fix(service): keep explicit backend failure fail-closed

A this-invocation output.md is preserved and never retried over, but
Success=false stays a step failure so truncated writes cannot pass.
Prior-round residue without a trusted patch is also not success.

Co-authored-by: Cursor <cursoragent@cursor.com>

.doc/ops/short-term/2026-09-13-document-revision-prelim-output.md

@@ -40,7 +40,7 @@ mutex: document-revision-prelim-output
 4. [x] 纳入现场时序：write 已流出但 `terminated`；SDK retry 1–3 Connection error；`auto_retry_end success=false`；仍 `agent_settled`；trace `success=true`
 5. [x] settle-policy + Go 分类 + `runOneStep` 有界全新重试 + 单测
 6. [x] `.dockerignore` 放行 `templates/**/*.md`；Docker context 实测 `/app/templates/docs/prelim_design.md`
-7. [x] 已有目标不覆盖；仅 provider+缺文件或 runner 明确失败才重试
+7. [x] 本次写入的目标不覆盖、不再调用；backend 明确失败仍 FAIL（不把文件存在当成功）；上一轮残留不当成功
 8. [ ] 占槽后真实 UI 重跑（本轮不做）
 
 ## 验收标准

.doc/remediation/2026-09-13-document-revision-prelim-output.md

@@ -27,7 +27,7 @@ updated: 2026-09-13
 | Wave 0 | Runner `rel_path` 归一化；缺产物则 `Success=false` | 绝对 / `./` / 重复子目录能落盘；空 patches 失败 |
 | Wave 0 | `runner.mjs`：`auto_retry_end` 失败或目标不存在不得 `ok:true` | 与现场 `agent_settled` 假成功对齐 |
 | Wave 1 | `settle-policy`：`terminated` / `Connection error` / `auto_retry_end` 后 settle 失败，返回 `retryable` | 干净 settle + 缺文件 → `retryable=false` |
-| Wave 1 | Go `AssessStepFailure` + `runOneStep` 有界全新 invocation（≤2） | 仅 `(provider terminated/connection + 缺目标)` 或 runner 明确失败；已有目标不覆盖；永久缺失 1 次；耗尽 fail-closed |
+| Wave 1 | Go `AssessStepFailure` + `runOneStep` 有界全新 invocation（≤2） | 仅 `(provider terminated/connection + 缺目标)` 或 runner 明确失败；本次写入不覆盖且失败不翻成功；上一轮残留不当成功；耗尽 fail-closed |
 | Wave 1 | 旧 runner 假成功：扫描 `execution.log` 识别 settled failure | 不依赖本轮 env-service 重建 |
 
 ## 取舍

internal/agent/step_retry.go

@@ -44,8 +44,9 @@ type StepFailureSignals struct {
 	WorkDir       string
 	ExtraText     string
 	TargetMissing bool
-	// TargetExists is a non-empty deliverable already on disk. Retry must
-	// not overwrite it.
+	// TargetExists means this invocation wrote a non-empty file that differs
+	// from the pre-invoke baseline. Do not retry or overwrite it. The step
+	// still fails unless the backend already reported a trusted success.
 	TargetExists bool
 }
 

internal/squad/evaluator_test.go

@@ -1,6 +1,7 @@
 package squad
 
 import (
+	"bytes"
 	"context"
 	"os"
 	"path/filepath"
@@ -235,5 +236,9 @@ func (b *testEchoBackend) Execute(ctx context.Context, req agent.Request) (*agen
 	if _, err := os.Stat(p); os.IsNotExist(err) {
 		_ = os.WriteFile(p, []byte("## 步骤执行交付产物\n\n已完成任务要求论证与数据沉淀。\n"), 0o644)
 	}
-	return &agent.Result{Success: true}, nil
+	content, err := os.ReadFile(p)
+	if err != nil || len(bytes.TrimSpace(content)) == 0 {
+		return &agent.Result{Success: false, ErrorReason: "missing target deliverable " + target}, nil
+	}
+	return &agent.Result{Success: true, Patches: map[string][]byte{target: content}}, nil
 }

internal/squad/step_retry_test.go

@@ -71,6 +71,11 @@ func (b *countingBackend) Execute(ctx context.Context, req agent.Request) (*agen
 		_ = os.WriteFile(filepath.Join(req.WorkDir, "output.md"), body, 0o644)
 		return &agent.Result{Success: false, ErrorReason: "Connection error.", Retryable: true}, nil
 
