elastic · githoober · Mar 19, 2026 · Mar 19, 2026
diff --git a/agent/native/ext/ConfigManager.cpp b/agent/native/ext/ConfigManager.cpp
@@ -117,6 +117,7 @@ typedef struct DurationOptionAdditionalMetadata DurationOptionAdditionalMetadata
 struct SizeOptionAdditionalMetadata
 {
     SizeUnits defaultUnits = sizeUnits_byte;
+    Int64 minValidValueInBytes = INT64_MIN;
 };
 typedef struct SizeOptionAdditionalMetadata SizeOptionAdditionalMetadata;
 
@@ -427,7 +428,14 @@ static ResultCode parseSizeValue( const OptionMetadata* optMeta, String rawValue
     ResultCode parseResultCode = parseSize( stringToView( rawValue )
                                                 , optMeta->additionalData.sizeData.defaultUnits
                                                 , /* out */ &parsedValue->u.sizeValue );
-    if ( parseResultCode == resultSuccess ) parsedValue->type = parsedOptionValueType_size;
+    if ( parseResultCode == resultSuccess )
+    {
+        if ( sizeToBytes( parsedValue->u.sizeValue ) < optMeta->additionalData.sizeData.minValidValueInBytes )
+        {
+            return resultParsingFailed;
+        }
+        parsedValue->type = parsedOptionValueType_size;
+    }
     return parseResultCode;
 }
 
@@ -667,7 +675,7 @@ static OptionMetadata buildDurationOptionMetadata(
     };
 }
 
-[[maybe_unused]] static OptionMetadata buildSizeOptionMetadata(
+static OptionMetadata buildSizeOptionMetadata(
         String name
         , StringView iniName
         , bool isSecret
@@ -676,6 +684,7 @@ static OptionMetadata buildDurationOptionMetadata(
         , SetConfigSnapshotFieldFunc setFieldFunc
         , GetConfigSnapshotFieldFunc getFieldFunc
         , SizeUnits defaultUnits
+        , Int64 minValidValueInBytes
 )
 {
     return (OptionMetadata)
@@ -692,7 +701,7 @@ static OptionMetadata buildDurationOptionMetadata(
         .setField = setFieldFunc,
         .getField = getFieldFunc,
         .parsedValueToZval = &parsedSizeValueToZval,
-        .additionalData = (OptionAdditionalMetadata){ .sizeData = (SizeOptionAdditionalMetadata){ .defaultUnits = defaultUnits } }
+        .additionalData = (OptionAdditionalMetadata){ .sizeData = (SizeOptionAdditionalMetadata){ .defaultUnits = defaultUnits, .minValidValueInBytes = minValidValueInBytes } }
     };
 }
 
@@ -827,6 +836,7 @@ ELASTIC_APM_DEFINE_ENUM_FIELD_ACCESS_FUNCS( LogLevel, logLevelSyslog )
 #   ifdef PHP_WIN32
 ELASTIC_APM_DEFINE_ENUM_FIELD_ACCESS_FUNCS( LogLevel, logLevelWinSysDebug )
 #   endif
+ELASTIC_APM_DEFINE_FIELD_ACCESS_FUNCS( sizeValue, maxSendQueueSize )
 #   if ( ELASTIC_APM_MEMORY_TRACKING_ENABLED_01 != 0 )
 ELASTIC_APM_DEFINE_ENUM_FIELD_ACCESS_FUNCS( MemoryTrackingLevel, memoryTrackingLevel )
 #   endif
@@ -882,6 +892,9 @@ ELASTIC_APM_DEFINE_FIELD_ACCESS_FUNCS( stringValue, debugDiagnosticsFile )
 #define ELASTIC_APM_INIT_DURATION_METADATA( fieldName, optName, defaultValue, defaultUnits, isNegativeValid ) \
     ELASTIC_APM_INIT_METADATA_EX( buildDurationOptionMetadata, fieldName, optName, /* isSecret */ false, /* isDynamic */ false, defaultValue, defaultUnits, isNegativeValid )
 
+#define ELASTIC_APM_INIT_SIZE_METADATA( fieldName, optName, defaultValue, defaultUnits, minValidValueInBytes ) \
+    ELASTIC_APM_INIT_METADATA_EX( buildSizeOptionMetadata, fieldName, optName, /* isSecret */ false, /* isDynamic */ false, defaultValue, defaultUnits, minValidValueInBytes )
+
 #define ELASTIC_APM_INIT_SECRET_METADATA( buildFunc, fieldName, optName, defaultValue ) \
     ELASTIC_APM_INIT_METADATA_EX( buildFunc, fieldName, optName, /* isSecret */ true, /* isDynamic */ false, defaultValue )
 
