hapi/web/src/chat/modelConfig.ts

import { isClaudeModelPreset } from '@hapi/protocol'

/**
 * Context windows vary by model/provider and may change over time.
 *
 * The UI only needs this to compute a conservative "context remaining" warning.
 * We intentionally keep a headroom budget to avoid false confidence near the limit
 * (system prompts, tool overhead, and other hidden tokens can consume extra space).
 *
 * If/when the server provides an explicit per-session context limit, prefer that
 * and use this only as a fallback.
 */
const CONTEXT_HEADROOM_TOKENS = 10_000
const DEFAULT_CLAUDE_CONTEXT_WINDOW_TOKENS = 200_000
const LARGE_CLAUDE_CONTEXT_WINDOW_TOKENS = 1_000_000
// Fallback for Codex sessions when the server has not reported an explicit modelContextWindow.
// The value matches the context window currently reported by Codex App Server token-count events.
const DEFAULT_CODEX_CONTEXT_WINDOW_TOKENS = 258_400

export function getContextBudgetTokens(model: string | null | undefined, flavor?: string | null): number | null {
    if (flavor === 'codex') {
        return Math.max(1, DEFAULT_CODEX_CONTEXT_WINDOW_TOKENS - CONTEXT_HEADROOM_TOKENS)
    }

    if (flavor !== 'claude') {
        return null
    }

    const trimmedModel = model?.trim()
    const windowTokens = (() => {
        if (!trimmedModel) {
            return DEFAULT_CLAUDE_CONTEXT_WINDOW_TOKENS
        }
        if (isClaudeModelPreset(trimmedModel)) {
            return trimmedModel.endsWith('[1m]')
                ? LARGE_CLAUDE_CONTEXT_WINDOW_TOKENS
                : DEFAULT_CLAUDE_CONTEXT_WINDOW_TOKENS
        }
        if (trimmedModel.startsWith('claude-')) {
            return DEFAULT_CLAUDE_CONTEXT_WINDOW_TOKENS
        }
        return null
    })()

    if (!windowTokens) return null
    return Math.max(1, windowTokens - CONTEXT_HEADROOM_TOKENS)
}