feat(spanmapper): add preview endpoint for span attribute mapping

Adds POST /api/v1/span_mapper_groups/preview so users can see what their
attribute mappings will do to a span before/while configuring them.

You send a sample input either a set of attributes, a single OTLP
span, or a full OTLP trace and get back the transformed result in the
same form. By default it runs against all the org's enabled mappings;
pass groupId to scope it to one group.

The actual signozspanmapper collector processor not merged yet
https://github.com/SigNoz/signoz-otel-collector/pull/796

docs/api/openapi.yml

@@ -7178,6 +7178,41 @@ components:
       - operation
       - priority
       type: object
+    SpantypesSpanMappingPreviewGroup:
+      properties:
+        group:
+          $ref: '#/components/schemas/SpantypesPostableSpanMapperGroup'
+        mappers:
+          items:
+            $ref: '#/components/schemas/SpantypesPostableSpanMapper'
+          nullable: true
+          type: array
+      required:
+      - group
+      - mappers
+      type: object
+    SpantypesSpanMappingPreviewRequest:
+      properties:
+        groupId:
+          nullable: true
+          type: string
+        groups:
+          items:
+            $ref: '#/components/schemas/SpantypesSpanMappingPreviewGroup'
+          nullable: true
+          type: array
+        span:
+          additionalProperties: {}
+          nullable: true
+          type: object
+      type: object
+    SpantypesSpanMappingPreviewResponse:
+      properties:
+        span:
+          additionalProperties: {}
+          nullable: true
+          type: object
+      type: object
     SpantypesUpdatableSpanMapper:
       properties:
         config:
@@ -12783,6 +12818,69 @@ paths:
       summary: Update a span mapper
       tags:
       - spanmapper
+  /api/v1/span_mapper_groups/preview:
+    post:
+      deprecated: false
+      description: Previews how attribute mappings would transform a sample span.
+      operationId: PreviewSpanMapping
+      requestBody:
+        content:
+          application/json:
+            schema:
+              $ref: '#/components/schemas/SpantypesSpanMappingPreviewRequest'
+      responses:
+        "200":
+          content:
+            application/json:
+              schema:
+                properties:
+                  data:
+                    $ref: '#/components/schemas/SpantypesSpanMappingPreviewResponse'
+                  status:
+                    type: string
+                required:
+                - status
+                - data
+                type: object
+          description: OK
+        "400":
+          content:
+            application/json:
+              schema:
+                $ref: '#/components/schemas/RenderErrorResponse'
+          description: Bad Request
+        "401":
+          content:
+            application/json:
+              schema:
+                $ref: '#/components/schemas/RenderErrorResponse'
+          description: Unauthorized
+        "403":
+          content:
+            application/json:
+              schema:
+                $ref: '#/components/schemas/RenderErrorResponse'
+          description: Forbidden
+        "404":
+          content:
+            application/json:
+              schema:
+                $ref: '#/components/schemas/RenderErrorResponse'
+          description: Not Found
+        "500":
+          content:
+            application/json:
+              schema:
+                $ref: '#/components/schemas/RenderErrorResponse'
+          description: Internal Server Error
+      security:
+      - api_key:
+        - VIEWER
+      - tokenizer:
+        - VIEWER
+      summary: Preview span attribute mapping against a sample span
+      tags:
+      - spanmapper
   /api/v1/testChannel:
     post:
       deprecated: true

frontend/src/api/generated/services/sigNoz.schemas.ts

@@ -8430,6 +8430,56 @@ export interface SpantypesSpanMapperDTO {
 	updatedBy?: string;
 }
 
