diff --git a/android/Android.bp b/android/Android.bp
index 94d2c04f6..2ccccf05c 100644
--- a/android/Android.bp
+++ b/android/Android.bp
@@ -19,6 +19,7 @@ bootstrap_go_package {
         "soong-shared",
         "soong-starlark",
         "soong-starlark-format",
+        "soong-ui-bp2build_metrics_proto",
         "soong-ui-metrics_proto",
         "soong-android-allowlists",
 
diff --git a/android/bazel.go b/android/bazel.go
index e631ed474..1bfbdec00 100644
--- a/android/bazel.go
+++ b/android/bazel.go
@@ -17,8 +17,10 @@ package android
 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 @@ type BazelConversionStatus struct {
 
 	// 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 @@ type Bazelable interface {
 	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 @@ func (b *BazelModuleBase) GetBazelLabel(ctx BazelConversionPathContext, module b
 	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 bp2buildDefaultTrueRecursively(packagePath string, config allowlists.Bp2Bui
 }
 
 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) {
diff --git a/android/defaults.go b/android/defaults.go
index 31d601498..e0e6e5cf1 100644
--- a/android/defaults.go
+++ b/android/defaults.go
@@ -17,6 +17,7 @@ package android
 import (
 	"reflect"
 
+	"android/soong/ui/metrics/bp2build_metrics_proto"
 	"github.com/google/blueprint"
 	"github.com/google/blueprint/proptools"
 )
@@ -179,7 +180,10 @@ func (d *DefaultsModuleBase) GenerateAndroidBuildActions(ctx ModuleContext) {}
 
 // ConvertWithBp2build to fulfill Bazelable interface; however, at this time defaults module are
 // *NOT* converted with bp2build
-func (defaultable *DefaultsModuleBase) ConvertWithBp2build(ctx TopDownMutatorContext) {}
+func (defaultable *DefaultsModuleBase) ConvertWithBp2build(ctx TopDownMutatorContext) {
+	// Defaults types are never convertible.
+	ctx.MarkBp2buildUnconvertible(bp2build_metrics_proto.UnconvertedReasonType_TYPE_UNSUPPORTED, "")
+}
 
 func InitDefaultsModule(module DefaultsModule) {
 	commonProperties := &commonProperties{}
diff --git a/android/filegroup.go b/android/filegroup.go
index 3522f8060..3b866550d 100644
--- a/android/filegroup.go
+++ b/android/filegroup.go
@@ -21,6 +21,7 @@ import (
 
 	"android/soong/bazel"
 	"android/soong/bazel/cquery"
+	"android/soong/ui/metrics/bp2build_metrics_proto"
 
 	"github.com/google/blueprint"
 )
@@ -111,6 +112,7 @@ func (fg *fileGroup) ConvertWithBp2build(ctx TopDownMutatorContext) {
 			if len(srcs.Value.Includes) > 1 {
 				ctx.ModuleErrorf("filegroup '%s' cannot contain a file with the same name", fg.Name())
 			}
+			ctx.MarkBp2buildUnconvertible(bp2build_metrics_proto.UnconvertedReasonType_SRC_NAME_COLLISION, "")
 			return
 		}
 	}
diff --git a/android/module.go b/android/module.go
index ba327108f..73783d84d 100644
--- a/android/module.go
+++ b/android/module.go
@@ -30,6 +30,7 @@ import (
 	"text/scanner"
 
 	"android/soong/bazel"
+	"android/soong/ui/metrics/bp2build_metrics_proto"
 
 	"github.com/google/blueprint"
 	"github.com/google/blueprint/proptools"
@@ -565,6 +566,8 @@ type Module interface {
 
 	// IsConvertedByBp2build returns whether this module was converted via bp2build
 	IsConvertedByBp2build() bool
+	GetUnconvertedReason() *UnconvertedReason
+
 	// Bp2buildTargets returns the target(s) generated for Bazel via bp2build for this module
 	Bp2buildTargets() []bp2buildInfo
 	GetUnconvertedBp2buildDeps() []string
@@ -1591,14 +1594,31 @@ func (b bp2buildInfo) BazelAttributes() []interface{} {
 }
 
 func (m *ModuleBase) addBp2buildInfo(info bp2buildInfo) {
+	if m.commonProperties.BazelConversionStatus.UnconvertedReason != nil {
+		panic(fmt.Errorf("bp2build: module '%s' marked unconvertible and also is converted", m.Name()))
+	}
 	m.commonProperties.BazelConversionStatus.Bp2buildInfo = append(m.commonProperties.BazelConversionStatus.Bp2buildInfo, info)
 }
 
+func (m *ModuleBase) setBp2buildUnconvertible(reasonType bp2build_metrics_proto.UnconvertedReasonType, detail string) {
+	if len(m.commonProperties.BazelConversionStatus.Bp2buildInfo) > 0 {
+		panic(fmt.Errorf("bp2build: module '%s' marked unconvertible and also is converted", m.Name()))
+	}
+	m.commonProperties.BazelConversionStatus.UnconvertedReason = &UnconvertedReason{
+		ReasonType: int(reasonType),
+		Detail:     detail,
+	}
+}
+
 // IsConvertedByBp2build returns whether this module was converted via bp2build.
 func (m *ModuleBase) IsConvertedByBp2build() bool {
 	return len(m.commonProperties.BazelConversionStatus.Bp2buildInfo) > 0
 }
 
