feat: add Loki log query support to lab-monitoring

Add 3 opt-in Loki tools (query_logs, list_labels, list_label_values)
that are registered when LOKI_URL is configured. Includes Loki HTTP
client, CLI commands (logs, labels), NixOS module option, formatting,
and tests.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

internal/mcp/server.go

@@ -83,7 +83,7 @@ This ensures package information matches the nixpkgs version the project actuall
 func DefaultMonitoringConfig() ServerConfig {
 	return ServerConfig{
 		Name:    "lab-monitoring",
-		Version: "0.1.0",
+		Version: "0.2.0",
 		Mode:    ModeCustom,
 		Instructions: `Lab Monitoring MCP Server - Query Prometheus metrics and Alertmanager alerts.
 
@@ -92,8 +92,9 @@ Tools for querying your monitoring stack:
 - List and inspect alerts from Alertmanager
 - View scrape target health status
 - Manage alert silences
+- Query logs via LogQL (when Loki is configured)
 
-All queries are executed against live Prometheus and Alertmanager HTTP APIs.`,
+All queries are executed against live Prometheus, Alertmanager, and Loki HTTP APIs.`,
 	}
 }
 

internal/monitoring/format.go

@@ -294,6 +294,126 @@ func formatMetricSearch(names []string, metadata map[string][]PromMetadata) stri
 	return sb.String()
 }
 
+const maxLabelValues = 100
+const maxLineLength = 500
+
+// formatLogStreams formats Loki log query results as grouped markdown.
+func formatLogStreams(data *LokiQueryData) string {
+	if data == nil || len(data.Result) == 0 {
+		return "No log results."
+	}
+
+	var sb strings.Builder
+
+	totalEntries := 0
+	for _, s := range data.Result {
+		totalEntries += len(s.Values)
+	}
+	sb.WriteString(fmt.Sprintf("**%d stream(s), %d total log entries**\n\n", len(data.Result), totalEntries))
+
+	for _, stream := range data.Result {
+		// Stream labels header
+		var labels []string
+		for k, v := range stream.Stream {
+			labels = append(labels, fmt.Sprintf("%s=%q", k, v))
+		}
+		sort.Strings(labels)
+		sb.WriteString(fmt.Sprintf("## {%s}\n\n", strings.Join(labels, ", ")))
+
+		if len(stream.Values) == 0 {
+			sb.WriteString("No entries.\n\n")
+			continue
+		}
+
+		sb.WriteString("| Timestamp | Log Line |\n")
+		sb.WriteString("| --- | --- |\n")
+
+		truncated := false
+		for i, entry := range stream.Values {
+			if i >= maxRows {
+				truncated = true
+				break
+			}
+
+			ts := formatNanosecondTimestamp(entry[0])
+			line := entry[1]
+			if len(line) > maxLineLength {
+				line = line[:maxLineLength] + "..."
+			}
+			// Escape pipe characters in log lines for markdown table
+			line = strings.ReplaceAll(line, "|", "\\|")
+			// Replace newlines with spaces for table compatibility
+			line = strings.ReplaceAll(line, "\n", " ")
+
+			sb.WriteString(fmt.Sprintf("| %s | %s |\n", ts, line))
+		}
+
+		if truncated {
+			sb.WriteString(fmt.Sprintf("\n*Showing %d of %d entries (truncated)*\n", maxRows, len(stream.Values)))
+		}
+		sb.WriteString("\n")
+	}
+
+	return sb.String()
+}
+
+// formatLabels formats a list of label names as a bullet list.
+func formatLabels(labels []string) string {
+	if len(labels) == 0 {
+		return "No labels found."
+	}
+
+	sort.Strings(labels)
+
+	var sb strings.Builder
+	sb.WriteString(fmt.Sprintf("**%d label(s)**\n\n", len(labels)))
+
+	for _, label := range labels {
+		sb.WriteString(fmt.Sprintf("- `%s`\n", label))
+	}
+
+	return sb.String()
+}
+
+// formatLabelValues formats label values as a bullet list.
+func formatLabelValues(label string, values []string) string {
+	if len(values) == 0 {
+		return fmt.Sprintf("No values found for label '%s'.", label)
+	}
+
+	sort.Strings(values)
+
+	var sb strings.Builder
+	sb.WriteString(fmt.Sprintf("**%d value(s) for label `%s`**\n\n", len(values), label))
+
+	truncated := false
+	for i, v := range values {
+		if i >= maxLabelValues {
+			truncated = true
+			break
+		}
+		sb.WriteString(fmt.Sprintf("- `%s`\n", v))
+	}
+
+	if truncated {
+		sb.WriteString(fmt.Sprintf("\n*Showing %d of %d values (truncated)*\n", maxLabelValues, len(values)))
+	}
+
+	return sb.String()
+}
+
+// formatNanosecondTimestamp converts a nanosecond Unix timestamp string to RFC3339.
+func formatNanosecondTimestamp(nsStr string) string {
+	var ns int64
+	for _, c := range nsStr {
+		if c >= '0' && c <= '9' {
+			ns = ns*10 + int64(c-'0')
+		}
+	}
+	t := time.Unix(0, ns)
+	return t.UTC().Format(time.RFC3339)
+}
+
 // formatMetricMetadata formats metadata for a single metric.
 func formatMetricMetadata(name string, metas []PromMetadata) string {
 	if len(metas) == 0 {

