diff --git a/kernel/relayflowd-core/src/machine.rs b/kernel/relayflowd-core/src/machine.rs
index e7579e714..255b53645 100644
--- a/kernel/relayflowd-core/src/machine.rs
+++ b/kernel/relayflowd-core/src/machine.rs
@@ -60,8 +60,9 @@ pub struct AttemptResult {
     /// Execution failures bypass verification but still follow retry policy.
     pub failure_reason: Option<CompletionReason>,
     /// Why the attempt was rejected, in the vocabulary of whoever rejected it.
-    /// A failed completion journals a null `output` — this is the only place a
-    /// rejection can name its own cause, so a dropped detail is a lost error.
+    /// The structured `output` (exit code + captured stdout/stderr tails) also
+    /// survives into the failed completion record (#292); this human-readable
+    /// detail complements it rather than being the only surviving cause.
     pub failure_detail: Option<String>,
 }
 
@@ -387,6 +388,13 @@ pub fn completion_actions(
         .as_ref()
         .is_some_and(|record| record.verdict == crate::entry::VerificationVerdict::Pass);
     let may_retry = semantic_executions.saturating_add(1) < step.max_iterations;
+    // Preserve `result.output` for successful completions, and for FAILED
+    // deterministic completions specifically — deterministic attempts journal
+    // `{exit_code, stdout_tail, stderr_tail}` so the CLI can render the
+    // diagnostic (#292 unblocks #276). LLM/agent step outputs remain nulled on
+    // verification failure: their `result.output` is the rejected parsed value,
+    // and the existing invariant is that it never survives to the journal.
+    let preserve_failure_output = step.step_type() == StepType::Deterministic;
     let (reason, disposition, output, next_attempt_at_ms) = if verified {
         (
             CompletionReason::Success,
@@ -402,7 +410,7 @@ pub fn completion_actions(
                 .failure_reason
                 .unwrap_or(CompletionReason::VerificationFailed),
             Disposition::Retry,
-            Value::Null,
+            if preserve_failure_output { result.output } else { Value::Null },
             Some(now_ms.saturating_add(delay as i64)),
         )
     } else {
@@ -411,7 +419,7 @@ pub fn completion_actions(
                 .failure_reason
                 .unwrap_or(CompletionReason::RetriesExhausted),
             Disposition::StepDone,
-            Value::Null,
+            if preserve_failure_output { result.output } else { Value::Null },
             None,
         )
     };
diff --git a/kernel/relayflowd-core/src/machine/tests.rs b/kernel/relayflowd-core/src/machine/tests.rs
index 3907712e4..26100af4b 100644
--- a/kernel/relayflowd-core/src/machine/tests.rs
+++ b/kernel/relayflowd-core/src/machine/tests.rs
@@ -62,6 +62,73 @@ fn verification_failure_schedules_a_durable_retry() {
     ));
 }
 
+#[test]
+fn failed_deterministic_completion_preserves_exit_code_and_stderr(
+) {
+    // #292: failed attempts used to journal `output: null`, so the CLI
+    // could not surface the actual exit code or stderr excerpt. Both the
+    // retry branch and the terminal branch must now preserve the captured
+    // shape verbatim from `AttemptResult.output`.
+    let spec = retrying_spec();
+    let mut retryable_step = spec.steps[0].clone();
+    retryable_step.max_iterations = 2;
+    let retry_output = json!({
+        "exit_code": 7,
+        "stdout_tail": "",
+        "stderr_tail": "shakedown intentional failure",
+    });
+    let retry_actions = completion_actions(
+        "run",
+        &retryable_step,
+        1,
+        0,
+        AttemptResult::successful(retry_output.clone(), "kernel"),
+        1_000,
+    );
+    let Action::Append(retry_completed) = &retry_actions[0] else {
+        panic!("failed attempt must append a typed completion");
+    };
+    let retry_payload: StepCompletedPayload =
+        serde_json::from_value(retry_completed.payload.clone())
+            .expect("failed retry completion must deserialize");
+    assert_eq!(retry_payload.disposition, Disposition::Retry);
+    assert_eq!(
+        retry_payload.output, retry_output,
+        "retry path drops the captured exit_code/stderr_tail"
+    );
+
+    let mut terminal_step = spec.steps[0].clone();
+    terminal_step.max_iterations = 1;
+    let terminal_output = json!({
+        "exit_code": 7,
+        "stdout_tail": "",
+        "stderr_tail": "final attempt failed",
+    });
+    let terminal_actions = completion_actions(
+        "run",
+        &terminal_step,
+        1,
+        0,
+        AttemptResult::successful(terminal_output.clone(), "kernel"),
+        2_000,
+    );
+    let Action::Append(terminal_completed) = &terminal_actions[0] else {
+        panic!("terminal failure must append a typed completion");
+    };
+    let terminal_payload: StepCompletedPayload =
+        serde_json::from_value(terminal_completed.payload.clone())
+            .expect("terminal failure completion must deserialize");
+    assert_eq!(terminal_payload.disposition, Disposition::StepDone);
+    assert_eq!(
+        terminal_payload.completion_reason,
+        CompletionReason::RetriesExhausted
+    );
+    assert_eq!(
+        terminal_payload.output, terminal_output,
+        "terminal failure path drops the captured exit_code/stderr_tail"
+    );
+}
+
 #[test]
 fn every_failed_run_terminates_with_declared_completion_reasons() {
     let failure_reasons = [
diff --git a/kernel/relayflowd/src/exec_det.rs b/kernel/relayflowd/src/exec_det.rs
index e907945d7..3a6ef16b6 100644
--- a/kernel/relayflowd/src/exec_det.rs
+++ b/kernel/relayflowd/src/exec_det.rs
@@ -209,7 +209,14 @@ mod tests {
         let Action::Append(completed) = &actions[0] else {
             panic!("expected completion")
         };
-        assert_eq!(completed.payload["output"], serde_json::Value::Null);
+        // Post-#292: structured output survives on failed deterministic completions
+        // so the CLI diagnostic (#276 / merged as #366) can render exit code + stderr
+        // directly, without falling back to trajectory_tail.
