diff --git a/changelog.d/fixes/12885-semantic-cache-no-truncated-writes.md b/changelog.d/fixes/12885-semantic-cache-no-truncated-writes.md
new file mode 100644
index 0000000000..ad643d50bd
--- /dev/null
+++ b/changelog.d/fixes/12885-semantic-cache-no-truncated-writes.md
@@ -0,0 +1 @@
+- fix(cache): never write a truncated completion (`finish_reason: "length"`/`max_tokens`) into the semantic cache — a partial answer cached under a temperature:0 signature was served to every later identical request, permanently returning a mid-sentence reply that no retry cleared (#12885)
diff --git a/open-sse/handlers/chatCore/semanticCacheStore.ts b/open-sse/handlers/chatCore/semanticCacheStore.ts
index 4c5f44c622..00111eef88 100644
--- a/open-sse/handlers/chatCore/semanticCacheStore.ts
+++ b/open-sse/handlers/chatCore/semanticCacheStore.ts
@@ -12,6 +12,7 @@ import {
   generateSignature as defaultGenerateSignature,
   setCachedResponse as defaultSetCachedResponse,
   isCacheableForWrite as defaultIsCacheableForWrite,
+  isTruncatedCompletion as defaultIsTruncatedCompletion,
 } from "@/lib/semanticCache";
 import { isSmallEnoughForSemanticCache as defaultIsSmallEnough } from "../../utils/estimateSize.ts";
 import { getSemanticCacheManager } from "../../services/cache/semanticCacheManager.ts";
@@ -32,6 +33,8 @@ type UsageLike = { prompt_tokens?: number; completion_tokens?: number } | null |
 
 export interface SemanticCacheStoreDeps {
   isCacheableForWrite: typeof defaultIsCacheableForWrite;
+  /** Optional so pre-existing callers/tests with partial deps keep working. */
+  isTruncatedCompletion?: typeof defaultIsTruncatedCompletion;
   isSmallEnoughForSemanticCache: typeof defaultIsSmallEnough;
   generateSignature: typeof defaultGenerateSignature;
   setCachedResponse: typeof defaultSetCachedResponse;
@@ -39,6 +42,7 @@ export interface SemanticCacheStoreDeps {
 
 const DEFAULT_DEPS: SemanticCacheStoreDeps = {
   isCacheableForWrite: defaultIsCacheableForWrite,
+  isTruncatedCompletion: defaultIsTruncatedCompletion,
   isSmallEnoughForSemanticCache: defaultIsSmallEnough,
   generateSignature: defaultGenerateSignature,
   setCachedResponse: defaultSetCachedResponse,
@@ -61,6 +65,7 @@ export function storeSemanticCacheResponse(
   if (
     !args.enabled ||
     !deps.isCacheableForWrite(args.body, args.headers) ||
+    (deps.isTruncatedCompletion ?? defaultIsTruncatedCompletion)(args.translatedResponse) ||
     !deps.isSmallEnoughForSemanticCache(args.translatedResponse)
   ) {
     return;
diff --git a/open-sse/handlers/chatCore/streamingSemanticCacheStore.ts b/open-sse/handlers/chatCore/streamingSemanticCacheStore.ts
index 58f6df5b96..26832ba7d6 100644
--- a/open-sse/handlers/chatCore/streamingSemanticCacheStore.ts
+++ b/open-sse/handlers/chatCore/streamingSemanticCacheStore.ts
@@ -13,6 +13,7 @@ import {
   generateSignature as defaultGenerateSignature,
   setCachedResponse as defaultSetCachedResponse,
   isCacheableForWrite as defaultIsCacheableForWrite,
+  isTruncatedCompletion as defaultIsTruncatedCompletion,
 } from "@/lib/semanticCache";
 import { isSmallEnoughForSemanticCache as defaultIsSmallEnough } from "../../utils/estimateSize.ts";
 import { getSemanticCacheManager } from "../../services/cache/semanticCacheManager.ts";
@@ -31,6 +32,8 @@ type CacheBody = {
 
 export interface StreamingSemanticCacheStoreDeps {
   isCacheableForWrite: typeof defaultIsCacheableForWrite;
