diff --git a/backend/scripts/p2-acceptance.mjs b/backend/scripts/p2-acceptance.mjs
index 1dd5fd66..1a279a56 100644
--- a/backend/scripts/p2-acceptance.mjs
+++ b/backend/scripts/p2-acceptance.mjs
@@ -776,6 +776,7 @@ async function writeInstallationFiles(ctx) {
       },
       "workspace-maintenance": {
         image: ctx.coreImageTag,
+        environment: { P2_CHILD_DEBUG: "1" },
         extra_hosts: ["host.docker.internal:host-gateway"],
       },
       qdrant: {
diff --git a/backend/src/workspace-maintenance.ts b/backend/src/workspace-maintenance.ts
index e50d1e5e..7a94171d 100644
--- a/backend/src/workspace-maintenance.ts
+++ b/backend/src/workspace-maintenance.ts
@@ -249,8 +249,15 @@ function createProductionService(): WorkspacePreprocessingService {
           let stderr = "";
           child.stdout?.on("data", (chunk: Buffer) => { stdout += chunk.toString("utf8"); });
           child.stderr?.on("data", (chunk: Buffer) => { stderr += chunk.toString("utf8"); });
-          child.on("close", (code) => resolve({ exitCode: code ?? 0, stdout, stderr }));
-          child.on("error", (error) => resolve({ exitCode: 1, stdout, stderr: String(error.message) }));
+          child.on("close", (code) => {
+            const capped = { exitCode: code ?? 0, stdout, stderr: stderr.slice(0, 64 * 1024) };
+            if (process.env.P2_CHILD_DEBUG === "1" && (code ?? 0) !== 0) {
+              capped.stdout = stdout.slice(0, 4096);
+              capped.stderr = stderr.slice(0, 4096);
+            }
+            resolve(capped);
+          });
+          child.on("error", (error) => resolve({ exitCode: 1, stdout, stderr: String(error.message).slice(0, 4096) }));
         });
       } finally {
         closeSync(configFd);
diff --git a/backend/src/workspaces/preprocessing-service.ts b/backend/src/workspaces/preprocessing-service.ts
index da5d7bc6..2a63e486 100644
--- a/backend/src/workspaces/preprocessing-service.ts
+++ b/backend/src/workspaces/preprocessing-service.ts
@@ -427,7 +427,13 @@ export class WorkspacePreprocessingService {
 
   private async runJsonStage(runtime: ActiveRuntime, argv: string[]): Promise<Record<string, unknown>> {
     const result = await this.deps.runChild({ argv, configPath: runtime.configLease.path });
-    if (result.exitCode !== 0) throw new Error("workspace child failed");
+    if (result.exitCode !== 0) {
+      if (process.env.P2_CHILD_DEBUG === "1") {
+        const detail = `${result.stderr ?? ""}${result.stdout ?? ""}`.replace(/[\r\n]+/g, " ").slice(0, 2048);
+        throw new Error(`workspace child failed: ${detail}`);
+      }
+      throw new Error("workspace child failed");
+    }
     return JSON.parse(result.stdout) as Record<string, unknown>;
   }