diff --git a/packages/adapter-utils/src/execution-target-sandbox.test.ts b/packages/adapter-utils/src/execution-target-sandbox.test.ts
index 19ce7fd727..002e418d28 100644
--- a/packages/adapter-utils/src/execution-target-sandbox.test.ts
+++ b/packages/adapter-utils/src/execution-target-sandbox.test.ts
@@ -148,7 +148,22 @@ describe("sandbox adapter execution targets", () => {
     throw new Error(message);
   }
 
-  async function runProxyWithInput(command: string, input: string): Promise<{ stdout: string; stderr: string; code: number | null }> {
+  type ProxyRunResult = {
+    stdout: string;
+    stderr: string;
+    code: number | null;
+    /**
+     * How long the exchange took. The bridge and the proxy both run on 5s
+     * budgets, which is generous locally and tight on a CI runner sharing a
+     * box with 19 other lanes. A run that returns fast and empty is a
+     * different fault from one that nearly hit the ceiling, and the numbers
+     * are the only way to tell them apart after the fact.
+     */
+    elapsedMs: number;
+  };
+
+  async function runProxyWithInput(command: string, input: string): Promise<ProxyRunResult> {
+    const startedAt = performance.now();
     const child = spawn(command, [], { stdio: ["pipe", "pipe", "pipe"] });
     let stdout = "";
     let stderr = "";
@@ -175,7 +190,65 @@ describe("sandbox adapter execution targets", () => {
         resolve(exitCode);
       });
     });
-    return { stdout, stderr, code };
+    return { stdout, stderr, code, elapsedMs: Math.round(performance.now() - startedAt) };
+  }
+
+  /**
+   * A failure report for a proxy exchange, attached to the assertions below.
+   *
+   * `execution-target-sandbox` has failed twice in CI and never once in a few
+   * hundred local runs, so the next occurrence has to carry its own evidence -
+   * a second unreproducible failure teaches nothing. The observed signature was
+   * an empty stdout with exit code 0, meaning the child exited cleanly having
+   * produced nothing, which is what a lost stdin frame looks like from here.
+   *
+   * The runtime tree is the part that discriminates. The stdin queue files are
+   * written by the host and deleted by the wrapper once parsed, so what remains
+   * says whether the frame was never written, written and never consumed, or
+   * consumed normally and the reply lost on the way back.
+   */
+  async function describeProxyRun(result: ProxyRunResult, runtimeRootDir: string): Promise<string> {
+    const lines = [
+      `proxy exit=${result.code} elapsedMs=${result.elapsedMs}`,
+      `proxy stdout=${JSON.stringify(result.stdout)}`,
+      `proxy stderr=${JSON.stringify(result.stderr)}`,
+    ];
+    const walk = async (dir: string, depth: number): Promise<void> => {
+      // Deep enough to reach the queue frames, which are the point. They sit
+      // at process-sessions/<id>/stdin/<seq>.json — depth 4 from the runtime
+      // root — so a cap of 3 listed the `stdin/` directory and stopped, making
+      // "the queue is empty" and "the walk never looked" print identically.
+      if (depth > 5) return;
+      let entries;
+      try {
+        entries = await readdir(dir, { withFileTypes: true });
+      } catch (error) {
+        lines.push(`${"  ".repeat(depth)}<unreadable ${dir}: ${(error as Error).message}>`);
+        return;
+      }
+      for (const entry of entries.sort((a, b) => a.name.localeCompare(b.name))) {
+        const full = path.join(dir, entry.name);
+        if (entry.isDirectory()) {
+          lines.push(`${"  ".repeat(depth)}${entry.name}/`);
+          await walk(full, depth + 1);
+          continue;
+        }
+        // Small files are the queue and event frames, and their contents are
+        // the point. Anything larger is a child script or a log; the size is
+        // enough to say it exists.
+        let detail = "";
+        try {
+          const raw = await readFile(full, "utf8");
+          detail = raw.length <= 400 ? ` ${JSON.stringify(raw)}` : ` <${raw.length}B>`;
+        } catch (error) {
+          detail = ` <unreadable: ${(error as Error).message}>`;
+        }
+        lines.push(`${"  ".repeat(depth)}${entry.name}${detail}`);
+      }
+    };
+    lines.push(`runtime tree under ${runtimeRootDir}:`);
+    await walk(runtimeRootDir, 1);
+    return lines.join("\n");
   }
 
   function combinedStream(
@@ -729,9 +802,10 @@ describe("sandbox adapter execution targets", () => {
 
     try {
       const result = await runProxyWithInput(bridge!.agentCommand, "hello\n");
-      expect(result.code).toBe(0);
-      expect(result.stdout).toBe("out:hello\n");
-      expect(result.stderr).toBe("err:hello\n");
+      const report = await describeProxyRun(result, path.posix.join(rootDir, ".paperclip-runtime", "acpx"));
+      expect(result.code, report).toBe(0);
+      expect(result.stdout, report).toBe("out:hello\n");
+      expect(result.stderr, report).toBe("err:hello\n");
     } finally {
       await bridge?.stop();
     }
@@ -1026,9 +1100,10 @@ describe("sandbox adapter execution targets", () => {
 
       try {
         const result = await runProxyWithInput(bridge!.agentCommand, "hello\n");
-        expect(result.code).toBe(0);
-        expect(result.stdout).toBe("out:hello\n");
-        expect(result.stderr).toBe("err:hello\n");
+        const report = await describeProxyRun(result, path.posix.join(rootDir, ".paperclip-runtime", "acpx"));
+        expect(result.code, report).toBe(0);
+        expect(result.stdout, report).toBe("out:hello\n");
+        expect(result.stderr, report).toBe("err:hello\n");
       } finally {
         await bridge?.stop();
       }
@@ -1087,8 +1162,9 @@ describe("sandbox adapter execution targets", () => {
         // frame flows, so it is observable as soon as the handle resolves.
         expect(spanNames).toContain("sandbox.agentProcess");
         const result = await runProxyWithInput(bridge!.agentCommand, "hello\n");
-        expect(result.code).toBe(0);
-        expect(result.stdout).toBe("out:hello\n");
+        const report = await describeProxyRun(result, path.posix.join(rootDir, ".paperclip-runtime", "acpx"));
+        expect(result.code, report).toBe(0);
+        expect(result.stdout, report).toBe("out:hello\n");
       } finally {
         await bridge?.stop();
       }
@@ -1480,7 +1556,10 @@ describe("sandbox adapter execution targets", () => {
         // Round-trip one input so a stdin-delivery control exec runs and gets
         // recorded before the assertions below.
         const result = await runProxyWithInput(bridge!.agentCommand, "hello\n");
-        expect(result.stdout).toBe("out:hello\n");
+        expect(
+          result.stdout,
+          await describeProxyRun(result, path.posix.join(rootDir, ".paperclip-runtime", "acpx")),
+        ).toBe("out:hello\n");
 
         // Exactly one exec runs on the persistent session: the long-lived agent
         // command. It streams its output through the session log stream, so it