients
   assert.equal(call.body.tools[0].cache_control, undefined);
 });
-test("chatCore supplements a missing message cache breakpoint for native Claude Code requests", async () => {
+test("chatCore advances a message cache breakpoint for native Claude Code requests", async () => {
   await settingsDb.updateSettings({ alwaysPreserveClientCache: "auto" });
   invalidateCacheControlSettingsCache();
 
@@ -1294,7 +1297,16 @@ test("chatCore supplements a missing message cache breakpoint for native Claude
         },
       ],
       messages: [
-        { role: "user", content: [{ type: "text", text: "first turn" }] },
+        {
+          role: "user",
+          content: [
+            {
+              type: "text",
+              text: "first turn",
+              cache_control: { type: "ephemeral" },
+            },
+          ],
+        },
         { role: "assistant", content: [{ type: "text", text: "first response" }] },
         { role: "user", content: [{ type: "text", text: "latest turn" }] },
       ],
@@ -1313,7 +1325,7 @@ test("chatCore supplements a missing message cache breakpoint for native Claude
 
   assert.deepEqual(call.body.messages[2].content[0].cache_control, {
     type: "ephemeral",
-    ttl: "1h",
+    ttl: "5m",
   });
   assert.equal(call.body.tools[0].cache_control, undefined);
 });
@@ -1401,10 +1413,12 @@ test("chatCore disables raw Claude passthrough when cache preservation is off an
     ),
     true
   );
-  // Cache preservation is on for native Claude, so cache markers are intact
+  // Cache preservation is on for native Claude, so cache markers are intact. This PR:
+  // an omitted TTL now defaults to "5m" once a "5m" boundary breakpoint (the system
+  // block above) has already appeared, instead of always defaulting to "1h".
   assert.deepEqual(call.body.messages[0].content[0].cache_control, {
     type: "ephemeral",
-    ttl: "1h",
+    ttl: "5m",
   });
   // Tools disable flag is applied
   assert.equal("_disableToolPrefix" in call.body, false);
diff --git a/tests/unit/claude-code-parity.test.ts b/tests/unit/claude-code-parity.test.ts
index 0e58e9a3f3..07a1e5e2d1 100644
--- a/tests/unit/claude-code-parity.test.ts
+++ b/tests/unit/claude-code-parity.test.ts
@@ -360,7 +360,7 @@ describe("ensureCacheControlOnLastUserMessage", () => {
     assert.deepEqual(body.messages[2].content[0].cache_control, { type: "ephemeral" });
   });
 
-  it("keeps an existing message breakpoint without adding another", () => {
+  it("keeps an existing message breakpoint and advances one to the last user message", () => {
     const body = {
       messages: [
         {
@@ -377,9 +377,33 @@ describe("ensureCacheControlOnLastUserMessage", () => {
       ],
     };
 
+    ensureCacheControlOnLastUserMessage(body);
     ensureCacheControlOnLastUserMessage(body);
 
-    assert.equal(body.messages[1].content[0].cache_control, undefined);
+    assert.deepEqual(body.messages[0].content[0].cache_control, { type: "ephemeral" });
+    assert.deepEqual(body.messages[1].content[0].cache_control, { type: "ephemeral" });
+    assert.equal(
+      body.messages.flatMap((message) => message.content).filter((block) => block.cache_control)
+        .length,
+      2
+    );
+  });
+
+  it("keeps a new tail breakpoint at 5m after an existing 5m breakpoint", () => {
+    const body = {
+      system: [
+        { type: "text", text: "long", cache_control: { type: "ephemeral", ttl: "1h" } },
+        { type: "text", text: "short", cache_control: { type: "ephemeral", ttl: "5m" } },
+      ],
+      messages: [{ role: "user", content: [{ type: "text", text: "Follow up" }] }],
+    };
+
+    ensureCacheControlOnLastUserMessage(body);
+
+    assert.deepEqual(body.messages[0].content[0].cache_control, {
+      type: "ephemeral",
+      ttl: "5m",
+    });
   });
 
   it("does not exceed four surviving system and message breakpoints", () => {
@@ -452,6 +476,25 @@ describe("normalizeCacheControlTtl", () => {
     });
   });
 
+  it("defaults missing ttl to 5m after a 5m breakpoint", () => {
+    const body = {
+      system: [{ type: "text", text: "stable", cache_control: { type: "ephemeral", ttl: "5m" } }],
+      messages: [
+        {
+          role: "user",
+          content: [{ type: "text", text: "follow up", cache_control: { type: "ephemeral" } }],
+        },
+      ],
+    };
+
+    normalizeCacheControlTtl(body);
+
+    assert.deepEqual(body.messages[0].content[0].cache_control, {
+      type: "ephemeral",
+      ttl: "5m",
+    });
+  });
+
   it("leaves blocks without cache_control untouched", () => {
     const body = {
       system: [{ type: "text", text: "no cache_control here" }],