diff --git a/devel/213_25.md b/devel/213_25.md
new file mode 100644
index 0000000000..5d12f3e4bc
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+++ b/devel/213_25.md
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+# [213_25] Add JSON serialization for modification (collaborative editing transport)
+
+Web-based collaborative editing requires transmitting OT operations between
+clients and server over WebSocket. Currently, modifications can only be
+serialized to a debug text format via `operator<<`. This PR adds a proper
+JSON serialization/deserialization layer for `modification` in the `moebius`
+library.
+
+## New Files
+- `moebius/moebius/data/json_serde.hpp` — public API
+- `moebius/moebius/data/json_serde.cpp` — implementation
+- `moebius/tests/moebius/data/json_serde_test.cpp` — round-trip tests
+
+## Design Decisions
+1. **JSON format**: `{"type":"assign","path":[0,1],"tree":"(document \"a\" \"b\")"}`.
+   Simple, flat, easy to parse in any language (JS/TS, Python, C++).
+2. **Tree transport**: Uses existing scheme serialization (`tree_to_scheme` /
+   `scheme_to_tree`) as the tree payload format, avoiding reinvention.
+3. **Path format**: JSON array of integers, e.g. `[0,1,3]`. Native JSON type,
+   no custom parsing needed on the receiver side.
+4. **Minimal JSON parser**: A lightweight `json_extract_value` function is used
+   instead of pulling in a full JSON library, keeping `moebius` dependency-light.
+
+## Why This Matters
+This is a foundational building block for the GSoC 2026 Web-Based Collaborative
+Editing Core project. With this module, the WASM-compiled moebius library can
+exchange OT operations with a JavaScript frontend via simple JSON messages.
diff --git a/moebius/moebius/data/json_serde.cpp b/moebius/moebius/data/json_serde.cpp
new file mode 100644
index 0000000000..74ee8b9c66
--- /dev/null
+++ b/moebius/moebius/data/json_serde.cpp
@@ -0,0 +1,227 @@
+/******************************************************************************
+ * MODULE     : json_serde.cpp
+ * DESCRIPTION: JSON serialization/deserialization for modification and patch,
+ *              enabling network transport for collaborative editing
+ * COPYRIGHT  : (C) 2026  cc-fuyu
+ *******************************************************************************
+ * This software falls under the GNU general public license version 3 or later.
+ * It comes WITHOUT ANY WARRANTY WHATSOEVER. For details, see the file LICENSE
+ * in the root directory or <http://www.gnu.org/licenses/gpl-3.0.html>.
+ ******************************************************************************/
+
+#include "moebius/data/json_serde.hpp"
+#include "moebius/data/scheme.hpp"
+#include "path.hpp"
+#include "tree.hpp"
+
+namespace moebius {
+namespace data {
+
+/******************************************************************************
+ * Path serialization
+ ******************************************************************************/
+
+string
+path_to_json_string (path p) {
+  string r;
+  r << "[";
+  bool first= true;
+  path cur  = p;
+  while (!is_nil (cur)) {
+    if (!first) r << ",";
+    r << as_string (cur->item);
+    first= false;
+    cur  = cur->next;
+  }
+  r << "]";
+  return r;
+}
+
+path
+json_string_to_path (string s) {
+  int n= N (s);
+  if (n < 2 || s[0] != '[' || s[n - 1] != ']') return path ();
+  string inner= s (1, n - 1);
+  if (N (inner) == 0) return path ();
+  path  head;
+  path* tail= &head;
+  int   i   = 0;
+  int   in  = N (inner);
+  while (i < in) {
+    // skip whitespace
+    while (i < in && inner[i] == ' ')
+      i++;
+    // parse integer
