Mastra `@mastra/core@1.41.0` adds per-request sandbox routing

PaulieScanlon published `@mastra/core@1.41.0` on June 5, 2026, with three headline changes to the Mastra agent framework's Workspace and streaming APIs. The most significant addition is per-request sandbox routing: the Workspace `sandbox` config now accepts either a static `WorkspaceSandbox` instance or a resolver function `({ requestContext }) => WorkspaceSandbox`, allowing a single Workspace to provision isolated sandboxes per user or role. By default, building workspace instructions no longer invokes the resolver — instead, stable placeholder text is inserted into the agent's system prompt to keep it cache-friendly. Callers can opt into concrete per-request instructions via `instructions.dynamicSandbox: 'resolve'` or a custom function. When using a resolver, the caller owns the returned sandbox's lifecycle; `mounts` throws an `INVALID_CONFIG` error and `lsp: true` is disabled with a warning because both require a concrete sandbox instance upfront.

The second addition, `sandboxCacheKey`, solves continuity for background processes in dynamic-sandbox deployments.

The second addition, `sandboxCacheKey`, solves continuity for background processes in dynamic-sandbox deployments. Setting `sandboxCacheKey` to a stable identifier (such as a thread ID from `requestContext`) ensures that `execute_command({ background: true })`, `get_process_output`, and `kill_process` all attach to the same sandbox across follow-up requests. Failed resolver calls are automatically evicted from the cache so retries can succeed, and `workspace.clearSandboxCache(cacheKey)` is available for manual lifecycle management.

The third change consolidates streaming behavior: `stream()` and `resumeStream()` now accept `untilIdle: true` or `untilIdle: { maxIdleMs }` to keep streams open across background-task continuations. The previously dedicated `streamUntilIdle()` and `resumeStreamUntilIdle()` methods — and their `/stream-until-idle` and `/resume-stream-until-idle` server endpoints — are deprecated but continue to work internally by delegating to the new option. The same `untilIdle` field is supported across server endpoints and client SDKs. The release also includes a patch to `@mastra/acp@0.2.1` fixing ACP tools to keep their default session alive across executions. No breaking changes are noted in this release.

Key facts

1. 01Workspace `sandbox` now accepts a resolver function `({ requestContext }) => WorkspaceSandbox` for per-request sandbox routing in multi-tenant deployments.
2. 02`sandboxCacheKey` keeps background process tools (`execute_command` with `background: true`, `get_process_output`, `kill_process`) attached to the same sandbox across follow-up requests.
3. 03Stable placeholder text is used in agent system prompts by default when a resolver is configured; opt into dynamic instructions via `instructions.dynamicSandbox`.
4. 04`mounts` throws `INVALID_CONFIG` and `lsp: true` is disabled with a warning when a sandbox resolver is used.
5. 05`stream()` and `resumeStream()` now accept `untilIdle: true` or `untilIdle: { maxIdleMs }` to keep streams open across background-task continuations.
6. 06`streamUntilIdle()`, `resumeStreamUntilIdle()`, and their dedicated server endpoints are deprecated in favor of the new `untilIdle` option.
7. 07`@mastra/acp@0.2.1` patches ACP tools to maintain their default session across executions.

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