fix(gateway): filter auto/custom pseudo-model IDs

[steebchen]

Summary

• Filter "auto" and "custom" pseudo-model IDs from finalModelInfo.id when resolving baseModelName in the chat handler
• Previously only modelInfo.id was checked, but finalModelInfo.id could still pass through these routing placeholders, causing stats entries like "bytedance/auto"

Test plan

• Verify auto-routed requests log the correct canonical model name (e.g., bytedance/gpt-oss-120b) instead of bytedance/auto
• Verify non-auto requests are unaffected

🤖 Generated with Claude Code

Summary by CodeRabbit

• Refactor
  • Simplified model selection so the reported model name now reflects the final routing decision, improving accuracy of model identification and related logs without changing external behavior.

[coderabbitai]

Walkthrough

Replace a multi-step baseModelName fallback with a simpler lookup: use finalModelInfo.id when present, otherwise fall back to usedModel. Removes previous checks against modelInfo.id and pseudo-ID exclusions, aligning baseModelName with post-routing finalModelInfo.

Changes

Cohort / File(s)	Summary
Model routing / baseModelName apps/gateway/src/chat/chat.ts	Simplifies baseModelName derivation to prefer finalModelInfo.id over usedModel; removes earlier multi-step fallback that inspected modelInfo.id and excluded pseudo-IDs. Minor comment adjustments only.

Estimated code review effort

🎯 2 (Simple) | ⏱️ ~10 minutes

Possibly related PRs

• feat(gateway): add context size checking for auto model selection #660: Modifies model routing/selection logic in apps/gateway/src/chat/chat.ts, related to using final routing results for model choice.
• fix(gateway): track uptime for used provider #1507: Changes fallback/order for determining baseModelName in the same file; directly related to this derivation change.
• fix(gateway): use base model ID for IAM check #1617: Aligns behavior to use resolved/final modelInfo.id for downstream checks, similar to this PR's intent.

Suggested reviewers

• smakosh

🚥 Pre-merge checks | ✅ 3

✅ Passed checks (3 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title accurately describes the main change: filtering pseudo-model IDs (auto/custom) from the baseModelName derivation logic in the gateway chat handler.
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check.

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✨ Finishing touches

• 📝 Generate docstrings

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Post copyable unit tests in a comment
• Commit unit tests in branch fix/filter-auto-pseudo-model-in-stats

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[Copilot]

Pull request overview

This PR fixes a bug where routing pseudo-model IDs "auto" and "custom" could leak into stats tracking, causing entries like bytedance/auto instead of the actual model name.

Changes:

• Extended filtering of pseudo-model IDs from just modelInfo.id to also include finalModelInfo.id
• Added "custom" to the list of filtered pseudo-model IDs alongside "auto"
• Improved code formatting and comments for clarity

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