+func (m *ModuleBase) GetUnconvertedReason() *UnconvertedReason {
+	return m.commonProperties.BazelConversionStatus.UnconvertedReason
+}
+
 // Bp2buildTargets returns the Bazel targets bp2build generated for this module.
 func (m *ModuleBase) Bp2buildTargets() []bp2buildInfo {
 	return m.commonProperties.BazelConversionStatus.Bp2buildInfo
diff --git a/android/mutator.go b/android/mutator.go
index 41853154f..2ec051e59 100644
--- a/android/mutator.go
+++ b/android/mutator.go
@@ -16,6 +16,7 @@ package android
 
 import (
 	"android/soong/bazel"
+	"android/soong/ui/metrics/bp2build_metrics_proto"
 
 	"github.com/google/blueprint"
 )
@@ -271,6 +272,10 @@ type TopDownMutatorContext interface {
 	// any platform for which this bool attribute is false.
 	CreateBazelTargetModuleWithRestrictions(bazel.BazelTargetModuleProperties, CommonAttributes, interface{}, bazel.BoolAttribute)
 
+	// MarkBp2buildUnconvertible registers the current module as "unconvertible to bp2build" for the
+	// given reason.
+	MarkBp2buildUnconvertible(reasonType bp2build_metrics_proto.UnconvertedReasonType, detail string)
+
 	// CreateBazelTargetAliasInDir creates an alias definition in `dir` directory.
 	// This function can be used to create alias definitions in a directory that is different
 	// from the directory of the visited Soong module.
@@ -718,6 +723,12 @@ func (t *topDownMutatorContext) CreateBazelTargetModuleWithRestrictions(
 	t.createBazelTargetModule(bazelProps, commonAttrs, attrs, enabledProperty)
 }
 
+func (t *topDownMutatorContext) MarkBp2buildUnconvertible(
+	reasonType bp2build_metrics_proto.UnconvertedReasonType, detail string) {
+	mod := t.Module()
+	mod.base().setBp2buildUnconvertible(reasonType, detail)
+}
+
 var (
 	bazelAliasModuleProperties = bazel.BazelTargetModuleProperties{
 		Rule_class: "alias",
diff --git a/bp2build/build_conversion.go b/bp2build/build_conversion.go
index a86048418..46a5bd8cb 100644
--- a/bp2build/build_conversion.go
+++ b/bp2build/build_conversion.go
@@ -28,6 +28,7 @@ import (
 	"android/soong/android"
 	"android/soong/bazel"
 	"android/soong/starlark_fmt"
+	"android/soong/ui/metrics/bp2build_metrics_proto"
 	"github.com/google/blueprint"
 	"github.com/google/blueprint/proptools"
 )
@@ -283,12 +284,15 @@ func GenerateBazelTargets(ctx *CodegenContext, generateFilegroups bool) (convers
 				// target in a BUILD file, we don't autoconvert them.
 
 				// Log the module.
-				metrics.AddConvertedModule(m, moduleType, dir, Handcrafted)
+				metrics.AddUnconvertedModule(m, moduleType, dir,
+					android.UnconvertedReason{
+						ReasonType: int(bp2build_metrics_proto.UnconvertedReasonType_DEFINED_IN_BUILD_FILE),
+					})
 			} else if aModule, ok := m.(android.Module); ok && aModule.IsConvertedByBp2build() {
 				// Handle modules converted to generated targets.
 
 				// Log the module.
-				metrics.AddConvertedModule(aModule, moduleType, dir, Generated)
+				metrics.AddConvertedModule(aModule, moduleType, dir)
 
 				// Handle modules with unconverted deps. By default, emit a warning.
 				if unconvertedDeps := aModule.GetUnconvertedBp2buildDeps(); len(unconvertedDeps) > 0 {
@@ -324,8 +328,18 @@ func GenerateBazelTargets(ctx *CodegenContext, generateFilegroups bool) (convers
 			} else if _, ok := ctx.Config().BazelModulesForceEnabledByFlag()[m.Name()]; ok && m.Name() != "" {
 				err := fmt.Errorf("Force Enabled Module %s not converted", m.Name())
 				errs = append(errs, err)
+			} else if aModule, ok := m.(android.Module); ok {
+				reason := aModule.GetUnconvertedReason()
+				if reason == nil {
+					panic(fmt.Errorf("module '%s' was neither converted nor marked unconvertible with bp2build", aModule.Name()))
+				} else {
+					metrics.AddUnconvertedModule(m, moduleType, dir, *reason)
+				}
+				return
 			} else {
-				metrics.AddUnconvertedModule(moduleType)
+				metrics.AddUnconvertedModule(m, moduleType, dir, android.UnconvertedReason{
+					ReasonType: int(bp2build_metrics_proto.UnconvertedReasonType_TYPE_UNSUPPORTED),
+				})
 				return
 			}
 		case QueryView:
diff --git a/bp2build/metrics.go b/bp2build/metrics.go
index a02065081..00f21c8a7 100644
--- a/bp2build/metrics.go
+++ b/bp2build/metrics.go
@@ -38,6 +38,7 @@ func CreateCodegenMetrics() CodegenMetrics {
 			RuleClassCount:           make(map[string]uint64),
 			ConvertedModuleTypeCount: make(map[string]uint64),
 			TotalModuleTypeCount:     make(map[string]uint64),
+			UnconvertedModules:       make(map[string]*bp2build_metrics_proto.UnconvertedReason),
 		},
 		convertedModulePathMap: make(map[string]string),
 	}
@@ -149,11 +150,6 @@ func (metrics *CodegenMetrics) AddEvent(event *bp2build_metrics_proto.Event) {
 	metrics.serialized.Events = append(metrics.serialized.Events, event)
 }
 
