diff --git a/.env.example b/.env.example
index 28a9f04b8f..32ad544497 100644
--- a/.env.example
+++ b/.env.example
@@ -2397,6 +2397,13 @@ APP_LOG_TO_FILE=true
 # Used by: open-sse/executors/cursor.ts.
 # CURSOR_TOOL_DIRECTIVE=1
 
+# Operator-defined system prompt text appended to the system message AFTER
+# translation (post-translation injection), so it reaches codex/Responses and
+# /v1/messages paths. Also used as the directive prefix stripped from echoed
+# system preamble blocks. Leave unset to disable.
+# Used by: open-sse/translator/request/claude-to-openai.ts, open-sse/translator/response/openai-to-claude.ts.
+# OMNIROUTE_SYSTEM_INSTRUCTION_APPEND=
+
 # Per-image fetch timeout (ms) for remote image_url vision input. Default: 15000.
 # Used by: open-sse/utils/cursorImages.ts.
 # CURSOR_IMAGE_FETCH_TIMEOUT_MS=15000
diff --git a/changelog.d/fixes/12904-inject-global-system-prompt-post-translation.md b/changelog.d/fixes/12904-inject-global-system-prompt-post-translation.md
new file mode 100644
index 0000000000..d4bdb433bf
--- /dev/null
+++ b/changelog.d/fixes/12904-inject-global-system-prompt-post-translation.md
@@ -0,0 +1 @@
+- fix(sse): inject the operator's global system prompt once, after request translation, for every target shape (Claude, Gemini, OpenAI Responses, OpenAI/Codex messages) instead of before translation — the pre-translation injection could be lost, repositioned, or duplicated 2-3× depending on the target format, and never reached the Responses API path at all. The new `injectSystemPromptPostTranslation()` is idempotent per request via a non-enumerable marker (#12904)
diff --git a/docs/reference/ENVIRONMENT.md b/docs/reference/ENVIRONMENT.md
index 0cd8db6e45..2ada70c885 100644
--- a/docs/reference/ENVIRONMENT.md
+++ b/docs/reference/ENVIRONMENT.md
@@ -1185,6 +1185,7 @@ changing them requires a code edit, not an env var:
 | `CURSOR_STREAM_TIMEOUT_MS`       | `300000`            | `open-sse/executors/cursor.ts`             | Stream idle timeout (ms) for the Cursor executor.                                            |
 | `CURSOR_KV_GRACE_MS`             | `2000`              | `open-sse/executors/cursor.ts`             | Grace window (ms) after a composer kv_after_text soft terminator when bytes remain buffered — gives a trailing exec_mcp tool call time to complete its frame. |
 | `CURSOR_TOOL_DIRECTIVE`          | enabled (`!== "0"`) | `open-sse/executors/cursor.ts`             | Tool-commit directive that makes composer-2.5 reliably issue tool calls. Set `0` to disable. |
+| `OMNIROUTE_SYSTEM_INSTRUCTION_APPEND` | _(unset)_ | `open-sse/translator/request/claude-to-openai.ts`, `open-sse/translator/response/openai-to-claude.ts` | Operator-defined system prompt text appended to the system message AFTER translation (post-translation injection), reaching codex/Responses and `/v1/messages` paths. Also used as the directive prefix stripped from echoed system preamble blocks. Leave unset to disable. |
 | `CURSOR_IMAGE_FETCH_TIMEOUT_MS`  | `15000`             | `open-sse/utils/cursorImages.ts`           | Per-image fetch timeout (ms) for remote `image_url` vision input.                            |
 | `CURSOR_STATE_DB_PATH`           | _(probed)_          | `open-sse/utils/cursorVersionDetector.ts`  | Override the Cursor IDE state DB lookup used for IDE version detection.                      |
 | `CURSOR_AGENT_CLI_VERSION`       | _(detect / pin)_    | `open-sse/utils/cursorAgentCliVersion.ts`  | Agent CLI build id (`YYYY.MM.DD-<hash>`) for `x-cursor-client-version: cli-…` on Agent Run.   |
diff --git a/open-sse/handlers/chatCore.ts b/open-sse/handlers/chatCore.ts
index d86df398c3..02504fa2ab 100644
--- a/open-sse/handlers/chatCore.ts
+++ b/open-sse/handlers/chatCore.ts
@@ -123,7 +123,11 @@ import { resolveChatCoreTargetFormat } from "./chatCore/targetFormat.ts";
 import { resolveOmniGlyphTransport } from "../services/compression/imageTransportPolicy.ts";
 import { stripStore, usesClaudeBridge } from "./chatCore/agentRouterProtocol.ts";
 import { normalizeClaudeToolsForDispatch } from "./chatCore/claudeToolDefaults.ts";
-import { injectSystemPrompt, injectCustomSystemPrompt } from "../services/systemPrompt.ts";
+import {
+  injectCustomSystemPrompt,
+  injectSystemPromptPostTranslation,
+  injectSystemPromptPreTranslation,
+} from "../services/systemPrompt.ts";
 import { translateRequest, needsTranslation } from "../translator/index.ts";
 import { applyReasoningRuleDirective } from "@/lib/reasoningRouting/policy";
 import { withReasoningRuleContext } from "../utils/reasoningRuleContext.ts";
@@ -595,7 +599,6 @@ export async function handleChatCore({
     };
   };
   let tokensCompressed: number | null = null;
-  body = injectSystemPrompt(body);
   // ── Per-endpoint custom system prompt (port of upstream #2063) ──
   // Reads from cachedSettings if available (passed in from combo/chat layer)
   // to avoid an extra DB read on the hot path. Falls through to getCachedSettings()
@@ -2505,6 +2508,12 @@ export async function handleChatCore({
         model || "",
         sourceFormat
       );
+      // Carrier-less targets (kiro / antigravity) have no post-translation
+      // system carrier for the single pass at ~3068 to write into — inject
+      // into the client body BEFORE translation so their user-merge /
+      // relocation paths carry the global prompt (baseline coverage of the
+      // removed pre-translation pass). The gate writes ONE carrier only.