internal/monitoring/handlers.go

@@ -62,7 +62,7 @@ type HandlerOptions struct {
 }
 
 // RegisterHandlers registers all monitoring tool handlers on the MCP server.
-func RegisterHandlers(server *mcp.Server, prom *PrometheusClient, am *AlertmanagerClient, opts HandlerOptions) {
+func RegisterHandlers(server *mcp.Server, prom *PrometheusClient, am *AlertmanagerClient, loki *LokiClient, opts HandlerOptions) {
 	server.RegisterTool(listAlertsTool(), makeListAlertsHandler(am))
 	server.RegisterTool(getAlertTool(), makeGetAlertHandler(am))
 	server.RegisterTool(searchMetricsTool(), makeSearchMetricsHandler(prom))
@@ -73,6 +73,11 @@ func RegisterHandlers(server *mcp.Server, prom *PrometheusClient, am *Alertmanag
 	if opts.EnableSilences {
 		server.RegisterTool(createSilenceTool(), makeCreateSilenceHandler(am))
 	}
+	if loki != nil {
+		server.RegisterTool(queryLogsTool(), makeQueryLogsHandler(loki))
+		server.RegisterTool(listLabelsTool(), makeListLabelsHandler(loki))
+		server.RegisterTool(listLabelValuesTool(), makeListLabelValuesHandler(loki))
+	}
 }
 
