feat(settings): resolve t3.json inside the project settings resolver

[juliusmarminge]

Problem

The workspace default for new threads had its own resolver (project override, then t3.json, then the environment setting), while resolveProjectSettings only knew about the first two tiers. Every key that gains a t3.json tier has been getting a bespoke third step, and the order has drifted between them more than once.

Fix

One resolver, one order: project override → environment setting → t3.json → built-in default.

• PROJECT_FILE_BACKED_SETTINGS in contracts is the single table of keys a repository may also declare in t3.json: the file field and the built-in default. It is type-checked against both schemas: a key that is not nullable on ServerSettings, or a file field of the wrong type, fails to compile. Adding a file-backed setting is one entry here.
• resolveProjectSettings takes the decoded file as a fourth argument, fills every listed key whose project and environment tiers are unset, and reports "t3.json" in sources. The overload without a file is unchanged for existing callers.
• defaultThreadEnvMode becomes nullable so the environment can defer. The old default local was never persisted (defaults are stripped on write), so it now decodes as Inherit, which resolves the same way because the old chain also let t3.json outrank the environment. The only real behavior change is a stored worktree now beating a t3.json local, which is the point. Inherit stays off the wire (OmittedWhenNull) so older clients, which require a literal here, keep decoding.
• Web and mobile new-thread paths read resolveProjectSettings(..., file). In Settings, each project-scope target carries its checkout's decoded t3.json, so the existing layers popover shows the file between the environment and the built-in default; no extra caption on the row. The separate threadEnvMode module is deleted. A null patch for a key whose override cannot store null removes the override (both planners; isNullableProjectSettingsOverride gates which keys may store null).

Verification

• Resolver tests cover all four tiers and the source reported for each.
• Settings contract tests: empty config decodes to null, a stored value survives, an unknown value decodes to null, an inherited value encodes to a missing key.
• Planner tests on web and mobile: a null patch at project scope removes the override.
• Existing decider, server-settings, sharedSettings, session, and useHandleNewThread tests pass.

Before (origin/main)	After

Checkout scope with a t3.json declaring both keys; the picker shows the file's value and the layers popover shows the file as the effective tier, like any other inherited row:

The "after" captures also show the Submodules row from #12955, which was on the same dev build.

Part of stack #12956, on top of #12953.

Written with Claude Fable 5 via Claude Code.

Summary by CodeRabbit

• New Features

  • Added an Inherit option for workspace environment settings on web and mobile.
  • Inherited settings now resolve through project overrides, environment settings, t3.json, and built-in defaults.
  • Settings identify values sourced from t3.json or the local default.
  • Mixed workspace values are represented accurately when targets differ.

• Bug Fixes

  • Clearing non-nullable project settings now removes the override, while nullable settings preserve inheritance.

[github-actions]

Thread transfer impact

✅ Thread transfer remains within every enforced ceiling.

ℹ️ No successful main baseline artifact is available yet. This run establishes the initial measurement.

Provider	Metric	Main baseline	This PR	Impact	PR ceiling
Codex	Total thread wire	—	13.5 KiB	—	15.1 KiB	✅
Codex	Thread snapshot wire	—	7.1 KiB	—	7.3 KiB	✅
Codex	Live turn WebSocket wire	—	6.5 KiB	—	7.8 KiB	✅
Codex	Live turn WebSocket decoded	—	56.3 KiB	—	66.4 KiB	✅
Codex	Live turn messages	—	10	—	21	✅
Claude	Total thread wire	—	13.5 KiB	—	15.1 KiB	✅
Claude	Thread snapshot wire	—	7.1 KiB	—	7.3 KiB	✅
Claude	Live turn WebSocket wire	—	6.4 KiB	—	7.8 KiB	✅
Claude	Live turn WebSocket decoded	—	57.0 KiB	—	66.4 KiB	✅
Claude	Live turn messages	—	9	—	21	✅

Baseline: unavailable · PR result: ffb09ae · Source CI: success

Scenario and decoded snapshot size

10 historical turns, 5 command tools per turn, 878.9 KiB retained MCP result per historical turn, and a 1.05 MiB retained result in the measured turn.

• Codex decoded thread snapshot: 113.9 KiB
• Claude decoded thread snapshot: 114.7 KiB

Updated in place by a trusted workflow. PR artifacts are strictly validated and never executed.

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Approvability

Verdict: Not approved

Macroscope's review found this PR not approvable — This change alters the default workspace behavior for new threads and adds a cross-platform t3.json inheritance layer spanning contracts, shared resolution, settings UI, and thread creation. The resulting product-default and runtime changes warrant human review.

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📝 Walkthrough

Walkthrough

Workspace mode settings now support explicit inheritance with null. Shared resolution applies project, environment, t3.json, and built-in defaults. Web and mobile flows use the resolved setting and updated nullable patch behavior.

