build-web-service

name	build-web-service
description	Use when you want to create a new web view for the user -- a page, dashboard, or app they can open as a tab. Runs an interactive flow: confirm the look and feel on a cheap throwaway mock first, then build the real site to a usable state, then harden it in the background. Covers scaffolding a new Flask service (canonical path) and the escape hatch for wrapping a pre-existing third-party server.
metadata	{"crystallized":true}

How to build a web service

A "web service" here is something the user can click on as a tab in the desktop client and see render at /service/<name>/, proxied through the system_interface.

There is one canonical path (scaffold a new Flask lib) and one escape hatch (wrap a pre-existing third-party server). Modify/remove flows go through the update-service skill.

This is the web specialization of the interactive-delivery shape

Read .agents/shared/references/interactive-delivery.md first. Building a web view is not a "scaffold, implement, ship" recipe -- it is an interactive flow: you confirm the look-and-feel on a cheap throwaway mock before building the real thing, build to a usable state in the foreground, and defer the thorough testing + review gates to a background worker. The phases below fill in that shared skeleton for web work. The single biggest mistake this skill exists to prevent is building (and testing, and hardening) a whole site before the user has confirmed the basic shape is what they want.

Map of the flow:

• Step 0 -- clarify and plan (skeleton phases 1-3): blocking questions only, in business terms; a small plan; wait for approval.
• Step 1 -- scaffold + throwaway mock (skeleton phases 4-6): scaffold the service, put a mock UI in front of the user, loop to explicit confirmation of the look-and-feel. Hard gate.
• Step 2-4 -- build to a usable site (the existing build mechanics, run after confirmation): implement real routes, verify, surface the tab.
• Step 5 -- finalize in the background (skeleton phase 7): once the user confirms the working site looks right, hand thorough testing + the review gates to a background worker. The main agent never runs those itself.

If you were sent here by fetch-process-show for a web view over fetched data, the data sample is already confirmed -- but you still run your own mock confirmation here, because the data sample confirms the data shape, not the UI shape. Render the handed-off sample.json in the mock so the user judges the UI against real data.

Step 0: Clarify and plan (business terms only)

Ask only the questions that genuinely block -- a fork that is both genuinely uncertain and expensive to reverse later. Most web views have none: default to the simplest conventional choice and to a single user, state each default in one line, and move on. Cheap-to-reverse choices (persistence, auto-reload vs. reload-to-refresh, latest-only vs. history) are not P0 -- pick the obvious default and let them surface during the mock loop or as a later follow-up surface, where the user can react to something concrete rather than answer "should this update on its own?" in the abstract.

If you do hit a real blocker, phrase it as the user-visible consequence that motivates it -- never a technical term (this system serves non-technical users): "should everyone see the same list?" not "do we need multi-tenancy?".

Record your stated defaults -- they are the architecture you build once, after the mock converges. Do not build any of it yet. Then propose a small plan and wait for approval.

Decide which path applies

• Authoring routes yourself (the common case): use the Flask scaffolder in Step 1. The scaffolder picks correct defaults so most framework gotchas don't fire.
• Wrapping a pre-existing third-party server (Jupyter, Grafana, an npx-installed dashboard, anything with its own start command): skip the scaffolder, jump to "Escape hatch: wrap an existing server" below.

If you would otherwise scaffold a Flask lib whose only job is to shell out to a third-party tool, do not do that -- the system_interface already proxies /service/<name>/... to whatever URL you register. Adding a Python proxy in front of the third-party server adds a hop, costs an extra process, and complicates WebSocket and streaming behavior. Use the escape hatch instead.

Do not extend apps/system_interface/ to add a new view. That app runs the top-level workspace UI; new web views go in their own scaffolded lib under libs/<your-package>/ so they get an isolated tab and prefix.

Pre-flight (both paths)

• Pick a kebab-case service name. Becomes the URL segment /service/<name>/. Short and descriptive (news, docs-viewer) beats clever. Avoid names already used in supervisord.conf (system_interface, browser, etc. are reserved by the scaffolder).
• Pick a free port. ss -tln lists what's bound. The scaffolder picks the lowest free port at or above 8080 by parsing supervisord.conf and runtime/applications.toml; if you're choosing manually, avoid 8000 (system_interface) and 8081 (the browser service).
• Bind to 127.0.0.1 (not 0.0.0.0). The forwarder reaches your app from inside the same container; binding to all interfaces is noise. The scaffolder does this. For the wrap-existing path, many Node frameworks default to 0.0.0.0 -- pass an explicit host (HOST=127.0.0.1, app.listen(port, "127.0.0.1"), etc.) if your third-party tool's default isn't loopback. Python defaults are usually loopback already.

Step 1: Run the scaffolder (canonical path)

uv run .agents/skills/build-web-service/scripts/scaffold_flask_lib.py \
    --name <service-name> \
    --description "<one-liner>" \
    [--port <int>] \
    [--extra-dep <pkg>] [--extra-dep <pkg>] ...

