Conciseness

The ~35-line body is lean with zero padding: it never explains what Flutter, OAuth, or PKCE are, and every line is directive ("Read `pubspec.yaml`...", "Never trust a redirect merely because its scheme matches"). This matches the 'every token earns its place' anchor; the level below would require some trimming, and none is evident.

Actionability

For an instruction-only skill the guidance is largely concrete: preflight names exact artifacts to read, the flow section prescribes "authorization-code flow with PKCE and an external user-agent" and "correlate the response using state and, for OIDC, nonce", and verification enumerates a specific scenario matrix. It falls short of the top anchor because guidance stays at the policy level — no specific commands, APIs, or plugin/package examples to make it fully copy-paste executable.

Workflow Clarity

A clear implicit sequence (Preflight → conditional references → boundary routing → Verification) with strong explicit checkpoints — the verification section is a detailed checklist ("State which provider, client type, platform, account state, redirect, storage, and backend checks were actually observed"). It is not a 5 because steps are section-implied rather than explicitly sequenced and there is no feedback loop for what to do when a verification scenario fails.

Progressive Disclosure

The body is a concise overview and all three bundle files in references/ are real, one level deep (no nested links), each surfaced under 'Load references conditionally' with a clear description of exactly when to read it — matching the well-signaled one-level-deep anchor; the level below would require buried or unclearly signaled references.

Specificity

The description lists multiple concrete actions ("Implement, repair, or review") and comprehensively enumerates capabilities: "OAuth or OIDC redirects, PKCE, token refresh, logout, account switching, passkeys, and device re-authentication" — full coverage of the domain rather than 1-2 actions, matching the comprehensive anchor rather than the 'several actions with minor gaps' anchor below it.

Completeness

Explicitly answers both: what ("Implement, repair, or review Flutter user authentication and session lifecycle...") and when ("Use when sign-in identity or session behavior is the task"), plus routing guidance for adjacent skills — matching the anchor requiring both what and when with concrete trigger phrasing, not the level below where 'when' is only adequately explicit.

Trigger Term Quality

Good natural keyword coverage: "sign-in", "token refresh", "logout", "account switching", "passkeys", "session", "OAuth". A few common variations are missing (e.g., "login"/"log in" as a synonym for sign-in), so it sits just below the comprehensive-synonyms anchor.

Distinctiveness Conflict Risk

Clear niche (identity and session behavior) with explicit disambiguation — "route broad threat audits to flutter-security and request transport to flutter-networking" — minimizing conflict risk with the most likely overlapping siblings, which fits the minimal-conflict anchor rather than the 'minor overlap risk' anchor.