diff --git a/solver/jobs.go b/solver/jobs.go
index b23a853a6..7fa06be10 100644
--- a/solver/jobs.go
+++ b/solver/jobs.go
@@ -20,7 +20,7 @@ import (
 	"go.opentelemetry.io/otel/trace"
 )
 
-// ResolveOpFunc finds an Op implementation for a Vertex
+// ResolveOpFunc finds an Op implementation for a Vertex.
 type ResolveOpFunc func(Vertex, Builder) (Op, error)
 
 type Builder interface {
@@ -261,6 +261,7 @@ type SolverOpt struct {
 	ResultSource  ResultSource
 	RefIDStore    *RefIDStore
 	CommitRefFunc CommitRefFunc
+	IsRunOnceFunc IsRunOnceFunc
 }
 
 func NewSolver(opts SolverOpt) *Solver {
@@ -280,6 +281,7 @@ func NewSolver(opts SolverOpt) *Solver {
 		solver,
 		opts.RefIDStore,
 		opts.ResultSource,
+		opts.IsRunOnceFunc,
 	)
 	solver.simple = simple
 
diff --git a/solver/llbsolver/simple.go b/solver/llbsolver/simple.go
new file mode 100644
index 000000000..3646c5677
--- /dev/null
+++ b/solver/llbsolver/simple.go
@@ -0,0 +1,29 @@
+package llbsolver
+
+import (
+	"github.com/moby/buildkit/solver"
+	"github.com/moby/buildkit/solver/llbsolver/ops"
+)
+
+// isRunOnce returns a function that can be called to determine if a Vertex
+// contains an operation that must be run at least once per build.
+func (s *Solver) isRunOnceOp() solver.IsRunOnceFunc {
+	return func(v solver.Vertex, b solver.Builder) (bool, error) {
+		w, err := s.resolveWorker()
+		if err != nil {
+			return false, err
+		}
+
+		op, err := w.ResolveOp(v, s.Bridge(b), s.sm)
+		if err != nil {
+			return false, err
+		}
+
+		switch op.(type) {
+		case *ops.SourceOp:
+			return true, nil
+		default:
+			return false, nil
+		}
+	}
+}
diff --git a/solver/llbsolver/solver.go b/solver/llbsolver/solver.go
index 12dae8052..4c228e354 100644
--- a/solver/llbsolver/solver.go
+++ b/solver/llbsolver/solver.go
@@ -144,6 +144,7 @@ func New(opt Opt) (*Solver, error) {
 
 	s.solver = solver.NewSolver(solver.SolverOpt{
 		ResolveOpFunc: s.resolver(),
+		IsRunOnceFunc: s.isRunOnceOp(),
 		DefaultCache:  opt.CacheManager,
 		ResultSource:  sources,
 		CommitRefFunc: worker.FinalizeRef,
diff --git a/solver/simple.go b/solver/simple.go
index 5fa347c5b..577ee46ed 100644
--- a/solver/simple.go
+++ b/solver/simple.go
@@ -10,6 +10,7 @@ import (
 	"sync"
 	"time"
 
+	"github.com/hashicorp/golang-lru/simplelru"
 	"github.com/moby/buildkit/util/bklog"
 	"github.com/moby/buildkit/util/progress"
 	"github.com/moby/buildkit/util/tracing"
@@ -21,21 +22,52 @@ import (
 // CommitRefFunc can be used to finalize a Result's ImmutableRef.
 type CommitRefFunc func(ctx context.Context, result Result) error
 
+// IsRunOnceFunc determines if the vertex represents an operation that needs to
+// be run at least once.
+type IsRunOnceFunc func(Vertex, Builder) (bool, error)
+
 // ResultSource can be any source (local or remote) that allows one to load a
 // Result using a cache key digest.
 type ResultSource interface {
 	Load(ctx context.Context, cacheKey digest.Digest) (Result, bool, error)
 }
 
+type runOnceCtrl struct {
+	lru *simplelru.LRU
+	mu  sync.Mutex
+}
+
+func newRunOnceCtrl() *runOnceCtrl {
+	lru, _ := simplelru.NewLRU(1e3, nil) // Error impossible on positive first argument.
+	return &runOnceCtrl{lru: lru}
+}
+
+// hasRun: Here, we use an LRU cache to whether we need to execute the source
+// operation for this job. The jobs may be re-run if the LRU size is exceeded,
+// but this shouldn't have a big impact on the build. The trade-off is
+// worthwhile given the memory-friendliness of LRUs.
+func (s *runOnceCtrl) hasRun(d digest.Digest, sessionID string) bool {
+	s.mu.Lock()
+	defer s.mu.Unlock()
+
+	key := fmt.Sprintf("%s:%s", sessionID, d)
+	ret := s.lru.Contains(key)
+
+	s.lru.Add(key, struct{}{})
+
+	return ret
+}
+
 type simpleSolver struct {
 	resolveOpFunc   ResolveOpFunc
+	isRunOnceFunc   IsRunOnceFunc
 	commitRefFunc   CommitRefFunc
 	solver          *Solver
 	parallelGuard   *parallelGuard
 	refIDStore      *RefIDStore
 	resultSource    ResultSource
 	cacheKeyManager *cacheKeyManager
-	mu              sync.Mutex
+	runOnceCtrl     *runOnceCtrl
 }
 