-func (metrics *CodegenMetrics) AddUnconvertedModule(moduleType string) {
-	metrics.serialized.UnconvertedModuleCount += 1
-	metrics.serialized.TotalModuleTypeCount[moduleType] += 1
-}
-
 func (metrics *CodegenMetrics) SetSymlinkCount(n uint64) {
 	if m := metrics.serialized.WorkspaceSymlinkCount; m != 0 {
 		fmt.Fprintf(os.Stderr, "unexpected non-zero workspaceSymlinkCount of %d", m)
@@ -187,7 +183,7 @@ const (
 	Handcrafted
 )
 
-func (metrics *CodegenMetrics) AddConvertedModule(m blueprint.Module, moduleType string, dir string, conversionType ConversionType) {
+func (metrics *CodegenMetrics) AddConvertedModule(m blueprint.Module, moduleType string, dir string) {
 	//a package module has empty name
 	if moduleType == "package" {
 		return
@@ -198,10 +194,25 @@ func (metrics *CodegenMetrics) AddConvertedModule(m blueprint.Module, moduleType
 	metrics.convertedModulePathMap[moduleName] = "//" + dir
 	metrics.serialized.ConvertedModuleTypeCount[moduleType] += 1
 	metrics.serialized.TotalModuleTypeCount[moduleType] += 1
+	metrics.serialized.GeneratedModuleCount += 1
+}
 
-	if conversionType == Handcrafted {
+func (metrics *CodegenMetrics) AddUnconvertedModule(m blueprint.Module, moduleType string, dir string,
+	reason android.UnconvertedReason) {
+	//a package module has empty name
+	if moduleType == "package" {
+		return
+	}
+	// Undo prebuilt_ module name prefix modifications
+	moduleName := android.RemoveOptionalPrebuiltPrefix(m.Name())
+	metrics.serialized.UnconvertedModules[moduleName] = &bp2build_metrics_proto.UnconvertedReason{
+		Type:   bp2build_metrics_proto.UnconvertedReasonType(reason.ReasonType),
+		Detail: reason.Detail,
+	}
+	metrics.serialized.UnconvertedModuleCount += 1
+	metrics.serialized.TotalModuleTypeCount[moduleType] += 1
+
+	if reason.ReasonType == int(bp2build_metrics_proto.UnconvertedReasonType_DEFINED_IN_BUILD_FILE) {
 		metrics.serialized.HandCraftedModuleCount += 1
-	} else if conversionType == Generated {
-		metrics.serialized.GeneratedModuleCount += 1
 	}
 }
diff --git a/bp2build/testing.go b/bp2build/testing.go
index f3263a1b2..140b214cf 100644
--- a/bp2build/testing.go
+++ b/bp2build/testing.go
@@ -78,7 +78,7 @@ type Bp2buildTestCase struct {
 	ModuleTypeUnderTestFactory android.ModuleFactory
 	// Text to add to the toplevel, root Android.bp file. If Dir is not set, all
 	// ExpectedBazelTargets are assumed to be generated by this file.
-	Blueprint            string
+	Blueprint string
 	// ExpectedBazelTargets compares the BazelTargets generated in `Dir` (if not empty).
 	// Otherwise, it checks the BazelTargets generated by `Blueprint` in the root directory.
 	ExpectedBazelTargets []string
@@ -146,6 +146,10 @@ func runBp2BuildTestCaseWithSetup(t *testing.T, extraPreparer android.FixturePre
 				})
 			}
 			ctx.RegisterBp2BuildConfig(bp2buildConfig)
+			// This setting is added to bp2build invocations. It prevents bp2build
+			// from cloning modules to their original state after mutators run. This
+			// would lose some data intentionally set by these mutators.
+			ctx.SkipCloneModulesAfterMutators = true
 		}),
 		android.FixtureModifyEnv(func(env map[string]string) {
 			if tc.UnconvertedDepsMode == errorModulesUnconvertedDeps {
@@ -210,7 +214,11 @@ func (b *bazelTestRunner) PostParseProcessor(result android.CustomTestResult) {
 		return
 	}
 
