---
model: claude-fable-5[1m]
service: claude
session: 638d5a98-abd2-4217-a447-19f2330eb334
timestamp: 2026-06-25T23:25:06Z
git_ref: 3a5cfde0
scope: code
substantive: true
raw_file: 20260625T232506Z_3a5cfde0_prompt_io.raw.md
---

## Prompt

Follow-up to a `/code-review` of PR #460 (this branch) which
surfaced lower-severity findings in the `.. d2::` sphinx
directive. The user's driving instructions across the session,

> yup do both *at least*, then report back.

(re: "implement #7 (+ optionally #8) on the branch") and,

> well we're on a clean main so why not just do these now too?

(re: also doing #9). The three findings being addressed,

- #7: a `d2` render that is *attempted and fails* (binary
  present but errors on the source) silently degraded to the
  stale committed SVG and the build still exited 0,
- #8: `parallel_*_safe = True` was declared while `run()`
  writes the shared output SVG during the read phase (torn
  write under `sphinx-build -j`),
- #9: output keyed only by `src.stem` so two `.d2` sources
  with the same stem collide.

## Response summary

Reworked `docs/_ext/d2diagrams.py`,

- `render_svg()` now returns a `RenderResult` tristate
  (`OK` / `FELL_BACK` / `NO_OUTPUT` / `FAILED`) so `run()` can
  distinguish a graceful no-binary fallback from a real render
  failure; `FAILED` emits a `reporter.error` (build fails under
  `-W`),
- the render writes into a sibling temp-file then `os.replace()`
  (atomic) so a failed/torn render can never clobber a good
  committed SVG,
- a per-build `_seen_outputs` map (reset on `builder-inited`)
  errors on a same-stem output collision.

Verified: a corrupted `.d2` errors under `-W` (exit 1) with the
committed SVG byte-unchanged; the collision guard fires on a
duplicate stem; `nix run nixpkgs#d2` render path + atomic swap
leave no temp residue; `ruff` clean.

## Response summary (cont.) — Files changed

- `docs/_ext/d2diagrams.py` — tristate render result, atomic
  temp-file render, output-collision guard, doc-string policy
  update.

## Human edits

None — committed as generated (`3a5cfde0`). The user noted
follow-up refinements may come in later commits.