 // Tool definitions
@@ -485,3 +490,183 @@ func makeCreateSilenceHandler(am *AlertmanagerClient) mcp.ToolHandler {
 func parseJSON(s string, v interface{}) error {
 	return json.Unmarshal([]byte(s), v)
 }
+
+// Loki tool definitions
+
+func queryLogsTool() mcp.Tool {
+	return mcp.Tool{
+		Name:        "query_logs",
+		Description: "Execute a LogQL range query against Loki to search and retrieve log entries",
+		InputSchema: mcp.InputSchema{
+			Type: "object",
+			Properties: map[string]mcp.Property{
+				"logql": {
+					Type:        "string",
+					Description: `LogQL query expression (e.g., '{job="varlogs"}', '{job="nginx"} |= "error"')`,
+				},
+				"start": {
+					Type:        "string",
+					Description: "Start time: relative duration (e.g., '1h', '30m'), RFC3339 timestamp, or Unix epoch seconds. Default: 1h ago",
+				},
+				"end": {
+					Type:        "string",
+					Description: "End time: relative duration (e.g., '5m'), RFC3339 timestamp, or Unix epoch seconds. Default: now",
+				},
+				"limit": {
+					Type:        "integer",
+					Description: "Maximum number of log entries to return (default: 100)",
+					Default:     100,
+				},
+				"direction": {
+					Type:        "string",
+					Description: "Sort order for log entries: 'backward' (newest first) or 'forward' (oldest first)",
+					Enum:        []string{"backward", "forward"},
+				},
+			},
+			Required: []string{"logql"},
+		},
+	}
+}
+
+func listLabelsTool() mcp.Tool {
+	return mcp.Tool{
+		Name:        "list_labels",
+		Description: "List available label names from Loki",
+		InputSchema: mcp.InputSchema{
+			Type:       "object",
+			Properties: map[string]mcp.Property{},
+		},
+	}
+}
+
+func listLabelValuesTool() mcp.Tool {
+	return mcp.Tool{
+		Name:        "list_label_values",
+		Description: "List values for a specific label from Loki",
+		InputSchema: mcp.InputSchema{
+			Type: "object",
+			Properties: map[string]mcp.Property{
+				"label": {
+					Type:        "string",
+					Description: "Label name to get values for (e.g., 'job', 'instance')",
+				},
+			},
+			Required: []string{"label"},
+		},
+	}
+}
+
+// Loki handler constructors
+
+func makeQueryLogsHandler(loki *LokiClient) mcp.ToolHandler {
+	return func(ctx context.Context, args map[string]interface{}) (mcp.CallToolResult, error) {
+		logql, _ := args["logql"].(string)
+		if logql == "" {
+			return mcp.ErrorContent(fmt.Errorf("logql is required")), nil
+		}
+
+		now := time.Now()
+		start := now.Add(-time.Hour)
+		end := now
+
+		if startStr, ok := args["start"].(string); ok && startStr != "" {
+			parsed, err := parseTimeArg(startStr, now.Add(-time.Hour))
+			if err != nil {
+				return mcp.ErrorContent(fmt.Errorf("invalid start time: %w", err)), nil
+			}
+			start = parsed