+	case "success-without-patch":
+		return &agent.Result{Success: true, Patches: map[string][]byte{
+			"execution.log": []byte("Outcome: SUCCESS\n"),
+		}}, nil
+
 	default:
 		return &agent.Result{Success: false, ErrorReason: "unknown mode"}, nil
 	}
@@ -174,14 +179,14 @@ func TestWorkflowDriver_DoesNotOverwriteExistingTarget(t *testing.T) {
 	wsDir, driver := prepareSingleWriterWS(t)
 	backend := &countingBackend{mode: "fail-after-writing-target"}
 	res, err := driver.Execute(context.Background(), wsDir, backend, squad.ExecuteOptions{Timeout: 5 * time.Second})
-	if err != nil {
-		t.Fatalf("existing target must be accepted, not retried: %v", err)
-	}
-	if !res.Success {
-		t.Fatalf("want success from existing file, got %s", res.ErrorReason)
+	if err == nil || res.Success {
+		t.Fatal("backend explicit failure must fail-closed; a possibly truncated file is not success")
 	}
 	if backend.calls != 1 {
-		t.Fatalf("must not re-invoke over an existing target, calls=%d", backend.calls)
+		t.Fatalf("must not re-invoke over this-invocation target, calls=%d", backend.calls)
+	}
+	if !strings.Contains(err.Error(), "Connection error.") {
+		t.Fatalf("want backend failure preserved, got %v", err)
 	}
 	body, err := os.ReadFile(filepath.Join(wsDir, "tasks", "01_draft", "output.md"))
 	if err != nil {
@@ -192,6 +197,29 @@ func TestWorkflowDriver_DoesNotOverwriteExistingTarget(t *testing.T) {
 	}
 }
 
+func TestWorkflowDriver_PriorResidueIsNotSuccess(t *testing.T) {
+	wsDir, driver := prepareSingleWriterWS(t)
+	leftover := []byte("# leftover from previous round\n")
+	if err := os.WriteFile(filepath.Join(wsDir, "tasks", "01_draft", "output.md"), leftover, 0o644); err != nil {
+		t.Fatal(err)
+	}
+	backend := &countingBackend{mode: "success-without-patch"}
+	res, err := driver.Execute(context.Background(), wsDir, backend, squad.ExecuteOptions{Timeout: 5 * time.Second})
+	if err == nil || res.Success {
+		t.Fatal("prior-round residue must not count as this-invocation success")
+	}
+	if backend.calls != 1 {
+		t.Fatalf("residue without trusted patch is a permanent contract miss, calls=%d", backend.calls)
+	}
+	got, err := os.ReadFile(filepath.Join(wsDir, "tasks", "01_draft", "output.md"))
+	if err != nil {
+		t.Fatal(err)
+	}
+	if string(got) != string(leftover) {
+		t.Fatalf("residue must stay on disk unchanged: %s", got)
+	}
+}
+
 func TestWorkflowDriver_BoundedTransientRetryThenFailClosed(t *testing.T) {
 	wsDir, driver := prepareSingleWriterWS(t)
 	backend := &countingBackend{mode: "always-transient", failText: "Connection error."}

