[ananthsub]

What does this PR do?

Lets Simple Agent run inside an Environment Server episode through agent sessions, gives the example single-tool-call Resources Server typed sessions, and adds a config that runs the example through the single-agent-turn Environment Server end to end. /run is unchanged.

Simple Agent sessions:

• Simple Agent implements only the base agent's session hooks, _seed_agent_session_state and _close_agent_session_state. The base agent's shared bookkeeping from #3554 handles the rest: idempotent seed, binding a session to its seed, the session cookie, retries, and close receipts. The cookie key is one constant, AGENT_SESSION_COOKIE_KEY, in nemo_gym.base_responses_api_agent.
• During the session, Simple Agent calls tools through the direct HTTP tool access the Environment Server grants, which carries the Resources session cookie. It accepts at most one direct HTTP grant and rejects sandbox access and required MCP access, which it does not support. A session without a grant never falls back to the configured Resources Server, which has no session for the episode: a tool call there fails the activation.
• When observability is on, the session returns its agent invocation record at close, where the Environment Server stores it as the episode's agent observations.
• Sessions live in the worker's memory, so seeding one requires num_workers: 1. /run keeps no session and still supports several workers.
• resources_server is optional in the Simple Agent config, because a session gets its tools from the Environment Server. /run and /aggregate_metrics still require it.

Example single-tool-call Resources Server:

• Implements typed, idempotent seed and close by resources_session_id. Close overrides the close_resources_session hook; the example no longer registers its own /close_session route, which the base route takes precedence over.
• configs/example_single_tool_call_single_agent_turn.yaml fronts Simple Agent with single_agent_turn_legacy and grants it direct HTTP access to the example's tools. The existing config, which the quickstart and training docs use, still runs through /run.

single_agent_turn_legacy accepts a task_source that names its Resources Server, its agent, or itself. Collation sets task_source to the instance that declares the dataset, usually the agent, so the adapter rejected collated rows before.

The close_resources_session docstring states what a close override must do: succeed for a session that is already closed or was never seeded, keep a later seed from recreating a closed session, and raise when releasing fails so the caller can retry. It also notes that the default seed does not tie the session cookie to resources_session_id.

Contract checks:

• nemo_gym.testing.session_conformance.check_resources_session_contract is an integration test of a server's session lifecycle: it seeds, re-seeds and closes a session through the server's real routes and middleware, in process, and checks the close rules, so it also catches a shadowed route. The base default and the example run it; a migrated server calls it from its own tests. nemo_gym.testing holds helpers that tests import.
• tests/unit_tests/test_session_contract_repo.py reads every agent and Resources Server's source, since servers run in their own environments. It fails when an agent overrides the public seed or close instead of the hooks, when an agent implements the seed hook without the close hook, when a Resources Server registers its own /close_session, or when a typed seed has no close_resources_session.

Issue

No issue is linked; this is part of the Environment Server work.

Validation

• pytest responses_api_agents/simple_agent/tests -q: 26 passed. A seed after close now returns 409 from the base bookkeeping instead of a 500. A session tool call without a grant fails and reaches no Resources Server; that test fails without the fix.
• pytest resources_servers/example_single_tool_call/tests -q: 5 passed, including the session contract check.
• pytest environment_servers/single_agent_turn/tests environment_servers/single_agent_turn_legacy/tests -q: 30 and 12 passed, including task_source naming the agent or the Resources Server, and rejecting another instance.
• pytest resources_servers/swebench_pro/tests -q: 96 passed. pytest responses_api_agents/hermes_agent/tests -q (its own venv): 219 passed.
• pytest tests/unit_tests/test_session_contract_repo.py tests/unit_tests/test_base_resources_server.py tests/unit_tests/test_agent_sessions.py tests/unit_tests/test_agent_session_retries.py tests/unit_tests/test_base_responses_api_agent.py -q: all passed. The repo-wide checks and the contract check each fail when the example registers its own /close_session again; the repo-wide agent check fails when Simple Agent overrides close_agent_session.
• pytest tests/unit_tests -q: all passed except tests that need packages this environment lacks (opensandbox, tenacity, mlflow, wandb) and a Slurm test that calls the system pip.
• pre-commit run on the changed files.

Rollouts

gym dataset collate on the new config stamped task_source with the agent's name, and the five collated example_single_tool_call tasks ran with gpt-5.6-sol through single_agent_turn_legacy with no failures. Three called get_weather and scored 1.0; the other two prompts are ambiguous ("get the weather for 3 cities in the us", "how's the outside?"), and the model asked which city was meant, so they scored 0.0 as the verifier intends.

A stateful check, run locally with example_session_state_mgmt given a typed seed and close (that change is a separate PR) and a single_agent_turn config like the one here: its counter starts at the task's initial_count, so a reward of 1.0 needs the seed, the agent's tool calls, and verification to reach the same Resources session. With the direct HTTP grant, all five collated tasks scored 1.0 (for example 3 seeded + 1 + 2 = 6), which confirms the grant's cookie reaches the seeded session without forwarding cookies on the agent seed request. With no grant, the earlier code scored 0.0 on all five because tool calls landed in fresh sessions; with the fix, those episodes fail with "Model called tool 'increment_counter', but this agent session has no direct HTTP tool access".

Compatibility

/run, its request and result shapes, and existing Simple Agent configurations are unchanged. resources_server becomes optional in the Simple Agent config, and configurations that set it behave as before.

[github-actions]

🔄 Merge queue validation started! You can track the progress here: https://github.com/NVIDIA-NeMo/Gym/actions/runs/37380663103