feat: auto-detect and display context length for custom providers #2513

[InB4DevOps]

What does this PR do?

This PR adds two related improvements to the custom provider workflow:

1. Automatic context length detection when users save custom providers via /model without specifying a context length. The system now attempts to auto-detect it using get_model_context_length() and displays the result with appropriate visual indicators.

2. Enhanced flag parsing for /model command, supporting --context-length, --base-url, and --api-key flags with both = and space separators.

3. Improved credential resolution for direct endpoints (custom providers with base_url), requiring explicit delegation.api_key instead of falling back to OPENAI_API_KEY env var.

4. Better test coverage for the new features and edge cases.

Changes

Core Implementation

hermes_cli/cli.py

• Added flag parsing for --context-length, --base-url, --api-key (supports both = and space separators)
• Improved custom endpoint detection by checking base_url_flag first
• Pass context_length to validate_requested_model() when saving custom providers
• Auto-detect and display context length when context_length is None and model name is provided

hermes_cli/main.py

• Added context length auto-detection in _model_flow_custom() for custom providers
• Displays detected length with 💡 emoji or fallback with 📏 emoji for clear visual feedback

hermes_cli/models.py

• Fixed suggested_base_url logic in probe_api_models() - changed condition from alternate_base != candidate_base to is_fallback
• Added context_length parameter to validate_requested_model()

tools/delegate_tool.py

• Modified _resolve_delegation_credentials() to require explicit delegation.api_key for direct endpoints
• Changed fallback behavior: only uses OPENROUTER_API_KEY if provider is explicitly openrouter, not OPENAI_API_KEY for custom endpoints
• Updated error message to clarify: "Set delegation.api_key in your config. Note: OPENAI_API_KEY environment variable is NOT used for direct endpoints"

tools/transcription_tools.py

• Modified _resolve_openai_api_key() to check only VOICE_TOOLS_OPENAI_KEY (not OPENAI_API_KEY)
• Added auto-detection fallback in _get_provider() that checks OPENAI_API_KEY for backwards compatibility

Tests

tests/test_model_context_length_detection.py (new file)

New comprehensive test suite with 127 lines covering:

• Context length auto-detection success case
• Context length fallback when detection fails
• probe_api_models() behavior with fallback URLs
• DEFAULT_FALLBACK_CONTEXT constant validation

tests/tools/test_delegate.py

• Renamed test from test_direct_endpoint_falls_back_to_openai_api_key_env to test_direct_endpoint_requires_explicit_api_key_not_openai_env
• Updated behavior: direct endpoints now require explicit delegation.api_key instead of falling back to OPENAI_API_KEY

tests/tools/test_transcription.py

• Improved test for test_no_key with proper temp file cleanup
• Added module namespace clearing to ensure clean test state

Documentation

AGENTS.md

Added new policy under "Known Pitfalls" documenting:

• When context length auto-detection runs
• What visual indicators users will see (💡 and 📏 emojis)
• Reference to implementation location

User Experience

Before:

✓ Custom provider saved.

After:

✓ Custom provider saved. 💡 Context length auto-detected: 200,000 tokens

Or when fallback is used:

✓ Custom provider saved. 📏 Context length: Using default of 128,000 tokens

Flag Support

Now supports both formats:

# With = separator
/model openrouter:anthropic/claude-opus-4.6 --base-url=https://example.com/api/v1 --api-key=sk-... --context-length=200k

# With space separator
/model openrouter:anthropic/claude-opus-4.6 --base-url https://example.com/api/v1 --api-key sk-... --context-length 200k

Technical Details

Context Length Auto-Detection

• Uses existing get_model_context_length() from agent.model_metadata
• Respects DEFAULT_FALLBACK_CONTEXT constant (128K) for consistency
• Auto-detection only runs when:
  • context_length is None
  • model_name is provided
  • Provider is set to "custom"

Enhanced Flag Parsing

The /model command now parses flags before extracting the model part, supporting:

• --context-length or --context_length (both with = or space separator)
• --base-url or --base_url (both with = or space separator)
• --api-key or --api_key (both with = or space separator)

Credential Resolution

Direct endpoints (custom providers with base_url) now:

1. Use configured delegation.api_key first
2. Fall back to OPENROUTER_API_KEY only if provider is explicitly "openrouter"
3. Raise ValueError if no key found, with clear guidance to use delegation.api_key

Testing

• New test suite in tests/test_model_context_length_detection.py
• Manual testing via /model command with custom providers
• Updated existing tests for changed behavior in delegation and transcription tools

Related

• Addresses the need for better user feedback when saving custom providers
• Aligns with the principle of providing clear visual indicators for system behavior

Related Issue

Fixes #2513

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Type of Change: ✨ New feature (non-breaking change that adds functionality)