fix(vertex): send every system message as systemInstruction

src/providers/google-vertex-ai/chatComplete.ts

@@ -261,39 +261,20 @@ export const VertexGoogleChatCompleteConfig: ProviderConfig = {
                     )
                 )
                     return;
-                const firstMessage = params.messages?.[0] || null;
-                if (!firstMessage) return;
-
-                if (
-                    SYSTEM_MESSAGE_ROLES.includes(firstMessage.role) &&
-                    typeof firstMessage.content === "string"
-                ) {
-                    return {
-                        parts: [
-                            {
-                                text: firstMessage.content,
-                            },
-                        ],
-                        role: "system",
-                    };
-                }
-
-                if (
-                    SYSTEM_MESSAGE_ROLES.includes(firstMessage.role) &&
-                    typeof firstMessage.content === "object" &&
-                    firstMessage.content?.[0]?.text
-                ) {
-                    return {
-                        parts: [
-                            {
-                                text: firstMessage.content?.[0].text,
-                            },
-                        ],
-                        role: "system",
-                    };
-                }
-
-                return;
+                // contents skips every system message, so all of them go here.
+                const parts = (params.messages ?? [])
+                    .filter((message) =>
+                        SYSTEM_MESSAGE_ROLES.includes(message.role),
+                    )
+                    .flatMap((message) =>
+                        typeof message.content === "string"
+                            ? [{ text: message.content }]
+                            : (message.content ?? [])
+                                  .filter((part) => part.text)
+                                  .map((part) => ({ text: part.text })),
+                    );
+                if (!parts.length) return;
+                return { parts, role: "system" };
             },
         },
     ],

tests/unit/src/services/transformToProviderRequest.node.ts

@@ -1,6 +1,11 @@
 import assert from 'node:assert/strict';
 import { describe, it } from 'node:test';
-import { AZURE_OPEN_AI, BEDROCK, OPEN_AI } from '../../../../src/globals';
+import {
+  AZURE_OPEN_AI,
+  BEDROCK,
+  GOOGLE_VERTEX_AI,
+  OPEN_AI,
+} from '../../../../src/globals';
 import { BedrockConverseAnthropicChatCompletionsParams } from '../../../../src/providers/bedrock/chatComplete';
 import { Params } from '../../../../src/types/requestBody';
 import { transformToProviderRequest } from '../../../../src/services/transformToProviderRequest';
@@ -153,3 +158,30 @@ describe('Claude Converse parameters', () => {
     assert.equal('additionalModelRequestFields' in request, false);
   });
 });
+
+describe('Vertex Gemini system messages', () => {
+  it('sends every system message and text part as systemInstruction', () => {
+    const request = chatRequest(GOOGLE_VERTEX_AI, {
+      model: 'gemini-3-flash-preview',
+      messages: [
+        { role: 'system', content: 'Answer in German.' },
+        {
+          role: 'system',
+          content: [
+            { type: 'text', text: 'Be brief.' },
+            { type: 'text', text: 'End with BANANE.' },
+          ],
+        },
+        { role: 'user', content: 'Name a colour.' },
+      ],
+    });
+    assert.deepEqual(request.systemInstruction, {
+      parts: [
+        { text: 'Answer in German.' },
+        { text: 'Be brief.' },
+        { text: 'End with BANANE.' },
+      ],
+      role: 'system',
+    });
+  });
+});