(MOT-3925) fix(harness): inherit parent provider only when the spawn model is inherited too

[andersonleal]

Problem

harness::spawn with an explicit model but no provider silently attached the parent turn's provider to the child. A parent running zai::glm-5.2 spawning model=claude-sonnet-4-6 produced the incoherent pair zai::claude-sonnet-4-6. The router honors an explicit provider without catalog membership checks (documented intent — decide() step 1 is cold-catalog tolerant), so provider-zai sent the claude model to Z.AI, which rejected it upstream:

{"error":{"code":"1211","message":"Unknown Model, please check the model code."}}

The child session died with the raw error as its result, and the parent could not recover by picking another Anthropic model — the retry inherited zai again.

Observed in a live multi-agent run: children spawned in-turn (fact-extractor-b4k9, summarizer-b4k9) inherited provider=zai and failed, while children spawned through the parentless reactive path got provider=None and ran the same model successfully via catalog routing to anthropic. Reproducible with iii trigger router::complete model=claude-sonnet-4-6 provider=zai messages=[…].

Fix

seed_child now resolves the provider through child_provider(): an explicit request wins; otherwise the parent's provider is inherited only when the model is inherited too — model and provider stay a coherent pair. A spawn that names its own model gets no provider and the router routes it by catalog.

spawn model	spawn provider	before	after
explicit	explicit	both used	unchanged
inherited	inherited	parent pair	unchanged
inherited	explicit	explicit provider	unchanged
explicit	absent	parent's provider	none → router decides

harness::send's inject path already reuses the last turn's options as one bundle, so spawn was the only place the pair could split.

Test

provider_is_inherited_only_together_with_the_model in subagent.rs covers the failing case, coherent-pair inheritance, explicit-provider-wins, and parentless spawns. Full harness lib suite: 194 passed.

https://claude.ai/code/session_0155foth69LSS75ebk2gjZ8U

Summary by CodeRabbit

• Bug Fixes
  • Improved provider selection for newly spawned sessions to keep model and provider choices consistent.
  • Explicit provider selections now take priority, while model-only requests no longer inherit an incompatible provider.
  • Sessions without a parent now start without inherited settings.
  • Updated validation coverage for the supported model/provider combinations.

[vercel]

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[github-actions]

skill-check — worker

0 verified, 41 skipped (no docs/).

Layer	Result
structure	✓
vale	✓
ai	✓
render	✓

Four for four. Nicely done.

[coderabbitai]

📝 Walkthrough

Walkthrough

The provider derivation logic in seed_child was refactored into a new child_provider helper that enforces coherent model+provider pairing rules: explicit provider always wins, explicit model without provider prevents inheritance, and unset model/provider inherits the parent's provider. Tests were added to validate this matrix.

Changes

Provider Inheritance Rule

Layer / File(s)	Summary
Provider inheritance helper and wiring harness/src/subagent.rs	Adds child_provider helper enforcing model+provider coherence rules and updates seed_child to delegate provider resolution to it.
Inheritance matrix tests harness/src/subagent.rs	Adds spawn_request test helper and a new test validating provider inheritance behavior across explicit/unset model and provider combinations, including parentless spawns.

Estimated code review effort: 2 (Simple) | ~10 minutes

Sequence Diagram(s)

sequenceDiagram
    participant Caller
    participant SeedChild
    participant ChildProvider
    participant ParentRecord

    Caller->>SeedChild: spawn request (model?, provider?)
    SeedChild->>ChildProvider: child_provider(req, parent_record)
    ChildProvider->>ParentRecord: read parent provider
    alt provider explicitly set
        ChildProvider-->>SeedChild: use request provider
    else model set, provider unset
        ChildProvider-->>SeedChild: return None (router resolves)
    else model and provider unset
        ChildProvider-->>SeedChild: inherit parent provider
    end
    SeedChild-->>Caller: child session with resolved provider

Loading

Poem

A hop, a nibble, a coherent pair,
model and provider matched with care,
no borrowed provider when model's declared,
but silence inherits what parent shared.
🐰 tests all green, my carrot's spared!

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check.
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title clearly matches the main fix: provider inheritance only when the spawn model is inherited too.

✨ Finishing Touches

📝 Generate docstrings

• Create stacked PR
• Commit on current branch

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch fix/harness-spawn-provider-inheritance

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