Function derive_context 

pub async fn derive_context(
    session: &Session,
    provider: Arc<dyn Provider>,
    model: &str,
    system_prompt: &str,
    tools: &[ToolDefinition],
    event_tx: Option<&Sender<SessionEvent>>,
    force_keep_last: Option<usize>,
) -> Result<DerivedContext>

Derive an ephemeral DerivedContext from session’s pure history.

The canonical Session::messages buffer is never touched — session is borrowed immutably.

Arguments

• session — The owning session (read-only borrow).
• provider — Caller’s primary provider.
• model — Caller’s primary model identifier.
• system_prompt — Included in token estimates.
• tools — Tool definitions, included in token estimates.
• event_tx — Optional channel for compaction lifecycle events.
• force_keep_last — When Some(n), skip the adaptive budget cascade and force a single compress_messages_keep_last call.

Errors

Propagates any error from the underlying compression pipeline that the recovery cascade cannot absorb.