diff --git a/apps/backend/internal/agent/runtime/agentctl/launcher/launcher.go b/apps/backend/internal/agent/runtime/agentctl/launcher/launcher.go
index 7f58f3d4fe..06400a12b9 100644
--- a/apps/backend/internal/agent/runtime/agentctl/launcher/launcher.go
+++ b/apps/backend/internal/agent/runtime/agentctl/launcher/launcher.go
@@ -518,21 +518,94 @@ func (l *Launcher) pipeOutput(name string, scanner *bufio.Scanner) {
 	}
 }
 
-// childLogLevel extracts the level token from a line emitted by the agentctl
-// child's console-format zap logger: "<ts>\t<LEVEL>\t<caller>\t<msg>", where
-// LEVEL is capitalized and may be wrapped in ANSI color codes. It returns the
-// uppercased level ("INFO"/"WARN"/…) or "" when the line does not match that
-// shape.
+// childLogLevel extracts a recognized level from the agentctl child's trusted
+// structured log formats. It returns the uppercased level ("INFO"/"WARN"/…)
+// or "" when the line does not match one of those formats.
 func childLogLevel(line string) string {
 	// A well-formed console record is "<ts>\t<LEVEL>\t<caller>\t<msg>", so the
 	// level token must be bounded by at least a following caller field. Requiring
 	// three segments rejects truncated lines (e.g. "<ts>\t<token>") whose second
 	// field is not actually a level, so they fall back to WARN.
 	fields := strings.SplitN(line, "\t", 4)
-	if len(fields) < 3 {
+	if len(fields) >= 3 {
+		if level := recognizedChildLogLevel(stripANSI(fields[1])); level != "" {
+			return level
+		}
+	}
+
+	if level := childSlogTextLevel(line); level != "" {
+		return level
+	}
+
+	// slog.Default uses the standard log package's date/time prefix followed by
+	// the level and message, for example "2026/08/12 10:00:00 INFO message".
+	defaultFields := strings.Fields(line)
+	if len(defaultFields) < 3 {
 		return ""
 	}
-	level := strings.ToUpper(stripANSI(fields[1]))
+	if _, err := time.Parse("2006/01/02 15:04:05", defaultFields[0]+" "+defaultFields[1]); err != nil {
+		return ""
+	}
+	return recognizedChildLogLevel(stripANSI(defaultFields[2]))
+}
+
+func childSlogTextLevel(line string) string {
+	fields, ok := splitSlogTextFields(line)
+	if !ok || len(fields) < 3 || !strings.HasPrefix(fields[0], "time=") ||
+		!strings.HasPrefix(fields[1], "level=") {
+		return ""
+	}
+	for _, field := range fields[2:] {
+		if strings.HasPrefix(field, "msg=") {
+			return recognizedChildLogLevel(stripANSI(strings.TrimPrefix(fields[1], "level=")))
+		}
+	}
+	return ""
+}
+
+func splitSlogTextFields(line string) ([]string, bool) {
+	fields := make([]string, 0, 4)
+	start := -1
+	inQuotes := false
+	escaped := false
+	for i := 0; i < len(line); i++ {
+		ch := line[i]
+		if start < 0 {
+			if ch == ' ' || ch == '\t' {
+				continue
+			}
+			start = i
+		}
+		if inQuotes {
+			switch {
+			case escaped:
+				escaped = false
+			case ch == '\\':
+				escaped = true
+			case ch == '"':
+				inQuotes = false
+			}
+			continue
+		}
+		switch ch {
+		case '"':
+			inQuotes = true
+		case ' ', '\t':
+			fields = append(fields, line[start:i])
+			start = -1
+		}
+	}
+	if inQuotes || escaped {
+		return nil, false
+	}
+	if start >= 0 {
+		fields = append(fields, line[start:])
+	}
+	return fields, true
+}
+
+func recognizedChildLogLevel(level string) string {
+	level = strings.ToUpper(level)
 	switch level {
 	case "DEBUG", "INFO", "WARN", "ERROR", "FATAL", "PANIC", "DPANIC":
 		return level
diff --git a/apps/backend/internal/agent/runtime/agentctl/launcher/launcher_loglevel_test.go b/apps/backend/internal/agent/runtime/agentctl/launcher/launcher_loglevel_test.go
index abec2d454e..f09f2fb7b6 100644
--- a/apps/backend/internal/agent/runtime/agentctl/launcher/launcher_loglevel_test.go
+++ b/apps/backend/internal/agent/runtime/agentctl/launcher/launcher_loglevel_test.go
@@ -1,6 +1,12 @@
 package launcher
 
-import "testing"
+import (
+	"bytes"
+	stdlog "log"
+	"log/slog"
+	"strings"
+	"testing"
+)
 
 func TestChildLogLevel(t *testing.T) {
 	tests := []struct {
@@ -78,6 +84,98 @@ func TestChildLogLevel(t *testing.T) {
 	}
 }
 
+func TestChildLogLevelSlogTextRecords(t *testing.T) {
+	tests := []struct {
+		name  string
+		level string
+		want  string
+	}{
+		{name: "debug", level: "DEBUG", want: "DEBUG"},
+		{name: "info", level: "INFO", want: "INFO"},
+		{name: "warn", level: "WARN", want: "WARN"},
+		{name: "error", level: "ERROR", want: "ERROR"},
+	}
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			line := "time=2026-08-12T10:00:00.000Z level=" + tt.level +
+				" source=main.go:97 msg=\"agentctl record\""
+			if got := childLogLevel(line); got != tt.want {
+				t.Fatalf("childLogLevel(%q) = %q, want %q", line, got, tt.want)
+			}
+		})
+	}
+}
+
+func TestChildLogLevelSlogDefaultRecords(t *testing.T) {
+	var output bytes.Buffer
+	originalWriter := stdlog.Writer()
+	originalFlags := stdlog.Flags()
+	stdlog.SetOutput(&output)
+	stdlog.SetFlags(stdlog.LstdFlags)
+	t.Cleanup(func() {
+		stdlog.SetOutput(originalWriter)
+		stdlog.SetFlags(originalFlags)
+	})
+
+	tests := []struct {
+		name  string
+		write func()
+		want  string
+	}{
+		{name: "info", write: func() { slog.Default().Info("agentctl record", "component", "acp-conn") }, want: "INFO"},
+		{name: "warn", write: func() { slog.Default().Warn("agentctl record", "component", "acp-conn") }, want: "WARN"},
+		{name: "error", write: func() { slog.Default().Error("agentctl record", "component", "acp-conn") }, want: "ERROR"},
+	}
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			output.Reset()
+			tt.write()
+			line := strings.TrimSpace(output.String())
+			if line == "" {
+				t.Fatal("slog.Default produced no output")
+			}
+			if got := childLogLevel(line); got != tt.want {
+				t.Fatalf("childLogLevel(%q) = %q, want %q", line, got, tt.want)
+			}
+		})
+	}
+}
+
+func TestChildLogLevelRejectsMalformedSlogTextRecords(t *testing.T) {
+	tests := []struct {
+		name string
+		line string
+	}{
+		{
+			name: "missing time anchor",
+			line: "level=INFO source=main.go:97 msg=\"agentctl record\"",
+		},
+		{
+			name: "missing level anchor",
+			line: "time=2026-08-12T10:00:00.000Z msg=\"level=INFO\"",
+		},
+		{
+			name: "missing message field",
+			line: "time=2026-08-12T10:00:00.000Z level=INFO source=main.go:97",
+		},
+		{
+			name: "level anchor is not second field",
+			line: "time=2026-08-12T10:00:00.000Z source=main.go:97 level=INFO msg=\"record\"",
+		},
+		{
+			name: "message-looking key is inside a quoted attribute",
+			line: `time=2026-08-12T10:00:00.000Z level=INFO attr="foo msg=bar" component=acp-conn`,
+		},
+	}
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			if got := childLogLevel(tt.line); got != "" {
+				t.Fatalf("childLogLevel(%q) = %q, want no recognized level", tt.line, got)
+			}
+		})
+	}
+}
+
 func TestStripANSI(t *testing.T) {
 	tests := []struct {
 		name string
diff --git a/apps/backend/internal/mcp/handlers/task_plan_handlers_test.go b/apps/backend/internal/mcp/handlers/task_plan_handlers_test.go
index acb997efba..bc85ed8661 100644
--- a/apps/backend/internal/mcp/handlers/task_plan_handlers_test.go
+++ b/apps/backend/internal/mcp/handlers/task_plan_handlers_test.go
@@ -4,6 +4,7 @@ import (
 	"context"
 	"encoding/json"
 	"path/filepath"
+	"strings"
 	"testing"
 	"time"
 
@@ -13,6 +14,7 @@ import (
 	"github.com/kandev/kandev/internal/events/bus"
 	"github.com/kandev/kandev/internal/task/models"
 	"github.com/kandev/kandev/internal/task/repository"
+	sqliterepo "github.com/kandev/kandev/internal/task/repository/sqlite"
 	"github.com/kandev/kandev/internal/task/service"
 	v1 "github.com/kandev/kandev/pkg/api/v1"
 	ws "github.com/kandev/kandev/pkg/websocket"
@@ -36,6 +38,11 @@ const (
 // newMCPPlanTestHandlers builds Handlers with only the plan service wired —
 // the plan actions touch nothing else.
 func newMCPPlanTestHandlers(t *testing.T) *Handlers {
+	h, _ := newMCPPlanTestHandlersWithRepo(t)
+	return h
+}
+
+func newMCPPlanTestHandlersWithRepo(t *testing.T) (*Handlers, *sqliterepo.Repository) {
 	t.Helper()
 	dbConn, err := db.OpenSQLite(filepath.Join(t.TempDir(), "test.db"))
 	if err != nil {
@@ -91,7 +98,7 @@ func newMCPPlanTestHandlers(t *testing.T) *Handlers {
 		}
 	}
 
-	return &Handlers{planService: service.NewPlanService(repo, eventBus, log), logger: log}
+	return &Handlers{planService: service.NewPlanService(repo, eventBus, log), logger: log}, repo
 }
 
 func mcpPlanMsg(t *testing.T, action string, payload string) *ws.Message {
@@ -202,6 +209,47 @@ func TestMCPPlanActionsReportMissingPlan(t *testing.T) {
 	})
 }
 
