diff --git a/scripts/anthropic-prompt-probe.ts b/scripts/anthropic-prompt-probe.ts
index 1deb21fbf8d..1efd66efec1 100644
--- a/scripts/anthropic-prompt-probe.ts
+++ b/scripts/anthropic-prompt-probe.ts
@@ -120,6 +120,7 @@ type StoppableGatewayChild = {
 };
 
 type ClosableLogFile = {
+  appendFile?(data: string | Uint8Array): Promise<void>;
   close(): Promise<void>;
 };
 
@@ -520,11 +521,32 @@ async function startGatewayProcess(params: {
       stdio: ["ignore", "pipe", "pipe"],
     },
   );
-  child.stdout.on("data", (chunk) => void logFile.appendFile(chunk));
-  child.stderr.on("data", (chunk) => void logFile.appendFile(chunk));
+  const pendingLogWrites = new Set<Promise<void>>();
+  const logWriteErrors: unknown[] = [];
+  const trackLogWrite = (chunk: Buffer) => {
+    const write = logFile.appendFile(chunk).catch((error: unknown) => {
+      logWriteErrors.push(error);
+      throw error;
+    });
+    pendingLogWrites.add(write);
+    void write
+      .finally(() => {
+        pendingLogWrites.delete(write);
+      })
+      .catch(() => undefined);
+  };
+  child.stdout.on("data", trackLogWrite);
+  child.stderr.on("data", trackLogWrite);
   return {
     async stop(): Promise<boolean> {
-      return await stopGatewayPromptChild(child, logFile);
+      return await stopGatewayPromptChild(
+        child,
+        logFile,
+        1_500,
+        1_500,
+        pendingLogWrites,
+        logWriteErrors,
+      );
     },
   };
 }
@@ -534,6 +556,8 @@ async function stopGatewayPromptChild(
   logFile: ClosableLogFile,
   sigintTimeoutMs = 1_500,
   sigkillTimeoutMs = 1_500,
+  pendingLogWrites: Iterable<Promise<void>> = [],
+  logWriteErrors: readonly unknown[] = [],
 ): Promise<boolean> {
   let exited = child.exitCode !== null || child.signalCode !== null;
   const exitPromise = exited
@@ -560,7 +584,14 @@ async function stopGatewayPromptChild(
       () => false,
     );
   }
+  const failedLogWrite = (await Promise.allSettled(pendingLogWrites)).find(
+    (result): result is PromiseRejectedResult => result.status === "rejected",
+  );
   await logFile.close();
+  const logWriteError = failedLogWrite?.reason ?? logWriteErrors[0];
+  if (logWriteError) {
+    throw new Error(`Anthropic prompt gateway log write failed: ${String(logWriteError)}`);
+  }
   return exited;
 }
 
diff --git a/test/scripts/dev-tooling-safety.test.ts b/test/scripts/dev-tooling-safety.test.ts
index 79b62fdd43d..d4e620075c2 100644
--- a/test/scripts/dev-tooling-safety.test.ts
+++ b/test/scripts/dev-tooling-safety.test.ts
@@ -422,6 +422,42 @@ describe("script-specific dev tooling hardening", () => {
     expect(closeCalls).toBe(1);
   });
 
+  it("waits for Anthropic prompt gateway log writes before closing the log file", async () => {
+    let resolveWrite: (() => void) | undefined;
+    const order: string[] = [];
+    const pendingWrite = new Promise<void>((resolve) => {
+      resolveWrite = () => {
+        order.push("write");
+        resolve();
+      };
+    });
+    const stop = promptProbeTesting.stopGatewayPromptChild(
+      {
+        exitCode: 0,
+        signalCode: null,
+        kill: () => true,
+        once(_event: "exit", _listener: () => void) {},
+      },
+      {
+        close: async () => {
+          order.push("close");
+        },
+      },
+      1,
+      1,
+      [pendingWrite],
+    );
+
+    await new Promise((resolve) => {
+      setTimeout(resolve, 10);
+    });
+    expect(order).toEqual([]);
+
+    resolveWrite?.();
+    await expect(stop).resolves.toBe(true);
+    expect(order).toEqual(["write", "close"]);
+  });
+
   it("uses exact Claude cookie host matchers instead of broad substring matches", () => {
     expect(claudeUsageTesting.CLAUDE_COOKIE_HOST_SQL).toContain("host_key = 'claude.ai'");
     expect(claudeUsageTesting.CLAUDE_COOKIE_HOST_SQL).toContain("LIKE '%.claude.ai'");