fix(model): serialize assistant tool calls with empty content

src/model/providers/openai/request.ts

@@ -132,14 +132,16 @@ function toOpenAIMessages(message: CanonicalMessage, messageIndex: number): Open
       block.type !== "tool_call" &&
       block.type !== "thinking",
   );
+  const requiresEmptyAssistantContent =
+    message.role === "assistant" && (thinkingBlocks.length > 0 || assistantToolCalls.length > 0);
 
   const messages: OpenAIMessage[] = [];
   if (normalContent.length > 0 || assistantToolCalls.length > 0 || thinkingBlocks.length > 0) {
     const msg: OpenAIMessage = {
       role: message.role,
       content: normalContent.length > 0
         ? toOpenAIContent(normalContent)
-        : (message.role === "assistant" && thinkingBlocks.length > 0 ? "" : undefined),
+        : (requiresEmptyAssistantContent ? "" : undefined),
       tool_calls: assistantToolCalls.length > 0 ? assistantToolCalls : undefined,
     };
     // DeepSeek V4 requires reasoning_content to be passed back on assistant

tests/model/providers/openai/request.test.ts

@@ -0,0 +1,76 @@
+import assert from "node:assert/strict";
+import test from "node:test";
+
+import { buildOpenAIRequest } from "../../../../src/model/providers/openai/request.js";
+import type {
+  CanonicalModelRequest,
+  ModelDefinition,
+} from "../../../../src/model/protocol/canonical.js";
+
+const model: ModelDefinition = {
+  id: "test-model",
+  capabilities: {
+    supportsToolUse: true,
+    supportsStreaming: true,
+    supportsParallelToolCalls: true,
+    supportsThinking: true,
+    supportsJsonSchema: true,
+    supportsSystemPrompt: true,
+    supportsPromptCache: false,
+    maxContextTokens: 8192,
+    maxOutputTokens: 1024,
+  },
+  multimodal: {
+    input: ["text"],
+  },
+};
+
+test("serializes assistant tool calls with explicit empty content", () => {
+  const request: CanonicalModelRequest = {
+    model: "test-model",
+    provider: "openai",
+    stream: true,
+    messages: [
+      {
+        role: "user",
+        content: [{ type: "text", text: "Call a tool." }],
+      },
+      {
+        role: "assistant",
+        content: [
+          {
+            type: "tool_call",
+            id: "call_test_1",
+            name: "noop",
+            input: {},
+          },
+        ],
+      },
+      {
+        role: "user",
+        content: [
+          {
+            type: "tool_result",
+            toolCallId: "call_test_1",
+            content: [{ type: "text", text: "ok" }],
+          },
+        ],
+      },
+    ],
+  };
+
+  const body = buildOpenAIRequest(request, model);
+  const assistantMessage = body.messages.find((message) => message.role === "assistant");
+
+  assert.equal(assistantMessage?.content, "");
+  assert.deepEqual(assistantMessage?.tool_calls, [
+    {
+      id: "call_test_1",
+      type: "function",
+      function: {
+        name: "noop",
+        arguments: "{}",
+      },
+    },
+  ]);
+});