+func TestMCPPlanCreateReportsMissingTask(t *testing.T) {
+	h := newMCPPlanTestHandlers(t)
+	out, err := h.handleCreateTaskPlan(context.Background(), mcpPlanMsg(t, ws.ActionMCPCreateTaskPlan,
+		`{"task_id":"task-plan-mcp-missing","content":"body"}`))
+	assertMCPPlanError(t, out, err, ws.ErrorCodeNotFound, "Task not found")
+	if strings.Contains(strings.ToLower(string(out.Payload)), "constraint") {
+		t.Fatalf("error payload leaks storage details: %s", out.Payload)
+	}
+}
+
+func TestMCPPlanUpdateReportsMissingTask(t *testing.T) {
+	h, repo := newMCPPlanTestHandlersWithRepo(t)
+	ctx := context.Background()
+	if _, err := h.planService.CreatePlan(ctx, service.CreatePlanRequest{
+		TaskID: mcpPlanTaskID, Content: "initial", CreatedBy: "agent",
+	}); err != nil {
+		t.Fatalf("CreatePlan: %v", err)
+	}
+	h.planService = service.NewPlanService(&missingTaskOnPlanWriteRepo{Repository: repo}, nil, h.logger)
+
+	out, err := h.handleUpdateTaskPlan(ctx, mcpPlanMsg(t, ws.ActionMCPUpdateTaskPlan,
+		`{"task_id":"`+mcpPlanTaskID+`","content":"updated"}`))
+	assertMCPPlanError(t, out, err, ws.ErrorCodeNotFound, "Task not found")
+	if strings.Contains(strings.ToLower(string(out.Payload)), "constraint") {
+		t.Fatalf("error payload leaks storage details: %s", out.Payload)
+	}
+}
+
+type missingTaskOnPlanWriteRepo struct {
+	*sqliterepo.Repository
+}
+
+func (r *missingTaskOnPlanWriteRepo) WritePlanRevision(
+	context.Context,
+	*models.TaskPlan,
+	*models.TaskPlanRevision,
+	*string,
+) error {
+	return repository.ErrTaskNotFound
+}
+
 // TestMCPPlanActionsSucceed pins the success payloads across a full CRUD round
 // trip. Two replies deliberately differ from the browser surface and must not
 // be "unified" away: get returns {} rather than null for a task with no plan,
diff --git a/apps/backend/internal/orchestrator/reconcile_liveness.go b/apps/backend/internal/orchestrator/reconcile_liveness.go
index 3952991f0f..bd79dc0098 100644
--- a/apps/backend/internal/orchestrator/reconcile_liveness.go
+++ b/apps/backend/internal/orchestrator/reconcile_liveness.go
@@ -186,14 +186,14 @@ func (s *Service) repairDeadRowLiveness(ctx context.Context, running *models.Exe
 	} else {
 		err = s.repo.RepairExecutorRunningDead(ctx, running.SessionID)
 	}
-	if err != nil && !errors.Is(err, models.ErrExecutorRunningNotFound) {
+	if errors.Is(err, models.ErrExecutorRunningNotFound) || errors.Is(err, models.ErrExecutionRotated) {
+		return nil
+	}
+	if err != nil {
 		s.logger.Warn("failed to repair executor row liveness during reconciliation",
 			zap.String("session_id", running.SessionID),
 			zap.Error(err))
 	}
-	if errors.Is(err, models.ErrExecutorRunningNotFound) || errors.Is(err, models.ErrExecutionRotated) {
-		return nil
-	}
 	return err
 }
 
diff --git a/apps/backend/internal/orchestrator/reconcile_liveness_test.go b/apps/backend/internal/orchestrator/reconcile_liveness_test.go
index cbcf3a5590..45afdaccb5 100644
--- a/apps/backend/internal/orchestrator/reconcile_liveness_test.go
+++ b/apps/backend/internal/orchestrator/reconcile_liveness_test.go
@@ -13,6 +13,7 @@ import (
 	"github.com/kandev/kandev/internal/common/logger"
 	"github.com/kandev/kandev/internal/task/models"
 	"go.uber.org/zap"
+	"go.uber.org/zap/zapcore"
 	"go.uber.org/zap/zaptest/observer"
 )
 
@@ -135,6 +136,71 @@ func TestStopRuntimeForStartupCleanupPreservesUnknownRowsWithoutHandle(t *testin
 	}
 }
 
+func TestRepairDeadRowLivenessLogSeverity(t *testing.T) {
+	t.Run("rotated execution is expected", func(t *testing.T) {
+		repo := setupTestRepo(t)
+		ctx := context.Background()
+		seedSession(t, repo, "task-rotated", "session-rotated", "")
+		if err := repo.UpsertExecutorRunning(ctx, &models.ExecutorRunning{
+			ID: "session-rotated", SessionID: "session-rotated", TaskID: "task-rotated",
+			AgentExecutionID: "new-execution", Status: models.ExecutorRunningStatusRunning,
+		}); err != nil {
+			t.Fatalf("UpsertExecutorRunning: %v", err)
+		}
+		current, err := repo.GetExecutorRunningBySessionID(ctx, "session-rotated")
+		if err != nil {
+			t.Fatalf("GetExecutorRunningBySessionID: %v", err)
+		}
+		stale := *current
+		stale.AgentExecutionID = "old-execution"
+
+		service := createTestServiceWithAgent(repo, newMockStepGetter(), newMockTaskRepo(), &mockAgentManager{})
+		core, logs := observer.New(zapcore.DebugLevel)
+		log, err := logger.NewFromZap(zap.New(core))
+		if err != nil {
+			t.Fatalf("NewFromZap: %v", err)
+		}
+		service.logger = log
+
+		if err := service.repairDeadRowLiveness(ctx, &stale); err != nil {
+			t.Fatalf("repairDeadRowLiveness: %v", err)
+		}
+		if entries := logs.FilterLevelExact(zapcore.WarnLevel).All(); len(entries) != 0 {
+			t.Fatalf("rotated repair warning count = %d, want 0", len(entries))
+		}
+		current, err = repo.GetExecutorRunningBySessionID(ctx, "session-rotated")
+		if err != nil {
+			t.Fatalf("GetExecutorRunningBySessionID after repair: %v", err)
+		}
+		if current.AgentExecutionID != "new-execution" || current.Status != models.ExecutorRunningStatusRunning {
+			t.Fatalf("rotated row = %+v, want newer running execution unchanged", current)
+		}
+	})
+
+	t.Run("unrelated repair error remains warning", func(t *testing.T) {
+		repo := setupTestRepo(t)
+		service := createTestServiceWithAgent(repo, newMockStepGetter(), newMockTaskRepo(), &mockAgentManager{})
+		core, logs := observer.New(zapcore.DebugLevel)
+		log, err := logger.NewFromZap(zap.New(core))
+		if err != nil {
+			t.Fatalf("NewFromZap: %v", err)
+		}
+		service.logger = log
+
+		err = service.repairDeadRowLiveness(context.Background(), &models.ExecutorRunning{})
+		if err == nil {
+			t.Fatal("repairDeadRowLiveness returned nil for an invalid repair request")
+		}
+		entries := logs.FilterMessage("failed to repair executor row liveness during reconciliation").All()
+		if len(entries) != 1 {
+			t.Fatalf("unrelated repair warning count = %d, want 1", len(entries))
+		}
+		if entries[0].Level != zapcore.WarnLevel {
+			t.Fatalf("unrelated repair log level = %s, want warn", entries[0].Level)
+		}
+	})
+}
+
 func containsAll(value string, needles ...string) bool {
 	for _, needle := range needles {
 		if !strings.Contains(value, needle) {
diff --git a/apps/backend/internal/task/handlers/task_plan_handlers_test.go b/apps/backend/internal/task/handlers/task_plan_handlers_test.go
index 1cf33fb10d..a8ea8ba17c 100644
--- a/apps/backend/internal/task/handlers/task_plan_handlers_test.go
+++ b/apps/backend/internal/task/handlers/task_plan_handlers_test.go
@@ -4,6 +4,7 @@ import (
 	"context"
 	"encoding/json"
 	"path/filepath"
+	"strings"
 	"testing"
 	"time"
 
@@ -13,6 +14,7 @@ import (
 	"github.com/kandev/kandev/internal/events/bus"
 	"github.com/kandev/kandev/internal/task/models"
 	"github.com/kandev/kandev/internal/task/repository"
+	sqliterepo "github.com/kandev/kandev/internal/task/repository/sqlite"
 	"github.com/kandev/kandev/internal/task/service"
 	v1 "github.com/kandev/kandev/pkg/api/v1"
 	ws "github.com/kandev/kandev/pkg/websocket"
@@ -36,6 +38,11 @@ const (
 // newPlanTestHandlers builds TaskHandlers with only the plan service wired —
 // the plan actions touch nothing else.
 func newPlanTestHandlers(t *testing.T) *TaskHandlers {
+	h, _ := newPlanTestHandlersWithRepo(t)
+	return h
+}
+
+func newPlanTestHandlersWithRepo(t *testing.T) (*TaskHandlers, *sqliterepo.Repository) {
 	t.Helper()
 	dbConn, err := db.OpenSQLite(filepath.Join(t.TempDir(), "test.db"))
 	if err != nil {
@@ -91,7 +98,7 @@ func newPlanTestHandlers(t *testing.T) *TaskHandlers {
 		}
 	}
 
-	return &TaskHandlers{planService: service.NewPlanService(repo, eventBus, log), logger: log}
+	return &TaskHandlers{planService: service.NewPlanService(repo, eventBus, log), logger: log}, repo
 }
 
 func planMsg(t *testing.T, action string, payload string) *ws.Message {
@@ -186,6 +193,47 @@ func TestPlanWSActionsReportMissingPlan(t *testing.T) {
 	})
 }
 
+func TestPlanWSCreateReportsMissingTask(t *testing.T) {
+	h := newPlanTestHandlers(t)
+	out, err := h.wsCreateTaskPlan(context.Background(), planMsg(t, ws.ActionTaskPlanCreate,
+		`{"task_id":"task-plan-ws-missing","content":"body"}`))
+	assertPlanError(t, out, err, ws.ErrorCodeNotFound, "Task not found")
+	if strings.Contains(strings.ToLower(string(out.Payload)), "constraint") {
+		t.Fatalf("error payload leaks storage details: %s", out.Payload)
+	}
+}
+
+func TestPlanWSUpdateReportsMissingTask(t *testing.T) {
+	h, repo := newPlanTestHandlersWithRepo(t)
+	ctx := context.Background()
+	if _, err := h.planService.CreatePlan(ctx, service.CreatePlanRequest{
+		TaskID: planTaskID, Content: "initial", CreatedBy: "user",
+	}); err != nil {
+		t.Fatalf("CreatePlan: %v", err)
+	}
+	h.planService = service.NewPlanService(&missingTaskOnPlanWriteRepo{Repository: repo}, nil, h.logger)
+
+	out, err := h.wsUpdateTaskPlan(ctx, planMsg(t, ws.ActionTaskPlanUpdate,
+		`{"task_id":"`+planTaskID+`","content":"updated"}`))
+	assertPlanError(t, out, err, ws.ErrorCodeNotFound, "Task not found")
+	if strings.Contains(strings.ToLower(string(out.Payload)), "constraint") {
+		t.Fatalf("error payload leaks storage details: %s", out.Payload)
+	}
+}
+
+type missingTaskOnPlanWriteRepo struct {
+	*sqliterepo.Repository
+}
+
+func (r *missingTaskOnPlanWriteRepo) WritePlanRevision(
+	context.Context,
+	*models.TaskPlan,
+	*models.TaskPlanRevision,
+	*string,
+) error {
+	return repository.ErrTaskNotFound
+}
+
 // TestPlanWSActionsSucceed pins the success payloads across a full CRUD round
 // trip, including the browser surface's null reply for a task with no plan and
 // its {"success":true} delete reply.
diff --git a/apps/backend/internal/task/planws/errors.go b/apps/backend/internal/task/planws/errors.go
index 33a331bb02..f886c778e5 100644
--- a/apps/backend/internal/task/planws/errors.go
+++ b/apps/backend/internal/task/planws/errors.go
@@ -13,6 +13,7 @@ package planws
 import (
 	"errors"
 
