fix(google): prevent empty contents error for gemini (#74465)

* fix(google): prevent empty contents error for gemini

* test(google): cover empty Gemini contents fallback

* docs(changelog): note Gemini empty content fallback

---------

Co-authored-by: Peter Steinberger <steipete@gmail.com>

extensions/google/transport-stream.test.ts

@@ -514,4 +514,54 @@ describe("google transport stream", () => {
 
     expect(params.cachedContent).toBe("cachedContents/prebuilt-context");
   });
+
+  it("uses a non-empty text placeholder for empty user text", () => {
+    const params = buildGoogleGenerativeAiParams(buildGeminiModel(), {
+      messages: [
+        { role: "user", content: "", timestamp: 0 },
+        {
+          role: "user",
+          content: [{ type: "text", text: "" }],
+          timestamp: 1,
+        },
+      ],
+    } as never);
+
+    expect(params.contents).toEqual([
+      { role: "user", parts: [{ text: " " }] },
+      { role: "user", parts: [{ text: " " }] },
+    ]);
+  });
+
+  it("uses a text placeholder when user parts are filtered out for text-only models", () => {
+    const params = buildGoogleGenerativeAiParams(buildGeminiModel({ input: ["text"] }), {
+      messages: [
+        {
+          role: "user",
+          content: [{ type: "image", mimeType: "image/png", data: "png-bytes" }],
+          timestamp: 0,
+        },
+      ],
+    } as never);
+
+    expect(params.contents).toEqual([{ role: "user", parts: [{ text: " " }] }]);
+  });
+
+  it("uses a user placeholder when converted Gemini contents would otherwise be empty", () => {
+    const params = buildGoogleGenerativeAiParams(buildGeminiModel(), {
+      messages: [
+        {
+          role: "assistant",
+          provider: "google",
+          api: "google-generative-ai",
+          model: "gemini-2.5-pro",
+          stopReason: "stop",
+          timestamp: 0,
+          content: [{ type: "text", text: "   " }],
+        },
+      ],
+    } as never);
+
+    expect(params.contents).toEqual([{ role: "user", parts: [{ text: " " }] }]);
+  });
 });

extensions/google/transport-stream.ts

@@ -315,14 +315,14 @@ function convertGoogleMessages(model: GoogleTransportModel, context: Context) {
       if (typeof msg.content === "string") {
         contents.push({
           role: "user",
-          parts: [{ text: sanitizeTransportPayloadText(msg.content) }],
+          parts: [{ text: sanitizeTransportPayloadText(msg.content) || " " }],
         });
         continue;
       }
       const parts = msg.content
         .map((item) =>
           item.type === "text"
-            ? { text: sanitizeTransportPayloadText(item.text) }
+            ? { text: sanitizeTransportPayloadText(item.text) || " " }
             : {
                 inlineData: {
                   mimeType: item.mimeType,
@@ -331,9 +331,10 @@ function convertGoogleMessages(model: GoogleTransportModel, context: Context) {
               },
         )
         .filter((item) => model.input.includes("image") || !("inlineData" in item));
-      if (parts.length > 0) {
-        contents.push({ role: "user", parts });
+      if (parts.length === 0) {
+        parts.push({ text: " " });
       }
+      contents.push({ role: "user", parts });
       continue;
     }
 
@@ -437,6 +438,9 @@ function convertGoogleMessages(model: GoogleTransportModel, context: Context) {
       }
     }
   }
+  if (contents.length === 0) {
+    contents.push({ role: "user", parts: [{ text: " " }] });
+  }
   return contents;
 }