Summary

This PR resolves the launch-blocking context window crash by implementing a 3-phase fix across Gateway and Web client:

• Phase 0: Human-readable, actionable context-overflow messaging and recovery actions
• Phase 1: Near-limit warning surfaced to users before hard failure
• Phase 2: Proactive Gateway-side context management with deterministic compaction/summarization

What changed

Phase 0 (immediate user recovery)

• Replaced technical overflow text with user-safe actionable copy in:
  • builds/typescript/engine/errors.ts
  • builds/typescript/engine/loop.ts
• Added overflow-specific chat actions:
  • Start New Conversation
  • Continue in New Conversation
• Suppressed irrelevant Open Settings route for context-overflow errors

Phase 1 (near-limit prevention)

• Added Gateway context warning metadata on /message response headers
• Parsed warning metadata in client adapter/hook
• Rendered “session getting long” warning indicator in chat UI

Phase 2 (proactive context management)

• Added Gateway context-window manager:
  • Token estimation for prompt + tool schemas
  • Deterministic replay-unit budgeting
  • Preservation of assistant tool-call + tool-result continuity
  • Automatic compaction/summarization of older turns
  • Summary artifact persistence under memory conversations

Files touched (high level)

• Gateway:
  • builds/typescript/gateway/context-window.ts (new)
  • builds/typescript/gateway/server.ts
• Engine:
  • builds/typescript/engine/errors.ts
  • builds/typescript/engine/loop.ts
• Web client:
  • builds/typescript/client_web/src/api/gateway-adapter.ts
  • builds/typescript/client_web/src/api/useGatewayChat.ts
  • builds/typescript/client_web/src/components/chat/ChatPanel.tsx
  • builds/typescript/client_web/src/components/chat/ErrorMessage.tsx
  • related type and test files

Tests

Added/updated tests for:

• Context overflow error classification and messaging
• Gateway context-window compaction and summary artifact behavior
• Header-driven near-limit warning propagation
• Hook/UI overflow-specific action paths and warning rendering

Validation run

• Backend:
  • npm run build
  • npm test -- engine/errors.test.ts gateway/context-window.test.ts
• Web:
  • npm run typecheck
  • npm test -- src/api/gateway-adapter.test.ts src/api/useGatewayChat.test.tsx src/components/chat/ChatPanel.test.tsx

All passed locally.

Architecture alignment

No contract drift introduced:

• Gateway->Engine handoff remains messages + metadata
• Context budgeting remains Gateway-owned
• Engine stream event taxonomy unchanged
• No request-metadata config side channel introduced