# Agent Note: SDK max output tokens

Status: implemented

English | [中文](2026-07-28-sdk-max-output-tokens.zh.md)

## Problem

The Python and TypeScript SDKs could select a provider and model but could not bound conversation-model output. The runtime therefore omitted `GenerateOptions.maxTokens`, leaving provider defaults in control even when an evaluation host required a fixed output budget. `compact-basic.maxTokens` could not fill this role because it limits only compaction-summary calls.

## Decision

The high-level SDKs expose one optional process-wide output cap: Python names it `max_tokens`, TypeScript names it `maxTokens`, and the shared `initialize` wire payload carries `maxTokens`. The JSON-RPC server rejects values that are not positive safe integers and stores the accepted cap with its provider/model route.

Each SDK-created root Agent receives the cap through `AgentOptions.maxTokens`. Agent Loop places that value in the initial `LlmCallConfig`, logs it in the request header, and reconstructs every dispatched conversation request from that durable header. Omitting the option leaves `maxTokens` absent so the selected provider retains its default.

In-process subagents inherit the parent's provider, model, and output cap. An explicit `SubagentStartRequest.agentOptions.maxTokens`, including one configured by `dsh-tool-subagent`, overrides the inherited value for that child and its descendants. Out-of-process providers own the configuration of their separate runtime; `subagent-dsh-sdk` therefore exposes its own optional `maxTokens` and forwards it through that child runtime's SDK handshake.

Compaction, session-title generation, web search, and other auxiliary calls keep their independently owned output limits. `maxTokensAsSuccess` remains outcome mapping only: it does not set or alter the cap.

## Alternatives considered

**Set an adapter environment variable.** This would be DeepSeek-adapter-specific, invisible in the session request header, ineffective for intercepted or alternate adapters, and easy to confuse with a provider default. The cap belongs in provider-neutral request configuration.

**Add `maxTokens` to every `session/prompt`.** Per-turn mutation would enlarge the wire and introduce request-config transitions that callers do not need for the current evaluation use case. A runtime initialization option gives every session in one SDK process the same reproducible budget.

**Reuse `compact-basic.maxTokens`.** The compaction value controls summary generation, not ordinary conversation requests. Sharing it would couple two different token budgets and make tuning one silently change the other.

## Consequences

SDK callers can bound model output without editing Cordis composition, and direct Agent creation uses the same validated `AgentOptions` contract. The cap is visible in durable request headers and reaches provider adapters as `GenerateOptions.maxTokens`; DeepSeek serialization maps it to `max_tokens`.

One SDK runtime has one default cap. A caller needing different caps runs separate runtime instances or explicitly overrides an in-process child through its agent options. Reaching the cap still produces the existing `max-tokens` stop reason, whose `ok` or `error` mapping remains deployment policy.
