fix(chat): render subagent content as collapsible thinking blocks by waleedlatif1 · Pull Request #3602 · simstudioai/sim

waleedlatif1 · 2026-03-15T11:13:22Z

Summary

Fix SSE content routing so subagent text goes to subagent_text blocks instead of leaking into main chat text
Add SubagentThinkingBlock component with streaming text carousel, shimmer animation, and collapsible "Thought for Xs" label
Thread duration and isStreaming through AgentGroup and MessageContent
Handle reasoning SSE events correctly (only render inside subagents, drop otherwise)

Type of Change

Bug fix
New feature

Testing

Tested manually

Checklist

Code follows project style guidelines
Self-reviewed my changes
Tests added/updated and passing
No new warnings introduced
I confirm that I have read and agree to the terms outlined in the Contributor License Agreement (CLA)

cursor · 2026-03-15T11:13:28Z

PR Summary

Medium Risk
Touches SSE streaming block construction and persistence, so regressions could mis-route assistant text or break replay of partial streams; UI changes are localized to subagent rendering.

Overview
Subagent output is now rendered as a dedicated collapsible “thinking” UI instead of plain text. AgentGroup replaces raw <p> rendering with a new SubagentThinkingBlock that supports shimmer “Thinking” state, character-by-character streaming, auto-scroll, and a collapsed “Thought for X” label once complete.

Streaming/persistence now track subagent text separately and record subagent durations. use-chat routes content/reasoning events into subagent_text blocks only when a subagent is active (preventing leakage into main chat text), stamps elapsed time between subagent_start/subagent_end onto the subagent marker, and threads duration through stored block types and MessageContent into AgentGroup to drive the completed label.

^{Written by Cursor Bugbot for commit 1805b36. Configure here.}

vercel · 2026-03-15T11:13:28Z

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apps/sim/app/workspace/[workspaceId]/home/components/message-content/message-content.tsx

waleedlatif1 · 2026-03-15T11:21:21Z

Fixed the cursor bot's finding — when the subagent block arrives for an already-open group, we now copy block.duration onto the existing group instead of skipping it entirely.

…ping

waleedlatif1 · 2026-03-15T11:21:46Z

@greptile

waleedlatif1 · 2026-03-15T11:21:49Z

@cursor review

greptile-apps · 2026-03-15T11:26:16Z

Greptile Summary

This PR fixes SSE content routing so subagent text and reasoning tokens are correctly directed to subagent_text blocks instead of leaking into the main chat text, and introduces a new SubagentThinkingBlock component that renders subagent output as a collapsible "Thought for Xs" thinking block with a streaming shimmer animation — mirroring the existing copilot ThinkingBlock UX pattern.

Key changes:

use-chat.ts: content/reasoning events inside an active subagent are routed to subagent_text blocks; subagentStartTime is used to calculate elapsed duration which is stamped onto the subagent marker block on subagent_end; streamingContentRef assignment correctly moved to the main-text branch only
subagent-thinking-block.tsx: New component with rAF-driven streaming text carousel and CSS-module shimmer animation; userCollapsedRef prevents the auto-expand from overriding an explicit user collapse mid-stream
agent-group.tsx: duration and isStreaming props threaded through; duration === undefined used as a proxy to identify the still-active subagent among a group
message-content.tsx: parseBlocks updated to propagate duration from subagent marker blocks into AgentGroupSegment; global isStreaming passed to all AgentGroup instances
types.ts / tasks.ts: duration?: number added to ContentBlock and TaskStoredContentBlock for persistence

Confidence Score: 4/5

Safe to merge with minor style and UX polish items to address.
The core SSE routing fix and duration-stamping logic are sound and well-constructed. Previous review threads have been addressed (CSS module shimmer, streamingContentRef placement). Remaining concerns are all non-blocking: the lastTextIdx scan is a minor style/perf note, the silent drop of orphaned subagent_text is a robustness gap rather than a production bug, and the button-enabled-before-content issue is an edge-case UX regression that only affects users who click the "Thinking" label before any content arrives.
subagent-thinking-block.tsx (button disabled condition) and message-content.tsx (subagent_text silent drop) warrant a second look before merge.