-	codegenCtx := NewCodegenContext(config, ctx.Context, Bp2Build, "")
+	codegenMode := Bp2Build
+	if ctx.Config().BuildMode == android.ApiBp2build {
+		codegenMode = ApiBp2build
+	}
+	codegenCtx := NewCodegenContext(config, ctx.Context, codegenMode, "")
 	res, errs := GenerateBazelTargets(codegenCtx, false)
 	if bazelResult.CollateErrs(errs) {
 		return
diff --git a/cc/cc.go b/cc/cc.go
index 348400026..d67f3ad14 100644
--- a/cc/cc.go
+++ b/cc/cc.go
@@ -24,6 +24,7 @@ import (
 	"strconv"
 	"strings"
 
+	"android/soong/ui/metrics/bp2build_metrics_proto"
 	"github.com/google/blueprint"
 	"github.com/google/blueprint/proptools"
 
@@ -4103,6 +4104,8 @@ func (c *Module) ConvertWithBp2build(ctx android.TopDownMutatorContext) {
 		} else {
 			sharedOrStaticLibraryBp2Build(ctx, c, false)
 		}
+	default:
+		ctx.MarkBp2buildUnconvertible(bp2build_metrics_proto.UnconvertedReasonType_TYPE_UNSUPPORTED, "")
 	}
 }
 
diff --git a/java/base.go b/java/base.go
index 677928320..2b81f5e36 100644
--- a/java/base.go
+++ b/java/base.go
@@ -20,6 +20,7 @@ import (
 	"strconv"
 	"strings"
 
+	"android/soong/ui/metrics/bp2build_metrics_proto"
 	"github.com/google/blueprint/pathtools"
 	"github.com/google/blueprint/proptools"
 
@@ -2205,5 +2206,7 @@ func (j *Module) ConvertWithBp2build(ctx android.TopDownMutatorContext) {
 		if testHost, ok := ctx.Module().(*TestHost); ok {
 			javaTestHostBp2Build(ctx, testHost)
 		}
+	default:
+		ctx.MarkBp2buildUnconvertible(bp2build_metrics_proto.UnconvertedReasonType_TYPE_UNSUPPORTED, "")
 	}
 }
diff --git a/java/sdk_library.go b/java/sdk_library.go
index 89da19a19..6d8e2ebfb 100644
--- a/java/sdk_library.go
+++ b/java/sdk_library.go
@@ -24,6 +24,7 @@ import (
 	"strings"
 	"sync"
 
+	"android/soong/ui/metrics/bp2build_metrics_proto"
 	"github.com/google/blueprint"
 	"github.com/google/blueprint/proptools"
 
@@ -2178,6 +2179,7 @@ type bazelSdkLibraryAttributes struct {
 // java_sdk_library bp2build converter
 func (module *SdkLibrary) ConvertWithBp2build(ctx android.TopDownMutatorContext) {
 	if ctx.ModuleType() != "java_sdk_library" {
+		ctx.MarkBp2buildUnconvertible(bp2build_metrics_proto.UnconvertedReasonType_TYPE_UNSUPPORTED, "")
 		return
 	}
 
diff --git a/linkerconfig/linkerconfig.go b/linkerconfig/linkerconfig.go
index 412a23b26..165697dfa 100644
--- a/linkerconfig/linkerconfig.go
+++ b/linkerconfig/linkerconfig.go
@@ -19,6 +19,7 @@ import (
 	"sort"
 	"strings"
 
+	"android/soong/ui/metrics/bp2build_metrics_proto"
 	"github.com/google/blueprint/proptools"
 
 	"android/soong/android"
@@ -109,6 +110,7 @@ type linkerConfigAttributes struct {
 func (l *linkerConfig) ConvertWithBp2build(ctx android.TopDownMutatorContext) {
 	if l.properties.Src == nil {
 		ctx.PropertyErrorf("src", "empty src is not supported")
+		ctx.MarkBp2buildUnconvertible(bp2build_metrics_proto.UnconvertedReasonType_UNSUPPORTED, "")
 		return
 	}
 	src := android.BazelLabelForModuleSrcSingle(ctx, *l.properties.Src)
diff --git a/ui/metrics/bp2build_metrics_proto/bp2build_metrics.pb.go b/ui/metrics/bp2build_metrics_proto/bp2build_metrics.pb.go
index 4821043a3..b34c2b6b7 100644
--- a/ui/metrics/bp2build_metrics_proto/bp2build_metrics.pb.go
+++ b/ui/metrics/bp2build_metrics_proto/bp2build_metrics.pb.go
@@ -14,8 +14,8 @@
 
 // Code generated by protoc-gen-go. DO NOT EDIT.
 // versions:
-// 	protoc-gen-go v1.28.0
-// 	protoc        v3.21.7
+// 	protoc-gen-go v1.30.0
+// 	protoc        v3.21.12
 // source: bp2build_metrics.proto
 
 package bp2build_metrics_proto
@@ -34,6 +34,78 @@ const (
 	_ = protoimpl.EnforceVersion(protoimpl.MaxVersion - 20)
 )
 
