diff --git a/base/bucket.go b/base/bucket.go
index abdaaec8ea..6ed2675823 100644
--- a/base/bucket.go
+++ b/base/bucket.go
@@ -18,6 +18,7 @@ import (
 	"io"
 	"net/http"
 	"net/url"
+	"regexp"
 	"strconv"
 	"strings"
 	"time"
@@ -541,9 +542,9 @@ func GetSourceID(ctx context.Context, bucket Bucket) (string, error) {
 	}
 	gocbBucket, err := AsGocbV2Bucket(bucket)
 	if err != nil {
-		// for rosmar bucket and testing, use the bucket name as the source ID to make it easier to identify the source
+		// for rosmar bucket and testing, base the source ID on the bucket name to make it easier to identify the source
 		if underGoTest() {
-			return bucket.GetName(), nil
+			return testSourceID(bucket.GetName(), bucketUUID)
 		}
 		serverUUID := ""
 		return CreateEncodedSourceID(bucketUUID, serverUUID)
@@ -557,6 +558,30 @@ func GetSourceID(ctx context.Context, bucket Bucket) (string, error) {
 	return CreateEncodedSourceID(bucketUUID, serverUUID)
 }
 
+// encodedSourceIDLength is the length of a source ID in its encoded form: 16 bytes of unpadded
+// base64.  Couchbase Lite rejects a version whose source ID is any other length, or any string that
+// is not the canonical encoding of 16 bytes.
+const encodedSourceIDLength = 22
+
+// testSourceIDName matches a bucket name testSourceID can embed: letters and digits only, so it never
+// contains the '+' padding, and short enough to leave room for the final 'A'.
+var testSourceIDName = regexp.MustCompile(`^[A-Za-z0-9]{0,21}$`)
+
+// testSourceID returns a source ID for a test bucket that still reads as the bucket's name, so that
+// a real Couchbase Lite in a test accepts the versions Sync Gateway writes.  The name is padded with
+// '+', which is visually quiet and never part of a name, so distinct names always give distinct IDs:
+// "rosmar1" becomes "rosmar1++++++++++++++A".  A name that cannot be embedded that way gets the same
+// encoded ID production would.
+func testSourceID(bucketName, bucketUUID string) (string, error) {
+	if !testSourceIDName.MatchString(bucketName) {
+		return CreateEncodedSourceID(bucketUUID, "")
+	}
+	// The final character carries only the last 2 of the 128 bits, and its other 4 bits must be zero,
+	// so it cannot be another '+'.  'A' is the zero digit: it is valid there and sets no bits of its own,
+	// so it reads as an end marker rather than as part of the ID.
+	return bucketName + strings.Repeat("+", encodedSourceIDLength-1-len(bucketName)) + "A", nil
+}
+
 // CreateEncodedSourceID will hash the bucket UUID and cluster UUID using md5 hash function then will base64 encode it
 // This function is in sync with xdcr implementation of UUIDstoDocumentSource https://github.com/couchbase/goxdcr/blob/dfba7a5b4251d93db46e2b0b4b55ea014218931b/hlv/hlv.go#L51
 func CreateEncodedSourceID(bucketUUID, clusterUUID string) (string, error) {
diff --git a/base/bucket_test.go b/base/bucket_test.go
index 5c6d97a3d1..d4cff1598f 100644
--- a/base/bucket_test.go
+++ b/base/bucket_test.go
@@ -16,6 +16,7 @@ import (
 	"crypto/rand"
 	"crypto/x509"
 	"crypto/x509/pkix"
+	"encoding/base64"
 	"encoding/pem"
 	"math/big"
 	"os"
@@ -453,6 +454,42 @@ func TestTLSConfig(t *testing.T) {
 	assert.Empty(t, conf)
 }
 
+func TestTestSourceID(t *testing.T) {
+	const bucketUUID = "a1b2c3d4e5f60718293a4b5c6d7e8f90"
+	encoded, err := CreateEncodedSourceID(bucketUUID, "")
+	require.NoError(t, err)
+
+	tests := []struct {
+		bucketName string
+		expected   string
+	}{
+		{bucketName: "rosmar1", expected: "rosmar1++++++++++++++A"},
+		{bucketName: "rosmar12", expected: "rosmar12+++++++++++++A"},
+		{bucketName: "rosmar1A", expected: "rosmar1A+++++++++++++A"},
+		{bucketName: "abcdefghijklmnopqrstu", expected: "abcdefghijklmnopqrstuA"},
+		{bucketName: "", expected: "+++++++++++++++++++++A"},
+		// No room left for the final 'A'.
+		{bucketName: "abcdefghijklmnopqrstuv", expected: encoded},
+		{bucketName: "sg_int_0", expected: encoded},
+		{bucketName: "rosmar+1", expected: encoded},
+	}
+	for _, test := range tests {
+		t.Run(test.bucketName, func(t *testing.T) {
+			sourceID, err := testSourceID(test.bucketName, bucketUUID)
+			require.NoError(t, err)
+			assert.Equal(t, test.expected, sourceID)
+
+			// Couchbase Lite only accepts the canonical, unpadded encoding of 16 bytes, and reserves
+			// the all-zero ID for itself.
+			require.Len(t, sourceID, encodedSourceIDLength)
+			decoded, err := base64.RawStdEncoding.Strict().DecodeString(sourceID)
+			require.NoError(t, err)
+			require.Len(t, decoded, 16)
+			assert.NotEqual(t, make([]byte, 16), decoded)
+		})
+	}
+}
+
 func TestBaseBucket(t *testing.T) {
 
