diff --git a/android/bazel.go b/android/bazel.go
index e631ed4..1bfbdec 100644
--- a/android/bazel.go
+++ b/android/bazel.go
@@ -17,8 +17,10 @@
 import (
 	"bufio"
 	"errors"
+	"fmt"
 	"strings"
 
+	"android/soong/ui/metrics/bp2build_metrics_proto"
 	"github.com/google/blueprint"
 	"github.com/google/blueprint/proptools"
 
@@ -73,6 +75,21 @@
 
 	// MissingBp2buildDep stores the module names of direct dependency that were not found
 	MissingDeps []string `blueprint:"mutated"`
+
+	// If non-nil, indicates that the module could not be converted successfully
+	// with bp2build. This will describe the reason the module could not be converted.
+	UnconvertedReason *UnconvertedReason
+}
+
+// The reason a module could not be converted to a BUILD target via bp2build.
+// This should match bp2build_metrics_proto.UnconvertedReason, but omits private
+// proto-related fields that prevent copying this struct.
+type UnconvertedReason struct {
+	// Should correspond to a valid value in bp2build_metrics_proto.UnconvertedReasonType.
+	// A raw int is used here instead, because blueprint logic requires that all transitive
+	// fields of module definitions be primitives.
+	ReasonType int
+	Detail     string
 }
 
 type BazelModuleProperties struct {
@@ -137,6 +154,12 @@
 	GetBazelLabel(ctx BazelConversionPathContext, module blueprint.Module) string
 	ShouldConvertWithBp2build(ctx BazelConversionContext) bool
 	shouldConvertWithBp2build(ctx bazelOtherModuleContext, module blueprint.Module) bool
+
+	// ConvertWithBp2build either converts the module to a Bazel build target or
+	// declares the module as unconvertible (for logging and metrics).
+	// Modules must implement this function to be bp2build convertible. The function
+	// must either create at least one Bazel target module (using ctx.CreateBazelTargetModule or
+	// its related functions), or declare itself unconvertible using ctx.MarkBp2buildUnconvertible.
 	ConvertWithBp2build(ctx TopDownMutatorContext)
 
 	// namespacedVariableProps is a map from a soong config variable namespace
@@ -232,7 +255,7 @@
 	if b.ShouldConvertWithBp2build(ctx) {
 		return bp2buildModuleLabel(ctx, module)
 	}
-	return "" // no label for unconverted module
+	panic(fmt.Errorf("requested non-existent label for module ", module.Name()))
 }
 
 type Bp2BuildConversionAllowlist struct {
@@ -533,22 +556,32 @@
 }
 
 func registerBp2buildConversionMutator(ctx RegisterMutatorsContext) {
-	ctx.TopDown("bp2build_conversion", convertWithBp2build).Parallel()
+	ctx.TopDown("bp2build_conversion", bp2buildConversionMutator).Parallel()
 }
 
-func convertWithBp2build(ctx TopDownMutatorContext) {
+func bp2buildConversionMutator(ctx TopDownMutatorContext) {
 	if ctx.Config().HasBazelBuildTargetInSource(ctx) {
 		// Defer to the BUILD target. Generating an additional target would
 		// cause a BUILD file conflict.
+		ctx.MarkBp2buildUnconvertible(bp2build_metrics_proto.UnconvertedReasonType_DEFINED_IN_BUILD_FILE, "")
 		return
 	}
 
 	bModule, ok := ctx.Module().(Bazelable)
-	if !ok || !bModule.shouldConvertWithBp2build(ctx, ctx.Module()) {
+	if !ok {
+		ctx.MarkBp2buildUnconvertible(bp2build_metrics_proto.UnconvertedReasonType_TYPE_UNSUPPORTED, "")
 		return
 	}
-
+	// TODO: b/285631638 - Differentiate between denylisted modules and missing bp2build capabilities.
+	if !bModule.shouldConvertWithBp2build(ctx, ctx.Module()) {
+		ctx.MarkBp2buildUnconvertible(bp2build_metrics_proto.UnconvertedReasonType_UNSUPPORTED, "")
+		return
+	}
 	bModule.ConvertWithBp2build(ctx)
+
+	if !ctx.Module().base().IsConvertedByBp2build() && ctx.Module().base().GetUnconvertedReason() == nil {
+		panic(fmt.Errorf("illegal bp2build invariant: module '%s' was neither converted nor marked unconvertible", ctx.ModuleName()))
+	}
 }
 
 func registerApiBp2buildConversionMutator(ctx RegisterMutatorsContext) {