+type UnconvertedReasonType int32
+
+const (
+	// Bp2build does not know how to convert this specific module for some reason
+	// not covered by other reason types. The reason detail should explain the
+	// specific issue.
+	UnconvertedReasonType_UNSUPPORTED UnconvertedReasonType = 0
+	// The module was already defined in a BUILD file available in the source tree.
+	UnconvertedReasonType_DEFINED_IN_BUILD_FILE UnconvertedReasonType = 1
+	// The module was explicitly denylisted by name.
+	UnconvertedReasonType_DENYLISTED UnconvertedReasonType = 2
+	// The module's type has no bp2build implementation.
+	UnconvertedReasonType_TYPE_UNSUPPORTED UnconvertedReasonType = 3
+	// The module has a property not yet supported. The detail field should
+	// name the unsupported property name.
+	UnconvertedReasonType_PROPERTY_UNSUPPORTED UnconvertedReasonType = 4
+	// The module has an unconverted dependency. The detail should consist of
+	// the name of the unconverted module.
+	UnconvertedReasonType_UNCONVERTED_DEP UnconvertedReasonType = 5
+	// The module has a source file with the same name as the module itself.
+	UnconvertedReasonType_SRC_NAME_COLLISION UnconvertedReasonType = 6
+)
+
+// Enum value maps for UnconvertedReasonType.
+var (
+	UnconvertedReasonType_name = map[int32]string{
+		0: "UNSUPPORTED",
+		1: "DEFINED_IN_BUILD_FILE",
+		2: "DENYLISTED",
+		3: "TYPE_UNSUPPORTED",
+		4: "PROPERTY_UNSUPPORTED",
+		5: "UNCONVERTED_DEP",
+		6: "SRC_NAME_COLLISION",
+	}
+	UnconvertedReasonType_value = map[string]int32{
+		"UNSUPPORTED":           0,
+		"DEFINED_IN_BUILD_FILE": 1,
+		"DENYLISTED":            2,
+		"TYPE_UNSUPPORTED":      3,
+		"PROPERTY_UNSUPPORTED":  4,
+		"UNCONVERTED_DEP":       5,
+		"SRC_NAME_COLLISION":    6,
+	}
+)
+
+func (x UnconvertedReasonType) Enum() *UnconvertedReasonType {
+	p := new(UnconvertedReasonType)
+	*p = x
+	return p
+}
+
+func (x UnconvertedReasonType) String() string {
+	return protoimpl.X.EnumStringOf(x.Descriptor(), protoreflect.EnumNumber(x))
+}
+
+func (UnconvertedReasonType) Descriptor() protoreflect.EnumDescriptor {
+	return file_bp2build_metrics_proto_enumTypes[0].Descriptor()
+}
+
+func (UnconvertedReasonType) Type() protoreflect.EnumType {
+	return &file_bp2build_metrics_proto_enumTypes[0]
+}
+
+func (x UnconvertedReasonType) Number() protoreflect.EnumNumber {
+	return protoreflect.EnumNumber(x)
+}
+
+// Deprecated: Use UnconvertedReasonType.Descriptor instead.
+func (UnconvertedReasonType) EnumDescriptor() ([]byte, []int) {
+	return file_bp2build_metrics_proto_rawDescGZIP(), []int{0}
+}
+
 type Bp2BuildMetrics struct {
 	state         protoimpl.MessageState
 	sizeCache     protoimpl.SizeCache
@@ -53,6 +125,8 @@ type Bp2BuildMetrics struct {
 	RuleClassCount map[string]uint64 `protobuf:"bytes,4,rep,name=ruleClassCount,proto3" json:"ruleClassCount,omitempty" protobuf_key:"bytes,1,opt,name=key,proto3" protobuf_val:"varint,2,opt,name=value,proto3"`
 	// List of converted modules
 	ConvertedModules []string `protobuf:"bytes,5,rep,name=convertedModules,proto3" json:"convertedModules,omitempty"`
+	// Unconverted modules, mapped to the reason the module was not converted.
+	UnconvertedModules map[string]*UnconvertedReason `protobuf:"bytes,11,rep,name=unconvertedModules,proto3" json:"unconvertedModules,omitempty" protobuf_key:"bytes,1,opt,name=key,proto3" protobuf_val:"bytes,2,opt,name=value,proto3"`
 	// Counts of converted modules by module type.
 	ConvertedModuleTypeCount map[string]uint64 `protobuf:"bytes,6,rep,name=convertedModuleTypeCount,proto3" json:"convertedModuleTypeCount,omitempty" protobuf_key:"bytes,1,opt,name=key,proto3" protobuf_val:"varint,2,opt,name=value,proto3"`
 	// Counts of total modules by module type.
@@ -143,6 +217,13 @@ func (x *Bp2BuildMetrics) GetConvertedModules() []string {
 	return nil
 }
 
+func (x *Bp2BuildMetrics) GetUnconvertedModules() map[string]*UnconvertedReason {
+	if x != nil {
+		return x.UnconvertedModules
+	}
+	return nil
+}
+
 func (x *Bp2BuildMetrics) GetConvertedModuleTypeCount() map[string]uint64 {
 	if x != nil {
 		return x.ConvertedModuleTypeCount
@@ -233,13 +314,72 @@ func (x *Event) GetRealTime() uint64 {
 	return 0
 }
 
