diff --git a/open-sse/translator/request/openai-responses.ts b/open-sse/translator/request/openai-responses.ts
index 5c9f98d738..18e8f41940 100644
--- a/open-sse/translator/request/openai-responses.ts
+++ b/open-sse/translator/request/openai-responses.ts
@@ -227,7 +227,7 @@ export function openaiResponsesToOpenAIRequest(
     const itemType = toString(item.type) || (item.role ? "message" : "");
 
     if (itemType === "message") {
-      const role = toString(item.role);
+      const role = toString(item.role) === "agent_message" ? "assistant" : toString(item.role);
 
       if (role !== "assistant") {
         if (currentAssistantMsg) {
@@ -484,6 +484,13 @@ export function openaiResponsesToOpenAIRequest(
       continue;
     }
 
+    // Defense in depth for Responses/subagent fallback: agent_message is
+    // Responses-only. Normalization should already have rewritten or dropped it;
+    // never throw a 5xx-looking unsupported-feature error if a shape slips through.
+    if (itemType === "agent_message" || toString(item.role) === "agent_message") {
+      continue;
+    }
+
     throw unsupportedFeature(
       `Unsupported Responses API feature: input item type '${itemType || "missing"}' cannot be represented in Chat Completions`
     );
@@ -773,7 +780,10 @@ export function openaiResponsesToOpenAIRequest(
   // ("When using tool_choice, tools must be set"). Contradictory choices like "required"
   // or forced functions are preserved so the upstream error remains visible.
   const finalChatTools = Array.isArray(result.tools) ? result.tools : [];
-  if (finalChatTools.length === 0 && (result.tool_choice === "auto" || result.tool_choice === "none")) {
+  if (
+    finalChatTools.length === 0 &&
+    (result.tool_choice === "auto" || result.tool_choice === "none")
+  ) {
     delete result.tool_choice;
   }
 
diff --git a/open-sse/utils/responsesInputNormalization.ts b/open-sse/utils/responsesInputNormalization.ts
index 490080c3db..d56acb6590 100644
--- a/open-sse/utils/responsesInputNormalization.ts
+++ b/open-sse/utils/responsesInputNormalization.ts
@@ -1,12 +1,20 @@
 type JsonRecord = Record<string, unknown>;
 
-function normalizeAgentMessageForChat(item: JsonRecord): JsonRecord | null {
-  if (item.type !== "agent_message") return null;
+function isAgentMessageItem(item: JsonRecord): boolean {
+  return item.type === "agent_message" || item.role === "agent_message";
+}
 
+function collectAgentMessageText(item: JsonRecord): string | null {
+  if (typeof item.content === "string") return item.content;
+  if (typeof item.text === "string") return item.text;
   if (!Array.isArray(item.content)) return null;
 
   const textParts: string[] = [];
   for (const partValue of item.content) {
+    if (typeof partValue === "string") {
+      textParts.push(partValue);
+      continue;
+    }
     if (!partValue || typeof partValue !== "object" || Array.isArray(partValue)) {
       return null;
     }
@@ -17,12 +25,21 @@ function normalizeAgentMessageForChat(item: JsonRecord): JsonRecord | null {
       // partial plaintext envelope or forward an opaque payload the model cannot use.
       return null;
     }
-    if (part.type !== "input_text" || typeof part.text !== "string") return null;
+    if (part.type !== "input_text" && part.type !== "output_text" && part.type !== "text") {
+      return null;
+    }
+    if (typeof part.text !== "string") return null;
     textParts.push(part.text);
   }
 
-  const text = textParts.join("\n");
-  if (!text.trim()) return null;
+  return textParts.join("\n");
+}
+
+function normalizeAgentMessageForChat(item: JsonRecord): JsonRecord | null {
+  if (!isAgentMessageItem(item)) return null;
+
+  const text = collectAgentMessageText(item);
+  if (typeof text !== "string" || !text.trim()) return null;
 
   return {
     type: "message",
@@ -125,7 +142,7 @@ function normalizeResponsesInputItemForChat(value: unknown): unknown {
 
   const agentMessage = normalizeAgentMessageForChat(item);
   if (agentMessage) return agentMessage;
-  if (item.type === "agent_message") {
+  if (isAgentMessageItem(item)) {
     // Encrypted or malformed agent messages have no lossless Chat equivalent.
     // Treat them like other Responses-only metadata instead of failing the whole turn.
     return { type: "reasoning" };
diff --git a/tests/unit/responses-chat-translation-gaps.test.ts b/tests/unit/responses-chat-translation-gaps.test.ts
index 093594e160..b7892b1478 100644
--- a/tests/unit/responses-chat-translation-gaps.test.ts
+++ b/tests/unit/responses-chat-translation-gaps.test.ts
@@ -178,6 +178,47 @@ test("Responses -> Chat skips encrypted or mixed agent_message items", () => {
   ]);
 });
 
+test("Responses -> Chat converts string-content agent_message items to assistant history", () => {
+  const result = translate({
+    input: [
+      { type: "message", role: "user", content: [{ type: "input_text", text: "Run the task" }] },
+      { type: "agent_message", author: "worker", content: "Task completed" },
+    ],
+  });
+
+  assert.deepEqual(result.messages, [
+    { role: "user", content: [{ type: "text", text: "Run the task" }] },
+    { role: "assistant", content: [{ type: "text", text: "Task completed" }] },
+  ]);
+});
+
+test("Responses -> Chat converts role-based agent_message items without a type field", () => {
+  const result = translate({
+    input: [
+      { type: "message", role: "user", content: [{ type: "input_text", text: "Run the task" }] },
+      { role: "agent_message", content: [{ type: "text", text: "Worker reply" }] },
+    ],
+  });
+
+  assert.deepEqual(result.messages, [
+    { role: "user", content: [{ type: "text", text: "Run the task" }] },
+    { role: "assistant", content: [{ type: "text", text: "Worker reply" }] },
+  ]);
+});
+
+test("Responses -> Chat does not throw when an agent_message item slips past normalize", () => {
+  const result = translate({
+    input: [
+      { type: "message", role: "user", content: [{ type: "input_text", text: "Run the task" }] },
+      { type: "agent_message", content: [{ type: "unknown_part" }] },
+    ],
+  });
+
+  assert.deepEqual(result.messages, [
+    { role: "user", content: [{ type: "text", text: "Run the task" }] },
+  ]);
+});
+
 test("Responses -> Chat consumes additional_tools input items without emitting messages", () => {
   const result = translate({
     input: [