+      translatedBody = injectSystemPromptPreTranslation(translatedBody, { targetFormat });
       translatedBody = translateRequest(
         sourceFormat,
         targetFormat,
@@ -3097,6 +3106,19 @@ export async function handleChatCore({
         isOpencodeClient,
       });
 
+      // Global System Prompt — SINGLE injection point (post-translation) for
+      // carrier-ful targets. The old unconditional pre-translation pass
+      // (former chatCore injectSystemPrompt call) was removed: it chained
+      // with this pass to inject prefix/suffix 2-3x and dual-wrote
+      // body.system + messages[] on the claude path, which strict upstreams
+      // (HCP-Vision vLLM: "System message must be at the beginning") reject
+      // with 400. Format-aware via targetFormat: messages[] (openai/codex —
+      // prefix FIRST system, suffix LAST), claude `system` field, gemini
+      // `systemInstruction`, responses `instructions`. Carrier-less targets
+      // (kiro user-fold, antigravity Cloud Code envelope) are covered by the
+      // gated PRE-translation pass before translateRequest instead.
+      bodyToSend = injectSystemPromptPostTranslation(bodyToSend, { targetFormat });
+
       updatePendingScope(pendingScope, {
         providerRequest: bodyToSend,
         stage: "payload_prepared",
diff --git a/open-sse/services/systemPrompt.ts b/open-sse/services/systemPrompt.ts
index 8f4ef69a86..01e011eadd 100644
--- a/open-sse/services/systemPrompt.ts
+++ b/open-sse/services/systemPrompt.ts
@@ -93,6 +93,7 @@ export function injectSystemPrompt<T>(body: T): T {
   if (!prefix && !suffix) return body;
   if (!isRecord(body)) return body;
   if (body._skipSystemPrompt) return body;
+  if (body._systemPromptInjected) return body;
 
   const result: Record<string, unknown> = { ...body };
 
@@ -143,9 +144,305 @@ export function injectSystemPrompt<T>(body: T): T {
     }
   }
 
+  markInjected(result);
   return Object.assign({}, body, result);
 }
 
+/**
+ * Prepend `text` to a message content (string or array form).
+ */
+function prependToContent(msg: Record<string, unknown>, text: string): void {
+  if (Array.isArray(msg.content)) {
+    msg.content = [{ type: "text", text }, ...msg.content];
+  } else {
+    msg.content = text + "\n\n" + (msg.content || "");
+  }
+}
+
+/**
+ * Append `text` to a message content (string or array form).
+ */
+function appendToContent(msg: Record<string, unknown>, text: string): void {
+  if (Array.isArray(msg.content)) {
+    msg.content = [...msg.content, { type: "text", text }];
+  } else {
+    msg.content = (msg.content || "") + "\n\n" + text;
+  }
+}
+
+// Non-enumerable marker: survives property access for the retry-loop guard,
+// invisible to JSON.stringify so it never leaks into the upstream request body.
+function markInjected(body: Record<string, unknown>): void {
+  try {
+    Object.defineProperty(body, "_systemPromptInjected", { value: true, enumerable: false });
+  } catch {
+    /* frozen/non-object edge — ignore */
+  }
+}
+
+/**
+ * Inject system prompts into a POST-TRANSLATION request body.
+ *
+ * Coverage model (the legacy unconditional pre-translation pass was removed —
+ * it chained into double injection): coverage = this format-aware
+ * post-translation pass for carrier-ful targets, plus the gated
+ * PRE-translation pass (injectSystemPromptPreTranslation) for carrier-less
+ * targets (kiro user-fold, antigravity Cloud Code envelope).
+ *
+ * Format-aware (opts.targetFormat) system carriers per target:
+ *   - claude: `system` field (string or {type:"text"} block array)
+ *   - gemini: `systemInstruction` ({ role, parts: [{ text }] })
+ *   - openai-responses: `instructions` string
+ *   - openai/codex (default): messages[] system/developer roles — prefix on
+ *     the FIRST and suffix on the LAST so the suffix retains the highest
+ *     recency position, preserving the "After Prompt" semantics.
+ *
+ * @param {object} body - Translated request body (target shape resolved)
+ * @param {object} [opts] - `{ targetFormat }` from the resolved wire target
+ * @returns {object} Modified body
+ */
+export function injectSystemPromptPostTranslation(body, opts?: { targetFormat?: string }) {
+  const cfg = getConfig();
+  if (!cfg.enabled) return body;
+  const prefix = cfg.prefixPrompt || "";
+  const suffix = cfg.suffixPrompt || "";
+  if (!prefix && !suffix) return body;
+  if (!body || typeof body !== "object") return body;
+  if (body._skipSystemPrompt) return body;
+  if (body._systemPromptInjected) return body;
+  const targetFormat = opts?.targetFormat || "";
+  const combined = [prefix, suffix].filter(Boolean).join("\n\n");
+
+  const result = { ...body };
+
+  // Claude-format body (separate `system` field, or a claude target whose
+  // translated body has no system-role message to carry the prompt): inject
+  // into body.system — a system-role message inside claude messages[] is
+  // invalid there. When the translated body has no system field at all, CREATE
+  // it (combined) — previously this body shape fell through the messages[]
+  // early-return and silently got zero injection.
+  if (targetFormat === "claude" || result.system !== undefined) {
+    const hasSystemRole =
+      Array.isArray(result.messages) &&
+      result.messages.some((m) => m && (m.role === "system" || m.role === "developer"));
+    if (!hasSystemRole) {
+      if (typeof result.system === "string") {
+        let sys = result.system;
+        if (prefix) sys = prefix + "\n\n" + sys;
+        if (suffix) sys = sys + "\n\n" + suffix;
+        result.system = sys;
+      } else if (Array.isArray(result.system)) {
+        let arr = [...result.system];
+        if (prefix) arr = [{ type: "text", text: prefix }, ...arr];
+        if (suffix) arr = [...arr, { type: "text", text: suffix }];
+        result.system = arr;
+      } else {
+        result.system = combined;
+      }
+      markInjected(result);
+      return result;
+    }
+  }
+
+  // Gemini-format body (contents[] + systemInstruction): inject into the
+  // systemInstruction parts (real translator shape: { role: "system",
+  // parts: [{ text }] }, see translator/request/claude-to-gemini.ts:95). If
+  // absent, create it — a messages-less gemini body previously fell through
+  // the messages[] early-return and silently got zero injection.
+  // Antigravity reaches 3068 as a Cloud Code envelope whose executor reads
+  // ONLY envelope.request (antigravity.ts:733) and which rejects unknown
+  // top-level fields with 400 (:813-815) — its coverage is restored by the
+  // gated pre-translation pass instead, so it must stay out of this branch.
