Appendix A — Browser AI Field Guide: Setup, APIs, States, Diagnostics and Configuration

This appendix consolidates the contracts, states, and procedures distributed through the argument into one working reference. Prose explains; this section is for the keyboard.

Publication snapshot: September 2026. Browser flags, origin trials, API availability, and experimental configuration change. Flag values and external documentation details below reproduce the tested builds of this edition. Anything the book’s own laboratories did not observe is marked as such.

A.1 Development setup

Flag values follow the laboratory run-book convention of this edition, not a permanent browser contract.

A.2 Availability and lifecycle states

One hierarchy, observed in the book’s laboratories:

available is not a ready session, and operational completion is not behavioral acceptance.

A.3 Built-in AI API matrix

Globals, operations, and streaming support as implemented in the book’s adapter registry:

Session verbs observed in code: create, prompt, clone, destroy. The extension worker in this edition executes prompt; other operations return an explicit unsupported marker rather than failing silently.

A.4 Prompt API configuration quick reference

A.5 Sampling modes

Per current Chrome Prompt API documentation (external, September 2026 — not lab-observed except as noted):

Book laboratory default: most-predictable. Reason: controlled experimental comparison and compatibility with the runtime observed during the book’s experiments, including the recorded speculative-decoding rejection that named it alongside topK: 1 and temperature: 0. Not a universal recommendation: creative or writing features may legitimately choose another supported profile.

A.6 Session lifecycle

Exposure, creation, ready, usage, clone, destroy, failure — see the table in Chapter 9, Section 6. Application rules: measure creation separately from inference, snapshot usage and quota, branch experiments with clone() rather than reprompting, end with explicit destroy(), and record why destruction occurred.

A.7 Observatory event dictionary

Every trace in this book validates against one closed vocabulary. Canonical lifecycle events:

Coordination, authority, policy, and context events:

A.8 Capture and privacy modes

Metrics-only capture is the default: sizes and outcomes are recorded without retaining prompts or page text. Content capture is explicit and visible. Private units are excluded by default and cannot cross an external provider boundary; exclusions carry reasons rather than failing silently.

A.9 Structured-output validation gates

Parse, schema, domain, policy, authority, then bounded repair — see the table in Chapter 16, Section 11. A model may propose a structured action; only deterministic policy and appropriate human authority may admit it for execution.

A.10 WebMCP quick reference

The live surface is document.modelContext.registerTool(definition, { signal }), withdrawn via AbortController. Registration requires the origin trial or the testing flag, a secure top-level context, and an exposed global; the book’s probe records available or absent rather than assuming. Observed in the tested profile: WebMCP absent — the capstone never depends on it being present.

A.11 Authority states

A grant binds one normalized effect — origin, tool, arguments, policy version — under a SHA-256 digest, with a TTL and single use. Lifecycle: bound, consumed exactly once at the coordinator START boundary, or expired. Replay is denied before execution; mutation is rejected on digest mismatch. Policy requests authority; it never manufactures it.

A.12 Diagnostic decision table

A.13 Lens modes

See the comparison table in Chapter 24, Section 2. One line each: Understand interprets, Explore retrieves bounded evidence, Govern applies user policy reversibly, Act proposes and executes through admission, policy, authority, and coordinator boundaries.

A.14 What to record in every experiment

Selection and inputs, provider and data route, evidence with provenance, policy version and decision, authority state for effects, worker and execution lineage, the validated trace — enough that a stranger can replay the run and reach the same verdict about what happened.

A.15 Laminated card

UNDERSTAND interpret      EXPLORE retrieve bounded evidence
GOVERN policy, reversible ACT propose, never self-authorize

MODEL CAN: perceive, classify, generate, rank, propose
MODEL CANNOT: grant authority, change policy, bypass admission,
  consume a grant twice, invent provenance, turn uncertainty
  into certainty

AVAILABILITY: absent < exposed < downloadable < available
SESSION: create, run, clone, destroy — measure each
GRANT: exact effect, once, expiring — consumed at START
TRACE: every claim validates or it did not happen