diff --git a/langfuse/model.py b/langfuse/model.py
index 69d721597..62e308954 100644
--- a/langfuse/model.py
+++ b/langfuse/model.py
@@ -195,9 +195,11 @@ def _escape_json_for_langchain(text: str) -> str:
 
             # ---------- opening brace ----------
             if ch == "{":
-                # leave existing “{{ …” untouched
+                # Leave existing “{{ …” untouched, but push a marker so a later
+                # “}}” can tell a pre-escaped pair from a JSON closing brace.
                 if i + 1 < n and text[i + 1] == "{":
                     out.append("{{")
+                    stack.append(None)  # pre-escaped “{{ … }}” pair
                     i += 2
                     continue
 
@@ -214,13 +216,31 @@ def _escape_json_for_langchain(text: str) -> str:
 
             # ---------- closing brace ----------
             elif ch == "}":
-                # leave existing “… }}” untouched
+                # A “}}” pair is pre-escaped (left untouched) unless its first
+                # “}” closes a JSON “{” that was doubled above (stack top True):
+                # each doubled “{” needs its own escaped “}}”.
                 if i + 1 < n and text[i + 1] == "}":
+                    top = stack.pop() if stack else False
+                    if top is True:
+                        # First "}" closes the doubled JSON brace: emit its
+                        # escaped pair, then reprocess the second "}".
+                        out.append("}}")
+                        i += 1
+                        continue
+                    # Pre-escaped pair (None), non-JSON close (False), or stray
+                    # pair: leave untouched.
                     out.append("}}")
                     i += 2
                     continue
 
-                is_json = stack.pop() if stack else False
+                # A lone "}" never closes a pre-escaped "{{": skip any stale
+                # markers to find the nearest unmatched "{".
+                is_json = False
+                while stack:
+                    top = stack.pop()
+                    if top is not None:
+                        is_json = top
+                        break
                 out.append("}}" if is_json else "}")
                 i += 1
                 continue
diff --git a/tests/unit/test_prompt_compilation.py b/tests/unit/test_prompt_compilation.py
index 1b96a14dd..7bdc789ea 100644
--- a/tests/unit/test_prompt_compilation.py
+++ b/tests/unit/test_prompt_compilation.py
@@ -932,3 +932,55 @@ def test_tool_calls_preservation_in_message_placeholder():
     # Final user message with compiled variable
     assert compiled_messages[4]["role"] == "user"
     assert compiled_messages[4]["content"] == "Help me with weather inquiry"
+
+
+def test_compact_nested_json_keeps_closing_braces():
+    """Regression test: compact nested JSON must not lose closing braces.
+
+    Covers langfuse/langfuse#18131: _escape_json_for_langchain treated every
+    ``}}`` as a pre-escaped pair and never popped the stack entry pushed for
+    the JSON ``{`` it doubled, so ``{"a": {"b": 1}}`` rendered as
+    ``{"a": {"b": 1}`` (one closing brace short).
+    """
+    prompt_string = 'Example: {"a": {"b": 1}}'
+
+    prompt = TextPromptClient(
+        Prompt_Text(
+            type="text",
+            name="compact_nested_json_test",
+            version=1,
+            config={},
+            tags=[],
+            labels=[],
+            prompt=prompt_string,
+        )
+    )
+
+    langchain_prompt_string = prompt.get_langchain_prompt()
+    assert langchain_prompt_string == 'Example: {{"a": {{"b": 1}}}}'
+
+    langchain_prompt = PromptTemplate.from_template(langchain_prompt_string)
+    assert langchain_prompt.format() == prompt_string
+
+
+def test_pre_escaped_placeholder_inside_nested_json():
+    """A pre-escaped ``{{name}}`` placeholder next to JSON closing braces must
+    be left alone while each doubled JSON ``{`` still gets its own ``}}``.
+    """
+    prompt_string = 'Reply with JSON like {"user": {"name": "{name}"}}'
+
+    prompt = TextPromptClient(
+        Prompt_Text(
+            type="text",
+            name="placeholder_inside_nested_json_test",
+            version=1,
+            config={},
+            tags=[],
+            labels=[],
+            prompt=prompt_string,
+        )
+    )
+
+    langchain_prompt_string = prompt.get_langchain_prompt()
+    langchain_prompt = PromptTemplate.from_template(langchain_prompt_string)
+    assert langchain_prompt.format(name="Ann") == 'Reply with JSON like {"user": {"name": "Ann"}}'