diff --git a/.env.example b/.env.example
index 1bd121a..1f6663d 100644
--- a/.env.example
+++ b/.env.example
@@ -24,11 +24,9 @@ ADMIN_IDS=
 GEMINI_API_KEY=
 
 # SOURCE_COMMIT (commit SHA) is read at startup for the deploynotify owner DM.
-# Do NOT set it here. On Coolify (Docker Compose) it is a predefined variable
-# that reaches the container only because compose.yml references it
-# (`SOURCE_COMMIT: ${SOURCE_COMMIT:-}`) — Coolify supplies the value via
-# --env-file. Local `docker compose up` has none, so deploynotify reports
-# "unknown".
+# Do NOT set it here. Coolify provides it at runtime. Keep "Include Source
+# Commit in Build" disabled so Docker layer cache survives across commits.
+# Local `docker compose up` has none, so deploynotify reports "unknown".
 
 # ====================== Leave UNSET on self-host ==================
 # Defaults are correct for self-host:
diff --git a/Dockerfile b/Dockerfile
index 9fac1ec..aa011e5 100644
--- a/Dockerfile
+++ b/Dockerfile
@@ -6,9 +6,9 @@ RUN go mod download
 
 COPY . .
 
-# The deploy-notify commit SHA comes from the SOURCE_COMMIT runtime env that
-# Coolify injects into the container (see compose.yml), not from a build
-# arg — Coolify does not pass build args here. The binary is built plain.
+# The deploy-notify commit SHA comes from Coolify's SOURCE_COMMIT runtime env,
+# not from a build arg. Keep it out of the build so Docker cache survives across
+# commits. The binary is built plain.
 RUN CGO_ENABLED=0 GOOS=linux go build \
     -ldflags="-s -w" \
     -o /out/server \
diff --git a/cmd/server/main_test.go b/cmd/server/main_test.go
index ba55f18..d10fc09 100644
--- a/cmd/server/main_test.go
+++ b/cmd/server/main_test.go
@@ -1,6 +1,9 @@
 package main
 
 import (
+	"os"
+	"path/filepath"
+	"strings"
 	"testing"
 
 	"github.com/tiennm99/miti99bot/internal/modules"
@@ -29,6 +32,22 @@ func TestResolveCommitSHA(t *testing.T) {
 	}
 }
 
+func TestComposeDoesNotOverrideSourceCommit(t *testing.T) {
+	b, err := os.ReadFile(filepath.Join("..", "..", "compose.yml"))
+	if err != nil {
+		t.Fatalf("read compose.yml: %v", err)
+	}
+	for _, line := range strings.Split(string(b), "\n") {
+		trimmed := strings.TrimSpace(line)
+		if trimmed == "" || strings.HasPrefix(trimmed, "#") {
+			continue
+		}
+		if strings.HasPrefix(trimmed, "SOURCE_COMMIT:") || strings.HasPrefix(trimmed, "- SOURCE_COMMIT") {
+			t.Fatalf("compose.yml must not declare SOURCE_COMMIT; Coolify supplies it at runtime and an explicit Compose value can override it with empty")
+		}
+	}
+}
+
 func TestFactoriesIncludesGoldAndCoin(t *testing.T) {
 	catalog := factories()
 	if catalog["gold"] == nil {
diff --git a/compose.yml b/compose.yml
index 1ff48de..fbc8bdc 100644
--- a/compose.yml
+++ b/compose.yml
@@ -16,13 +16,9 @@ services:
       ADMIN_IDS: ${ADMIN_IDS}              # CSV of admin Telegram user ids
       # --- Optional ---
       GEMINI_API_KEY: ${GEMINI_API_KEY}    # only the twentyq module needs it
-      # SOURCE_COMMIT (commit SHA, read at startup for the deploynotify owner DM).
-      # For Docker Compose, Coolify passes predefined vars via --env-file for
-      # interpolation ONLY — the var reaches the container only if referenced
-      # here (per Coolify docs). The :- default keeps local `docker compose up`
-      # (where SOURCE_COMMIT is unset) quiet; on Coolify the real value wins, so
-      # deploynotify reports "unknown" only outside Coolify.
-      SOURCE_COMMIT: ${SOURCE_COMMIT:-}
+      # SOURCE_COMMIT is intentionally not declared here. Coolify provides it
+      # at runtime via its generated env file; declaring it here with Compose
+      # interpolation can override the runtime value with an empty string.
       # Storage auto-selects mongodb because MONGO_URL is set — no KV_PROVIDER.
       # The in-process cron scheduler runs by default — no CRON_MODE.
       # PORT defaults to 8080 (internal health server) — omit unless overriding.
diff --git a/docs/deploy-coolify-selfhosted.md b/docs/deploy-coolify-selfhosted.md
index b3c76c6..8e53c49 100644
--- a/docs/deploy-coolify-selfhosted.md
+++ b/docs/deploy-coolify-selfhosted.md
@@ -82,13 +82,13 @@ Copy [`.env.example`](../.env.example) → `.env` (gitignored) and fill in.
    double-fires crons. Prefer **stop-first redeploys** so two containers never
    overlap near a cron time.
 5. **deploynotify commit SHA:** `SOURCE_COMMIT` is a Coolify predefined
-   variable. For Docker Compose, Coolify passes predefined vars via `--env-file`
-   for interpolation only, so the value reaches the container **only because
-   `compose.yml` references it** (`SOURCE_COMMIT: ${SOURCE_COMMIT:-}`).
-   The bot reads it at startup and DMs the owner on every boot; outside Coolify
-   (local `docker compose up`) it is unset and the DM shows `unknown`. The
-   "Include Source Commit in Build" Coolify setting affects build args only and
-   is **not** needed for this runtime path.
+   variable. The bot reads it at startup and DMs the owner on every boot;
+   outside Coolify (local `docker compose up`) it is unset and the DM shows
+   `unknown`. Keep "Include Source Commit in Build" disabled: that setting
+   affects build args only, is not needed for this runtime path, and would
+   invalidate Docker cache on every commit. Do not add `SOURCE_COMMIT` to
+   `compose.yml`; an interpolated empty value can override Coolify's runtime
+   env-file value.
 6. **Health check:** use Coolify's HTTP monitor against `GET /` (returns
    `text/plain` `miti99bot ok`). Do **not** use a compose `healthcheck` — the
    distroless image has no shell/curl and `cmd/server` has no `-healthcheck`