+		}
+
+		if endStr, ok := args["end"].(string); ok && endStr != "" {
+			parsed, err := parseTimeArg(endStr, now)
+			if err != nil {
+				return mcp.ErrorContent(fmt.Errorf("invalid end time: %w", err)), nil
+			}
+			end = parsed
+		}
+
+		limit := 100
+		if l, ok := args["limit"].(float64); ok && l > 0 {
+			limit = int(l)
+		}
+
+		direction := "backward"
+		if d, ok := args["direction"].(string); ok && d != "" {
+			direction = d
+		}
+
+		data, err := loki.QueryRange(ctx, logql, start, end, limit, direction)
+		if err != nil {
+			return mcp.ErrorContent(fmt.Errorf("log query failed: %w", err)), nil
+		}
+
+		return mcp.CallToolResult{
+			Content: []mcp.Content{mcp.TextContent(formatLogStreams(data))},
+		}, nil
+	}
+}
+
+func makeListLabelsHandler(loki *LokiClient) mcp.ToolHandler {
+	return func(ctx context.Context, args map[string]interface{}) (mcp.CallToolResult, error) {
+		labels, err := loki.Labels(ctx)
+		if err != nil {
+			return mcp.ErrorContent(fmt.Errorf("failed to list labels: %w", err)), nil
+		}
+
+		return mcp.CallToolResult{
+			Content: []mcp.Content{mcp.TextContent(formatLabels(labels))},
+		}, nil
+	}
+}
+
+func makeListLabelValuesHandler(loki *LokiClient) mcp.ToolHandler {
+	return func(ctx context.Context, args map[string]interface{}) (mcp.CallToolResult, error) {
+		label, _ := args["label"].(string)
+		if label == "" {
+			return mcp.ErrorContent(fmt.Errorf("label is required")), nil
+		}
+
+		values, err := loki.LabelValues(ctx, label)
+		if err != nil {
+			return mcp.ErrorContent(fmt.Errorf("failed to list label values: %w", err)), nil
+		}
+
+		return mcp.CallToolResult{
+			Content: []mcp.Content{mcp.TextContent(formatLabelValues(label, values))},
+		}, nil
+	}
+}
+
+// parseTimeArg parses a time argument that can be:
+// - A relative duration (e.g., "1h", "30m", "2h30m") — interpreted as that duration ago from now
+// - An RFC3339 timestamp (e.g., "2024-01-15T10:30:00Z")
+// - A Unix epoch in seconds (e.g., "1705312200")
+// If parsing fails, returns the provided default time.
+func parseTimeArg(s string, defaultTime time.Time) (time.Time, error) {
+	// Try as relative duration first
+	if d, err := time.ParseDuration(s); err == nil {
+		return time.Now().Add(-d), nil
+	}
+
+	// Try as RFC3339
+	if t, err := time.Parse(time.RFC3339, s); err == nil {
+		return t, nil
+	}
+
+	// Try as Unix epoch seconds
+	var epoch int64
+	validDigits := true
+	for _, c := range s {
+		if c >= '0' && c <= '9' {
+			epoch = epoch*10 + int64(c-'0')
+		} else {
+			validDigits = false
+			break
+		}
+	}
+	if validDigits && len(s) > 0 {
+		return time.Unix(epoch, 0), nil
+	}
+
+	return defaultTime, fmt.Errorf("cannot parse time '%s': use relative duration (e.g., '1h'), RFC3339, or Unix epoch seconds", s)
+}