+export interface SpantypesSpanMappingPreviewGroupDTO {
+	group: SpantypesPostableSpanMapperGroupDTO;
+	/**
+	 * @type array,null
+	 */
+	mappers: SpantypesPostableSpanMapperDTO[] | null;
+}
+
+export type SpantypesSpanMappingPreviewRequestDTOSpanAnyOf = {
+	[key: string]: unknown;
+};
+
+/**
+ * @nullable
+ */
+export type SpantypesSpanMappingPreviewRequestDTOSpan =
+	SpantypesSpanMappingPreviewRequestDTOSpanAnyOf | null;
+
+export interface SpantypesSpanMappingPreviewRequestDTO {
+	/**
+	 * @type string,null
+	 */
+	groupId?: string | null;
+	/**
+	 * @type array,null
+	 */
+	groups?: SpantypesSpanMappingPreviewGroupDTO[] | null;
+	/**
+	 * @type object,null
+	 */
+	span?: SpantypesSpanMappingPreviewRequestDTOSpan;
+}
+
+export type SpantypesSpanMappingPreviewResponseDTOSpanAnyOf = {
+	[key: string]: unknown;
+};
+
+/**
+ * @nullable
+ */
+export type SpantypesSpanMappingPreviewResponseDTOSpan =
+	SpantypesSpanMappingPreviewResponseDTOSpanAnyOf | null;
+
+export interface SpantypesSpanMappingPreviewResponseDTO {
+	/**
+	 * @type object,null
+	 */
+	span?: SpantypesSpanMappingPreviewResponseDTOSpan;
+}
+
 export interface SpantypesUpdatableSpanMapperDTO {
 	config?: SpantypesSpanMapperConfigDTO;
 	/**
@@ -9736,6 +9786,14 @@ export type UpdateSpanMapperPathParameters = {
 	groupId: string;
 	mapperId: string;
 };
+export type PreviewSpanMapping200 = {
+	data: SpantypesSpanMappingPreviewResponseDTO;
+	/**
+	 * @type string
+	 */
+	status: string;
+};
+
 export type GetTraceAggregationsPathParameters = {
 	traceID: string;
 };

frontend/src/api/generated/services/spanmapper/index.ts

@@ -27,9 +27,11 @@ import type {
 	ListSpanMapperGroupsParams,
 	ListSpanMappers200,
 	ListSpanMappersPathParameters,
+	PreviewSpanMapping200,
 	RenderErrorResponseDTO,
 	SpantypesPostableSpanMapperDTO,
 	SpantypesPostableSpanMapperGroupDTO,
+	SpantypesSpanMappingPreviewRequestDTO,
 	SpantypesUpdatableSpanMapperDTO,
 	SpantypesUpdatableSpanMapperGroupDTO,
 	UpdateSpanMapperGroupPathParameters,
@@ -780,3 +782,86 @@ export const useUpdateSpanMapper = <
 > => {
 	return useMutation(getUpdateSpanMapperMutationOptions(options));
 };
+/**
+ * Previews how attribute mappings would transform a sample span.
+ * @summary Preview span attribute mapping against a sample span
+ */
+export const previewSpanMapping = (
