fix(agent-runtime): guard prompt cache tool names

src/agents/embedded-agent-runner/prompt-cache-observability.test.ts

@@ -17,6 +17,25 @@ describe("prompt cache observability", () => {
     ).toEqual(["read", "write"]);
   });
 
+  it("skips unreadable tool name rows", () => {
+    const tools: Array<{ name?: string }> = [{ name: "read" }];
+    Object.defineProperty(tools, "1", {
+      get() {
+        throw new Error("prompt cache tool row exploded");
+      },
+    });
+    tools.length = 3;
+    Object.defineProperty(tools, "2", {
+      value: {
+        get name() {
+          throw new Error("prompt cache tool name exploded");
+        },
+      },
+    });
+
+    expect(collectPromptCacheToolNames(tools)).toEqual(["read"]);
+  });
+
   it("tracks cache-relevant changes and reports a real cache-read drop", () => {
     const first = beginPromptCacheObservation({
       sessionId: "session-1",

src/agents/embedded-agent-runner/prompt-cache-observability.ts

@@ -138,7 +138,30 @@ function diffSnapshots(
 }
 
 export function collectPromptCacheToolNames(tools: Array<{ name?: string }>): string[] {
-  return tools.map((tool) => tool.name?.trim()).filter((name): name is string => Boolean(name));
+  const names: string[] = [];
+  for (const key of Object.keys(tools)) {
+    const index = Number(key);
+    if (!Number.isInteger(index)) {
+      continue;
+    }
+    let tool: { name?: string } | undefined;
+    try {
+      tool = tools[index];
+    } catch {
+      continue;
+    }
+    let rawName: string | undefined;
+    try {
+      rawName = tool?.name;
+    } catch {
+      continue;
+    }
+    const name = rawName?.trim();
+    if (name) {
+      names.push(name);
+    }
+  }
+  return names;
 }
 
 export function beginPromptCacheObservation(params: {