Required:

• --name: kebab-case (lowercase letters/digits with single hyphens).
• --description: becomes the lib pyproject.toml description.

Optional:

• --port: explicit port; auto-picked if omitted.
• --extra-dep: repeatable. Add libraries beyond flask/flask-sock (e.g. --extra-dep "jinja2>=3.1" --extra-dep "anthropic>=0.40").
• --skip-uv-sync: skip the final uv sync --all-packages (for fast iteration / dry runs).

The scaffolder fails non-zero with a clear stderr message if the lib already exists, the name is reserved or invalid, the requested port is taken, or uv sync fails.

What gets generated:

• libs/<package>/pyproject.toml -- declares [project.scripts] <name> = "<package>.runner:main".
• libs/<package>/src/<package>/__init__.py -- empty.
• libs/<package>/src/<package>/runner.py -- sync Flask starter. Builds a Flask app and serves it with werkzeug.serving.run_simple(..., threaded=True). It serves at /; the system_interface proxy handles the /service/<name>/ prefixing, so no root_path/ROOT_PATH is needed. It also defines a DATA_DIR constant (defaults to runtime/<name>/, overridable via the <PACKAGE_UPPER>_DATA_DIR env var) -- route all persistent state through it (see File-path conventions below) -- and a PORT constant (defaults to this service's assigned port, overridable via the <PACKAGE_UPPER>_PORT env var) bound in run_simple. Both overrides are what let a future edit boot a throwaway instance on a spare port against a data copy (see update-service).
• libs/<package>/test_<package>_ratchets.py -- standard ratchets at zero.
• libs/<package>/README.md -- one-line description.

What gets updated:

• Root pyproject.toml -- adds <service-name> to [project].dependencies, libs/<package> to [tool.uv.workspace].members, and <service-name> = { workspace = true } to [tool.uv.sources].

• supervisord.conf -- appends a program block:

  [program:<name>]
  command=python3 scripts/oom_tag_service.py user bash -c "python3 scripts/forward_port.py --url http://localhost:<port> --name <name> && uv run <name>"
  directory=/mngr/code
  autostart=true
  autorestart=true
  # plus rotated stdout/stderr logfiles under /var/log/supervisor/<name>-*.log
  

  The Flask app serves at / and needs no prefix env var: the system_interface proxy handles /service/<name>/ prefixing (it rewrites absolute paths in served HTML and installs a scoped service worker that prepends the prefix to the page's own fetches). The bash -c "..." wrapper is required because supervisord runs commands directly (no shell) and this one chains forward_port.py with &&. The oom_tag_service.py user prefix tags this user-created service so it is shed before any built-in service under memory pressure (see libs/oom_priority/README.md).

supervisord does not watch the config, so tell it to pick up the new program, then confirm it is running:

supervisorctl reread && supervisorctl update
supervisorctl status <name>

If it isn't RUNNING, read its log (/var/log/supervisor/<name>-stderr.log) or run supervisorctl tail <name> stderr.

Put a throwaway mock in front of the user (the confirmation gate; looped)

Scaffolding the service is fine before confirmation -- it is cheap and reversible. Building the real data layer or state architecture before the user confirms the look-and-feel is the tripwire: do not. Instead, serve a throwaway mock of the proposed UI as a route inside the scaffolded service, so the user sees it as a real tab and reacts to the actual look-and-feel.

This is skeleton phase 5 (the cheap throwaway artifact). Keep it disposable:

• The mock renders static / hard-coded content that demonstrates the proposed layout and interactions -- no real fetching, no persistence, no backend logic. Invoke the frontend-design skill before writing the markup (see Step 2).
• If you were handed a confirmed sample.json (the fetch-process-show hybrid), render that real data in the mock so the user judges the UI against real content. Otherwise use representative placeholder data that covers the shapes the real view will show (including an empty state and a busy/overflow state).
• layout.py open <name> to surface it (see Step 4 for the command), then loop: present -> take feedback -> update the mock so the change is visible -> re-present. Do not accept feedback and move on having only asserted you'll apply it.
• Loop until the user explicitly confirms the look-and-feel is right.

The user may respond to the mock with a request for functionality that requires updated backend support. Your mocks should remain mostly frontend code but demonstrate how things would likely look and feel once that updated backend code is implemented. Be careful to confirm that the user will be happy with how things look and feel and approximately function prior to doing the heavy work of building out backend code.

Hard gate (skeleton phase 6). Do not implement real routes, data, or state (Step 2 onward) until that confirmation. The mock is the single source of truth for the UI shape: if later work changes the look-and-feel, re-confirm before calling the site done.

For the escape-hatch path (wrapping a third-party tool) there is no markup you author, so there is no mock to build -- the demonstration is the wrapped tool itself. Stand it up, show it to the user, and confirm it's what they wanted before investing in configuration or integration around it.

Step 2: Build the real routes to a usable site (after confirmation)

Everything from here runs only after the user has confirmed the mock. The goal of the foreground work is a usable site the user can actually try -- not a fully hardened one. Implement the real routes (replacing the mock), wire in the data/state architecture you recorded in Step 0, run the Step 3 smoke verify, and surface the tab (Step 4). Then stop and hand the running site to the user -- the thorough testing and review gates happen in the background (Step 5), not here.