+	spantypesSpanMappingPreviewRequestDTO?: BodyType<SpantypesSpanMappingPreviewRequestDTO>,
+	signal?: AbortSignal,
+) => {
+	return GeneratedAPIInstance<PreviewSpanMapping200>({
+		url: `/api/v1/span_mapper_groups/preview`,
+		method: 'POST',
+		headers: { 'Content-Type': 'application/json' },
+		data: spantypesSpanMappingPreviewRequestDTO,
+		signal,
+	});
+};
+
+export const getPreviewSpanMappingMutationOptions = <
+	TError = ErrorType<RenderErrorResponseDTO>,
+	TContext = unknown,
+>(options?: {
+	mutation?: UseMutationOptions<
+		Awaited<ReturnType<typeof previewSpanMapping>>,
+		TError,
+		{ data?: BodyType<SpantypesSpanMappingPreviewRequestDTO> },
+		TContext
+	>;
+}): UseMutationOptions<
+	Awaited<ReturnType<typeof previewSpanMapping>>,
+	TError,
+	{ data?: BodyType<SpantypesSpanMappingPreviewRequestDTO> },
+	TContext
+> => {
+	const mutationKey = ['previewSpanMapping'];
+	const { mutation: mutationOptions } = options
+		? options.mutation &&
+			'mutationKey' in options.mutation &&
+			options.mutation.mutationKey
+			? options
+			: { ...options, mutation: { ...options.mutation, mutationKey } }
+		: { mutation: { mutationKey } };
+
+	const mutationFn: MutationFunction<
+		Awaited<ReturnType<typeof previewSpanMapping>>,
+		{ data?: BodyType<SpantypesSpanMappingPreviewRequestDTO> }
+	> = (props) => {
+		const { data } = props ?? {};
+
+		return previewSpanMapping(data);
+	};
+
+	return { mutationFn, ...mutationOptions };
+};
+
+export type PreviewSpanMappingMutationResult = NonNullable<
+	Awaited<ReturnType<typeof previewSpanMapping>>
+>;
+export type PreviewSpanMappingMutationBody =
+	| BodyType<SpantypesSpanMappingPreviewRequestDTO>
+	| undefined;
+export type PreviewSpanMappingMutationError = ErrorType<RenderErrorResponseDTO>;
+
+/**
+ * @summary Preview span attribute mapping against a sample span
+ */
+export const usePreviewSpanMapping = <
+	TError = ErrorType<RenderErrorResponseDTO>,
+	TContext = unknown,
+>(options?: {
+	mutation?: UseMutationOptions<
+		Awaited<ReturnType<typeof previewSpanMapping>>,
+		TError,
+		{ data?: BodyType<SpantypesSpanMappingPreviewRequestDTO> },
+		TContext
+	>;
+}): UseMutationResult<
+	Awaited<ReturnType<typeof previewSpanMapping>>,
+	TError,
+	{ data?: BodyType<SpantypesSpanMappingPreviewRequestDTO> },
+	TContext
+> => {
+	return useMutation(getPreviewSpanMappingMutationOptions(options));
+};

