Reproducibility¶

By default, Tectonic embeds the current wall-clock time in the PDF metadata (creation date, modification date), so identical inputs produce non-byte-identical PDFs. For most workflows this is fine; for some (release archives, deterministic CI snapshots, content-addressed distribution) it isn't.

Enabling reproducible mode¶

latex_document(
    name = "cv",
    main = "cv.tex",
    srcs = ["cv.tex"],
    reproducible = True,
)

When reproducible = True:

The action's environment includes SOURCE_DATE_EPOCH=0. Tectonic honours this and uses 1970-01-01 for the PDF's embedded creation/modification dates.
The tectonic invocation includes -Z deterministic-mode. This normalises a handful of other engine-level non-determinisms (random number generator seeds, etc.).

The combination is enough to produce byte-identical PDFs across clean builds.

Verifying it¶

CI verifies this on every push:

bazel build //:cv_reproducible
sha256sum bazel-bin/cv_reproducible.pdf
bazel clean --expunge
bazel build //:cv_reproducible
sha256sum bazel-bin/cv_reproducible.pdf
# (same)

See the CI workflow for the exact step.

Caveats¶

Mutually exclusive with SyncTeX¶

Tectonic's -Z deterministic-mode deliberately disables SyncTeX output, because SyncTeX records absolute filesystem paths in the .synctex.gz file (sandbox execroot paths) which aren't stable across machines.

If you try to set both:

latex_document(
    name = "cv",
    reproducible = True,
    synctex = True,  # ERROR at analysis time
    ...
)

…the rule fails at analysis with an explanatory message. Pick one or the other.

Not byte-identical across rules_latex versions¶

A rules_latex point release that changes the bundled tectonic version will produce different PDFs (different engine version → different output). Hold rules_latex version constant across the builds you want to compare.

Embedded fonts can vary across platforms¶

In rare cases involving system fonts (e.g. when a document uses \setmainfont{Arial} on macOS), the embedded font subset can differ between macOS and Linux. Avoid fontspec system-font lookups in documents that need to be byte-identical across platforms — stick with Tectonic's bundled fonts.