Guard source previews against content version substitution

Why: Opening a processed source must not show changed current bytes as the quoted version.

Evidence: SELF TaskSourceContent regression fails before and passes after expected-hash enforcement; artifact regressions and live compilation pass.

Impact: Optional content hash narrows the existing authorized artifact read; controlled fixtures select a genuinely provided quotation.

Refs: fast-slow-u2-u3 source-version acceptance

internal/server/server.go

@@ -1382,6 +1382,13 @@ func (s *Server) artifactContent(w http.ResponseWriter, r *http.Request) {
 		writeStoreErr(w, err)
 		return
 	}
+	if expected := r.URL.Query().Get("expectedHash"); expected != "" {
+		sum := sha256.Sum256(content)
+		if hex.EncodeToString(sum[:]) != expected {
+			http.Error(w, "来源已更新，请从文件列表查看当前版本；任务中的引用片段仍可用于历史核验", http.StatusConflict)
+			return
+		}
+	}
 	ct := contentTypeOf(a)
 	w.Header().Set("Content-Type", ct)
 	setArtifactHeaders(w, ct)

internal/server/task_sources_test.go

@@ -0,0 +1,38 @@
+package server
+
+import (
+	"agent-system-backend/internal/store"
+	core "agent-system-core"
+	"crypto/sha256"
+	"encoding/hex"
+	"net/http"
+	"net/http/httptest"
+	"testing"
+)
+
+func TestTaskSourceContentDoesNotSubstituteLatestVersion(t *testing.T) {
+	srv, st := newTestServer(t)
+	original := []byte("上线六月")
+	a := core.Artifact{Owner: store.OwnerDefault, ProjectID: "p", Name: "input.txt"}
+	if e := st.CreateArtifact(&a, original); e != nil {
+		t.Fatal(e)
+	}
+	sum := sha256.Sum256(original)
+	hash := hex.EncodeToString(sum[:])
+	get := func() *httptest.ResponseRecorder {
+		r := httptest.NewRequest("GET", "/api/artifacts/"+a.ID+"/content?expectedHash="+hash, nil)
+		r.SetPathValue("id", a.ID)
+		w := httptest.NewRecorder()
+		srv.artifactContent(w, r)
+		return w
+	}
+	if w := get(); w.Code != http.StatusOK || w.Body.String() != string(original) {
+		t.Fatal("original version not readable", w.Code, w.Body.String())
+	}
+	if e := st.UpdateArtifactContent(store.OwnerDefault, a.ID, []byte("上线七月")); e != nil {
+		t.Fatal(e)
+	}
+	if w := get(); w.Code != http.StatusConflict {
+		t.Fatal("latest content masquerades as processed version", w.Code, w.Body.String())
+	}
+}

internal/server/work_checkpoint_eval_test.go

@@ -69,7 +69,8 @@ func evalMechanismStream(ctx context.Context, model string, msgs []agentllm.Mess
 				count++
 			}
 		}
-		matches := regexp.MustCompile(`sourceId=(src_[a-f0-9]+) blockIndex=(\d+)`).FindAllStringSubmatch(sourceText, -1)
+		matches := regexp.MustCompile(`sourceId=(src_[a-f0-9]+) blockIndex=(\d+)[^\n]*\n引用定位片段：([^\n]*)`).FindAllStringSubmatch(sourceText, -1)
+		matches = filterCheckpointSources(matches)
 		if count < 2 && len(matches) > 0 {
 			var block int
 			fmt.Sscan(matches[0][2], &block)
@@ -318,3 +319,13 @@ func evalProviderStream(route map[string]any, dir string) agentllm.StreamFunc {
 		return out
 	}
 }
+
+func filterCheckpointSources(matches [][]string) [][]string {
+	var out [][]string
+	for _, match := range matches {
+		if len(match) > 3 && strings.Contains(match[3], "上线六月") {
+			out = append(out, match)
+		}
+	}
+	return out
+}