internal/monitoring/handlers_test.go

@@ -14,7 +14,7 @@ import (
 )
 
 // setupTestServer creates a test MCP server with monitoring handlers backed by test HTTP servers.
-func setupTestServer(t *testing.T, promHandler, amHandler http.HandlerFunc) (*mcp.Server, func()) {
+func setupTestServer(t *testing.T, promHandler, amHandler http.HandlerFunc, lokiHandler ...http.HandlerFunc) (*mcp.Server, func()) {
 	t.Helper()
 
 	promSrv := httptest.NewServer(promHandler)
@@ -26,11 +26,22 @@ func setupTestServer(t *testing.T, promHandler, amHandler http.HandlerFunc) (*mc
 
 	prom := NewPrometheusClient(promSrv.URL)
 	am := NewAlertmanagerClient(amSrv.URL)
-	RegisterHandlers(server, prom, am, HandlerOptions{EnableSilences: true})
+
+	var loki *LokiClient
+	var lokiSrv *httptest.Server
+	if len(lokiHandler) > 0 && lokiHandler[0] != nil {
+		lokiSrv = httptest.NewServer(lokiHandler[0])
+		loki = NewLokiClient(lokiSrv.URL)
+	}
+
+	RegisterHandlers(server, prom, am, loki, HandlerOptions{EnableSilences: true})
 
 	cleanup := func() {
 		promSrv.Close()
 		amSrv.Close()
+		if lokiSrv != nil {
+			lokiSrv.Close()
+		}
 	}
 
 	return server, cleanup
@@ -305,8 +316,9 @@ func TestHandler_ToolCount(t *testing.T) {
 	defer cleanup()
 
 	tools := listTools(t, server)
+	// Without Loki: 7 base + 1 silence = 8
 	if len(tools) != 8 {
-		t.Errorf("expected 8 tools with silences enabled, got %d", len(tools))
+		t.Errorf("expected 8 tools with silences enabled (no Loki), got %d", len(tools))
 		for _, tool := range tools {
 			t.Logf("  tool: %s", tool.Name)
 		}
@@ -325,6 +337,37 @@ func TestHandler_ToolCount(t *testing.T) {
 	}
 }
 