 	tests := []struct {
diff --git a/db/blip_handler.go b/db/blip_handler.go
index 3e13448b48..e39240777b 100644
--- a/db/blip_handler.go
+++ b/db/blip_handler.go
@@ -1755,5 +1755,5 @@ func GetHLVFromRevMessage(msg *blip.Message) (*HybridLogicalVector, []string, er
 			versionVectorStr += ";" + historyStr
 		}
 	}
-	return extractHLVFromBlipString(versionVectorStr)
+	return ExtractHLVFromBlipString(versionVectorStr)
 }
diff --git a/db/blip_sync_context.go b/db/blip_sync_context.go
index b6cace07d6..d1a5149a39 100644
--- a/db/blip_sync_context.go
+++ b/db/blip_sync_context.go
@@ -970,7 +970,7 @@ func (bsc *BlipSyncContext) getKnownRevs(ctx context.Context, docID string, know
 		if revID, ok := knownRevsArray[0].(string); ok {
 			if bsc.useHLV() && !base.IsRevTreeID(revID) {
 				// extract cv from the known revs array
-				msgHLV, _, deltaSrcErr := extractHLVFromBlipString(revID)
+				msgHLV, _, deltaSrcErr := ExtractHLVFromBlipString(revID)
 				if deltaSrcErr != nil {
 					base.DebugfCtx(ctx, base.KeySync, "Invalid known rev format for hlv on doc: %s falling back to full body replication. Err: %v KnownRev: %s", base.UD(docID), deltaSrcErr, revID)
 					deltaSrcRev = "" // will force falling back to full body replication below
@@ -987,7 +987,7 @@ func (bsc *BlipSyncContext) getKnownRevs(ctx context.Context, docID string, know
 	for _, rev := range knownRevsArray {
 		if revID, ok := rev.(string); ok {
 			// extract cv from the known revs array
-			msgHLV, _, err := extractHLVFromBlipString(revID)
+			msgHLV, _, err := ExtractHLVFromBlipString(revID)
 			if err != nil {
 				// assume we have received legacy rev if the following conditions are met:
 				//  - we cannot parse cv from known revs
diff --git a/db/crud.go b/db/crud.go
index 996580c383..2353d809af 100644
--- a/db/crud.go
+++ b/db/crud.go
@@ -4314,7 +4314,7 @@ func (db *DatabaseCollectionWithUser) CheckProposedVersion(ctx context.Context,
 		// Temporary (CBG-4466): check the full HLV that's being sent by CBL with proposeChanges messages.
 		// If the current server cv is dominated by the incoming HLV (i.e. the incoming HLV has an entry for the same source
 		// with a version that's greater than or equal to the server's cv), then we can accept the proposed version.
-		proposedHLV, _, err := extractHLVFromBlipString(proposedHLVString)
+		proposedHLV, _, err := ExtractHLVFromBlipString(proposedHLVString)
 		if err != nil {
 			base.WarnfCtx(ctx, "CheckProposedVersion for doc %s unable to extract proposedHLV from rev message, will be treated as conflict: %v", base.UD(docid), err)
 		} else if proposedHLV.DominatesSource(localDocCV) {
diff --git a/db/hybrid_logical_vector.go b/db/hybrid_logical_vector.go
index 1d6554e452..66274e4e86 100644
--- a/db/hybrid_logical_vector.go
+++ b/db/hybrid_logical_vector.go
@@ -590,7 +590,7 @@ func (hlv *HybridLogicalVector) toHistoryForHLV(sortFunc func(HLVVersions) iter.
 	return s.String()
 }
 
