.forgejo/workflows/android.yml

@@ -19,18 +19,9 @@ name: Android
 
 on:
   push:
+    # NO `paths:` FILTER — the `decide` job below reads the real file set instead.
+    # See desktop.yml for why, and 85ead4d for what the duplication cost.
     branches: [dev, main]
-    paths:
-      - "android/**"
-      # The Rust the .so is built from. A core change reaches the phone exactly
-      # as it reaches the desktop, so this lane has to rebuild on it.
-      - "core/**"
-      - "Cargo.toml"
-      - "Cargo.lock"
-      # The version deriver — see the note in desktop.yml. This lane did not run on
-      # 85ead4d, which changed it.
-      - "packaging/**"
-      - ".forgejo/workflows/android.yml"
   workflow_dispatch:
 
 concurrency:
@@ -45,8 +36,46 @@ env:
   JAVA_TOOL_OPTIONS: "--enable-native-access=ALL-UNNAMED"
 
 jobs:
+  # Does the APK need rebuilding, or is the channel already serving this source?
+  # See the equivalent job in desktop.yml — same reasoning, same replacement of a
+  # hand-kept `paths:` filter with the one file set in `packaging/version.sh`.
+  #
+  # The guard runs here so it covers the skip path too (§6.3).
+  #
+  # NOTE THE COUPLING WITH ci.yml: when this lane builds, its last step dispatches
+  # ci.yml so the image bakes in the APK just published. When it SKIPS, no dispatch
+  # happens — and that is correct, because ci.yml's `gate` stands down only when the
+  # push touched Android's files, which is the same condition that makes this build.
+  # The two decisions agree because they read the same fact; they are still two
+  # readers of it, which is why the gate's grep carries a comment pointing here.
+  decide:
+    name: Build, or is the channel already serving this?
+    runs-on: python-ci
+    container:
+      image: git.fabledsword.com/bvandeusen/ci-python:3.14
+    outputs:
+      build: ${{ steps.d.outputs.build }}
+    steps:
+      - uses: actions/checkout@v6
+        with:
+          fetch-depth: 0
+
+      - name: Decide
+        id: d
+        env:
+          GITHUB_TOKEN: ${{ github.token }}
+        run: |
+          case "$GITHUB_REF_NAME" in
+            main) channel=stable ;;
+            *)    channel=dev ;;
+          esac
+          sh packaging/guard-forward.sh android "$channel"
+          echo "build=$(sh packaging/should-build.sh android "$channel")" >> $GITHUB_OUTPUT
+
   build:
     name: Kotlin + Rust (APK)
+    needs: [decide]
+    if: needs.decide.outputs.build == 'true'
     # runs-on is only a scheduling label (Label Model B). flutter-ci is the
     # proven-working label that can pull our container images.
     runs-on: flutter-ci
@@ -126,26 +155,6 @@ jobs:
             echo "apk=android/app/build/outputs/apk/debug/app-debug.apk" >> $GITHUB_OUTPUT
           fi
 
-      # THE ONE CHECK THAT LOOKS AT REALITY (note 3127 §6.3). Everything else in this
-      # lane derives a number and trusts it; this compares the derived value against
-      # what the channel is actually serving, and fails the lane if it went DOWN.
-      #
-      # Placed before the build, not after: a bad derivation should cost seconds, not
-      # a five-minute compile and a publish that has to be undone. Too-low is the
-      # unrecoverable direction — every installed client reports "up to date" forever
-      # and no later build fixes it until one climbs back above the bad number
-      # (#2183, #2993).
-      - name: Guard — the version must not go backwards
-        if: github.ref == 'refs/heads/dev' || github.ref == 'refs/heads/main'
-        env:
-          GITHUB_TOKEN: ${ github.token }
-        run: |
-          case "$GITHUB_REF_NAME" in
-            main) channel=stable ;;
-            *)    channel=dev ;;
-          esac
-          sh ../packaging/guard-forward.sh android "$channel"
-
       - name: Make gradlew executable
         run: chmod +x ./gradlew
 

