Hey, thanks for the report. This is a confirmed bug. additional_context in postToolUse hooks is accepted and logged correctly, but it isn’t actually delivered to the model context. For non-MCP tools, the hook response is discarded fire-and-forget. For MCP, only updated_mcp_tool_output is used.
The same report is already here: Native postToolUse hooks accept and log additional_context successfully, but the injected context is not surfaced to the model
The team is aware. For now, the only hook where additional_context works end-to-end is sessionStart. If the context you need to inject is fairly static or can be determined at session start, you can use that as a workaround.
Closing this as a duplicate. We’ll post updates in the original thread.