diff --git a/logctx/handler.go b/logctx/handler.go
index 2ad15d6..a351f19 100644
--- a/logctx/handler.go
+++ b/logctx/handler.go
@@ -3,7 +3,8 @@ package logctx
 import (
 	"context"
 	"log/slog"
-	"os"
+
+	"instant.dev/common/buildinfo"
 )
 
 // Field-name constants — never inline these strings in tests or callers.
@@ -17,18 +18,18 @@ const (
 	FieldTeamID   = "team_id"
 )
 
-// commitID returns the build's git SHA. Track 1 of the observability rollout
-// adds a real `instant.dev/common/buildinfo` package whose GitSHA var is set
-// via `-ldflags -X`. Until that package merges, we fall back to the
-// COMMIT_ID env var (set by the Dockerfile / k8s deployment) so this package
-// does not block on track 1. The sentinel "dev" matches the buildinfo
-// package's planned default so log readers see a single consistent value
-// across both implementations.
+// commitID returns the build's git SHA, sourced from
+// `instant.dev/common/buildinfo.GitSHA`. The buildinfo var is set at link
+// time via `-ldflags -X` by the Dockerfiles / CI; un-flagged local builds
+// fall back to the buildinfo sentinel ("dev").
+//
+// Historical note: this used to read os.Getenv("COMMIT_ID") as a
+// decoupling shim from when logctx shipped before buildinfo. Both packages
+// now live on the same module, so we collapse to a direct import. This
+// eliminates the divergence where /healthz returned the real SHA (from
+// buildinfo) but slog lines emitted commit_id="dev" (env var unset).
 func commitID() string {
-	if v := os.Getenv("COMMIT_ID"); v != "" {
-		return v
-	}
-	return "dev"
+	return buildinfo.GitSHA
 }
 
 // Handler wraps an underlying slog.Handler and injects the five mandatory
diff --git a/logctx/handler_test.go b/logctx/handler_test.go
index 4a3b744..9429ca2 100644
--- a/logctx/handler_test.go
+++ b/logctx/handler_test.go
@@ -8,6 +8,8 @@ import (
 	"strings"
 	"testing"
 	"time"
+
+	"instant.dev/common/buildinfo"
 )
 
 // newTestHandler builds a logctx Handler over a fresh JSON handler writing to
@@ -142,6 +144,26 @@ func TestHandler_EnabledPassthrough(t *testing.T) {
 	}
 }
 
+// Test 6: commit_id is sourced from instant.dev/common/buildinfo.GitSHA.
+// Confirms the logctx <-> buildinfo wiring: when the buildinfo var is
+// patched (in production this happens via `-ldflags -X` at link time),
+// every emitted log line carries that same SHA — keeping slog output in
+// lock-step with /healthz and /api/v1/buildinfo.
+func TestHandler_CommitIDFromBuildinfo(t *testing.T) {
+	prev := buildinfo.GitSHA
+	t.Cleanup(func() { buildinfo.GitSHA = prev })
+	buildinfo.GitSHA = "test-sha-abc"
+
+	buf, h := newTestHandler(t, "api")
+	if err := h.Handle(context.Background(), newRecord("hello")); err != nil {
+		t.Fatalf("Handle: %v", err)
+	}
+	rec := decode(t, buf)
+	if rec[FieldCommitID] != "test-sha-abc" {
+		t.Errorf("commit_id = %v, want test-sha-abc", rec[FieldCommitID])
+	}
+}
+
 // Bonus: WithAttrs / WithGroup preserve the injected service+commit_id on
 // the returned child handler. Belt-and-braces guard against regressions
 // where someone refactors the struct and forgets to copy the fields.