diff --git a/CHANGELOG.md b/CHANGELOG.md
index 61f85cf1b7..c20d13ec19 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -21,6 +21,8 @@
 
 ### 🐛 Bug Fixes
 
+- **fix(providers):** GitLab Duo tool-calling follow-up turns no longer fail upstream with `422 {"detail":"Validation error"}` (tokens 0/0, rejected pre-inference). The #6234 tool-result-feedback fix serialized the **entire** multi-turn conversation into GitLab's single-file `code_suggestions` (`small_file`) generation endpoint — folded history that turn-N sent as an oversized `current_file.content_above_cursor` **and** duplicated verbatim into `user_instruction`, tripping the AI-Gateway's `small_file` validation guard. The executor now **bounds** that prompt: it keeps system + latest user message + the most-recent tool round (dropping older turns), caps oversized tool results, and stops duplicating the full prompt into `user_instruction` (which now carries only the short latest user message) — while still feeding the most-recent tool result back so the agent continues (`open-sse/executors/gitlab.ts`, [#6220](https://github.com/diegosouzapw/OmniRoute/issues/6220)). The unit test covers the bounding logic; the upstream 422→200 clearing is VPS-only (Hard Rule #18). Regression guard: `tests/unit/gitlab-tool-exchange-bounded-6220.test.ts`.
+
 - **fix(i18n):** the provider-detail (`/dashboard/providers/[id]`) connection-status filter labels no longer render as `__MISSING__:All` / `__MISSING__:Active` / `__MISSING__:Error` / `__MISSING__:Banned` / `__MISSING__:CreditsExhausted` in non-English locales (notably pt-BR) ([#6290](https://github.com/diegosouzapw/OmniRoute/issues/6290)). Root cause was **not** the namespace mismatch the issue guessed — the `providers.filter*` keys resolve correctly in `en.json`; the debt lived in the locale mirrors (`src/i18n/messages/*.json`), where these five keys carried the `__MISSING__:` sync sentinel in ~15 locales and were absent entirely in ~26 others, so next-intl found the key and echoed the sentinel verbatim. All 40 non-English/-Chinese mirrors now ship real translations for the five `providers.filter*` labels. Regression guard: `tests/unit/i18n-provider-filter-keys-6290.test.ts`. (thanks @diegosouzapw)
 
 - **fix(providers):** the `copilot-m365-web` streaming executor now emits `debug`-level WebSocket diagnostics ([#6210](https://github.com/diegosouzapw/OmniRoute/issues/6210)) — the outbound WS URL (with the `access_token` **redacted** via `redactWsUrl()`), handshake success/failure, and each received SignalR frame's `type`/`target`. Previously the streaming path logged nothing, so an empty `content:null` response (the M365 Education / Starter tier symptom fixed in #6234) was undiagnosable even at `APP_LOG_LEVEL=debug`. The change is debug-level and side-effect-free — it does not alter streaming behavior or the frame parser, and the token never reaches the logs. Regression guard: `tests/unit/copilot-m365-web-logging-6210.test.ts` (thanks @qpeyba)
diff --git a/open-sse/executors/gitlab.ts b/open-sse/executors/gitlab.ts
index 510a8a39e3..594dfa7e47 100644
--- a/open-sse/executors/gitlab.ts
+++ b/open-sse/executors/gitlab.ts
@@ -43,6 +43,22 @@ type OpenAIMessage = {
 
 type JsonRecord = Record<string, unknown>;
 
+/**
+ * Bounds for GitLab's `code_suggestions` `small_file` generation contract. Its AI-Gateway
+ * validation guard rejects an oversized/over-structured prompt with
+ * `422 {"detail":"Validation error"}` (tokens 0/0, pre-inference). Turn-1 is small and
+ * passes; a long folded tool-exchange history trips it — so the serialized prompt (and the
+ * `user_instruction` field) must stay bounded (#6220).
+ */
+const MAX_TOOL_EXCHANGE_CHARS = 24_000;
+const MAX_TOOL_RESULT_CHARS = 8_000;
+const MAX_USER_INSTRUCTION_CHARS = 4_000;
+
+function capText(text: string, max: number): string {
+  if (text.length <= max) return text;
+  return `${text.slice(0, max)}\n…[truncated ${text.length - max} chars]`;
+}
+
 type GitLabRequestTarget = {
   mode: "monolith" | "direct";
   url: string;
@@ -145,26 +161,76 @@ function renderConversationTurn(message: OpenAIMessage, role: string, text: stri
   return null;
 }
 
