diff --git a/apps/decoder.c b/apps/decoder.c
index 270c644db6f9b481e464714d194cb1cb3886d8b4..3180647b1c85cbbbbcbb5310ee201043fa64d1c6 100644
--- a/apps/decoder.c
+++ b/apps/decoder.c
@@ -163,6 +163,9 @@ static ivas_error decodeG192( DecArguments arg, BS_READER_HANDLE hBsReader, IVAS
 static ivas_error decodeVoIP( DecArguments arg, BS_READER_HANDLE hBsReader, IVAS_DEC_HRTF_BINARY_WRAPPER *hHrtf, RotFileReader *headRotReader, RotFileReader *externalOrientationFileReader, RotFileReader *refRotReader, Vector3PairFileReader *referenceVectorReader, ObjectEditFileReader *objectEditFileReader, ISAR_SPLIT_REND_BITS_DATA *splitRendBits, IVAS_DEC_HANDLE *phIvasDec, int16_t *pcmBuf );
 static ivas_error load_hrtf_from_file( IVAS_DEC_HRTF_BINARY_WRAPPER *hHrtfBinary, IVAS_DEC_HANDLE hIvasDec, const IVAS_AUDIO_CONFIG OutputConfig, const int32_t output_Fs );
 static void do_object_editing_fx( IVAS_EDITABLE_PARAMETERS *editableParameters, ObjectEditFileReader *objectEditFileReader );
+#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
 
 
 /*------------------------------------------------------------------------------------------*
@@ -203,7 +206,6 @@ int main(
     reset_mem( USE_BYTES );
 #endif
 
-
     splitRendBits.bits_buf = splitRendBitsBuf;
 
     /*------------------------------------------------------------------------------------------*
@@ -453,7 +455,6 @@ int main(
         }
     }
 
-
     /*-------------------------------------------------------------------*
      * Load renderer configuration from file
      *--------------------------------------------------------------------*/
@@ -538,6 +539,7 @@ int main(
             fprintf( stderr, "Failed to get directivity patterns for one or more of IDs: %d %d %d %d\n\n", arg.directivityPatternId[0], arg.directivityPatternId[1], arg.directivityPatternId[2], arg.directivityPatternId[3] );
             goto cleanup;
         }
+
         if ( ( error = RenderConfigReader_getDistanceAttenuation( renderConfigReader, renderConfig.distAtt_fx ) ) != IVAS_ERR_OK )
         {
             fprintf( stderr, "Failed to get Distance Attenuation \n\n" );
@@ -608,6 +610,7 @@ int main(
             hLsCustomData.azimuth_fx[i] = (Word32) ( hLsCustomData.azimuth[i] * ( 1u << 22 ) );
             hLsCustomData.elevation_fx[i] = (Word32) ( hLsCustomData.elevation[i] * ( 1u << 22 ) );
         }
+
         if ( ( error = IVAS_DEC_FeedCustomLsData( hIvasDec, hLsCustomData ) ) != IVAS_ERR_OK )
         {
             fprintf( stderr, "\nIVAS_DEC_FeedCustomLsData failed: %s\n", IVAS_DEC_GetErrorMessage( error ) );
@@ -686,7 +689,6 @@ int main(
         goto cleanup;
     }
 
-
     /*------------------------------------------------------------------------------------------*
      * Close files and deallocate resources
      *------------------------------------------------------------------------------------------*/
@@ -734,7 +736,6 @@ cleanup:
     print_wmops();
     print_mem( NULL );
 #endif
-
     if ( !arg.quietModeEnabled )
     {
         printf( "\n" );
@@ -2026,7 +2027,11 @@ static ivas_error decodeG192(
     int16_t isSplitRend, isSplitCoded;
     IVAS_RENDER_CONFIG_DATA renderConfig;
     RenderConfigReader *renderConfigReader = NULL;
+#ifdef FIX_FMSW_DEC_EXT
+    bool restartNeeded = false;
+#else
     bool restartNeeded;
+#endif
 
 #ifdef VARIABLE_SPEED_DECODING
     if ( arg.tsmEnabled )
@@ -2117,6 +2122,7 @@ static ivas_error decodeG192(
         fprintf( stderr, "\nError getting render frame size in samples\n" );
         return error;
     }
+
     if ( ( error = IVAS_DEC_GetReferencesUpdateFrequency( hIvasDec, &vec_pos_len ) ) != IVAS_ERR_OK )
     {
         fprintf( stderr, "\nError getting render frame size in samples\n" );
@@ -2412,6 +2418,18 @@ 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 )
         {
@@ -2805,7 +2823,11 @@ 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 };
@@ -3292,6 +3314,18 @@ 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 )
         {
@@ -3900,4 +3934,80 @@ static ivas_error load_hrtf_from_file(
     return IVAS_ERR_OK;
 }
 
+#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
+
 #undef WMC_TOOL_SKIP
diff --git a/lib_com/options.h b/lib_com/options.h
index a1b758cfc563aca2adef722e54399cc2355c02f1..95272a536476586c757eb0e7473cd5c4aecbc98e 100644
--- a/lib_com/options.h
+++ b/lib_com/options.h
@@ -106,6 +106,7 @@
 #define FIX_BASOP_2599_TCA_OVERFLOW                     /* VA: issue 2599, Fix TCA overflow */
 #define FIX_2588_MISSING_CONDITIONS                     /* VA: Proposed fix to 2588, addition of a condition in lsf_dec */
 #define FIX_BASOP_2442_MASA2TC_TO_MONO_AND_AMBI         /* Nokia: BASOP issue 2442: Increase accuracy of computations and add additional gain clamp for low energy decorrelated signal rendering. */
+#define FIX_FMSW_DEC_EXT                                /* float issue 1566: fix EXT output in format switching */
 
 /* ##################### End NON-BE switches ########################### */