Summary

Fixes the confirmed findings from a full architecture review of all three packages, developed test-first (10+ new tests written red-first).

P0 — correctness

• Middleware session isolation: a middleware created at module scope previously shared one Janitor across every conversation it served — token-usage feeds, E10 compression suppression, and circuit-breaker counts leaked between users. Janitor state is now pooled per session (providerOptions: { contextChef: { sessionId } } in the AI SDK middleware, ctx.conversationId in TanStack). Calls without a session identity share the default slot (prior behavior, zero breaking change). New maxSessions cap (default 256, LRU, validated).
• Gemini tool-call correlation: fromGemini's correlation state was scoped to a single message, so functionResponses arriving in a later content entry always fell back to the -0 ID — repeat calls of the same tool produced duplicate IDs and misattributed results. Now per-name counters + FIFO queues span the conversation.

P1 — failure-path hardening

• Compression-model failures no longer inject the raw error text into the LLM-bound fallback summary (goes to logger instead).
• onCompress and onBeforeCompress now share one degradation stance: a throwing hook is caught and logged, never fails compile() (JSDoc contracts updated).
• Invalid session keys (empty string / non-string / malformed namespace) warn once and route to the default slot — semantics unified across both middleware packages.
• Janitor missing-config nags fire once per middleware instead of once per session.
• SessionPool rejects non-positive maxSize (would silently disable pooling).

Cleanup

• Non-text tool-result parts flatten to typed placeholders ([tool result file: image/png (chart.png)]) instead of being silently dropped; the truncator's duplicate extraction delegates to the same implementation.
• Shared role-flattening extracted to core as flattenForCompression (both middlewares reuse it; ~120 lines of copy-paste removed).
• OpenAIAdapter.compile drops the per-message JSON.parse(JSON.stringify(...)) round-trip for a direct clone (honors toJSON, so Date still lands as ISO string).
• Placeholder vocabulary centralized in Prompts; peerDependencies tightened to ^4.0.0 / ^7.0.0.

Changesets included for all three packages (core minor, ai-sdk-middleware minor, tanstack-ai minor).

Test plan

• 925 tests pass (core 733, ai-sdk-middleware 123, tanstack-ai 69) — includes new red-first tests for every behavioral fix
• pnpm -r run typecheck clean
• biome check . clean
• pnpm -r run build clean