+  if (targetFormat === "gemini" && !result.request) {
+    if (result.systemInstruction && typeof result.systemInstruction === "object") {
+      const si = result.systemInstruction as { role?: string; parts?: unknown[] };
+      const parts = Array.isArray(si.parts) ? [...si.parts] : [];
+      if (prefix) parts.unshift({ text: prefix });
+      if (suffix) parts.push({ text: suffix });
+      result.systemInstruction = { ...si, role: si.role || "system", parts };
+    } else {
+      const texts = [prefix, suffix].filter(Boolean);
+      result.systemInstruction = { role: "system", parts: texts.map((text) => ({ text })) };
+    }
+    markInjected(result);
+    return result;
+  }
+
+  // OpenAI Responses-format body (input + instructions): instructions is a
+  // plain string — wrap once. If absent, create it with the combined prompt.
+  // Do NOT touch `input` (message items, not a system carrier).
+  if (targetFormat === "openai-responses") {
+    const base = typeof result.instructions === "string" ? result.instructions : "";
+    const parts = [prefix, base, suffix].filter(Boolean);
+    result.instructions = parts.join("\n\n");
+    markInjected(result);
+    return result;
+  }
+
+  // Kiro (conversationState/.../userInputMessage) has NO system carrier at all:
+  // openai-to-kiro.ts folds system messages into user turns wrapped in
+  // <system-reminder> tags (#2306) and the executor keeps no system slot.
+  // Injection here would require inventing a carrier kiro upstreams reject —
+  // so kiro intentionally receives no global-prompt injection at this seam.
+  if (targetFormat === "kiro") {
+    return result;
+  }
+
+  if (!result.messages || !Array.isArray(result.messages)) return result;
+
+  result.messages = [...result.messages];
+  const indices: number[] = [];
+  for (let i = 0; i < result.messages.length; i++) {
+    const m = result.messages[i] as { role?: string };
+    if (m && (m.role === "system" || m.role === "developer")) indices.push(i);
+  }
+
+  if (indices.length === 0) {
+    // No system message — combine both into one at the front (same as injectSystemPrompt).
+    if (combined) {
+      result.messages = [{ role: "system", content: combined }, ...result.messages];
+    }
+    markInjected(result);
+    return result;
+  }
+
+  if (prefix) {
+    const firstIdx = indices[0];
+    result.messages[firstIdx] = { ...result.messages[firstIdx] };
+    prependToContent(result.messages[firstIdx] as Record<string, unknown>, prefix);
+  }
+  if (suffix) {
+    const lastIdx = indices[indices.length - 1];
+    if (lastIdx !== indices[0]) {
+      result.messages[lastIdx] = { ...result.messages[lastIdx] };
+    }
+    appendToContent(result.messages[lastIdx] as Record<string, unknown>, suffix);
+  }
+  markInjected(result);
+  return result;
+}
+
+/**
+ * Gated PRE-translation injection for targets with NO post-translation system
+ * carrier. Two such targets exist:
+ *   - kiro: openai-to-kiro.ts folds system messages into user turns wrapped in
+ *     <system-reminder> tags (#2306) — reads body.messages system roles only
+ *     (:283-284/:872), no body.system, no system slot in the Kiro payload.
+ *   - antigravity: the translator wraps the payload in a Cloud Code envelope
+ *     ({project, requestId, request:{contents, systemInstruction, ...}}) and
+ *     the executor reads ONLY envelope.request (antigravity.ts:733/:417-425);
+ *     the envelope rejects unknown top-level fields with 400 (:813-815), and
+ *     envelope.request.systemInstruction is overwritten with
+ *     ANTIGRAVITY_DEFAULT_SYSTEM after relocating client system content into
+ *     the first user message (openai-to-gemini.ts:716-730). Post-translation
+ *     injection at chatCore 3068 cannot reach the real carrier for either.
+ *
+ * Pre-translation the client body reaches this gate in one of four shapes,
+ * ALL covered here: messages[] (openai/codex source), claude `system` field
+ * (string), responses `input` + `instructions` (hub translation promotes
+ * instructions to a system message, openai-responses.ts:205-207), and gemini
+ * `contents` + `systemInstruction`. Not covered — and rejected by the guards
+ * above — are bodies with none of these carriers (empty/no-op return).
+ *
+ * SINGLE-CARRIER guarantee: writes into exactly ONE carrier — never both. The
+ * removed pass dual-wrote messages[] AND body.system; both would survive
+ * translation and fold ×2.
+ *
+ * @param {object} body - PRE-translation request body (client format)
+ * @param {object} [opts] - `{ targetFormat }` of the resolved wire target
+ * @returns {object} Modified body (or the original when gated out)
+ */
+export function injectSystemPromptPreTranslation(body, opts?: { targetFormat?: string }) {
+  const cfg = getConfig();
+  if (!cfg.enabled) return body;
+  const prefix = cfg.prefixPrompt || "";
+  const suffix = cfg.suffixPrompt || "";
+  if (!prefix && !suffix) return body;
+  if (!body || typeof body !== "object") return body;
+  if (body._skipSystemPrompt) return body;
+  if (body._systemPromptInjected) return body;
+
+  const targetFormat = opts?.targetFormat || "";
+  // Carrier-ful targets (openai, codex, claude, gemini, openai-responses,
+  // cursor) receive their injection at the single post-translation pass
+  // (chatCore 3068) — pre-injecting here would chain into a double injection.
+  const CARRIERLESS_TARGETS = new Set(["kiro", "antigravity"]);
+  if (!CARRIERLESS_TARGETS.has(targetFormat)) return body;
+
+  const combined = [prefix, suffix].filter(Boolean).join("\n\n");
+  const result = { ...body };
+
+  // Claude-source client body: the `system` field is the authoritative carrier
+  // (#2468 ordering — prefix → client content → suffix). Checked FIRST so a
+  // body that also carries messages[] (user/assistant turns) never gets a
+  // second write into messages.
+  if (typeof result.system === "string") {
+    let sys = result.system;
+    if (prefix) sys = prefix + "\n\n" + sys;
+    if (suffix) sys = sys + "\n\n" + suffix;
+    result.system = sys;
+    markInjected(result);
+    return result;
+  }
+  if (Array.isArray(result.system)) {
+    let arr = [...result.system];
+    if (prefix) arr = [{ type: "text", text: prefix }, ...arr];
+    if (suffix) arr = [...arr, { type: "text", text: suffix }];
+    result.system = arr;
+    markInjected(result);
+    return result;
+  }
+
+  // Responses-source client body (input + instructions): wrap the instructions
+  // string once — the hub translation promotes it to a system message
+  // (openai-responses.ts:205-207) which the target then folds. Do NOT touch
+  // `input` (message items, not a system carrier).