 func newSimpleSolver(
@@ -44,6 +76,7 @@ func newSimpleSolver(
 	solver *Solver,
 	refIDStore *RefIDStore,
 	resultSource ResultSource,
+	isRunOnceFunc IsRunOnceFunc,
 ) *simpleSolver {
 	return &simpleSolver{
 		cacheKeyManager: newCacheKeyManager(),
@@ -53,6 +86,8 @@ func newSimpleSolver(
 		solver:          solver,
 		refIDStore:      refIDStore,
 		resultSource:    resultSource,
+		isRunOnceFunc:   isRunOnceFunc,
+		runOnceCtrl:     newRunOnceCtrl(),
 	}
 }
 
@@ -75,6 +110,14 @@ func (s *simpleSolver) build(ctx context.Context, job *Job, e Edge) (CachedResul
 			return nil, err
 		}
 
+		// Release previous result as this is not the final return value.
+		if ret != nil {
+			err := ret.Release(ctx)
+			if err != nil {
+				return nil, err
+			}
+		}
+
 		ret = res
 
 		// Hijack the CacheKey type in order to export a reference from the new cache key to the ref ID.
@@ -87,6 +130,11 @@ func (s *simpleSolver) build(ctx context.Context, job *Job, e Edge) (CachedResul
 		})
 	}
 
+	err := s.commitRefFunc(ctx, ret)
+	if err != nil {
+		return nil, err
+	}
+
 	return NewCachedResult(ret, expKeys), nil
 }
 
@@ -121,14 +169,25 @@ func (s *simpleSolver) buildOne(ctx context.Context, d digest.Digest, vertex Ver
 	defer done()
 	<-wait
 
-	v, ok, err := s.resultSource.Load(ctx, cacheKey)
+	isRunOnce, err := s.isRunOnceFunc(vertex, job)
 	if err != nil {
 		return nil, "", err
 	}
 
-	if ok && v != nil {
-		notifyError(ctx, st, true, nil)
-		return v, cacheKey, nil
+	// Special case for source operations. They need to be run once per build or
+	// content changes will not be reliably detected.
+	mayLoadCache := !isRunOnce || isRunOnce && s.runOnceCtrl.hasRun(cacheKey, job.SessionID)
+
+	if mayLoadCache {
+		v, ok, err := s.resultSource.Load(ctx, cacheKey)
+		if err != nil {
+			return nil, "", err
+		}
+
+		if ok && v != nil {
+			notifyError(ctx, st, true, nil)
+			return v, cacheKey, nil
+		}
 	}
 
 	results, _, err := st.op.Exec(ctx, inputs)
@@ -136,17 +195,17 @@ func (s *simpleSolver) buildOne(ctx context.Context, d digest.Digest, vertex Ver
 		return nil, "", err
 	}
 
-	// Ensure all results are finalized (committed to cache). It may be better
-	// to background these calls at some point.
-	for _, res := range results {
-		err = s.commitRefFunc(ctx, res)
-		if err != nil {
-			return nil, "", err
+	res := results[int(e.Index)]
+
+	for i := range results {
+		if i != int(e.Index) {
+			err = results[i].Release(ctx)
+			if err != nil {
+				return nil, "", err
+			}
 		}
 	}
 
-	res := results[int(e.Index)]
-
 	err = s.refIDStore.Set(ctx, cacheKey, res.ID())
 	if err != nil {
 		return nil, "", err
@@ -306,6 +365,14 @@ func (s *simpleSolver) preprocessInputs(ctx context.Context, st *state, vertex V
 			}
 		}
 
+		// The result can be released now that the preprocess & slow cache
+		// digest functions have been run. This is crucial as failing to do so
+		// will lead to full file copying from mutable snapshots.
+		err = res.Release(ctx)
+		if err != nil {
+			return nil, err
+		}
+
 		// Add input references to the struct as to link dependencies.
 		scm.inputs[i] = in.Vertex.Digest()