diff --git a/.gitea/workflows/docker-publish.yml b/.gitea/workflows/docker-publish.yml
index 7ca3adc..6f1aefe 100644
--- a/.gitea/workflows/docker-publish.yml
+++ b/.gitea/workflows/docker-publish.yml
@@ -159,8 +159,9 @@ jobs:
       - name: Derive platform slug
         id: platform
         run: |
-          PLATFORM_PAIR="${{ matrix.platform }}"
-          echo "pair=${PLATFORM_PAIR//\//-}" >> $GITHUB_OUTPUT
+          # POSIX-safe slash substitution — act's runner container ships
+          # /bin/sh as dash, which doesn't support bash's ${VAR//a/b}.
+          echo "pair=$(echo '${{ matrix.platform }}' | tr / -)" >> $GITHUB_OUTPUT
 
       - name: Set up QEMU
         if: matrix.platform != 'linux/amd64'
@@ -220,8 +221,9 @@ jobs:
       - name: Derive platform slug
         id: platform
         run: |
-          PLATFORM_PAIR="${{ matrix.platform }}"
-          echo "pair=${PLATFORM_PAIR//\//-}" >> $GITHUB_OUTPUT
+          # POSIX-safe slash substitution — act's runner container ships
+          # /bin/sh as dash, which doesn't support bash's ${VAR//a/b}.
+          echo "pair=$(echo '${{ matrix.platform }}' | tr / -)" >> $GITHUB_OUTPUT
 
       - name: Set up QEMU
         if: matrix.platform != 'linux/amd64'
diff --git a/CHANGELOG.md b/CHANGELOG.md
index 3da9343..c051607 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -6,6 +6,15 @@ Tags follow `v{opencode_version}[letter]` — bare tag for the first build on a
 
 ---
 
+## v1.14.31c — 2026-05-01
+
+**CI: fix bash-specific parameter expansion and bump omos size threshold.**
+
+- **Fix:** `Derive platform slug` step in the per-arch matrix build jobs (`build-base`, `build-omos`) used `${PLATFORM_PAIR//\//-}` which is a bash parameter-expansion. The runner container executes step scripts via `/bin/sh` (dash), which errored with `Bad substitution`. Rewrote using `tr / -` which is POSIX and behaves identically. Both `build-base` and `build-omos` matrix jobs were blocked on this on `v1.14.31b`.
+- **Fix:** smoke-test image-size threshold for the `omos` variant bumped from 3000 MB to 3200 MB. The mempalace-toolkit bake-in added ~100 MB to omos; measured 3107 MB on `v1.14.31b`. All functional smoke checks (opencode, node, mempalace CLIs, toolkit wrappers, oh-my-opencode-slim) pass — this is a guardrail recalibration, not a performance concession. The underlying image genuinely grew.
+- The runner-disk reclaim step from v1.14.31b did its job: `smoke-base` and `validate-base` now pass cleanly. Only `smoke-omos` was blocked this iteration, and only on the threshold.
+- No image changes beyond what shipped in v1.14.31. Rebuild of v1.14.31 content only.
+
 ## v1.14.31b — 2026-05-01
 
 **CI: reclaim runner disk before `load: true` smoke builds.**
diff --git a/scripts/smoke-test.sh b/scripts/smoke-test.sh
index bba3046..6b4fa34 100755
--- a/scripts/smoke-test.sh
+++ b/scripts/smoke-test.sh
@@ -214,9 +214,12 @@ SIZE_BYTES=$(docker image inspect --format='{{.Size}}' "$IMAGE")
 SIZE_MB=$((SIZE_BYTES / 1024 / 1024))
 echo "  Uncompressed size: ${SIZE_MB} MB"
 
-# Thresholds (uncompressed): base 2500 MB, omos 3000 MB. Adjust as image content evolves.
+# Thresholds (uncompressed): base 2500 MB, omos 3200 MB. Adjust as image content evolves.
+# omos bumped 3000→3200 on v1.14.31c — mempalace-toolkit bake-in pushed the
+# omos variant to ~3.1 GB. Functional smoke checks all pass; this is a
+# guardrail, not a performance limit.
 THRESHOLD=2500
-[ "$VARIANT" = "omos" ] && THRESHOLD=3000
+[ "$VARIANT" = "omos" ] && THRESHOLD=3200
 if [ "$SIZE_MB" -gt "$THRESHOLD" ]; then
   fail "image size ${SIZE_MB} MB exceeds threshold ${THRESHOLD} MB for variant=$VARIANT"
 else