+	"github.com/kandev/kandev/internal/task/repository"
 	"github.com/kandev/kandev/internal/task/service"
 	ws "github.com/kandev/kandev/pkg/websocket"
 )
@@ -35,6 +36,7 @@ type mapping struct {
 // MCP tools already receive, so editing one now moves both surfaces together.
 var (
 	taskIDRequired       = mapping{service.ErrTaskIDRequired, ws.ErrorCodeValidation, "task_id is required"}
+	taskNotFound         = mapping{repository.ErrTaskNotFound, ws.ErrorCodeNotFound, "Task not found"}
 	sessionIDRequired    = mapping{service.ErrSessionIDRequired, ws.ErrorCodeValidation, "session_id is required"}
 	sessionTaskMismatch  = mapping{service.ErrSessionTaskMismatch, ws.ErrorCodeValidation, "Session does not belong to task"}
 	planNotFound         = mapping{service.ErrTaskPlanNotFound, ws.ErrorCodeNotFound, "Task plan not found"}
@@ -46,6 +48,7 @@ var (
 	// plan or revision failure.
 	allMappings = []mapping{
 		taskIDRequired,
+		taskNotFound,
 		sessionIDRequired,
 		sessionTaskMismatch,
 		planNotFound,
@@ -73,9 +76,11 @@ func errorResponse(msg *ws.Message, err error, fallback string, recognized []map
 //
 // ErrTaskPlanNotFound is deliberately absent: CreatePlan only returns it when
 // the plan vanishes between the write and the read-back, which is a server-side
-// inconsistency rather than the caller naming a plan that does not exist.
+// inconsistency rather than the caller naming a plan that does not exist. The
+// repository's ErrTaskNotFound is different: it means the write target task
+// did not exist and is safe to report as a not-found response.
 func CreateError(msg *ws.Message, err error) (*ws.Message, error) {
-	return errorResponse(msg, err, "Failed to create task plan: "+err.Error(), []mapping{taskIDRequired})
+	return errorResponse(msg, err, "Failed to create task plan: "+err.Error(), []mapping{taskIDRequired, taskNotFound})
 }
 
 // GetError maps a PlanService.GetPlan failure. The fallback omits err.Error()
@@ -87,7 +92,7 @@ func GetError(msg *ws.Message, err error) (*ws.Message, error) {
 
 // UpdateError maps a PlanService.UpdatePlan failure.
 func UpdateError(msg *ws.Message, err error) (*ws.Message, error) {
-	return errorResponse(msg, err, "Failed to update task plan: "+err.Error(), []mapping{taskIDRequired, planNotFound})
+	return errorResponse(msg, err, "Failed to update task plan: "+err.Error(), []mapping{taskIDRequired, taskNotFound, planNotFound})
 }
 
 // DeleteError maps a PlanService.DeletePlan failure.
diff --git a/apps/backend/internal/task/planws/errors_test.go b/apps/backend/internal/task/planws/errors_test.go
index e1565ffabe..a34997a088 100644
--- a/apps/backend/internal/task/planws/errors_test.go
+++ b/apps/backend/internal/task/planws/errors_test.go
@@ -6,6 +6,7 @@ import (
 	"fmt"
 	"testing"
 
+	"github.com/kandev/kandev/internal/task/repository"
 	"github.com/kandev/kandev/internal/task/service"
 	ws "github.com/kandev/kandev/pkg/websocket"
 )
@@ -36,6 +37,7 @@ func decode(t *testing.T, msg *ws.Message, err error) ws.ErrorPayload {
 // production message changes, which it cannot do if both read the same symbol.
 const (
 	wantTaskIDRequired   = "task_id is required"
+	wantTaskNotFound     = "Task not found"
 	wantTaskPlanNotFound = "Task plan not found"
 )
 
@@ -57,6 +59,7 @@ func TestMappersCoverEverySentinel(t *testing.T) {
 		message string
 	}{
 		{"create/task_id", CreateError, service.ErrTaskIDRequired, ws.ErrorCodeValidation, wantTaskIDRequired},
+		{"create/task_not_found", CreateError, repository.ErrTaskNotFound, ws.ErrorCodeNotFound, wantTaskNotFound},
 		{"create/fallback", CreateError, boom, ws.ErrorCodeInternalError, "Failed to create task plan: disk on fire"},
 		// CreatePlan's ErrTaskPlanNotFound means the read-back after a
 		// successful write found nothing — a server fault, not a missing plan.
@@ -67,6 +70,7 @@ func TestMappersCoverEverySentinel(t *testing.T) {
 		{"get/fallback", GetError, boom, ws.ErrorCodeInternalError, "Failed to get task plan"},
 
 		{"update/task_id", UpdateError, service.ErrTaskIDRequired, ws.ErrorCodeValidation, wantTaskIDRequired},
+		{"update/task_not_found", UpdateError, repository.ErrTaskNotFound, ws.ErrorCodeNotFound, wantTaskNotFound},
 		{"update/plan_not_found", UpdateError, service.ErrTaskPlanNotFound, ws.ErrorCodeNotFound, wantTaskPlanNotFound},
 		{"update/fallback", UpdateError, boom, ws.ErrorCodeInternalError, "Failed to update task plan: disk on fire"},
 
@@ -99,6 +103,7 @@ func TestErrorCoversFullVocabulary(t *testing.T) {
 		message string
 	}{
 		{service.ErrTaskIDRequired, ws.ErrorCodeValidation, wantTaskIDRequired},
+		{repository.ErrTaskNotFound, ws.ErrorCodeNotFound, wantTaskNotFound},
 		{service.ErrSessionIDRequired, ws.ErrorCodeValidation, "session_id is required"},
 		{service.ErrSessionTaskMismatch, ws.ErrorCodeValidation, "Session does not belong to task"},
 		{service.ErrTaskPlanNotFound, ws.ErrorCodeNotFound, wantTaskPlanNotFound},
diff --git a/apps/backend/internal/task/repository/sqlite/document_plan_postgres_test.go b/apps/backend/internal/task/repository/sqlite/document_plan_postgres_test.go
index c3c0baf208..545dd47e75 100644
--- a/apps/backend/internal/task/repository/sqlite/document_plan_postgres_test.go
+++ b/apps/backend/internal/task/repository/sqlite/document_plan_postgres_test.go
@@ -203,3 +203,27 @@ func TestPostgresWritePlanRevisionUpsertAndImplementationMarker(t *testing.T) {
 		t.Errorf("GetTaskPlan(missing) = %+v, %v; want nil, nil", missing, err)
 	}
 }
+
+func TestPostgresWritePlanRevisionMissingTask(t *testing.T) {
+	db := testutil.OpenIsolatedPostgres(t, testutil.PostgresDSNFromEnv(t))
+	repo, err := NewWithDB(db, db, nil)
+	if err != nil {
+		t.Fatalf("init postgres schema: %v", err)
+	}
+	ctx := context.Background()
+	taskID := "task-plan-pg-missing"
+
+	err = repo.WritePlanRevision(ctx,
+		&models.TaskPlan{ID: "plan-pg-missing", TaskID: taskID, Title: "Plan", Content: "body"},
+		&models.TaskPlanRevision{TaskID: taskID, Title: "Plan", Content: "body", AuthorKind: "agent"},
+		nil)
+	if !errors.Is(err, ErrTaskNotFound) {
+		t.Fatalf("WritePlanRevision error = %v, want ErrTaskNotFound", err)
+	}
+	if got := countRows(t, repo, `SELECT COUNT(1) FROM task_plans WHERE task_id = ?`, taskID); got != 0 {
+		t.Fatalf("plan HEAD rows = %d, want 0 after rollback", got)
+	}
+	if got := countRows(t, repo, `SELECT COUNT(1) FROM task_plan_revisions WHERE task_id = ?`, taskID); got != 0 {
+		t.Fatalf("plan revision rows = %d, want 0 after rollback", got)
+	}
+}
diff --git a/apps/backend/internal/task/repository/sqlite/plan.go b/apps/backend/internal/task/repository/sqlite/plan.go
index 05a75fd13c..c3e3b8b424 100644
--- a/apps/backend/internal/task/repository/sqlite/plan.go
+++ b/apps/backend/internal/task/repository/sqlite/plan.go
@@ -8,6 +8,7 @@ import (
 
 	"github.com/google/uuid"
 	"github.com/jmoiron/sqlx"
+	internaldb "github.com/kandev/kandev/internal/db"
 	"github.com/kandev/kandev/internal/task/models"
 )
 
@@ -333,6 +334,9 @@ func upsertPlanHead(ctx context.Context, tx *sqlx.Tx, db *sqlx.DB, head *models.
 			created_by = excluded.created_by,
 			updated_at = excluded.updated_at
 	`), head.ID, head.TaskID, head.Title, head.Content, head.CreatedBy, head.CreatedAt, head.UpdatedAt); err != nil {
+		if internaldb.IsForeignKeyViolation(err) {
+			return fmt.Errorf("upsert task plan head for task %s: %w", head.TaskID, ErrTaskNotFound)
+		}
 		return fmt.Errorf("upsert task plan head: %w", err)
 	}
 	return nil
diff --git a/apps/backend/internal/task/repository/sqlite/plan_test.go b/apps/backend/internal/task/repository/sqlite/plan_test.go
index c1455bf2f7..28e2b73e8c 100644
--- a/apps/backend/internal/task/repository/sqlite/plan_test.go
+++ b/apps/backend/internal/task/repository/sqlite/plan_test.go
@@ -582,6 +582,26 @@ func TestWritePlanRevisionCreatesHeadAndFirstRevision(t *testing.T) {
 	assertPlanRevisionEqual(t, gotRev, rev)
 }
 
