diff --git a/apps/decoder.c b/apps/decoder.c
index 3d294790b190446b05f467d59abf938fdaa0dada..45375f38ed2ace294768b98352f2360b12daa9ec 100644
--- a/apps/decoder.c
+++ b/apps/decoder.c
@@ -188,6 +188,9 @@ static int16_t app_own_random( int16_t *seed );
 #endif
 static void do_object_editing( IVAS_EDITABLE_PARAMETERS *editableParameters, ObjectEditFileReader *objectEditFileReader );
 static ivas_error restartDecoder( IVAS_DEC_HANDLE *phIvasDec, const IVAS_DEC_MODE decMode, DecArguments *arg, IVAS_RENDER_CONFIG_DATA *renderConfig, IVAS_CUSTOM_LS_DATA *hLsCustomData );
+#ifdef FIX_FMSW_DEC_EXT
+static ivas_error updateOnFormatSwitching( IVAS_DEC_HANDLE hIvasDec, IVAS_DEC_BS_FORMAT *pBsFormat, const char *outputWavFilename, MasaFileWriter **ppMasaWriter, IsmFileWriter *ismWriters[IVAS_MAX_NUM_OBJECTS], int16_t *pNumOutChannels, uint16_t *pNumObj, const bool delayCompensationEnabled );
+#endif
 
 
 /*------------------------------------------------------------------------------------------*
@@ -2286,8 +2289,12 @@ static ivas_error decodeG192(
     SplitFileReadWrite *splitRendWriter = NULL;
     int16_t isSplitRend, isSplitCoded;
 #ifdef FIX_FLOAT_1539_G192_FORMAT_SWITCH
+#ifdef FIX_FMSW_DEC_EXT
+    bool restartNeeded = false;
+#else
     bool restartNeeded;
 #endif
+#endif
 
 #ifdef VARIABLE_SPEED_DECODING
     if ( arg.tsmEnabled )
@@ -2774,6 +2781,18 @@ static ivas_error decodeG192(
             }
         }
 
+#ifdef FIX_FMSW_DEC_EXT
+        /* Output writing update in case of format switching and EXTERNAL output */
+        if ( restartNeeded && arg.outputConfig == IVAS_AUDIO_CONFIG_EXTERNAL )
+        {
+            if ( ( error = updateOnFormatSwitching( hIvasDec, &bsFormat, arg.outputWavFilename, &masaWriter, ismWriters, &nOutChannels, &numObj, arg.delayCompensationEnabled ) ) != IVAS_ERR_OK )
+            {
+                fprintf( stderr, "Error in updateOnFormatSwitching(): %s\n", IVAS_DEC_GetErrorMessage( error ) );
+                goto cleanup;
+            }
+        }
+#endif
+
         /* Write current frame */
         if ( decodedGoodFrame )
         {
@@ -3256,7 +3275,11 @@ static ivas_error decodeVoIP(
     int16_t i;
     IVAS_DEC_HANDLE hIvasDec = *phIvasDec;
 
+#ifdef FIX_FMSW_DEC_EXT
+    bool restartNeeded = false;
+#else
     bool restartNeeded;
+#endif
     IVAS_RTP ivasRtp = { 0 };
     IVAS_RTP srRtp = { 0 };
     IVAS_RTP_SR_INFO srInfo = { true, false, 0, 20, IVAS_SR_TRANSPORT_LCLD };
@@ -3762,6 +3785,18 @@ static ivas_error decodeVoIP(
             }
         }
 
+#ifdef FIX_FMSW_DEC_EXT
+        /* Output writing update in case of format switching and EXTERNAL output */
+        if ( restartNeeded && arg.outputConfig == IVAS_AUDIO_CONFIG_EXTERNAL )
+        {
+            if ( ( error = updateOnFormatSwitching( hIvasDec, &bsFormat, arg.outputWavFilename, &masaWriter, ismWriters, &nOutChannels, &numObj, arg.delayCompensationEnabled ) ) != IVAS_ERR_OK )
+            {
+                fprintf( stderr, "Error in updateOnFormatSwitching(): %s\n", IVAS_DEC_GetErrorMessage( error ) );
+                goto cleanup;
+            }
+        }
+#endif
+
         /* Write current frame */
         if ( !srRtp.hPack && decodedGoodFrame )
         {
@@ -4365,6 +4400,81 @@ static ivas_error load_hrtf_from_file(
 }
 
