diff --git a/src/process/supervisor/supervisor.test.ts b/src/process/supervisor/supervisor.test.ts
index a5aaabd3edd..08bd10ab0f4 100644
--- a/src/process/supervisor/supervisor.test.ts
+++ b/src/process/supervisor/supervisor.test.ts
@@ -290,4 +290,42 @@ describe("process supervisor", () => {
     expect(streamed).toBe("streamed");
     expect(exit.stdout).toBe("");
   });
+
+  it("bounds retained stdout and stderr while streaming full chunks", async () => {
+    const adapter = createStubChildAdapter();
+    createChildAdapterMock.mockResolvedValue(adapter);
+
+    const supervisor = createProcessSupervisor();
+    let streamedStdout = "";
+    let streamedStderr = "";
+    const stdoutChunk = `${"a".repeat(300)}stdout-tail`;
+    const stderrChunk = `${"b".repeat(300)}stderr-tail`;
+    const run = await spawnChild(supervisor, {
+      sessionId: "s-capture-cap",
+      argv: createWriteStdoutArgv(stdoutChunk),
+      timeoutMs: 1_000,
+      stdinMode: "pipe-closed",
+      maxCapturedOutputChars: 256,
+      onStdout: (chunk) => {
+        streamedStdout += chunk;
+      },
+      onStderr: (chunk) => {
+        streamedStderr += chunk;
+      },
+    });
+
+    adapter.emitStdout(stdoutChunk);
+    adapter.emitStderr(stderrChunk);
+    adapter.settle(0);
+
+    const exit = await run.wait();
+    expect(streamedStdout).toBe(stdoutChunk);
+    expect(streamedStderr).toBe(stderrChunk);
+    expect(exit.stdout.length).toBeLessThanOrEqual(256);
+    expect(exit.stderr.length).toBeLessThanOrEqual(256);
+    expect(exit.stdout).toContain("captured stdout truncated");
+    expect(exit.stderr).toContain("captured stderr truncated");
+    expect(exit.stdout.endsWith("stdout-tail")).toBe(true);
+    expect(exit.stderr.endsWith("stderr-tail")).toBe(true);
+  });
 });
diff --git a/src/process/supervisor/supervisor.ts b/src/process/supervisor/supervisor.ts
index a111ec67170..3f7ef1587cd 100644
--- a/src/process/supervisor/supervisor.ts
+++ b/src/process/supervisor/supervisor.ts
@@ -21,6 +21,7 @@ type ActiveRun = {
 };
 
 const GRACEFUL_CANCEL_TIMEOUT_MS = 5000;
+const DEFAULT_MAX_CAPTURED_OUTPUT_CHARS = 1024 * 1024;
 
 let supervisorLogRuntimePromise: Promise<SupervisorLogRuntime> | undefined;
 
@@ -36,6 +37,28 @@ function clampTimeout(value?: number): number | undefined {
   return Math.max(1, Math.floor(value));
 }
 
+function clampCapturedOutputChars(value?: number): number {
+  if (typeof value !== "number" || !Number.isFinite(value) || value <= 0) {
+    return DEFAULT_MAX_CAPTURED_OUTPUT_CHARS;
+  }
+  return Math.max(256, Math.floor(value));
+}
+
+function appendCapturedOutput(
+  current: string,
+  chunk: string,
+  stream: "stdout" | "stderr",
+  maxChars: number,
+) {
+  const next = current + chunk;
+  if (next.length <= maxChars) {
+    return next;
+  }
+  const marker = `[openclaw: captured ${stream} truncated to last ${maxChars} chars]\n`;
+  const tailChars = Math.max(0, maxChars - marker.length);
+  return `${marker}${next.slice(-tailChars)}`;
+}
+
 function isTimeoutReason(reason: TerminationReason) {
   return reason === "overall-timeout" || reason === "no-output-timeout";
 }
@@ -95,6 +118,7 @@ export function createProcessSupervisor(): ProcessSupervisor {
     let noOutputTimer: NodeJS.Timeout | null = null;
     let forceKillTimer: NodeJS.Timeout | null = null;
     const captureOutput = input.captureOutput !== false;
+    const maxCapturedOutputChars = clampCapturedOutputChars(input.maxCapturedOutputChars);
 
     const overallTimeoutMs = clampTimeout(input.timeoutMs);
     const noOutputTimeoutMs = clampTimeout(input.noOutputTimeoutMs);
@@ -198,14 +222,14 @@ export function createProcessSupervisor(): ProcessSupervisor {
 
       adapter.onStdout((chunk) => {
         if (captureOutput) {
-          stdout += chunk;
+          stdout = appendCapturedOutput(stdout, chunk, "stdout", maxCapturedOutputChars);
         }
         input.onStdout?.(chunk);
         touchOutput();
       });
       adapter.onStderr((chunk) => {
         if (captureOutput) {
-          stderr += chunk;
+          stderr = appendCapturedOutput(stderr, chunk, "stderr", maxCapturedOutputChars);
         }
         input.onStderr?.(chunk);
         touchOutput();
diff --git a/src/process/supervisor/types.ts b/src/process/supervisor/types.ts
index 24a4f03bfff..ce06299a588 100644
--- a/src/process/supervisor/types.ts
+++ b/src/process/supervisor/types.ts
@@ -81,6 +81,11 @@ type SpawnBaseInput = {
    * When false, stdout/stderr are streamed via callbacks only and not retained in RunExit payload.
    */
   captureOutput?: boolean;
+  /**
+   * Maximum retained stdout/stderr characters per stream when captureOutput is enabled.
+   * Streaming callbacks still receive full chunks.
+   */
+  maxCapturedOutputChars?: number;
   onStdout?: (chunk: string) => void;
   onStderr?: (chunk: string) => void;
 };