feat(runtimes): mastra sub-agent streaming via a public-API stream tee - #961
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…eam tee Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>
…d-mount finding Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>
…the bridge consumes them Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>
…CALL_* and attributed child deltas Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>
…tream Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>
Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>
Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>
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…, superseded marker, tee edge tests - Correct the emitter line-count claim in the blog post and adapter-choice docs from a stale "125 to 310" to the measured "190 to 360" (aws-strands 189, microsoft-agent-framework 309, mastra emitter+tee 287+76=363). - Mark the wire-capture-subagents.md verdict-summary row for incremental child streaming as superseded, pointing at the After streaming section that documents the actual (later) behavior. - Add streaming-tee tests: consumer early break on the wrapped fullStream propagates to the source generator's return(); traceId/usage defined as prototype getters still resolve through the result proxy. - Add a subagent-emitter test: an agent-* tool-call chunk with no prior start/step-start synthesizes TOOL_CALL_START with the parentMessageId key entirely absent, not present-but-undefined. - Annotate the tee's has() trap as a deliberate restatement of default behavior rather than dead code. Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>
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Summary
The Mastra demo's subagent card now streams the child's deltas live, flipping the matrix cell from Partial to Yes. Spec:
docs/superpowers/specs/2026-09-02-mastra-subagent-streaming-design.md; plan:docs/superpowers/plans/2026-09-02-mastra-subagent-streaming.md.Research verdict (why this layer):
@ag-ui/mastra@1.1.2is the latest pin and drops everytool-outputchunk (case "tool-output": break) while buffering the delegation'sTOOL_CALL_START/ARGS/ENDuntil the tool result. There is no alternative upstream layer, the officialonChunkhook excludestool-output, and a bridge subclass would couple to three TS-private methods plus aclone()that rebuilds the base class. The child's chunks are, however, public and typed on the parentfullStream({type:'tool-output', payload:{output:<childChunk>, toolCallId, toolName}}), so the fix observes them through a public-API seam. Pins unchanged (@ag-ui/mastra@1.1.2,@mastra/core@1.63.2); the bridge is not modified.What changed
deployments/ag-ui-mastra/streaming-tee.mjs(new):withDelegationTee(agent, observe)— aProxyover the realAgentthat forwards every member bound to the real instance (#privatefields and the bridge's'getMemory' in agentcheck keep working) and wraps onlystream()/resumeStream()results sofullStreamis an async generator that callsobserve(chunk)before yielding to the bridge. One reader, one iteration, so observer writes are ordered ahead of the bridge's events for the same chunk.deployments/ag-ui-mastra/subagent-emitter.mjs: the per-run injector gainschunk(c)alongsideeventsFor(e). The delegationtool-callchunk synthesizes eagerTOOL_CALL_START/ARGS/END(args are complete there;parentMessageIdfrom the laststart/step-startchunk) +SUBAGENT_STARTED;tool-outputinnertext-start/delta/endbecome attributedTEXT_MESSAGE_START/CONTENT/END;tool-result/tool-error/tool-call-suspendedclose the card. The bridge's later bufferedTOOL_CALL_START/ARGS/ENDcopies for a synthesized id are dropped ineventsFor(itsTOOL_CALL_RESULTpasses through). The old single-chunk synthesis stays only as the no-deltas fallback.deployments/ag-ui-mastra/server.mjs: builds the proxy per request; both inputs write through the same SSE frame writer.choosing-an-adapter, runtimes intro, Mastra overview and how-it-connects, both runtime blog posts.Evidence
fullStream, 248 chunks):tool-call(args complete) →tool-outputx135 (explicittext-start, 108text-delta,text-end) →tool-result;tool-call-suspendednever appears;chat-tool-callsanchors cards to entries in the parenttoolCalls()list, so the eagerTOOL_CALL_STARTis required for the card to mount.TOOL_CALL_START+SUBAGENT_STARTEDat t=1578 ms; 94 attributedTEXT_MESSAGE_CONTENT(399 chars) t=6266–7183 ms;SUBAGENT_FINISHEDbeforeTOOL_CALL_RESULTat t=7382 ms; exactly oneTOOL_CALL_STARTfor the delegation id.data-state="running"): 67 → 110 → 188 → 261 → 279 → 306 → 330 → 457 → 494 chars, thendone. Screenshot:cockpit/runtimes/mastra/angular/e2e/manual/subagent-card-live.png.Tallies
deployments/ag-ui-mastra): 42/42 (8 tee, 24 injector, 10 service).cockpit/runtimes/mastra/angular: 5/5, assertions unchanged.npx nx test website: green.Caveats
TOOL_CALL_STARTis already painted when a child suspends (the bridge would have retracted its buffered copy); the injector closes the card withSUBAGENT_FINISHED {outcome:{type:'suspended'}}. The demo's delegation cannot suspend; the path is unit-tested only.tool-output).tool-outputmapping patch against the@ag-ui/mastraintegration, referencing #2402/#2403 for the eager-START half.🤖 Generated with Claude Code