+func TestWritePlanRevisionMissingTask(t *testing.T) {
+	repo := newRepoForEntityTests(t)
+	ctx := context.Background()
+	taskID := "task-writeplan-missing"
+
+	err := repo.WritePlanRevision(ctx,
+		&models.TaskPlan{ID: "plan-missing", TaskID: taskID, Title: "Plan", Content: "body"},
+		&models.TaskPlanRevision{TaskID: taskID, Title: "Plan", Content: "body", AuthorKind: "agent"},
+		nil)
+	if !errors.Is(err, ErrTaskNotFound) {
+		t.Fatalf("WritePlanRevision error = %v, want ErrTaskNotFound", err)
+	}
+	if got := countRows(t, repo, `SELECT COUNT(1) FROM task_plans WHERE task_id = ?`, taskID); got != 0 {
+		t.Fatalf("plan HEAD rows = %d, want 0 after rollback", got)
+	}
+	if got := countRows(t, repo, `SELECT COUNT(1) FROM task_plan_revisions WHERE task_id = ?`, taskID); got != 0 {
+		t.Fatalf("plan revision rows = %d, want 0 after rollback", got)
+	}
+}
+
 func TestWritePlanRevisionUpsertsHeadAndAppendsRevisions(t *testing.T) {
 	repo := newRepoForEntityTests(t)
 	ctx := context.Background()
diff --git a/apps/backend/internal/task/service/plan_service.go b/apps/backend/internal/task/service/plan_service.go
index 743b2c2e37..70dd6391f3 100644
--- a/apps/backend/internal/task/service/plan_service.go
+++ b/apps/backend/internal/task/service/plan_service.go
@@ -238,7 +238,7 @@ func (s *PlanService) upsertPlan(ctx context.Context, req CreatePlanRequest) (*m
 	}
 
 	if err := s.repo.WritePlanRevision(ctx, plan, rev, coalesceID); err != nil {
-		s.logger.Error("write plan revision", zap.String("task_id", req.TaskID), zap.Error(err))
+		s.logPlanWriteError(req.TaskID, err)
 		return nil, err
 	}
 
@@ -497,6 +497,7 @@ func (s *PlanService) RevertPlan(ctx context.Context, req RevertPlanRequest) (*m
 		RevertOfRevisionID: &targetID,
 	}
 	if err := s.repo.WritePlanRevision(ctx, plan, rev, nil); err != nil {
+		s.logPlanWriteError(req.TaskID, err)
 		return nil, err
 	}
 
@@ -513,6 +514,15 @@ func (s *PlanService) RevertPlan(ctx context.Context, req RevertPlanRequest) (*m
 	return rev, nil
 }
 
+func (s *PlanService) logPlanWriteError(taskID string, err error) {
+	fields := []zap.Field{zap.String("task_id", taskID), zap.Error(err)}
+	if errors.Is(err, repository.ErrTaskNotFound) {
+		s.logger.Debug("write plan revision", fields...)
+		return
+	}
+	s.logger.Error("write plan revision", fields...)
+}
+
 func (s *PlanService) publishPlanEvent(ctx context.Context, eventType string, plan *models.TaskPlan) {
 	if s.eventBus == nil {
 		return
diff --git a/apps/backend/internal/task/service/plan_service_logging_test.go b/apps/backend/internal/task/service/plan_service_logging_test.go
new file mode 100644
index 0000000000..d63bf11710
--- /dev/null
+++ b/apps/backend/internal/task/service/plan_service_logging_test.go
@@ -0,0 +1,145 @@
+package service
+
+import (
+	"context"
+	"errors"
+	"testing"
+
+	"github.com/kandev/kandev/internal/task/models"
+	"github.com/kandev/kandev/internal/task/repository/repoerrors"
+	"go.uber.org/zap/zapcore"
+)
+
+func TestPlanServiceMissingTaskWriteLogSeverity(t *testing.T) {
+	svc, _, _ := createTestPlanService(t)
+	log, logs := newObservedServiceLogger(t)
+	svc.logger = log
+
+	_, err := svc.CreatePlan(context.Background(), CreatePlanRequest{
+		TaskID: "task-plan-service-missing", Content: "body",
+	})
+	if !errors.Is(err, repoerrors.ErrTaskNotFound) {
+		t.Fatalf("CreatePlan error = %v, want ErrTaskNotFound", err)
+	}
+	entries := logs.FilterMessage("write plan revision").All()
+	if len(entries) != 1 {
+		t.Fatalf("write-plan log count = %d, want 1", len(entries))
+	}
+	if entries[0].Level != zapcore.DebugLevel {
+		t.Fatalf("write-plan log level = %s, want debug", entries[0].Level)
+	}
+	if errorsCount := logs.FilterLevelExact(zapcore.ErrorLevel).Len(); errorsCount != 0 {
+		t.Fatalf("error log count = %d, want 0 for missing task", errorsCount)
+	}
+}
+
+func TestPlanServiceOtherWriteErrorLogSeverity(t *testing.T) {
+	svc, _, repo := createTestPlanService(t)
+	injected := errors.New("database unavailable")
+	svc.repo = &planWriteErrorRepo{planRepo: repo, err: injected}
+	log, logs := newObservedServiceLogger(t)
+	svc.logger = log
+
+	_, err := svc.CreatePlan(context.Background(), CreatePlanRequest{
+		TaskID: "task-plan-service-error", Content: "body",
+	})
+	if !errors.Is(err, injected) {
+		t.Fatalf("CreatePlan error = %v, want injected error", err)
+	}
+	entries := logs.FilterMessage("write plan revision").All()
+	if len(entries) != 1 {
+		t.Fatalf("write-plan log count = %d, want 1", len(entries))
+	}
+	if entries[0].Level != zapcore.ErrorLevel {
+		t.Fatalf("write-plan log level = %s, want error", entries[0].Level)
+	}
+}
+
+func TestPlanServiceRevertMissingTaskWriteLogSeverity(t *testing.T) {
+	svc, _, repo := createTestPlanService(t)
+	ctx := context.Background()
+	seedTask(t, ctx, repo, "task-plan-revert-missing")
+
+	_, err := svc.CreatePlan(ctx, CreatePlanRequest{
+		TaskID: "task-plan-revert-missing", Content: "body",
+	})
+	if err != nil {
+		t.Fatalf("CreatePlan error = %v", err)
+	}
+	revision, err := repo.GetLatestTaskPlanRevision(ctx, "task-plan-revert-missing")
+	if err != nil {
+		t.Fatalf("GetLatestTaskPlanRevision error = %v", err)
+	}
+
+	svc.repo = &planWriteErrorRepo{planRepo: repo, err: repoerrors.ErrTaskNotFound}
+	log, logs := newObservedServiceLogger(t)
+	svc.logger = log
+
+	_, err = svc.RevertPlan(ctx, RevertPlanRequest{
+		TaskID:           "task-plan-revert-missing",
+		TargetRevisionID: revision.ID,
+	})
+	if !errors.Is(err, repoerrors.ErrTaskNotFound) {
+		t.Fatalf("RevertPlan error = %v, want ErrTaskNotFound", err)
+	}
+	entries := logs.FilterMessage("write plan revision").All()
+	if len(entries) != 1 {
+		t.Fatalf("write-plan log count = %d, want 1", len(entries))
+	}
+	if entries[0].Level != zapcore.DebugLevel {
+		t.Fatalf("write-plan log level = %s, want debug", entries[0].Level)
+	}
+	if errorsCount := logs.FilterLevelExact(zapcore.ErrorLevel).Len(); errorsCount != 0 {
+		t.Fatalf("error log count = %d, want 0 for missing task", errorsCount)
+	}
+}
+
+func TestPlanServiceRevertOtherWriteErrorLogSeverity(t *testing.T) {
+	svc, _, repo := createTestPlanService(t)
+	ctx := context.Background()
+	seedTask(t, ctx, repo, "task-plan-revert-error")
+
+	_, err := svc.CreatePlan(ctx, CreatePlanRequest{
+		TaskID: "task-plan-revert-error", Content: "body",
+	})
+	if err != nil {
+		t.Fatalf("CreatePlan error = %v", err)
+	}
+	revision, err := repo.GetLatestTaskPlanRevision(ctx, "task-plan-revert-error")
+	if err != nil {
+		t.Fatalf("GetLatestTaskPlanRevision error = %v", err)
+	}
+	injected := errors.New("database unavailable")
+	svc.repo = &planWriteErrorRepo{planRepo: repo, err: injected}
+	log, logs := newObservedServiceLogger(t)
+	svc.logger = log
+
+	_, err = svc.RevertPlan(ctx, RevertPlanRequest{
+		TaskID:           "task-plan-revert-error",
+		TargetRevisionID: revision.ID,
+	})
+	if !errors.Is(err, injected) {
+		t.Fatalf("RevertPlan error = %v, want injected error", err)
+	}
+	entries := logs.FilterMessage("write plan revision").All()
+	if len(entries) != 1 {
+		t.Fatalf("write-plan log count = %d, want 1", len(entries))
+	}
+	if entries[0].Level != zapcore.ErrorLevel {
+		t.Fatalf("write-plan log level = %s, want error", entries[0].Level)
+	}
+}
+
+type planWriteErrorRepo struct {
+	planRepo
+	err error
+}
+
+func (r *planWriteErrorRepo) WritePlanRevision(
+	context.Context,
+	*models.TaskPlan,
+	*models.TaskPlanRevision,
+	*string,
+) error {
+	return r.err
+}
diff --git a/apps/backend/internal/task/service/service_turns.go b/apps/backend/internal/task/service/service_turns.go
index 825fe0a2ba..151ab6910d 100644
--- a/apps/backend/internal/task/service/service_turns.go
+++ b/apps/backend/internal/task/service/service_turns.go
@@ -20,6 +20,7 @@ import (
 	"github.com/kandev/kandev/internal/events/bus"
 	"github.com/kandev/kandev/internal/steptelemetry"
 	"github.com/kandev/kandev/internal/task/models"
+	taskrepo "github.com/kandev/kandev/internal/task/repository"
 	"github.com/kandev/kandev/internal/worktree"
 )
 
