diff --git a/open-sse/translator/response/openai-responses.ts b/open-sse/translator/response/openai-responses.ts
index 3a9a1299da..6e62715f84 100644
--- a/open-sse/translator/response/openai-responses.ts
+++ b/open-sse/translator/response/openai-responses.ts
@@ -7,38 +7,16 @@ import { FORMATS } from "../formats.ts";
 import { appendToolCallArgumentDelta } from "../../utils/toolCallArguments.ts";
 import { fallbackToolCallId } from "../helpers/toolCallHelper.ts";
 import { shouldParseTextualReasoningTags } from "../../handlers/responseSanitizer.ts";
+import {
+  normalizeToolName,
+  stripEmptyOptionalToolArgs,
+  normalizeOutputIndex,
+  normalizeUpstreamFailure,
+  extractResponsesReasoningSummaryText,
+} from "./openai-responses/pureHelpers.ts";
 
-function normalizeToolName(value) {
-  return typeof value === "string" ? value.trim() : "";
-}
-
-function stripEmptyOptionalToolArgs(value, toolName) {
-  if (value == null) return value;
-
-  if (typeof value === "string") {
-    // JSON-string cleanup is intentionally scoped to Claude Code's Read tool.
-    // For arbitrary tools, empty strings/arrays may be valid user payloads.
-    if (toolName !== "Read") return value;
-    try {
-      const parsed = JSON.parse(value);
-      if (Array.isArray(parsed) || typeof parsed !== "object" || parsed === null) return value;
-      const cleaned = stripEmptyOptionalToolArgs(parsed, toolName);
-      return JSON.stringify(cleaned ?? {});
-    } catch {
-      return value;
-    }
-  }
-
-  if (Array.isArray(value) || typeof value !== "object") return value;
-
-  const cleaned = { ...value };
-  for (const [key, entry] of Object.entries(cleaned)) {
-    if (entry === "" || (Array.isArray(entry) && entry.length === 0)) {
-      delete cleaned[key];
-    }
-  }
-  return cleaned;
-}
+// normalizeUpstreamFailure is re-exported for external importers (tests).
+export { normalizeUpstreamFailure } from "./openai-responses/pureHelpers.ts";
 
 /**
  * Translate OpenAI chunk to Responses API events
@@ -192,11 +170,6 @@ export function openaiToOpenAIResponsesResponse(chunk, state) {
 }
 
 // Normalize output_index to a non-negative integer (replaces fragile parseInt calls)
-function normalizeOutputIndex(outputIndex) {
-  const normalized = Number(outputIndex);
-  return Number.isInteger(normalized) && normalized >= 0 ? normalized : 0;
-}
-
 // Record a finalized item keyed by output_index so buildDenseOutput can sort later
 function recordCompletedItem(state, outputIndex, item) {
   if (!Array.isArray(state.completedOutputItems)) {
@@ -564,50 +537,6 @@ function flushEvents(state) {
   return events;
 }
 
-export function normalizeUpstreamFailure(data, fallbackType = "server_error") {
-  const response = data?.response && typeof data.response === "object" ? data.response : null;
-  const error =
-    response?.error && typeof response.error === "object"
-      ? response.error
-      : data?.error && typeof data.error === "object"
-        ? data.error
-        : null;
-
-  const code = typeof error?.code === "string" ? error.code : "";
-  const message =
-    typeof error?.message === "string"
-      ? error.message
-      : typeof data?.message === "string"
-        ? data.message
-        : "Upstream failure";
-
-  // Preserve upstream error semantics:
-  // - context_length_exceeded → 400 (client can retry with smaller context)
-  // - rate_limit_exceeded → 429 (client should back off)
-  // - Everything else → 502 (upstream failure)
-  const isContextOverflow = code === "context_length_exceeded";
-  const isRateLimit = code === "rate_limit_exceeded" || code === "rate_limited";
-  let status: number;
-  let type: string;
-  if (isRateLimit) {
-    status = 429;
-    type = "rate_limit_error";
-  } else if (isContextOverflow) {
-    status = 400;
-    type = "invalid_request_error";
-  } else {
-    status = 502;
-    type = fallbackType;
-  }
-
-  return {
-    status,
-    type,
-    code: code || (isRateLimit ? "rate_limit_exceeded" : "bad_gateway"),
-    message,
-  };
-}
-
 /**
  * OpenAI Chat Completions streams announce the assistant role on the FIRST delta
  * (e.g. `{ "role": "assistant", "content": "" }` or `{ "role": "assistant",
@@ -680,15 +609,6 @@ function buildResponsesReasoningDeltaChunk(state, text) {
   };
 }
 
