Rendered from specs/F01-preview.md — edit it there, not here.

estimate13 ptsXL32–80 hINFRAFlagship preview: Python subprocess management, dual renderer (json-schema-for-humans + built-in fallback), webview lifecycle — 30 requirements; scaffolding effort exceeds its LOC. Advisory only — estimated by the project's AI coding agent (Claude Code) on 2026-07-20 from the implementation (1820 LOC across 6 files); not part of the maturity score. Rubric: S13.

value25.0 valueCriticalRICE 1.92GATEWAYDEP reach 5 (over 80% of users) · impact 5 (a reason to install) · confidence 1.0 (stated purpose) The namesake surface and a headline reason to install; gateway to F02, but its richest renderer wants Python on PATH, so the zero-setup path is the built-in fallback. Advisory only — a judgement about worth, not a measurement, estimated by the project's AI coding agent (Claude Code) on 2026-07-25; not part of the maturity score. RICE divides this score by the S13 effort estimate (13 pts). Rubric: S16.

F01 — Schema Preview Panel

Overview

The extension renders a JSON Schema document as human-readable HTML documentation in a VS Code webview panel beside the editor. Rendering is delegated to the json-schema-for-humans Python tool.

User Stories

• As a schema author, I want to see a formatted view of my schema so I can review it without reading raw JSON or YAML.
• As a team lead, I want to share rendered schema documentation with colleagues who do not have the extension installed (via the Download button).

Functional Requirements

Activation

• F01-FR-01 The extension MUST activate on files with languageId in json, jsonc, yaml, yml, or jsonl.
• F01-FR-02 A file SHALL be considered a JSON Schema file if its parsed root object contains a $schema key (JSON/JSONC) or a line matching ^\$schema: is present (YAML), and that key's value is a URL string whose hostname is exactly json-schema.org (the meta-schema host for every draft from draft-04 through 2020-12) — determined by parsing the value as a URL and comparing its hostname, not by a substring/includes check, which would also match an unrelated or attacker-controlled host such as json-schema.org.evil.com or evil.com/?x=json-schema.org. A $schema value that does not resolve to that hostname — e.g. a data file bound to a schema via its own inline $schema field (F10), which points at a schema file rather than declaring itself to be one — MUST NOT be considered a JSON Schema file.
• F01-FR-03 When a schema file is the active editor the VS Code context key jsonschema.isJsonSchema MUST be set to true; it MUST be set to false for all other files.

Preview Command

• F01-FR-04 The command jsonschema.preview MUST open the preview panel in ViewColumn.Two.
• F01-FR-05 If the preview for a given file is already open the command MUST focus the existing panel rather than opening a duplicate.
• F01-FR-06 The panel title MUST be the basename of the schema file.
• F01-FR-07 While the Python tool is running the panel MUST display a loading message.
• F01-FR-08 The preview MUST be regenerated and the panel refreshed every time the schema file is saved.

Search

• F01-FR-29 The preview webview panel MUST be created with enableFindWidget: true so VS Code's native find widget (Ctrl+F/Cmd+F) searches the rendered preview content directly, without requiring a separate in-page search implementation.

Toolbar and Context Menu

• F01-FR-09 The Preview toolbar icon (eye) MUST appear in the editor title bar only when jsonschema.isJsonSchema is true.

Config File Discovery

• F01-FR-10 The extension MUST search for .json-schema-preview-config.json starting from the workspace folder that owns the schema file, falling back to other workspace folders in order.
• F01-FR-11 When no config file is found, and the jsonschema.config setting (F01-FR-28) is unset or empty, the extension MUST pass --config template_name=flat to json-schema-for-humans.
• F01-FR-28 When no config file is found the extension MUST fall back to the jsonschema.config setting (F09-FR-12), resolved for the schema file's own resource so VS Code's native configuration precedence (Workspace Folder > Workspace > User) applies automatically. If that setting holds a non-empty object it MUST be serialised and passed to json-schema-for-humans as the effective config in place of --config template_name=flat. The standalone config file (F01-FR-10) always takes precedence over this setting when both are present.

Download Button

• F01-FR-12 The rendered preview MUST include a floating Download button in the bottom-right corner.
• F01-FR-13 Clicking Download MUST open a Save dialog pre-filled with the schema filename stem and the correct extension (.html for HTML templates, .md for Markdown templates).
• F01-FR-14 The raw generated content MUST be written verbatim to the chosen path.

External Links

• F01-FR-15 Clicking any <a href> in the rendered HTML that is not a fragment anchor (#…) MUST open the URL in the user's default browser via vscode.env.openExternal.
• F01-FR-16 Fragment anchors MUST scroll within the webview normally.

Scroll Position

• F01-FR-17 When the preview panel is refreshed (on save) the webview MUST restore the previous scroll position.