@@ -852,10 +853,16 @@ func (s *Service) GetWorkspaceInfoForSession(ctx context.Context, taskID, sessio
 	if session.TaskEnvironmentID != "" {
 		env, envErr := s.taskEnvironments.GetTaskEnvironment(ctx, session.TaskEnvironmentID)
 		if envErr != nil {
-			s.logger.Warn("failed to get task environment for session",
+			logFields := []zap.Field{
 				zap.String("session_id", sessionID),
 				zap.String("task_environment_id", session.TaskEnvironmentID),
-				zap.Error(envErr))
+				zap.Error(envErr),
+			}
+			if errors.Is(envErr, taskrepo.ErrTaskEnvironmentNotFound) {
+				s.logger.Debug("failed to get task environment for session", logFields...)
+			} else {
+				s.logger.Warn("failed to get task environment for session", logFields...)
+			}
 		} else {
 			taskEnv = env
 		}
diff --git a/apps/backend/internal/task/service/service_turns_logging_test.go b/apps/backend/internal/task/service/service_turns_logging_test.go
new file mode 100644
index 0000000000..0f60134401
--- /dev/null
+++ b/apps/backend/internal/task/service/service_turns_logging_test.go
@@ -0,0 +1,102 @@
+package service
+
+import (
+	"context"
+	"errors"
+	"testing"
+
+	commonlogger "github.com/kandev/kandev/internal/common/logger"
+	"github.com/kandev/kandev/internal/task/models"
+	"github.com/kandev/kandev/internal/task/repository"
+	"go.uber.org/zap"
+	"go.uber.org/zap/zapcore"
+	"go.uber.org/zap/zaptest/observer"
+)
+
+func TestGetWorkspaceInfoForSession_MissingEnvironmentLogSeverity(t *testing.T) {
+	svc, _, repo := createTestService(t)
+	ctx := context.Background()
+	setupTestTask(t, repo)
+	if err := repo.CreateTaskSession(ctx, &models.TaskSession{
+		ID: "session-stale-environment", TaskID: "task-123",
+		TaskEnvironmentID: "env-stale", State: models.TaskSessionStateCompleted,
+	}); err != nil {
+		t.Fatalf("CreateTaskSession: %v", err)
+	}
+	createTestEnvironment(t, repo, "env-current", "task-123")
+
+	log, logs := newObservedServiceLogger(t)
+	svc.logger = log
+
+	info, err := svc.GetWorkspaceInfoForSession(ctx, "task-123", "session-stale-environment")
+	if err != nil {
+		t.Fatalf("GetWorkspaceInfoForSession: %v", err)
+	}
+	if info.TaskEnvironmentID != "env-current" {
+		t.Fatalf("TaskEnvironmentID = %q, want env-current", info.TaskEnvironmentID)
+	}
+	if entries := logs.FilterLevelExact(zapcore.WarnLevel).All(); len(entries) != 0 {
+		t.Fatalf("stale environment warning count = %d, want 0", len(entries))
+	}
+	entries := logs.FilterMessage("failed to get task environment for session").All()
+	if len(entries) != 1 {
+		t.Fatalf("stale environment debug evidence count = %d, want 1", len(entries))
+	}
+	if entries[0].Level != zapcore.DebugLevel {
+		t.Fatalf("stale environment log level = %s, want debug", entries[0].Level)
+	}
+}
+
+func TestGetWorkspaceInfoForSession_UnexpectedEnvironmentErrorLogSeverity(t *testing.T) {
+	svc, _, repo := createTestService(t)
+	ctx := context.Background()
+	setupTestTask(t, repo)
+	if err := repo.CreateTaskSession(ctx, &models.TaskSession{
+		ID: "session-environment-error", TaskID: "task-123",
+		TaskEnvironmentID: "env-unavailable", State: models.TaskSessionStateCompleted,
+	}); err != nil {
+		t.Fatalf("CreateTaskSession: %v", err)
+	}
+	createTestEnvironment(t, repo, "env-current", "task-123")
+	svc.taskEnvironments = &workspaceInfoTaskEnvironmentRepo{
+		TaskEnvironmentRepository: repo,
+		directErr:                 errors.New("environment store unavailable"),
+	}
+
+	log, logs := newObservedServiceLogger(t)
+	svc.logger = log
+
+	info, err := svc.GetWorkspaceInfoForSession(ctx, "task-123", "session-environment-error")
+	if err != nil {
+		t.Fatalf("GetWorkspaceInfoForSession: %v", err)
+	}
+	if info.TaskEnvironmentID != "env-current" {
+		t.Fatalf("TaskEnvironmentID = %q, want env-current", info.TaskEnvironmentID)
+	}
+	entries := logs.FilterMessage("failed to get task environment for session").All()
+	if len(entries) != 1 {
+		t.Fatalf("unexpected environment warning count = %d, want 1", len(entries))
+	}
+	if entries[0].Level != zapcore.WarnLevel {
+		t.Fatalf("unexpected environment log level = %s, want warn", entries[0].Level)
+	}
+}
+
+type workspaceInfoTaskEnvironmentRepo struct {
+	repository.TaskEnvironmentRepository
+	directErr error
+}
+
+func (r *workspaceInfoTaskEnvironmentRepo) GetTaskEnvironment(context.Context, string) (*models.TaskEnvironment, error) {
+	return nil, r.directErr
+}
+
+func newObservedServiceLogger(t *testing.T) (*commonlogger.Logger, *observer.ObservedLogs) {
+	t.Helper()
+	core, logs := observer.New(zapcore.DebugLevel)
+	log, err := commonlogger.NewFromZap(zap.New(core))
+	if err != nil {
+		t.Fatalf("NewFromZap: %v", err)
+	}
+	return log, logs
+}
diff --git a/docs/plans/backend-diagnostic-signal-quality/plan.md b/docs/plans/backend-diagnostic-signal-quality/plan.md
new file mode 100644
index 0000000000..a74a3a9f3a
--- /dev/null
+++ b/docs/plans/backend-diagnostic-signal-quality/plan.md
@@ -0,0 +1,98 @@
+---
+spec: docs/specs/platform/requirements/expected-runtime-log-severity.md
+created: 2026-08-28
+status: implemented
+---
+
+# Implementation Plan: Backend diagnostic signal quality
+
+## Overview
+
+This repair removes false warning and error entries from four confirmed backend
+paths. It preserves warnings for unknown child output and unexpected storage
+errors.
+
+The work changes no successful API payload, process lifecycle, workspace
+selection, or runtime cleanup result. A plan write for a missing task changes
+from an internal storage error to the stable `not_found` contract.
+
+## Specifications
+
+- [Expected runtime log severity requirements](../../specs/platform/requirements/expected-runtime-log-severity.md)
+- [Expected runtime log severity design](../../specs/platform/system-design/expected-runtime-log-severity.md)
+- [Task runtime cleanup requirements](../../specs/tasks/requirements/runtime-cleanup.md)
+- [Task runtime cleanup design](../../specs/tasks/system-design/runtime-cleanup.md)
+- [Task documents requirements](../../specs/tasks/requirements/documents.md)
+- [Task plan write lifecycle design](../../specs/tasks/system-design/plan-write-lifecycle.md)
+
+## Implementation order
+
+Wave 1 contains four independent backend corrections:
+
+- [x] [Task 01: Preserve child log severity](task-01-preserve-child-log-severity.md)
+- [x] [Task 02: Quiet stale environment fallback](task-02-quiet-stale-environment-fallback.md)
+- [x] [Task 03: Quiet rotated liveness repair](task-03-quiet-rotated-liveness-repair.md)
+- [x] [Task 04: Classify missing-task plan writes](task-04-classify-missing-task-plan-writes.md)
+
+Wave 2 depends on Task 04:
+
+- [x] [Task 05: Map missing-task plan responses](task-05-map-missing-task-plan-responses.md)
+
+Implementation remains sequential in the primary conversation unless the user
+explicitly authorizes delegation.
+
+## TDD strategy
+
+Each work order starts with a focused regression that fails on the current
+behavior. The implementation then makes that regression pass without changing
+the neighboring error path.
+
+Repository coverage runs against SQLite. The existing environment-gated
+PostgreSQL suite proves dialect parity for the plan foreign-key mapping.
+
+No browser end-to-end test is required. The changed behavior is a backend log
+contract and a shared WebSocket error response. Handler integration tests cover
+both browser and MCP surfaces.
+
+## Final verification
+
+Run these commands from the repository root after all work orders are complete:
+
+```bash
+cd apps/backend && go test ./internal/agent/runtime/agentctl/launcher ./internal/task/service ./internal/orchestrator ./internal/task/repository/sqlite ./internal/task/planws ./internal/task/handlers ./internal/mcp/handlers -count=1
+make -C apps/backend lint
+git diff --check
+```
+
+If `KANDEV_TEST_POSTGRES_DSN` is available, also run:
+
+```bash
+cd apps/backend && go test ./internal/task/repository/sqlite -run TestPostgresWritePlanRevisionMissingTask -count=1
+```
+
+## Risks
+
+- A permissive child-log parser can hide unstructured stderr. The parser must
+  accept only anchored Zap and Go `slog` records.
+- A broad not-found check can hide a database outage. The service must match the
+  typed task-environment sentinel only.
+- A liveness repair can race a newer execution. The fix must preserve the newer
+  row and change only the log classification.
+- Foreign-key text differs by database. The repository must use
+  `internal/db.IsForeignKeyViolation` instead of matching new strings.
+- A missing-task plan rejection can race after access validation. The write
+  transaction remains the authoritative check.
+
+## Out of scope
+
+- Enabling authentication or changing bind addresses.
+- Changing authentication warning text or severity.
+- Repairing third-party plugin webhook defaults or provider outages.
+- Changing the designed host-utility idle-repair behavior.
+- Reclassifying network disconnect warnings without a confirmed caller defect.
+- Changing raw ACP collection, retention, or diagnostic bundle size.
+- Migrating plans to the generalized task-documents model.
+
+## Open questions
+
+None.
diff --git a/docs/plans/backend-diagnostic-signal-quality/task-01-preserve-child-log-severity.md b/docs/plans/backend-diagnostic-signal-quality/task-01-preserve-child-log-severity.md
new file mode 100644
index 0000000000..b2111eb807
--- /dev/null
+++ b/docs/plans/backend-diagnostic-signal-quality/task-01-preserve-child-log-severity.md
@@ -0,0 +1,78 @@
+---
+id: "01-preserve-child-log-severity"
+title: "Preserve child log severity"
+status: done
+wave: 1
+depends_on: []
+plan: "plan.md"
+spec: "../../specs/platform/requirements/expected-runtime-log-severity.md"
+---
+
+# Task 01: Preserve child log severity
+
+Extend the agentctl launcher parser to recognize the Go `slog` formats used by
+the child process.
+
+## Scope
+
+- Extend `childLogLevel` with anchored Go `slog` TextHandler and default-handler
+  parsers.
+- Keep the existing Zap console parser and ANSI handling.
+- Keep unrecognized stderr on the warning fallback.
+
+## Exclusions
+
+- Do not change child logger configuration.
+- Do not parse arbitrary level-like words from messages.
+- Do not change stdout handling.
+
+## Traceability
+
+- `REQ-PLATFORM-EXPECTED-RUNTIME-LOG-SEVERITY-001`
+- `AC-PLATFORM-EXPECTED-RUNTIME-LOG-SEVERITY-001.9`
+- `AC-PLATFORM-EXPECTED-RUNTIME-LOG-SEVERITY-001.10`
+- `AC-PLATFORM-EXPECTED-RUNTIME-LOG-SEVERITY-001.13`
+- `AC-PLATFORM-EXPECTED-RUNTIME-LOG-SEVERITY-001.14`
+- Design: `docs/specs/platform/system-design/expected-runtime-log-severity.md`
+
+## Acceptance
+
+- Recognized Go `slog` `DEBUG`, `INFO`, `WARN`, and `ERROR` records use the
+  matching parent severity.
+- Both `slog.TextHandler` key/value output and the default handler's standard
+  log date/time output are recognized.
+- Existing Zap console records keep their current classification.
+- Malformed and unstructured stderr remains at warning level.
+
+## Verification
+
+```bash
+cd apps/backend && go test ./internal/agent/runtime/agentctl/launcher -run 'Test(ChildLogLevel|PipeOutput)' -count=1
+```
+
+The new `slog` case must fail before the production change because the current
+parser accepts only tab-separated Zap console records.
+
+## Files likely touched
+
+- `apps/backend/internal/agent/runtime/agentctl/launcher/launcher.go`
+- `apps/backend/internal/agent/runtime/agentctl/launcher/launcher_loglevel_test.go`
+
+## Dependencies
+
+None.
+
+## Results
+
+Implemented anchored Go `slog.TextHandler` and default-handler parsing while
+preserving the existing Zap parser and warning fallback. The text parser is
+quote-aware so a `msg=` substring inside an attribute cannot change severity.
+Added coverage for actual `slog.Default()` output, all standard TextHandler
+levels, and malformed/unstructured records.
+
+Verification:
+
+```text
+cd apps/backend && go test ./internal/agent/runtime/agentctl/launcher -run 'Test(ChildLogLevel|PipeOutput)' -count=1
+Go test: 28 passed in 1 packages
+```
diff --git a/docs/plans/backend-diagnostic-signal-quality/task-02-quiet-stale-environment-fallback.md b/docs/plans/backend-diagnostic-signal-quality/task-02-quiet-stale-environment-fallback.md
new file mode 100644
index 0000000000..f7b8686707
--- /dev/null
+++ b/docs/plans/backend-diagnostic-signal-quality/task-02-quiet-stale-environment-fallback.md
@@ -0,0 +1,75 @@
+---
+id: "02-quiet-stale-environment-fallback"
+title: "Quiet stale environment fallback"
+status: done
+wave: 1
+depends_on: []
+plan: "plan.md"
+spec: "../../specs/platform/requirements/expected-runtime-log-severity.md"
+---
+
+# Task 02: Quiet stale environment fallback
+
+Classify a stale session environment reference as an expected fallback.