-// extractHLVFromBlipMessage extracts the full HLV a string in the format seen over Blip
+// ExtractHLVFromBlipString extracts the full HLV from a string in the format seen over Blip
 // blip string may be the following formats
 //  1. cv only:    		cv
 //  2. cv and pv:  		cv;pv
@@ -607,7 +607,7 @@ func (hlv *HybridLogicalVector) toHistoryForHLV(sortFunc func(HLVVersions) iter.
 //
 // Function will return list of revIDs if legacy rev ID was found in the HLV history section (PV)
 // TODO: CBG-3662 - Optimise once we've settled on and tested the format with CBL
-func extractHLVFromBlipString(versionVectorStr string) (*HybridLogicalVector, []string, error) {
+func ExtractHLVFromBlipString(versionVectorStr string) (*HybridLogicalVector, []string, error) {
 	hlv := &HybridLogicalVector{}
 
 	vectorFields := strings.Split(versionVectorStr, ";")
diff --git a/db/hybrid_logical_vector_test.go b/db/hybrid_logical_vector_test.go
index c2b045336c..bdf7be44ea 100644
--- a/db/hybrid_logical_vector_test.go
+++ b/db/hybrid_logical_vector_test.go
@@ -259,7 +259,7 @@ func createHLVForTest(tb *testing.T, input string) *HybridLogicalVector {
 	if input == "" {
 		return NewHybridLogicalVector()
 	}
-	hlv, _, err := extractHLVFromBlipString(input)
+	hlv, _, err := ExtractHLVFromBlipString(input)
 	require.NoError(tb, err)
 	return hlv
 }
@@ -838,7 +838,7 @@ func TestInvalidHLVInBlipMessageForm(t *testing.T) {
 	for _, testCase := range testCases {
 		t.Run(testCase.name, func(t *testing.T) {
 			require.NotEmpty(t, testCase.errMsg) // make sure err msg is specified
-			hlv, legacyRevs, err := extractHLVFromBlipString(testCase.hlv)
+			hlv, legacyRevs, err := ExtractHLVFromBlipString(testCase.hlv)
 			require.ErrorContains(t, err, testCase.errMsg, "expected err for %s", testCase.hlv)
 			require.Nil(t, hlv)
 			require.Nil(t, legacyRevs)
@@ -1100,12 +1100,12 @@ func getHLVTestCases(t testing.TB) []extractHLVFromBlipMsgBMarkCases {
 }
 
 // TestExtractHLVFromChangesMessage:
-//   - Each test case gets run through extractHLVFromBlipString and assert that the resulting HLV
+//   - Each test case gets run through ExtractHLVFromBlipString and assert that the resulting HLV
 //     is correct to what is expected
 func TestExtractHLVFromChangesMessage(t *testing.T) {
 	for _, test := range getHLVTestCases(t) {
 		t.Run(test.name, func(t *testing.T) {
-			hlv, legacyRevs, err := extractHLVFromBlipString(test.hlvString)
+			hlv, legacyRevs, err := ExtractHLVFromBlipString(test.hlvString)
 			require.NoError(t, err)
 
 			require.Equal(t, test.expectedHLV, *hlv, "HLV not parsed correctly for %s", test.hlvString)
@@ -1176,7 +1176,7 @@ func BenchmarkExtractHLVFromBlipMessage(b *testing.B) {
 	for _, bm := range getHLVTestCases(b) {
 		b.Run(bm.name, func(b *testing.B) {
 			for b.Loop() {
-				_, _, _ = extractHLVFromBlipString(bm.hlvString)
+				_, _, _ = ExtractHLVFromBlipString(bm.hlvString)
 			}
 		})
 	}
@@ -1497,7 +1497,7 @@ func TestHLVAddVersion(t *testing.T) {
 	for _, tc := range testCases {
 		t.Run(tc.name, func(t *testing.T) {
 
-			hlv, _, err := extractHLVFromBlipString(tc.initialHLV)
+			hlv, _, err := ExtractHLVFromBlipString(tc.initialHLV)
 			require.NoError(t, err, "unable to parse initialHLV")
 			newVersion, err := ParseVersion(tc.newVersion)
 			require.NoError(t, err)
@@ -1505,7 +1505,7 @@ func TestHLVAddVersion(t *testing.T) {
 			err = hlv.AddVersion(newVersion)
 			require.NoError(t, err)
 
-			expectedHLV, _, err := extractHLVFromBlipString(tc.expectedHLV)
+			expectedHLV, _, err := ExtractHLVFromBlipString(tc.expectedHLV)
 			require.NoError(t, err)
 			require.True(t, hlv.Equal(expectedHLV), "expected %#v does not match actual %#v", expectedHLV, hlv)
 
@@ -1625,9 +1625,9 @@ func TestHLVIsInConflict(t *testing.T) {
 	}
 	for _, tc := range testCases {
 		t.Run(tc.name, func(t *testing.T) {
-			localHLV, _, err := extractHLVFromBlipString(tc.localHLV)
+			localHLV, _, err := ExtractHLVFromBlipString(tc.localHLV)
 			require.NoError(t, err)
-			incomingHLV, _, err := extractHLVFromBlipString(tc.incomingHLV)
+			incomingHLV, _, err := ExtractHLVFromBlipString(tc.incomingHLV)
 			require.NoError(t, err)
 
 			require.Equal(t, tc.conflict, IsInConflict(t.Context(), localHLV, incomingHLV))