+  /** Optional so pre-existing callers/tests with partial deps keep working. */
+  isTruncatedCompletion?: typeof defaultIsTruncatedCompletion;
   isSmallEnoughForSemanticCache: typeof defaultIsSmallEnough;
   generateSignature: typeof defaultGenerateSignature;
   setCachedResponse: typeof defaultSetCachedResponse;
@@ -38,6 +41,7 @@ export interface StreamingSemanticCacheStoreDeps {
 
 const DEFAULT_DEPS: StreamingSemanticCacheStoreDeps = {
   isCacheableForWrite: defaultIsCacheableForWrite,
+  isTruncatedCompletion: defaultIsTruncatedCompletion,
   isSmallEnoughForSemanticCache: defaultIsSmallEnough,
   generateSignature: defaultGenerateSignature,
   setCachedResponse: defaultSetCachedResponse,
@@ -113,7 +117,8 @@ export function storeStreamingSemanticCacheResponse(
     !args.enabled ||
     args.streamStatus !== 200 ||
     !args.streamResponseBody ||
-    !deps.isCacheableForWrite(args.body, args.headers)
+    !deps.isCacheableForWrite(args.body, args.headers) ||
+    (deps.isTruncatedCompletion ?? defaultIsTruncatedCompletion)(args.streamResponseBody)
   ) {
     return;
   }
diff --git a/src/lib/semanticCache.ts b/src/lib/semanticCache.ts
index 03fe8938ed..3ff0d8e978 100644
--- a/src/lib/semanticCache.ts
+++ b/src/lib/semanticCache.ts
@@ -469,3 +469,56 @@ export function isCacheableForWrite(body, headers) {
   if (body.temperature !== 0) return false;
   return true;
 }
+
+/**
+ * A response cut short by the output-token ceiling is a partial answer, not a
+ * reusable one. Caching it under a temperature:0 signature pins the truncation
+ * for every later identical request — the caller sees a mid-sentence reply that
+ * no retry clears, because each retry is served the same poisoned entry.
+ *
+ * Only `length` (and its Claude-side spelling `max_tokens`) is treated as
+ * truncation. `stop`, `tool_calls`, and a missing/unknown reason are complete
+ * responses and stay cacheable, so this never narrows the cache beyond the bug.
+ */
+const TRUNCATED_FINISH_REASONS = new Set(["length", "max_tokens"]);
+
+export function isTruncatedCompletion(response: unknown): boolean {
+  if (!response || typeof response !== "object") return false;
+  const r = response as {
+    choices?: Array<{ finish_reason?: unknown }>;
+    stop_reason?: unknown;
+  };
+  if (Array.isArray(r.choices)) {
+    for (const choice of r.choices) {
+      const reason = choice?.finish_reason;
+      if (typeof reason === "string" && TRUNCATED_FINISH_REASONS.has(reason)) return true;
+    }
+  }
+  // Claude-format responses carry the reason at the top level instead.
+  if (typeof r.stop_reason === "string" && TRUNCATED_FINISH_REASONS.has(r.stop_reason)) {
+    return true;
+  }
+  return false;
+}
+
+/**
+ * Streaming variant: the assembled SSE body is scanned for a truncating
+ * finish_reason. Parsing is intentionally tolerant — an unparseable chunk is
+ * treated as "not known to be truncated" so a malformed frame never silently
+ * disables caching.
+ */
+export function isTruncatedStreamBody(streamBody: unknown): boolean {
+  if (typeof streamBody !== "string" || streamBody.length === 0) return false;
+  for (const line of streamBody.split("\n")) {
+    const trimmed = line.trim();
+    if (!trimmed.startsWith("data:")) continue;
+    const payload = trimmed.slice(5).trim();
+    if (!payload || payload === "[DONE]") continue;
+    try {
+      if (isTruncatedCompletion(JSON.parse(payload))) return true;
+    } catch {
+      // Non-JSON frame — ignore.