+type UnconvertedReason struct {
+	state         protoimpl.MessageState
+	sizeCache     protoimpl.SizeCache
+	unknownFields protoimpl.UnknownFields
+
+	// The type of reason that the module could not be converted.
+	Type UnconvertedReasonType `protobuf:"varint,1,opt,name=type,proto3,enum=soong_build_bp2build_metrics.UnconvertedReasonType" json:"type,omitempty"`
+	// Descriptive details describing why the module could not be converted.
+	// This detail should be kept very short and should be in the context of
+	// the type. (Otherwise, this would significantly bloat metrics.)
+	Detail string `protobuf:"bytes,2,opt,name=detail,proto3" json:"detail,omitempty"`
+}
+
+func (x *UnconvertedReason) Reset() {
+	*x = UnconvertedReason{}
+	if protoimpl.UnsafeEnabled {
+		mi := &file_bp2build_metrics_proto_msgTypes[2]
+		ms := protoimpl.X.MessageStateOf(protoimpl.Pointer(x))
+		ms.StoreMessageInfo(mi)
+	}
+}
+
+func (x *UnconvertedReason) String() string {
+	return protoimpl.X.MessageStringOf(x)
+}
+
+func (*UnconvertedReason) ProtoMessage() {}
+
+func (x *UnconvertedReason) ProtoReflect() protoreflect.Message {
+	mi := &file_bp2build_metrics_proto_msgTypes[2]
+	if protoimpl.UnsafeEnabled && x != nil {
+		ms := protoimpl.X.MessageStateOf(protoimpl.Pointer(x))
+		if ms.LoadMessageInfo() == nil {
+			ms.StoreMessageInfo(mi)
+		}
+		return ms
+	}
+	return mi.MessageOf(x)
+}
+
+// Deprecated: Use UnconvertedReason.ProtoReflect.Descriptor instead.
+func (*UnconvertedReason) Descriptor() ([]byte, []int) {
+	return file_bp2build_metrics_proto_rawDescGZIP(), []int{2}
+}
+
+func (x *UnconvertedReason) GetType() UnconvertedReasonType {
+	if x != nil {
+		return x.Type
+	}
+	return UnconvertedReasonType_UNSUPPORTED
+}
+
+func (x *UnconvertedReason) GetDetail() string {
+	if x != nil {
+		return x.Detail
+	}
+	return ""
+}
+
 var File_bp2build_metrics_proto protoreflect.FileDescriptor
 
 var file_bp2build_metrics_proto_rawDesc = []byte{
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@@ -266,50 +406,84 @@ var file_bp2build_metrics_proto_rawDesc = []byte{
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+	0x72, 0x69, 0x63, 0x73, 0x2e, 0x55, 0x6e, 0x63, 0x6f, 0x6e, 0x76, 0x65, 0x72, 0x74, 0x65, 0x64,
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+	0x12, 0x16, 0x0a, 0x06, 0x64, 0x65, 0x74, 0x61, 0x69, 0x6c, 0x18, 0x02, 0x20, 0x01, 0x28, 0x09,
+	0x52, 0x06, 0x64, 0x65, 0x74, 0x61, 0x69, 0x6c, 0x2a, 0xb0, 0x01, 0x0a, 0x15, 0x55, 0x6e, 0x63,
+	0x6f, 0x6e, 0x76, 0x65, 0x72, 0x74, 0x65, 0x64, 0x52, 0x65, 0x61, 0x73, 0x6f, 0x6e, 0x54, 0x79,
+	0x70, 0x65, 0x12, 0x0f, 0x0a, 0x0b, 0x55, 0x4e, 0x53, 0x55, 0x50, 0x50, 0x4f, 0x52, 0x54, 0x45,
+	0x44, 0x10, 0x00, 0x12, 0x19, 0x0a, 0x15, 0x44, 0x45, 0x46, 0x49, 0x4e, 0x45, 0x44, 0x5f, 0x49,
+	0x4e, 0x5f, 0x42, 0x55, 0x49, 0x4c, 0x44, 0x5f, 0x46, 0x49, 0x4c, 0x45, 0x10, 0x01, 0x12, 0x0e,
+	0x0a, 0x0a, 0x44, 0x45, 0x4e, 0x59, 0x4c, 0x49, 0x53, 0x54, 0x45, 0x44, 0x10, 0x02, 0x12, 0x14,
+	0x0a, 0x10, 0x54, 0x59, 0x50, 0x45, 0x5f, 0x55, 0x4e, 0x53, 0x55, 0x50, 0x50, 0x4f, 0x52, 0x54,
+	0x45, 0x44, 0x10, 0x03, 0x12, 0x18, 0x0a, 0x14, 0x50, 0x52, 0x4f, 0x50, 0x45, 0x52, 0x54, 0x59,
+	0x5f, 0x55, 0x4e, 0x53, 0x55, 0x50, 0x50, 0x4f, 0x52, 0x54, 0x45, 0x44, 0x10, 0x04, 0x12, 0x13,
+	0x0a, 0x0f, 0x55, 0x4e, 0x43, 0x4f, 0x4e, 0x56, 0x45, 0x52, 0x54, 0x45, 0x44, 0x5f, 0x44, 0x45,
+	0x50, 0x10, 0x05, 0x12, 0x16, 0x0a, 0x12, 0x53, 0x52, 0x43, 0x5f, 0x4e, 0x41, 0x4d, 0x45, 0x5f,
+	0x43, 0x4f, 0x4c, 0x4c, 0x49, 0x53, 0x49, 0x4f, 0x4e, 0x10, 0x06, 0x42, 0x31, 0x5a, 0x2f, 0x61,
+	0x6e, 0x64, 0x72, 0x6f, 0x69, 0x64, 0x2f, 0x73, 0x6f, 0x6f, 0x6e, 0x67, 0x2f, 0x75, 0x69, 0x2f,
+	0x6d, 0x65, 0x74, 0x72, 0x69, 0x63, 0x73, 0x2f, 0x62, 0x70, 0x32, 0x62, 0x75, 0x69, 0x6c, 0x64,
+	0x5f, 0x6d, 0x65, 0x74, 0x72, 0x69, 0x63, 0x73, 0x5f, 0x70, 0x72, 0x6f, 0x74, 0x6f, 0x62, 0x06,
+	0x70, 0x72, 0x6f, 0x74, 0x6f, 0x33,
 }
 