-/** Serialize the full turn history so the model sees the tool result (#6220). */
+/**
+ * Keep only what the `small_file` generation contract can carry: every system message,
+ * the latest user message, and the most-recent tool round (the last assistant `tool_calls`
+ * turn onward). Older turns are dropped so `content_above_cursor` stays bounded (#6220).
+ */
+function selectBoundedMessages(messages: OpenAIMessage[]): OpenAIMessage[] {
+  let lastUserIdx = -1;
+  let lastToolRoundIdx = -1;
+  messages.forEach((message, idx) => {
+    const role = String(message?.role || "user").toLowerCase();
+    if (role === "user") lastUserIdx = idx;
+    if (role === "assistant" && Array.isArray(message?.tool_calls) && message.tool_calls.length) {
+      lastToolRoundIdx = idx;
+    }
+  });
+  const tailStart = lastToolRoundIdx >= 0 ? lastToolRoundIdx : lastUserIdx;
+  const kept: OpenAIMessage[] = [];
+  messages.forEach((message, idx) => {
+    const role = String(message?.role || "user").toLowerCase();
+    if (role === "system" || role === "developer") {
+      kept.push(message);
+    } else if (idx === lastUserIdx || idx >= tailStart) {
+      kept.push(message);
+    }
+  });
+  return kept;
+}
+
+/**
+ * Serialize the most-recent turn history so the model sees the tool result (#6220), but
+ * BOUNDED: only the last tool round is kept, each tool result is capped, and the whole
+ * prompt is capped — otherwise the folded history trips GitLab's `small_file` 422 guard.
+ */
 function buildToolExchangePrompt(messages: OpenAIMessage[]): string {
   const systemParts: string[] = [];
   const convo: string[] = [];
-  for (const message of messages) {
+  for (const message of selectBoundedMessages(messages)) {
     const role = String(message?.role || "user").toLowerCase();
-    const text = extractTextContent(message?.content);
+    let text = extractTextContent(message?.content);
     if (role === "system" || role === "developer") {
       if (text) systemParts.push(text);
       continue;
     }
+    if (role === "tool") text = capText(text, MAX_TOOL_RESULT_CHARS);
     const line = renderConversationTurn(message, role, text);
     if (line) convo.push(line);
   }
   const header = systemParts.length
     ? `System instructions:\n${systemParts.join("\n\n")}\n\n`
     : "";
-  return `${header}${convo.join(
+  const body = `${header}${convo.join(
     "\n\n"
   )}\n\nContinue the response using the tool result above; do not repeat the tool call.`.trim();
+  return capText(body, MAX_TOOL_EXCHANGE_CHARS);
+}
+
+/**
+ * The user's actual instruction — the latest user message, capped. Kept separate from the
+ * folded history so it is NOT duplicated into an oversized `user_instruction`, the likely
+ * offending 422 field on tool-calling follow-up turns (#6220).
+ */
+function buildLatestUserInstruction(messages: OpenAIMessage[]): string {
+  for (let i = messages.length - 1; i >= 0; i--) {
+    const role = String(messages[i]?.role || "user").toLowerCase();
+    if (role === "user") {
+      const text = extractTextContent(messages[i]?.content);
+      if (text) return capText(text, MAX_USER_INSTRUCTION_CHARS);
+    }
+  }
+  return "";
 }
 
 /**
@@ -329,7 +395,16 @@ export class GitlabExecutor extends BaseExecutor {
     _stream: boolean,
     credentials: ExecuteInput["credentials"]
   ): Record<string, unknown> {
-    const prompt = buildPrompt(body.messages as OpenAIMessage[] | undefined);
+    const messages = body.messages as OpenAIMessage[] | undefined;
+    const prompt = buildPrompt(messages);
+    // On a tool-exchange follow-up, `content_above_cursor` already carries the (bounded)
+    // folded history — sending the same long text again as `user_instruction` is the
+    // likely field that trips GitLab's `small_file` 422 guard. Send only the short latest
+    // user message there instead (#6220). Simple turns keep the legacy behavior.
+    const isToolExchange = Array.isArray(messages) && hasToolExchange(messages);
+    const userInstruction = isToolExchange
+      ? buildLatestUserInstruction(messages as OpenAIMessage[])
+      : prompt;
     const providerData =
       credentials?.providerSpecificData && typeof credentials.providerSpecificData === "object"
         ? credentials.providerSpecificData
@@ -354,7 +429,7 @@ export class GitlabExecutor extends BaseExecutor {
       generation_type: "small_file",
       stream: false,
       ...(projectPath ? { project_path: projectPath } : {}),
-      ...(prompt ? { user_instruction: prompt } : {}),
+      ...(userInstruction ? { user_instruction: userInstruction } : {}),
     };
   }
 