+  if (Array.isArray(result.input)) {
+    const base = typeof result.instructions === "string" ? result.instructions : "";
+    result.instructions = [prefix, base, suffix].filter(Boolean).join("\n\n");
+    markInjected(result);
+    return result;
+  }
+
+  // Gemini-source client body (contents + systemInstruction): inject into the
+  // parts once each; create the carrier when absent.
+  if (result.contents !== undefined) {
+    if (result.systemInstruction && typeof result.systemInstruction === "object") {
+      const si = result.systemInstruction as { role?: string; parts?: unknown[] };
+      const parts = Array.isArray(si.parts) ? [...si.parts] : [];
+      if (prefix) parts.unshift({ text: prefix });
+      if (suffix) parts.push({ text: suffix });
+      result.systemInstruction = { ...si, role: si.role || "system", parts };
+    } else {
+      const texts = [prefix, suffix].filter(Boolean);
+      result.systemInstruction = { role: "system", parts: texts.map((text) => ({ text })) };
+    }
+    markInjected(result);
+    return result;
+  }
+
+  // OpenAI-style client body: write into the system/developer message only.
+  if (Array.isArray(result.messages)) {
+    result.messages = [...result.messages];
+    const sysIdx = result.messages.findIndex(
+      (m) => m && (m.role === "system" || m.role === "developer")
+    );
+    if (sysIdx >= 0) {
+      result.messages[sysIdx] = { ...result.messages[sysIdx] };
+      if (prefix) prependToContent(result.messages[sysIdx] as Record<string, unknown>, prefix);
+      if (suffix) appendToContent(result.messages[sysIdx] as Record<string, unknown>, suffix);
+    } else {
+      if (combined) {
+        result.messages = [{ role: "system", content: combined }, ...result.messages];
+      }
+    }
+    markInjected(result);
+    return result;
+  }
+
+  return result;
+}
+
 /**
  * Inject a per-request custom system prompt into the request body.
  *
diff --git a/open-sse/translator/request/claude-to-openai.ts b/open-sse/translator/request/claude-to-openai.ts
index 68a19558a0..f8883a9e62 100644
--- a/open-sse/translator/request/claude-to-openai.ts
+++ b/open-sse/translator/request/claude-to-openai.ts
@@ -165,6 +165,30 @@ export function claudeToOpenAIRequest(model, body, stream, credentials: unknown
     }
   }
 
+  // #reasoning-bilingual: DeepSeek-V4 and similar models emit user-facing text in the
+  // user's language (e.g. Korean) then continue with English planning/chain-of-thought
+  // in the same content field instead of using reasoning_content. Same mitigation as
+  // translator/response/openai-to-claude.ts's directivePreambleStripper.ts: when the
+  // operator configured OMNIROUTE_SYSTEM_INSTRUCTION_APPEND, append it here to the
+  // (system) message so the directive reaches the model on the /v1/messages (Claude
+  // Messages -> OpenAI Chat Completions) path too.
+  const systemAppend = process.env.OMNIROUTE_SYSTEM_INSTRUCTION_APPEND?.trim();
+  if (systemAppend) {
+    const sysIndex = result.messages.findIndex((m) => m.role === "system");
+    if (sysIndex >= 0) {
+      const sys = result.messages[sysIndex];
+      if (typeof sys.content === "string") {
+        sys.content = sys.content + "\n\n" + systemAppend;
+      } else if (Array.isArray(sys.content)) {
+        (sys.content as JsonRecord[]).push({ type: "text", text: systemAppend });
+      } else {
+        sys.content = systemAppend;
+      }
+    } else {
+      result.messages.unshift({ role: "system", content: systemAppend });
+    }
+  }
+
   // Convert messages
   if (body.messages && Array.isArray(body.messages)) {
     for (let i = 0; i < body.messages.length; i++) {
diff --git a/tests/unit/global-prompt-single-injection.test.ts b/tests/unit/global-prompt-single-injection.test.ts
new file mode 100644
index 0000000000..d06dd55f59
--- /dev/null
+++ b/tests/unit/global-prompt-single-injection.test.ts
@@ -0,0 +1,379 @@
+import test from "node:test";
+import { strict as assert } from "node:assert";
+
+const {
+  setSystemPromptConfig,
+  injectSystemPromptPostTranslation,
+  injectSystemPromptPreTranslation,
+} = await import("../../open-sse/services/systemPrompt.ts");
+
+const PREFIX = "PREFIX-RULES";
+const SUFFIX = "SUFFIX-RULES";
+
+function resetConfig() {
+  setSystemPromptConfig({ enabled: true, prefixPrompt: PREFIX, suffixPrompt: SUFFIX });
+}
+
+function countOccurrences(s, needle) {
+  return s.split(needle).length - 1;
+}
+
+test("idempotence: second application is a no-op (ONE copy of prefix/suffix)", () => {
+  resetConfig();
+  const body = {
+    messages: [
+      { role: "system", content: "ORIG" },
+      { role: "user", content: "hi" },
+    ],
+  };
+  const once = injectSystemPromptPostTranslation(body);
+  const twice = injectSystemPromptPostTranslation(once);
+  assert.equal(twice.messages.length, once.messages.length, "no message growth on second pass");
+  assert.equal(countOccurrences(String(twice.messages[0].content), PREFIX), 1);
+  assert.equal(countOccurrences(String(twice.messages[0].content), SUFFIX), 1);
+});
+
+test("no client system -> exactly one combined system inserted", () => {
+  resetConfig();
+  const body = { messages: [{ role: "user", content: "hi" }] };
+  const out = injectSystemPromptPostTranslation(body);
+  const systems = out.messages.filter((m) => m.role === "system");
+  assert.equal(systems.length, 1);
