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claude-code - 💡(How to fix) Fix [BUG] Long-running session degradation since v2.1.139, compounded by silent fast-mode model swap in v2.1.142 — no changelog, breaks pinned-model workflows, quantified waste

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Error Message

Error Messages/Logs

Root Cause

Session 7b7f4c91 is the primary evidence session where degradation was identified, debugged, and root causes traced. Session ec1c8982 (6.4 MB, May 11) is the earliest large session after v2.1.139 — the first one showing the pattern.

Fix Action

Fix / Workaround

Degradation patterns that emerge in long-running sessions (not short ones):

Workaround

RAW_BUFFERClick to expand / collapse

Preflight Checklist

  • I have searched existing issues and this hasn't been reported yet
  • This is a single bug report (please file separate reports for different bugs)
  • I am using the latest version of Claude Code

What's Wrong?

Title: Long-running session degradation since v2.1.139, compounded by silent fast-mode model swap in v2.1.142

Summary

Starting with Claude Code v2.1.139, long-running sessions exhibit progressive behavioral degradation that was not present in v2.1.138. This was then compounded by v2.1.142, which silently changed the fast-mode default model from Opus 4.6 to Opus 4.7 without any changelog entry or user notification.

The combination produces sessions that start well but progressively lose tool-calling discipline, begin hallucinating capabilities, and enter unproductive loops — consuming tokens and user time without producing useful output.

Symptoms

Degradation patterns that emerge in long-running sessions (not short ones):

  • Synthesis-over-verification drift: Model confidently asserts facts without calling available tools to check them — worsens as session length increases
  • Phantom capability claims: Claims to have invoked tools/features it never actually called
  • Failure loop blindness: Retries the same broken action 3+ times without changing approach
  • Reduced tool-calling discipline: Reasons about what a tool would return rather than calling it
  • Bulk-assertion without evidence: Marks multiple verification criteria as "complete" in a single text block with no interleaved tool calls
  • Context compaction artifacts: After compaction events, behavioral rules from system prompts are partially forgotten or deprioritized

Version Timeline

VersionReleasedStatus
2.1.1382026-05-09Last known-good for long-running session stability
2.1.1392026-05-11First version exhibiting long-session degradation
2.1.1402026-05-12Degradation continues
2.1.1412026-05-13Degradation continues
2.1.1422026-05-14Silent fast-mode model swap (Opus 4.6 → 4.7) — compounds the issue
2.1.1432026-05-15Both issues present

Quantified Impact (Max Subscriber, since v2.1.139)

MetricValue
Duration of degradation6 days (2026-05-11 to 2026-05-16)
Sessions in affected window~850
Total session data46.9 MB
Estimated tokens consumed~4.1M
Estimated tokens wasted (degraded behavior)~1.4M
Per-token cost equivalent~$73
User hours spent on corrections/debugging~7 hours
Quality score before v2.1.1397–8/10 (consistent, monitored)
Quality score during affected period4.5/10 (sustained, monitored)
Sessions requiring full restart (context exhaustion)4+

Waste rate is conservative (35%) — large sessions showed 50–65% waste (hallucinations, repeated identical failures, user corrections, forced restarts). At $200/month subscription cost, this represents over a third of a billing cycle's value lost to degraded output.

What Should Happen?

What Should Happen?

  1. Long-running sessions should maintain consistent tool-calling discipline and behavioral rule adherence throughout the entire session, including after context compaction events. A rule present in the system prompt at turn 1 should still be followed at turn 50.

  2. Model version changes in any mode (fast, standard) should be documented in release notes and visible to the user — not silently swapped server-side.

  3. Context compaction should preserve the full fidelity of system prompts and CLAUDE.md operational rules. If compaction necessarily drops context, system-level instructions should be the last thing dropped, not the first.

  4. Paid subscribers ($200/month Max) should have a stable, first-class mechanism to pin model versions — not undocumented environment variables discovered by reverse-engineering regressions.

  5. Behavioral regressions of this magnitude (quality drop from 7-8/10 to 4.5/10) should be caught by Anthropic's internal testing before release, not discovered by subscribers debugging wasted sessions.

Error Messages/Logs

Steps to Reproduce

Steps to Reproduce

  1. Run Claude Code v2.1.139+ with fast mode enabled
  2. Start a session with structured operational rules (system prompts, CLAUDE.md with explicit verification requirements)
  3. Work through a multi-phase task requiring 10+ tool calls
  4. Observe progressive degradation: early turns follow rules, later turns drift
  5. After context compaction, observe partial loss of behavioral rules
  6. Compare identical workflow on v2.1.138 — no degradation

Workaround

export CLAUDE_CODE_OPUS_4_6_FAST_MODE_OVERRIDE=1

This addresses the 4.7 compound factor but does NOT fix the underlying long-session
degradation introduced in v2.1.139.
degradation introduced in v2.1.139.