.forgejo/workflows/ci.yml

@@ -1,12 +1,21 @@
 # CI runs first; build only proceeds if lint + typecheck pass.
 #
-# Push to dev:       typecheck + lint + test + build :dev + :<sha>
-# Push to main:      typecheck + lint + test + build :latest + :<sha>
-# Tag v* (release):  typecheck + lint + test + build :latest + :<version> + :<sha>
+# Push to dev:   typecheck + lint + test + build :dev
+# Push to main:  typecheck + lint + test + build :latest + :<sha>
 #
-# main is the production line, so a merge to main rebuilds and moves :latest to its
-# tip (family rule 46) — no version release required. The :<sha> image is the
-# immutable rollback unit for every build.
+# THAT IS THE COMPLETE TAG SET (rule 145). No version-shaped image tag in any lane:
+# nothing pins one — verified by looking for a consumer, not for whether one is
+# imaginable — and the git release tag is a different object in a different system
+# (step 7). The image is addressed by CHANNEL or by COMMIT; the release by date.
+#
+# A `v*` tag builds nothing at all. The merge to main already published everything,
+# so a tag rebuilding that same source would re-push :<sha> with different bytes,
+# which rule 145 forbids even when they match.
+#
+# main is the production line, so a merge moves :latest to its tip (family rule 46)
+# — no version release required. :<sha> is the immutable rollback unit, and it is
+# on main ONLY: a sha tag per dev push is a rollback target nobody has ever pulled,
+# accumulating forever, for a channel whose entire contract is that it moves.
 #
 # Required secret (repo -> Settings -> Secrets -> Actions):
 #   REGISTRY_TOKEN  -- Forgejo PAT with write:packages scope
@@ -17,7 +26,6 @@ name: CI & Build
 on:
   push:
     branches: [dev, main]
-    tags: ["v*"]
     paths:
       - "src/**"
       - "frontend/**"
@@ -34,11 +42,10 @@ on:
   # `gate` job below for the other half.
   workflow_dispatch:
 
-# Cancel older runs on the same branch when a newer push lands. Tag runs get their
-# own group implicitly and are never cancelled.
+# Cancel older runs on the same branch when a newer push lands.
 concurrency:
   group: ci-${{ github.ref }}
-  cancel-in-progress: ${{ !startsWith(github.ref, 'refs/tags/') }}
+  cancel-in-progress: true
 
 permissions:
   contents: read
@@ -66,7 +73,7 @@ jobs:
   # than a config so at least it is inspectable in the log.
   gate:
     name: Build now, or wait for Android?
-    if: github.ref == 'refs/heads/dev' || github.ref == 'refs/heads/main' || startsWith(github.ref, 'refs/tags/v')
+    if: github.ref == 'refs/heads/dev' || github.ref == 'refs/heads/main'
     runs-on: python-ci
     container:
       image: git.fabledsword.com/bvandeusen/ci-python:3.14
@@ -91,17 +98,6 @@ jobs:
             exit 0
           fi
 
-          # A tag. The Android lane does not run on tags, so nothing would ever
-          # call back — standing down here would mean a release tag that never
-          # produces an image at all.
-          case "${{ github.ref }}" in
-            refs/tags/*)
-              echo "Tag build — the Android lane does not run on tags. Building."
-              echo "build=true" >> $GITHUB_OUTPUT
-              exit 0
-              ;;
-          esac
-
           # No parent (first commit, or a force-push that orphaned it) — nothing to
           # compare, so build rather than stall.
           if ! git rev-parse --verify -q HEAD^ >/dev/null; then
@@ -127,7 +123,12 @@ jobs:
           echo "Changed in this push:"
           echo "$changed" | sed 's/^/    /'
 
-          if echo "$changed" | grep -qE '^(android/|core/|Cargo\.toml$|Cargo\.lock$|\.forgejo/workflows/android\.yml$)'; then
+          # MUST match android's file set in packaging/version.sh. `packaging/` was
+          # missing here after step 4 added it there — so a packaging-only push had
+          # the Android lane rebuild and dispatch while this gate ALSO let the image
+          # build, producing two images for one commit and, on main, a second push of
+          # the same :<sha> with different bytes. Rule 145's exact prohibition.
+          if echo "$changed" | grep -qE '^(android/|core/|packaging/|Cargo\.toml$|Cargo\.lock$|\.forgejo/workflows/android\.yml$)'; then
             echo ""
             echo "This push also changes the Android client. Standing down: the"
             echo "Android lane will publish a new APK and dispatch this workflow,"
@@ -142,7 +143,7 @@ jobs:
 