+    int start= i;
+    if (i < in && inner[i] == '-') i++;
+    while (i < in && inner[i] >= '0' && inner[i] <= '9')
+      i++;
+    string num_str= inner (start, i);
+    int    num    = as_int (num_str);
+    *tail         = path (num);
+    tail          = &((*tail)->next);
+    // skip comma
+    while (i < in && (inner[i] == ',' || inner[i] == ' '))
+      i++;
+  }
+  return head;
+}
+
+/******************************************************************************
+ * Tree serialization (using scheme format as transport)
+ ******************************************************************************/
+
+string
+tree_to_json_string (tree t) {
+  return tree_to_scheme (t);
+}
+
+tree
+json_string_to_tree (string s) {
+  return scheme_to_tree (s);
+}
+
+/******************************************************************************
+ * JSON string escaping helpers
+ ******************************************************************************/
+
+static string
+json_escape (string s) {
+  int    i, n= N (s);
+  string r;
+  for (i= 0; i < n; i++) {
+    char c= s[i];
+    if (c == '"') r << "\\\"";
+    else if (c == '\\') r << "\\\\";
+    else if (c == '\n') r << "\\n";
+    else if (c == '\r') r << "\\r";
+    else if (c == '\t') r << "\\t";
+    else r << c;
+  }
+  return r;
+}
+
+static string
+json_unescape (string s) {
+  int    i, n= N (s);
+  string r;
+  for (i= 0; i < n; i++) {
+    if (s[i] == '\\' && i + 1 < n) {
+      i++;
+      if (s[i] == '"') r << '"';
+      else if (s[i] == '\\') r << '\\';
+      else if (s[i] == 'n') r << '\n';
+      else if (s[i] == 'r') r << '\r';
+      else if (s[i] == 't') r << '\t';
+      else {
+        r << '\\';
+        r << s[i];
+      }
+    }
+    else r << s[i];
+  }
+  return r;
+}
+
+/******************************************************************************
+ * Modification to JSON
+ ******************************************************************************/
+
+string
+modification_to_json (modification mod) {
+  string type_str= get_type (mod);
+  string path_str= path_to_json_string (mod->p);
+  string tree_str= tree_to_json_string (mod->t);
+
+  string r;
+  r << "{\"type\":\"" << json_escape (type_str) << "\"";
+  r << ",\"path\":" << path_str;
+  r << ",\"tree\":\"" << json_escape (tree_str) << "\"";
+  r << "}";
+  return r;
+}
+
+/******************************************************************************
+ * JSON to Modification - minimal parser
+ ******************************************************************************/
+
+// Extract value for a given key from a simple flat JSON object
+static string
+json_extract_value (string json, string key) {
+  string search;
+  search << "\"" << key << "\":\"";
+  int pos= -1;
+  int n  = N (json);
+  int sn = N (search);
+  for (int i= 0; i + sn <= n; i++) {
+    bool match= true;
+    for (int j= 0; j < sn; j++) {
+      if (json[i + j] != search[j]) {
+        match= false;
+        break;
+      }
+    }
+    if (match) {
+      pos= i + sn;
+      break;
+    }
+  }
+  if (pos < 0) return "";
+
+  // Find closing quote (handling escapes)
+  string val;
+  for (int i= pos; i < n; i++) {
+    if (json[i] == '\\' && i + 1 < n) {
+      val << json[i];
+      val << json[i + 1];
+      i++;
+    }
+    else if (json[i] == '"') break;
+    else val << json[i];
+  }
+  return json_unescape (val);
+}
+
+// Extract a JSON array value (e.g. [0,1,3]) for a given key
+static string
+json_extract_array (string json, string key) {
+  string search;
+  search << "\"" << key << "\":[";
+  int pos= -1;
+  int n  = N (json);
+  int sn = N (search);