diff --git a/tests/unit/gitlab-tool-exchange-bounded-6220.test.ts b/tests/unit/gitlab-tool-exchange-bounded-6220.test.ts
new file mode 100644
index 0000000000..75c8566aa5
--- /dev/null
+++ b/tests/unit/gitlab-tool-exchange-bounded-6220.test.ts
@@ -0,0 +1,95 @@
+/**
+ * Regression test for #6220 (follow-up) — the tool-exchange feedback fix
+ * (gitlab-tool-result-feedback-6220) taught the GitLab Duo executor to serialize the
+ * FULL multi-turn conversation into the code_suggestions prompt so the model sees the
+ * tool result. But GitLab's AI-Gateway `code_suggestions` endpoint is a single-file
+ * `generation` API with a `small_file` validation guard: once the folded history grew
+ * large, `transformRequest` sent an oversized `content_above_cursor` AND duplicated the
+ * whole thing into `user_instruction`, and the gateway rejected turn-N with
+ * `422 {"detail":"Validation error"}` (tokens 0/0, pre-inference).
+ *
+ * Fix: BOUND the serialized tool-exchange prompt — keep system + latest user message +
+ * the most-recent tool round, cap oversized tool results, and stop duplicating the full
+ * prompt into `user_instruction` (it now carries only the short latest user message).
+ * The most-recent tool result must still be present so the model keeps its observation.
+ *
+ * The upstream 422→200 clearing is VPS-only (Hard Rule #18); this unit test covers the
+ * bounding logic (char/length caps + tool-result presence), which is the root cause.
+ */
+import test from "node:test";
+import assert from "node:assert/strict";
+import { GitlabExecutor, buildPrompt } from "../../open-sse/executors/gitlab.ts";
+
+// A tool result large enough to blow the small_file generation contract if unbounded.
+const HUGE_TOOL_RESULT =
+  "TOOLRESULT_START " + "x".repeat(60_000) + " TOOLRESULT_END";
+
+function buildLongToolConversation() {
+  const messages: Array<Record<string, unknown>> = [
+    { role: "system", content: "You are a helpful coding assistant." },
+    { role: "user", content: "List the files and summarize the repo." },
+  ];
+  // ≥10 messages: several assistant tool_calls + tool results folded back.
+  for (let i = 0; i < 5; i++) {
+    messages.push({
+      role: "assistant",
+      content: "",
+      tool_calls: [
+        {
+          id: `call_${i}`,
+          type: "function",
+          function: { name: "read_file", arguments: `{"path":"file_${i}.ts"}` },
+        },
+      ],
+    });
+    messages.push({
+      role: "tool",
+      tool_call_id: `call_${i}`,
+      name: "read_file",
+      content: i === 4 ? HUGE_TOOL_RESULT : `contents of file_${i}`,
+    });
+  }
+  return messages;
+}
+
+test("buildPrompt: long tool-exchange history is bounded (char cap) [#6220]", () => {
+  const messages = buildLongToolConversation();
+  const prompt = buildPrompt(messages);
+  // Sane bound for the small_file generation contract.
+  assert.ok(
+    prompt.length < 30_000,
+    `bounded prompt should stay under 30k chars, got ${prompt.length}`
+  );
+  // The most-recent tool result must still be present (its head survives the cap).
+  assert.match(prompt, /TOOLRESULT_START/);
+});
+
+test("transformRequest: content_above_cursor and user_instruction are both bounded [#6220]", () => {
+  const executor = new GitlabExecutor("gitlab-duo");
+  const messages = buildLongToolConversation();
+  const out = executor.transformRequest(
+    "gitlab-duo/model",
+    { messages },
+    false,
+    {} as never
+  ) as Record<string, unknown>;
+
+  const currentFile = out.current_file as Record<string, unknown>;
+  const contentAbove = String(currentFile.content_above_cursor || "");
+  const userInstruction = String(out.user_instruction || "");
+
+  // content_above_cursor carries the (bounded) folded history + the tool observation.
+  assert.ok(
+    contentAbove.length < 30_000,
+    `content_above_cursor should be bounded, got ${contentAbove.length}`
+  );
+  assert.match(contentAbove, /TOOLRESULT_START/);
+
+  // user_instruction must NOT duplicate the whole huge prompt — the duplication was the
+  // likely offending 422 field. It carries only the short latest user message.
+  assert.ok(
+    userInstruction.length < 5_000,
+    `user_instruction should be short (not the full prompt), got ${userInstruction.length}`
+  );
+  assert.match(userInstruction, /summarize the repo/);
+});