+
+## Scope
+
+- Match `taskrepo.ErrTaskEnvironmentNotFound` before the warning log.
+- Use debug severity for that typed condition.
+- Preserve the task-based environment fallback and workspace result.
+- Keep warnings for all other lookup errors.
+
+## Exclusions
+
+- Do not change session or task-environment persistence.
+- Do not remove the fallback lookup.
+- Do not classify errors by message text.
+
+## Traceability
+
+- `REQ-PLATFORM-EXPECTED-RUNTIME-LOG-SEVERITY-001`
+- `AC-PLATFORM-EXPECTED-RUNTIME-LOG-SEVERITY-001.11`
+- `AC-PLATFORM-EXPECTED-RUNTIME-LOG-SEVERITY-001.12`
+- `AC-PLATFORM-EXPECTED-RUNTIME-LOG-SEVERITY-001.15`
+- `AC-PLATFORM-EXPECTED-RUNTIME-LOG-SEVERITY-001.16`
+- Design: `docs/specs/platform/system-design/expected-runtime-log-severity.md`
+
+## Acceptance
+
+- A typed missing environment emits no warning and still returns the task
+  fallback result.
+- An unexpected environment lookup error still emits one warning.
+- The fallback keeps the authoritative task-environment identifier.
+
+## Verification
+
+```bash
+cd apps/backend && go test ./internal/task/service -run 'TestGetWorkspaceInfoForSession_(AlignsTaskEnvironmentIDOnFallback|MissingEnvironmentLogSeverity|UnexpectedEnvironmentErrorLogSeverity)' -count=1
+```
+
+The missing-environment log test must fail before the production change because
+the current branch records every direct lookup error as a warning.
+
+## Files likely touched
+
+- `apps/backend/internal/task/service/service_turns.go`
+- `apps/backend/internal/task/service/service_turns_test.go`
+- `apps/backend/internal/task/service/service_turns_logging_test.go`
+
+## Dependencies
+
+None.
+
+## Results
+
+Implemented typed not-found classification before logging the stale session
+environment lookup. The task fallback remains authoritative, with debug
+evidence for the expected miss and warning-level evidence for unexpected
+lookup errors.
+
+Verification:
+
+```text
+cd apps/backend && go test ./internal/task/service -run 'TestGetWorkspaceInfoForSession_(AlignsTaskEnvironmentIDOnFallback|MissingEnvironmentLogSeverity|UnexpectedEnvironmentErrorLogSeverity)' -count=1
+Go test: 3 passed in 1 packages
+```
diff --git a/docs/plans/backend-diagnostic-signal-quality/task-03-quiet-rotated-liveness-repair.md b/docs/plans/backend-diagnostic-signal-quality/task-03-quiet-rotated-liveness-repair.md
new file mode 100644
index 0000000000..200c0daa42
--- /dev/null
+++ b/docs/plans/backend-diagnostic-signal-quality/task-03-quiet-rotated-liveness-repair.md
@@ -0,0 +1,68 @@
+---
+id: "03-quiet-rotated-liveness-repair"
+title: "Quiet rotated liveness repair"
+status: done
+wave: 1
+depends_on: []
+plan: "plan.md"
+spec: "../../specs/tasks/requirements/runtime-cleanup.md"
+---
+
+# Task 03: Quiet rotated liveness repair
+
+Treat a lost liveness compare-and-set as expected reconciliation convergence.
+
+## Scope
+
+- Classify `models.ErrExecutionRotated` before the warning branch.
+- Return success without changing the newer execution row.
+- Keep the warning and returned error for unrelated repair failures.
+
+## Exclusions
+
+- Do not change the compare-and-set repository operation.
+- Do not retry against the newer execution identity.
+- Do not change row deletion or resume-safety rules.
+
+## Traceability
+
+- `REQ-TASKS-RUNTIME-CLEANUP-001`
+- `AC-TASKS-RUNTIME-CLEANUP-001.8`
+- Design: `docs/specs/tasks/system-design/runtime-cleanup.md`
+
+## Acceptance
+
+- `models.ErrExecutionRotated` returns success and emits no warning.
+- `models.ErrExecutorRunningNotFound` keeps its existing successful behavior.
+- An unrelated repair error returns the error and emits one warning.
+
+## Verification
+
+```bash
+cd apps/backend && go test ./internal/orchestrator -run TestRepairDeadRowLivenessLogSeverity -count=1
+```
+
+The rotated case must fail before the production change because the current
+branch emits a warning before it returns success.
+
+## Files likely touched
+
+- `apps/backend/internal/orchestrator/reconcile_liveness.go`
+- `apps/backend/internal/orchestrator/reconcile_liveness_test.go`
+
+## Dependencies
+
+None.
+
+## Results
+
+Moved the expected `models.ErrExecutionRotated` and existing missing-row
+classification ahead of the warning branch. The newer execution row remains
+unchanged, while unrelated repair errors still return and emit one warning.
+
+Verification:
+
+```text
+cd apps/backend && go test ./internal/orchestrator -run TestRepairDeadRowLivenessLogSeverity -count=1
+Go test: 3 passed in 1 packages
+```
diff --git a/docs/plans/backend-diagnostic-signal-quality/task-04-classify-missing-task-plan-writes.md b/docs/plans/backend-diagnostic-signal-quality/task-04-classify-missing-task-plan-writes.md
new file mode 100644
index 0000000000..cc77f42484
--- /dev/null
+++ b/docs/plans/backend-diagnostic-signal-quality/task-04-classify-missing-task-plan-writes.md
@@ -0,0 +1,88 @@
+---
+id: "04-classify-missing-task-plan-writes"
+title: "Classify missing-task plan writes"
+status: done
+wave: 1
+depends_on: []
+plan: "plan.md"
+spec: "../../specs/tasks/requirements/documents.md"
+---
+
+# Task 04: Classify missing-task plan writes
+
+Translate a plan foreign-key rejection to the shared missing-task sentinel.
+
+## Scope
+
+- Use `internal/db.IsForeignKeyViolation` at the plan-head write boundary.
+- Return `repoerrors.ErrTaskNotFound` with wrapped operation context.
+- Preserve transaction rollback for the plan head and revision.
+- Record the expected service rejection at debug level for both the upsert and
+  revert write paths.
+- Add SQLite coverage and environment-gated PostgreSQL parity coverage.
+
+## Exclusions
+
+- Do not add a separate task existence query before the transaction.
+- Do not change successful plan coalescing or revision numbering.
+- Do not change other foreign-key mappings.
+
+## Traceability
+
+- `REQ-TASKS-DOCUMENTS-001`
+- `AC-TASKS-DOCUMENTS-001.2`
+- Design: `docs/specs/tasks/system-design/plan-write-lifecycle.md`
+
+## Acceptance
+
+- A plan write for a missing task returns an error that matches
+  `repoerrors.ErrTaskNotFound` on SQLite and PostgreSQL.
+- The rejected transaction creates no plan head or revision.
+- The plan service emits a debug entry for that sentinel from both
+  `upsertPlan` and `RevertPlan`. Other write errors retain
+  their error-level entry. Existing create/update coverage remains intact.
+
+## Verification
+
+```bash
+cd apps/backend && go test ./internal/task/repository/sqlite -run 'TestWritePlanRevisionMissingTask|TestPostgresWritePlanRevisionMissingTask' -count=1
+cd apps/backend && go test ./internal/task/service -run 'TestPlanService(MissingTaskWriteLogSeverity|OtherWriteErrorLogSeverity|RevertMissingTaskWriteLogSeverity|RevertOtherWriteErrorLogSeverity)' -count=1
+```
+
+The repository test must fail before the production change because the current
+error exposes the raw foreign-key constraint failure.
+
+The PostgreSQL case skips when `KANDEV_TEST_POSTGRES_DSN` is not set. CI runs
+the environment-gated parity suite.
+
+## Files likely touched
+
+- `apps/backend/internal/task/repository/sqlite/plan.go`
+- `apps/backend/internal/task/repository/sqlite/plan_test.go`
+- `apps/backend/internal/task/repository/sqlite/document_plan_postgres_test.go`
+- `apps/backend/internal/task/service/plan_service.go`
+- `apps/backend/internal/task/service/plan_service_logging_test.go`
+
+## Dependencies
+
+None.
+
+## Results
+
+Implemented dialect-aware foreign-key classification at the plan-head write
+boundary. Missing-task writes now return `repoerrors.ErrTaskNotFound`, roll
+back both plan tables, and log at debug in `PlanService`; unrelated write
+errors retain error-level logging. Both `upsertPlan` and `RevertPlan` use
+the same classifier, with observer-backed coverage for the missing-task and
+unrelated-error paths. Added SQLite, environment-gated PostgreSQL, and service
+severity coverage without removing create/update coverage.
+
+Verification:
+
+```text
+cd apps/backend && go test ./internal/task/repository/sqlite -run 'TestWritePlanRevisionMissingTask|TestPostgresWritePlanRevisionMissingTask' -count=1
+Go test: 1 passed in 1 packages
+
+cd apps/backend && go test ./internal/task/service -run 'TestPlanService(MissingTaskWriteLogSeverity|OtherWriteErrorLogSeverity|RevertMissingTaskWriteLogSeverity|RevertOtherWriteErrorLogSeverity)' -count=1
+Go test: 4 passed in 1 packages
+```
diff --git a/docs/plans/backend-diagnostic-signal-quality/task-05-map-missing-task-plan-responses.md b/docs/plans/backend-diagnostic-signal-quality/task-05-map-missing-task-plan-responses.md
new file mode 100644
index 0000000000..cc5474ade0
--- /dev/null
+++ b/docs/plans/backend-diagnostic-signal-quality/task-05-map-missing-task-plan-responses.md
@@ -0,0 +1,78 @@
+---
+id: "05-map-missing-task-plan-responses"
+title: "Map missing-task plan responses"
+status: done
+wave: 2
+depends_on:
+  - "04-classify-missing-task-plan-writes"
+plan: "plan.md"
+spec: "../../specs/tasks/requirements/documents.md"
+---
+
+# Task 05: Map missing-task plan responses
+
+Return the stable `not_found` contract from both task-plan handler surfaces.
+
+## Scope
+
+- Add the shared task-not-found sentinel to `planws` create and update mappings.
+- Keep `ErrTaskPlanNotFound` semantics unchanged.
+- Add shared mapper coverage and integration coverage for browser and MCP plan
+  creation against a missing task.
+- Assert that responses contain no database constraint details.
+
+## Exclusions
+
+- Do not change success payloads or action names.
+- Do not change plan-get or plan-delete behavior.
+- Do not duplicate error mapping in each handler package.
+
+## Traceability
+
+- `REQ-TASKS-DOCUMENTS-001`
+- `AC-TASKS-DOCUMENTS-001.2`
+- Design: `docs/specs/tasks/system-design/plan-write-lifecycle.md`
+
+## Acceptance
+
+- Browser and MCP plan creation for a missing task returns
+  `ws.ErrorCodeNotFound`.
+- Both responses use a stable task-not-found message and omit storage details.
+- Unrelated plan write errors remain `ws.ErrorCodeInternalError`.
+
+## Verification
+
+```bash
+cd apps/backend && go test ./internal/task/planws -run TestMappersCoverEverySentinel -count=1
+cd apps/backend && go test ./internal/task/handlers ./internal/mcp/handlers -run 'Test.*Plan.*MissingTask' -count=1
+```
+
+The handler tests must fail before the production change because the current
+create mapping returns `internal_error` with the foreign-key message.
+
+## Files likely touched
+
+- `apps/backend/internal/task/planws/errors.go`
+- `apps/backend/internal/task/planws/errors_test.go`
+- `apps/backend/internal/task/handlers/task_plan_handlers_test.go`
+- `apps/backend/internal/mcp/handlers/task_plan_handlers_test.go`
+
+## Dependencies
+
+- Task 04 must expose `repository.ErrTaskNotFound` from the write boundary.
+
+## Results
+
+Implemented the shared missing-task mapping for create and update plan
+responses. Browser and MCP creation and update tests now verify the stable
+`not_found` message and reject leaked database constraint details.
+
+Verification:
+
+```text
+cd apps/backend && go test ./internal/task/planws -run TestMappersCoverEverySentinel -count=1
+Go test: 14 passed in 1 packages
+
+cd apps/backend && go test ./internal/task/handlers ./internal/mcp/handlers -run 'Test.*Plan.*MissingTask' -count=1
+Go test: 14 passed in 2 packages
+```
diff --git a/docs/specs/platform/README.md b/docs/specs/platform/README.md
index 5a9e57b0ca..d598e6b29b 100644
--- a/docs/specs/platform/README.md
+++ b/docs/specs/platform/README.md
@@ -78,6 +78,7 @@ localization, feature toggles, health, and shared session recovery services.
 - [Bounded Task Status Delivery](system-design/bounded-task-status-delivery.md)
 - [Diagnostic logging System Design Part 1](system-design/diagnostic-logging-01.md)
 - [Diagnostic logging System Design Part 2](system-design/diagnostic-logging-02.md)
+- [Expected runtime log severity](system-design/expected-runtime-log-severity.md)
 - [Health Endpoint — Surface the Running Version](system-design/health-endpoint-version.md)
 - [Internationalization (i18n)](system-design/i18n.md)
 - [LSP File Intelligence System Design Part 1](system-design/lsp-file-intelligence-01.md)
diff --git a/docs/specs/platform/requirements/expected-runtime-log-severity.md b/docs/specs/platform/requirements/expected-runtime-log-severity.md
index fbdb436b05..6e9cb34b23 100644
--- a/docs/specs/platform/requirements/expected-runtime-log-severity.md
+++ b/docs/specs/platform/requirements/expected-runtime-log-severity.md
@@ -2,6 +2,7 @@
 status: active
 system: platform
 created: 2026-08-23
+updated: 2026-08-28
 owners:
   - kandev
 ---
@@ -9,7 +10,8 @@ owners:
 