@@ -1115,6 +1128,13 @@ static void initOptionsMetadata( OptionMetadata* optsMeta )
             ELASTIC_APM_CFG_OPT_NAME_LOG_LEVEL_WIN_SYS_DEBUG );
     #endif
 
+    ELASTIC_APM_INIT_SIZE_METADATA(
+            maxSendQueueSize,
+            ELASTIC_APM_CFG_OPT_NAME_MAX_SEND_QUEUE_SIZE,
+            /* defaultValue: */ makeSize( 10, sizeUnits_mebibyte ),
+            /* defaultUnits: */ sizeUnits_mebibyte,
+            /* minValidValueInBytes: */ 1 );
+
     #if ( ELASTIC_APM_MEMORY_TRACKING_ENABLED_01 != 0 )
     ELASTIC_APM_ENUM_INIT_METADATA(
             /* fieldName: */ memoryTrackingLevel,

diff --git a/agent/native/ext/ConfigManager.h b/agent/native/ext/ConfigManager.h
@@ -96,6 +96,7 @@ enum OptionId
     #ifdef PHP_WIN32
     optionId_logLevelWinSysDebug,
     #endif
+    optionId_maxSendQueueSize,
     #if ( ELASTIC_APM_MEMORY_TRACKING_ENABLED_01 != 0 )
     optionId_memoryTrackingLevel,
     #endif
@@ -303,6 +304,8 @@ const ConfigSnapshot* getGlobalCurrentConfigSnapshot();
 #define ELASTIC_APM_CFG_OPT_NAME_LOG_LEVEL_WIN_SYS_DEBUG "log_level_win_sys_debug"
 #   endif
 
+#define ELASTIC_APM_CFG_OPT_NAME_MAX_SEND_QUEUE_SIZE "max_send_queue_size"
+
 /**
  * Internal configuration option (not included in public documentation)
  */

diff --git a/agent/native/ext/ConfigSnapshot.h b/agent/native/ext/ConfigSnapshot.h
@@ -25,6 +25,7 @@
 #include "LogLevel.h"
 #include "OptionalBool.h"
 #include "time_util.h" // Duration
+#include "util.h" // Size
 #include "elastic_apm_assert_enabled.h"
 
 struct ConfigSnapshot
@@ -68,6 +69,7 @@ struct ConfigSnapshot
         #ifdef PHP_WIN32
     LogLevel logLevelWinSysDebug = logLevel_off;
         #endif
+    Size maxSendQueueSize;
         #if ( ELASTIC_APM_MEMORY_TRACKING_ENABLED_01 != 0 )
     MemoryTrackingLevel memoryTrackingLevel = memoryTrackingLevel_off;
         #endif

diff --git a/agent/native/ext/backend_comm.cpp b/agent/native/ext/backend_comm.cpp
@@ -660,15 +660,14 @@ static void freeDataToSendQueue( DataToSendQueue* dataQueue )
     }
 }
 
-#define ELASTIC_APM_MAX_QUEUE_SIZE_IN_BYTES (10 * 1024 * 1024)
-
 struct BackgroundBackendComm
 {
     Mutex* mutex;
     ConditionVariable* condVar;
     Thread* thread;
     DataToSendQueue dataToSendQueue;
     size_t dataToSendTotalSize;
+    UInt64 maxQueueSizeInBytes;
     size_t nextEventsBatchId;
     bool shouldExit;
     TimeSpec shouldExitBy;
@@ -1051,13 +1050,17 @@ ResultCode newBackgroundBackendComm( const ConfigSnapshot* config, BackgroundBac
 
     ResultCode resultCode;
     BackgroundBackendComm* backgroundBackendComm = NULL;
+    Int64 maxQueueSizeInBytes = sizeToBytes( config->maxSendQueueSize );
+
+    ELASTIC_APM_ASSERT( maxQueueSizeInBytes > 0, "maxQueueSizeInBytes: %" PRId64, maxQueueSizeInBytes );
 