internal/squad/workflow.go

@@ -1,6 +1,7 @@
 package squad
 
 import (
+	"bytes"
 	"context"
 	"encoding/json"
 	"fmt"
@@ -512,13 +513,9 @@ func (d *WorkflowDriver) runOneStep(
 	)
 	for attempt := 1; attempt <= agent.MaxTransientStepInvocations; attempt++ {
 		onDisk, _ := os.ReadFile(prevOutPath)
-		produced := len(strings.TrimSpace(string(onDisk))) > 0 && string(onDisk) != prevContent
-		if attempt > 1 && produced {
-			ok = true
-			matErr = nil
-			if res != nil {
-				res.Success = true
-			}
+		wroteThisInvocation := deliverableWroteThisInvocation(prevContent, onDisk)
+		if attempt > 1 && wroteThisInvocation {
+			// Preserve a this-invocation file (may be truncated). Do not call again.
 			break
 		}
 		if attempt > 1 && streamer != nil {
@@ -536,32 +533,34 @@ func (d *WorkflowDriver) runOneStep(
 		if res != nil {
 			incoming = res.Patches
 		}
+		hasTrustedPatch := incomingHasDeliverable(subDir, deliverableName, incoming)
 
 		if execErr == nil && res != nil && res.Success {
 			if _, matErr = agent.MaterializeTargets(subDir, []string{deliverableName}, incoming); matErr != nil {
 				res.Success = false
 				res.ErrorReason = matErr.Error()
+			} else if !hasTrustedPatch {
+				after, _ := os.ReadFile(prevOutPath)
+				if !deliverableWroteThisInvocation(prevContent, after) {
+					matErr = fmt.Errorf("missing target deliverable %s", deliverableName)
+					res.Success = false
+					res.ErrorReason = matErr.Error()
+				}
 			}
 		}
 
 		onDisk, _ = os.ReadFile(prevOutPath)
-		produced = len(strings.TrimSpace(string(onDisk))) > 0 && string(onDisk) != prevContent
+		wroteThisInvocation = deliverableWroteThisInvocation(prevContent, onDisk)
 		newContent := ""
-		if produced {
-			newContent = string(onDisk)
-		} else if res != nil {
+		if hasTrustedPatch && res != nil {
 			if b, has := res.Patches[deliverableName]; has {
 				newContent = string(b)
 			}
 		}
-		if newContent == "" {
+		if newContent == "" && wroteThisInvocation {
 			newContent = string(onDisk)
 		}
-		ok = (execErr == nil && res != nil && res.Success && matErr == nil) || produced
-		if produced && res != nil {
-			res.Success = true
-			matErr = nil
-		}
+		ok = execErr == nil && res != nil && res.Success && matErr == nil
 		sim := OutputSimilarity(prevContent, newContent)
 		if mu != nil {
 			mu.Lock()
@@ -604,8 +603,8 @@ func (d *WorkflowDriver) runOneStep(
 			Err:           execErr,
 			Result:        res,
 			WorkDir:       subDir,
-			TargetMissing: !produced && (matErr != nil || (res != nil && strings.Contains(res.ErrorReason, "missing target deliverable"))),
-			TargetExists:  produced,
+			TargetMissing: !wroteThisInvocation && (matErr != nil || (res != nil && strings.Contains(res.ErrorReason, "missing target deliverable"))),
+			TargetExists:  wroteThisInvocation,
 		})
 		if assess.Class != agent.FailureTransient || attempt >= agent.MaxTransientStepInvocations {
 			break
@@ -711,6 +710,25 @@ func (d *WorkflowDriver) runOneStep(
 	return nil
 }
 
+func deliverableWroteThisInvocation(baseline string, onDisk []byte) bool {
+	if len(bytes.TrimSpace(onDisk)) == 0 {
+		return false
+	}
+	return string(onDisk) != baseline
+}
+
+func incomingHasDeliverable(workDir, target string, incoming map[string][]byte) bool {
+	for k, v := range incoming {
+		if len(bytes.TrimSpace(v)) == 0 {
+			continue
+		}
+		if agent.MatchDeliverableRel(workDir, k, target) {
+			return true
+		}
+	}
+	return false
+}
+
 func (d *WorkflowDriver) applyDynamicAdjustment(
 	ctx context.Context,
 	wsDir string,