 ## Overview
 
-Normal review and first-launch timing conditions currently look like backend failures in the logs. This obscures actionable errors and makes diagnostic bundles harder to triage.
+Expected runtime conditions and forwarded child logs can use the wrong severity.
+This behavior obscures actionable errors and makes diagnostic bundles harder to triage.
 
 ## Requirements
 
@@ -27,6 +29,14 @@ Normal review and first-launch timing conditions currently look like backend fai
 - **AC-PLATFORM-EXPECTED-RUNTIME-LOG-SEVERITY-001.6:** **GIVEN** an active session and a dependency failure unrelated to a missing file, **WHEN** `workspace.file.get` requests a path, **THEN** the response has the `internal_error` error code and an error-level entry is emitted.
 - **AC-PLATFORM-EXPECTED-RUNTIME-LOG-SEVERITY-001.7:** **GIVEN** a physical worktree has been materialized before its task environment row exists, **WHEN** worktree persistence receives `ErrEnvironmentNotResolved`, **THEN** the call succeeds, emits a debug-level deferred-persistence entry, and does not remove the physical worktree.
 - **AC-PLATFORM-EXPECTED-RUNTIME-LOG-SEVERITY-001.8:** **GIVEN** worktree persistence receives any other store error, **WHEN** the manager handles it, **THEN** the existing error and cleanup behavior remains unchanged.
+- **AC-PLATFORM-EXPECTED-RUNTIME-LOG-SEVERITY-001.9:** The launcher shall preserve a recognized child log level from Zap console records and Go `slog` text records.
+- **AC-PLATFORM-EXPECTED-RUNTIME-LOG-SEVERITY-001.10:** The launcher shall record unrecognized child stderr at warning level so that unstructured panics and tracebacks remain visible.
+- **AC-PLATFORM-EXPECTED-RUNTIME-LOG-SEVERITY-001.11:** A missing task environment referenced by a session shall use the existing task fallback without a warning entry.
+- **AC-PLATFORM-EXPECTED-RUNTIME-LOG-SEVERITY-001.12:** A task-environment lookup error that is not a typed not-found condition shall retain its warning entry.
+- **AC-PLATFORM-EXPECTED-RUNTIME-LOG-SEVERITY-001.13:** **GIVEN** a child emits a recognized `INFO` record, **WHEN** the launcher forwards stderr, **THEN** the parent records the line at info level.
+- **AC-PLATFORM-EXPECTED-RUNTIME-LOG-SEVERITY-001.14:** **GIVEN** a child emits unstructured stderr, **WHEN** the launcher forwards the line, **THEN** the parent records it at warning level.
+- **AC-PLATFORM-EXPECTED-RUNTIME-LOG-SEVERITY-001.15:** **GIVEN** a session references a missing task environment, **WHEN** workspace information is loaded, **THEN** the task fallback succeeds without a warning entry.
+- **AC-PLATFORM-EXPECTED-RUNTIME-LOG-SEVERITY-001.16:** **GIVEN** a task-environment lookup returns another error, **WHEN** workspace information is loaded, **THEN** the existing fallback and warning behavior remains unchanged.
 
 ## Migrated source detail
 
@@ -49,6 +59,10 @@ bundles harder to triage.
   owns persistence of its record.
 - Other worktree persistence failures keep their existing return, cleanup, and
   logging behavior.
+- Forwarded child stderr keeps recognized Zap and Go `slog` levels. Unknown
+  stderr stays at warning level.
+- A missing session task environment uses the existing task fallback without a
+  warning. Other lookup errors keep their warning.
 
 ## API surface
 
@@ -71,6 +85,12 @@ status and message path.
   remove the newly created physical worktree.
 - Any other worktree store failure remains an actual persistence failure and
   follows the current cleanup and retry boundary.
+- The launcher accepts only anchored child log formats. Unrecognized stderr
+  remains visible at warning level.
+- A typed missing task environment is an expected stale-reference condition.
+  The task fallback remains authoritative for workspace information.
+- Other task-environment lookup errors can indicate a storage problem and
+  remain warnings.
 
 ## Persistence guarantees
 
@@ -95,10 +115,21 @@ persistence, or cleanup behavior across backend restarts.
 - **GIVEN** worktree persistence receives any other store error, **WHEN** the
   manager handles it, **THEN** the existing error and cleanup behavior remains
   unchanged.
+- **GIVEN** a child emits a recognized `INFO` record, **WHEN** the launcher
+  forwards stderr, **THEN** the parent records the line at info level.
+- **GIVEN** a child emits unstructured stderr, **WHEN** the launcher forwards
+  the line, **THEN** the parent records it at warning level.
+- **GIVEN** a session references a missing task environment, **WHEN** workspace
+  information is loaded, **THEN** the task fallback succeeds without a warning
+  entry.
+- **GIVEN** a task-environment lookup returns another error, **WHEN** workspace
+  information is loaded, **THEN** the existing fallback and warning behavior
+  remains unchanged.
 
