🤖 fix: enable reasoning for Claude Opus 4.5

[ThomasK33]

Summary

Fixes missing reasoning traces in the UI when using Claude Opus 4.5.

Problem

Opus 4.5 supports two separate but complementary parameters for reasoning:

1. effort (new in Opus 4.5): Controls how eagerly Claude spends tokens across ALL output (text, tool calls, and thinking). Values: low, medium, high.

2. thinking (extended thinking): Enables visible reasoning traces with a token budget. This is what makes the "Thinking..." UI appear.

We were only passing effort, which controls token spend but doesn't enable the reasoning traces to be returned from the API. The thinking parameter must also be set for reasoning to be visible.

Solution

For Opus 4.5, now pass both parameters:

{
  anthropic: {
    thinking: {
      type: "enabled",
      budgetTokens: 20000,  // Enables visible reasoning traces
    },
    effort: "high",  // Controls token spend eagerness
  }
}

How these parameters interact

From Anthropic's docs:

The effort parameter works alongside the thinking token budget when extended thinking is enabled. These two controls serve different purposes:

• Effort parameter: Controls how Claude spends all tokens—including thinking tokens, text responses, and tool calls
• Thinking token budget: Sets a maximum limit on thinking tokens specifically

The effort parameter can be used with or without extended thinking enabled.

Configuration	Behavior
effort only	Controls token spend, no visible reasoning
thinking only	Shows reasoning with default budget
effort + thinking	Shows reasoning WITH token spend control

Testing

• Verified reasoning traces now appear in UI when using Opus 4.5
• make typecheck passes

Generated with mux

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Codex Review: Didn't find any major issues. 🎉

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