pkg/apiserver/signozapiserver/spanmapper.go

@@ -51,6 +51,26 @@ func (provider *provider) addSpanMapperRoutes(router *mux.Router) error {
 		return err
 	}
 
+	if err := router.Handle("/api/v1/span_mapper_groups/preview", handler.New(
+		provider.authzMiddleware.ViewAccess(provider.spanMapperHandler.PreviewMapping),
+		handler.OpenAPIDef{
+			ID:                  "PreviewSpanMapping",
+			Tags:                []string{"spanmapper"},
+			Summary:             "Preview span attribute mapping against a sample span",
+			Description:         "Previews how attribute mappings would transform a sample span.",
+			Request:             new(spantypes.SpanMappingPreviewRequest),
+			RequestContentType:  "application/json",
+			Response:            new(spantypes.SpanMappingPreviewResponse),
+			ResponseContentType: "application/json",
+			SuccessStatusCode:   http.StatusOK,
+			ErrorStatusCodes:    []int{http.StatusBadRequest, http.StatusNotFound},
+			Deprecated:          false,
+			SecuritySchemes:     newSecuritySchemes(types.RoleViewer),
+		},
+	)).Methods(http.MethodPost).GetError(); err != nil {
+		return err
+	}
+
 	if err := router.Handle("/api/v1/span_mapper_groups/{groupId}", handler.New(
 		provider.authzMiddleware.AdminAccess(provider.spanMapperHandler.UpdateGroup),
 		handler.OpenAPIDef{

pkg/modules/spanmapper/implspanmapper/handler.go

@@ -273,6 +273,35 @@ func (h *handler) DeleteMapper(rw http.ResponseWriter, r *http.Request) {
 	render.Success(rw, http.StatusNoContent, nil)
 }
 
