fix(combos): name per-target exclusion reasons on combo_unavailable

[vadymhimself]

Summary

• A combo refusal made before any target was tried returned only No available targets for combo: <id> (503 combo_unavailable), with no indication of why each target was skipped. In production this cost three wrong guesses before the cause turned out to be a request-eligibility filter.
• The message now names one coarse reason per target, e.g. No available targets for combo: all (gone/model-gone: provider missing; off/model-off: provider disabled; spent/model-spent: quota exhausted; cooling/model-cooling: cooldown 42s; metered/model-metered: ineligible for this request).
• comboExclusionReasons in src/combos/resolve.ts reuses the picker's own checks, in picker order. targetProviderIsUsable now delegates to the same providerExclusionReason, so the label and the selection decision cannot drift apart. A throwing eligibility check is labelled ineligible for this request; it never turns the 503 into a 500.
• Wired at every zero-attempt exit: the three comboUnavailable sites in core-combo.ts (initial pick, encrypted-task recovery pick, JEV), and NoAvailableComboTargetsError (from tryPickComboModel and the metered re-pick in request-prepare.ts), whose reasons the three request-prepare.ts catch sites forward. The post-attempt exhaustion 503 in exhaustedFailure is unchanged.
• Privacy: each label contains only the configured provider/model and a fixed reason string. No account labels, emails, tokens or request content.
• Usage ledger: not changed. RequestLogContext has no free-text error slot for a zero-attempt refusal, and adding one would change the persisted schema, which is out of scope here.
• Docs: added a sentence plus an example to docs-site guides/combos.md ("Why do I get combo_unavailable?"), and documented the contract in structure/runtime.md, which still fits its 600-line budget.

Verification

• Red on upstream/dev (b6dffa0) with the new test only: bun test tests/responses/responses-combo-exhausted-error.test.ts -t "zero-attempt" gave 1 fail, with Received: "No available targets for combo: all".
• Green with the fix: bun test tests/responses/responses-combo-exhausted-error.test.ts gave 16 pass, 0 fail.
• bun test tests/server/server-combo-reasoning-replay-eligibility.test.ts tests/server/server-combo-failover-e2e.test.ts tests/codex-integration/combos.test.ts: all pass. One exact-string assertion (missing provider row) was updated to the new message, ... free (missing/bad-model: provider missing).
• bun x tsc --noEmit: clean.
• bun run structure:check: passed.
• bun run privacy:scan: passed.
• bun run test:changed: 38888 pass, 107 skip, 7 fail across 1775 files. All 7 failures are local-environment-only and also fail on clean dev on this machine: connected-client runtime probe scope, Node launcher PATH fallback activation (x3), real PATH Bun validation (x2), and resolveCodexRuntime > treats missing persisted and resolved versions. The full bun run test was not run separately. test:changed already covered 1775 files through the import graph, and the remainder is left to CI.
• No GUI changes.

Checklist

• Scope stays focused and avoids unrelated cleanup.
• Docs or release notes were updated when needed.
• Security-sensitive changes were reviewed for secrets, auth, and unsafe defaults.

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This PR stays in draft until every box below is ticked. Tick all four boxes once the requirements are met:

• Required local validation passed; commands, results, and any full-suite exception are documented.

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Summary by CodeRabbit

• Bug Fixes

  • Combo-unavailable errors now explain why each target was skipped, including missing or disabled providers, exhausted quota, active cooldowns, and request ineligibility.
  • Error responses continue to include cooldown retry timing when applicable.

• Documentation

  • Added guidance on interpreting target exclusion reasons when no target was attempted.

[coderabbitai]

📝 Walkthrough

Combo target resolution now derives exclusion reasons when no target can be selected. Combo-unavailable errors and server responses include those reasons, and related tests and documentation have been updated.

Changes: Combo exclusion diagnostics

Layer / File(s)	Summary
Derive target exclusion reasons src/combos/resolve.ts, src/combos/index.ts	The resolver reports the first applicable exclusion for each target: provider missing or disabled, quota exhausted, cooldown, or request ineligibility. When no target is picked, the error includes the reasons.
Propagate reasons to unavailable responses src/server/responses/core-errors.ts, src/server/responses/core-combo.ts, src/server/responses/request-prepare.ts, tests/responses/responses-combo-exhausted-error.test.ts, tests/server/server-combo-reasoning-replay-eligibility.test.ts, docs-site/src/content/docs/guides/combos.md, structure/runtime.md	Response paths pass exclusion reasons into combo-unavailable errors. Tests cover five exclusion reasons and verify that no target is dispatched. The guide and runtime documentation describe the error details.

