diff --git a/benchmarks/gdpval/README.md b/benchmarks/gdpval/README.md
index 7b7be3e4c0a..3372ec3d7c2 100644
--- a/benchmarks/gdpval/README.md
+++ b/benchmarks/gdpval/README.md
@@ -451,6 +451,47 @@ gym eval run \
   `--resume` reruns; Pi does not set it yet. So a timeout counts against Pi's score, but not against Stirrup's.
 - The prompt and the harness differ from the certified Stirrup setup, so these are not AA v2 scores.
 
+`benchmarks/gdpval/codex.yaml` runs [Codex](../../responses_api_agents/codex_agent/README.md) the same way. Codex sends
+no sampling or output limit to a Gym model server, so set them on the model server, for example on `openai_model` in
+front of vLLM:
+
+```bash
+export PERSIST_DELIVERABLES_DIR=$PWD/output/gdpval_codex/deliverables
+gym eval prepare --benchmark gdpval/codex
+gym eval run \
+    --model-type openai_model \
+    --benchmark gdpval/codex \
+    --split benchmark \
+    --output results/gdpval_codex.jsonl \
+    --model-url <vllm_base_url> \
+    --model-api-key <vllm_api_key> \
+    --model <served_model_name> \
+    '++policy_model.responses_api_models.openai_model.extra_body={temperature: 1.0, top_p: 0.95, max_output_tokens: 32768}' \
+    '++policy_model.responses_api_models.openai_model.propagate_upstream_http_status_codes=[400]'
+```
+
+- The rows, the task tools and the deliverables directory work as for Pi, and these are not AA v2 scores either.
+  Codex installs Node.js and Codex in the task sandbox at the start of each episode, from nodejs.org and the npm
+  registry (about 190 MB). The model sees the tools as `mcp__gdpval_resources_server__<tool>`, and rollouts record
+  the calls by the tool name alone, such as `finish`. Codex keeps about 12 KB of each tool result, so the model sees
+  only the start and end of a long `fetch_web_page` result.
+- `extra_body` pins the Stirrup agent's temperature and top_p and limits each model call to 32,768 output tokens.
+- Codex checks `model_auto_compact_token_limit` after each reply that calls a tool, so the context can pass it by
+  about a reply and its tool output before Codex compacts, and the compaction call reserves 32,768 tokens for its own
+  reply. So keep the limit at about the window minus twice the output limit or lower: `codex.yaml` uses 196,608 for
+  a 262,144-token window. A near-full reply with a large tool result can still overflow the compaction call, and the
+  run then fails.
+- Codex does not compact on a context-length error; the run fails. `propagate_upstream_http_status_codes: [400]`
+  makes it fail within seconds with vLLM's message. By default `openai_model` returns HTTP 500, and Codex sends the
+  request 30 times before failing with a "high demand" error.
+- `finish` does not end a Codex run: Codex continues until the model replies without a tool call, and a later
+  `finish` or `abandon_task_finish` replaces the recorded call. The files are collected when the run ends, so a run
+  that fails after `finish`, for example on a context-length error, is not judged.
+- The Codex agent rejects sampling and output limits in the request, so every row fails if `gym eval run` gets
+  `--temperature`, `--top-p` or `--max-output-tokens`. Change Codex's own settings, such as `model_context_window`
+  and `model_auto_compact_token_limit` for another window, with
+  `++gdpval_codex_agent.responses_api_agents.codex_agent.<key>=<value>`.
+
 ## Multi-judge panel
 
