feat(mcp): expose current environment diagnostics

[juliusmarminge]

Problem

Agents could not inspect the identity or safe operational state of the T3 environment backing their current conversation without relying on unrelated settings payloads.

Change

Add t3_environment_read, backed by the existing environment, provider registry, and settings services. It returns a paged provider-health summary, explicit safe capabilities, and a narrow server-owned preference projection.

Behavior

The tool is current-environment only, read-only, and closed-world. It reads cached provider state without refreshing, installing, or authenticating providers. Human-readable diagnostics are Unicode-safe and bounded; credentials, provider configuration, model inventories, paths, raw diagnostics, and administrative fields never enter the result. Environment mismatch is checked before provider/settings reads, and finite public failure codes do not expose internal causes.

Focused validation

• allowlisted service projection, capability/mismatch denial, unavailable-service, and Unicode-bound tests
• production HTTP tools/list root-object and annotation coverage
• Claude read-only allowlist and shared presentation coverage
• 6 focused files / 18 tests passed across the complete native stack
• server and contracts scoped typechecks, targeted formatting, and git diff --check

Dependency

Bottom layer of native stack 8782, based on t3code/codex-turn-mapping at the pinned rollout target. #8728 adds the dependent preference mutation.

Implemented by GPT-5.6-Sol via Codex in T3 Code.

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[macroscopeapp]

Reviewed the new EnvironmentMcpService, its toolkit wiring, and the environmentMcp contract against the Effect service conventions. Module layout (namespace imports of Effect subpaths, inline Context.Service interface, make then layer, dependencies acquired via yield*) looks right; two findings on the error model.

^{Posted via Macroscope — Effect Service Conventions}

[github-actions]

Thread transfer impact

⚠️ The latest CI run did not produce a thread transfer result for 512391d.

This comment will update automatically after the next completed run.

[macroscopeapp]

Approvability

Verdict: Not approved

Macroscope's review found this PR not approvable — This PR introduces and registers a new authenticated MCP capability that exposes environment, provider health, authentication summaries, and server-owned preferences to provider sessions. Its read-only and allowlisted design limits risk, but the new production data boundary and session-tool availability require human review.

Notes:

• Diff unchanged. Approvability was decided on eligibility alone.

No code changes detected at 512391d. Prior analysis still applies.

^{You can add or adjust custom eligibility rules. Learn more.}

[macroscopeapp]

One retained finding: EnvironmentMcpFailure still drops the underlying failure when the service wraps a real error. See the inline comment.

^{Posted via Macroscope — Effect Service Conventions}

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[juliusmarminge]

Superseded as the active MCP proposal by #10560 in the rebuilt MCP stack, following the maintainer request to review thin service wrappers separately from service hardening.

This is not a full feature-parity replacement. Updates require a live full-access/default caller under the existing ThreadCommandExecutor. Provider configuration, secrets and admin controls are not exposed. No new settings store or lock service.

Closing this mixed proposal without merging. The remote branch agents/mcp-environment/read and exact head 512391db91759dc9f4a7b1e93ca3075fe1e90dea are retained so the service changes can be extracted for separate review later.