perf(workflow): reduce doStreamStep step-boundary payload

[VaguelySerious]

Summary

Port of vercel/workflow#1932 to @ai-sdk/workflow.

doStreamStep built the full StepResult inside the step and also returned a per-chunk snapshot array. Both were serialized into the durable event log on every step. But:

• the iterator never read the chunk array (pure dead payload), and
• StepResult carries redundant copies of data: content (= text + tool calls), the duplicate toolCalls/dynamicToolCalls lists, reasoningText (= joined reasoning), and the always-empty *ToolResults arrays.

Change

The step now returns only the minimal raw aggregates needed to rebuild the StepResult — text, reasoning, responseMetadata, warnings — alongside the existing finish and toolCalls. The iterator reconstructs the full StepResult via a new buildStepResult helper outside the step boundary.

• The user-facing StepResult (yielded steps, onStepEnd, telemetry) is byte-for-byte the same shape; only the durable event-log payload shrinks.
• runtimeContext / toolsContext / stepNumber no longer cross the boundary either — the iterator already holds them and applies them during reconstruction.

Tests

• stream-text-iterator.test.ts mocks updated to the new { toolCalls, finish, raw, providerExecutedToolResults } shape; the contexts test now asserts the contexts are applied to the reconstructed step and yield value (rather than passed across the boundary).
• The unmocked end-to-end workflow-agent tests exercise the real doStreamStep → buildStepResult path and pass unchanged, confirming the reconstruction is faithful.
• pnpm --filter @ai-sdk/workflow build && test pass (node + edge).

Related

• Sibling PR: [ai] Reduce doStreamStep step boundary payload workflow#1932 (the @workflow/ai copy)
• Original issue: DurableAgent/WorkflowAgent duplicates doStreamStep return size workflow#1929

Opened as a draft — this is an AI-assisted port. The two copies of this package have diverged (vercel/ai's doStreamStep is streamModelCall-based and built the StepResult inline, with no separate chunksToStep), so this applies the same optimization rather than cherry-picking the diff; please review the adaptation before merge.

🤖 Generated with Claude Code

[gr2m]

Reviewed: the payload reduction is a faithful field-for-field move of the StepResult construction out of the step boundary, the dropped chunks array was dead payload, and doStreamStep is internal-only. Replay compat across the shape change is acceptable while the package is experimental.

[github-actions]

🚀 Published in:

Package	Version
ai	7.0.17 github npm
@ai-sdk/amazon-bedrock	5.0.13 github npm
@ai-sdk/angular	3.0.17 github npm
@ai-sdk/anthropic	4.0.9 github npm
@ai-sdk/anthropic-aws	2.0.1 github npm
@ai-sdk/gateway	4.0.13 github npm
@ai-sdk/google	4.0.9 github npm
@ai-sdk/google-vertex	5.0.12 github npm
@ai-sdk/harness	1.0.20 github npm
@ai-sdk/harness-claude-code	1.0.20 github npm
@ai-sdk/harness-codex	1.0.21 github npm
@ai-sdk/harness-deepagents	1.0.19 github npm
@ai-sdk/harness-opencode	1.0.20 github npm
@ai-sdk/harness-pi	1.0.20 github npm
@ai-sdk/langchain	3.0.17 github npm
@ai-sdk/llamaindex	3.0.17 github npm
@ai-sdk/mcp	2.0.8 github npm
@ai-sdk/mistral	4.0.6 github npm
@ai-sdk/otel	1.0.17 github npm
@ai-sdk/policy-opa	1.0.17 github npm
@ai-sdk/react	4.0.18 github npm
@ai-sdk/rsc	3.0.17 github npm
@ai-sdk/sandbox-just-bash	1.0.20 github npm
@ai-sdk/sandbox-vercel	1.0.20 github npm
@ai-sdk/svelte	5.0.17 github npm
@ai-sdk/tui	1.0.17 github npm
@ai-sdk/vue	4.0.17 github npm
@ai-sdk/workflow	1.0.17 github npm
@ai-sdk/workflow-harness	1.0.20 github npm
@ai-sdk/xai	4.0.8 github npm