 By default every GDPVal deliverable is graded by a **panel** of frontier LLM
diff --git a/benchmarks/gdpval/codex.yaml b/benchmarks/gdpval/codex.yaml
new file mode 100644
index 00000000000..62015ec64e4
--- /dev/null
+++ b/benchmarks/gdpval/codex.yaml
@@ -0,0 +1,61 @@
+# SPDX-FileCopyrightText: Copyright (c) 2026 NVIDIA CORPORATION & AFFILIATES. All rights reserved.
+# SPDX-License-Identifier: Apache-2.0
+# GDPVal with Codex in place of Stirrup: same resources server, judges and multi-stage driver as config.yaml.
+# Codex runs in the task sandbox the resources server lends it and reaches the task tools
+# (finish, abandon_task_finish, web_search, fetch_web_page) over MCP. The rows use the harness-neutral prompt.
+# See "Run another harness" in README.md for the run command and the model server settings Codex needs.
+# Codex has no finish_tool_names: finish does not end its run. Codex continues until the model replies without a
+# tool call, a later finish or abandon_task_finish replaces the recorded call, and the resources server collects
+# the files when the run ends.
+config_paths:
+  - benchmarks/gdpval/config.yaml
+
+# Drop the Stirrup agent; the environment server below runs Codex instead.
+gdpval_stirrup_agent: null
+
+gdpval_resources_server:
+  resources_servers:
+    gdpval:
+      allowed_agents: [codex_agent]
+      expose_tools_over_mcp: true
+
+gdpval_codex_agent:
+  responses_api_agents:
+    codex_agent:
+      entrypoint: app.py
+      description: GDPVal professional deliverable tasks with Codex in place of Stirrup
+      model_server:
+        type: responses_api_models
+        name: policy_model
+      model: ${policy_model_name}
+      codex_version: 0.144.4
+      # Concurrent Codex runs, as Stirrup's concurrency. Keep it at least the environment server's
+      # max_concurrent_episodes: an episode beyond it waits with its sandbox started and its deadline running.
+      concurrency: 32
+      model_context_window: 262144
+      # Codex checks this after each reply that calls a tool, so the context can pass it by about a reply and its
+      # tool output. The compaction call re-sends the context without the tool definitions, and vLLM reserves
+      # 32,768 tokens for its reply: 196,608 + 32,768 + 32,768 = 262,144 fits a full reply with a short tool
+      # result. At 229,376 the compaction call would usually overflow the window.
+      model_auto_compact_token_limit: 196608
+      # The Codex process, inside the environment server's default_episode_timeout_seconds.
+      timeout: 10800
+      session_close_timeout_seconds: 90
+      # A Codex MCP call that times out keeps running on the server and holds back the next call to it, so keep
+      # this well above a slow web call: httpx gives each step of a search or fetch up to 180 s.
+      mcp_tool_timeout_seconds: 900
+      datasets:
+        - name: gdpval
+          type: benchmark
+          jsonl_fpath: benchmarks/gdpval/data/gdpval_benchmark_harness_neutral.jsonl
+          prompt_config: benchmarks/gdpval/prompts/harness-neutral.yaml
+          prepare_script: benchmarks/gdpval/prepare_harness_neutral.py
+          num_repeats: 1
+
+gdpval_environment_server:
+  environment_servers:
+    single_agent_turn_legacy:
+      agent_server:
+        type: responses_api_agents
+        name: gdpval_codex_agent
+      resources_tool_transports: [mcp]
diff --git a/benchmarks/gdpval/prepare_harness_neutral.py b/benchmarks/gdpval/prepare_harness_neutral.py
index 543791be3aa..774e1b0ea55 100644
--- a/benchmarks/gdpval/prepare_harness_neutral.py
+++ b/benchmarks/gdpval/prepare_harness_neutral.py
@@ -3,8 +3,8 @@
 """Prepare the GDPVal rows for harnesses that get the task tools from the resources server.
 
 The rows match ``prepare.py``'s but carry no tool list, which session harnesses such as Pi and Codex reject in the
-request. Pi reaches finish, abandon_task_finish, web_search and fetch_web_page through the resources server's MCP
-grant. Use these rows with ``prompts/harness-neutral.yaml``.
+request. Pi and Codex reach finish, abandon_task_finish, web_search and fetch_web_page through the resources server's
+MCP grant. Use these rows with ``prompts/harness-neutral.yaml``.
 """
 
 from __future__ import annotations
diff --git a/resources_servers/gdpval/tests/test_harness_configs.py b/resources_servers/gdpval/tests/test_harness_configs.py
index 47331a7862c..c6cb18fa24a 100644
--- a/resources_servers/gdpval/tests/test_harness_configs.py
+++ b/resources_servers/gdpval/tests/test_harness_configs.py
@@ -9,12 +9,13 @@
 
 from nemo_gym.benchmarks import BenchmarkConfig
 from nemo_gym.global_config import GlobalConfigDictParser, GlobalConfigDictParserConfig
+from responses_api_agents.codex_agent.app import CodexAgentConfig
 from responses_api_agents.pi_agent.app import PiAgentConfig
 
 
 _BENCHMARK_DIR = Path(__file__).resolve().parents[3] / "benchmarks" / "gdpval"
 # Each harness config here (<harness>.yaml), with its agent's config class.
-_HARNESS_CONFIGS = {"pi": PiAgentConfig}
+_HARNESS_CONFIGS = {"pi": PiAgentConfig, "codex": CodexAgentConfig}
 
 
 @pytest.fixture(autouse=True)
@@ -49,6 +50,8 @@ def test_harness_replaces_stirrup_and_reaches_the_task_tools_over_mcp(harness):
     # The agent ignores keys its config class does not define, so a misspelled setting would be dropped silently.
     # Gym reads datasets and description itself.
     assert agent.keys() - {"datasets", "description"} - _HARNESS_CONFIGS[harness].model_fields.keys() == set()
+    # Building the config also runs its validators, such as the limits on timeouts and compaction.
+    _HARNESS_CONFIGS[harness].model_validate({**agent, "name": benchmark.agent_name})
     # Only the agent, the agents the resources server allows and the tool transport differ from config.yaml, so the
     # judges, scoring, sandbox and multi-stage driver are the ones Stirrup runs with.
     overrides = OmegaConf.to_container(OmegaConf.load(path))