diff --git a/backend/tests/test_nginx_langgraph_body_size.py b/backend/tests/test_nginx_langgraph_body_size.py
new file mode 100644
index 000000000..7e32f0be7
--- /dev/null
+++ b/backend/tests/test_nginx_langgraph_body_size.py
@@ -0,0 +1,130 @@
+"""Regression coverage for issue #3952: long chat prompts through nginx's
+``/api/langgraph/`` route failing with a raw HTTP 500 (or 413) before the
+request ever reaches Gateway's application-level error handling.
+
+Root cause (confirmed by a live nginx reproduction during triage, not just
+config reading): nginx's defaults are unfit for a text-chat proxy route.
+
+- ``client_max_body_size`` defaults to ``1m``, well under a long pasted
+  prompt -- nginx rejects anything larger with a 413 before the body is
+  even read.
+- ``proxy_request_buffering`` defaults to ``on``, which spools any request
+  body larger than the in-memory ``client_body_buffer_size`` (~16k) to a
+  temp file (``client_body_temp``, under the nginx ``-p`` prefix) before
+  proxying it upstream. On a non-root local run -- the common ``make dev``
+  case, and the same class of problem already documented elsewhere in these
+  same config files for *response* buffering -- that temp directory can be
+  unwritable, which makes nginx fail the request with a raw
+  "500 Internal Server Error" page and a "Permission denied" line in the
+  error log, matching the reporter's exact symptom (nginx error page, no
+  DeerFlow JSON/SSE error) and reproduced independently while diagnosing
+  this issue.
+
+The uploads location (``/api/threads/{id}/uploads``) already carries both
+settings for file uploads. This locks the same settings onto
+``/api/langgraph/`` for chat prompts, sized for text rather than binary
+uploads (see the range check below), across all three places this nginx
+config is maintained: the Docker production config, the local-dev config
+used by ``make dev``, and the Kubernetes/Helm ConfigMap template.
+"""
+
+from __future__ import annotations
+
+import re
+from pathlib import Path
+
+import pytest
+
+REPO_ROOT = Path(__file__).resolve().parents[2]
+NGINX_CONFIGS = (
+    "docker/nginx/nginx.conf",
+    "docker/nginx/nginx.local.conf",
+    "deploy/helm/deer-flow/templates/configmap-nginx.yaml",
+)
+
+# Text prompts never carry binary file attachments (those go through the
+# dedicated uploads route), so the ceiling here is intentionally well below
+# the uploads route's 100M -- generous for even a very long pasted document,
+# while avoiding needlessly letting one chat request force Gateway to buffer
+# and JSON-parse an arbitrarily large body in memory.
+_MIN_EXPECTED_BODY_SIZE_BYTES = 5 * 1024 * 1024
+_MAX_EXPECTED_BODY_SIZE_BYTES = 100 * 1024 * 1024
+
+_SIZE_MULTIPLIERS = {"": 1, "k": 1024, "m": 1024**2, "g": 1024**3}
+
+
+def _read(path: str) -> str:
+    return (REPO_ROOT / path).read_text(encoding="utf-8")
+
+
+def _extract_location_block(content: str, location_selector: str) -> str:
+    """Extract a single nginx ``location <location_selector> { ... }`` block
+    by brace-depth matching, so assertions target only that location and
+    can't be satisfied by a directive that merely appears elsewhere in the
+    file (e.g. the neighboring uploads location, which already has both
+    settings and must not make the langgraph-route assertions pass by
+    accident)."""
