bug(v2): Fable/Zen request-size 400 bypasses auto-compaction #35013

Description

OpenCode V2 auto-compaction does not trigger for long Fable sessions that exceed the Zen/Fable request byte envelope before they approach the advertised token context limit.

Observed behavior:

• Model: opencode/claude-fable-5#high through Zen.
• Catalog limit for Fable: context=1000000, output=128000.
• V2 auto-compaction threshold: context - max(output, buffer) = 872000 estimated tokens.
• A real long session translated to 646 LLM messages and about 1.31 MB of request JSON before system/tools.
• Token.estimate(JSON.stringify({ system, messages, tools })) for messages-only shape was about 328000, below the 872000 threshold, so SessionCompaction.compactIfNeeded returned false.
• The provider then failed before any assistant output with HTTP 400:

Provider request failed with HTTP 400: {"type":"error","error":{"type":"invalid_request_error","message":"Error from provider (Console): Upstream request failed"}}

No session.next.compaction.* events were emitted. The turn became session.next.step.failed.2.

Why This Happens

The proactive path in packages/core/src/session/compaction.ts only checks estimated tokens against the advertised model context:

estimate({ system: input.request.system, messages: input.request.messages, tools: input.request.tools }) <=
  context - Math.max(output, config.buffer)

The reactive overflow path in packages/core/src/session/runner/llm.ts only recovers when isContextOverflowFailure(...) is true. The Zen/Fable response body says only Upstream request failed, so packages/llm/src/provider-error.ts does not classify it as context overflow.

Synthetic Reproduction

Against https://opencode.ai/zen/v1/messages with model=claude-fable-5, max_tokens=1, and no private content:

• 700000 characters in one user text message succeeds.
• 1000000 characters in one user text message returns the same generic HTTP 400 Upstream request failed.
• 650 tiny alternating messages ending in a user turn succeeds, so this is not simply message count.
• Small Anthropic tool-use/tool-result shapes also succeed, so this is not obviously invalid tool-result ordering.

Expected Behavior

V2 should compact before sending requests that are too large for the route/provider envelope, or classify this generic Zen/Fable 400 as recoverable overflow when it happens before assistant output.

Possible fixes:

• Add a byte-envelope guard to SessionCompaction.compactIfNeeded, perhaps route/model configurable and separate from token context.
• Add provider/route metadata for max request bytes or a lower effective compaction threshold for Zen/Fable.
• Improve Zen/Fable error classification so this generic request-size failure becomes context-overflow when appropriate.

Related Issues

Related but not the same:

• Sub-agents hang indefinitely on context overflow — no compaction triggered #25187: sub-agent context overflow and incomplete overflow classification.
• [Network connection lost] due to Context overflow at model limit misclassified — no compaction, no retry, session dies!! #33376: context overflow misclassified through an unrelated error path.
• fix(compaction): output-budget reservation capped at 20K for limit.input models, risking overflow #32656: output-budget reservation issue in the older compaction path.

This report is specifically about V2 packages/core/src/session/compaction.ts using token context only while Zen/Fable appears to fail at a smaller request-byte envelope.