diff --git a/internal/gitea/files.go b/internal/gitea/files.go
index e579dd4..3fcd229 100644
--- a/internal/gitea/files.go
+++ b/internal/gitea/files.go
@@ -92,13 +92,24 @@ type FileWriteResult struct {
 	} `json:"commit"`
 }
 
+// UpsertFile creates a file when args.Sha is empty (POST) or updates an existing
+// file when args.Sha is set (PUT). Gitea routes both operations by HTTP method on
+// the same /contents/{path} URL, and rejects PUT without a sha.
 func (c *Client) UpsertFile(ctx context.Context, owner, repo, path string, args UpsertFileArgs) (*FileWriteResult, error) {
 	p := fmt.Sprintf("/api/v1/repos/%s/%s/contents/%s", owner, repo, path)
 	payload, err := json.Marshal(args)
 	if err != nil {
 		return nil, err
 	}
-	body, status, err := c.PutJSON(ctx, p, payload)
+	var (
+		body   []byte
+		status int
+	)
+	if args.Sha == "" {
+		body, status, err = c.PostJSON(ctx, p, payload)
+	} else {
+		body, status, err = c.PutJSON(ctx, p, payload)
+	}
 	if err != nil {
 		return nil, err
 	}
diff --git a/internal/tools/file_write_branch_test.go b/internal/tools/file_write_branch_test.go
index d7f7e12..483a1fd 100644
--- a/internal/tools/file_write_branch_test.go
+++ b/internal/tools/file_write_branch_test.go
@@ -39,9 +39,9 @@ func TestFileWriteBranchCreatesBranchAndFile(t *testing.T) {
 		_, _ = w.Write([]byte(createBranchResp))
 	})
 
-	// Upsert file → 201
+	// New file (no sha) → POST to /contents/{path}
 	mux.HandleFunc("/api/v1/repos/owner/myrepo/contents/doc.md", func(w http.ResponseWriter, r *http.Request) {
-		require.Equal(t, http.MethodPut, r.Method)
+		require.Equal(t, http.MethodPost, r.Method)
 		w.WriteHeader(http.StatusCreated)
 		_, _ = w.Write([]byte(upsertFileResp))
 	})
@@ -64,6 +64,39 @@ func TestFileWriteBranchCreatesBranchAndFile(t *testing.T) {
 	assert.Equal(t, "cmt1", result["commit_sha"])
 }
 
+func TestFileWriteBranchUsesPutWhenShaProvided(t *testing.T) {
+	mux := http.NewServeMux()
+
+	// Branch exists
+	mux.HandleFunc("/api/v1/repos/owner/myrepo/branches/feat/existing", func(w http.ResponseWriter, r *http.Request) {
+		w.Header().Set("Content-Type", "application/json")
+		_, _ = w.Write([]byte(branchCheckExistsResp))
+	})
+
+	// Existing file (sha provided) → PUT
+	mux.HandleFunc("/api/v1/repos/owner/myrepo/contents/doc.md", func(w http.ResponseWriter, r *http.Request) {
+		require.Equal(t, http.MethodPut, r.Method)
+		w.WriteHeader(http.StatusOK)
+		_, _ = w.Write([]byte(upsertFileResp))
+	})
+
+	srv := httptest.NewServer(mux)
+	defer srv.Close()
+
+	tool := tools.NewFileWriteBranch(gitea.NewClient(srv.URL, "tok"), allowlist.New([]string{"owner"}))
+	out, err := tool.Call(context.Background(), json.RawMessage(`{
+		"owner":"owner","name":"myrepo","path":"doc.md",
+		"content":"hello","branch":"feat/existing",
+		"sha":"oldsha","message":"update doc.md"
+	}`))
+	require.NoError(t, err)
+
+	var result map[string]any
+	require.NoError(t, json.Unmarshal(out, &result))
+	assert.Equal(t, "feat/existing", result["branch"])
+	assert.Equal(t, "cmt1", result["commit_sha"])
+}
+
 func TestFileWriteBranchUsesDefaultBaseWhenBaseEmpty(t *testing.T) {
 	var createBody []byte
 	mux := http.NewServeMux()