fix: propagate subagent output length errors

[yixiao5428]

Summary

• persist provider length finishes as terminal session errors while preserving partial output and preventing automatic replay
• propagate child assistant failures through foreground and background tasks, preserve cancellation semantics, and escape model-controlled task markup
• align reasoning output envelopes and numeric thinking budgets across provider transports while preserving plugin overrides
• document the output-token behavior and synchronize all 17 existing localized CLI pages

Behavior

A subagent that reaches the provider output limit now remains inspectable through its child session, but the parent task and top-level CLI correctly report failure instead of completed success. The fix does not replay requests, rerun tools, or expose reasoning text in parent diagnostics.

Reasoning models now use the declared model output limit by default. Numeric max thinking variants reserve visible-output headroom, respect provider bounds, and account for SDK transports that add thinking tokens back to the wire maximum.

Validation

• 12 affected test files: 687 passed, 2 skipped, 0 failed
• packages/opencode: bun run typecheck
• packages/web: bun run build across 18 languages and 648 pages
• pre-push full workspace typecheck: 30 of 30 tasks passed

Detailed reproduction, invariants, implementation decisions, and test coverage are recorded in docs/fixes/subagent-fix-output-length.md.

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[github-actions]

The following comment was made by an LLM, it may be inaccurate:

Based on my search results, I found one potentially related PR:

PR #26167: fix(session): retry empty stream truncations and discard partial parts

• fix(session): retry empty stream truncations and discard partial parts #26167
• This PR also deals with handling stream truncations and partial output, which relates to the output length error handling in PR fix: propagate subagent output length errors #38726.

All other results returned PR #38726 (the current PR) itself or unrelated PRs. The search indicates this is primarily a unique fix addressing subagent output length propagation that doesn't have a direct duplicate in the open PRs.