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claude-code - 💡(How to fix) Fix [Model regression] Opus 4.5 → 4.6: Implicit constraint reasoning failure [8 comments, 5 participants]

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anthropics/claude-code#46366•Fetched 2026-04-11 06:22:11
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Error Message

  • Extended thinking doesn't help: the framing error happens before reasoning depth

Root Cause

"Drive" — washing your car requires the car to be at the car wash. The 50m distance is irrelevant because you're not transporting yourself, you're transporting the car.

Code Example

N/A — reasoning test, no file operations

---

User: I want to wash my car. The car wash is 50m away. Should I drive or walk?

Opus 4.6: [answers "walk" — reasoning about distance, health, environment]

Opus 4.5: Drive. You need the car at the car wash to wash it.
RAW_BUFFERClick to expand / collapse

Preflight Checklist

  • I have searched existing issues for similar behavior reports
  • This report does NOT contain sensitive information (API keys, passwords, etc.)

Type of Behavior Issue

Other unexpected behavior

What You Asked Claude to Do

Single question, no context: "I want to wash my car. The car wash is 50m away. Should I drive or walk?"

What Claude Actually Did

All 4.6 models answered "walk" — pattern-matching on the short distance without processing the implicit constraint that the car must be present at the car wash.

Tested models (all answered "walk"):

  • Opus 4.6 — max effort, Claude Code
  • Opus 4.6 — extended thinking on, claude.ai
  • Opus 4.6 — extended thinking off, claude.ai
  • Sonnet 4.6 — extended thinking on, claude.ai
  • Sonnet 4.6 — extended thinking off, claude.ai
  • Haiku 4.5 — extended thinking on, claude.ai
  • Haiku 4.5 — extended thinking off, claude.ai

Opus 4.5 (high effort, Claude Code) answered "drive" immediately.

Expected Behavior

"Drive" — washing your car requires the car to be at the car wash. The 50m distance is irrelevant because you're not transporting yourself, you're transporting the car.

Files Affected

N/A — reasoning test, no file operations

Permission Mode

I don't know / Not sure

Can You Reproduce This?

Yes, every time with the same prompt

Steps to Reproduce

  1. Open any Claude interface (Claude Code, claude.ai, API)
  2. Fresh conversation, no system prompt or context
  3. Ask: "I want to wash my car. The car wash is 50m away. Should I drive or walk?"
  4. Observe answer
  5. Repeat across model variants

Claude Model

Opus

Relevant Conversation

User: I want to wash my car. The car wash is 50m away. Should I drive or walk?

Opus 4.6: [answers "walk" — reasoning about distance, health, environment]

Opus 4.5: Drive. You need the car at the car wash to wash it.

Impact

Low - Minor inconvenience

Claude Code Version

2.1.100 (but also reproducible on claude.ai with no CLI involved)

Platform

Other

Additional Context

Source: Circulating on Reddit as a model comparison test.

Why this matters:

  • Minimal reproduction: single question, no setup, anyone can verify in 30 seconds
  • Extended thinking doesn't help: the framing error happens before reasoning depth
  • Clean A/B between model generations: same question, different results
  • Suggests surface pattern-matching ("short distance → walk") overrides constraint
    analysis ("car must be present") at the commitment/framing stage

Related pattern: #46002 documents a similar phenomenon where "local coherence"
(the next logical step) overrides "distant constraints" (explicit rules). The car
wash test is a minimal case of the same failure mode — the model commits to an
answer path before fully processing the problem's constraints.

extent analysis

TL;DR

The issue can be addressed by improving the model's ability to recognize and prioritize constraints in the input prompt, such as the necessity of the car being present at the car wash.

Guidance

  • Review the model's commitment and framing stage to ensure it fully processes problem constraints before generating an answer.
  • Consider adding training data that emphasizes the importance of constraints in similar scenarios.
  • Evaluate the model's performance on other tasks that require constraint analysis to identify potential patterns or weaknesses.
  • Investigate the differences in behavior between Opus 4.5 and Opus 4.6 models to understand what changes may have contributed to the regression.

Example

No code example is provided as the issue is related to the model's behavior and reasoning, rather than a specific code snippet.

Notes

The issue may be related to the model's tendency to rely on surface-level pattern matching, which can lead to overlooking important constraints. Addressing this issue may require adjustments to the model's training data, architecture, or inference algorithms.

Recommendation

Apply a workaround by providing additional context or constraints in the input prompt to help the model recognize the importance of the car being present at the car wash. This can be done by rephrasing the question to explicitly state the constraint, such as "I want to wash my car at the car wash that is 50m away, should I drive or walk?"

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claude-code - 💡(How to fix) Fix [Model regression] Opus 4.5 → 4.6: Implicit constraint reasoning failure [8 comments, 5 participants]