 var (
@@ -324,24 +498,31 @@ func file_bp2build_metrics_proto_rawDescGZIP() []byte {
 	return file_bp2build_metrics_proto_rawDescData
 }
 
-var file_bp2build_metrics_proto_msgTypes = make([]protoimpl.MessageInfo, 5)
+var file_bp2build_metrics_proto_enumTypes = make([]protoimpl.EnumInfo, 1)
+var file_bp2build_metrics_proto_msgTypes = make([]protoimpl.MessageInfo, 7)
 var file_bp2build_metrics_proto_goTypes = []interface{}{
-	(*Bp2BuildMetrics)(nil), // 0: soong_build_bp2build_metrics.Bp2BuildMetrics
-	(*Event)(nil),           // 1: soong_build_bp2build_metrics.Event
-	nil,                     // 2: soong_build_bp2build_metrics.Bp2BuildMetrics.RuleClassCountEntry
-	nil,                     // 3: soong_build_bp2build_metrics.Bp2BuildMetrics.ConvertedModuleTypeCountEntry
-	nil,                     // 4: soong_build_bp2build_metrics.Bp2BuildMetrics.TotalModuleTypeCountEntry
+	(UnconvertedReasonType)(0), // 0: soong_build_bp2build_metrics.UnconvertedReasonType
+	(*Bp2BuildMetrics)(nil),    // 1: soong_build_bp2build_metrics.Bp2BuildMetrics
+	(*Event)(nil),              // 2: soong_build_bp2build_metrics.Event
+	(*UnconvertedReason)(nil),  // 3: soong_build_bp2build_metrics.UnconvertedReason
+	nil,                        // 4: soong_build_bp2build_metrics.Bp2BuildMetrics.RuleClassCountEntry
+	nil,                        // 5: soong_build_bp2build_metrics.Bp2BuildMetrics.UnconvertedModulesEntry
+	nil,                        // 6: soong_build_bp2build_metrics.Bp2BuildMetrics.ConvertedModuleTypeCountEntry
+	nil,                        // 7: soong_build_bp2build_metrics.Bp2BuildMetrics.TotalModuleTypeCountEntry
 }
 var file_bp2build_metrics_proto_depIdxs = []int32{
-	2, // 0: soong_build_bp2build_metrics.Bp2BuildMetrics.ruleClassCount:type_name -> soong_build_bp2build_metrics.Bp2BuildMetrics.RuleClassCountEntry
-	3, // 1: soong_build_bp2build_metrics.Bp2BuildMetrics.convertedModuleTypeCount:type_name -> soong_build_bp2build_metrics.Bp2BuildMetrics.ConvertedModuleTypeCountEntry
-	4, // 2: soong_build_bp2build_metrics.Bp2BuildMetrics.totalModuleTypeCount:type_name -> soong_build_bp2build_metrics.Bp2BuildMetrics.TotalModuleTypeCountEntry
-	1, // 3: soong_build_bp2build_metrics.Bp2BuildMetrics.events:type_name -> soong_build_bp2build_metrics.Event
-	4, // [4:4] is the sub-list for method output_type
-	4, // [4:4] is the sub-list for method input_type
-	4, // [4:4] is the sub-list for extension type_name
-	4, // [4:4] is the sub-list for extension extendee
-	0, // [0:4] is the sub-list for field type_name
+	4, // 0: soong_build_bp2build_metrics.Bp2BuildMetrics.ruleClassCount:type_name -> soong_build_bp2build_metrics.Bp2BuildMetrics.RuleClassCountEntry
+	5, // 1: soong_build_bp2build_metrics.Bp2BuildMetrics.unconvertedModules:type_name -> soong_build_bp2build_metrics.Bp2BuildMetrics.UnconvertedModulesEntry
+	6, // 2: soong_build_bp2build_metrics.Bp2BuildMetrics.convertedModuleTypeCount:type_name -> soong_build_bp2build_metrics.Bp2BuildMetrics.ConvertedModuleTypeCountEntry
+	7, // 3: soong_build_bp2build_metrics.Bp2BuildMetrics.totalModuleTypeCount:type_name -> soong_build_bp2build_metrics.Bp2BuildMetrics.TotalModuleTypeCountEntry
+	2, // 4: soong_build_bp2build_metrics.Bp2BuildMetrics.events:type_name -> soong_build_bp2build_metrics.Event
+	0, // 5: soong_build_bp2build_metrics.UnconvertedReason.type:type_name -> soong_build_bp2build_metrics.UnconvertedReasonType
+	3, // 6: soong_build_bp2build_metrics.Bp2BuildMetrics.UnconvertedModulesEntry.value:type_name -> soong_build_bp2build_metrics.UnconvertedReason
+	7, // [7:7] is the sub-list for method output_type
+	7, // [7:7] is the sub-list for method input_type
+	7, // [7:7] is the sub-list for extension type_name
+	7, // [7:7] is the sub-list for extension extendee
+	0, // [0:7] is the sub-list for field type_name
 }
 