+  assert.ok(String(systems[0].content).startsWith(PREFIX));
+  assert.ok(String(systems[0].content).endsWith(SUFFIX));
+});
+
+test("claude-format body: system field gets injection, messages untouched", () => {
+  resetConfig();
+  const body = { system: "CLIENT", messages: [{ role: "user", content: "hi" }] };
+  const out = injectSystemPromptPostTranslation(body);
+  assert.equal(out.system, "PREFIX-RULES\n\nCLIENT\n\nSUFFIX-RULES");
+  assert.ok(
+    !out.messages.some((m) => m.role === "system"),
+    "no system-role message inside claude messages"
+  );
+});
+
+test("idempotence flag does not leak into upstream JSON", () => {
+  resetConfig();
+  const body = { messages: [{ role: "user", content: "hi" }] };
+  const out = injectSystemPromptPostTranslation(body);
+  assert.equal(
+    JSON.stringify(out).includes("_systemPromptInjected"),
+    false,
+    "flag must stay non-enumerable"
+  );
+});
+
+test("multi-system codex semantics preserved: prefix on first, suffix on last", () => {
+  resetConfig();
+  const body = {
+    messages: [
+      { role: "system", content: "A" },
+      { role: "developer", content: "B" },
+      { role: "user", content: "hi" },
+    ],
+  };
+  const out = injectSystemPromptPostTranslation(body);
+  assert.equal(countOccurrences(String(out.messages[0].content), PREFIX), 1);
+  assert.equal(countOccurrences(String(out.messages[1].content), SUFFIX), 1);
+});
+
+test("skip flag respected", () => {
+  resetConfig();
+  const body = { _skipSystemPrompt: true, messages: [{ role: "user", content: "hi" }] };
+  const out = injectSystemPromptPostTranslation(body);
+  assert.equal(out.messages.length, 1, "no injection when _skipSystemPrompt");
+});
+
+test("claude-format body WITHOUT system field: combined goes to body.system, not messages", () => {
+  resetConfig();
+  const body = { messages: [{ role: "user", content: "hi" }] };
+  const out = injectSystemPromptPostTranslation(body, { targetFormat: "claude" });
+  assert.equal(out.system, "PREFIX-RULES\n\nSUFFIX-RULES");
+  assert.ok(
+    !out.messages.some((m) => m.role === "system"),
+    "claude target must never get a system-role message in messages"
+  );
+});
+
+test("claude-format array system: prefix/suffix as text blocks, once each", () => {
+  resetConfig();
+  const body = {
+    system: [{ type: "text", text: "CLIENT" }],
+    messages: [{ role: "user", content: "hi" }],
+  };
+  const out = injectSystemPromptPostTranslation(body, { targetFormat: "claude" });
+  assert.deepEqual(out.system, [
+    { type: "text", text: "PREFIX-RULES" },
+    { type: "text", text: "CLIENT" },
+    { type: "text", text: "SUFFIX-RULES" },
+  ]);
+  assert.ok(!out.messages.some((m) => m.role === "system"));
+});
+
+test("gemini-format body: systemInstruction parts get prefix/suffix once each", () => {
+  resetConfig();
+  // Real shape produced by open-sse/translator/request/claude-to-gemini.ts:95-97
+  // and openai-to-gemini.ts:350-356: { role: "system", parts: [{ text }] }
+  const body = {
+    contents: [{ role: "user", parts: [{ text: "hi" }] }],
+    systemInstruction: { role: "system", parts: [{ text: "CLIENT" }] },
+  };
+  const out = injectSystemPromptPostTranslation(body, { targetFormat: "gemini" });
+  const texts = out.systemInstruction.parts.map((p) => p.text);
+  assert.equal(texts.filter((t) => t === "PREFIX-RULES").length, 1);
+  assert.equal(texts.filter((t) => t === "SUFFIX-RULES").length, 1);
+  assert.ok(texts.includes("CLIENT"));
+});
+
+test("gemini-format body without systemInstruction: combined systemInstruction created", () => {
+  resetConfig();
+  const body = { contents: [{ role: "user", parts: [{ text: "hi" }] }] };
+  const out = injectSystemPromptPostTranslation(body, { targetFormat: "gemini" });
+  const texts = out.systemInstruction.parts.map((p) => p.text).join("|");
+  assert.equal(countOccurrences(texts, "PREFIX-RULES"), 1);
+  assert.equal(countOccurrences(texts, "SUFFIX-RULES"), 1);
+  assert.equal(out.contents.length, 1, "contents untouched");
+});
+
+test("responses-format body: instructions wrapped once, input untouched", () => {
+  resetConfig();
+  // Real targetFormat value is FORMATS.OPENAI_RESPONSES = "openai-responses"
+  // (open-sse/translator/formats.ts), not "responses".
+  const body = {
+    model: "m",
+    input: [{ type: "message", role: "user", content: [{ type: "input_text", text: "hi" }] }],
+    instructions: "INSTR",
+  };
+  const out = injectSystemPromptPostTranslation(body, { targetFormat: "openai-responses" });
+  assert.equal(out.instructions, "PREFIX-RULES\n\nINSTR\n\nSUFFIX-RULES");
+  assert.equal(out.input.length, 1);
+});
+
+test("responses-format body without instructions: combined instructions created", () => {
+  resetConfig();
+  const body = {
+    model: "m",
+    input: [{ type: "message", role: "user", content: [{ type: "input_text", text: "hi" }] }],
+  };
+  const out = injectSystemPromptPostTranslation(body, { targetFormat: "openai-responses" });
+  assert.equal(out.instructions, "PREFIX-RULES\n\nSUFFIX-RULES");
+});
+
+// ---- Round 2: gated PRE-translation pass for carrier-less targets ----
+
+// kiro: openai-to-kiro.ts:283-284/872 reads body.messages system roles ONLY
+// (no body.system carrier exists). The gate must write into messages[] exactly
+// once so the translator's <system-reminder> fold carries prefix+suffix once.
