fix(config): re-read model.context_length from config after /model switch (#40979)

[rodboev]

Summary

When a user sets model.context_length in config.yaml, the override is read at session start in agent_init.py and stored as agent._config_context_length. This value controls the compression threshold, how aggressively Hermes compresses history to stay within the context window.

When /model is used to switch models mid-session, switch_model() in agent/agent_runtime_helpers.py clears _config_context_length to None, correctly discarding any stale value from the previous model. It then re-reads custom_providers per-model context lengths from config, but did not re-read the global model.context_length key. The override was silently lost: the session continued with auto-detected context lengths for all subsequent turns, ignoring the user's explicit setting.

This PR extends the config re-read block inside switch_model() to also extract model.context_length and re-apply it before calling get_model_context_length(). The follow-up fixes keep that same context state consistent across the runtime lifecycle: /model persists the refreshed _config_context_length and live _custom_providers into _primary_runtime, initialization snapshots those fields before any /model switch can happen, and restore_primary_runtime() restores both fields when the agent exits fallback.

Changes

• agent/agent_runtime_helpers.py: re-read model.context_length during switch_model(), persist the refreshed _config_context_length and _custom_providers into _primary_runtime, and restore those fields in restore_primary_runtime()
• agent/agent_init.py: include the init-time config_context_length and a deep copy of custom_providers in the primary runtime snapshot
• tests/run_agent/test_switch_model_context.py: cover real temporary-HERMES_HOME config loading for the global override and refreshed custom-provider persistence after /model
• tests/run_agent/test_primary_runtime_restore.py: cover init snapshot contents, copy isolation, fallback before any /model switch, restore success, fallback reset, and restored context state

Validation

Scenario	Before	After
model.context_length: 65536 set, no /model switch	honored	honored and included in the initial primary snapshot
model.context_length: 65536 set, /model new-model called	lost, reverts to auto-detect	re-read from config.yaml, passed to get_model_context_length, and persisted into _primary_runtime
model.context_length not set, /model called	None passed	None passed, unchanged
custom_providers per-model context_length, /model called	honored during the immediate switch	honored and persisted into the primary runtime snapshot
fallback before any /model switch	restore could read missing snapshot keys	initial snapshot restores _config_context_length and a copied _custom_providers list
/model switch followed by fallback/restore	refreshed state lost on restore	refreshed _config_context_length and _custom_providers restored correctly

Test plan

• pytest tests/run_agent/test_switch_model_context.py tests/run_agent/test_primary_runtime_restore.py -v --timeout=0 — 41 focused tests pass

Why it's low-risk

The change stays scoped to /model, initialization, and runtime restore paths, and it only preserves state the branch already computes. Sessions without model.context_length set keep None; sessions without live custom providers snapshot and restore None.

Upstream

Closes #40979.
Reported by @bonaluo.

[tonydwb]

Code Review Summary (PR #41270)

Verdict: Approved

Looks Good

• Fixes issue #40979: model.context_length from config.yaml is re-read after /model switch and persisted into primary runtime state
• Handles both global model.context_length and per-model custom_providers context_length
• restore_primary_runtime now restores _config_context_length and _custom_providers (deep copy)
• Tests cover: restore after fallback, re-read on switch, custom_providers persistence

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Reviewed by Hermes Agent

[teknium1]

Thanks for tracing the global override through the /model path; current main still clears _config_context_length at agent/agent_runtime_helpers.py:1848 and then passes the unset value at :2056.

Problems

• The new restore assignments need corresponding initial snapshot entries. agent/agent_init.py:2105-2123 does not store config_context_length or custom_providers. Since fallback activation leaves _custom_providers intact (agent/chat_completion_helpers.py:1502-1636), restoring rt.get("custom_providers") after a fallback before any /model would replace it with None.
• The added restore test pre-populates those keys, so it misses that initial-session path. The config test also mocks load_config; a temporary-HERMES_HOME integration test would cover the required propagation path.

Suggested changes

• Snapshot both fields during initialization and add a fallback-without-/model preservation test.
• Add a real config-loading regression test for the global override.

Automated hermes-sweeper review.