agent's nbformat write failed: lost the cell tags papermill needs, parameters missing

TL;DR

Papermill finds its parameters cell by the parameters tag, so any agent transformation that rebuilds cells without copying metadata.tags silently unplugs papermill: the next run injects nowhere and fails downstream. Preserve the full cell metadata (or at minimum tags) through every transformation, and add a post-transform assertion that tagged cells still exist. Tags are load-bearing; treat them as such.

agent's nbformat write failed: lost the cell tags papermill needs, parameters missing

Use this when

• Papermill stops injecting after an agent rewrites the notebook.
• The rewritten notebook has no parameters tag.
• The agent rebuilds cells (split, merge, clean).

Not for this skill when

• The tag was never there. Tag the cell.
• Papermill injects but values are wrong. That is a typing issue.

Steps

1. Confirm the loss: compare tags before and after the transform. Verify: the parameters tag is gone.
2. Fix the transform to copy cell.metadata (deep) into rebuilt cells. Verify: tags survive.
3. Add a post-transform check: assert the tagged cells still exist with their tags. Verify: the check fails loudly on future regressions.
4. Re-run papermill on the transformed notebook. Verify: injection works.
5. Document tags as load-bearing in the agent's notebook guide. Verify: the next transform author knows.

Variant phrasings

papermill tags lost rewrite

Short form. Copy metadata through transforms.

cell tags dropped nbformat write

The mechanism phrasing. Deep-copy metadata.

parameters tag missing after agent

The symptom phrasing. Assert tags post-transform.

Compatibility: nbformat 5.x, papermill 2.x. Tags live in cell.metadata.tags.

Why it happens

Cell tags are invisible in normal rendering, so transforms that rebuild cells from source text naturally forget them; the author sees identical-looking cells and ships. But tags drive automation (papermill parameters, nbconvert exclusions, CI markers), and losing them breaks downstream tools with confusing errors far from the transform. Invisible-but-load-bearing metadata must be preserved by default.

Edge cases / pitfalls

• Shallow-copying metadata shares tag lists between cells. Deep-copy.
• Other load-bearing metadata (kernelspec, widgets state) deserves the same preservation.
• The assertion should name the missing tag. 'Tags missing' is less useful than 'parameters tag missing'.
• If the transform intentionally retags, log the change. Silent retagging is a bug factory.

Provenance

Resolved from the public thread: https://vectle.com/posts/pst7iYp4pHqx7nF4G2JChNHg