 ## Out of scope
 
 - Changing frontend diff rendering or review-file status behavior.
 - Changing worktree ownership, task-environment transactions, cleanup policy,
   or startup reconciliation.
-- Reclassifying unrelated integration, ACP, or startup warnings.
+- Reclassifying integration failures, raw ACP frames, host-utility idle repair,
+  network disconnects, or authentication warnings.
diff --git a/docs/specs/platform/system-design/expected-runtime-log-severity.md b/docs/specs/platform/system-design/expected-runtime-log-severity.md
new file mode 100644
index 0000000000..40e79320b3
--- /dev/null
+++ b/docs/specs/platform/system-design/expected-runtime-log-severity.md
@@ -0,0 +1,75 @@
+---
+status: current
+system: platform
+requirements:
+  - REQ-PLATFORM-EXPECTED-RUNTIME-LOG-SEVERITY-001
+created: 2026-08-28
+owners:
+  - kandev
+---
+# Expected runtime log severity System Design
+
+## Purpose and boundaries
+
+This design preserves the severity of recognized child logs. It also removes
+warnings from one expected task-environment fallback.
+
+The design does not change process control, workspace selection, or returned
+errors. Unknown child stderr remains visible at warning level.
+
+## Requirement mapping
+
+| Requirement | Design section |
+| --- | --- |
+| `REQ-PLATFORM-EXPECTED-RUNTIME-LOG-SEVERITY-001` | [Child log forwarding](#child-log-forwarding) and [Task-environment fallback](#task-environment-fallback) |
+
+## Child log forwarding
+
+`internal/agent/runtime/agentctl/launcher` owns the parent-side log forwarder.
+The forwarder accepts these anchored child record formats:
+
+- Zap console records with a tab-separated level field.
+- Go `slog` `TextHandler` records with `time`, `level`, and a message field.
+- Go `slog.Default` records with the standard log date/time prefix, level, and
+  message.
+
+The parser returns only recognized levels. The forwarder maps `DEBUG`, `INFO`,
+`WARN`, and error-class levels to the equivalent parent logger method.
+
+The parser does not infer a level from an arbitrary word. If a line does not
+match a trusted format, the forwarder records it at warning level. This fallback
+keeps panics, tracebacks, and malformed child output visible.
+
+## Task-environment fallback
+
+`internal/task/service.GetWorkspaceInfoForSession` first reads the environment
+that the session references. A stale reference can return the typed
+task-environment not-found error.
+
+The service classifies that typed error before it records a log entry. It uses
+the existing task-based fallback and records only debug evidence. A different
+lookup error keeps the current warning and fallback behavior.
+
+The fallback result, workspace metadata, and caller response do not change.
+
+## Failure and recovery
+
+An unknown child record remains a warning. This rule prevents a parser change
+from hiding unstructured failures.
+
+An unexpected task-environment lookup error remains a warning. The task-based
+fallback can still return usable workspace information, but the storage error
+remains diagnostic evidence.
+
+## Observability
+
+The parent log keeps the original child line and `stream` field. Only the parent
+severity changes when the parser recognizes the child severity.
+
+The stale task-environment path keeps the session and environment identifiers
+at debug level. Unexpected lookup errors keep the existing warning fields.
+
+## Test strategy
+
+Table tests cover Zap, Go `slog`, malformed, and unstructured child lines.
+Observer-backed service tests cover typed not-found and unexpected lookup errors.
diff --git a/docs/specs/tasks/README.md b/docs/specs/tasks/README.md
index 072a561eb1..e3469c058b 100644
--- a/docs/specs/tasks/README.md
+++ b/docs/specs/tasks/README.md
@@ -121,6 +121,7 @@ signals, and task-scoped scheduling contracts.
 - [Task model unification](system-design/model-unification.md)
 - [Remote Contribution Tasks](system-design/remote-contribution-tasks.md)
 - [Task Archive Confirmation](system-design/archive-confirmation.md)
+- [Task plan write lifecycle](system-design/plan-write-lifecycle.md)
 - [Task Runtime Cleanup](system-design/runtime-cleanup.md)
 - [Queued Run Scheduling](system-design/run-scheduling.md)
 - [Session Delete Preserves Task Workspaces](system-design/session-delete-resource-cleanup.md)
diff --git a/docs/specs/tasks/requirements/documents.md b/docs/specs/tasks/requirements/documents.md
index 01182082b3..6b26525051 100644
--- a/docs/specs/tasks/requirements/documents.md
+++ b/docs/specs/tasks/requirements/documents.md
@@ -2,6 +2,7 @@
 status: active
 system: tasks
 created: 2026-04-29
+updated: 2026-08-28
 owners:
   - cfl
 ---
@@ -20,6 +21,7 @@ This document is the migrated task-system source for the capability. The source
 #### Acceptance criteria
 
 - **AC-TASKS-DOCUMENTS-001.1:** When a consumer uses this capability, the system shall provide the observable behavior and exclusions documented below.
+- **AC-TASKS-DOCUMENTS-001.2:** When a plan write targets a missing task, the system shall return `not_found`, create no plan data, and expose no storage constraint details. This expected rejection shall create a debug entry and no error-level entry.
 
 ## Migrated source detail
 
@@ -103,10 +105,12 @@ POST   /tasks/:id/documents/:key/revisions/:revId/restore → restore from prior
 
 - **GIVEN** the existing MCP tool `plan_create`, **WHEN** called with a task ID and content, **THEN** it creates/updates the document with `key=plan` (backward compatible, no behavior change).
 
+- **GIVEN** a missing or deleted task, **WHEN** a consumer writes its plan, **THEN** the response is `not_found`, no plan data exists, and no storage error is exposed.
+
 ## Out of scope
 
 - Real-time collaborative editing (one writer at a time, last-write-wins)
 - Document templates (e.g. auto-populate spec template on creation)
 - Cross-task document linking (documents belong to one task)
 - Attachment revision history (attachments are replace-only, not versioned)
-- Image/file preview rendering in the UI (download link only for now)
\ No newline at end of file
+- Image/file preview rendering in the UI (download link only for now)
diff --git a/docs/specs/tasks/requirements/runtime-cleanup.md b/docs/specs/tasks/requirements/runtime-cleanup.md
index 1504798914..e6346acd3b 100644
--- a/docs/specs/tasks/requirements/runtime-cleanup.md
+++ b/docs/specs/tasks/requirements/runtime-cleanup.md
@@ -2,7 +2,7 @@
 status: draft
 system: tasks
 created: 2026-06-22
-updated: 2026-08-24
+updated: 2026-08-28
 owners:
   - cfl
 ---
@@ -32,3 +32,4 @@ and safe when runtimes or task rows are already gone.
   cleanup removed its physical worktree, resuming the task shall recreate or
   reactivate the recoverable task-owned worktree instead of requiring
   attach-only reuse of the deleted workspace.
+- **AC-TASKS-RUNTIME-CLEANUP-001.8:** When dead-row repair loses its compare-and-set to a newer execution, reconciliation shall preserve the newer row without a warning. Other repair errors shall remain warnings.
diff --git a/docs/specs/tasks/system-design/plan-write-lifecycle.md b/docs/specs/tasks/system-design/plan-write-lifecycle.md
new file mode 100644
index 0000000000..25d5b1a27d
--- /dev/null
+++ b/docs/specs/tasks/system-design/plan-write-lifecycle.md
@@ -0,0 +1,69 @@
+---
+status: current
+system: tasks
+requirements:
+  - REQ-TASKS-DOCUMENTS-001
+created: 2026-08-28
+owners:
+  - kandev
+---
+# Task plan write lifecycle System Design
+
+## Purpose and boundaries
+
+This design defines the missing-task boundary for backward-compatible task-plan
+writes. It does not implement the broader task-documents migration.
+
+## Requirement mapping
+
+| Requirement | Design section |
+| --- | --- |
+| `REQ-TASKS-DOCUMENTS-001` | [Transactional write](#transactional-write) and [Error contract](#error-contract) |
+
+## Components and responsibilities
+
+- `internal/task/repository/sqlite.Repository` owns the transaction and database
+  error classification for SQLite and PostgreSQL.
+- `internal/task/service.PlanService` owns operational logging and preserves
+  typed repository errors.
+- `internal/task/planws` owns the shared WebSocket error contract for browser
+  handlers and MCP handlers.
+
+## Transactional write
+
+`WritePlanRevision` writes the plan head before it writes or merges a revision.
+Both plan tables reference the task row with foreign keys.
+
+If the head write reports a foreign-key violation, the repository returns the
+shared `ErrTaskNotFound` sentinel. The transaction then rolls back. No plan head
+or revision remains.
+
+The repository uses `internal/db.IsForeignKeyViolation` for both supported
+database dialects. It does not inspect a raw database error outside that helper.
+
+## Error contract
+
+The plan service passes `ErrTaskNotFound` to its callers. It records the
+expected rejection at debug level. Other write errors remain error-level
+entries.
+
+`planws.CreateError` and `planws.UpdateError` map `ErrTaskNotFound` to the
+existing `not_found` WebSocket code. The response does not include a database
+constraint message.
+
+The browser and MCP surfaces use the same `planws` mapping. No request or
+success payload changes.
+
+## Failure and recovery
+
+A concurrent task deletion can occur after access validation and before the
+plan transaction. The foreign-key classification closes that race without a
+separate task existence query.
+
+An unrelated database error keeps its wrapped diagnostic context. The service
+records it at error level and the wire contract returns `internal_error`.
+
+## Test strategy
+
+SQLite and PostgreSQL repository tests cover the sentinel and rollback. Service
+tests cover log severity. Shared contract tests cover browser and MCP mappings.
diff --git a/docs/specs/tasks/system-design/runtime-cleanup.md b/docs/specs/tasks/system-design/runtime-cleanup.md
index 5e43247cfe..cc4b4acce5 100644
--- a/docs/specs/tasks/system-design/runtime-cleanup.md
+++ b/docs/specs/tasks/system-design/runtime-cleanup.md
@@ -114,10 +114,8 @@ worktrees, or executor rows behind and the machine slowly runs out of memory.
 - Archive, delete, cascade, workspace-delete, and quick-chat expiration persist a
   cleanup intent and resource snapshot before mutating or deleting task state.
   Cleanup is performed by a durable worker rather than a detached goroutine.
-- Stop-owner registration is advisory. It uses a non-blocking attempt to acquire
-  the per-session `cancelInFlightGuard` before it records teardown ownership.
-- If that guard is in use, registration skips the claim and the requested stop
-  continues. Durable cleanup and idempotent runtime teardown handle later races.
+- Advisory stop-owner registration skips a contended `cancelInFlightGuard` claim
+  without blocking; the requested stop and durable cleanup continue.
 - A durable cleanup job makes at most eight attempts. Failed attempts back off
   for 1 minute, 5 minutes, 15 minutes, 1 hour, 3 hours, 6 hours, and 12 hours
   before the next claim. An eighth failed attempt becomes terminal `failed`
@@ -255,8 +253,8 @@ primary stop operation when `agent_execution_id` is available. Fallback cleanup
 runtime-specific and must be bounded by context.
 
 `TaskExecutionStopper.RegisterExecutionStopOwner(sessionID, executionID, force)`
-must not wait for the per-session cancellation guard. Registration can skip a
-contended claim because it is advisory and does not replace the stop operation.
+must not wait for the session guard; it may skip a contended claim and never
+replaces `StopExecution`.
 
 ## State Machine
 
@@ -299,14 +297,15 @@ The durable cleanup job wraps that resource lifecycle:
   the task only if existing product behavior requires it, but destructive runtime
   cleanup must fail closed: do not remove runtime rows or worktrees based on an
   empty or partial inventory.
-- If stop-owner registration finds the session guard in use, it skips the claim.
-  The explicit stop continues, and durable cleanup preserves any retryable work.
+- A contended owner claim is skipped; explicit stop continues and durable cleanup
+  preserves retryable work.
 - If stopping a runtime execution times out, the process manager escalates to a
   process-group kill and waits for confirmation. If confirmation still fails, the
   runtime row remains retryable.
 - If a runtime row points at a missing in-memory execution, cleanup attempts the
   runtime-specific persisted handle when available. If no handle can be used, the
   row is preserved with a warning instead of being silently dropped.
+- Keep `models.ErrExecutionRotated` (newer won); warn on other errors.
 - If a stop operation reports the execution or session is not found and the owned
   row is a confirmed-dead local runtime, cleanup records the stop as successful,
   prunes or repairs the row under the resume-safety invariant, proceeds with any
@@ -476,6 +475,8 @@ The durable cleanup job wraps that resource lifecycle:
   a not-found stop is reclassified as successful, **THEN** the row is repaired in
   place (token and worktree preserved) rather than deleted, per
   `RowMustBePreserved`.
+- **WHEN** compare-and-set returns `models.ErrExecutionRotated`, **THEN** keep
+  the newer row silently.
 - **GIVEN** agentctl is stopped while an ACP child process ignores stdin EOF,
   **WHEN** the stop timeout expires, **THEN** the ACP process group is killed and
   no ACP child is reparented to PID 1.