microsoft/semantic-kernel · error · InvalidOperationException

The Bedrock agent requires a message to be invoked.

Error message

The Bedrock agent requires a message to be invoked.

What it means

Thrown at the top of the non-streaming InvokeAsync on BedrockAgent when the messages collection is empty (Count == 0). Bedrock agents require at least one user message to begin a session. This is a caller-contract violation, not a service error.

Solutions

1. Ensure at least one ChatMessageContent is passed to InvokeAsync.
2. Guard the call site: skip invocation when there is nothing to send, or supply a default user message.

Example fix

// before
await agent.InvokeAsync(new List<ChatMessageContent>()); // throws 173

// after
if (messages.Count > 0)
{
    await agent.InvokeAsync(messages);
}

if (messages is null || messages.Count == 0)
    throw new ArgumentException("Bedrock agent requires at least one message.");
await agent.InvokeAsync(messages, thread, options, ct);

static bool HasMessages(ICollection<ChatMessageContent> m) => m is not null && m.Count > 0;

try { await agent.InvokeAsync(messages, thread); }
catch (InvalidOperationException ex) when (ex.Message.Contains("requires a message"))
{ /* ensure the caller supplies at least one message */ }

When it happens

Trigger: Calling agent.InvokeAsync([]) or InvokeAsync with an empty list. Also if a caller passes a collection that was populated then cleared before the call.

Common situations: Caller built a message list conditionally and it ended up empty; test invoked the agent with no input; upstream message-routing logic dropped all messages.

Related errors

• Bedrock agents must be invoked with a user message
• Invalid role
• Message content cannot be null.
• The streaming configuration must be null for non-streaming…
• The streaming configuration must have StreamFinalResponse…

AI-assisted analysis of microsoft/semantic-kernel@c028a0c7dc (2026-08-13). Data as JSON: /api/errors/04598a82601514ec. Report an issue: GitHub.

Appendix: source

Thrown at dotnet/src/Agents/Bedrock/BedrockAgent.cs:112

    public IAsyncEnumerable<AgentResponseItem<ChatMessageContent>> InvokeAsync(
        ICollection<ChatMessageContent> messages,
        AgentThread? thread = null,
        BedrockAgentInvokeOptions? options = null,
        CancellationToken cancellationToken = default)
    {
        return this.InvokeAsync(messages, thread, (AgentInvokeOptions?)options, cancellationToken);
    }

    /// <inheritdoc/>
    public override async IAsyncEnumerable<AgentResponseItem<ChatMessageContent>> InvokeAsync(
        ICollection<ChatMessageContent> messages,
        AgentThread? thread = null,
        AgentInvokeOptions? options = null,
        [EnumeratorCancellation] CancellationToken cancellationToken = default)
    {
        if (messages.Count == 0)
        {
            throw new InvalidOperationException("The Bedrock agent requires a message to be invoked.");
        }

        // Create a thread if needed
        BedrockAgentThread bedrockThread = await this.EnsureThreadExistsWithMessagesAsync(
            messages,
            thread,
            () => new BedrockAgentThread(this.RuntimeClient),
            cancellationToken).ConfigureAwait(false);

        // Get the context contributions from the AIContextProviders.
#pragma warning disable SKEXP0110, SKEXP0130  // Type is for evaluation purposes only and is subject to change or removal in future updates. Suppress this diagnostic to proceed.
        AIContext providersContext = await bedrockThread.AIContextProviders.ModelInvokingAsync(messages, cancellationToken).ConfigureAwait(false);
#pragma warning restore SKEXP0110, SKEXP0130 // Type is for evaluation purposes only and is subject to change or removal in future updates. Suppress this diagnostic to proceed.

        // Ensure that the last message provided is a user message
        string message = this.ExtractUserMessage(messages.Last());

        // Build session state with conversation history and override instructions if needed

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