Important Files Changed

Filename	Overview
apps/sim/app/workspace/[workspaceId]/home/hooks/use-chat.ts	SSE routing refactored to send content/reasoning events inside active subagents to `subagent_text` blocks instead of the main text block; `subagentStartTime` tracks elapsed time and stamps `duration` onto the subagent marker on `subagent_end`; `streamingContentRef` correctly moved to the `else` branch. Logic is sound for sequential subagents.
apps/sim/app/workspace/[workspaceId]/home/components/message-content/components/agent-group/agent-group.tsx	Replaces plain `<p>` with `SubagentThinkingBlock`; uses `duration === undefined` as a proxy for "subagent still active" to derive per-item `isStreaming`. The `lastTextIdx` backward scan runs on every render without memoization — a minor style issue.
apps/sim/app/workspace/[workspaceId]/home/components/message-content/components/agent-group/subagent-thinking-block.tsx	New collapsible thinking block component with rAF-driven streaming text and shimmer animation. The toggle button is enabled during streaming even with no content yet, which can suppress the intended auto-expand via `userCollapsedRef`. CSS module shimmer (addressing previous thread) is correctly used.
apps/sim/app/workspace/[workspaceId]/home/components/message-content/message-content.tsx	`parseBlocks` updated to propagate `duration` from the subagent marker block to the segment, and `isStreaming` + `duration` are threaded down to `AgentGroup`. `subagent_text` blocks with no active group are silently discarded without a warning — minor robustness gap.
apps/sim/app/workspace/[workspaceId]/home/components/message-content/components/agent-group/subagent-thinking-block.module.css	New CSS module for shimmer animation; correctly extracted from inline `<style>` injection (per previous review). Keyframe and `.shimmer` utility look correct.
apps/sim/app/workspace/[workspaceId]/home/types.ts	Adds optional `duration?: number` to `ContentBlock`; straightforward and backward-compatible.
apps/sim/hooks/queries/tasks.ts	Adds optional `duration?: number` to `TaskStoredContentBlock` API interface; backward-compatible with existing stored messages that lack the field.

Sequence Diagram

sequenceDiagram
    participant BE as Backend SSE
    participant UC as useChat (processSSEStream)
    participant MC as MessageContent (parseBlocks)
    participant AG as AgentGroup
    participant STB as SubagentThinkingBlock

    BE->>UC: subagent_start { subagent: "agent-A" }
    UC->>UC: activeSubagent = "agent-A"<br/>subagentStartTime = Date.now()<br/>blocks.push({ type: "subagent", content: "agent-A" })
    UC->>MC: flush() → re-render

    BE->>UC: content / reasoning chunk
    UC->>UC: last block is subagent_text?<br/>→ append chunk<br/>else push new subagent_text block
    UC->>MC: flush() → re-render
    MC->>AG: segment { duration: undefined, isStreaming: true }
    AG->>STB: isStreaming={true}, content, duration={undefined}
    STB-->>STB: auto-expand, shimmer "Thinking" label

    BE->>UC: subagent_end
    UC->>UC: elapsed = Date.now() - subagentStartTime<br/>blocks[j].duration = elapsed (backward scan)<br/>activeSubagent = undefined
    UC->>MC: flush() → re-render
    MC->>AG: segment { duration: 2000, isStreaming: true }
    AG->>STB: isStreaming={false}, duration=2000
    STB-->>STB: collapse, label → "Thought for 2s"

    BE->>UC: stream end (done event)
    UC->>MC: isStreaming = false
    MC->>AG: isStreaming={false}
    AG->>STB: isStreaming={false} (no change)

Comments Outside Diff (1)

apps/sim/app/workspace/[workspaceId]/home/components/message-content/message-content.tsx, line 69-78 (link)

subagent_text silently dropped when no active group

When !group is true (e.g. a subagent_text block arrives before its corresponding subagent marker due to an out-of-order flush, or after the group has already been pushed to segments), the block's content is silently discarded with continue. This creates a silent data-loss path that is hard to debug in production.

A defensive console.warn or console.error would surface this quickly during development/staging without affecting production behavior:

_{Last reviewed commit: 5eb93dd}

...kspaceId]/home/components/message-content/components/agent-group/subagent-thinking-block.tsx

apps/sim/app/workspace/[workspaceId]/home/hooks/use-chat.ts

cursor

✅ Bugbot reviewed your changes and found no new issues!

Comment @cursor review or bugbot run to trigger another review on this PR

…ef to main-only branch

waleedlatif1 · 2026-03-15T11:34:41Z

@cursor review

waleedlatif1 · 2026-03-15T11:34:42Z

@greptile

...rkspace/[workspaceId]/home/components/message-content/components/agent-group/agent-group.tsx

...kspaceId]/home/components/message-content/components/agent-group/subagent-thinking-block.tsx

apps/sim/app/workspace/[workspaceId]/home/hooks/use-chat.ts

cursor

✅ Bugbot reviewed your changes and found no new issues!

