Claude Builder Weekly: Fallback Models, Middleware Support & Agent SDK Hardening

TypeScript SDK v0.102.0: Managed Agents Type Updates & Middleware Fix

• Released 2026-06-06, this minor bump ships small but breaking type changes to Managed Agents alongside the middleware-before-signing bug fix that also landed in the cloud SDKs.

TypeScript SDK v0.101.0: Middleware Support, Streaming Fixes & Timeout Scoping

• Released 2026-06-05, this release adds a first-class middleware hook for intercepting requests, fixes request timeouts to scope only to the inner fetch (not the full middleware chain), and corrects stop_details propagation and scientific-notation JSON parsing in streaming.

Bedrock SDK v0.30.0/v0.30.1: Middleware & Claude Opus 4.1 Deprecated

• Released 2026-06-05–06, the Bedrock TypeScript SDK gains middleware support, marks Claude Opus 4.1 as deprecated, and patches middleware running before request signing.

AWS SDK v0.4.0/v0.4.1: Middleware Support & Opus 4.1 Deprecation

• Released 2026-06-05–06, the AWS-flavor TypeScript SDK adds middleware support, deprecates Claude Opus 4.1, and fixes middleware ordering relative to request signing.

Vertex SDK v0.17.0: Middleware & Claude Opus 4.1 Deprecated

• Released 2026-06-05, the Vertex TypeScript SDK lands middleware support and marks Claude Opus 4.1 deprecated, matching parity with other cloud flavors.

Foundry SDK v0.3.0: Middleware & Claude Opus 4.1 Deprecated

• Released 2026-06-05, the Foundry TypeScript SDK receives middleware support and deprecates Claude Opus 4.1 in line with all other cloud SDK variants.

Claude Code v2.1.166: Fallback Models, Glob Deny Rules & Cross-Session Security

• The headline release of this cycle adds `fallbackModel` config for up to three ordered fallback models on overload, glob-pattern support in deny rules (`"*"` blocks all tools), hardened cross-session messaging that strips user authority from relayed `SendMessage` requests, and the ability to disable thinking via `MAX_THINKING_TOKENS=0`.
• Also fixes 20+ bugs spanning JetBrains flickering, Kitty keyboard protocol drops, Windows PowerShell hangs, and more.

Editor’s read: Must-read: fallback model config and the cross-session security hardening are both production-critical for multi-agent deployments.

Claude Code v2.1.163: Version Range Enforcement, `/plugin list` & Hook `additionalContext`

• Admins can now enforce `requiredMinimumVersion`/`requiredMaximumVersion` in managed settings, blocking out-of-range Claude Code installs at startup.
• Stop/SubagentStop hooks gain `additionalContext` to keep a turn alive without triggering a hook error.

Claude Code v2.1.162: `claude agents --json` `waitingFor` Field & Wide-Terminal Fixes

• The `claude agents --json` output now includes a `waitingFor` field showing exactly what a blocked session is waiting on (e.g.
• a permission prompt).
• Numerous `claude agents` UI fixes address truncated status text on wide terminals and Ctrl+V image paste.

Claude Code v2.1.161: OTEL Custom Dimensions & Parallel Tool Call Independence

• OTEL `RESOURCE_ATTRIBUTES` values are now propagated as metric labels for slicing usage by team or repo.
• A failed Bash command in a parallel batch no longer cancels sibling tool calls — each returns its own result.

Claude Code v2.1.160: Shell Startup File Write Prompt & `ultracode` Rename

• Claude Code now prompts before writing to shell startup files or build-tool configs that grant code execution.
• The dynamic-workflow trigger keyword is renamed from `workflow` to `ultracode`; the old keyword is now a no-op.

Claude Agent SDK TypeScript v0.3.166: MCP Runtime Server Injection Fixed

• Fixed MCP resource tools not being injected for servers added at runtime via the `mcp_set_servers` control request — a critical correctness fix for dynamic MCP topologies.

Claude Agent SDK TypeScript v0.3.163: `stop_task` Idempotency & Hook `additionalContext`

• `stop_task` now returns success when the target task is already gone, enabling reliable pruning of stale task chips.
• Stop and SubagentStop hooks support `additionalContext` in `hookSpecificOutput` for non-error turn continuation.

Claude Agent SDK TypeScript v0.3.162: Refusal Detection via `stop_reason` & Search Tool Defaults

• Refusal responses now carry `stop_reason: "refusal"` and `stop_details` on the assistant message, removing the need for error-text matching.
• SDK sessions on native builds now default to embedded find/grep instead of registering dedicated Grep/Glob tools.

Claude Agent SDK TypeScript v0.3.161: Idempotent `initialize` & Live Agent Switching

• A second `initialize` control request now returns the same success payload instead of an error.
• `applyFlagSettings` supports live agent switching mid-session — pass `null` to reset to the default agent.

Claude Agent SDK TypeScript v0.3.160: Abort-During-Hook No Longer Hangs

• Fixed SDK hook callbacks swallowing abort signals — aborting during a PostToolUse hook now cleanly ends the turn with a final `result` message rather than hanging the calling process.

Claude Agent SDK Python v0.2.88: Trio Compatibility for Session Stores

• Fixed a crash (`TypeError: trio.run received unrecognized yield message`) when using `session_store=` with `query()` or `ClaudeSDKClient` under trio — all session-store code paths are now ported to anyio.

Claude Agent SDK Python v0.2.91: Anyio Test Suite (asyncio + trio CI Coverage)

• Switched the test suite from `pytest-asyncio` to anyio's pytest plugin, running every async test under both asyncio and trio backends to catch backend-specific regressions in CI.

Claude Code v2.1.162: Sub-1000ms MCP Tool Timeout Floor Removed

• MCP per-server `timeout` values below 1000 ms were previously floored to a 1-second watchdog that aborted every tool call; sub-1000 ms values now fall back gracefully to `MCP_TOOL_TIMEOUT` or the default, and `claude mcp get` annotates them.

Claude Code v2.1.161: `/mcp` Hides Unused claude.ai Connectors

• The `/mcp` panel now collapses claude.ai connectors you've never signed into behind a 'Show unused connectors' row, reducing noise in the MCP configuration view.