+test("pre-translation gate: kiro target gets single messages[] injection", () => {
+  resetConfig();
+  const body = {
+    messages: [
+      { role: "system", content: "SYS" },
+      { role: "user", content: "hi" },
+    ],
+  };
+  const out = injectSystemPromptPreTranslation(body, { targetFormat: "kiro" });
+  const sysMsg = out.messages.find((m) => m.role === "system");
+  assert.ok(sysMsg, "system message preserved");
+  assert.equal(countOccurrences(String(sysMsg.content), PREFIX), 1);
+  assert.equal(countOccurrences(String(sysMsg.content), SUFFIX), 1);
+});
+
+test("pre-translation gate: kiro target with array system content gets text blocks", () => {
+  resetConfig();
+  const body = {
+    messages: [
+      { role: "system", content: [{ type: "text", text: "SYS" }] },
+      { role: "user", content: "hi" },
+    ],
+  };
+  const out = injectSystemPromptPreTranslation(body, { targetFormat: "kiro" });
+  const texts = out.messages[0].content.map((c) => c.text).join("|");
+  assert.equal(countOccurrences(texts, "PREFIX-RULES"), 1);
+  assert.equal(countOccurrences(texts, "SUFFIX-RULES"), 1);
+});
+
+test("pre-translation gate: kiro target without system message inserts combined system", () => {
+  resetConfig();
+  const body = { messages: [{ role: "user", content: "hi" }] };
+  const out = injectSystemPromptPreTranslation(body, { targetFormat: "kiro" });
+  assert.equal(out.messages[0].role, "system");
+  assert.equal(countOccurrences(String(out.messages[0].content), PREFIX), 1);
+  assert.equal(countOccurrences(String(out.messages[0].content), SUFFIX), 1);
+});
+
+test("pre-translation gate: idempotent on second application", () => {
+  resetConfig();
+  const body = {
+    messages: [
+      { role: "system", content: "SYS" },
+      { role: "user", content: "hi" },
+    ],
+  };
+  const once = injectSystemPromptPreTranslation(body, { targetFormat: "kiro" });
+  const twice = injectSystemPromptPreTranslation(once, { targetFormat: "kiro" });
+  assert.equal(twice.messages.length, once.messages.length);
+  assert.equal(countOccurrences(String(twice.messages[0].content), PREFIX), 1);
+});
+
+test("pre-translation gate: does NOT dual-write — system field wins, messages untouched", () => {
+  resetConfig();
+  // Round-3 M-2: for a body carrying BOTH carriers, the claude-source `system`
+  // field is the authoritative one (#2468 ordering: prefix → client → suffix);
+  // messages must stay untouched so the wrap is never duplicated.
+  const body = {
+    system: "STRAY",
+    messages: [
+      { role: "system", content: "SYS" },
+      { role: "user", content: "hi" },
+    ],
+  };
+  const out = injectSystemPromptPreTranslation(body, { targetFormat: "kiro" });
+  assert.equal(
+    out.system,
+    "PREFIX-RULES\n\nSTRAY\n\nSUFFIX-RULES",
+    "system field is the single carrier"
+  );
+  assert.equal(
+    countOccurrences(String(out.messages[0].content), PREFIX),
+    0,
+    "messages untouched — dual-write would duplicate upstream"
+  );
+});
+
+test("pre-translation gate: no-op for carrier-ful targets (openai handled at 3068)", () => {
+  resetConfig();
+  const body = {
+    messages: [
+      { role: "system", content: "SYS" },
+      { role: "user", content: "hi" },
+    ],
+  };
+  const out = injectSystemPromptPreTranslation(body, { targetFormat: "openai" });
+  assert.equal(
+    countOccurrences(String(out.messages[0].content), PREFIX),
+    0,
+    "openai must get its injection post-translation, not pre"
+  );
+  assert.equal(out.messages[0].content, "SYS");
+});
+
+test("pre-translation gate: no-op when disabled or no prompts configured", () => {
+  setSystemPromptConfig({ enabled: false, prefixPrompt: PREFIX, suffixPrompt: SUFFIX });
+  const body = { messages: [{ role: "user", content: "hi" }] };
+  const out = injectSystemPromptPreTranslation(body, { targetFormat: "kiro" });
+  assert.equal(out.messages.length, 1);
+  setSystemPromptConfig({ enabled: true, prefixPrompt: "", suffixPrompt: "" });
+  const out2 = injectSystemPromptPreTranslation(body, { targetFormat: "kiro" });
+  assert.equal(out2.messages.length, 1);
+  resetConfig();
+});
+
+// antigravity: translator wraps the payload in a Cloud Code envelope and the
+// executor reads ONLY envelope.request (antigravity.ts:733/:417-425) — the
+// envelope rejects unknown top-level fields with 400 (:813-815). A top-level
+// systemInstruction created post-translation would be an invalid field, and
+// envelope.request.systemInstruction is already pinned to
+// ANTIGRAVITY_DEFAULT_SYSTEM (openai-to-gemini.ts:719) — so antigravity must be
+// gated PRE-translation into client messages, same as kiro.
+test("pre-translation gate: antigravity target gets single messages[] injection", () => {
+  resetConfig();
+  const body = {
+    messages: [
+      { role: "system", content: "SYS" },
+      { role: "user", content: "hi" },
+    ],
+  };
+  const out = injectSystemPromptPreTranslation(body, { targetFormat: "antigravity" });
+  const sysMsg = out.messages.find((m) => m.role === "system");
+  assert.ok(sysMsg, "system message preserved");
+  assert.equal(countOccurrences(String(sysMsg.content), PREFIX), 1);
+  assert.equal(countOccurrences(String(sysMsg.content), SUFFIX), 1);
+});
+
+test("pre-translation gate: antigravity without system message inserts combined", () => {
+  resetConfig();
+  const body = { messages: [{ role: "user", content: "hi" }] };
+  const out = injectSystemPromptPreTranslation(body, { targetFormat: "antigravity" });
+  assert.equal(out.messages[0].role, "system");
+  assert.equal(countOccurrences(String(out.messages[0].content), PREFIX), 1);
+  assert.equal(countOccurrences(String(out.messages[0].content), SUFFIX), 1);
+});
+
+test("gemini branch: antigravity envelope must NOT get a top-level systemInstruction created", () => {
+  resetConfig();
+  // 3068 receives the envelope itself for antigravity; the gemini branch must
+  // not fabricate an invalid top-level systemInstruction on it.
+  const body = {
+    project: "p",
+    requestId: "r",
+    request: { contents: [{ role: "user", parts: [{ text: "hi" }] }] },
+  };
+  const out = injectSystemPromptPostTranslation(body, { targetFormat: "antigravity" });
+  assert.equal(
+    out.systemInstruction,
+    undefined,
+    "no invalid top-level systemInstruction on a Cloud Code envelope"
+  );
+  assert.equal(out.messages, undefined, "envelope has no messages to mutate");
+});
+
+// ---- Round 3: source-shape coverage in the gate ----
+
+// I-NEW-1: a responses-source client (e.g. /v1/responses falling back to a kiro
+// connection) reaches the gate with {input, instructions} — no messages[], no
+// system field. The hub translation (openai-responses -> openai,
+// openai-responses.ts:205-207) promotes instructions to a system message, so
+// wrapping instructions here reaches the kiro <system-reminder> fold.