+        assert_eq!(completed.payload["output"]["exit_code"], 7);
+        assert_eq!(
+            completed.payload["output"]["stderr_tail"],
+            "shakedown intentional failure"
+        );
         assert_eq!(completed.payload["trajectory_tail"]["exit_code"], 7);
         assert_eq!(
             completed.payload["trajectory_tail"]["stderr_tail"],
diff --git a/packages/sdk/src/cli/deterministic-failure.ts b/packages/sdk/src/cli/deterministic-failure.ts
index 53173d386..3e9176a78 100644
--- a/packages/sdk/src/cli/deterministic-failure.ts
+++ b/packages/sdk/src/cli/deterministic-failure.ts
@@ -28,9 +28,10 @@ export async function deterministicFailureDetails(
       // A later completion supersedes an earlier failed attempt.
       failures.delete(stepId);
       const payload = record(entry['payload']);
-      // Failed completions null reusable output; command evidence survives in
-      // trajectory_tail. Accept output as well for existing completion records.
-      const output = record(payload?.['trajectory_tail']) ?? record(payload?.['output']);
+      // Post-#292 the kernel preserves the captured `{exit_code, stdout_tail,
+      // stderr_tail}` in `output` on failed completions; prefer it. Fall back
+      // to `trajectory_tail` for journal records emitted before that fix.
+      const output = record(payload?.['output']) ?? record(payload?.['trajectory_tail']);
       const exitCode = output?.['exit_code'];
       if (payload?.['disposition'] !== 'step_done' || payload['completionReason'] === 'success'
         || typeof exitCode !== 'number' || !Number.isSafeInteger(exitCode) || exitCode === 0) continue;
diff --git a/packages/sdk/tests/deterministic-failure-diagnostic.test.ts b/packages/sdk/tests/deterministic-failure-diagnostic.test.ts
index fbcc060e1..ace8ba0fa 100644
--- a/packages/sdk/tests/deterministic-failure-diagnostic.test.ts
+++ b/packages/sdk/tests/deterministic-failure-diagnostic.test.ts
@@ -171,4 +171,27 @@ describe('deterministic failure diagnostic', () => {
       kind: 'step_failed', message: expect.stringContaining('invalid journal sequence'),
     });
   });
+
+  it('reads exit_code and stderr_tail from output (post-#292 canonical shape)', async () => {
+    // Post-#292 the kernel emits the captured shape in `output` on failed
+    // completions rather than routing it through `trajectory_tail`. The CLI
+    // must surface the diagnostic from that field even when trajectory_tail
+    // is absent.
+    const post292Completion = {
+      seq: 1, entry_type: 'step.completed', step_id: 'fail-command',
+      payload: {
+        completionReason: 'retries_exhausted', disposition: 'step_done',
+        output: { exit_code: 7, stdout_tail: '', stderr_tail: 'post-292 stderr' },
+        // trajectory_tail intentionally omitted — output is the canonical
+        // carrier once the kernel fix has landed.
+      },
+    };
+    const { client } = stub([[post292Completion]]);
+    const diagnostic = (await classify(client)).report.diagnostics.at(-1) as RunDiagnostic;
+    expect(diagnostic).toMatchObject({
+      kind: 'step_failed', stepId: 'fail-command', exitCode: 7,
+      stderrTail: 'post-292 stderr',
+      hint: 'flows replay run-failed --at fail-command',
+    });
+  });
 });