   typecheck:
     name: TypeScript typecheck
-    if: github.ref == 'refs/heads/dev' || github.ref == 'refs/heads/main' || startsWith(github.ref, 'refs/tags/v')
+    if: github.ref == 'refs/heads/dev' || github.ref == 'refs/heads/main'
     runs-on: python-ci
     container:
       image: git.fabledsword.com/bvandeusen/ci-python:3.14
@@ -159,7 +160,7 @@ jobs:
 
   lint:
     name: Python lint
-    if: github.ref == 'refs/heads/dev' || github.ref == 'refs/heads/main' || startsWith(github.ref, 'refs/tags/v')
+    if: github.ref == 'refs/heads/dev' || github.ref == 'refs/heads/main'
     runs-on: python-ci
     container:
       image: git.fabledsword.com/bvandeusen/ci-python:3.14
@@ -172,7 +173,7 @@ jobs:
 
   test:
     name: Python tests
-    if: github.ref == 'refs/heads/dev' || github.ref == 'refs/heads/main' || startsWith(github.ref, 'refs/tags/v')
+    if: github.ref == 'refs/heads/dev' || github.ref == 'refs/heads/main'
     runs-on: python-ci
     container:
       image: git.fabledsword.com/bvandeusen/ci-python:3.14
@@ -202,7 +203,7 @@ jobs:
   # discovery step below filters `docker ps` by it. Service hostnames are not routable
   # on this runner (rule 79), so the step resolves the container's bridge IP.
   integration:
-    if: github.ref == 'refs/heads/dev' || github.ref == 'refs/heads/main' || startsWith(github.ref, 'refs/tags/v')
+    if: github.ref == 'refs/heads/dev' || github.ref == 'refs/heads/main'
     runs-on: python-ci
     container:
       image: git.fabledsword.com/bvandeusen/ci-python:3.14
@@ -291,7 +292,6 @@ jobs:
         # run: steps execute under busybox sh (family rule 81), so use POSIX `case`,
         # NOT bash `[[ ]]`.
         run: |
-          TAGS="${{ env.IMAGE }}:${{ github.sha }}"
           # The image's version is DERIVED from its own shipped files — including the
           # Android client it bakes in, which is why an APK-only change re-versions
           # it. One value and no ordering key: nothing compares a server image, so
@@ -303,15 +303,14 @@ jobs:
           BUILD_VERSION="$(sh packaging/version.sh display server)"
           case "${{ github.ref }}" in
             refs/heads/dev)
-              TAGS="$TAGS,${{ env.IMAGE }}:dev"
+              TAGS="${{ env.IMAGE }}:dev"
               ;;
             refs/heads/main)
-              # Production line: :latest tracks main's tip (rule 46). No :main tag;
-              # the :<sha> above is the rollback unit.
-              TAGS="$TAGS,${{ env.IMAGE }}:latest"
+              TAGS="${{ env.IMAGE }}:latest,${{ env.IMAGE }}:${{ github.sha }}"
               ;;
-            refs/tags/*)
-              TAGS="$TAGS,${{ env.IMAGE }}:latest,${{ env.IMAGE }}:${{ github.ref_name }}"
+            *)
+              echo "::error::This lane builds images for dev and main only."
+              exit 1
               ;;
           esac
           echo "value=$TAGS" >> $GITHUB_OUTPUT
@@ -349,8 +348,8 @@ jobs:
           # from it. Not a versioning gap; a plain defect, fixed here because this
           # is the step that gave `stable` an APK to point at.
           case "${{ github.ref_name }}" in
-            main|v*) channel=stable ;;
-            *)       channel=dev ;;
+            main) channel=stable ;;
+            *)    channel=dev ;;
           esac
           echo "Baking in the $channel client."
           base="${{ github.server_url }}/${{ github.repository }}/releases/download/$channel"

