Claude "Prompt is too long" (terminal_reason blocking_limit) is shown as "Usage limit reached" #19832

What happened

A long Claude (Opus 5.5) thread in the desktop app stopped with "Prompt is too long", and T3 Code showed "Usage limit reached." plus the banner "Usage limit reached · Reset time unavailable; retry manually". Every follow-up message ("kontynuuj", sent 3 times) failed the same way within ~40 ms. The plan was not usage-limited: the latest rate_limit_event reported status: "allowed" (5h window 73%, 7d window 40%). The real cause was a full context window (auto-compact was disabled in the user's Claude Code settings), so "retry manually" can never succeed and the UI does not point the user to /compact or a new thread.

Diagnosis

The Claude SDK ended the turn with terminal_reason: "blocking_limit", is_error: true, result: "Prompt is too long", and the synthetic assistant message carried error: "invalid_request". In this case blocking_limit meant the context blocking limit, not a plan or rate limit.

apps/server/src/orchestration-v2/Adapters/ClaudeAdapterV2.ts treats every blocking_limit as a usage limit:

• L2820: message.terminal_reason === "blocking_limit" || ... ? "usage_limit" : "provider_error" sets the failure class to usage_limit, which drives the "Usage limit reached" row, the warning banner and the usage-limit recovery/resume UI.
• L2638: terminalResultError maps blocking_limit to "Claude stopped: a usage limit blocked the request."
• ClaudeAdapterV2.test.ts:4644 asserts blocking_limit → usage_limit, so the mapping is locked in.

Suggested direction: don't classify blocking_limit as usage_limit on its own. Use usage_limit only when there is rate-limit evidence (api_error_status === 429, an assistant error: "rate_limit", or a rate_limit_event with a non-allowed status). When the result is "Prompt is too long" / error: "invalid_request", report a context-window failure (as prompt_too_long already is: "the prompt exceeds the model's context window") and offer /compact or a new thread.

Steps to reproduce

1. In ~/.claude/settings.json set "autoCompactEnabled": false.
2. In the T3 Code desktop app, run a Claude thread (Opus 5.5, 1M context) until the conversation fills the context window (here: 189 turns, a long refactor with many file reads and builds).
3. When the turn fails, observe "Prompt is too long" followed by "Usage limit reached." and the "Usage limit reached · Reset time unavailable; retry manually" banner.
4. Send any follow-up message: it fails immediately with the same pair of messages, while the provider's rate-limit state is allowed.

Version

0.0.46-nightly.20261015.3145 (desktop app, local server; tag commit 339179f, same as main at triage time)

Environment

macOS (Darwin 27.0.0, arm64), Node v26.8.2, @anthropic-ai/claude-agent-sdk 0.3.293, model claude-opus-5-5 (1M context)

Evidence

# provider/events.<thread>.log (trimmed)
08:36:10.675Z rate_limit_event  {"status":"allowed","rateLimitType":"five_hour","unifiedWindows":{"five_hour":{"utilization":0.73},"seven_day":{"utilization":0.4}}}
08:36:10.699Z assistant         {"model":"<synthetic>","content":[{"type":"text","text":"Prompt is too long"}],"error":"invalid_request","is_api_error_message":true}
08:36:10.710Z result            {"subtype":"success","is_error":true,"terminal_reason":"blocking_limit","api_error_status":null,"result":"Prompt is too long","num_turns":189}
08:38:00.634Z prompt.offer      "kontynuuj"
08:38:00.689Z assistant         {"model":"<synthetic>","text":"Prompt is too long","error":"invalid_request"}
08:38:00.695Z result            {"is_error":true,"terminal_reason":"blocking_limit","result":"Prompt is too long","duration_ms":37}
08:39:48.2xxZ (same again for the third "kontynuuj")

UI: "Prompt is too long" → "⚠ Usage limit reached." (×3), banner "Usage limit reached · Reset time unavailable; retry manually". A screenshot will be attached below.

Related issues

#10320 (a different Claude error misclassified: expired login or usage limit shown as "gave up after repeated API errors"; closed). #19823 (configurable compaction threshold, not about error classification). No existing issue covers blocking_limit / "Prompt is too long" being shown as a usage limit.

Fix applied or workaround

No code changes. Workaround: run /compact in the thread or start a new thread; re-enable autoCompactEnabled in ~/.claude/settings.json to avoid hitting the limit.

Filed by

Claude Code (Claude Opus 5.5, claude-opus-5-5) via t3 triage