     ELASTIC_APM_MALLOC_INSTANCE_IF_FAILED_GOTO( BackgroundBackendComm, /* out */ backgroundBackendComm );
     backgroundBackendComm->condVar = NULL;
     backgroundBackendComm->mutex = NULL;
     backgroundBackendComm->thread = NULL;
     initDataToSendQueue( &( backgroundBackendComm->dataToSendQueue ) );
     backgroundBackendComm->dataToSendTotalSize = 0;
+    backgroundBackendComm->maxQueueSizeInBytes = (UInt64) maxQueueSizeInBytes;
     backgroundBackendComm->nextEventsBatchId = 1;
     backgroundBackendComm->shouldExit = false;
     ELASTIC_APM_CALL_IF_FAILED_GOTO( newMutex( &( backgroundBackendComm->mutex ), /* dbgDesc */ "Background backend communications" ) );
@@ -1179,15 +1182,21 @@ ResultCode enqueueEventsToSendToApmServer( StringView userAgentHttpHeader, Strin
     bool shouldUnlockMutex = false;
     UInt64 id;
     BackgroundBackendComm* backgroundBackendComm = g_backgroundBackendComm;
+    UInt64 totalSizeAfterEnqueue;
 
     ELASTIC_APM_CALL_IF_FAILED_GOTO( lockMutex( backgroundBackendComm->mutex, &shouldUnlockMutex, __FUNCTION__ ) );
 
-    if ( backgroundBackendComm->dataToSendTotalSize >= ELASTIC_APM_MAX_QUEUE_SIZE_IN_BYTES )
+    totalSizeAfterEnqueue = (UInt64) backgroundBackendComm->dataToSendTotalSize + (UInt64) serializedEvents.length;
+    if ( totalSizeAfterEnqueue > backgroundBackendComm->maxQueueSizeInBytes )
     {
         ELASTIC_APM_LOG_ERROR(
-                "Already queued events are above max queue size - dropping these events"
+                "Queueing these events would exceed max queue size - dropping these events"
                 "; size of already queued events: %" PRIu64
-                , (UInt64) backgroundBackendComm->dataToSendTotalSize );
+                "; size of events to queue: %" PRIu64
+                "; max queue size: %" PRIu64
+                , (UInt64) backgroundBackendComm->dataToSendTotalSize
+                , (UInt64) serializedEvents.length
+                , backgroundBackendComm->maxQueueSizeInBytes );
         ELASTIC_APM_SET_RESULT_CODE_AND_GOTO_FAILURE();
     }
 

diff --git a/agent/native/ext/elastic_apm.cpp b/agent/native/ext/elastic_apm.cpp
@@ -173,6 +173,7 @@ PHP_INI_BEGIN()
     #ifdef PHP_WIN32
     ELASTIC_APM_INI_ENTRY( ELASTIC_APM_CFG_OPT_NAME_LOG_LEVEL_WIN_SYS_DEBUG )
     #endif
+    ELASTIC_APM_INI_ENTRY( ELASTIC_APM_CFG_OPT_NAME_MAX_SEND_QUEUE_SIZE )
     #if ( ELASTIC_APM_MEMORY_TRACKING_ENABLED_01 != 0 )
     ELASTIC_APM_INI_ENTRY( ELASTIC_APM_CFG_OPT_NAME_MEMORY_TRACKING_LEVEL )
     #endif

diff --git a/agent/native/ext/tests/config_defaults.phpt b/agent/native/ext/tests/config_defaults.phpt
@@ -8,6 +8,7 @@ ELASTIC_APM_LOG_LEVEL=
 ELASTIC_APM_LOG_LEVEL_FILE=
 ELASTIC_APM_LOG_LEVEL_SYSLOG=
 ELASTIC_APM_LOG_LEVEL_WIN_SYS_DEBUG=
+ELASTIC_APM_MAX_SEND_QUEUE_SIZE=
 ELASTIC_APM_SECRET_TOKEN=
 ELASTIC_APM_SERVER_URL=
 ELASTIC_APM_SERVICE_NAME=
@@ -78,6 +79,15 @@ if (elasticApmIsOsWindows()) {
     elasticApmAssertSame("elastic_apm_get_config_option_by_name('log_level_win_sys_debug')", elastic_apm_get_config_option_by_name('log_level_win_sys_debug'), ELASTIC_APM_LOG_LEVEL_NOT_SET);
 }
 