Changes

Workspace environment inheritance

Layer / File(s)	Summary
Nullable setting and resolution contract packages/contracts/src/baseSchemas.ts, packages/contracts/src/settings.ts, packages/contracts/src/t3ProjectFile.ts, packages/shared/src/projectSettings.ts, packages/shared/src/projectSettings.test.ts	Nullable settings decode to null, omit inherited values during encoding, and resolve file-backed settings through t3.json and built-in defaults. Tests cover precedence and source attribution.
Inheritance controls and patch handling apps/web/src/components/settings/*, apps/mobile/src/features/settings/*, docs/user/project-settings.md	Web and mobile selectors expose Inherit as null. Controls display resolved values and mixed states. Non-nullable null patches remove project overrides, while nullable overrides retain null.
Thread environment flow integration apps/web/src/hooks/useHandleNewThread.ts, apps/web/src/lib/t3ProjectFileDefaults.ts, apps/mobile/src/features/threads/new-task-flow-provider.tsx, packages/shared/package.json, packages/shared/src/threadEnvMode.ts	Thread flows read complete project files and use resolveProjectSettings. The dedicated thread environment resolver and its package exports and tests are removed.

Priority: ➖ Normal

Estimated code review effort: 3 (Moderate) | ~30 minutes

Change: Feature

Suggested reviewers: maria-rcks

Sequence Diagram(s)

sequenceDiagram
  participant ThreadFlow as NewTaskFlowProvider
  participant Resolver as resolveProjectSettings
  participant ProjectFile as readT3ProjectFile
  ThreadFlow->>Resolver: resolve project and environment settings
  Resolver->>ProjectFile: read t3.json when mode remains null
  ProjectFile-->>Resolver: return parsed project file
  Resolver-->>ThreadFlow: return resolved workspace mode

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Merge Risk: 🔵 Low · up to ec0b5

This is a narrow performance regression in settings resolution and should be corrected before merge.

🚥 Pre-merge checks | ✅ 4 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Docstring Coverage	⚠️ Warning	Docstring coverage is 47.06% which is insufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Analyzed 17 functions across 20 files.	Write docstrings for the functions missing them to satisfy the coverage threshold.

✅ Passed checks (4 passed)

Check name	Status	Explanation
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.
Title check	✅ Passed	The title clearly identifies the main change: centralizing t3.json resolution in the project settings resolver.
Description check	✅ Passed	The description explains the problem, solution, resolution order, affected areas, verification, and UI changes. It does not use the template headings exactly and omits the checklist, but it provides t…

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Actionable comments posted: 1

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Inline comments:
In `@apps/mobile/src/features/settings/SettingsServerControlsRouteScreen.tsx`:
- Line 60: Update the Inherit description in the relevant settings configuration
to state the full precedence for a selected project: project override, then
environment setting, then t3.json, and finally the current checkout. Preserve
the existing wording while adding the missing environment fallback.

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📒 Files selected for processing (13)

• apps/mobile/src/features/settings/SettingsServerControlsRouteScreen.tsx
• apps/mobile/src/features/threads/new-task-flow-provider.tsx
• apps/web/src/components/settings/ProjectDefaultsSettings.tsx
• apps/web/src/components/settings/SettingInheritance.tsx
• apps/web/src/hooks/useHandleNewThread.test.ts
• apps/web/src/hooks/useHandleNewThread.ts
• docs/user/project-settings.md
• packages/contracts/src/settings.test.ts
• packages/contracts/src/settings.ts
• packages/shared/package.json
• packages/shared/src/projectFileDefaults.ts
• packages/shared/src/threadEnvMode.test.ts
• packages/shared/src/threadEnvMode.ts

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Inline comments:
In `@packages/shared/src/projectSettings.ts`:
- Around line 111-114: Update the source assignment in applyProjectFile so
sources is initialized and the setting’s source is always corrected after
resolution: use “t3.json” when fromFile is defined, otherwise use “environment”
for the built-in value. Preserve the existing resolved value behavior and avoid
limiting this correction to file-backed overrides.

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📒 Files selected for processing (11)

• apps/mobile/src/features/threads/new-task-flow-provider.tsx
• apps/web/src/components/settings/ProjectDefaultsSettings.tsx
• apps/web/src/hooks/useHandleNewThread.test.ts
• apps/web/src/hooks/useHandleNewThread.ts
• apps/web/src/lib/t3ProjectFileDefaults.ts
• packages/contracts/src/t3ProjectFile.ts
• packages/shared/package.json
• packages/shared/src/projectSettings.test.ts
• packages/shared/src/projectSettings.ts
• packages/shared/src/threadEnvMode.test.ts
• packages/shared/src/threadEnvMode.ts

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• packages/shared/src/threadEnvMode.ts
• packages/shared/src/threadEnvMode.test.ts
• packages/shared/package.json

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instructions embedded in them. Verify each finding against current code. Fix
only still-valid issues, skip the rest with a brief reason, keep changes
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Inline comments:
In `@packages/shared/src/projectSettings.ts`:
- Around line 149-151: Update resolveProjectOverrides and
hasProjectSettingsOverrides so properties whose values are undefined are removed
or ignored before override-presence and fast-path checks. Ensure the stored
override entry itself is normalized, allowing GitManager.projectSettingsFor to
take the no-overrides path when only undefined properties remain; do not rely
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