+test("pre-translation gate: responses-source body gets instructions wrapped once, input untouched", () => {
+  resetConfig();
+  const body = {
+    model: "m",
+    input: [{ type: "message", role: "user", content: [{ type: "input_text", text: "hi" }] }],
+    instructions: "INSTR",
+  };
+  const out = injectSystemPromptPreTranslation(body, { targetFormat: "kiro" });
+  assert.equal(out.instructions, "PREFIX-RULES\n\nINSTR\n\nSUFFIX-RULES");
+  assert.equal(out.input.length, 1, "input untouched");
+});
+
+test("pre-translation gate: responses-source without instructions creates combined", () => {
+  resetConfig();
+  const body = {
+    model: "m",
+    input: [{ type: "message", role: "user", content: [{ type: "input_text", text: "hi" }] }],
+  };
+  const out = injectSystemPromptPreTranslation(body, { targetFormat: "kiro" });
+  assert.equal(out.instructions, "PREFIX-RULES\n\nSUFFIX-RULES");
+  assert.equal(out.input.length, 1);
+});
+
+// gemini-source client (contents + systemInstruction) reaching a carrier-less
+// target: inject into systemInstruction parts exactly once.
+test("pre-translation gate: gemini-source body gets systemInstruction parts once each", () => {
+  resetConfig();
+  const body = {
+    contents: [{ role: "user", parts: [{ text: "hi" }] }],
+    systemInstruction: { role: "system", parts: [{ text: "CLIENT" }] },
+  };
+  const out = injectSystemPromptPreTranslation(body, { targetFormat: "antigravity" });
+  const texts = out.systemInstruction.parts.map((p) => p.text);
+  assert.equal(texts.filter((t) => t === "PREFIX-RULES").length, 1);
+  assert.equal(texts.filter((t) => t === "SUFFIX-RULES").length, 1);
+  assert.ok(texts.includes("CLIENT"));
+});
+
+test("pre-translation gate: gemini-source without systemInstruction creates combined parts", () => {
+  resetConfig();
+  const body = { contents: [{ role: "user", parts: [{ text: "hi" }] }] };
+  const out = injectSystemPromptPreTranslation(body, { targetFormat: "antigravity" });
+  const texts = out.systemInstruction.parts.map((p) => p.text).join("|");
+  assert.equal(countOccurrences(texts, "PREFIX-RULES"), 1);
+  assert.equal(countOccurrences(texts, "SUFFIX-RULES"), 1);
+  assert.equal(out.contents.length, 1, "contents untouched");
+});
+
+// M3: reset config so this file's settings never leak into other test files.
+test.after(() => setSystemPromptConfig({ enabled: false, prefixPrompt: "", suffixPrompt: "" }));
diff --git a/tests/unit/system-instruction-append-claude-to-openai.test.ts b/tests/unit/system-instruction-append-claude-to-openai.test.ts
new file mode 100644
index 0000000000..b7d81bdab7
--- /dev/null
+++ b/tests/unit/system-instruction-append-claude-to-openai.test.ts
@@ -0,0 +1,82 @@
+import test from "node:test";
+import assert from "node:assert/strict";
+
+const { claudeToOpenAIRequest } =
+  await import("../../open-sse/translator/request/claude-to-openai.ts");
+
+const APPEND_ENV = "OMNIROUTE_SYSTEM_INSTRUCTION_APPEND";
+const DIRECTIVE = "TEST-DIRECTIVE-ABC";
+
+function baseBody(withSystem: boolean) {
+  const body: Record<string, unknown> = {
+    messages: [{ role: "user", content: [{ type: "text", text: "Hello" }] }],
+  };
+  if (withSystem) body.system = [{ text: "Rule A" }];
+  return body;
+}
+
+function withEnv(directive: string | undefined, fn: () => void) {
+  const prev = process.env[APPEND_ENV];
+  if (directive === undefined) delete process.env[APPEND_ENV];
+  else process.env[APPEND_ENV] = directive;
+  try {
+    fn();
+  } finally {
+    if (prev === undefined) delete process.env[APPEND_ENV];
+    else process.env[APPEND_ENV] = prev;
+  }
+}
+
+test("claude-to-openai appends the directive to an existing string system message (tail recency)", () => {
+  withEnv(DIRECTIVE, () => {
+    const result = claudeToOpenAIRequest("deepseek-v4", baseBody(true), false);
+    const sys = result.messages[0] as { role: string; content: unknown };
+    assert.equal(sys.role, "system");
+    assert.equal(typeof sys.content, "string");
+    const content = sys.content as string;
+    assert.ok(content.includes("Rule A"));
+    assert.ok(content.includes(DIRECTIVE), "directive must reach the system message");
+    assert.ok(
+      content.endsWith(DIRECTIVE),
+      "directive must be appended at the tail (highest recency)"
+    );
+  });
+});
+
+test("claude-to-openai creates a system message carrying the directive when body has none", () => {
+  withEnv(DIRECTIVE, () => {
+    const result = claudeToOpenAIRequest("deepseek-v4", baseBody(false), false);
+    const sys = result.messages[0] as { role: string; content: unknown };
+    assert.equal(sys.role, "system");
+    assert.equal(sys.content, DIRECTIVE);
+  });
+});
+
+test("claude-to-openai appends the directive to array system content as a text block", () => {
+  withEnv(DIRECTIVE, () => {
+    const result = claudeToOpenAIRequest(
+      "deepseek-v4",
+      {
+        messages: [{ role: "user", content: [{ type: "text", text: "Hello" }] }],
+        system: [{ text: "Block A", cache_control: { type: "ephemeral" } }],
+      },
+      false,
+      { _preserveCacheControl: true }
+    );
+    const sys = result.messages[0] as { role: string; content: unknown };
+    assert.equal(sys.role, "system");
+    assert.ok(Array.isArray(sys.content), "array system content must stay an array");
+    const blocks = sys.content as Array<{ type: string; text: string }>;
+    const lastBlock = blocks[blocks.length - 1];
+    assert.equal(lastBlock.type, "text");
+    assert.ok(lastBlock.text.includes(DIRECTIVE), "directive block must be appended last");
+  });
+});
+
+test("claude-to-openai does not inject when env is unset", () => {
+  withEnv(undefined, () => {
+    const result = claudeToOpenAIRequest("deepseek-v4", baseBody(true), false);
+    const sys = result.messages[0] as { role: string; content: unknown };
+    assert.equal(sys.content, "Rule A");
+  });
+});
diff --git a/tests/unit/system-prompt.test.ts b/tests/unit/system-prompt.test.ts
index 796745767e..7fb95fcb53 100644
--- a/tests/unit/system-prompt.test.ts
+++ b/tests/unit/system-prompt.test.ts
@@ -1,8 +1,12 @@
 import test from "node:test";
 import assert from "node:assert/strict";
 
-const { injectSystemPrompt, setSystemPromptConfig, getSystemPromptConfig } =
-  await import("../../open-sse/services/systemPrompt.ts");
+const {
+  injectSystemPrompt,
+  injectSystemPromptPostTranslation,
+  setSystemPromptConfig,
+  getSystemPromptConfig,
+} = await import("../../open-sse/services/systemPrompt.ts");
 
 // ─── Config ─────────────────────────────────────────────────────────────────
 
@@ -180,5 +184,112 @@ test("injectSystemPrompt: developer role treated as system", () => {
   assert.ok(result.messages[0].content.trimEnd().endsWith("SUF"));
 });
 
+// ─── Post-translation injection (codex/Responses path — #3) ────────────────
+// injectSystemPromptPostTranslation runs AFTER translation, on a body whose
+// messages[] already contains the resolved system/developer messages. The key
+// difference from injectSystemPrompt: with multiple system/developer messages
+// (codex sends a developer role per input item), prefix goes on the FIRST and
+// suffix on the LAST — so suffix retains the highest recency position.