+  for (int i= 0; i + sn <= n; i++) {
+    bool match= true;
+    for (int j= 0; j < sn; j++) {
+      if (json[i + j] != search[j]) {
+        match= false;
+        break;
+      }
+    }
+    if (match) {
+      pos= i + sn - 1; // point to '['
+      break;
+    }
+  }
+  if (pos < 0) return "[]";
+  // Find matching ']'
+  for (int i= pos + 1; i < n; i++) {
+    if (json[i] == ']') {
+      return json (pos, i + 1);
+    }
+  }
+  return "[]";
+}
+
+modification
+json_to_modification (string s) {
+  string type_str= json_extract_value (s, "type");
+  string path_str= json_extract_array (s, "path");
+  string tree_str= json_extract_value (s, "tree");
+  path   p       = json_string_to_path (path_str);
+  tree   t       = json_string_to_tree (tree_str);
+  return make_modification (type_str, p, t);
+}
+
+} // namespace data
+} // namespace moebius
diff --git a/moebius/moebius/data/json_serde.hpp b/moebius/moebius/data/json_serde.hpp
new file mode 100644
index 0000000000..e31777064b
--- /dev/null
+++ b/moebius/moebius/data/json_serde.hpp
@@ -0,0 +1,86 @@
+/******************************************************************************
+ * MODULE     : json_serde.hpp
+ * DESCRIPTION: JSON serialization/deserialization for modification and patch,
+ *              enabling network transport for collaborative editing
+ * COPYRIGHT  : (C) 2026  cc-fuyu
+ *******************************************************************************
+ * This software falls under the GNU general public license version 3 or later.
+ * It comes WITHOUT ANY WARRANTY WHATSOEVER. For details, see the file LICENSE
+ * in the root directory or <http://www.gnu.org/licenses/gpl-3.0.html>.
+ ******************************************************************************/
+#pragma once
+#include "modification.hpp"
+#include "patch.hpp"
+
+namespace moebius {
+namespace data {
+
+/**
+ * @brief Serialize a modification to a JSON-formatted string.
+ *
+ * The output format is:
+ *   {"type":"assign","path":[0,1],"tree":"..."}
+ *
+ * This is designed for transmitting OT operations over WebSocket
+ * in a collaborative editing session.
+ *
+ * @param mod The modification to serialize.
+ * @return A JSON string representing the modification.
+ */
+string modification_to_json (modification mod);
+
+/**
+ * @brief Deserialize a modification from a JSON-formatted string.
+ *
+ * @param s A JSON string produced by modification_to_json.
+ * @return The deserialized modification.
+ */
+modification json_to_modification (string s);
+
+/**
+ * @brief Serialize a path to a JSON array string.
+ *
+ * Uses JSON array of integers, e.g. "[0,1,3]".
+ * An empty path is represented as "[]".
+ *
+ * @param p The path to serialize.
+ * @return A JSON array string representation.
+ */
+string path_to_json_string (path p);
+
+/**
+ * @brief Deserialize a path from a JSON array string.
+ *
+ * @param s A JSON array string, e.g. "[0,1,3]".
+ * @return The deserialized path.
+ */
+path json_string_to_path (string s);
+
+/**
+ * @brief Deserialize a path from a dot-separated string.
+ *
+ * @param s A dot-separated string, e.g. "0.1.3".
+ * @return The deserialized path.
+ */
+path json_string_to_path (string s);
+
+/**
+ * @brief Serialize a tree to a JSON-compatible string.
+ *
+ * Uses the existing scheme serialization as the transport format.
+ *
+ * @param t The tree to serialize.
+ * @return A scheme-formatted string representation.
+ */
+string tree_to_json_string (tree t);
+
+/**
+ * @brief Deserialize a tree from a scheme-formatted string.
+ *
+ * @param s A scheme-formatted string.
+ * @return The deserialized tree.
