fix: restore the trimmed execution trace

[dmadisetti]

📝 Summary

This fix is a followup to #9617 and #9616 and restores the truncated stacktrace to the user's notebook.

---

Before fix:

After fix:

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Project	Deployment	Actions	Updated (UTC)
marimo-docs	Ready	Preview, Comment	May 20, 2026 6:27pm

[cubic-dev-ai]

No issues found across 5 files

Architecture diagram

sequenceDiagram
    participant UserCell as "User Cell Code"
    participant Executor as "DefaultExecutor"
    participant CauseChain as "Exception Chain"
    participant MarimoRuntime as "MarimoRuntimeException"
    participant Hooks as "Post-Execution Hooks"
    participant Traceback as "write_traceback()"
    participant Frontend as "User Notebook UI"

    Note over UserCell,Frontend: Runtime Exception Flow

    UserCell->>Executor: execute_cell() or execute_cell_async()
    Executor->>Executor: exec(cell.body, glbls)
    alt Exception raised in user code
        UserCell-->>Executor: BaseException (e.g., ValueError)
        Executor->>CauseChain: Strip executor.py frame from e.__traceback__
        Note over Executor,CauseChain: Sets e.__traceback__ = e.__traceback__.tb_next
        Executor->>MarimoRuntime: raise MarimoRuntimeException from e
        Note over Executor,MarimoRuntime: User's traceback starts at user code, not executor.py
    end

    MarimoRuntime-->>Hooks: run_result.exception (MarimoRuntimeException)
    Hooks->>Hooks: traceback.format_exception(run_result.exception)
    Hooks->>CauseChain: Access __cause__.__traceback__ (user-code frames only)
    CauseChain-->>Hooks: Formatted traceback string
    Hooks->>Traceback: write_traceback(formatted_traceback)
    
    alt Code mode
        Traceback->>Frontend: broadcast_notification with raw traceback data
    else Run mode
        alt show_tracebacks enabled
            Traceback->>Frontend: sys.stderr._write_with_mimetype (highlighted)
        else show_tracebacks disabled
            Traceback->>Frontend: No traceback sent
        end
    end

    Note over UserCell,Frontend: Traceback now includes full execution path from user code

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_{Re-trigger cubic}

[Copilot]

Pull request overview

Restores user-facing (trimmed) execution tracebacks after the executor refactor by stripping the internal DefaultExecutor frame at the exception source, and removing downstream string-based traceback trimming in messaging/hooks.

Changes:

• Strip the executor’s own frame from caught exceptions in DefaultExecutor (sync + async) before wrapping in MarimoRuntimeException.
• Remove _trim_traceback usage and implementation so traceback formatting relies on the already-trimmed exception.
• Update/add tests to assert the executor frame is absent and adjust snapshots accordingly.

Reviewed changes

Copilot reviewed 6 out of 6 changed files in this pull request and generated 4 comments.

Show a summary per file

File	Description
marimo/_runtime/executor/executor.py	Strips the executor frame from exception tracebacks before wrapping.
marimo/_messaging/tracebacks.py	Removes string-based traceback trimming; highlights/broadcasts the passed traceback as-is.
marimo/_runtime/runner/hooks_post_execution.py	Stops trimming formatted tracebacks before highlighting.
tests/_runtime/test_executor_evaluator.py	Adds assertions that executor frames are removed from cause tracebacks (sync/async).
tests/_server/test_scratchpad_integration.py	Updates snapshot expectations to match trimmed tracebacks.
tests/_messaging/test_tracebacks.py	Removes unit test coverage for deleted _trim_traceback.