.forgejo/workflows/desktop.yml

@@ -16,35 +16,16 @@ name: Desktop (Tauri)
 
 on:
   push:
+    # NO `paths:` FILTER. It was a second, independent statement of this artifact's
+    # file set, hand-kept beside the one in `packaging/version.sh`, and it drifted
+    # from it within a day (85ead4d). The `decide` job below reads the real set and
+    # skips in seconds when nothing moved — one definition, one reader (§3).
+    #
+    # The cost is that this workflow starts on every push rather than on a matching
+    # one. That is a ~15s container for a decision, against a lane that cannot
+    # silently fail to run.
     branches: [dev, main]
     tags: ["v*"]
-    paths:
-      - "desktop/**"
-      # The shared client core (store + sync engine) the desktop wraps. Its own
-      # crate since the Android client binds the same code, so a change there is a
-      # change to this app even though nothing under desktop/ moved.
-      - "core/**"
-      # The Android uniffi shim. It builds no desktop artifact, but it is a
-      # workspace member, so this lane's `cargo clippy --all-targets` is what
-      # compiles and lints it — and until the Android lane exists (M12 step 5),
-      # it is the ONLY thing that does.
-      - "android/**"
-      # The workspace manifest and lockfile, which now live at the repo root.
-      - "Cargo.toml"
-      - "Cargo.lock"
-      # The whole frontend, not just the adapter/bridge seam: it is compiled INTO
-      # the desktop binary, so any part of it changing means the shipped app is out
-      # of date. Config and lockfile included — a dependency bump changes the bundle
-      # as surely as a component does.
-      - "frontend/**"
-      # The version deriver. It decides what this artifact CLAIMS to be, so a change
-      # to it is a change to the artifact — and `packaging/version.sh` lists this
-      # same set from the other side. Two places holding one decision, which is why
-      # step 6 replaces these filters with skip-if-exists. Until then: edit one, edit
-      # the other. Learned the direct way — 85ead4d changed the deriver and this lane
-      # did not run at all.
-      - "packaging/**"
-      - ".forgejo/workflows/desktop.yml"
   workflow_dispatch:
 
