fix(agents): keep Anthropic tool cache breakpoint advancing

src/agents/anthropic-payload-policy.test.ts

@@ -100,12 +100,13 @@ describe("anthropic payload policy", () => {
           type: "tool_result",
           tool_use_id: "tool_1",
           content: "done",
+          cache_control: { type: "ephemeral", ttl: "1h" },
         },
       ],
     });
   });
 
-  it("keeps the conversation cache marker on user text through trailing tool results", () => {
+  it("keeps a stable user marker while advancing through trailing tool results", () => {
     const policy = resolveAnthropicPayloadPolicy({
       provider: "anthropic",
       api: "anthropic-messages",
@@ -157,7 +158,14 @@ describe("anthropic payload policy", () => {
     });
     expect(payload.messages[4]).toEqual({
       role: "user",
-      content: [{ type: "tool_result", tool_use_id: "tool_2", content: "next chunk" }],
+      content: [
+        {
+          type: "tool_result",
+          tool_use_id: "tool_2",
+          content: "next chunk",
+          cache_control: { type: "ephemeral" },
+        },
+      ],
     });
   });
 

src/agents/anthropic-payload-policy.ts

@@ -185,6 +185,9 @@ function applyAnthropicCacheControlToMessages(
       const blockRecord = block as Record<string, unknown>;
       if (blockRecord.type === "text" || blockRecord.type === "image") {
         blockRecord.cache_control = cacheControl;
+        if (fallbackToolResult) {
+          fallbackToolResult.cache_control = cacheControl;
+        }
         return;
       }
       if (blockRecord.type === "tool_result" && fallbackToolResult === undefined) {

src/llm/providers/anthropic.ts

@@ -1193,6 +1193,13 @@ function convertMessages(
           if (block.type === "text" || block.type === "image") {
             (block as typeof block & { cache_control?: typeof cacheControl }).cache_control =
               cacheControl;
+            if (fallbackToolResult) {
+              (
+                fallbackToolResult as typeof fallbackToolResult & {
+                  cache_control?: typeof cacheControl;
+                }
+              ).cache_control = cacheControl;
+            }
             return params;
           }
           if (block.type === "tool_result" && fallbackToolResult === undefined) {