+//////////////////////////////////////////////
+///////////////  max_send_queue_size
+
+elasticApmAssertSame("getenv('ELASTIC_APM_MAX_SEND_QUEUE_SIZE')", getenv('ELASTIC_APM_MAX_SEND_QUEUE_SIZE'), false);
+
+elasticApmAssertEqual("ini_get('elastic_apm.max_send_queue_size')", ini_get('elastic_apm.max_send_queue_size'), false);
+
+elasticApmAssertSame("elastic_apm_get_config_option_by_name('max_send_queue_size')", elastic_apm_get_config_option_by_name('max_send_queue_size'), 10.0 * 1024 * 1024);
+
 //////////////////////////////////////////////
 ///////////////  secret_token
 

diff --git a/agent/native/ext/tests/config_ini_has_higher_precedence_than_env_vars.phpt b/agent/native/ext/tests/config_ini_has_higher_precedence_than_env_vars.phpt
@@ -4,8 +4,10 @@ Configuration in ini file has higher precedence than environment variables
 ELASTIC_APM_LOG_LEVEL_STDERR=CRITICAL
 ELASTIC_APM_LOG_FILE=log_file_from_env_vars.txt
 ELASTIC_APM_LOG_LEVEL_FILE=off
+ELASTIC_APM_MAX_SEND_QUEUE_SIZE=123MB
 --INI--
 elastic_apm.log_file=log_file_from_ini.txt
+elastic_apm.max_send_queue_size=456MB
 elastic_apm.bootstrap_php_part_file=../../php/bootstrap_php_part.php
 --FILE--
 <?php
@@ -18,6 +20,12 @@ elasticApmAssertSame("ini_get('elastic_apm.log_file')", ini_get('elastic_apm.log
 
 elasticApmAssertSame("elastic_apm_get_config_option_by_name('log_file')", elastic_apm_get_config_option_by_name('log_file'), 'log_file_from_ini.txt');
 
+elasticApmAssertSame("getenv('ELASTIC_APM_MAX_SEND_QUEUE_SIZE')", getenv('ELASTIC_APM_MAX_SEND_QUEUE_SIZE'), '123MB');
+
+elasticApmAssertSame("ini_get('elastic_apm.max_send_queue_size')", ini_get('elastic_apm.max_send_queue_size'), '456MB');
+
+elasticApmAssertSame("elastic_apm_get_config_option_by_name('max_send_queue_size')", elastic_apm_get_config_option_by_name('max_send_queue_size'), 456.0 * 1024 * 1024);
+
 echo 'Test completed'
 ?>
 --EXPECT--

diff --git a/agent/native/ext/tests/config_invalid_ini_fallback_default_not_env_var.phpt b/agent/native/ext/tests/config_invalid_ini_fallback_default_not_env_var.phpt
@@ -3,9 +3,11 @@ When value in ini is invalid the fallback is the default and not environment var
 --ENV--
 ELASTIC_APM_LOG_LEVEL_STDERR=CRITICAL
 ELASTIC_APM_ASSERT_LEVEL=O_n
+ELASTIC_APM_MAX_SEND_QUEUE_SIZE=123MB
 ELASTIC_APM_MEMORY_TRACKING_LEVEL=ALL
 ELASTIC_APM_VERIFY_SERVER_CERT=false
 --INI--
+elastic_apm.max_send_queue_size=0
 elastic_apm.memory_tracking_level=not a valid memory tracking level
 elastic_apm.verify_server_cert=not a valid bool
 elastic_apm.bootstrap_php_part_file=../../php/bootstrap_php_part.php
@@ -18,6 +20,10 @@ require __DIR__ . '/../tests_util/tests_util.php';
 elasticApmAssertSame("elastic_apm_get_config_option_by_name('assert_level')", elastic_apm_get_config_option_by_name('assert_level'), ELASTIC_APM_ASSERT_LEVEL_O_N);
 elasticApmAssertSame("getenv('ELASTIC_APM_ASSERT_LEVEL')", getenv('ELASTIC_APM_ASSERT_LEVEL'), 'O_n');
 
+// max_send_queue_size is set in ini but the value is invalid so it falls back on default (which is `10MB`) and not the value set by env vars (which is `123MB`)
+elasticApmAssertSame("elastic_apm_get_config_option_by_name('max_send_queue_size')", elastic_apm_get_config_option_by_name('max_send_queue_size'), 10.0 * 1024 * 1024);
+elasticApmAssertSame("getenv('ELASTIC_APM_MAX_SEND_QUEUE_SIZE')", getenv('ELASTIC_APM_MAX_SEND_QUEUE_SIZE'), '123MB');
+
 // memory_tracking_level is set in ini but the value is invalid so it falls back on default (which is `ELASTIC_APM_MEMORY_TRACKING_LEVEL_NOT_SET) and not the value set by env vars (which is ELASTIC_APM_MEMORY_TRACKING_LEVEL_ALL)
 elasticApmAssertSame("elastic_apm_get_config_option_by_name('memory_tracking_level')", elastic_apm_get_config_option_by_name('memory_tracking_level'), ELASTIC_APM_MEMORY_TRACKING_LEVEL_NOT_SET);
 elasticApmAssertSame("getenv('ELASTIC_APM_MEMORY_TRACKING_LEVEL')", getenv('ELASTIC_APM_MEMORY_TRACKING_LEVEL'), 'ALL');