 concurrency:
@@ -58,9 +39,56 @@ permissions:
   contents: write
 
 jobs:
+  # Does anything need building at all?
+  #
+  # ONE reader of ONE definition — the file sets in `packaging/version.sh` — replacing
+  # the `paths:` filters that used to state the same fact a second time. They drifted
+  # from it within a day: `packaging/` was added to the sets and not to the filters,
+  # so the commit fixing a derivation bug never ran on the two lanes it fixed
+  # (85ead4d). Note 3127 §3 warns about exactly that duplication.
+  #
+  # THE GUARD RUNS HERE, so it runs on every path INCLUDING the skip one (§6.3).
+  # Skipping because "the channel already serves this version" is indistinguishable
+  # from "we derived a stale value that happens to match" unless something checks.
+  decide:
+    name: Build, or is the channel already serving this?
+    if: github.ref == 'refs/heads/dev' || github.ref == 'refs/heads/main' || startsWith(github.ref, 'refs/tags/v')
+    runs-on: python-ci
+    container:
+      image: git.fabledsword.com/bvandeusen/ci-python:3.14
+    outputs:
+      build: ${{ steps.d.outputs.build }}
+    steps:
+      - uses: actions/checkout@v6
+        with:
+          # Derives a version — depth-1 is silently wrong (§6.1).
+          fetch-depth: 0
+
+      - name: Decide
+        id: d
+        env:
+          GITHUB_TOKEN: ${{ github.token }}
+        run: |
+          # A tag has no channel to compare against and still builds bundles until
+          # step 7 retires that lane. Always build, never skip.
+          case "${{ github.ref }}" in
+            refs/tags/*)
+              echo "Tag build — no channel to compare against."
+              echo "build=true" >> $GITHUB_OUTPUT
+              exit 0
+              ;;
+          esac
+          case "$GITHUB_REF_NAME" in
+            main) channel=stable ;;
+            *)    channel=dev ;;
+          esac
+          sh packaging/guard-forward.sh desktop "$channel"
+          echo "build=$(sh packaging/should-build.sh desktop "$channel")" >> $GITHUB_OUTPUT
+
   build:
     name: Tauri desktop (Linux)
-    if: github.ref == 'refs/heads/dev' || github.ref == 'refs/heads/main' || startsWith(github.ref, 'refs/tags/v')
+    needs: [decide]
+    if: needs.decide.outputs.build == 'true'
     runs-on: python-ci
     container:
       image: git.fabledsword.com/bvandeusen/ci-tauri:1.97
@@ -126,26 +154,6 @@ jobs:
       # so making it conditional is what lets the pipeline stay green before the
       # operator has added the secret. With the key present, each bundle gets a
       # `.sig` beside it — the file the updater actually verifies against.
-      # THE ONE CHECK THAT LOOKS AT REALITY (note 3127 §6.3). Everything else in this
-      # lane derives a number and trusts it; this compares the derived value against
-      # what the channel is actually serving, and fails the lane if it went DOWN.
-      #
-      # Placed before the build, not after: a bad derivation should cost seconds, not
-      # a five-minute compile and a publish that has to be undone. Too-low is the
-      # unrecoverable direction — every installed client reports "up to date" forever
-      # and no later build fixes it until one climbs back above the bad number
-      # (#2183, #2993).
-      - name: Guard — the version must not go backwards
-        if: github.ref == 'refs/heads/dev' || github.ref == 'refs/heads/main'
-        env:
-          GITHUB_TOKEN: ${ github.token }
-        run: |
-          case "$GITHUB_REF_NAME" in
-            main) channel=stable ;;
-            *)    channel=dev ;;
-          esac
-          sh packaging/guard-forward.sh desktop "$channel"
-
       - name: Tauri build (deb + AppImage)
         env:
           TAURI_SIGNING_PRIVATE_KEY: ${{ secrets.TAURI_SIGNING_PRIVATE_KEY }}
@@ -306,7 +314,8 @@ jobs:
   # built, not that it runs. A real-machine check stays mandatory before trusting it.
   windows:
     name: Windows installer (cross-compiled)
-    if: github.ref == 'refs/heads/dev' || github.ref == 'refs/heads/main' || startsWith(github.ref, 'refs/tags/v')
+    needs: [decide]
+    if: needs.decide.outputs.build == 'true'
     runs-on: python-ci
     container:
       image: git.fabledsword.com/bvandeusen/ci-tauri-win:1.97
@@ -348,26 +357,6 @@ jobs:
       # --runner cargo-xwin swaps cargo for the cross-compiling driver (it supplies
       # the MSVC CRT/SDK, pre-warmed into the image, and links with lld-link).
       # Frontend already built above; skip the beforeBuildCommand rebuild.
-      # THE ONE CHECK THAT LOOKS AT REALITY (note 3127 §6.3). Everything else in this
-      # lane derives a number and trusts it; this compares the derived value against
-      # what the channel is actually serving, and fails the lane if it went DOWN.
-      #
-      # Placed before the build, not after: a bad derivation should cost seconds, not
-      # a five-minute compile and a publish that has to be undone. Too-low is the
-      # unrecoverable direction — every installed client reports "up to date" forever
-      # and no later build fixes it until one climbs back above the bad number
-      # (#2183, #2993).
-      - name: Guard — the version must not go backwards
-        if: github.ref == 'refs/heads/dev' || github.ref == 'refs/heads/main'
-        env:
-          GITHUB_TOKEN: ${ github.token }
-        run: |
-          case "$GITHUB_REF_NAME" in
-            main) channel=stable ;;
-            *)    channel=dev ;;
-          esac
-          sh packaging/guard-forward.sh desktop "$channel"
-
       - name: Tauri build (NSIS installer)
         env:
           TAURI_SIGNING_PRIVATE_KEY: ${{ secrets.TAURI_SIGNING_PRIVATE_KEY }}

packaging/guard-forward.sh

@@ -3,7 +3,8 @@
 # Refuse to publish a version lower than the one already on the channel.
 #
 #   guard-forward.sh <desktop|android> <dev|stable>
-#   guard-forward.sh compare <a> <b>     exit 0 iff a sorts strictly below b
+#   guard-forward.sh compare   <a> <b>              exit 0 iff a sorts below b
+#   guard-forward.sh published <artifact> <channel> print what the channel serves
 #
 # Note 3127 §6.3. Everything else in this milestone derives a number and trusts it;
 # this is the one thing that checks the answer against reality before a user gets it.
@@ -53,6 +54,64 @@ version_lt() {
   return 1   # equal
 }
 
