Bug: `StreamingMode.SSE` causes agent to drop tool calls in subsequent turns

[iyedb]

Description

When StreamingMode.SSE is enabled (without PROGRESSIVE_SSE_STREAMING), an agent that produces
both a text response and one or more tool calls in the same turn works correctly on the first
user turn but silently drops tool calls on every subsequent turn.

Steps to reproduce

1. Create an LlmAgent with two lightweight tools, for example:

   • showForecastChart — accepts a location string, returns "Chart displayed."
   • showQuickActions — accepts an array of action labels, returns "Actions displayed."

2. Write a system prompt that instructs the agent to always reply with a short text summary
   and call both tools on every turn.

3. Run the agent with streamingMode: StreamingMode.NONE (the default). Send two consecutive
   user messages. Verify that both tools are called on both turns.

4. Run the exact same agent and conversation with streamingMode: StreamingMode.SSE
   (PROGRESSIVE_SSE_STREAMING disabled, which is the default). Send the same two messages.

5. Observe that on the later turns one or more tool calls that appeared in step 3 are absent.
   Comparing candidatesTokenCount and thoughtsTokenCount in usageMetadata between the two
   modes confirms the model actually generates a shorter response: the missing tool
   calls are not produced by Gemini API.

Expected behavior

The agent should call the same tools in SSE mode as it does with SSE disabled.

Environment

• @google/adk version: 1.1.0
• Model: gemini-2.5-flash
• Runtime: Node.js
• PROGRESSIVE_SSE_STREAMING: disabled (default)

Note: possibly related to #289

[Varun-S10]

Hi @iyedb, Thank you for raising the issue. We have tried to reproduce the issue exactly how you mentioned. On our end, in both NONE and SSE modes, the code calls both tools as expected on Turn 2.

As a workaround, please ensure you are fully consuming the stream in your code to its absolute completion before sending the next user message. If the stream is interrupted early, the final tool calls may not be saved to the default session history.

Could you please let us know if this works for you, or if you still face the issue, provide the exact code snippet you are using so that it will help us investigate further?