fix(transport): strip empty-string reasoning_content from OpenRouter assistant replay

DeepSeek V4 via OpenRouter injects reasoning_content: "" on assistant
messages that contain tool_calls.  The sanitizer only deleted the field
when it was not a string, so empty strings slipped through and were
replayed on follow-up turns.  OpenRouter rejects the field with an
HTTP 500 Internal Server Error instead of a descriptive 4xx, breaking
every subsequent tool-call turn for the session.

Also strip empty-string reasoning for the same reason.

Closes #82150

src/agents/openai-transport-stream.test.ts

@@ -5609,6 +5609,60 @@ describe("buildOpenAICompletionsParams sanitizes reasoning replay fields", () =>
     expect(assistant.reasoning_content).toBe("Need to answer politely.");
   });
 
+  it("strips empty-string reasoning_content from OpenRouter assistant replay", () => {
+    // DeepSeek V4 via OpenRouter injects reasoning_content: "" on tool-call
+    // assistant messages. An empty string is technically a "string" but the
+    // Chat Completions schema does not accept it, and OpenRouter rejects the
+    // replayed message with HTTP 500.
+    const params = buildOpenAICompletionsParams(
+      openRouterModel,
+      {
+        systemPrompt: "system",
+        messages: [
+          { role: "user", content: "read config" },
+          {
+            role: "assistant",
+            provider: "openrouter",
+            api: "openai-completions",
+            model: "deepseek/deepseek-v4-pro",
+            stopReason: "toolUse",
+            timestamp: 0,
+            content: [
+              {
+                type: "thinking",
+                thinking: "",
+                thinkingSignature: "reasoning_content",
+              },
+              {
+                type: "toolCall",
+                id: "call_1",
+                name: "read_file",
+                arguments: { path: "config.json" },
+              },
+            ],
+          },
+          {
+            role: "toolResult",
+            toolCallId: "call_1",
+            toolName: "read_file",
+            content: [{ type: "text", text: "{ }" }],
+            isError: false,
+            timestamp: 1,
+          },
+          { role: "user", content: "continue" },
+        ],
+        tools: [],
+      } as never,
+      undefined,
+    ) as { messages: unknown };
+
+    const msgs = params.messages as Record<string, unknown>[];
+    const assistantMsgs = msgs.filter((m) => m.role === "assistant");
+    for (const msg of assistantMsgs) {
+      expect(msg).not.toHaveProperty("reasoning_content");
+    }
+  });
+
   it("preserves OpenRouter array reasoning_details from tool-call signatures", () => {
     const reasoningDetail = { type: "reasoning.encrypted", id: "rs_1", data: "ciphertext" };
     const params = buildOpenAICompletionsParams(

src/agents/openai-transport-stream.ts

@@ -2448,10 +2448,16 @@ function sanitizeOpenRouterReasoningReplayFields(record: Record<string, unknown>
     delete record.reasoning_details;
   }
 
-  if ("reasoning" in record && typeof record.reasoning !== "string") {
+  // Strip non-string AND empty-string reasoning fields — empty strings are
+  // response artifacts that upstream providers (OpenRouter, DeepSeek) reject
+  // with HTTP 500 when replayed on follow-up turns.
+  if ("reasoning" in record && (typeof record.reasoning !== "string" || record.reasoning === "")) {
     delete record.reasoning;
   }
-  if ("reasoning_content" in record && typeof record.reasoning_content !== "string") {
+  if (
+    "reasoning_content" in record &&
+    (typeof record.reasoning_content !== "string" || record.reasoning_content === "")
+  ) {
     delete record.reasoning_content;
   }