-function extractResponsesReasoningSummaryText(item) {
-  if (!item || !Array.isArray(item.summary)) return "";
-  return item.summary
-    .map((part) =>
-      part && typeof part === "object" && typeof part.text === "string" ? part.text : ""
-    )
-    .join("");
-}
-
 /**
  * Translate OpenAI Responses API chunk to OpenAI Chat Completions format
  * This is for when Codex returns data and we need to send it to an OpenAI-compatible client
diff --git a/open-sse/translator/response/openai-responses/pureHelpers.ts b/open-sse/translator/response/openai-responses/pureHelpers.ts
new file mode 100644
index 0000000000..3ed559fb55
--- /dev/null
+++ b/open-sse/translator/response/openai-responses/pureHelpers.ts
@@ -0,0 +1,92 @@
+// Pure, stateless helpers for the OpenAI Responses <-> Chat response translator.
+// Extracted verbatim from response/openai-responses.ts (no host imports, no stream state).
+
+export function normalizeToolName(value) {
+  return typeof value === "string" ? value.trim() : "";
+}
+
+export function stripEmptyOptionalToolArgs(value, toolName) {
+  if (value == null) return value;
+
+  if (typeof value === "string") {
+    // JSON-string cleanup is intentionally scoped to Claude Code's Read tool.
+    // For arbitrary tools, empty strings/arrays may be valid user payloads.
+    if (toolName !== "Read") return value;
+    try {
+      const parsed = JSON.parse(value);
+      if (Array.isArray(parsed) || typeof parsed !== "object" || parsed === null) return value;
+      const cleaned = stripEmptyOptionalToolArgs(parsed, toolName);
+      return JSON.stringify(cleaned ?? {});
+    } catch {
+      return value;
+    }
+  }
+
+  if (Array.isArray(value) || typeof value !== "object") return value;
+
+  const cleaned = { ...value };
+  for (const [key, entry] of Object.entries(cleaned)) {
+    if (entry === "" || (Array.isArray(entry) && entry.length === 0)) {
+      delete cleaned[key];
+    }
+  }
+  return cleaned;
+}
+
+export function normalizeOutputIndex(outputIndex) {
+  const normalized = Number(outputIndex);
+  return Number.isInteger(normalized) && normalized >= 0 ? normalized : 0;
+}
+
+export function normalizeUpstreamFailure(data, fallbackType = "server_error") {
+  const response = data?.response && typeof data.response === "object" ? data.response : null;
+  const error =
+    response?.error && typeof response.error === "object"
+      ? response.error
+      : data?.error && typeof data.error === "object"
+        ? data.error
+        : null;
+
+  const code = typeof error?.code === "string" ? error.code : "";
+  const message =
+    typeof error?.message === "string"
+      ? error.message
+      : typeof data?.message === "string"
+        ? data.message
+        : "Upstream failure";
+
+  // Preserve upstream error semantics:
+  // - context_length_exceeded → 400 (client can retry with smaller context)
+  // - rate_limit_exceeded → 429 (client should back off)
+  // - Everything else → 502 (upstream failure)
+  const isContextOverflow = code === "context_length_exceeded";
+  const isRateLimit = code === "rate_limit_exceeded" || code === "rate_limited";
+  let status: number;
+  let type: string;
+  if (isRateLimit) {
+    status = 429;
+    type = "rate_limit_error";
+  } else if (isContextOverflow) {
