microsoft/semantic-kernel · error · InvalidOperationException

Message content cannot be null.

Error message

Message content cannot be null.

What it means

Thrown by BedrockAgentChannel while building the Bedrock session-state conversation history from the channel's message backlog. For every message except the last (which is sent separately), Content must be non-null. A null Content breaks the Bedrock message model, so InvalidOperationException is thrown. Note this runs over this._history, i.e., the accumulated channel history, not just the input.

Source

Thrown at dotnet/src/Agents/Bedrock/BedrockAgentChannel.cs:223

    }

    private SessionState ParseHistoryToSessionState()
    {
        SessionState sessionState = new();

        // We don't take the last message as it needs to be sent separately in another parameter.
        if (this._history.Count > 1)
        {
            sessionState.ConversationHistory = new()
            {
                Messages = []
            };

            foreach (var message in this._history.Take(this._history.Count - 1))
            {
                if (message.Content is null)
                {
                    throw new InvalidOperationException("Message content cannot be null.");
                }
                if (message.Role != AuthorRole.Assistant && message.Role != AuthorRole.User)
                {
                    throw new InvalidOperationException("Message role must be either Assistant or User.");
                }

                sessionState.ConversationHistory.Messages.Add(new()
                {
                    Role = message.Role == AuthorRole.Assistant
                        ? Amazon.BedrockAgentRuntime.ConversationRole.Assistant
                        : Amazon.BedrockAgentRuntime.ConversationRole.User,
                    Content = [
                        new Amazon.BedrockAgentRuntime.Model.ContentBlock()
                        {
                            Text = message.Content,
                        },
                    ],
                });

View on GitHub (pinned to c028a0c7dc)

Solutions

1. Ensure every message added to the channel history has non-null Content (use string.Empty rather than null).
2. Filter out metadata-only messages before they enter the Bedrock channel history.
3. If converting from another provider, default null Content to an empty string.

Example fix

// before
history.Add(new ChatMessageContent(AuthorRole.Assistant, content: null)); // -> throws 179

// after
history.Add(new ChatMessageContent(AuthorRole.Assistant, content: "") );

foreach (var m in history.Take(history.Count - 1))
{
    if (m.Content is null)
        throw new InvalidOperationException($"Message (role {m.Role}) has null Content; use string.Empty.");
}

static bool HistoryHasNoNullContent(IReadOnlyList<ChatMessageContent> h) =>
    h.Take(h.Count - 1).All(m => m.Content is not null);

try { /* invoke bedrock agent */ }
catch (InvalidOperationException ex) when (ex.Message.Contains("Message content cannot be null"))
{ /* replace null Content with string.Empty in the channel history */ }

When it happens

Trigger: The agent channel history contains a ChatMessageContent whose Content is null (e.g., a message carrying only function-call metadata with no text, or a placeholder). This is checked before role validation in the same loop.

Common situations: A tool-call or function-step message with null Content was added to the channel history; a deserialized/round-tripped message lost its content; an empty assistant ack got persisted.

Related errors

• The Bedrock agent requires a message to be invoked.
• The streaming configuration must be null for non-streaming r
• The streaming configuration must have StreamFinalResponse se
• Bedrock agents must be invoked with a user message
• Invalid role: {authorRole}

AI-assisted analysis of microsoft/semantic-kernel@c028a0c7dc (2026-08-13). Data as JSON: /api/errors/abeef5df7770c4de. Report an issue: GitHub.