 func init() { file_bp2build_metrics_proto_init() }
@@ -374,19 +555,32 @@ func file_bp2build_metrics_proto_init() {
 				return nil
 			}
 		}
+		file_bp2build_metrics_proto_msgTypes[2].Exporter = func(v interface{}, i int) interface{} {
+			switch v := v.(*UnconvertedReason); i {
+			case 0:
+				return &v.state
+			case 1:
+				return &v.sizeCache
+			case 2:
+				return &v.unknownFields
+			default:
+				return nil
+			}
+		}
 	}
 	type x struct{}
 	out := protoimpl.TypeBuilder{
 		File: protoimpl.DescBuilder{
 			GoPackagePath: reflect.TypeOf(x{}).PkgPath(),
 			RawDescriptor: file_bp2build_metrics_proto_rawDesc,
-			NumEnums:      0,
-			NumMessages:   5,
+			NumEnums:      1,
+			NumMessages:   7,
 			NumExtensions: 0,
 			NumServices:   0,
 		},
 		GoTypes:           file_bp2build_metrics_proto_goTypes,
 		DependencyIndexes: file_bp2build_metrics_proto_depIdxs,
+		EnumInfos:         file_bp2build_metrics_proto_enumTypes,
 		MessageInfos:      file_bp2build_metrics_proto_msgTypes,
 	}.Build()
 	File_bp2build_metrics_proto = out.File
diff --git a/ui/metrics/bp2build_metrics_proto/bp2build_metrics.proto b/ui/metrics/bp2build_metrics_proto/bp2build_metrics.proto
index 9ff4ae26e..49cb2b46e 100644
--- a/ui/metrics/bp2build_metrics_proto/bp2build_metrics.proto
+++ b/ui/metrics/bp2build_metrics_proto/bp2build_metrics.proto
@@ -39,6 +39,9 @@ message Bp2BuildMetrics {
   // List of converted modules
   repeated string convertedModules = 5;
 
+  // Unconverted modules, mapped to the reason the module was not converted.
+  map<string, UnconvertedReason> unconvertedModules = 11;
+
   // Counts of converted modules by module type.
   map<string, uint64> convertedModuleTypeCount = 6;
 
@@ -63,3 +66,40 @@ message Event {
   // The number of nanoseconds elapsed since start_time.
   uint64 real_time = 3;
 }
+
+message UnconvertedReason {
+  // The type of reason that the module could not be converted.
+  UnconvertedReasonType type = 1;
+
+  // Descriptive details describing why the module could not be converted.
+  // This detail should be kept very short and should be in the context of
+  // the type. (Otherwise, this would significantly bloat metrics.)
+  string detail = 2;
+}
+
+enum UnconvertedReasonType {
+  // Bp2build does not know how to convert this specific module for some reason
+  // not covered by other reason types. The reason detail should explain the
+  // specific issue.
+  UNSUPPORTED = 0;
+
+  // The module was already defined in a BUILD file available in the source tree.
+  DEFINED_IN_BUILD_FILE = 1;
+
+  // The module was explicitly denylisted by name.
+  DENYLISTED = 2;
+
+  // The module's type has no bp2build implementation.
+  TYPE_UNSUPPORTED = 3;
+
+  // The module has a property not yet supported. The detail field should
+  // name the unsupported property name.
+  PROPERTY_UNSUPPORTED = 4;
+
+  // The module has an unconverted dependency. The detail should consist of
+  // the name of the unconverted module.
+  UNCONVERTED_DEP = 5;
+
+  // The module has a source file with the same name as the module itself.
+  SRC_NAME_COLLISION = 6;
+}
\ No newline at end of file