diff --git a/open-sse/translator/formats/claude.js b/open-sse/translator/formats/claude.js
index e52bef9d..a9bbd19d 100644
--- a/open-sse/translator/formats/claude.js
+++ b/open-sse/translator/formats/claude.js
@@ -26,24 +26,32 @@ export function lastCacheableToolIndex(tools) {
 }
 
 // Check if message has valid non-empty content
+// A block type outside this list makes the whole message count as empty and be
+// dropped by prepareClaudeRequest — so anything the caller can legitimately
+// send alone must be listed. container_upload (Files API) is one of those:
+// a user turn whose only block is a file reference is valid Anthropic input
+// (#4316), and dropping it forwarded `messages: []` to the provider.
+const CONTENTFUL_BLOCKS = new Set([
+  CLAUDE_BLOCK.TOOL_USE,
+  CLAUDE_BLOCK.TOOL_RESULT,
+  CLAUDE_BLOCK.IMAGE,
+  CLAUDE_BLOCK.DOCUMENT,
+  CLAUDE_BLOCK.CONTAINER_UPLOAD,
+]);
+
+function isContentfulBlock(block) {
+  if (!block) return false;
+  if (block.type === CLAUDE_BLOCK.TEXT) return !!block.text?.trim();
+  return CONTENTFUL_BLOCKS.has(block.type);
+}
+
 export function hasValidContent(msg) {
   if (typeof msg.content === "string" && msg.content.trim()) return true;
   if (msg.content && typeof msg.content === "object" && !Array.isArray(msg.content)) {
-    const block = msg.content;
-    return !!((block.type === CLAUDE_BLOCK.TEXT && block.text?.trim()) ||
-      block.type === CLAUDE_BLOCK.TOOL_USE ||
-      block.type === CLAUDE_BLOCK.TOOL_RESULT ||
-      block.type === CLAUDE_BLOCK.IMAGE ||
-      block.type === CLAUDE_BLOCK.DOCUMENT);
+    return isContentfulBlock(msg.content);
   }
   if (Array.isArray(msg.content)) {
-    return msg.content.some(block =>
-      (block.type === CLAUDE_BLOCK.TEXT && block.text?.trim()) ||
-      block.type === CLAUDE_BLOCK.TOOL_USE ||
-      block.type === CLAUDE_BLOCK.TOOL_RESULT ||
-      block.type === CLAUDE_BLOCK.IMAGE ||
-      block.type === CLAUDE_BLOCK.DOCUMENT
-    );
+    return msg.content.some(isContentfulBlock);
   }
   return false;
 }
diff --git a/open-sse/translator/schema/blocks.js b/open-sse/translator/schema/blocks.js
index 91c71c4a..d2bb783c 100644
--- a/open-sse/translator/schema/blocks.js
+++ b/open-sse/translator/schema/blocks.js
@@ -18,6 +18,7 @@ export const CLAUDE_BLOCK = {
   DOCUMENT: "document",
   TOOL_USE: "tool_use",
   TOOL_RESULT: "tool_result",
+  CONTAINER_UPLOAD: "container_upload",
   THINKING: "thinking",
   REDACTED_THINKING: "redacted_thinking",
   SERVER_TOOL_USE: "server_tool_use",
diff --git a/tests/unit/claude-container-upload.test.js b/tests/unit/claude-container-upload.test.js
new file mode 100644
index 00000000..0b74dc2f
--- /dev/null
+++ b/tests/unit/claude-container-upload.test.js
@@ -0,0 +1,55 @@
+// #4316: a user message whose only content block is `container_upload`
+// (Anthropic Files API) was dropped whole, so the provider received
+// `messages: []` and the request still returned 200 with no indication that
+// the user turn had vanished.
+//
+// hasValidContent() enumerated the block types that count as content, and any
+// type outside that list made the message look empty — prepareClaudeRequest
+// then filtered it out. container_upload is valid Anthropic input on its own,
+// and on the Claude→Claude route no translation runs at all, so the block
+// should reach the provider untouched.
+import { describe, expect, it } from "vitest";
+
+import { hasValidContent, prepareClaudeRequest } from "../../open-sse/translator/formats/claude.js";
+
+const uploadBlock = { type: "container_upload", file_id: "file_abc123" };
+
+describe("container_upload keeps the user turn alive (#4316)", () => {
+  it("counts a lone container_upload block as content", () => {
+    expect(hasValidContent({ role: "user", content: [uploadBlock] })).toBe(true);
+  });
+
+  it("counts a bare container_upload object as content", () => {
+    expect(hasValidContent({ role: "user", content: uploadBlock })).toBe(true);
+  });
+
+  it("does not forward messages: [] for a container_upload-only request", () => {
+    const body = {
+      model: "claude-sonnet-4-5",
+      max_tokens: 64,
+      messages: [{ role: "user", content: [uploadBlock] }],
+    };
+    const prepared = prepareClaudeRequest(body);
+    expect(prepared.messages).toHaveLength(1);
+    expect(prepared.messages[0].role).toBe("user");
+    expect(prepared.messages[0].content).toContainEqual(expect.objectContaining({
+      type: "container_upload",
+      file_id: "file_abc123",
+    }));
+  });
+
+  it("still drops a genuinely empty message", () => {
+    expect(hasValidContent({ role: "user", content: [] })).toBe(false);
+    expect(hasValidContent({ role: "user", content: [{ type: "text", text: "   " }] })).toBe(false);
+  });
+
+  it("keeps a container_upload alongside text", () => {
+    const body = {
+      model: "claude-sonnet-4-5",
+      max_tokens: 64,
+      messages: [{ role: "user", content: [uploadBlock, { type: "text", text: "summarise this" }] }],
+    };
+    const prepared = prepareClaudeRequest(body);
+    expect(prepared.messages).toHaveLength(1);
+  });
+});