diff --git a/api/controllers/openapi/_errors.py b/api/controllers/openapi/_errors.py
index 84b76091c3..c910b17623 100644
--- a/api/controllers/openapi/_errors.py
+++ b/api/controllers/openapi/_errors.py
@@ -14,8 +14,13 @@ mandated by the OAuth spec)::
 error-handler path funnels through one builder, and it also rewrites
 ``e.data`` because flask-restx ``Api.handle_error`` lets a pre-existing
 ``e.data`` override the registered handler's return value.
+
+The transport-generic enum members, ``_CODE_BY_STATUS`` and the
+``OpenApiError``/``OpenApiErrorFormatter`` bases are openapi-only today;
+promote them to ``libs`` if a second surface adopts ``ErrorBody``.
 """
 
+import logging
 from enum import StrEnum
 from typing import Any
 
@@ -96,6 +101,8 @@ _CODE_BY_STATUS: dict[int, OpenApiErrorCode] = {
 
 _GENERIC_500_MESSAGE = "Internal Server Error"
 
+logger = logging.getLogger(__name__)
+
 
 class OpenApiError(HTTPException):
     """Dedicated throwable for the /openapi/v1 surface.
@@ -136,14 +143,24 @@ class OpenApiErrorFormatter:
         exc_data = getattr(e, "data", None)
         merged: dict[str, Any] = {**data, **exc_data} if isinstance(exc_data, dict) else dict(data)
 
-        body = ErrorBody(
-            code=self._resolve_code(e, status_code),
-            message=self._resolve_message(merged, status_code),
-            status=status_code,
-            hint=self._resolve_hint(e),
-            details=self._extract_details(e, merged),
-        )
-        wire = body.model_dump(mode="json", exclude_none=True)
+        # finalize runs inside the framework error handler: raising here would
+        # replace the response with an unformatted 500, so fall back instead
+        try:
+            body = ErrorBody(
+                code=self._resolve_code(e, status_code),
+                message=self._resolve_message(merged, status_code),
+                status=status_code,
+                hint=self._resolve_hint(e),
+                details=self._extract_details(e, merged),
+            )
+            wire = body.model_dump(mode="json", exclude_none=True)
+        except Exception:
+            logger.exception("error-body build failed; emitting fallback body")
+            wire = {
+                "code": str(_CODE_BY_STATUS.get(status_code, OpenApiErrorCode.UNKNOWN)),
+                "message": http_status_message(status_code) or "request failed",
+                "status": status_code,
+            }
 
         # flask-restx Api.handle_error does `data = getattr(e, "data", default_data)`
         # AFTER our handler returns, so a pre-existing e.data (flask_restx.abort,
diff --git a/api/libs/external_api.py b/api/libs/external_api.py
index 077edad6b7..fc5286bf80 100644
--- a/api/libs/external_api.py
+++ b/api/libs/external_api.py
@@ -32,7 +32,9 @@ def register_external_error_handlers(api: Api, body_formatter: ErrorBodyFormatte
     def handle_http_exception(e: HTTPException):
         got_request_exception.send(current_app, exception=e)
 
-        # If Werkzeug already prepared a Response, just use it.
+        # If Werkzeug already prepared a Response, just use it. This bypasses
+        # body_formatter entirely — surfaces with a formatter must not raise
+        # exceptions carrying a pre-built response.
         if e.response is not None:
             return e.response
 
diff --git a/api/tests/unit_tests/controllers/openapi/test_error_contract.py b/api/tests/unit_tests/controllers/openapi/test_error_contract.py
index e3ae80ca6d..7da539d264 100644
--- a/api/tests/unit_tests/controllers/openapi/test_error_contract.py
+++ b/api/tests/unit_tests/controllers/openapi/test_error_contract.py
@@ -126,6 +126,17 @@ class TestOpenApiErrorFormatter:
 
         assert second == first
 
+    def test_malformed_canonical_details_falls_back_instead_of_raising(self, fmt):
+        # finalize runs inside the framework error handler; a ValidationError
+        # escaping it would replace the response with an unformatted 500
+        e = UnprocessableEntity()
+        e.data = {"message": "broken", "details": [{"bad": "shape"}]}
+        data = {"code": "unprocessable_entity", "message": "broken", "status": 422}
+
+        wire = fmt.finalize(e, data, 422)
+
+        assert wire == {"code": "invalid_param", "message": "Unprocessable Entity", "status": 422}
+
     def test_base_http_exception_error_code_wins_over_status_map(self, fmt):
         e = ProviderQuotaExceededError()
         data = dict(e.data)
diff --git a/cli/src/errors/format.test.ts b/cli/src/errors/format.test.ts
index bebbfb0c73..33591a6da3 100644
--- a/cli/src/errors/format.test.ts
+++ b/cli/src/errors/format.test.ts
@@ -67,6 +67,25 @@ describe('formatErrorForCli — human', () => {
     expect(formatErrorForCli(noServerHint, { isErrTTY: false })).toContain('run difyctl auth login')
   })
 
+  it('omits the loc prefix when a detail has no loc', () => {
+    const err = new HttpClientError({
+      code: ErrorCode.Server4xxOther,
+      message: 'Request validation failed',
+      httpStatus: 422,
+      serverError: {
+        code: 'invalid_param',
+        message: 'Request validation failed',
+        status: 422,
+        details: [{ type: 'invalid', loc: [], msg: 'body required' }],
+      },
+    })
+
+    const out = formatErrorForCli(err, { isErrTTY: false })
+
+    expect(out).toContain('- body required (invalid)')
+    expect(out).not.toContain('- : body required')
+  })
+
   it('renders request and http_status lines', () => {
     const err = new HttpClientError({
       code: ErrorCode.Server5xx,
diff --git a/cli/src/errors/format.ts b/cli/src/errors/format.ts
index 63a0a0b28b..281229cb6d 100644
--- a/cli/src/errors/format.ts
+++ b/cli/src/errors/format.ts
@@ -50,8 +50,10 @@ function renderHuman(env: ErrorEnvelope, isErrTTY: boolean): string {
   const server = e.server
   const headerCode = server?.code ?? e.code
   const lines: string[] = [`${headerCode}: ${e.message}`]
-  for (const d of server?.details ?? [])
-    lines.push(`  - ${(d.loc ?? []).join('.')}: ${d.msg} (${d.type})`)
+  for (const d of server?.details ?? []) {
+    const loc = (d.loc ?? []).join('.')
+    lines.push(`  - ${loc ? `${loc}: ` : ''}${d.msg} (${d.type})`)
+  }
   const hint = server?.hint ?? e.hint
   if (hint !== undefined && hint !== null)
     lines.push(`${cs.magenta('hint:')} ${cs.cyan(hint)}`)