+# Auth if we have it, anonymous if not — the releases are public, but a token costs
+# nothing and keeps this working if that ever changes.
+#
+# MISSING CURL IS FATAL, not empty. Every fetch here ends in `|| true` so a network
+# blip reads as "nothing published yet" and passes — which is right for a genuinely
+# empty channel and catastrophic for a runner image without curl, where it would
+# silently turn the guard into a no-op that reports success on every build.
+if ! command -v curl >/dev/null 2>&1; then
+  echo "guard-forward.sh: curl is not on PATH — refusing to run, because every" >&2
+  echo "                  lookup here would read as 'nothing published' and this" >&2
+  echo "                  guard would pass without checking anything." >&2
+  exit 1
+fi
+
+fetch() {
+  if [ -n "${GITHUB_TOKEN:-}" ]; then
+    curl -fsSL -H "Authorization: token $GITHUB_TOKEN" "$1" 2>/dev/null || true
+  else
+    curl -fsSL "$1" 2>/dev/null || true
+  fi
+}
+
+# What the channel is serving, per artifact. ONE definition of where to look, shared
+# with `should-build.sh` — the skip decision and the guard must agree about what is
+# published, and two readers of one fact is how this repo keeps producing #2181-2183.
+published_for() {
+  case "$1" in
+    desktop)
+      # What the UPDATER reads. The manifest is the thing that decides whether a
+      # client is offered a build, so it is the authority on what is published.
+      fetch "$SERVER/$REPO/releases/download/$2/latest.json" \
+        | grep -oE '"version"[[:space:]]*:[[:space:]]*"[^"]+"' | head -1 \
+        | sed -E 's/.*"([^"]+)"$/\1/'
+      ;;
+    android)
+      fetch "$SERVER/$REPO/releases/download/$2/thoughtsync-android.json" \
+        | grep -oE '"version_code"[[:space:]]*:[[:space:]]*[0-9]+' | head -1 \
+        | grep -oE '[0-9]+$'
+      ;;
+  esac
+}
+
+# The NAME the channel serves, which is the commit-derived value. Separate from
+# `published_for` because the guard compares ordering KEYS and the skip decision
+# compares identity — for Android those are different fields, and conflating them
+# would make every build look like a change (the code is build-time; it always moves).
+published_name() {
+  case "$1" in
+    desktop) published_for desktop "$2" ;;
+    android)
+      fetch "$SERVER/$REPO/releases/download/$2/thoughtsync-android.json" \
+        | grep -oE '"version_name"[[:space:]]*:[[:space:]]*"[^"]+"' | head -1 \
+        | sed -E 's/.*"([^"]+)"$/\1/'
+      ;;
+  esac
+}
+
+
 # An explicit comparison mode, so the ordering logic is testable without a network
 # and inspectable without a push. Read-only and bypasses nothing — it is the same
 # function the guard itself uses, which is the point: a test of a reimplementation
@@ -63,6 +122,13 @@ if [ "$artifact" = "compare" ]; then
   if version_lt "$a" "$b"; then exit 0; else exit 1; fi
 fi
 
+if [ "$artifact" = "published" ]; then
+  a2="${2:?usage: guard-forward.sh published <artifact> <channel>}"
+  c2="${3:?usage: guard-forward.sh published <artifact> <channel>}"
+  published_name "$a2" "$c2"
+  exit 0
+fi
+
 channel="${2:?usage: guard-forward.sh <desktop|android> <dev|stable>}"
 
 case "$artifact" in desktop|android) : ;; *)
@@ -72,17 +138,9 @@ case "$channel" in dev|stable) : ;; *)
   echo "guard-forward.sh: unknown channel '$channel'" >&2; exit 2 ;;
 esac
 
-BASE="$SERVER/$REPO/releases/download/$channel"
 
-# Auth if we have it, anonymous if not — the releases are public, but a token costs
-# nothing and keeps this working if that ever changes.
-fetch() {
-  if [ -n "${GITHUB_TOKEN:-}" ]; then
-    curl -fsSL -H "Authorization: token $GITHUB_TOKEN" "$1" 2>/dev/null || true
-  else
-    curl -fsSL "$1" 2>/dev/null || true
-  fi
-}
+
+
 
 
 case "$artifact" in
@@ -90,9 +148,7 @@ case "$artifact" in
     derived="$(sh "$ROOT/packaging/version.sh" key desktop)"
     # What the UPDATER reads, not what the release happens to hold — the manifest is
     # the thing that decides whether a client is offered this build.
-    published="$(fetch "$BASE/latest.json" \
-      | grep -oE '"version"[[:space:]]*:[[:space:]]*"[^"]+"' | head -1 \
-      | sed -E 's/.*"([^"]+)"$/\1/')"
+    published="$(published_for desktop "$channel")"
     # COMMIT time, so EQUALITY IS THE ORDINARY CASE: an unchanged source derives
     # exactly what it derived last time, and `<=` would fail every no-change build.
     # §6.3 says *strictly* less for exactly this reason.
@@ -100,9 +156,7 @@ case "$artifact" in
     ;;
   android)
     derived="$(sh "$ROOT/packaging/version.sh" key android)"
-    published="$(fetch "$BASE/thoughtsync-android.json" \
-      | grep -oE '"version_code"[[:space:]]*:[[:space:]]*[0-9]+' | head -1 \
-      | grep -oE '[0-9]+$')"
+    published="$(published_for android "$channel")"
     # BUILD time, so equality is NOT ordinary — it means two builds landed in the
     # same minute, and Android refuses to install an APK whose versionCode does not
     # RISE. So this one requires strictly greater.