 
+#ifdef FIX_FMSW_DEC_EXT
+/*---------------------------------------------------------------------*
+ * updateOnFormatSwitching()
+ *
+ * In case of IVAS format switching and EXTERNAL output,
+ * update decoder wrt. output writing
+ *---------------------------------------------------------------------*/
+
+static ivas_error updateOnFormatSwitching(
+    IVAS_DEC_HANDLE hIvasDec,                        /* i/o: IVAS decoder handle                             */
+    IVAS_DEC_BS_FORMAT *pBsFormat,                   /* o  : format of currently decoded bitstream           */
+    const char *outputWavFilename,                   /* i  : name of the output audio file                   */
+    MasaFileWriter **ppMasaWriter,                   /* o  : MasaFileWriter handle                           */
+    IsmFileWriter *ismWriters[IVAS_MAX_NUM_OBJECTS], /* o  : 1-4 ismWriters handles                          */
+    int16_t *pNumOutChannels,                        /* o  : number of output channels                       */
+    uint16_t *pNumObj,                               /* o  : number of ISM objects                           */
+    const bool delayCompensationEnabled              /* i  : is delay compensation enabled                   */
+)
+{
+    ivas_error error;
+
+    /* in case of EXT output, the number of output audio channels can change */
+    if ( ( error = IVAS_DEC_GetNumOutputChannels( hIvasDec, pNumOutChannels ) ) != IVAS_ERR_OK )
+    {
+        fprintf( stderr, "\nError in IVAS_DEC_GetNumOutputChannels, code: %d\n", error );
+        return error;
+    }
+
+    /* Get the format of currently decoded bitstream */
+    if ( ( error = IVAS_DEC_GetFormat( hIvasDec, pBsFormat ) ) != IVAS_ERR_OK )
+    {
+        fprintf( stderr, "\nError in IVAS_DEC_GetFormat, code: %d\n", error );
+        return error;
+    }
+
+    *pNumObj = 0;
+    if ( *pBsFormat == IVAS_DEC_BS_OBJ || *pBsFormat == IVAS_DEC_BS_MASA_ISM || *pBsFormat == IVAS_DEC_BS_SBA_ISM )
+    {
+        if ( ( error = IVAS_DEC_GetNumObjects( hIvasDec, pNumObj ) ) != IVAS_ERR_OK )
+        {
+            fprintf( stderr, "\nError in IVAS_DEC_GetNumObjects: %s\n", IVAS_DEC_GetErrorMessage( error ) );
+            return error;
+        }
+
+        /* If outputting ISM/OMASA/OSBA, ensure the output metadata files are opened */
+        for ( int16_t i = 0; i < *pNumObj; ++i )
+        {
+            if ( ismWriters[i] == NULL )
+            {
+                if ( ( error = IsmFileWriter_open( outputWavFilename, i, &ismWriters[i] ) ) != IVAS_ERR_OK )
+                {
+                    fprintf( stderr, "\nError: Error opening ISM decoded metadata file %s\n", IsmFileWriter_getFilePath( ismWriters[i] ) );
+                    return error;
+                }
+            }
+        }
+    }
+
+    /* If outputting MASA, ensure the output file metadata is opened */
+    if ( *pBsFormat == IVAS_DEC_BS_MASA || *pBsFormat == IVAS_DEC_BS_MASA_ISM )
+    {
+        if ( ppMasaWriter != NULL )
+        {
+            if ( ( error = MasaFileWriter_open( outputWavFilename, delayCompensationEnabled, ppMasaWriter ) ) != IVAS_ERR_OK )
+            {
+                fprintf( stderr, "\nError: Error opening MASA decoded metadata file %s\n", MasaFileWriter_getFilePath( *ppMasaWriter ) );
+                return error;
+            }
+        }
+    }
+
+    return IVAS_ERR_OK;
+}
+#endif
+
 /*---------------------------------------------------------------------*
  * restartDecoder()
  *
diff --git a/lib_com/options.h b/lib_com/options.h
index c5197b92a6fd206f1a3f3df7767919daa44e076c..c8bb88ad67308add50fb2aecc7d5783e1f2eca2e 100644
--- a/lib_com/options.h
+++ b/lib_com/options.h
@@ -178,6 +178,7 @@
 #define FIX_1540_EXPOSE_PT_IN_RTP_HEADER_API            /* Expose Payload Type setting in RTP Header */
 #define FIX_1563_FIX_STEREO_SW                          /* VA: float issue 1563: fix clicks in stereo switching */
 #define FIX_1562_DTX_CRASH_DECODER                      /* VA: float issue 1562: fix crash in stereo decoding in DTX and bitrate switching */
+#define FIX_FMSW_DEC_EXT                                /* float issue 1566: fix EXT output in format switching */
 
 /* ##################### End NON-BE switches ########################### */