diff --git a/internal/docrevision/commit_idempotent_test.go b/internal/docrevision/commit_idempotent_test.go
new file mode 100644
index 00000000..cb6099ea
--- /dev/null
+++ b/internal/docrevision/commit_idempotent_test.go
@@ -0,0 +1,74 @@
+package docrevision_test
+
+import (
+	"context"
+	"testing"
+
+	"chat-agent-system/internal/docrevision"
+)
+
+func TestCommitAgentVersion_IdempotentWhenAlreadyCommitted(t *testing.T) {
+	ctx := context.Background()
+	svc := docrevision.NewService(nil)
+	docID := "doc_commit_idem"
+	base := "# base\n\n法规 GB9706\n"
+	st, err := svc.OpenTemplateVersion(ctx, docID, base, docrevision.ActorSnapshot{DisplayName: "t"}, 1, "")
+	if err != nil {
+		t.Fatal(err)
+	}
+	cand, _, err := svc.CreateAgentCandidate(ctx, docrevision.AgentCandidateRequest{
+		DocID: docID, RunID: "idem-1", InputSnapshotID: st.HeadSnapshotID,
+		OutputMarkdown: "# base\n\n", ActorKind: docrevision.ActorAgentSquad,
+	})
+	if err != nil && cand == nil {
+		t.Fatal(err)
+	}
+	if cand.Status != docrevision.CandidateNeedsConfirmation {
+		t.Fatalf("want needs_confirmation got %s", cand.Status)
+	}
+	st1, err := svc.CommitAgentVersion(ctx, docID, cand.ID, docrevision.CommitAgentDecision{
+		Decision: "commit", Confirmed: true, ActorUserID: 1,
+		ExpectedInputSHA: st.HeadSHA256,
+	})
+	if err != nil {
+		t.Fatalf("first commit: %v", err)
+	}
+	got, err := svc.GetCandidate(ctx, docID, cand.ID)
+	if err != nil || got.Status != docrevision.CandidateCommitted {
+		t.Fatalf("want committed got %+v err=%v", got, err)
+	}
+
+	// Retry with same hashes must succeed (projection/governance compensation path).
+	st2, err := svc.CommitAgentVersion(ctx, docID, cand.ID, docrevision.CommitAgentDecision{
+		Decision: "commit", Confirmed: true, ActorUserID: 1,
+		ExpectedInputSHA: st.HeadSHA256,
+	})
+	if err != nil {
+		t.Fatalf("idempotent recommit: %v", err)
+	}
+	if st2 == nil || st2.HeadSHA256 == "" || st2.HeadSHA256 != st1.HeadSHA256 {
+		t.Fatalf("head mismatch after idempotent commit: %#v vs %#v", st2, st1)
+	}
+
+	// Wrong expected output hash must not silently succeed.
+	outSHA := ""
+	if got.OutputSnapshotID != nil {
+		// CommitAgentVersion validates ExpectedOutputSHA against output snapshot.
+		// Use a bogus hash.
+		outSHA = "deadbeef"
+	}
+	_, err = svc.CommitAgentVersion(ctx, docID, cand.ID, docrevision.CommitAgentDecision{
+		Decision: "commit", Confirmed: true, ActorUserID: 1,
+		ExpectedInputSHA: st.HeadSHA256, ExpectedOutputSHA: outSHA,
+	})
+	if err == nil {
+		t.Fatal("mismatched expected_output_sha must fail on idempotent path")
+	}
+
+	_, err = svc.CommitAgentVersion(ctx, docID, cand.ID, docrevision.CommitAgentDecision{
+		Decision: "reject", Confirmed: true, ActorUserID: 1,
+	})
+	if err == nil {
+		t.Fatal("reject after commit must fail")
+	}
+}
diff --git a/internal/docrevision/service.go b/internal/docrevision/service.go
index fba58a8d..e0c6597a 100644
--- a/internal/docrevision/service.go
+++ b/internal/docrevision/service.go
@@ -544,6 +544,25 @@ func (s *Service) CommitAgentVersion(ctx context.Context, docID string, candidat
 		return nil, ErrNotFound
 	}
 	if c.Status == CandidateCommitted {
+		// Idempotent retry after projection/governance failure: same candidate + matching
+		// hashes may continue ProjectOutputMarkdown / MarkProposalApplied.
+		if dec.Decision == "reject" {
+			return nil, fmt.Errorf("docrevision: candidate already committed")
+		}
+		if c.OutputSnapshotID == nil {
+			return nil, ErrNotFound
+		}
+		outSnap := s.snapshots[*c.OutputSnapshotID]
+		inSnap := s.snapshots[c.InputSnapshotID]
+		if outSnap == nil || inSnap == nil {
+			return nil, ErrNotFound
+		}
+		if dec.ExpectedInputSHA != "" && dec.ExpectedInputSHA != inSnap.ContentSHA256 {
+			return nil, ErrCandidateStale
+		}
+		if dec.ExpectedOutputSHA != "" && dec.ExpectedOutputSHA != outSnap.ContentSHA256 {
+			return nil, ErrCandidateStale
+		}
 		return s.mustStateLocked(c.DocID), nil
 	}
 	if c.Status == CandidateStale {