fix(agent-panel): recover prompt submits failures

[julio4]

📝 Summary

Fix two failure paths in the acp agent panel prompt-submission handler:

• Submitting from an unnamed notebook previously cleared the draft and attachments, changed the session title, and left the panel busy before returning with the existing naming warning. Validate the filename before mutating submission state so the user can name the notebook and submit again.
• Rejected agent prompts were only logged to the console. Instead we can log them in the existing error banner, preserving JSON-RPC authentication errors and their existing recovery action. Include prompt preparation inside same try/catch/finally so preparation failures also release loading state.

These failure paths were encountered while embedding acp-based cell. This change fixes the shared handler for both composer submissions and queued prompts; it adds no downstream-specific behavior, dependencies, or configuration.

📋 Pre-Review Checklist

• For large changes, or changes that affect the public API: this change was discussed or approved through an issue, on Discord, or the community discussions (Please provide a link if applicable).
• Any AI generated code has been reviewed line-by-line by the human PR author, who stands by it.
• Video or media evidence is provided for any visual changes (optional).

✅ Merge Checklist

• I have read the contributor guidelines.
• Documentation has been updated where applicable, including docstrings for API changes.
• Tests have been added for the changes made.

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Project	Deployment	Actions	Updated
marimo-docs	Ready	Preview	Sep 13, 2026 3:55pm UTC

[github-actions]

All contributors have signed the CLA ✍️ ✅
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[julio4]

I have read the CLA Document and I hereby sign the CLA

[mscolnick]

@cubic-dev-ai

[cubic-dev-ai]

@cubic-dev-ai

@mscolnick I have started the AI code review. It will take a few minutes to complete.

[Light2Dark]

this looks good, tq!

[cubic-dev-ai]

1 issue found across 2 files

Prompt for AI agents (unresolved issues)

Check if these issues are valid — if so, understand the root cause of each and fix them. If appropriate, use sub-agents to investigate and fix each issue separately.


<file name="frontend/src/components/chat/acp/agent-panel.tsx">

<violation number="1" location="frontend/src/components/chat/acp/agent-panel.tsx:907">
P2: When an unnamed notebook consumes a queued `pendingPrompt.submit`, this check returns after the effect has already cleared `pendingAiPromptAtom`, so the prompt is lost. Validate queued prompts before clearing them or restore the pending prompt on this failure.</violation>
</file>

<sub>Reply with feedback, questions, or to request a fix.

Re-trigger cubic</sub>

[cubic-dev-ai]

P2: When an unnamed notebook consumes a queued pendingPrompt.submit, this check returns after the effect has already cleared pendingAiPromptAtom, so the prompt is lost. Validate queued prompts before clearing them or restore the pending prompt on this failure.

Prompt for AI agents

Check if this issue is valid — if so, understand the root cause and fix it. At frontend/src/components/chat/acp/agent-panel.tsx, line 907:

<comment>When an unnamed notebook consumes a queued `pendingPrompt.submit`, this check returns after the effect has already cleared `pendingAiPromptAtom`, so the prompt is lost. Validate queued prompts before clearing them or restore the pending prompt on this failure.</comment>

<file context>
@@ -902,6 +902,18 @@ const AgentPanel: React.FC = () => {
 
+      let absoluteFilename: string;
+      try {
+        absoluteFilename = getAbsoluteFilename();
+      } catch {
+        toast({
</file context>

[Light2Dark]

Existing issue