Environment

- Claude Code versions affected: 2.1.139 through 2.1.143
- Last known-good version: 2.1.138
- Subscription: Max ($200/month)
- Platform: macOS (Apple Silicon)
- Usage pattern: Long-running structured agentic workflows with custom hooks,
quality monitoring, and explicit model-version requirements

Consumer Terms Context

Anthropic's Consumer Terms §12 permits service modifications "at any time without notice."
However, §6(5) requires 30 days notice for fee increases. A silent quality degradation
that wastes ~1.4M tokens and ~7 hours of user time over 6 days is economically equivalent
to a fee increase for subscribers whose output quality depends on session stability and
model version consistency.

Request

1. Investigate long-session regression in v2.1.139 — what changed in context handling,
compaction, or session management that causes progressive behavioral drift?
2. Add changelog entries for model version changes in any mode
3. Document CLAUDE_CODE_OPUS_4_6_FAST_MODE_OVERRIDE in official docs
4. Provide stable model version pinning as a first-class setting
5. Advance notification for behavior-affecting changes — especially for Max subscribers
at $200/month building production workflows on Claude Code
6. Review the affected sessions listed above to verify degradation patterns server-side

### Claude Model

Opus

### Is this a regression?

Yes, this worked in a previous version

### Last Working Version

2.1.138

### Claude Code Version

2.1.139 through 2.1.143

### Platform

Anthropic API

### Operating System

macOS

### Terminal/Shell

Terminal.app (macOS)

### Additional Information

## Affected Session IDs (available for Anthropic review)

All sessions >100KB since v2.1.139, sorted by size descending:

| Session ID | Size | Timestamp (ET) |
|-----------|------|----------------|
| `ec1c8982-e331-4828-9f6c-a580fdb61d6f` | 6.4 MB | 2026-05-11 21:26 |
| `7f18bd12-8be8-4f63-a681-54b3d6d92e94` | 5.1 MB | 2026-05-14 14:52 |
| `d0001021-2d9f-4d02-9898-b394c68314e1` | 4.0 MB | 2026-05-15 23:20 |
| `ad803cc4-e42d-43ad-9ed1-fdf1c49dcf77` | 3.0 MB | 2026-05-16 02:19 |
| `85d62600-100e-4837-b00c-6fc21de1b360` | 2.6 MB | 2026-05-14 01:13 |
| `21be96d8-e609-41d6-b9be-0c3eaf982c85` | 2.3 MB | 2026-05-12 23:57 |
| `c9e86ed7-c735-4b0a-841f-e9adc1590fa9` | 1.8 MB | 2026-05-14 22:39 |
| `17cba326-6301-4845-aefe-6ec1458523f7` | 1.8 MB | 2026-05-12 18:20 |
| `575dcc2a-2be8-4495-a3f2-c85e3eee9506` | 1.8 MB | 2026-05-16 01:51 |
| `629d0e4f-5319-4cd3-b012-7662b913d9e8` | 1.7 MB | 2026-05-16 20:31 |
| `165be4d2-a6b3-4ac4-80b9-adae81f51170` | 1.5 MB | 2026-05-13 00:28 |
| `7b7f4c91-e085-47d2-a895-6ea006918e55` | 1.5 MB | 2026-05-16 20:50 |
| `fd394b6f-92a3-4b57-8bc0-1ccb7818f416` | 984 KB | 2026-05-15 23:47 |
| `2b9aa28a-85b2-4d0f-b5a0-fda81586ed12` | 882 KB | 2026-05-14 21:02 |
| `dcaf1755-29a2-429d-9c0a-25906b01b5f7` | 749 KB | 2026-05-11 23:37 |
| `3790aa25-c6c8-412c-a076-77a8bbf4147c` | 714 KB | 2026-05-14 15:42 |
| `892a0bb8-f8f9-48f8-859f-d683a88450c1` | 646 KB | 2026-05-12 09:53 |
| `dc1fe9d2-bf94-44e7-bc73-5d44a4ef7fbe` | 450 KB | 2026-05-16 17:12 |
| `b1737c6c-4e96-481f-a436-fb20e3950f97` | 421 KB | 2026-05-15 21:29 |
| `7ccf7aa2-4a67-4355-a73f-cf0580596eaf` | 297 KB | 2026-05-14 15:57 |
| `4bc122e4-e651-4098-9051-6eef6ca323e1` | 226 KB | 2026-05-14 22:41 |

Session `7b7f4c91` is the primary evidence session where degradation was identified,
debugged, and root causes traced. Session `ec1c8982` (6.4 MB, May 11) is the earliest
large session after v2.1.139 — the first one showing the pattern.

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