Context window detection fails for third-party Anthropic-compatible providers

When using a third-party provider that implements the Anthropic API (e.g., MiniMax via https://api.minimax.io/anthropic), Claude Code's context window detection falls back to the hardcoded default of 200,000 tokens, even when the underlying model may support a larger context window.

Root Cause

In src/utils/context.ts, getContextWindowForModel() calls getModelCapability() to retrieve max_input_tokens from a cached capability list. However, getModelCapability() is gated by isFirstPartyAnthropicBaseUrl() in src/utils/model/modelCapabilities.ts:46-51:

function isModelCapabilitiesEligible(): boolean {
  if (process.env.USER_TYPE !== 'ant') return false
  if (getAPIProvider() !== 'firstParty') return false
  if (!isFirstPartyAnthropicBaseUrl()) return false  // ← MiniMax fails here
  return true
}

Since MiniMax's base URL is https://api.minimax.io/anthropic (not api.anthropic.com), isFirstPartyAnthropicBaseUrl() returns false, and getModelCapability() returns undefined. This causes getContextWindowForModel() to fall through to MODEL_CONTEXT_WINDOW_DEFAULT = 200_000.

Impact

• AutoCompact triggers too aggressively: With a 200K assumed window, AutoCompact threshold is 200,000 - 13,000 = 187,000 (93.5%). If MiniMax actually supports 1M, this is ~19% into the real window.
• Users hit context limits unexpectedly: The tool estimates the context is at 93.5% when it may actually be at only 18.7% for a 1M-capable model served through MiniMax.
• Manual /compact becomes necessary: Users report needing to run /compact manually when the auto-compact warning should have fired much earlier (or not at all).

Reproduction Steps

1. Set ANTHROPIC_BASE_URL=https://api.minimax.io/anthropic
2. Set ANTHROPIC_AUTH_TOKEN=<MiniMax token>
3. Set ANTHROPIC_MODEL=MiniMax-M2.7 (or any model served through MiniMax)
4. Observe that getContextWindowForModel("MiniMax-M2.7") returns 200000 regardless of the model's actual capabilities
5. Note that AutoCompact warning fires at ~187K tokens (based on 200K window) rather than at a proportional threshold for the actual window

Expected Behavior

Claude Code should either:

1. Detect actual context window for third-party providers (if the provider exposes model capabilities via their own endpoints)
2. Allow manual override via environment variable (CLAUDE_CODE_MAX_CONTEXT_TOKENS) or model configuration
3. At minimum, not assume the smallest possible window for unknown third-party providers — use a conservative estimate or probe the actual limit

Proposed Fix

Option A: Extend capability detection to third-party providers (medium effort)

Add a getThirdPartyModelCapability() path that tries to fetch from the provider's model list endpoint, or maintain a local override map for known MiniMax/Gateway models.

Option B: Environment variable override for specific models (simple, immediate)

Add support for per-model context window overrides in CLAUDE_CODE_MAX_CONTEXT_TOKENS:

CLAUDE_CODE_MAX_CONTEXT_TOKENS="MiniMax-M2.7:1000000,claude-opus-4-6:1000000"

Option C: Probe actual context limit on first use (most robust)

On the first API call with a new model, detect 413 Payload Too Large and learn the actual limit, persisting it locally. Already done for team memory (src/services/teamMemorySync/index.ts:529), could be generalized.

Workaround

Set CLAUDE_CODE_MAX_CONTEXT_TOKENS=1000000 in environment to override the auto-detected window for all models.

Additional Context

• AutoCompact already has a circuit breaker (3 consecutive failures) to prevent hammering the API when context is irrecoverably over the limit (src/services/compact/autoCompact.ts:70)
• The 200K default is documented at src/utils/context.ts:9 as a comment but may not reflect actual provider capabilities
• This issue affects any Anthropic-compatible third-party API (Azure, AWS Bedrock, Vertex, MiniMax, OpenRouter, etc.) where the provider URL doesn't match api.anthropic.com