packaging/should-build.sh

@@ -0,0 +1,76 @@
+#!/usr/bin/env sh
+#
+# Does this artifact need building, or is the channel already serving this exact
+# source? Prints `true` or `false`.
+#
+#   should-build.sh <desktop|android> <dev|stable>
+#
+# Note 3127 §4, skip-if-exists — adapted, because §4 assumes a registry keyed by
+# VERSION and rule 145 removed exactly that. There is no `:<version>` tag to ask
+# about. What there IS, for both clients, is a channel that publishes the version it
+# is serving, and that answers the same question: if the channel already serves what
+# this source derives, the artifact would be byte-identical and there is nothing to
+# build.
+#
+# WHAT THIS REPLACES, and why that matters more here than the cost saving: the
+# `paths:` filters in the workflows were a SECOND, independent statement of each
+# artifact's file set, hand-kept beside the one in `version.sh`. They disagreed
+# within a day of the sets being written — `packaging/` was added to the sets and
+# not to the filters, so the commit that fixed a derivation bug never ran on the two
+# lanes it fixed (85ead4d). §3 warns about exactly this duplication; one definition
+# with one reader is the fix, and the cost saving is a bonus.
+#
+# THE SERVER IS NOT LISTED HERE, DELIBERATELY. Its image build is ~15 seconds against
+# 6 and 9 minutes for the clients, so there is little to save — and always building
+# it is strictly better for a server that can face the internet, because it picks up
+# `python:3.12-slim` base updates on every push. That is also why the base-image
+# tension in §4 does not bite this project: the artifact most exposed to it never
+# skips. The clients' bases are CI runner images, pinned deliberately.
+set -eu
+
+ROOT="$(cd "$(dirname "$0")/.." && pwd)"
+
+artifact="${1:?usage: should-build.sh <desktop|android> <dev|stable>}"
+channel="${2:?usage: should-build.sh <desktop|android> <dev|stable>}"
+
+case "$artifact" in desktop|android) : ;; *)
+  echo "should-build.sh: unknown artifact '$artifact'" >&2; exit 2 ;;
+esac
+case "$channel" in dev|stable) : ;; *)
+  echo "should-build.sh: unknown channel '$channel'" >&2; exit 2 ;;
+esac
+
+# The value that answers "is this the same code?" — which is not the same as the one
+# the guard compares.
+#
+#   desktop  the ordering key IS the identity; one value, one clock.
+#   android  the NAME. Its versionCode is build-time and moves every run, so
+#            comparing that would report a change on every push and never skip.
+case "$artifact" in
+  desktop) derived="$(sh "$ROOT/packaging/version.sh" key desktop)" ;;
+  android) derived="$(sh "$ROOT/packaging/version.sh" display android)" ;;
+esac
+
+published="$(sh "$ROOT/packaging/guard-forward.sh" published "$artifact" "$channel")"
+
+if [ -z "$published" ]; then
+  echo "should-build: $channel serves no $artifact yet — building." >&2
+  echo true
+  exit 0
+fi
+
+if [ "$derived" = "$published" ]; then
+  # UNCHANGED. The channel is already serving this exact source, so a build would
+  # produce the same artifact under the same name and republish it for nothing.
+  #
+  # Skipping is safe here in a way it would not be if anything pinned: there is no
+  # immutable tag to re-push with different bytes (rule 145 removed version tags),
+  # so the immutability argument in §4.2 does not apply and this stands on cost
+  # alone — which is the smaller, honest claim.
+  echo "should-build: $channel already serves $artifact $derived — skipping." >&2
+  echo false
+  exit 0
+fi
+
+echo "should-build: $artifact moved $published -> $derived — building." >&2
+echo true