Budgets

tako-runtime ships an in-memory BudgetTracker that's consulted both before every provider call (using LlmProvider::estimate_cost_usd) and after (reconciling against the response's actual usage).

budget = tako.Budget(
    max_usd_per_request=5.0,
    max_usd_per_day=500.0,
    max_usd_per_tenant_per_day={"acme": 100.0, "beta": 1000.0},
)
client = tako.Client(providers=[...], budget=budget)

A BudgetExhausted error propagates when:

• The estimated cost would push a single request over max_usd_per_request.
• The reconciled cost would push the day's spend over max_usd_per_day.
• The reconciled cost would push a tenant over their per-tenant cap.

BudgetBackend

BudgetTracker delegates to a BudgetBackend:

#[async_trait]
pub trait BudgetBackend: Send + Sync + 'static + Debug {
    async fn current_usage(&self, tenant_id: &str) -> Result<TenantUsage, TakoError>;
    async fn record(&self, tenant_id: &str, usd: f64, tokens: u64) -> Result<(), TakoError>;
}

Two backends ship out of the box:

• InMemoryBudgetBackend — single-process deployments (default).
• RedisBudgetBackend — multi-replica deployments. Backed by a monotonic-write Lua script so a slow replica cannot clobber a higher water-mark. Behind the tako-runtime/redis cargo feature; mirror in Python is tako.Client(budget_backend=tako.RedisBudgetBackend(...)).

BudgetTracker is wired through tako.SingleAgent, tako.Conductor, tako.Trinity, and tako.guards.LlmJudge — every provider call counts against the same caps, including the judge in SelfCaller.

Cost estimation

Providers expose estimate_cost_usd(messages: &[Message]). Most provider crates use the per-million-token rates from their Capabilities struct; you can override these at builder time:

provider = tako.providers.OpenAI(
    model="gpt-5",
    api_key="...",
    capabilities=tako.Capabilities(
        max_context_tokens=128_000,
        usd_per_input_mtok=2.50,
        usd_per_output_mtok=10.00,
        ...
    ),
)

Estimation is conservative: it assumes the worst-case output length up to max_tokens (or 1024 if unset).