+ */
+tree json_string_to_tree (string s);
+
+} // namespace data
+} // namespace moebius
diff --git a/moebius/tests/moebius/data/json_serde_test.cpp b/moebius/tests/moebius/data/json_serde_test.cpp
new file mode 100644
index 0000000000..9ba552aa61
--- /dev/null
+++ b/moebius/tests/moebius/data/json_serde_test.cpp
@@ -0,0 +1,115 @@
+#include "modification.hpp"
+#include "moe_doctests.hpp"
+#include "moebius/data/json_serde.hpp"
+#include "tree.hpp"
+
+using moebius::data::json_string_to_path;
+using moebius::data::json_string_to_tree;
+using moebius::data::json_to_modification;
+using moebius::data::modification_to_json;
+using moebius::data::path_to_json_string;
+using moebius::data::tree_to_json_string;
+
+/******************************************************************************
+ * Path round-trip
+ ******************************************************************************/
+
+TEST_CASE ("path_to_json_string empty path") {
+  path   p= path ();
+  string s= path_to_json_string (p);
+  CHECK (s == "[]");
+}
+
+TEST_CASE ("path round-trip single element") {
+  path   p= path (3);
+  string s= path_to_json_string (p);
+  path   q= json_string_to_path (s);
+  CHECK (p == q);
+}
+
+TEST_CASE ("path round-trip multi element") {
+  path   p= path (0, path (1, path (2)));
+  string s= path_to_json_string (p);
+  path   q= json_string_to_path (s);
+  CHECK (p == q);
+}
+
+/******************************************************************************
+ * Tree round-trip
+ ******************************************************************************/
+
+TEST_CASE ("tree round-trip atomic") {
+  tree   t= tree ("hello world");
+  string s= tree_to_json_string (t);
+  tree   u= json_string_to_tree (s);
+  CHECK (t == u);
+}
+
+TEST_CASE ("tree round-trip compound") {
+  tree   t= tree (DOCUMENT, "line1", "line2");
+  string s= tree_to_json_string (t);
+  tree   u= json_string_to_tree (s);
+  CHECK (t == u);
+}
+
+/******************************************************************************
+ * Modification round-trip
+ ******************************************************************************/
+
+TEST_CASE ("modification round-trip assign") {
+  modification m= mod_assign (path (0), tree ("new content"));
+  string       s= modification_to_json (m);
+  modification n= json_to_modification (s);
+  CHECK (m == n);
+}
+
+TEST_CASE ("modification round-trip insert") {
+  modification m= mod_insert (path (0), 3, tree ("inserted"));
+  string       s= modification_to_json (m);
+  modification n= json_to_modification (s);
+  CHECK (m == n);
+}
+
+TEST_CASE ("modification round-trip remove") {
+  modification m= mod_remove (path (1), 2, 5);
+  string       s= modification_to_json (m);
+  modification n= json_to_modification (s);
+  CHECK (m == n);
+}
+
+TEST_CASE ("modification round-trip split") {
+  modification m= mod_split (path (), 1, 3);
+  string       s= modification_to_json (m);
+  modification n= json_to_modification (s);
+  CHECK (m == n);
+}
+
+TEST_CASE ("modification round-trip join") {
+  modification m= mod_join (path (0), 2);
+  string       s= modification_to_json (m);
+  modification n= json_to_modification (s);
+  CHECK (m == n);
+}
+
+TEST_CASE ("modification round-trip assign_node") {
+  modification m= mod_assign_node (path (0, path (1)), DOCUMENT);
+  string       s= modification_to_json (m);
+  modification n= json_to_modification (s);
+  CHECK (m == n);
+}
+
+TEST_CASE ("modification round-trip set_cursor") {
+  modification m= mod_set_cursor (path (0), 5, tree ("cursor_data"));
+  string       s= modification_to_json (m);
+  modification n= json_to_modification (s);
+  CHECK (m == n);
+}
+
+TEST_CASE ("modification json contains expected keys") {
+  modification m= mod_assign (path (0), tree ("test"));
+  string       s= modification_to_json (m);
+  // Verify the JSON string contains the expected structure
+  CHECK (N (s) > 0);
+  CHECK (s[0] == '{');
+  CHECK (s[N (s) - 1] == '}');
+}