diff --git a/Dockerfile b/Dockerfile
index bd385ab..7f37883 100644
--- a/Dockerfile
+++ b/Dockerfile
@@ -16,12 +16,48 @@ COPY . .
 # no libc. Trim symbols/DWARF to shrink the layer.
 RUN CGO_ENABLED=0 GOOS=linux go build -trimpath -ldflags='-s -w' -o /out/tapir ./cmd/tapir
 
+# ── ffmpeg ──────────────────────────────────────────────────────────────────
+# Audio upload probes files with ffprobe and converts them with ffmpeg (ADR-033,
+# #34 option A). Static builds, so they run in the distroless image below.
+#
+# Pinned by version AND sha256: a changed upstream file fails the build instead of
+# shipping. The checksum proves these are the bytes that were reviewed (2026-10-02;
+# the publisher's md5 matched too). It does NOT prove authenticity: the publisher
+# signs nothing we can check. amd64 only (the cluster is one amd64 node).
+#
+# These builds are GPLv3 (--enable-gpl --enable-version3). The service is private and
+# the image is not distributed; the licence text is copied into the image anyway.
+#
+# The release URL moves on when upstream releases a newer version, so the pinned
+# version is tried in releases/ and then old-releases/. If neither has it the build
+# fails; vendoring the verified tarball is the durable fix.
+FROM golang:1.26 AS ffmpeg
+
+ARG FFMPEG_VERSION=7.0.2
+ARG FFMPEG_SHA256=abda8d77ce8309141f83ab8edf0596834087c52467f6badf376a6a2a4c87cf67
+
+RUN set -eu; \
+    mkdir -p /out; \
+    for dir in releases old-releases; do \
+      curl -fsSL -o /out/ffmpeg.tar.xz "https://johnvansickle.com/ffmpeg/${dir}/ffmpeg-${FFMPEG_VERSION}-amd64-static.tar.xz" && break; \
+    done; \
+    echo "${FFMPEG_SHA256}  /out/ffmpeg.tar.xz" | sha256sum -c -; \
+    tar -xJf /out/ffmpeg.tar.xz -C /out --strip-components=1 --wildcards '*/ffmpeg' '*/ffprobe' '*/GPLv3.txt'; \
+    rm /out/ffmpeg.tar.xz
+
 # ── runtime ─────────────────────────────────────────────────────────────────
 # distroless static + nonroot: no shell, no package manager, runs as uid 65532.
 # ca-certificates are bundled, which the OIDC/HTTPS clients need.
 FROM gcr.io/distroless/static-debian12:nonroot
 
 COPY --from=build /out/tapir /tapir
+COPY --from=ffmpeg /out/ffmpeg /out/ffprobe /usr/local/bin/
+COPY --from=ffmpeg /out/GPLv3.txt /usr/share/doc/ffmpeg/GPLv3.txt
+
+# Where the service finds them. Uploads stay OFF until TAPIR_UPLOADS_ENABLED is set
+# (a deployment decision, not an image one).
+ENV TAPIR_FFMPEG_BIN=/usr/local/bin/ffmpeg \
+    TAPIR_FFPROBE_BIN=/usr/local/bin/ffprobe
 
 EXPOSE 8080
 USER nonroot:nonroot