+// PreviewMapping handles POST /api/v1/span_mapper_groups/preview.
+// used to get preview of attributes/resources after remapping.
+func (h *handler) PreviewMapping(rw http.ResponseWriter, r *http.Request) {
+	ctx, cancel := context.WithTimeout(r.Context(), 10*time.Second)
+	defer cancel()
+
+	claims, err := authtypes.ClaimsFromContext(ctx)
+	if err != nil {
+		render.Error(rw, err)
+		return
+	}
+
+	orgID := valuer.MustNewUUID(claims.OrgID)
+
+	req := new(spantypes.SpanMappingPreviewRequest)
+	if err := binding.JSON.BindBody(r.Body, req); err != nil {
+		render.Error(rw, err)
+		return
+	}
+
+	result, err := h.module.PreviewMapping(ctx, orgID, req)
+	if err != nil {
+		render.Error(rw, err)
+		return
+	}
+
+	render.Success(rw, http.StatusOK, result)
+}
+
 // groupIDFromPath extracts and validates the {id} or {groupId} path variable.
 func groupIDFromPath(r *http.Request) (valuer.UUID, error) {
 	vars := mux.Vars(r)

pkg/modules/spanmapper/implspanmapper/module.go

@@ -4,6 +4,7 @@ import (
 	"context"
 	"encoding/json"
 
+	"github.com/SigNoz/signoz/pkg/errors"
 	"github.com/SigNoz/signoz/pkg/modules/spanmapper"
 	"github.com/SigNoz/signoz/pkg/query-service/agentConf"
 	"github.com/SigNoz/signoz/pkg/types/opamptypes"
@@ -102,6 +103,97 @@ func (module *module) DeleteMapper(ctx context.Context, orgID, groupID, id value
 	return nil
 }
 
+// PreviewMapping resolves the mappings to preview (from the request body, a
+// saved group, or all enabled saved mappings) and returns the input span with
+// its "attributes" and "resource" maps transformed.
+func (module *module) PreviewMapping(ctx context.Context, orgID valuer.UUID, req *spantypes.SpanMappingPreviewRequest) (*spantypes.SpanMappingPreviewResponse, error) {
+	groups, err := module.resolvePreviewGroups(ctx, orgID, req)
+	if err != nil {
+		return nil, err
+	}
+
+	if len(req.Span) == 0 {
+		return nil, errors.New(errors.TypeInvalidInput, spantypes.ErrCodeMappingInvalidInput, "'span' must be provided")
+	}
+
+	outResource, outSpan := spantypes.SimulateMappingForAttributes(groups, spanAttrMap(req.Span["resource"]), spanAttrMap(req.Span["attributes"]))
+
+	result := make(map[string]any, len(req.Span))
+	for k, v := range req.Span {
+		result[k] = v
+	}
+	setAttrMap(result, "attributes", req.Span, outSpan)
+	setAttrMap(result, "resource", req.Span, outResource)
+
+	return &spantypes.SpanMappingPreviewResponse{Span: result}, nil
+}
+
+func spanAttrMap(v any) map[string]any {
+	if m, ok := v.(map[string]any); ok {
+		return m
+	}
+	return nil
+}
+
+func setAttrMap(dst map[string]any, key string, in map[string]any, transformed map[string]any) {
+	if _, present := in[key]; present || len(transformed) > 0 {
+		dst[key] = transformed
+	}
+}
+
+// resolvePreviewGroups picks the mappings to preview against: the groups in the
+// request body, else a specific saved group (GroupID), else all of the org's
+// enabled saved mappings.
+func (module *module) resolvePreviewGroups(ctx context.Context, orgID valuer.UUID, req *spantypes.SpanMappingPreviewRequest) ([]*spantypes.SpanMapperGroupWithMappers, error) {
+	hasGroups := len(req.Groups) > 0
+	hasGroupID := req.GroupID != nil && *req.GroupID != ""
+
+	if hasGroups && hasGroupID {
+		return nil, errors.New(errors.TypeInvalidInput, spantypes.ErrCodeMappingInvalidInput, "provide either 'groups' or 'groupId', not both")
+	}
+
+	if hasGroups {
+		groups := make([]*spantypes.SpanMapperGroupWithMappers, 0, len(req.Groups))
+		for _, spec := range req.Groups {
+			group := &spantypes.SpanMapperGroup{
+				OrgID:     orgID,
+				Name:      spec.Group.Name,
+				Condition: spec.Group.Condition,
+				Enabled:   spec.Group.Enabled,
+			}
+			mappers := make([]*spantypes.SpanMapper, 0, len(spec.Mappers))
+			for _, pm := range spec.Mappers {
+				mappers = append(mappers, &spantypes.SpanMapper{
+					Name:         pm.Name,
+					FieldContext: pm.FieldContext,
+					Config:       pm.Config,
+					Enabled:      pm.Enabled,
+				})
+			}
+			groups = append(groups, &spantypes.SpanMapperGroupWithMappers{Group: group, Mappers: mappers})
+		}
+		return groups, nil
+	}
+
+	if hasGroupID {
+		id, err := valuer.NewUUID(*req.GroupID)
+		if err != nil {
+			return nil, errors.Wrapf(err, errors.TypeInvalidInput, spantypes.ErrCodeMappingInvalidInput, "group id is not a valid uuid")
+		}
+		group, err := module.store.GetGroup(ctx, orgID, id)
+		if err != nil {
+			return nil, err
+		}
+		mappers, err := module.store.ListMappers(ctx, orgID, id)
+		if err != nil {
+			return nil, err
+		}
+		return []*spantypes.SpanMapperGroupWithMappers{{Group: group, Mappers: mappers}}, nil
+	}
+
+	return module.listEnabledGroupsWithMappers(ctx, orgID)
+}
+
 func (module *module) AgentFeatureType() agentConf.AgentFeatureType {
 	return spantypes.SpanAttrMappingFeatureType
 }