diff --git a/agent/native/ext/tests/config_setting_to_invalid_values_using_env_vars.phpt b/agent/native/ext/tests/config_setting_to_invalid_values_using_env_vars.phpt
@@ -4,6 +4,7 @@ Setting configuration option to invalid value via environment variables
 ELASTIC_APM_LOG_LEVEL_STDERR=CRITICAL
 ELASTIC_APM_ENABLED=not_valid_boolean_value
 ELASTIC_APM_ASSERT_LEVEL=|:/:\:|
+ELASTIC_APM_MAX_SEND_QUEUE_SIZE=0
 ELASTIC_APM_SECRET_TOKEN=\|<>|/
 ELASTIC_APM_SERVER_URL=<\/\/>
 ELASTIC_APM_SERVICE_NAME=/\><\/
@@ -30,6 +31,13 @@ elasticApmAssertSame("getenv('ELASTIC_APM_ASSERT_LEVEL')", getenv('ELASTIC_APM_A
 
 elasticApmAssertSame("elastic_apm_get_config_option_by_name('assert_level')", elastic_apm_get_config_option_by_name('assert_level'), ELASTIC_APM_ASSERT_LEVEL_NOT_SET);
 
+//////////////////////////////////////////////
+///////////////  max_send_queue_size
+
+elasticApmAssertSame("getenv('ELASTIC_APM_MAX_SEND_QUEUE_SIZE')", getenv('ELASTIC_APM_MAX_SEND_QUEUE_SIZE'), '0');
+
+elasticApmAssertSame("elastic_apm_get_config_option_by_name('max_send_queue_size')", elastic_apm_get_config_option_by_name('max_send_queue_size'), 10.0 * 1024 * 1024);
+
 //////////////////////////////////////////////
 ///////////////  secret_token
 

diff --git a/agent/native/ext/tests/config_setting_to_non-defaults_using_env_vars.phpt b/agent/native/ext/tests/config_setting_to_non-defaults_using_env_vars.phpt
@@ -8,6 +8,7 @@ ELASTIC_APM_LOG_LEVEL=CRITICAL
 ELASTIC_APM_LOG_LEVEL_FILE=TRACE
 ELASTIC_APM_LOG_LEVEL_SYSLOG=TRACE
 ELASTIC_APM_LOG_LEVEL_WIN_SYS_DEBUG=CRITICAL
+ELASTIC_APM_MAX_SEND_QUEUE_SIZE=123MB
 ELASTIC_APM_SECRET_TOKEN=non-default_secret_token_123
 ELASTIC_APM_SERVER_URL=https://non-default_server_url:4321/some/path
 ELASTIC_APM_SERVICE_NAME=Non-default Service Name
@@ -66,6 +67,13 @@ if (elasticApmIsOsWindows()) {
     elasticApmAssertSame("elastic_apm_get_config_option_by_name('log_level_win_sys_debug')", elastic_apm_get_config_option_by_name('log_level_win_sys_debug'), ELASTIC_APM_LOG_LEVEL_CRITICAL);
 }
 
+//////////////////////////////////////////////
+///////////////  max_send_queue_size
+
+elasticApmAssertSame("getenv('ELASTIC_APM_MAX_SEND_QUEUE_SIZE')", getenv('ELASTIC_APM_MAX_SEND_QUEUE_SIZE'), '123MB');
+
+elasticApmAssertSame("elastic_apm_get_config_option_by_name('max_send_queue_size')", elastic_apm_get_config_option_by_name('max_send_queue_size'), 123.0 * 1024 * 1024);
+
 //////////////////////////////////////////////
 ///////////////  secret_token
 

