fix(anthropic): treat a blank tool_use id as absent, not as an id

`block.id ?? synthesizeToolUseId()` only synthesizes when the field is
missing. An Anthropic-compatible relay that sends "" or "   " satisfies
the nullish check, so the call went out carrying a blank id and the
client had nothing to echo back — the next turn cannot pair the result
with its call.

usableToolUseId() treats blank as absent. Both the streaming
content_block_start path and the non-stream block path use it.

Last item of #765's tool-id half. Ablated: restoring the ?? form fails
the new blank-id test for both "" and "   ".

Author: lidge-jun

src/adapters/anthropic.ts

@@ -266,6 +266,16 @@ function synthesizeToolUseId(): string {
   return `toolu_${crypto.randomUUID().replace(/-/g, "").slice(0, 24)}`;
 }
 
+/**
+ * A tool_use id that a client can actually echo back. `??` only catches a missing
+ * field, so an Anthropic-compatible relay that sends `""` or `"   "` produced a
+ * call whose id round-trips as blank — the next turn then cannot pair the result
+ * with its call. Treat blank as absent and synthesize (#765).
+ */
+function usableToolUseId(id: unknown): string {
+  return typeof id === "string" && id.trim() ? id : synthesizeToolUseId();
+}
+
 function toolUseArguments(input: unknown): string {
   if (typeof input === "string") {
     const trimmed = input.trim();
@@ -824,7 +834,7 @@ export function createAnthropicAdapter(provider: OcxProviderConfig, cacheRetenti
                 if (!block) break;
                 currentBlockType = block.type;
                 if (block.type === "tool_use") {
-                  currentToolCallId = block.id ?? synthesizeToolUseId();
+                  currentToolCallId = usableToolUseId(block.id);
                   currentToolCallName = toolNames.fromWire(block.name ?? "");
                   currentToolCallJson = "";
                   yield { type: "tool_call_start", id: currentToolCallId, name: currentToolCallName };
@@ -938,7 +948,7 @@ export function createAnthropicAdapter(provider: OcxProviderConfig, cacheRetenti
           } else if (block.type === "redacted_thinking" && typeof block.data === "string") {
             events.push({ type: "redacted_thinking", data: block.data });
           } else if (block.type === "tool_use") {
-            const id = block.id ?? synthesizeToolUseId();
+            const id = usableToolUseId(block.id);
             events.push({ type: "tool_call_start", id, name: toolNames.fromWire(block.name ?? "") });
             events.push({ type: "tool_call_delta", arguments: toolUseArguments(block.input) });
             events.push({ type: "tool_call_end" });

tests/anthropic-stream-hardening.test.ts

@@ -67,6 +67,28 @@ describe("anthropic stream hardening", () => {
     expect(start && "id" in start && start.id.startsWith("toolu_")).toBe(true);
   });
 
+  // A relay that sends `""` or `"   "` is the case `??` misses: the field is present,
+  // so no synthesis happened and the call went out with an id the client cannot echo
+  // back. Blank must be treated as absent (#765).
+  test("tool_use with a blank id synthesizes one instead of shipping it", async () => {
+    for (const blank of ['""', '"   "']) {
+      const response = new Response([
+        "event: content_block_start\n",
+        `data: {"type":"content_block_start","content_block":{"type":"tool_use","id":${blank},"name":"get_weather"}}\n\n`,
+        "event: content_block_delta\n",
+        'data: {"type":"content_block_delta","delta":{"type":"input_json_delta","partial_json":"{}"}}\n\n',
+        "event: content_block_stop\n",
+        'data: {"type":"content_block_stop"}\n\n',
+        "event: message_stop\n",
+        'data: {"type":"message_stop"}\n\n',
+      ].join(""));
+      const events = await collect(createAnthropicAdapter(provider).parseStream(response));
+      const start = events.find(e => e.type === "tool_call_start");
+      expect(start && "id" in start && start.id.trim()).toBeTruthy();
+      expect(start && "id" in start && start.id.startsWith("toolu_")).toBe(true);
+    }
+  });
+
   test("empty EOF without content still errors", async () => {
     const response = new Response("");
     const events = await collect(createAnthropicAdapter(provider).parseStream(response));