Fixes#15779. Custom-provider per-model context_length (`custom_providers[].models.<id>.context_length`) is now honored across every resolution path, not just agent startup. Also adds 256K as the top probe tier and default fallback.
## What changed
New helper `hermes_cli.config.get_custom_provider_context_length()` — single source of truth for the per-model override lookup, with trailing-slash-insensitive base-url matching.
`agent.model_metadata.get_model_context_length()` gains an optional `custom_providers=` kwarg (step 0b — runs after explicit `config_context_length` but before every other probe).
Wired through five call sites that previously either duplicated the lookup or ignored it entirely:
- `run_agent.py` startup — refactored to use the new helper (dedups legacy inline loop, keeps invalid-value warning)
- `AIAgent.switch_model()` — re-reads custom_providers from live config on every /model switch
- `hermes_cli.model_switch.resolve_display_context_length()` — new `custom_providers=` kwarg
- `gateway/run.py` /model confirmation (picker callback + text path)
- `gateway/run.py` `_format_session_info` (/info)
## Context probe tiers
`CONTEXT_PROBE_TIERS = [256_000, 128_000, 64_000, 32_000, 16_000, 8_000]` — was `[128_000, ...]`. `DEFAULT_FALLBACK_CONTEXT` follows tier[0], so unknown models now default to 256K. The stale `128000` literal in the OpenRouter metadata-miss path is replaced with `DEFAULT_FALLBACK_CONTEXT` for consistency.
## Repro (from #15779)
```yaml
custom_providers:
- name: my-custom-endpoint
base_url: https://example.invalid/v1
model: gpt-5.5
models:
gpt-5.5:
context_length: 1050000
```
`/model gpt-5.5 --provider custom:my-custom-endpoint` → previously "Context: 128,000", now "Context: 1,050,000".
## Tests
- `tests/hermes_cli/test_custom_provider_context_length.py` — new file, 19 tests covering the helper, step-0b integration, and the 256K tier invariants
- `tests/hermes_cli/test_model_switch_context_display.py` — added regression tests for #15779 through the display resolver
- `tests/gateway/test_session_info.py` — updated default-fallback assertion (128K → 256K)
- `tests/agent/test_model_metadata.py` — updated tier assertions for the new top tier
When a new session starts in the gateway (via /new, /reset, or
auto-reset), send the user a summary of the detected configuration:
✨ Session reset! Starting fresh.
◆ Model: qwen3.5:27b-q4_K_M
◆ Provider: custom
◆ Context: 8K tokens (config)
◆ Endpoint: http://localhost:11434/v1
This makes misconfigured context length immediately visible — a user
running a local 8K model that falls to the 128K default will see:
◆ Context: 128K tokens (default — set model.context_length in config to override)
Instead of silently getting no compression and degrading responses.
- _format_session_info() resolves model, provider, context length,
and endpoint from config + runtime, matching the hygiene code's
resolution chain
- Local/custom endpoints shown; cloud endpoints hidden (not useful)
- Context source annotated: config, detected, or default with hint
- Appended to /new and /reset responses, and auto-reset notifications
- 9 tests covering all formatting paths and failure resilience
Addresses the user-facing side of #2708 — instead of trying to fix
every edge case in context detection, surface the values so users
can immediately see when something is wrong.