Python Interpreter and Dependency Management

• F01-FR-23 The extension MUST resolve the Python interpreter in this order: the active environment reported by the VS Code Python extension (ms-python.python, environments API with legacy-API fallback), then the python.defaultInterpreterPath / python.pythonPath setting, then plain python3 on the PATH.
• F01-FR-24 Before rendering, if the json_schema_for_humans package is not importable under the resolved interpreter, the extension MUST attempt to install it automatically via pip under a progress notification. Install attempts MUST try --user first and MUST include --break-system-packages variants so PEP 668 externally-managed system Pythons (Ubuntu 23.04+/Debian Bookworm+) are handled, before falling back to pip3/pip executables.
• F01-FR-25 Once the package is confirmed importable under an interpreter, the check MUST NOT re-run for that interpreter within the same extension-host session.
• F01-FR-26 If every automatic install attempt fails the resulting error MUST include manual installation instructions (a copy-pasteable pip command and a virtual-environment alternative).

Error Handling

• F01-FR-18 If json-schema-for-humans is not installed the error page MUST display a contextual hint with the pip install command.
• F01-FR-19 If the Python interpreter is not found the error page MUST display a hint directing the user to install Python 3 or select an interpreter via the VS Code Python extension.
• F01-FR-20 If generation times out the error page MUST show a timeout hint.

Fallback Renderer

• F01-FR-21 When the Python interpreter or the json-schema-for-humans package is unavailable, the extension MUST render a built-in, dependency-free HTML documentation page for the schema instead of only showing an error, so the schema remains previewable without Python. A schema-level error (as opposed to missing tooling) MUST still surface the error page.
• F01-FR-22 The fallback page MUST be self-contained (inline styles, no scripts, a static default-src 'none' CSP) and MUST HTML-escape every schema-derived string so schema content cannot inject markup.
• F01-FR-27 A jsonschema.preview.renderer setting MUST let the user choose the preview engine: "auto" (default) uses json-schema-for-humans when available and falls back to the built-in renderer otherwise (the F01-FR-21 behaviour); "builtin" MUST always use the built-in renderer and MUST NOT invoke Python at all — so users who don't have (or don't want) Python get the built-in output directly, with no interpreter probe or install prompt. The setting's value MUST resolve through a pure, testable helper that defaults to "auto" for any missing or unrecognised value.

Non-Functional Requirements

• F01-NFR-01 The panel MUST use retainContextWhenHidden: true to avoid re-running the Python tool on every tab switch (accepted memory tradeoff).
• F01-NFR-02 The webview's localResourceRoots MUST be scoped to the directory of the schema file being rendered.
• F01-NFR-03 The fallback page MUST expose the schema title as a top-level <h1> and nested property groups as sub-headings, and MUST NOT convey the required/optional distinction by colour alone (see S06-SR-04 / S06-SR-07).

Acceptance Criteria

1. Opening a .json file with "$schema": "http://json-schema.org/draft-07/schema#" and running JSON Schema: Preview shows a populated documentation panel within the timeout period.
2. Saving the schema file causes the preview to refresh automatically.
3. The Download button produces a file whose content matches what the Python tool generated.
4. Clicking an http://… link inside the preview opens a browser tab.
5. A data file whose own "$schema" value points at a schema path (e.g. "$schema": "./schemas/person.schema.json", written by inline binding — F10) is NOT treated as a JSON Schema file: jsonschema.isJsonSchema stays false, the Preview/Edit/Configure toolbar icons don't appear, and Validate/Infer icons show instead.
6. With the preview panel focused, pressing Ctrl+F/Cmd+F opens VS Code's native find widget over the panel and searches its rendered content.

Requirement traceability

Status of each requirement in specs/traceability.json. Test coverage is auto-discovered from [ID] tags in test titles and is not listed here.

