fix(core): broadcast channel output for runs without inbound request context

[CalebBarnes]

What this ships

Agent runs on a channel-backed thread (Slack, Discord, etc.) now post their output back to the channel even when the run did not start from an inbound platform message. Heartbeat-triggered runs, Studio runs, and custom-UI / user-code runs against a channel thread previously produced no channel output at all — they were silently dropped.

Why

ChatChannelOutputProcessor only rendered to the channel when AgentChannels stashed a render context on requestContext during an inbound webhook (processChatMessage, approve/decline). Any run that did not originate from a platform message had no such context, so the processor early-returned and nothing reached the channel. Channel delivery was effectively coupled to the inbound request path.

How

• ChatChannelOutputProcessor is now bound to its owning AgentChannels at construction.
• When no inbound render context is present, the processor reconstructs it from the run's threadId (provided by the processor-identity work) via a new AgentChannels.buildRenderContextForThread(threadId). That method reads the thread's persisted channel coordinates (channel_platform, channel_externalThreadId), materializes the live channel thread handle, and delegates to the same _buildRenderContext the inbound paths use — so both paths produce an identical render context (single source of truth).
• The inbound requestContext path is unchanged and still takes precedence (fast path).
• Non-channel threads pass through untouched. The resolved render context (or null) is cached on state so it is built once per run, not per chunk.

Nothing new is persisted: every non-serializable field (adapter, closures, logger) is rebuilt live from the bound AgentChannels.

Stacking

Stacked on #18552 (processor context identity), which threads threadId to the output processor. This PR targets that branch and stays a draft until #18552 merges.

Test plan

• pnpm build:core
• pnpm --filter ./packages/core check (typecheck clean)
• pnpm --filter ./packages/core test -- src/channels/ — all 238 channels tests pass, including 5 new fallback tests in output-processor.test.ts:
  • broadcasts on a channel-backed thread with no requestContext key
  • passes through when the thread has no channel metadata
  • passes through when the thread does not exist in storage
  • passes through when no threadId is provided
  • prefers the inbound requestContext render context over the fallback (no regression)

[changeset-bot]

🦋 Changeset detected

Latest commit: 7e6ca73

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 22 packages

Name	Type
@mastra/core	Patch
mastracode	Patch
@mastra/mcp-docs-server	Patch
@internal/playground	Patch
@mastra/client-js	Patch
@mastra/opencode	Patch
@mastra/longmemeval	Patch
mastra	Patch
@mastra/deployer-cloud	Patch
@mastra/react	Patch
@mastra/playground-ui	Patch
@mastra/server	Patch
@mastra/deployer	Patch
create-mastra	Patch
@mastra/express	Patch
@mastra/fastify	Patch
@mastra/hono	Patch
@mastra/koa	Patch
@mastra/nestjs	Patch
@mastra/next	Patch
@mastra/tanstack-start	Patch
@mastra/temporal	Patch

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Project	Deployment	Actions	Updated (UTC)
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[CalebBarnes]

Superseded by #18630, which collapses this two-PR stack into a single self-contained PR. Instead of threading new identity fields through the whole processor surface (old #18552), #18630 reads the run's threadId from the framework-populated MastraMemory request-context key via the validated parseMemoryRequestContext accessor — no processor API changes needed. The channels fix (this PR's content) is unchanged.