diff --git a/agent/tools/sandbox_diagnostics.ts b/agent/tools/sandbox_diagnostics.ts
new file mode 100644
index 0000000..27fa424
--- /dev/null
+++ b/agent/tools/sandbox_diagnostics.ts
@@ -0,0 +1,219 @@
+import { defineTool } from "eve/tools";
+import { z } from "zod";
+
+export default defineTool({
+  description:
+    "Run sandbox diagnostics to verify resources (CPU, memory), " +
+    "the project build toolchain (typecheck, build, test), and background " +
+    "process liveliness. Use this to confirm the sandbox meets the required " +
+    "operating caps before or after infrastructure changes.",
+  inputSchema: z.object({
+    skipBackgroundTest: z
+      .boolean()
+      .default(false)
+      .describe(
+        "Skip the background-process liveliness test. " +
+          "Set to true when you only need resource and build checks.",
+      ),
+  }),
+  outputSchema: z.object({
+    resources: z.object({
+      vcpus: z.number(),
+      memoryTotalMb: z.number(),
+      memoryAvailableMb: z.number(),
+    }),
+    toolchain: z.array(
+      z.object({
+        name: z.string(),
+        passed: z.boolean(),
+        timingMs: z.number(),
+        detail: z.string(),
+      }),
+    ),
+    backgroundTest: z
+      .object({
+        launched: z.boolean(),
+        completed: z.boolean(),
+        exitCode: z.number().nullable(),
+        detail: z.string(),
+      })
+      .nullable(),
+    summary: z.string(),
+  }),
+  async execute({ skipBackgroundTest }, ctx) {
+    const sandbox = await ctx.getSandbox();
+    const output: string[] = [];
+    const toolchainResults: Array<{
+      name: string;
+      passed: boolean;
+      timingMs: number;
+      detail: string;
+    }> = [];
+
+    // ===================================================================
+    // 1. Resources
+    // ===================================================================
+    const nproc = await sandbox.run({ command: "nproc" });
+    const free = await sandbox.run({ command: "free -m" });
+
+    const vcpus = Number(nproc.stdout.trim()) || 0;
+
+    // Parse free -m output: "Mem:  total  used  free  shared  buff/cache  available"
+    const memLine =
+      free.stdout
+        .split("\n")
+        .find((l) => l.startsWith("Mem:")) ?? "";
+    const memParts = memLine.split(/\s+/).filter(Boolean);
+    const memTotalMb = Number(memParts[1] ?? 0);
+    const memAvailableMb = Number(memParts[6] ?? 0);
+
+    output.push(
+      `Resources:\n  vCPUs: ${vcpus}\n  Memory: ${memTotalMb} MB total, ${memAvailableMb} MB available`,
+    );
+
+    // ===================================================================
+    // 2. Toolchain
+    // ===================================================================
+    const toolchainSteps = [
+      { name: "typecheck", command: "npm run typecheck", cwd: "/workspace" },
+      { name: "build", command: "npm run build", cwd: "/workspace" },
+      { name: "test", command: "npm test", cwd: "/workspace" },
+    ];
+
+    for (const step of toolchainSteps) {
+      const start = Date.now();
+      const result = await sandbox.run({
+        command: `cd ${step.cwd} && ${step.command}`,
+      });
+      const elapsed = Date.now() - start;
+      const passed = result.exitCode === 0;
+      toolchainResults.push({
+        name: step.name,
+        passed,
+        timingMs: elapsed,
+        detail: passed
+          ? `Passed (${elapsed}ms)`
+          : `Failed (exit ${result.exitCode}): ${
+              (result.stderr || result.stdout).slice(0, 300)
+            }`,
+      });
+      output.push(
+        `${step.name}: ${passed ? "PASS" : "FAIL"} (${elapsed}ms)` +
+          (passed ? "" : ` — exit ${result.exitCode}`),
+      );
+    }
+
+    // ===================================================================
+    // 3. Background-process liveliness
+    // ===================================================================