+    }
+  }
+  return false;
+}
diff --git a/tests/unit/semantic-cache-no-truncated-writes.test.ts b/tests/unit/semantic-cache-no-truncated-writes.test.ts
new file mode 100644
index 0000000000..47750e4107
--- /dev/null
+++ b/tests/unit/semantic-cache-no-truncated-writes.test.ts
@@ -0,0 +1,101 @@
+// Regression: a truncated upstream response (finish_reason "length") must never be
+// written to the semantic cache. A partial completion cached under a temperature:0
+// signature is served to every subsequent identical request, permanently returning
+// a mid-sentence answer that no retry can clear (only a cache flush).
+// Observed live on OmniRoute against github/gemini-3.5-flash: temperature:0 returned
+// finish_reason "length" at 93 completion tokens on every call, while the same request
+// with x-omniroute-no-cache:true returned a complete 239-token response. (#12885)
+import { test } from "node:test";
+import assert from "node:assert/strict";
+
+const { storeSemanticCacheResponse } =
+  await import("../../open-sse/handlers/chatCore/semanticCacheStore.ts");
+const { storeStreamingSemanticCacheResponse } =
+  await import("../../open-sse/handlers/chatCore/streamingSemanticCacheStore.ts");
+// Real truncation predicate — only the cache backend and the temperature/size
+// gates are stubbed, so these tests exercise the actual detection logic.
+const { isTruncatedCompletion } = await import("../../src/lib/semanticCache.ts");
+
+function deps(stored: unknown[]) {
+  return {
+    isCacheableForWrite: () => true,
+    isTruncatedCompletion,
+    isSmallEnoughForSemanticCache: () => true,
+    generateSignature: () => "sig",
+    setCachedResponse: (_s: unknown, _m: string, r: unknown, t: number) => stored.push({ r, t }),
+  } as never;
+}
+
+test("does not cache a non-streaming response truncated by max_tokens", () => {
+  const stored: unknown[] = [];
+  storeSemanticCacheResponse(
+    {
+      enabled: true,
+      body: { messages: [{ role: "user", content: "hi" }], temperature: 0 },
+      headers: undefined,
+      translatedResponse: {
+        choices: [{ finish_reason: "length", message: { content: "partial answ" } }],
+      },
+      model: "gemini-3.5-flash",
+      usage: { prompt_tokens: 10, completion_tokens: 93 },
+    },
+    deps(stored)
+  );
+  assert.equal(stored.length, 0, "truncated response must not be cached");
+});
+
+test("still caches a complete non-streaming response", () => {
+  const stored: unknown[] = [];
+  storeSemanticCacheResponse(
+    {
+      enabled: true,
+      body: { messages: [{ role: "user", content: "hi" }], temperature: 0 },
+      headers: undefined,
+      translatedResponse: {
+        choices: [{ finish_reason: "stop", message: { content: "complete answer" } }],
+      },
+      model: "gemini-3.5-flash",
+      usage: { prompt_tokens: 10, completion_tokens: 239 },
+    },
+    deps(stored)
+  );
+  assert.equal(stored.length, 1, "complete response must still be cached");
+});
+
+test("does not cache a streaming response truncated by max_tokens", () => {
+  const stored: unknown[] = [];
+  storeStreamingSemanticCacheResponse(
+    {
+      enabled: true,
+      streamStatus: 200,
+      streamResponseBody: {
+        choices: [{ finish_reason: "length", message: { content: "partial" } }],
+      },
+      body: { messages: [{ role: "user", content: "hi" }], temperature: 0 },
+      headers: undefined,
+      model: "gemini-3.5-flash",
+      streamUsage: { prompt_tokens: 10, completion_tokens: 93 },
+    },
+    deps(stored)
+  );
+  assert.equal(stored.length, 0, "truncated streaming response must not be cached");
+});
+
+test("still caches a complete streaming response", () => {
+  const stored: unknown[] = [];
+  storeStreamingSemanticCacheResponse(
+    {
+      enabled: true,
+      streamStatus: 200,
+      streamResponseBody: {
+        choices: [{ finish_reason: "stop", message: { content: "complete" } }],
+      },
+      body: { messages: [{ role: "user", content: "hi" }], temperature: 0 },
+      headers: undefined,
+      model: "gemini-3.5-flash",
+      streamUsage: { prompt_tokens: 10, completion_tokens: 239 },
+    },
+    deps(stored)
+  );
+  assert.equal(stored.length, 1, "complete streaming response must still be cached");
+});