diff --git a/open-sse/handlers/chatCore.ts b/open-sse/handlers/chatCore.ts
index 2a189975eccf..6ca8530676c8 100644
--- a/open-sse/handlers/chatCore.ts
+++ b/open-sse/handlers/chatCore.ts
@@ -1291,9 +1291,14 @@ function isCopilotClient(
 export function extractSystemRoleMessages(payload: Record<string, unknown>): void {
   if (!Array.isArray(payload.messages)) return;
   const messages = payload.messages as Array<{ role?: unknown; content?: unknown }>;
-  const systemMessages = messages.filter(
-    (m) => typeof m.role === "string" && m.role.toLowerCase() === "system"
-  );
+  // Treat both `system` and `developer` as system-equivalent (OpenAI's Responses
+  // API renamed system → developer). Anthropic rejects either as a chat role, so
+  // both must be lifted into the top-level `system` field — parity with the
+  // normal-path extractSystemMessagesToBody closure.
+  const isSystemRole = (role: unknown): boolean =>
+    typeof role === "string" &&
+    (role.toLowerCase() === "system" || role.toLowerCase() === "developer");
+  const systemMessages = messages.filter((m) => isSystemRole(m.role));
   if (systemMessages.length === 0) return;
 
   const extraBlocks: Array<Record<string, unknown>> = [];
@@ -1318,9 +1323,7 @@ export function extractSystemRoleMessages(payload: Record<string, unknown>): voi
       payload.system = extraBlocks;
     }
   }
-  payload.messages = messages.filter(
-    (m) => typeof m.role !== "string" || m.role.toLowerCase() !== "system"
-  );
+  payload.messages = messages.filter((m) => !isSystemRole(m.role));
 }
 
 export async function handleChatCore({
diff --git a/tests/unit/semantic-cache.test.ts b/tests/unit/semantic-cache.test.ts
index f27a23a5ec75..69185613a0d2 100644
--- a/tests/unit/semantic-cache.test.ts
+++ b/tests/unit/semantic-cache.test.ts
@@ -1,6 +1,10 @@
 import { describe, it } from "node:test";
 import assert from "node:assert/strict";
-import { generateSignature, isCacheable } from "../../src/lib/semanticCache.ts";
+import {
+  generateSignature,
+  isCacheableForRead,
+  isCacheableForWrite,
+} from "../../src/lib/semanticCache.ts";
 
 describe("Semantic Cache", () => {
   describe("generateSignature", () => {
@@ -69,35 +73,50 @@ describe("Semantic Cache", () => {
     });
   });
 
-  describe("isCacheable", () => {
-    it("returns true for non-streaming temp=0 requests", () => {
-      assert.equal(isCacheable({ stream: false, temperature: 0 }, null), true);
+  describe("isCacheableForRead", () => {
+    // #2536 superseded isCacheable with read/write variants that cache both
+    // streaming and non-streaming requests and require an explicit numeric
+    // temperature: 0 (omitted temperature is treated as non-deterministic).
+    it("returns true for temperature=0 (streaming or not)", () => {
+      assert.equal(isCacheableForRead({ stream: false, temperature: 0 }, null), true);
+      assert.equal(isCacheableForRead({ stream: true, temperature: 0 }, null), true);
     });
 
-    it("returns true when temperature is undefined (defaults to 0)", () => {
-      assert.equal(isCacheable({ stream: false }, null), true);
-    });
-
-    it("returns false for streaming requests", () => {
-      assert.equal(isCacheable({ stream: true, temperature: 0 }, null), false);
-    });
-
-    it("returns false when stream is not explicitly false", () => {
-      assert.equal(isCacheable({ temperature: 0 }, null), false);
+    it("returns false when temperature is omitted (provider default may be non-deterministic)", () => {
+      assert.equal(isCacheableForRead({ stream: false }, null), false);
     });
 
     it("returns false for non-zero temperature", () => {
-      assert.equal(isCacheable({ stream: false, temperature: 0.7 }, null), false);
+      assert.equal(isCacheableForRead({ temperature: 0.7 }, null), false);
     });
 
     it("returns false when no-cache header is set", () => {
       const headers = new Headers({ "x-omniroute-no-cache": "true" });
-      assert.equal(isCacheable({ stream: false, temperature: 0 }, headers), false);
+      assert.equal(isCacheableForRead({ temperature: 0 }, headers), false);
     });
 
     it("returns true when no-cache header is absent", () => {
       const headers = new Headers({});
-      assert.equal(isCacheable({ stream: false, temperature: 0 }, headers), true);
+      assert.equal(isCacheableForRead({ temperature: 0 }, headers), true);
+    });
+  });
+
+  describe("isCacheableForWrite", () => {
+    it("returns true for temperature=0 responses", () => {
+      assert.equal(isCacheableForWrite({ temperature: 0 }, null), true);
+    });
+
+    it("returns false when temperature is omitted", () => {
+      assert.equal(isCacheableForWrite({ stream: false }, null), false);
+    });
+
+    it("returns false for non-zero temperature", () => {
+      assert.equal(isCacheableForWrite({ temperature: 0.7 }, null), false);
+    });
+
+    it("returns false when no-cache header is set", () => {
+      const headers = new Headers({ "x-omniroute-no-cache": "true" });
+      assert.equal(isCacheableForWrite({ temperature: 0 }, headers), false);
     });
   });
 });
diff --git a/tests/unit/system-role-extraction.test.ts b/tests/unit/system-role-extraction.test.ts
index 306fb7feef27..e2f9f09381c0 100644
--- a/tests/unit/system-role-extraction.test.ts
+++ b/tests/unit/system-role-extraction.test.ts
@@ -16,6 +16,24 @@ test("extractSystemRoleMessages moves role=system to top-level system", () => {
   assert.deepEqual(payload.system, [{ type: "text", text: "Memory context: foo" }]);
 });
 
+test("extractSystemRoleMessages also lifts role=developer (OpenAI Responses system alias)", () => {
+  const payload = {
+    messages: [
+      { role: "developer", content: "Dev instructions" },
+      { role: "system", content: "Sys context" },
+      { role: "user", content: "hello" },
+    ],
+  };
+  extractSystemRoleMessages(payload);
+  // both developer and system are removed from messages and lifted into system
+  assert.equal(payload.messages.length, 1);
+  assert.equal(payload.messages[0].role, "user");
+  assert.deepEqual(payload.system, [
+    { type: "text", text: "Dev instructions" },
+    { type: "text", text: "Sys context" },
+  ]);
+});
+
 test("extractSystemRoleMessages merges with existing top-level system string", () => {
   const payload = {
     system: "You are Claude.",