Comment @cursor review or bugbot run to trigger another review on this PR

waleedlatif1 · 2026-03-15T11:50:55Z

@cursor review

waleedlatif1 · 2026-03-15T11:50:56Z

@greptile

…ion on mid-stream stop

greptile-apps · 2026-03-15T11:56:43Z

...rkspace/[workspaceId]/home/components/message-content/components/agent-group/agent-group.tsx

+  let lastTextIdx = -1
+  for (let i = items.length - 1; i >= 0; i--) {
+    if (items[i].type === 'text') {
+      lastTextIdx = i
+      break
+    }
+  }


lastTextIdx scan should be memoized

The backward linear scan runs on every render, which happens on every SSE flush during streaming. For an AgentGroup with many items this is fine today (small N), but the scan is executed inside the component body without any memoization guard. Since items is a prop that changes reference on every parent re-render, wrapping this in useMemo makes the intent explicit and avoids the scan on renders where items hasn't changed.

Suggested change

let lastTextIdx = -1

for (let i = items.length - 1; i >= 0; i--) {

if (items[i].type === 'text') {

lastTextIdx = i

break

}

}

const lastTextIdx = useMemo(() => {

for (let i = items.length - 1; i >= 0; i--) {

if (items[i].type === 'text') return i

}

return -1

}, [items])

You'll also need to add useMemo to the import at line 3:

import { useEffect, useMemo, useRef, useState } from 'react'

greptile-apps · 2026-03-15T11:56:47Z

...kspaceId]/home/components/message-content/components/agent-group/subagent-thinking-block.tsx

+        type='button'
+        onClick={toggle}
+        disabled={!hasContent && !isStreaming}
+        className='group inline-flex items-center gap-1 text-left text-[13px] text-[var(--text-secondary)] transition-colors hover:text-[var(--text-primary)]'


Button clickable but visually inert when streaming with empty content

disabled={!hasContent && !isStreaming} leaves the button enabled whenever isStreaming is true, even before any content has arrived. At that point there is no ChevronDown affordance and no visible content to expand — yet a click calls toggle(), sets expanded=true, and if the user clicks again it sets userCollapsedRef.current = true, permanently suppressing the auto-expand that fires when the first chunk arrives.

A user who clicks the "Thinking" label impatiently (before content) will then see the block never auto-expand, since userCollapsedRef has been set:

Suggested change

type='button'

onClick={toggle}

disabled={!hasContent && !isStreaming}

className='group inline-flex items-center gap-1 text-left text-[13px] text-[var(--text-secondary)] transition-colors hover:text-[var(--text-primary)]'

disabled={!hasContent}

This disables the button until content exists, regardless of streaming state. The auto-expand effect already handles opening the block as soon as hasContent becomes true during streaming, so there's no need for early interaction.

cursor

Cursor Bugbot has reviewed your changes and found 1 potential issue.

^{Bugbot Autofix is OFF. To automatically fix reported issues with cloud agents, enable autofix in the Cursor dashboard.}

cursor · 2026-03-15T11:58:11Z

apps/sim/app/workspace/[workspaceId]/home/hooks/use-chat.ts

+                    blocks[j].duration = elapsed
+                    break
+                  }
+                }


Replayed SSE events produce near-zero subagent durations

Low Severity

During SSE stream reconnection, all cached batch events are enqueued as a single chunk and processed synchronously. subagentStartTime is set to Date.now() when subagent_start is processed, and subagent_end computes Date.now() - subagentStartTime moments later, yielding ~0ms. The 1s floor causes every replayed subagent to display "Thought for 1s" regardless of actual duration.

Additional Locations (1)

apps/sim/app/workspace/[workspaceId]/home/hooks/use-chat.ts#L489-L490

fix(home-chat): render subagent content as collapsible thinking blocks

91e1edd

cursor bot reviewed Mar 15, 2026

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apps/sim/app/workspace/[workspaceId]/home/components/message-content/message-content.tsx Show resolved Hide resolved

fix(home-chat): copy duration to existing agent group instead of skip…

756e86d

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fix(home-chat): extract shimmer to CSS module, move streamingContentR…

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waleedlatif1 changed the title ~~fix(home-chat): render subagent content as collapsible thinking blocks~~ fix(chat): render subagent content as collapsible thinking blocks Mar 15, 2026

greptile-apps bot reviewed Mar 15, 2026

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fix(home-chat): only shimmer last text item in agent group

1805b36

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fix(home-chat): deduplicate flush blocks copy, persist subagent durat…

5eb93dd

…ion on mid-stream stop

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Conversation

waleedlatif1 commented Mar 15, 2026

Summary

Type of Change

Testing

Checklist

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Greptile Summary

Confidence Score: 4/5

Important Files Changed

Sequence Diagram

Comments Outside Diff (1)

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Replayed SSE events produce near-zero subagent durations

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