Clarify worker failures in operator UI

coordinator/internal/transport/http/templates/job.html

@@ -15,7 +15,7 @@
     <section class="summary"><div class="summary-head"><div><span id="status" class="status status-{{statusClass .Status}}">{{statusLabel .Status}}</span><p id="hint" class="hint">{{statusHint .Status}}</p></div><small>Summary refreshes automatically every two seconds.</small></div><div class="bar" aria-label="Progress"><span id="progress-bar" style="width:{{progressPercent .Completed .Failed .Total}}%"></span></div><p id="progress" class="muted">{{.Completed}} of {{.Total}} tasks complete</p><div class="numbers"><div class="number"><b id="total">{{.Total}}</b><small>total shards</small></div><div class="number"><b id="completed">{{.Completed}}</b><small>complete</small></div><div class="number"><b id="pending">{{.Pending}}</b><small>waiting</small></div><div class="number"><b id="active">{{add .Leased .Running}}</b><small>with workers</small></div><div class="number"><b id="failed">{{.Failed}}</b><small>failed</small></div></div><details><summary>Technical details</summary><p>Job ID: <code>{{.ID}}</code><br>Workload: <code>{{.Workload}}</code><br>Created: {{time .CreatedAt}}</p></details></section>
     <section class="notice"><strong>What can be downloaded now?</strong><br><code>partial_result</code> files come from individual shards. They are useful for checking the pipeline, but are not a merged final CSV because the reducer is not implemented yet.</section>
     <h2>Shard tasks</h2><p class="muted">If a task fails, its code and message appear here. Refresh the page to update the detailed rows.</p>
-    <div class="table-wrap"><table><tr><th>Shard</th><th>State</th><th>Attempt</th><th>Worker / lease</th><th>Error</th></tr>{{range .Tasks}}<tr><td>#{{.ChunkIndex}}</td><td><span class="status status-{{statusClass .Status}}">{{statusLabel .Status}}</span></td><td>{{.Attempt}} / {{.MaxAttempts}}</td><td>{{if .LeaseOwner}}<code>{{.LeaseOwner}}</code>{{if .LeaseExpiresAt}}<br><small>until {{time .LeaseExpiresAt}}</small>{{end}}{{else}}<span class="muted">—</span>{{end}}</td><td class="error">{{if .ErrorCode}}<code>{{.ErrorCode}}</code><br>{{.ErrorMessage}}{{else}}<span class="muted">—</span>{{end}}</td></tr>{{else}}<tr><td colspan="5" class="empty">No tasks have appeared yet.</td></tr>{{end}}</table></div>
+    <div class="table-wrap"><table><tr><th>Shard</th><th>State</th><th>Attempt</th><th>Worker / lease</th><th>Error</th></tr>{{range .Tasks}}<tr><td>#{{.ChunkIndex}}</td><td><span class="status status-{{statusClass .Status}}">{{statusLabel .Status}}</span></td><td>{{.Attempt}} / {{.MaxAttempts}}</td><td>{{if .LeaseOwner}}<code>{{.LeaseOwner}}</code>{{if .LeaseExpiresAt}}<br><small>until {{time .LeaseExpiresAt}}</small>{{end}}{{else}}<span class="muted">—</span>{{end}}</td><td class="error">{{if .ErrorCode}}<strong>{{taskErrorLabel .ErrorCode}}</strong><br><small>{{taskErrorHint .ErrorCode}}</small>{{else}}<span class="muted">—</span>{{end}}</td></tr>{{else}}<tr><td colspan="5" class="empty">No tasks have appeared yet.</td></tr>{{end}}</table></div>
     <h2>Coordinator artifacts</h2>
     <div class="table-wrap"><table><tr><th>Type</th><th>File</th><th>Size</th><th>Integrity check</th><th></th></tr>{{range .Artifacts}}<tr><td>{{if .Diagnostic}}<strong>Partial result</strong><br><small>diagnostic</small>{{else}}{{.Kind}}{{end}}</td><td>{{.Filename}}</td><td>{{bytes .SizeBytes}}</td><td><code>{{.SHA256}}</code></td><td>{{if .Downloadable}}<a class="download" href="/ui/jobs/{{$.ID}}/artifacts/{{.ID}}">Download CSV</a>{{else}}<span class="muted">Unavailable</span>{{end}}</td></tr>{{else}}<tr><td colspan="5" class="empty">No artifacts yet. The worker uploads a CSV after it completes a shard.</td></tr>{{end}}</table></div>
   </main>

coordinator/internal/transport/http/ui.go

@@ -23,6 +23,8 @@ var uiTemplates = template.Must(template.New("ui").Funcs(template.FuncMap{
 	"statusLabel":       uiStatusLabel,
 	"statusHint":        uiStatusHint,
 	"statusClass":       uiStatusClass,
+	"taskErrorLabel":    uiTaskErrorLabel,
+	"taskErrorHint":     uiTaskErrorHint,
 	"workerStatusLabel": uiWorkerStatusLabel,
 	"workloadLabel":     uiWorkloadLabel,
 	"progressPercent":   uiProgressPercent,
@@ -95,6 +97,43 @@ func uiWorkerStatusLabel(status string) string {
 	}
 }
 
+// uiTaskErrorLabel deliberately maps worker implementation errors to an
+// operator-facing diagnosis. Raw subprocess commands and local paths belong in
+// the worker terminal, not in the web UI.
+func uiTaskErrorLabel(errorCode string) string {
+	switch errorCode {
+	case "CalledProcessError":
+		return "Local calculation failed"
+	case "ValueError":
+		return "Task input could not be processed"
+	case "CoordinatorTransientError":
+		return "Coordinator connection was interrupted"
+	case "CoordinatorConflictError":
+		return "Worker lease was no longer valid"
+	case "FileNotFoundError":
+		return "Local task file is missing"
+	default:
+		return errorCode
+	}
+}
+
+func uiTaskErrorHint(errorCode string) string {
+	switch errorCode {
+	case "CalledProcessError":
+		return "The local SciMesh command stopped before it could upload a result. Check the worker terminal for the original error."
+	case "ValueError":
+		return "The coordinator task or its downloaded input did not meet the worker validation rules."
+	case "CoordinatorTransientError":
+		return "The worker will retry after the coordinator connection is available again."
+	case "CoordinatorConflictError":
+		return "Another worker or a lease timeout changed this task before completion."
+	case "FileNotFoundError":
+		return "The worker could not find one of its local task files. Restart it with an absolute --work-dir."
+	default:
+		return "Check the worker terminal for the original error details."
+	}
+}
+
 func uiWorkloadLabel(workload string) string {
 	switch workload {
 	case "similarity-search", "similarity_search":

coordinator/internal/transport/http/ui_test.go

@@ -33,3 +33,12 @@ func TestUIProgressPercent(t *testing.T) {
 		t.Errorf("empty progress = %d, want 0", got)
 	}
 }
+
+func TestUITaskErrorPresentationDoesNotExposeCommand(t *testing.T) {
+	if got := uiTaskErrorLabel("CalledProcessError"); got != "Local calculation failed" {
+		t.Errorf("error label = %q", got)
+	}
+	if got := uiTaskErrorHint("CalledProcessError"); got == "" {
+		t.Error("error hint must explain the failure")
+	}
+}