+    status = 400;
+    type = "invalid_request_error";
+  } else {
+    status = 502;
+    type = fallbackType;
+  }
+
+  return {
+    status,
+    type,
+    code: code || (isRateLimit ? "rate_limit_exceeded" : "bad_gateway"),
+    message,
+  };
+}
+
+export function extractResponsesReasoningSummaryText(item) {
+  if (!item || !Array.isArray(item.summary)) return "";
+  return item.summary
+    .map((part) =>
+      part && typeof part === "object" && typeof part.text === "string" ? part.text : ""
+    )
+    .join("");
+}
diff --git a/tests/unit/response-openai-responses-purehelpers-split.test.ts b/tests/unit/response-openai-responses-purehelpers-split.test.ts
new file mode 100644
index 0000000000..7b8485471e
--- /dev/null
+++ b/tests/unit/response-openai-responses-purehelpers-split.test.ts
@@ -0,0 +1,60 @@
+import { test } from "node:test";
+import assert from "node:assert/strict";
+import { readFileSync } from "node:fs";
+import { fileURLToPath } from "node:url";
+import { dirname, join } from "node:path";
+
+// Split-guard for the response/openai-responses pure-helper extraction.
+// The stateless helpers (normalizeToolName / stripEmptyOptionalToolArgs /
+// normalizeOutputIndex / normalizeUpstreamFailure / extractResponsesReasoningSummaryText)
+// live in the pure leaf `openai-responses/pureHelpers.ts` (no stream state, no host import).
+// The host imports them back and re-exports normalizeUpstreamFailure for external importers.
+const HERE = dirname(fileURLToPath(import.meta.url));
+const RESP = join(HERE, "../../open-sse/translator/response");
+const HOST = join(RESP, "openai-responses.ts");
+const LEAF = join(RESP, "openai-responses/pureHelpers.ts");
+
+test("leaf hosts the pure helpers, has no stream state and no host import", () => {
+  const src = readFileSync(LEAF, "utf8");
+  for (const sym of [
+    "normalizeToolName",
+    "stripEmptyOptionalToolArgs",
+    "normalizeOutputIndex",
+    "normalizeUpstreamFailure",
+    "extractResponsesReasoningSummaryText",
+  ]) {
+    assert.match(src, new RegExp(`export function ${sym}\\b`));
+  }
+  assert.doesNotMatch(src, /from "\.\.\/openai-responses\.ts"/);
+  // No stream-state parameter leaked into the pure leaf (ignore comments).
+  const code = src
+    .split("\n")
+    .filter((l) => !l.trim().startsWith("//"))
+    .join("\n");
+  assert.doesNotMatch(code, /\bstate\b/);
+});
+
+test("host imports helpers back and re-exports normalizeUpstreamFailure", () => {
+  const src = readFileSync(HOST, "utf8");
+  assert.match(src, /from "\.\/openai-responses\/pureHelpers\.ts"/);
+  assert.match(
+    src,
+    /export \{ normalizeUpstreamFailure \} from "\.\/openai-responses\/pureHelpers\.ts"/
+  );
+});
+
+test("normalizeUpstreamFailure preserves upstream error semantics", async () => {
+  const { normalizeUpstreamFailure } =
+    await import("../../open-sse/translator/response/openai-responses/pureHelpers.ts");
+  assert.equal(
+    normalizeUpstreamFailure({ error: { code: "rate_limit_exceeded", message: "slow down" } })
+      .status,
+    429
+  );
+  assert.equal(
+    normalizeUpstreamFailure({ error: { code: "context_length_exceeded", message: "too big" } })
+      .status,
+    400
+  );
+  assert.equal(normalizeUpstreamFailure({ message: "boom" }).status, 502);
+});