diff --git a/plugins/image_gen/openai-codex/__init__.py b/plugins/image_gen/openai-codex/__init__.py
index 26c7bb827b07..d8d5ea80d97e 100644
--- a/plugins/image_gen/openai-codex/__init__.py
+++ b/plugins/image_gen/openai-codex/__init__.py
@@ -48,14 +48,22 @@ _IMAGE_GENERATION_UNAVAILABLE_MESSAGE = (
     "Image generation is not enabled for the current Codex account. "
     "Switch the image provider to OpenAI API key, FAL, or xAI."
 )
+_IMAGE_GENERATION_UNSUPPORTED_ERROR = (
+    "Tool choice 'image_generation' not found in 'tools' parameter."
+)
 
 
 def _is_image_generation_unsupported_error(status_code: int, body: str) -> bool:
     """Match only Codex's account-capability rejection for the image tool."""
     if status_code != 400:
         return False
-    normalized = " ".join((body or "").lower().split())
-    return "tool choice 'image_generation' not found in 'tools' parameter" in normalized
+    try:
+        payload = json.loads(body)
+        error = payload.get("error") if isinstance(payload, dict) else None
+        message = error.get("message") if isinstance(error, dict) else None
+    except (TypeError, ValueError):
+        message = body
+    return isinstance(message, str) and message.strip() == _IMAGE_GENERATION_UNSUPPORTED_ERROR
 
 
 # ---------------------------------------------------------------------------
@@ -411,12 +419,13 @@ def _collect_image_b64(
                 response.raise_for_status()
             except httpx.HTTPStatusError as exc:
                 exc.response.read()
-                body = exc.response.text[:500]
+                full_body = exc.response.text
                 if _is_image_generation_unsupported_error(
                     exc.response.status_code,
-                    body,
+                    full_body,
                 ):
-                    raise CodexImageGenerationUnsupportedError(body) from exc
+                    raise CodexImageGenerationUnsupportedError(full_body) from exc
+                body = full_body[:500]
                 raise RuntimeError(
                     f"Codex Responses API returned HTTP {exc.response.status_code}: {body}"
                 ) from exc
diff --git a/tests/plugins/image_gen/test_openai_codex_provider.py b/tests/plugins/image_gen/test_openai_codex_provider.py
index eaf8f09264b2..338ea97418a7 100644
--- a/tests/plugins/image_gen/test_openai_codex_provider.py
+++ b/tests/plugins/image_gen/test_openai_codex_provider.py
@@ -9,6 +9,7 @@ endpoint.
 from __future__ import annotations
 
 import importlib
+import json
 from pathlib import Path
 
 import pytest
@@ -348,6 +349,50 @@ class TestCapabilityErrorDetection:
     def test_does_not_misclassify_other_failures(self, status_code, body):
         assert codex_plugin._is_image_generation_unsupported_error(status_code, body) is False
 
+    def test_does_not_match_error_message_with_extra_text(self):
+        body = json.dumps({
+            "error": {
+                "message": (
+                    "Tool choice 'image_generation' not found in 'tools' parameter "
+                    "because the request is malformed."
+                )
+            }
+        })
+
+        assert codex_plugin._is_image_generation_unsupported_error(400, body) is False
+
+    def test_collect_classifies_exact_http_error_after_large_metadata(self, monkeypatch):
+        import httpx
+
+        body = json.dumps({
+            "metadata": "x" * 600,
+            "error": {
+                "message": "Tool choice 'image_generation' not found in 'tools' parameter."
+            },
+        })
+
+        def _handler(request):
+            return httpx.Response(400, text=body, request=request)
+
+        real_client = httpx.Client
+        monkeypatch.setattr(
+            httpx,
+            "Client",
+            lambda *args, **kwargs: real_client(
+                transport=httpx.MockTransport(_handler),
+                headers=kwargs.get("headers"),
+                timeout=kwargs.get("timeout"),
+            ),
+        )
+
+        with pytest.raises(codex_plugin.CodexImageGenerationUnsupportedError):
+            codex_plugin._collect_image_b64(
+                "codex-token",
+                prompt="a cat",
+                size="1024x1024",
+                quality="low",
+            )
+
 
 # ── Plugin entry point ──────────────────────────────────────────────────────