Invalid Config, event_buffer_size must be non-negative

Error message

Invalid Config, event_buffer_size must be non-negative

What it means

Returned by convertServerConfig when the server's event_buffer_size setting is negative. The event broker buffer must hold zero or more events, so a negative size is rejected during agent config conversion/reload.

Source

Thrown at command/agent/agent.go:329

	}
	if agentConfig.Sentinel != nil {
		conf.SentinelConfig = agentConfig.Sentinel
	}
	if agentConfig.Server.NonVotingServer {
		conf.NonVoter = true
	}
	if agentConfig.Server.RedundancyZone != "" {
		conf.RedundancyZone = agentConfig.Server.RedundancyZone
	}
	if agentConfig.Server.UpgradeVersion != "" {
		conf.UpgradeVersion = agentConfig.Server.UpgradeVersion
	}
	if agentConfig.Server.EnableEventBroker != nil {
		conf.EnableEventBroker = *agentConfig.Server.EnableEventBroker
	}
	if agentConfig.Server.EventBufferSize != nil {
		if *agentConfig.Server.EventBufferSize < 0 {
			return nil, fmt.Errorf("Invalid Config, event_buffer_size must be non-negative")
		}
		conf.EventBufferSize = int64(*agentConfig.Server.EventBufferSize)
	}
	if agentConfig.Autopilot != nil {
		if agentConfig.Autopilot.CleanupDeadServers != nil {
			conf.AutopilotConfig.CleanupDeadServers = *agentConfig.Autopilot.CleanupDeadServers
		}
		if agentConfig.Autopilot.ServerStabilizationTime != 0 {
			conf.AutopilotConfig.ServerStabilizationTime = agentConfig.Autopilot.ServerStabilizationTime
		}
		if agentConfig.Autopilot.LastContactThreshold != 0 {
			conf.AutopilotConfig.LastContactThreshold = agentConfig.Autopilot.LastContactThreshold
		}
		if agentConfig.Autopilot.MaxTrailingLogs != 0 {
			conf.AutopilotConfig.MaxTrailingLogs = uint64(agentConfig.Autopilot.MaxTrailingLogs)
		}
		if agentConfig.Autopilot.MinQuorum != 0 {
			conf.AutopilotConfig.MinQuorum = uint(agentConfig.Autopilot.MinQuorum)

View on GitHub (pinned to 482b49bf1a)

Solutions

Set event_buffer_size to 0 (use default) or a positive number of events.
Remove the stanza to use the default buffer size.
Check any config generation logic for sign errors.

Example fix

// before
server {
  event_buffer_size = -100
}
// after
server {
  event_buffer_size = 100
}

Defensive patterns

Strategy: validation

Validate before calling

// Go: pre-validate
if v := cfg.Server.EventBufferSize; v != nil && *v < 0 {
    return fmt.Errorf("event_buffer_size must be >= 0")
}

Try / catch

conf, err := convertServerConfig(agentCfg)
if err != nil {
    if strings.Contains(err.Error(), "event_buffer_size") {
        return fmt.Errorf("event_buffer_size must be non-negative: %w", err)
    }
    return err
}

Prevention

Use 0 or positive values only; omit the option for the default.
Audit config templating for signed-int errors.
Note the check runs on reload too — validate before SIGHUP.

When it happens

Trigger: Setting server { event_buffer_size = -1 } (any negative int64) in agent config, then starting the agent or reloading it (serverConfig, handleReload, tests).

Common situations: Signed arithmetic or templating bugs producing negative buffer values; misunderstanding the option as an auto-size hint; copy-paste with a leading dash.

Related errors

AI-assisted analysis of hashicorp/nomad@482b49bf1a (2026-09-04). Data as JSON: /api/errors/0b8839e10a66177a. Report an issue: GitHub.