pkg/modules/spanmapper/spanmapper.go

@@ -27,6 +27,7 @@ type Module interface {
 	CreateMapper(ctx context.Context, orgID, groupID valuer.UUID, mapper *spantypes.SpanMapper) error
 	UpdateMapper(ctx context.Context, orgID, groupID, id valuer.UUID, fieldContext spantypes.FieldContext, config *spantypes.SpanMapperConfig, enabled *bool, updatedBy string) error
 	DeleteMapper(ctx context.Context, orgID, groupID, id valuer.UUID) error
+	PreviewMapping(ctx context.Context, orgID valuer.UUID, req *spantypes.SpanMappingPreviewRequest) (*spantypes.SpanMappingPreviewResponse, error)
 }
 
 // Handler defines the HTTP handler interface for mapping group and mapper endpoints.
@@ -42,4 +43,5 @@ type Handler interface {
 	CreateMapper(rw http.ResponseWriter, r *http.Request)
 	UpdateMapper(rw http.ResponseWriter, r *http.Request)
 	DeleteMapper(rw http.ResponseWriter, r *http.Request)
+	PreviewMapping(rw http.ResponseWriter, r *http.Request)
 }

pkg/types/spantypes/spanmappersimulator.go

@@ -0,0 +1,117 @@
+package spantypes
+
+import (
+	"strings"
+)
+
+// resourceSourcePrefix marks a source that reads from resource attributes:
+// buildAttributeRule prefixes those keys with "resource." (e.g. "resource.service.name").
+var resourceSourcePrefix = FieldContextResource.StringValue() + "."
+
+type SpanMappingPreviewGroup struct {
+	Group   PostableSpanMapperGroup `json:"group" required:"true"`
+	Mappers []PostableSpanMapper    `json:"mappers" required:"true" nullable:"true"`
+}
+
+type SpanMappingPreviewRequest struct {
+	Span    map[string]any            `json:"span" nullable:"true"`
+	Groups  []SpanMappingPreviewGroup `json:"groups" nullable:"true"`
+	GroupID *string                   `json:"groupId" nullable:"true"`
+}
+
+type SpanMappingPreviewResponse struct {
+	Span map[string]any `json:"span" nullable:"true"`
+}
+
+func SimulateMappingForAttributes(groups []*SpanMapperGroupWithMappers, resourceAttrs, spanAttrs map[string]any) (outResource, outSpan map[string]any) {
+	cfg := buildProcessorConfig(filterEnabledGroupsWithMappers(groups))
+
+	outResource = cloneAttrs(resourceAttrs)
+	outSpan = cloneAttrs(spanAttrs)
+
+	applyEnabledGroups(cfg, outSpan, outResource)
+	return outResource, outSpan
+}
+
+func filterEnabledGroupsWithMappers(groups []*SpanMapperGroupWithMappers) []*SpanMapperGroupWithMappers {
+	out := make([]*SpanMapperGroupWithMappers, 0, len(groups))
+	for _, gm := range groups {
+		if gm == nil || gm.Group == nil || !gm.Group.Enabled {
+			continue
+		}
+		enabled := make([]*SpanMapper, 0, len(gm.Mappers))
+		for _, m := range gm.Mappers {
+			if m != nil && m.Enabled {
+				enabled = append(enabled, m)
+			}
+		}
+		if len(enabled) > 0 {
+			out = append(out, &SpanMapperGroupWithMappers{Group: gm.Group, Mappers: enabled})
+		}
+	}
+	return out
+}
+
+func cloneAttrs(in map[string]any) map[string]any {
+	out := make(map[string]any, len(in))
+	for k, v := range in {
+		out[k] = v
+	}
+	return out
+}
+
+// The functions below are copied from signoz-otel-collector (processor/signozspanmappingprocessor, PR #796):
+// TODO(spanmapper-preview): delete them and call the real processor once that PR merges and the dependency is bumped.
+func applyEnabledGroups(cfg *spanMapperProcessorConfig, spanAttrs, resourceAttrs map[string]any) {
+	for i := range cfg.Groups {
+		g := &cfg.Groups[i]
+		if !spanMapperConditionMet(g.ExistsAny, spanAttrs, resourceAttrs) {
+			continue
+		}
+		for j := range g.Attributes {
+			applySpanMapperRule(&g.Attributes[j], spanAttrs, resourceAttrs)
+		}
+	}
+}
+
+func spanMapperConditionMet(cond spanMapperProcessorExistsAny, spanAttrs, resourceAttrs map[string]any) bool {
+	return anyKeyContains(spanAttrs, cond.Attributes) || anyKeyContains(resourceAttrs, cond.Resource)
+}
+
+func anyKeyContains(attrs map[string]any, patterns []string) bool {
+	for k := range attrs {
+		for _, p := range patterns {
+			if strings.Contains(k, p) {
+				return true
+			}
+		}
+	}
+	return false
+}
+
+func applySpanMapperRule(rule *spanMapperProcessorAttribute, spanAttrs, resourceAttrs map[string]any) {
+	dst := spanAttrs
+	if rule.Context == FieldContextResource.StringValue() {
+		dst = resourceAttrs
+	}
+
+	for i := range rule.Sources {
+		src := &rule.Sources[i]
+		sourceKey, isResource := strings.CutPrefix(src.Key, resourceSourcePrefix)
+
+		from := spanAttrs
+		if isResource {
+			from = resourceAttrs
+		}
+		val, ok := from[sourceKey]
+		if !ok {
+			continue
+		}
+
+		dst[rule.Target] = val
+		if src.Action == SpanMapperOperationMove.StringValue() {
+			delete(from, sourceKey)
+		}
+		return
+	}
+}