+    let backgroundTest: {
+      launched: boolean;
+      completed: boolean;
+      exitCode: number | null;
+      detail: string;
+    } | null = null;
+
+    if (!skipBackgroundTest) {
+      const bgStart = Date.now();
+      const dir = "/workspace/.evolve-tasks/bg-diag-test";
+
+      await sandbox.run({ command: `mkdir -p ${dir}` });
+      await sandbox.writeTextFile({
+        path: `${dir}/cmd.sh`,
+        content: "sleep 5 && echo background-test-passed\n",
+      });
+
+      // Launch a short-lived background process using the same mechanism as
+      // the start_background tool
+      const launch = await sandbox.run({
+        command:
+          `( setsid bash -l ${dir}/cmd.sh > ${dir}/output.log 2>&1; ` +
+          `echo $? > ${dir}/exitcode ) & ` +
+          `echo $! > ${dir}/pid && echo launched`,
+      });
+
+      if (!launch.stdout.includes("launched")) {
+        backgroundTest = {
+          launched: false,
+          completed: false,
+          exitCode: null,
+          detail: `Failed to launch: ${
+            (launch.stderr || launch.stdout).slice(0, 200)
+          }`,
+        };
+      } else {
+        // Give it a moment then check completion
+        await new Promise((r) => setTimeout(r, 7000));
+
+        const check = await sandbox.run({
+          command:
+            `if [ -f ${dir}/exitcode ]; then ` +
+            `echo "STATE:exited:$(cat ${dir}/exitcode)"; ` +
+            `tail -3 ${dir}/output.log; ` +
+            `elif kill -0 "$(cat ${dir}/pid 2>/dev/null)" 2>/dev/null; then ` +
+            `echo "STATE:running"; ` +
+            `else echo "STATE:crashed"; fi`,
+        });
+
+        const elapsed = Date.now() - bgStart;
+        const stateLine =
+          check.stdout
+            .split("\n")
+            .find((l) => l.startsWith("STATE:")) ?? "";
+
+        if (stateLine.startsWith("STATE:exited:")) {
+          const exitCode = Number(
+            stateLine.slice("STATE:exited:".length).trim(),
+          );
+          const completed = exitCode === 0;
+          backgroundTest = {
+            launched: true,
+            completed,
+            exitCode: Number.isFinite(exitCode) ? exitCode : null,
+            detail: completed
+              ? `Completed in ${elapsed}ms with exit code ${exitCode}`
+              : `Failed (exit ${exitCode}) after ${elapsed}ms`,
+          };
+          output.push(
+            `Background process: ${completed ? "PASS" : "FAIL"} (${elapsed}ms, exit ${exitCode})`,
+          );
+        } else if (stateLine.startsWith("STATE:running")) {
+          backgroundTest = {
+            launched: true,
+            completed: false,
+            exitCode: null,
+            detail: `Still running after ${elapsed}ms — waited 7s for a 5s sleep`,
+          };
+          output.push(`Background process: STILL RUNNING after ${elapsed}ms`);
+        } else {
+          backgroundTest = {
+            launched: true,
+            completed: false,
+            exitCode: null,
+            detail: `Process crashed (state: ${stateLine})`,
+          };
+          output.push("Background process: CRASHED");
+        }
+      }
+    } else {
+      output.push("Background test: skipped");
+    }
+
+    // ===================================================================
+    // Summary
+    // ===================================================================
+    const toolchainOk = toolchainResults.every((r) => r.passed);
+    const bgOk = backgroundTest === null || backgroundTest.completed;
+    const allOk = toolchainOk && bgOk;
+    const summary = allOk ? "ALL CHECKS PASSED" : "SOME CHECKS FAILED";
+
+    output.unshift(`=== Sandbox Diagnostics: ${summary} ===\n`);
+
+    return {
+      resources: { vcpus, memoryTotalMb: memTotalMb, memoryAvailableMb: memAvailableMb },
+      toolchain: toolchainResults,
+      backgroundTest,
+      summary,
+    };
+  },
+});