+func TestHandler_ToolCountWithLoki(t *testing.T) {
+	server, cleanup := setupTestServer(t,
+		func(w http.ResponseWriter, r *http.Request) {},
+		func(w http.ResponseWriter, r *http.Request) {},
+		func(w http.ResponseWriter, r *http.Request) {},
+	)
+	defer cleanup()
+
+	tools := listTools(t, server)
+	// With Loki: 7 base + 1 silence + 3 loki = 11
+	if len(tools) != 11 {
+		t.Errorf("expected 11 tools with silences and Loki enabled, got %d", len(tools))
+		for _, tool := range tools {
+			t.Logf("  tool: %s", tool.Name)
+		}
+	}
+
+	// Verify Loki tools are present
+	lokiTools := map[string]bool{"query_logs": false, "list_labels": false, "list_label_values": false}
+	for _, tool := range tools {
+		if _, ok := lokiTools[tool.Name]; ok {
+			lokiTools[tool.Name] = true
+		}
+	}
+	for name, found := range lokiTools {
+		if !found {
+			t.Errorf("expected %s tool when Loki enabled", name)
+		}
+	}
+}
+
 func TestHandler_ToolCountWithoutSilences(t *testing.T) {
 	promSrv := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {}))
 	amSrv := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {}))
@@ -337,7 +380,7 @@ func TestHandler_ToolCountWithoutSilences(t *testing.T) {
 
 	prom := NewPrometheusClient(promSrv.URL)
 	am := NewAlertmanagerClient(amSrv.URL)
-	RegisterHandlers(server, prom, am, HandlerOptions{EnableSilences: false})
+	RegisterHandlers(server, prom, am, nil, HandlerOptions{EnableSilences: false})
 
 	tools := listTools(t, server)
 	if len(tools) != 7 {
@@ -384,6 +427,110 @@ func listTools(t *testing.T, server *mcp.Server) []mcp.Tool {
 	return listResult.Tools
 }
 
+func TestHandler_QueryLogs(t *testing.T) {
+	server, cleanup := setupTestServer(t,
+		nil,
+		nil,
+		func(w http.ResponseWriter, r *http.Request) {
+			if r.URL.Path != "/loki/api/v1/query_range" {
+				http.NotFound(w, r)
+				return
+			}
+			w.Header().Set("Content-Type", "application/json")
+			_, _ = w.Write([]byte(`{
+				"status": "success",
+				"data": {
+					"resultType": "streams",
+					"result": [
+						{
+							"stream": {"job": "varlogs", "filename": "/var/log/syslog"},
+							"values": [
+								["1704067200000000000", "Jan  1 00:00:00 host kernel: test message"]
+							]
+						}
+					]
+				}
+			}`))
+		},
+	)
+	defer cleanup()
+
+	result := callTool(t, server, "query_logs", map[string]interface{}{
+		"logql": `{job="varlogs"}`,
+	})
+	if result.IsError {
+		t.Fatalf("unexpected error: %s", result.Content[0].Text)
+	}
+	if !strings.Contains(result.Content[0].Text, "varlogs") {
+		t.Errorf("expected output to contain 'varlogs', got: %s", result.Content[0].Text)
+	}
+	if !strings.Contains(result.Content[0].Text, "test message") {
+		t.Errorf("expected output to contain 'test message', got: %s", result.Content[0].Text)
+	}
+}
+
+func TestHandler_ListLabels(t *testing.T) {
+	server, cleanup := setupTestServer(t,
+		nil,
+		nil,
+		func(w http.ResponseWriter, r *http.Request) {
+			if r.URL.Path != "/loki/api/v1/labels" {
+				http.NotFound(w, r)
+				return
+			}
+			w.Header().Set("Content-Type", "application/json")
+			_, _ = w.Write([]byte(`{
+				"status": "success",
+				"data": ["job", "instance", "filename"]
+			}`))
+		},
+	)
+	defer cleanup()
+
+	result := callTool(t, server, "list_labels", map[string]interface{}{})
+	if result.IsError {
+		t.Fatalf("unexpected error: %s", result.Content[0].Text)
+	}
+	if !strings.Contains(result.Content[0].Text, "3 label") {
+		t.Errorf("expected output to contain '3 label', got: %s", result.Content[0].Text)
+	}
+	if !strings.Contains(result.Content[0].Text, "job") {
+		t.Errorf("expected output to contain 'job', got: %s", result.Content[0].Text)
+	}
+}
+
+func TestHandler_ListLabelValues(t *testing.T) {
+	server, cleanup := setupTestServer(t,
+		nil,
+		nil,
+		func(w http.ResponseWriter, r *http.Request) {
+			if r.URL.Path != "/loki/api/v1/label/job/values" {
+				http.NotFound(w, r)
+				return
+			}
+			w.Header().Set("Content-Type", "application/json")
+			_, _ = w.Write([]byte(`{
+				"status": "success",
+				"data": ["varlogs", "nginx", "systemd"]
+			}`))
+		},
+	)
+	defer cleanup()
+
+	result := callTool(t, server, "list_label_values", map[string]interface{}{
+		"label": "job",
+	})
+	if result.IsError {
+		t.Fatalf("unexpected error: %s", result.Content[0].Text)
+	}
+	if !strings.Contains(result.Content[0].Text, "3 value") {
+		t.Errorf("expected output to contain '3 value', got: %s", result.Content[0].Text)
+	}
+	if !strings.Contains(result.Content[0].Text, "nginx") {
+		t.Errorf("expected output to contain 'nginx', got: %s", result.Content[0].Text)
+	}
+}
+
 // callTool is a test helper that calls a tool through the MCP server.
 func callTool(t *testing.T, server *mcp.Server, name string, args map[string]interface{}) mcp.CallToolResult {
 	t.Helper()

