[Opus 4.6] BetaCompactionBlock deserialized as ParsedBetaTextBlock during streaming accumulation

[Julien-Abulfeda]

SDK Version [anthropic==0.79.0] (Python 3.12) via vertex

Description

When using the beta streaming API with compaction=True, the BetaCompactionBlock in the final ParsedBetaMessage.content is incorrectly typed as a ParsedBetaTextBlock instead of BetaCompactionBlock.

Reproduction

kwargs["betas"]=["compact-2026-01-12"]
kwargs["context_management"]={
    "edits": [{
        "type": "compact_20260112",
        "trigger": {"type": "input_tokens", "value": 50000},
        "pause_after_compaction": pause_after_compaction
    }]}

client = AsyncAnthropicVertex(
    project_id=project_id,
    region=region,
    credentials=credentials  # <-- Fix: credentials passées directement
)
async with client.beta.messages.stream(**kwargs) as stream:
    message = await stream.get_final_message()
    
    for block in message.content:
        print(type(block), block.type)
        # Expected: <class 'BetaCompactionBlock'> compaction
        # Actual:   <class 'ParsedBetaTextBlock'> compaction

Observed behavior

When a compaction event occurs, the streaming events (content_block_start, content_block_delta and content_block_stop for "compaction" are correctly typed but the final message contains:

ParsedBetaTextBlock(citations=None, text=None, type='compaction', parsed_output=None, content=None)

• type(block) is ParsedBetaTextBlock, not BetaCompactionBlock
• The object has parasitic fields from ParsedBetaTextBlock (text, citations, parsed_output) all set to None
• block.type is correctly "compaction"
• Calling model_dump() / model_dump_json() on the parent message triggers Pydantic serialization warnings for every variant in the union

Expected behavior

message.content[i] should be an instance of BetaCompactionBlock when block.type == "compaction".