Priority: ⬇️ Low

Estimated code review effort: 3 (Moderate) | ~20 minutes

Change: Bug fix

Sequence Diagram(s)

sequenceDiagram
  participant requestPrepare
  participant comboUnavailable
  participant NoAvailableComboTargetsError
  requestPrepare->>comboUnavailable: Pass combo ID and exclusion reasons
  comboUnavailable->>NoAvailableComboTargetsError: Construct unavailable message
  NoAvailableComboTargetsError-->>comboUnavailable: Return message
  comboUnavailable-->>requestPrepare: Return unavailable response

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Suggested reviewers: lidge-jun

Merge Risk: 🔵 Low · up to fe3ab

Some zero-attempt JEV responses still omit the promised target reason. This is a localized diagnostic gap; merging is reasonable with an explicit follow-up.

Pre-merge checks: 4 · 1

Failed checks: 1 warning

Check name	Status	Explanation	Resolution
Docstring Coverage		Docstring coverage is 37.50% which is insufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Analyzed 8 functions across 7 files. (2 skipped: 2…	Write docstrings for the functions missing them to satisfy the coverage threshold.

Passed checks: 4 passed

Check name	Status	Explanation
Linked Issues check		Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check		Check skipped because no linked issues were found for this pull request.
Description Check		Check skipped - CodeRabbit’s high-level summary is enabled.
Title check		The title clearly and concisely describes the main change: adding per-target exclusion reasons to combo_unavailable responses.

Full details: Docstring Coverage

Explanation

Docstring coverage is 37.50% which is insufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Analyzed 8 functions across 7 files. (2 skipped: 2 unsupported.)

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[vadymhimself]

@coderabbitai full review

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✅ Action performed

Full review finished.

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Actionable comments posted: 1

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• 🪄 Fix comments: Fix CodeRabbit comments on this PR

🤖 Prompt to fix review comments

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Inline comments:
Review comments at @src/server/responses/core-combo.ts:
- Line 576: Update the diagnostic predicate passed to comboExclusionReasons in
the zero-choice path to apply the supported reasoning-effort check for JEV
targets, so it reports why all eligible targets were discarded. Keep this check
limited to diagnostics without changing normal target selection, and add a test
for the all-choices-discarded case.

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ℹ️ Review info

⚙️ Run configuration

• Configuration used: Repository: lidge-jun/opencodex/.coderabbit.yaml
• Review profile: ASSERTIVE
• Plan: Advanced
• Run ID: 46b6b7c2-64a7-4be0-8a04-d7b2c94a1730

📥 Commits

Reviewing files that changed from the base of the PR and between 49ffccd and fe3ab50.

📒 Files selected for processing: 9

• docs-site/src/content/docs/guides/combos.md
• src/combos/index.ts
• src/combos/resolve.ts
• src/server/responses/core-combo.ts
• src/server/responses/core-errors.ts
• src/server/responses/request-prepare.ts
• structure/runtime.md
• tests/responses/responses-combo-exhausted-error.test.ts
• tests/server/server-combo-reasoning-replay-eligibility.test.ts

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[github-actions]

✅ Deterministic PR hygiene checks passed.

[lidge-jun]

Being carried into #7224 with your authorship preserved; this PR will be closed when the bundle lands.

[github-actions]

⏳ DRAFT

• review readiness checklist open (0/4 boxes ticked).

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• Tick all four boxes in the PR description once you're done (currently 0/4).

Review readiness checklist

• ⬜ Required local validation passed; commands, results, and any full-suite exception are documented.
• ⬜ I pushed my PR to a recent dev commit (at most 10 behind; a maintainer may still ask for the exact tip before merge).
• ⬜ I resolved all correct Codex and CodeRabbit findings.
• ⬜ My PR is ready for review.

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[lidge-jun]

Landed on dev in 6ccb47a via #7224 (your change is its own commit in that bundle, and the squash keeps your authorship as Co-authored-by). Thank you!