internal/monitoring/loki.go

@@ -0,0 +1,122 @@
+package monitoring
+
+import (
+	"context"
+	"encoding/json"
+	"fmt"
+	"io"
+	"net/http"
+	"net/url"
+	"strings"
+	"time"
+)
+
+// LokiClient is an HTTP client for the Loki API.
+type LokiClient struct {
+	baseURL    string
+	httpClient *http.Client
+}
+
+// NewLokiClient creates a new Loki API client.
+func NewLokiClient(baseURL string) *LokiClient {
+	return &LokiClient{
+		baseURL: strings.TrimRight(baseURL, "/"),
+		httpClient: &http.Client{
+			Timeout: 30 * time.Second,
+		},
+	}
+}
+
+// QueryRange executes a LogQL range query against Loki.
+func (c *LokiClient) QueryRange(ctx context.Context, logql string, start, end time.Time, limit int, direction string) (*LokiQueryData, error) {
+	params := url.Values{}
+	params.Set("query", logql)
+	params.Set("start", fmt.Sprintf("%d", start.UnixNano()))
+	params.Set("end", fmt.Sprintf("%d", end.UnixNano()))
+	if limit > 0 {
+		params.Set("limit", fmt.Sprintf("%d", limit))
+	}
+	if direction != "" {
+		params.Set("direction", direction)
+	}
+
+	body, err := c.get(ctx, "/loki/api/v1/query_range", params)
+	if err != nil {
+		return nil, fmt.Errorf("query range failed: %w", err)
+	}
+
+	var data LokiQueryData
+	if err := json.Unmarshal(body, &data); err != nil {
+		return nil, fmt.Errorf("failed to parse query data: %w", err)
+	}
+	return &data, nil
+}
+
+// Labels returns all available label names from Loki.
+func (c *LokiClient) Labels(ctx context.Context) ([]string, error) {
+	body, err := c.get(ctx, "/loki/api/v1/labels", nil)
+	if err != nil {
+		return nil, fmt.Errorf("labels failed: %w", err)
+	}
+
+	var labels []string
+	if err := json.Unmarshal(body, &labels); err != nil {
+		return nil, fmt.Errorf("failed to parse labels: %w", err)
+	}
+	return labels, nil
+}
+
+// LabelValues returns all values for a given label name from Loki.
+func (c *LokiClient) LabelValues(ctx context.Context, label string) ([]string, error) {
+	path := fmt.Sprintf("/loki/api/v1/label/%s/values", url.PathEscape(label))
+	body, err := c.get(ctx, path, nil)
+	if err != nil {
+		return nil, fmt.Errorf("label values failed: %w", err)
+	}
+
+	var values []string
+	if err := json.Unmarshal(body, &values); err != nil {
+		return nil, fmt.Errorf("failed to parse label values: %w", err)
+	}
+	return values, nil
+}
+
+// get performs a GET request and returns the "data" field from the Loki response envelope.
+// Loki uses the same {"status":"success","data":...} format as Prometheus.
+func (c *LokiClient) get(ctx context.Context, path string, params url.Values) (json.RawMessage, error) {
+	u := c.baseURL + path
+	if len(params) > 0 {
+		u += "?" + params.Encode()
+	}
+
+	req, err := http.NewRequestWithContext(ctx, http.MethodGet, u, nil)
+	if err != nil {
+		return nil, fmt.Errorf("failed to create request: %w", err)
+	}
+
+	resp, err := c.httpClient.Do(req)
+	if err != nil {
+		return nil, fmt.Errorf("request failed: %w", err)
+	}
+	defer resp.Body.Close() //nolint:errcheck // cleanup on exit
+
+	body, err := io.ReadAll(resp.Body)
+	if err != nil {
+		return nil, fmt.Errorf("failed to read response: %w", err)
+	}
+
+	if resp.StatusCode != http.StatusOK {
+		return nil, fmt.Errorf("unexpected status %d: %s", resp.StatusCode, string(body))
+	}
+
+	var promResp PromResponse
+	if err := json.Unmarshal(body, &promResp); err != nil {
+		return nil, fmt.Errorf("failed to parse response: %w", err)
+	}
+
+	if promResp.Status != "success" {
+		return nil, fmt.Errorf("loki error (%s): %s", promResp.ErrorType, promResp.Error)
+	}
+
+	return promResp.Data, nil
+}