Requirement	Status	Implementation	Note
F01-FR-01	implemented	package.json	activationEvents include onLanguage json/jsonc/yaml/yml/jsonl (manifest.test.ts)
F01-FR-02	implemented	src/PreviewWebPanel.ts	isJsonSchemaFile() requires the $schema value's URL hostname to be exactly json-schema.org (parsed via the URL constructor, not a substring/includes check — CodeQL js/incomplete-url-substring-sanitization), so an inline-bound data file (F10) isn't misclassified as a schema; unit-tested
F01-FR-03	implemented	src/extension.ts, src/PreviewWebPanel.ts	setContext jsonschema.isJsonSchema true/false (extension.test.ts)
F01-FR-04	manual	src/PreviewWebPanel.ts	VS Code API-bound; verified by manual / E2E testing
F01-FR-05	manual	src/PreviewWebPanel.ts	VS Code API-bound; verified by manual / E2E testing
F01-FR-06	manual	src/PreviewWebPanel.ts	VS Code API-bound; verified by manual / E2E testing
F01-FR-07	manual	src/PreviewWebPanel.ts	VS Code API-bound; verified by manual / E2E testing
F01-FR-08	manual	src/extension.ts	VS Code API-bound; verified by manual / E2E testing
F01-FR-09	implemented	package.json	editor/title preview icon gated on jsonschema.isJsonSchema
F01-FR-10	implemented	src/PreviewWebPanel.ts	findConfigFile() unit-tested
F01-FR-11	manual	src/PreviewWebPanel.ts	VS Code API-bound; verified by manual / E2E testing
F01-FR-12	manual	src/PreviewWebPanel.ts	VS Code API-bound; verified by manual / E2E testing
F01-FR-13	manual	src/PreviewWebPanel.ts	VS Code API-bound; verified by manual / E2E testing
F01-FR-14	manual	src/PreviewWebPanel.ts	VS Code API-bound; verified by manual / E2E testing
F01-FR-15	manual	src/PreviewWebPanel.ts	VS Code API-bound; verified by manual / E2E testing
F01-FR-16	manual	src/PreviewWebPanel.ts	VS Code API-bound; verified by manual / E2E testing
F01-FR-17	manual	src/PreviewWebPanel.ts	VS Code API-bound; verified by manual / E2E testing
F01-FR-18	manual	src/PreviewWebPanel.ts	VS Code API-bound; verified by manual / E2E testing
F01-FR-19	manual	src/PreviewWebPanel.ts	VS Code API-bound; verified by manual / E2E testing
F01-FR-20	manual	src/PreviewWebPanel.ts	VS Code API-bound; verified by manual / E2E testing
F01-FR-21	implemented	src/fallbackRenderer.ts, src/PreviewWebPanel.ts	renderSchemaHtml + isToolingUnavailable unit-tested; wired into generateDocHTML
F01-FR-22	implemented	src/fallbackRenderer.ts	static CSP, no scripts, sanitizeHtml on all schema text; unit-tested
F01-FR-23	manual	src/python.ts	getPythonInterpreter: Python-ext environments API -> legacy API -> settings -> python3; python.ts is subprocess-bound (coverage-excluded), verified manually
F01-FR-24	manual	src/python.ts	ensureInstalled: pip candidates incl. --user and --break-system-packages under withProgress; python.ts is subprocess-bound (coverage-excluded), verified manually
F01-FR-25	manual	src/python.ts	packageReadyFor per-interpreter set skips re-checking within the session; python.ts is subprocess-bound (coverage-excluded), verified manually
F01-FR-26	manual	src/python.ts	final throw includes pip command and venv alternative instructions; python.ts is subprocess-bound (coverage-excluded), verified manually
F01-FR-27	implemented	src/settings.ts, src/PreviewWebPanel.ts	jsonschema.preview.renderer setting; getPreviewRenderer() (auto|builtin, default auto) unit-tested in settings.test.ts; PreviewWebPanel.generateDocHTML short-circuits to the already-tested renderFallbackHTML when builtin, skipping the Python probe
F01-FR-28	implemented	src/PreviewWebPanel.ts	getSettingsConfig()/resolveConfigSource() unit-tested in preview.test.ts
F01-FR-29	implemented	src/PreviewWebPanel.ts	enableFindWidget:true passed to createWebviewPanel; unit-tested via the call's options argument
F01-NFR-01	manual	src/PreviewWebPanel.ts	VS Code API-bound; verified by manual / E2E testing
F01-NFR-02	manual	src/PreviewWebPanel.ts	VS Code API-bound; verified by manual / E2E testing
F01-NFR-03	implemented	src/fallbackRenderer.ts	h1 title + h3 nested groups + textual required marker; unit-tested

Documented in

Documentation sections tagged spec:F01 (or one of its requirement ids) — 705 of ~1046 expected words (67%). The expectation comes from this spec's evaluated complexity of 26.2 (32 documentable requirements — 29 FR, 3 NFR — weighted by kind and by each definition's length).

• JSON Schema PreviewREADME.md · 70 words
• ReadREADME.md · 83 words
• Commands/guide/commands · 75 words
• Configuration/guide/configuration · 37 words
• jsonschema.preview.syncScroll | Type | Default |/guide/configuration · 124 words
• Introduction/guide/ · 33 words
• Finding your way around/guide/ · 242 words
• Auto-Open Preview/guide/ · 19 words
• Searching the Preview/guide/ · 22 words

Changes over time

• d7ba5832026-08-11feat(preview): section-accurate scroll sync + native find in preview (F28, F01)
• 8c17c6a2026-07-26feat(preview): support jsonschema.config VS Code setting as config alternative
• e39da2d2026-07-14fix(F01-FR-02): match $schema hostname exactly, not a substring
• def09082026-07-14fix(F01-FR-02): require $schema to reference a meta-schema, not just exist
• c9e222a2026-07-10feat(preview): add jsonschema.preview.renderer to force the built-in renderer (F01-FR-27)
• a3820eb2026-07-06docs(specs): backfill status audit and add six planned feature specs
• 9bfbbe02026-07-04feat(preview): built-in pure-JS fallback renderer when Python is unavailable
• ac906af2026-06-23docs(specs): add software requirements specification