pkg/types/spantypes/spanmappersimulator_test.go

@@ -0,0 +1,159 @@
+package spantypes
+
+import (
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+)
+
+func simGroup(name string, attrCond, resCond []string, mappers ...*SpanMapper) *SpanMapperGroupWithMappers {
+	return &SpanMapperGroupWithMappers{
+		Group: &SpanMapperGroup{
+			Name:      name,
+			Condition: SpanMapperGroupCondition{Attributes: attrCond, Resource: resCond},
+			Enabled:   true,
+		},
+		Mappers: mappers,
+	}
+}
+
+func simMapper(target string, ctx FieldContext, sources ...SpanMapperSource) *SpanMapper {
+	return &SpanMapper{
+		Name:         target,
+		FieldContext: ctx,
+		Config:       SpanMapperConfig{Sources: sources},
+		Enabled:      true,
+	}
+}
+
+func simAttrSrc(key string, op SpanMapperOperation, priority int) SpanMapperSource {
+	return SpanMapperSource{Key: key, Context: FieldContextSpanAttribute, Operation: op, Priority: priority}
+}
+
+func simResSrc(key string, op SpanMapperOperation, priority int) SpanMapperSource {
+	return SpanMapperSource{Key: key, Context: FieldContextResource, Operation: op, Priority: priority}
+}
+
+func TestSimulate_MatchInSpanAttrs(t *testing.T) {
+	groups := []*SpanMapperGroupWithMappers{
+		simGroup("llm", []string{"model"}, nil,
+			simMapper("gen_ai.request.model", FieldContextSpanAttribute,
+				simAttrSrc("llm.model", SpanMapperOperationCopy, 1)),
+		),
+	}
+	_, outSpan := SimulateMappingForAttributes(groups, nil, map[string]any{"llm.model": "gpt-4", "gen_ai.llm.model": "gpt-40"})
+
+	assert.Equal(t, "gpt-4", outSpan["gen_ai.request.model"])
+}
+
+func TestSimulate_MatchInResourceAttrs(t *testing.T) {
+	groups := []*SpanMapperGroupWithMappers{
+		simGroup("llm", nil, []string{"service.name"},
+			simMapper("gen_ai.request.model", FieldContextSpanAttribute,
+				simResSrc("service.name", SpanMapperOperationCopy, 1)),
+		),
+	}
+	_, outSpan := SimulateMappingForAttributes(groups, map[string]any{"service.name": "my-llm-service"}, nil)
+
+	assert.Equal(t, "my-llm-service", outSpan["gen_ai.request.model"])
+}
+
+func TestSimulate_NoMatchSkipsGroup(t *testing.T) {
+	groups := []*SpanMapperGroupWithMappers{
+		simGroup("llm", []string{"model"}, nil,
+			simMapper("gen_ai.request.model", FieldContextSpanAttribute,
+				simAttrSrc("llm.model", SpanMapperOperationCopy, 1)),
+		),
+	}
+	_, outSpan := SimulateMappingForAttributes(groups, nil, map[string]any{"some.other.key": "value"})
+
+	_, ok := outSpan["gen_ai.request.model"]
+	assert.False(t, ok, "target must not be set when condition is not met")
+}
+
+func TestSimulate_SourceFirstMatchWins(t *testing.T) {
+	groups := []*SpanMapperGroupWithMappers{
+		simGroup("tokens", []string{"llm"}, nil,
+			simMapper("gen_ai.request.tokens", FieldContextSpanAttribute,
+				simAttrSrc("gen_ai.request_tokens", SpanMapperOperationCopy, 2),
+				simAttrSrc("llm.tokens", SpanMapperOperationCopy, 1)),
+		),
+	}
+	_, outSpan := SimulateMappingForAttributes(groups, nil, map[string]any{"gen_ai.request_tokens": "100", "llm.tokens": "200"})
+
+	assert.Equal(t, "100", outSpan["gen_ai.request.tokens"])
+}
+