internal/monitoring/loki_test.go

@@ -0,0 +1,162 @@
+package monitoring
+
+import (
+	"context"
+	"net/http"
+	"net/http/httptest"
+	"testing"
+	"time"
+)
+
+func TestLokiClient_QueryRange(t *testing.T) {
+	srv := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		if r.URL.Path != "/loki/api/v1/query_range" {
+			t.Errorf("unexpected path: %s", r.URL.Path)
+		}
+		if r.URL.Query().Get("query") != `{job="varlogs"}` {
+			t.Errorf("unexpected query param: %s", r.URL.Query().Get("query"))
+		}
+		if r.URL.Query().Get("direction") != "backward" {
+			t.Errorf("unexpected direction: %s", r.URL.Query().Get("direction"))
+		}
+		if r.URL.Query().Get("limit") != "10" {
+			t.Errorf("unexpected limit: %s", r.URL.Query().Get("limit"))
+		}
+
+		w.Header().Set("Content-Type", "application/json")
+		_, _ = w.Write([]byte(`{
+			"status": "success",
+			"data": {
+				"resultType": "streams",
+				"result": [
+					{
+						"stream": {"job": "varlogs", "filename": "/var/log/syslog"},
+						"values": [
+							["1234567890000000000", "line 1"],
+							["1234567891000000000", "line 2"]
+						]
+					}
+				]
+			}
+		}`))
+	}))
+	defer srv.Close()
+
+	client := NewLokiClient(srv.URL)
+	start := time.Unix(0, 1234567890000000000)
+	end := time.Unix(0, 1234567899000000000)
+	data, err := client.QueryRange(context.Background(), `{job="varlogs"}`, start, end, 10, "backward")
+	if err != nil {
+		t.Fatalf("unexpected error: %v", err)
+	}
+
+	if data.ResultType != "streams" {
+		t.Errorf("expected resultType=streams, got %s", data.ResultType)
+	}
+	if len(data.Result) != 1 {
+		t.Fatalf("expected 1 stream, got %d", len(data.Result))
+	}
+	if data.Result[0].Stream["job"] != "varlogs" {
+		t.Errorf("expected job=varlogs, got %s", data.Result[0].Stream["job"])
+	}
+	if len(data.Result[0].Values) != 2 {
+		t.Fatalf("expected 2 entries, got %d", len(data.Result[0].Values))
+	}
+	if data.Result[0].Values[0][1] != "line 1" {
+		t.Errorf("expected first line='line 1', got %s", data.Result[0].Values[0][1])
+	}
+}
+
+func TestLokiClient_QueryRangeError(t *testing.T) {
+	srv := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		w.Header().Set("Content-Type", "application/json")
+		_, _ = w.Write([]byte(`{
+			"status": "error",
+			"errorType": "bad_data",
+			"error": "invalid LogQL query"
+		}`))
+	}))
+	defer srv.Close()
+
+	client := NewLokiClient(srv.URL)
+	_, err := client.QueryRange(context.Background(), "invalid{", time.Now().Add(-time.Hour), time.Now(), 100, "backward")
+	if err == nil {
+		t.Fatal("expected error, got nil")
+	}
+	if !contains(err.Error(), "invalid LogQL query") {
+		t.Errorf("expected error to contain 'invalid LogQL query', got: %s", err.Error())
+	}
+}
+
+func TestLokiClient_Labels(t *testing.T) {
+	srv := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		if r.URL.Path != "/loki/api/v1/labels" {
+			t.Errorf("unexpected path: %s", r.URL.Path)
+		}
+
+		w.Header().Set("Content-Type", "application/json")
+		_, _ = w.Write([]byte(`{
+			"status": "success",
+			"data": ["job", "instance", "filename"]
+		}`))
+	}))
+	defer srv.Close()
+
+	client := NewLokiClient(srv.URL)
+	labels, err := client.Labels(context.Background())
+	if err != nil {
+		t.Fatalf("unexpected error: %v", err)
+	}
+
+	if len(labels) != 3 {
+		t.Fatalf("expected 3 labels, got %d", len(labels))
+	}
+	if labels[0] != "job" {
+		t.Errorf("expected first label=job, got %s", labels[0])
+	}
+}
+
+func TestLokiClient_LabelValues(t *testing.T) {
+	srv := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		if r.URL.Path != "/loki/api/v1/label/job/values" {
+			t.Errorf("unexpected path: %s, expected /loki/api/v1/label/job/values", r.URL.Path)
+		}
+
+		w.Header().Set("Content-Type", "application/json")
+		_, _ = w.Write([]byte(`{
+			"status": "success",
+			"data": ["varlogs", "nginx", "systemd"]
+		}`))
+	}))
+	defer srv.Close()
+
+	client := NewLokiClient(srv.URL)
+	values, err := client.LabelValues(context.Background(), "job")
+	if err != nil {
+		t.Fatalf("unexpected error: %v", err)
+	}
+
+	if len(values) != 3 {
+		t.Fatalf("expected 3 values, got %d", len(values))
+	}
+	if values[0] != "varlogs" {
+		t.Errorf("expected first value=varlogs, got %s", values[0])
+	}
+}
+
+func TestLokiClient_HTTPError(t *testing.T) {
+	srv := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		w.WriteHeader(http.StatusInternalServerError)
+		_, _ = w.Write([]byte("internal error"))
+	}))
+	defer srv.Close()
+
+	client := NewLokiClient(srv.URL)
+	_, err := client.QueryRange(context.Background(), `{job="test"}`, time.Now().Add(-time.Hour), time.Now(), 100, "backward")
+	if err == nil {
+		t.Fatal("expected error, got nil")
+	}
+	if !contains(err.Error(), "500") {
+		t.Errorf("expected error to contain status code, got: %s", err.Error())
+	}
+}

internal/monitoring/types.go

@@ -118,3 +118,20 @@ type Matcher struct {
 	IsRegex bool   `json:"isRegex"`
 	IsEqual *bool  `json:"isEqual,omitempty"`
 }
+
+// Loki API response types
+
+// LokiQueryData represents the data field for Loki query results.
+type LokiQueryData struct {
+	ResultType string       `json:"resultType"`
+	Result     []LokiStream `json:"result"`
+}
+
+// LokiStream represents a single log stream with its entries.
+type LokiStream struct {
+	Stream map[string]string `json:"stream"`
+	Values []LokiEntry       `json:"values"`
+}
+
+// LokiEntry represents a log entry as [nanosecond_timestamp, log_line].
+type LokiEntry [2]string