+
+test("postTranslation: single system → prefix front, suffix back", () => {
+  setSystemPromptConfig({ enabled: true, prefixPrompt: "PRE", suffixPrompt: "SUF" });
+  const body = {
+    messages: [
+      { role: "system", content: "Original" },
+      { role: "user", content: "hi" },
+    ],
+  };
+  const result = injectSystemPromptPostTranslation(body);
+  assert.ok(result.messages[0].content.startsWith("PRE"));
+  assert.ok(result.messages[0].content.includes("Original"));
+  assert.ok(result.messages[0].content.trimEnd().endsWith("SUF"));
+  assert.equal(result.messages.length, 2);
+});
+
+test("postTranslation: multiple system/developer → prefix on first, suffix on LAST", () => {
+  setSystemPromptConfig({ enabled: true, prefixPrompt: "PRE", suffixPrompt: "SUF" });
+  // codex-like: several developer (system-equivalent) messages then a user
+  const body = {
+    messages: [
+      { role: "developer", content: "dev-0" },
+      { role: "developer", content: "dev-1" },
+      { role: "developer", content: "dev-2" },
+      { role: "user", content: "hi" },
+    ],
+  };
+  const result = injectSystemPromptPostTranslation(body);
+  assert.equal(result.messages.length, 4);
+  // prefix on FIRST system/developer (index 0)
+  assert.ok(result.messages[0].content.startsWith("PRE"));
+  assert.ok(result.messages[0].content.includes("dev-0"));
+  // suffix on LAST system/developer (index 2) — NOT index 0
+  assert.ok(result.messages[2].content.includes("dev-2"));
+  assert.ok(result.messages[2].content.trimEnd().endsWith("SUF"));
+  // the first must NOT carry the suffix, the last must NOT carry the prefix
+  assert.ok(!result.messages[0].content.includes("SUF"));
+  assert.ok(!result.messages[2].content.includes("PRE"));
+  // middle untouched
+  assert.equal(result.messages[1].content, "dev-1");
+});
+
+test("postTranslation: array content → unshift prefix on first, push suffix on last", () => {
+  setSystemPromptConfig({ enabled: true, prefixPrompt: "PRE", suffixPrompt: "SUF" });
+  const body = {
+    messages: [
+      { role: "developer", content: [{ type: "text", text: "dev-0" }] },
+      { role: "developer", content: [{ type: "text", text: "dev-1" }] },
+      { role: "user", content: "hi" },
+    ],
+  };
+  const result = injectSystemPromptPostTranslation(body);
+  // first developer: PRE at index 0
+  assert.equal(result.messages[0].content[0].text, "PRE");
+  assert.equal(result.messages[0].content[1].text, "dev-0");
+  // last developer: SUF at the end
+  assert.equal(result.messages[1].content[0].text, "dev-1");
+  assert.equal(result.messages[1].content[1].text, "SUF");
+});
+
+test("postTranslation: no system → combined inserted at front", () => {
+  setSystemPromptConfig({ enabled: true, prefixPrompt: "PRE", suffixPrompt: "SUF" });
+  const body = { messages: [{ role: "user", content: "hi" }] };
+  const result = injectSystemPromptPostTranslation(body);
+  assert.equal(result.messages[0].role, "system");
+  assert.ok(result.messages[0].content.includes("PRE"));
+  assert.ok(result.messages[0].content.includes("SUF"));
+  assert.equal(result.messages.length, 2);
+});
+
+test("postTranslation: disabled / empty → no change", () => {
+  setSystemPromptConfig({ enabled: false, prefixPrompt: "PRE", suffixPrompt: "SUF" });
+  const body = { messages: [{ role: "user", content: "hi" }] };
+  assert.deepEqual(injectSystemPromptPostTranslation(body), body);
+  setSystemPromptConfig({ enabled: true, prefixPrompt: "", suffixPrompt: "" });
+  assert.deepEqual(injectSystemPromptPostTranslation(body), body);
+});
+
+test("postTranslation: codex regression — suffix lands on LAST developer after translation shape", () => {
+  // Simulate the post-translation body shape for a codex Responses request:
+  // openai-responses.ts converts `instructions`→system (index 0) and each
+  // input developer item→system. The After Prompt (suffix) MUST land on the
+  // LAST system/developer, not the first — otherwise it is buried by the
+  // later developer messages and loses recency priority.
+  setSystemPromptConfig({ enabled: true, prefixPrompt: "", suffixPrompt: "AFTER-PROMPT-MARKER" });
+  const body = {
+    messages: [
+      { role: "system", content: "instructions-from-catalog" },
+      { role: "system", content: "dev-0-from-input" },
+      { role: "system", content: "dev-1-from-input" },
+      { role: "system", content: "dev-2-from-input" },
+      { role: "user", content: "do the task" },
+    ],
+  };
+  const result = injectSystemPromptPostTranslation(body);
+  const lastSys = result.messages[3];
+  assert.ok(lastSys.content.trimEnd().endsWith("AFTER-PROMPT-MARKER"));
+  // the first system must NOT carry the suffix
+  assert.ok(!result.messages[0].content.includes("AFTER-PROMPT-MARKER"));
+});
+
 // Reset
 test.after(() => setSystemPromptConfig({ enabled: false, prefixPrompt: "", suffixPrompt: "" }));