useMemo fingerprint in CopilotChat ignores object-content messages (activity/tool messages never trigger re-render)

[dchmurny]

Description

CopilotChat (v2, packages/react-core/src/v2/components/chat/CopilotChat.tsx) memoizes the messages array it hands to its chatView slot using a fingerprint computed per message:

const contentKey = typeof m.content === "string" 
  ? m.content.length
  : Array.isArray(m.content) 
    ? m.content.length 
    : 0;

For messages whose content is a plain object (e.g. activity/progress messages shaped like { steps: [...] }), contentKey is always the constant 0 regardless of how the object's contents change. The dependency array of the useMemo therefore never changes for these messages, so CopilotChat never recomputes messages when only object-content fields mutate — downstream components (e.g. a custom chatView) receive a stale, frozen message even though the underlying agent state (via onActivitySnapshotEvent / onRunFinalized) is updating correctly.

This is distinct from #4676 (already fixed): that issue was about @ag-ui/client mutating message objects in place and MemoizedActivityMessage's shallow/reference comparator not detecting changes. That underlying fix is present and correct in 0.0.57 — but CopilotChat's own memo sits one layer above it and independently blocks the update via this fingerprinting bug.

Impact

Any custom activity/progress renderer registered via renderActivityMessages appears frozen on its first snapshot; a run's live step progress and completion state (e.g. "Finished in Xs") never render, even though the agent event stream is correct.

Reproduction

1. Register a custom activity renderer via renderActivityMessages that emits ACTIVITY_SNAPSHOT events with replace: true and object content (e.g. { steps: [{ name: 'step1', status: 'running' }] })
2. Emit a second ACTIVITY_SNAPSHOT with the same messageId but different content (e.g. { steps: [{ name: 'step1', status: 'done' }] })
3. Observe: the renderer receives the updated content in agent.messages (verifiable via useEffect(() => { console.log(agent.messages) }, [agent.messages])) but the UI never re-renders

Environment

• @copilotkit/react-core@1.64.1
• @ag-ui/client@0.0.57
• @ag-ui/core@0.0.57

Suggested Fix

Include a stable content-derived key for non-string/array content, e.g. JSON.stringify(m.content).length or a content hash, instead of falling back to a constant 0:

const contentKey = 
  typeof m.content === "string" 
    ? m.content.length
    : Array.isArray(m.content) 
      ? m.content.length 
      : JSON.stringify(m.content).length;

Or better yet, use the full JSON.stringify(m.content) for activity messages to ensure any content change is detected.

Workaround Used

Subscribe directly to the agent's raw event stream (agent.subscribe(...)) in a separate React Context provider mounted alongside chatView, bypassing CopilotChat's messages memo entirely.

[coderabbitai]

⚠️ Possible Duplicate Issue(s)

• CopilotChat keeps stale A2UI activity content after ACTIVITY_DELTA from a userAction run #6001

🔗 Related PRs

#2021 - fix: memoize nested components to not rerender when content changes [merged]
#2128 - fix infinite loop [merged]
#2236 - useCoagent performance tweaks [merged]
#2260 - fix: add backwards compatibility with new Headless UI implementation [merged]
#2914 - ensure custom messages re-render when state updates [merged]

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[copilotkit-outpost-bot]

⚠️ This is an AI-generated response. We've escalated this to our engineering team — someone will follow up in this thread shortly.

Thanks for the detailed report

This is a well-documented issue. To be transparent about what I can and can't confirm from my position:

What I can say:

• The fingerprinting logic you've quoted and the 0 fallback for plain-object content is consistent with the pattern described in the docs for how CopilotKit handles message memoization
• The distinction you've drawn from 🐛 Bug: ACTIVITY_SNAPSHOT with replace: true does not re-render — MemoizedActivityMessage and CopilotChat messages fingerprint break under in-place message mutation #4676 is clearly articulated — the engineering team will need to verify whether these are truly independent layers or interact in other ways
• Your reproduction steps are specific and actionable

What I won't do:

• Confirm this is a bug or that the root cause is exactly as described — that determination belongs to the maintainers who can read and run the actual code
• Endorse the suggested fix as correct — one possibility is that JSON.stringify(m.content).length would help, but there may be performance implications or edge cases (e.g. key ordering instability, circular refs in some message shapes) that make a different approach preferable

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Your workaround is reasonable in the meantime

Subscribing to the agent's raw event stream directly is a valid escape hatch. If you want to keep it closer to the standard rendering path, another approach to explore in your own code is reading messages directly from the agent and driving your custom renderer from there:

Code example (tsx)

import { useAgent } from "@copilotkit/react-core/v2";
import { useEffect, useState } from "react";

function ActivityRenderer() {
  const { agent } = useAgent({ agentId: "my-agent" });
  // agent.messages reflects the latest state from the agent store
  // without going through CopilotChat's memoization layer
  const activityMessages = agent.messages.filter(
    (m) => m.role === "activity" // adjust to your actual role/type discriminant
  );

  return (
    <>
      {activityMessages.map((m) => (
        <YourCustomActivityCard key={m.id} message={m} />
      ))}
    </>
  );
}

Note: The exact role discriminant and message shape depend on your @ag-ui/core version — check what your event stream is actually producing.

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For the maintainers reviewing this

The reporter has:

• Isolated the specific useMemo fingerprint logic
• Confirmed the agent event stream is correct (ruling out upstream issues)
• Provided a minimal reproduction path
• Distinguished this from the previously fixed 🐛 Bug: ACTIVITY_SNAPSHOT with replace: true does not re-render — MemoizedActivityMessage and CopilotChat messages fingerprint break under in-place message mutation #4676

The suggested JSON.stringify(m.content).length approach and the "full stringify" alternative are both worth evaluating against the performance profile of large activity message payloads.

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Was there anything else about the workaround or the rendering path you'd like to dig into while this is being reviewed?

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_{🤖 Generated by CopilotKit AI Support · Documentation}

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