+func TestSimulate_SourceFallsBackToSecond(t *testing.T) {
+	groups := []*SpanMapperGroupWithMappers{
+		simGroup("tokens", []string{"llm"}, nil,
+			simMapper("gen_ai.request.tokens", FieldContextSpanAttribute,
+				simAttrSrc("gen_ai.request_tokens", SpanMapperOperationCopy, 2),
+				simAttrSrc("llm.tokens", SpanMapperOperationCopy, 1)),
+		),
+	}
+	_, outSpan := SimulateMappingForAttributes(groups, nil, map[string]any{"llm.tokens": "200"})
+
+	assert.Equal(t, "200", outSpan["gen_ai.request.tokens"])
+}
+
+func TestSimulate_ActionMove(t *testing.T) {
+	groups := []*SpanMapperGroupWithMappers{
+		simGroup("input", []string{"gen_ai"}, nil,
+			simMapper("gen_ai.request.input", FieldContextSpanAttribute,
+				simAttrSrc("gen_ai.input", SpanMapperOperationMove, 1)),
+		),
+	}
+	_, outSpan := SimulateMappingForAttributes(groups, nil, map[string]any{"gen_ai.input": "hello"})
+
+	assert.Equal(t, "hello", outSpan["gen_ai.request.input"])
+	_, srcPresent := outSpan["gen_ai.input"]
+	assert.False(t, srcPresent, "source key must be deleted when action=move")
+}
+
+func TestSimulate_WriteToResourceContext(t *testing.T) {
+	groups := []*SpanMapperGroupWithMappers{
+		simGroup("llm", []string{"llm"}, nil,
+			simMapper("gen_ai.request.model", FieldContextResource,
+				simAttrSrc("llm.model", SpanMapperOperationCopy, 1)),
+		),
+	}
+	outResource, outSpan := SimulateMappingForAttributes(groups, nil, map[string]any{"llm.model": "gpt-4"})
+
+	assert.Equal(t, "gpt-4", outResource["gen_ai.request.model"], "target must be written to resource attributes")
+	_, inSpan := outSpan["gen_ai.request.model"]
+	assert.False(t, inSpan)
+}
+
+func TestSimulate_DisabledGroupsAndMappersSkipped(t *testing.T) {
+	disabledGroup := simGroup("g1", []string{"llm"}, nil,
+		simMapper("gen_ai.request.model", FieldContextSpanAttribute,
+			simAttrSrc("llm.model", SpanMapperOperationCopy, 1)))
+	disabledGroup.Group.Enabled = false
+
+	_, outSpan := SimulateMappingForAttributes([]*SpanMapperGroupWithMappers{disabledGroup}, nil, map[string]any{"llm.model": "gpt-4"})
+
+	_, ok := outSpan["gen_ai.request.model"]
+	assert.False(t, ok, "disabled groups must not be evaluated")
+}
+
+func TestSimulate_NoMappingsReturnsInputUnchanged(t *testing.T) {
+	outResource, outSpan := SimulateMappingForAttributes(nil, map[string]any{"host.name": "h1"}, map[string]any{"model": "gpt-5"})
+
+	assert.Equal(t, map[string]any{"host.name": "h1"}, outResource, "resource attributes returned unchanged")
+	assert.Equal(t, map[string]any{"model": "gpt-5"}, outSpan, "span attributes returned unchanged")
+}
+
+func TestSimulate_DoesNotMutateInput(t *testing.T) {
+	input := map[string]any{"gen_ai.input": "hi"}
+	groups := []*SpanMapperGroupWithMappers{
+		simGroup("input", []string{"gen_ai"}, nil,
+			simMapper("gen_ai.request.input", FieldContextSpanAttribute,
+				simAttrSrc("gen_ai.input", SpanMapperOperationMove, 1))),
+	}
+	_, _ = SimulateMappingForAttributes(groups, nil, input)
+
+	// Original input map must be untouched (move would have deleted the key).
+	_, ok := input["gen_ai.input"]
+	assert.True(t, ok, "input map must not be mutated by the preview")
+}