+    marker = re.compile(r"location\s+" + re.escape(location_selector) + r"\s*\{")
+    match = marker.search(content)
+    assert match, f"could not find `location {location_selector}` block"
+
+    start = match.end() - 1  # index of the opening brace
+    depth = 0
+    for i, ch in enumerate(content[start:], start=start):
+        if ch == "{":
+            depth += 1
+        elif ch == "}":
+            depth -= 1
+            if depth == 0:
+                return content[start : i + 1]
+
+    raise AssertionError(f"unbalanced braces in `location {location_selector}` block")
+
+
+def _parse_body_size_bytes(block: str) -> int:
+    match = re.search(r"client_max_body_size\s+(\d+)\s*([mMkKgG]?)\s*;", block)
+    assert match, "client_max_body_size value not found or not parseable"
+    value, unit = match.groups()
+    return int(value) * _SIZE_MULTIPLIERS[unit.lower()]
+
+
+@pytest.mark.parametrize("path", NGINX_CONFIGS)
+def test_langgraph_route_disables_request_buffering(path):
+    content = _read(path)
+    block = _extract_location_block(content, "/api/langgraph/")
+
+    assert "proxy_request_buffering off;" in block, (
+        f"{path}: /api/langgraph/ does not disable request buffering, so nginx "
+        "spools long chat-prompt request bodies to a temp file before proxying "
+        "them to Gateway, which can 500 with a permission error on non-root "
+        "local runs -- see issue #3952"
+    )
+
+
+@pytest.mark.parametrize("path", NGINX_CONFIGS)
+def test_langgraph_route_raises_body_size_limit_for_text_prompts(path):
+    content = _read(path)
+    block = _extract_location_block(content, "/api/langgraph/")
+
+    size_bytes = _parse_body_size_bytes(block)
+
+    assert _MIN_EXPECTED_BODY_SIZE_BYTES <= size_bytes <= _MAX_EXPECTED_BODY_SIZE_BYTES, (
+        f"{path}: /api/langgraph/ client_max_body_size is {size_bytes} bytes, "
+        f"expected between {_MIN_EXPECTED_BODY_SIZE_BYTES} and "
+        f"{_MAX_EXPECTED_BODY_SIZE_BYTES} bytes -- comfortably above nginx's 1m "
+        "default (the actual bug) but below the uploads route's 100M, since "
+        "this route only ever carries JSON chat text, never binary files"
+    )
+
+
+@pytest.mark.parametrize("path", NGINX_CONFIGS)
+def test_uploads_route_still_has_its_own_body_size_settings(path):
+    """Non-regression guard: confirms the extraction helper is precise (the
+    uploads location has both directives with its own, larger value) and that
+    fixing the langgraph route does not accidentally touch the upload route's
+    existing configuration."""
+    content = _read(path)
+    block = _extract_location_block(content, r"~ ^/api/threads/[^/]+/uploads")
+
+    assert "client_max_body_size 100M;" in block
+    assert "proxy_request_buffering off;" in block
diff --git a/deploy/helm/deer-flow/templates/configmap-nginx.yaml b/deploy/helm/deer-flow/templates/configmap-nginx.yaml
index 4fc4257d3..7d5a8fa54 100644
--- a/deploy/helm/deer-flow/templates/configmap-nginx.yaml
+++ b/deploy/helm/deer-flow/templates/configmap-nginx.yaml
@@ -74,6 +74,11 @@ data:
                 proxy_set_header X-Forwarded-Proto $forwarded_proto;
                 proxy_set_header Connection '';
                 proxy_set_header X-Accel-Buffering no;
+                # Long chat/text-prompt support (issue #3952): matches the
+                # uploads location's buffering fix below, sized for text
+                # prompts rather than binary file uploads.
+                client_max_body_size 20M;
+                proxy_request_buffering off;
                 proxy_connect_timeout 600s;
                 proxy_send_timeout 600s;
                 proxy_read_timeout 600s;
diff --git a/docker/nginx/nginx.conf b/docker/nginx/nginx.conf
index 09bca618f..c2fd40925 100644
--- a/docker/nginx/nginx.conf
+++ b/docker/nginx/nginx.conf
@@ -68,6 +68,17 @@ http {
             # SSE/Streaming support
             proxy_set_header X-Accel-Buffering no;
 
+            # Long chat/text-prompt support (issue #3952): a sufficiently long
+            # pasted prompt otherwise exceeds nginx's default 1m
+            # client_max_body_size, or gets spooled to a temp file via
+            # proxy_request_buffering before reaching Gateway -- on a non-root
+            # local run that temp directory may not be writable, producing a
+            # raw nginx 500 instead of a graceful application error. Mirrors
+            # the uploads location's fix below (same mechanism, sized for text
+            # prompts rather than binary file uploads).
+            client_max_body_size 20M;
+            proxy_request_buffering off;
+
             # Timeouts for long-running requests
             proxy_connect_timeout 600s;
             proxy_send_timeout 600s;
diff --git a/docker/nginx/nginx.local.conf b/docker/nginx/nginx.local.conf
index 16c7de0b1..664a45722 100644
--- a/docker/nginx/nginx.local.conf
+++ b/docker/nginx/nginx.local.conf
@@ -57,6 +57,17 @@ http {
             proxy_cache off;
             proxy_set_header X-Accel-Buffering no;
 
+            # Long chat/text-prompt support (issue #3952): a sufficiently long
+            # pasted prompt otherwise exceeds nginx's default 1m
+            # client_max_body_size, or gets spooled to a temp file via
+            # proxy_request_buffering before reaching Gateway -- on a non-root
+            # local run that temp directory may not be writable, producing a
+            # raw nginx 500 instead of a graceful application error. Mirrors
+            # the uploads location's fix below (same mechanism, sized for text
+            # prompts rather than binary file uploads).
+            client_max_body_size 20M;
+            proxy_request_buffering off;
+
             # Timeouts for long-running requests
             proxy_connect_timeout 600s;
             proxy_send_timeout 600s;