diff --git a/agent/native/ext/tests/config_setting_to_non-defaults_using_ini.phpt b/agent/native/ext/tests/config_setting_to_non-defaults_using_ini.phpt
@@ -9,6 +9,7 @@ elastic_apm.log_level=CRITICAL
 elastic_apm.log_level_file=TRACE
 elastic_apm.log_level_syslog=TRACE
 elastic_apm.log_level_win_sys_debug=CRITICAL
+elastic_apm.max_send_queue_size=456MB
 elastic_apm.secret_token=non-default_secret_token_123
 elastic_apm.server_url=https://non-default_server_url:4321/some/path
 elastic_apm.service_name=Non-default Service Name
@@ -65,6 +66,13 @@ if (elasticApmIsOsWindows()) {
     elasticApmAssertSame("elastic_apm_get_config_option_by_name('log_level_win_sys_debug')", elastic_apm_get_config_option_by_name('log_level_win_sys_debug'), ELASTIC_APM_LOG_LEVEL_CRITICAL);
 }
 
+//////////////////////////////////////////////
+///////////////  max_send_queue_size
+
+elasticApmAssertSame("ini_get('elastic_apm.max_send_queue_size')", ini_get('elastic_apm.max_send_queue_size'), '456MB');
+
+elasticApmAssertSame("elastic_apm_get_config_option_by_name('max_send_queue_size')", elastic_apm_get_config_option_by_name('max_send_queue_size'), 456.0 * 1024 * 1024);
+
 //////////////////////////////////////////////
 ///////////////  secret_token
 

diff --git a/agent/php/ElasticApm/Impl/Config/AllOptionsMetadata.php b/agent/php/ElasticApm/Impl/Config/AllOptionsMetadata.php
@@ -70,6 +70,11 @@ private static function buildPositiveOrZeroIntMetadata(int $default): IntOptionM
         return new IntOptionMetadata(/* min */ 0, /* max */ null, $default);
     }
 
+    private static function buildPositiveSizeMetadata(int $defaultInBytes, int $defaultUnits): SizeOptionMetadata
+    {
+        return new SizeOptionMetadata(/* min */ 1.0, /* max */ null, $defaultUnits, floatval($defaultInBytes));
+    }
+
     /**
      * @return array<string, OptionMetadata<mixed>> Option name to metadata
      */
@@ -103,6 +108,7 @@ public static function get(): array
             OptionNames::LOG_LEVEL                                  => new NullableLogLevelOptionMetadata(),
             OptionNames::LOG_LEVEL_STDERR                           => new NullableLogLevelOptionMetadata(),
             OptionNames::LOG_LEVEL_SYSLOG                           => new NullableLogLevelOptionMetadata(),
+            OptionNames::MAX_SEND_QUEUE_SIZE                        => self::buildPositiveSizeMetadata(OptionDefaultValues::MAX_SEND_QUEUE_SIZE, SizeUnits::MEGABYTES),
             OptionNames::NON_KEYWORD_STRING_MAX_LENGTH              => self::buildPositiveOrZeroIntMetadata(/* default */ 10 * 1024),
             OptionNames::PROFILING_INFERRED_SPANS_ENABLED           => new BoolOptionMetadata(/* default */ false),
             OptionNames::PROFILING_INFERRED_SPANS_MIN_DURATION      => self::buildDurationMetadataInMilliseconds(/* default */ 0),

diff --git a/agent/php/ElasticApm/Impl/Config/OptionDefaultValues.php b/agent/php/ElasticApm/Impl/Config/OptionDefaultValues.php
@@ -34,6 +34,7 @@ final class OptionDefaultValues
 {
     use StaticClassTrait;
 
+    public const MAX_SEND_QUEUE_SIZE = 10 * 1024 * 1024;
     public const SPAN_STACK_TRACE_MIN_DURATION = 5;
     public const STACK_TRACE_LIMIT = 50;
     public const TRANSACTION_MAX_SPANS = 500;

diff --git a/agent/php/ElasticApm/Impl/Config/OptionNames.php b/agent/php/ElasticApm/Impl/Config/OptionNames.php
@@ -57,6 +57,7 @@ final class OptionNames
     public const LOG_LEVEL = 'log_level';
     public const LOG_LEVEL_SYSLOG = 'log_level_syslog';
     public const LOG_LEVEL_STDERR = 'log_level_stderr';
+    public const MAX_SEND_QUEUE_SIZE = 'max_send_queue_size';
     public const NON_KEYWORD_STRING_MAX_LENGTH = 'non_keyword_string_max_length';
     public const PROFILING_INFERRED_SPANS_ENABLED = 'profiling_inferred_spans_enabled';
     public const PROFILING_INFERRED_SPANS_MIN_DURATION = 'profiling_inferred_spans_min_duration';