The starter runner.py has just GET / (a placeholder HTML page) and GET /health (returns {"status": "ok"}). Replace the placeholder with your real routes.

Use sync handlers (def, not async def). Flask handlers are sync def, and the starter runs on the threaded Werkzeug server (run_simple(..., threaded=True)), so concurrent requests are handled by separate threads -- no asyncio needed.

Rendering HTML for a human

If your service renders HTML that a person will look at (anything beyond a pure JSON API, a webhook receiver, or a transparent proxy of a third-party tool), you must invoke the frontend-design skill before writing the markup. Always do this before working on UI, regardless of the scope of the work.

Skip this step for routes that emit only JSON, only redirects, or that serve an existing third-party UI through the escape hatch below -- there's no markup to design.

Calling Claude from your service

If your service needs to call Claude (classify/summarize content, run a one-shot agentic task, or launch a full agent), follow the use-ai-integration skill: it picks the path (a keyed litellm call or the keyless claude_p.py helper), covers the claude -p environment fix and the cost model, and saves you from hand-rolling the call.

Always surface the raw data and its source

When a view renders data derived from underlying records (a summary, a reformatted list, extracted fields), include -- by default, without the user asking -- a "view raw" control showing the original record rendered in its native format (an HTML email as the rendered email, not escaped source; JSON pretty-printed; markdown rendered -- the faithful original minus your processing) plus, for records from an external service, an "open in " link back to the origin (e.g. open the email in Gmail). When you render untrusted third-party HTML (a raw email body is the common case), sandbox it -- a sandboxed iframe or a sanitizer -- so the view can't run scripts or phone home via tracking pixels.

This is the surfacing half of the preserve-and-surface principle (CLAUDE.md): the derived view inevitably leaves gaps (a field the agent didn't extract, a rendering it didn't anticipate), and the raw/source affordance lets the user bridge them without waiting for a rebuild. Design it in from the first version -- it depends on the data layer having persisted the raw payload and source reference (see the crystallize data-capture guidance), so confirm that's available and flag it if it isn't. Keep it unobtrusive (a small per-record control, not clutter) and don't call it out in chat -- always present, never announced.

File-path conventions

Two cases, two patterns:

• Persistent state (caches, cursors, last-visit timestamps, JSON snapshots, user records -- anything written and read across runs): read and write it under the generated DATA_DIR constant, never a hardcoded runtime/<name>/ at the call site. DATA_DIR defaults to runtime/<name>/ (cwd-relative, resolved from /mngr/code where the supervisord-managed service runs) but honors the <PACKAGE_UPPER>_DATA_DIR env var. That override is what makes a future edit safe: an agent changing the service can run a throwaway instance against a copy of the data instead of the live store (see update-service), so keep every read/write going through DATA_DIR -- a hardcoded runtime/<name>/ silently bypasses the override and re-exposes the live data. Do NOT use Path(__file__)-based paths for state.
• Static assets shipped alongside the .py file (templates, default configs, bundled JSON): Path(__file__).parent / "assets/..." is the right pattern.

Step 3: Verify

Both paths use the same verification recipe. See references/verify.md -- curl against http://127.0.0.1:8000/service/<name>/ then a Playwright assertion on a unique-to-your-app marker.

If verification surfaces something unexpected (502, "duplicated dockview tab bar", redirect loop, broken WebSockets), see references/cross-flow-gotchas.md -- it's symptom-indexed.

Step 4: Surface the view to the user

Once verification passes, tell the workspace UI to actually open the new tab. Without this step the user would have to discover it via the "+" dropdown -- skip the surfacing step only for services with no UI (pure JSON APIs, webhook receivers, etc.).

python3 scripts/layout.py open <name>

layout.py POSTs to a loopback-only workspace_server endpoint that broadcasts a layout_op message over its WebSocket. The frontend focuses the panel if a tab for <name> is already open, otherwise splits a new iframe alongside the primary chat (60% web / 40% chat). The script briefly waits for the service to appear in runtime/applications.toml so it's safe to run immediately after the forward_port.py call.

To force a reload of an already-open tab (e.g. after redeploying the service) without prompting the user to click Refresh:

python3 scripts/layout.py refresh <name>

You should always refresh services after making changes, to make sure the user can see the updates.

For anything beyond open / refresh -- splitting, moving, focusing, renaming, maximizing, replacing an iframe's URL, inspecting the live tree -- see the manage-layout skill. layout.py list is also useful when the user is asking about what tabs are available (it prints every user-facing registered service plus every mngr-level agent, with open/running flags; the workspace chrome's own system_interface entry is hidden).

Step 5: Finalize in the background (after the user confirms the working site)

The foreground work stops at a usable, surfaced site. The thorough pass -- extending Playwright coverage, the full test suite and ratchets, /autofix, and the code-guardian gates -- runs in a background harden worker, never in the main agent. This is skeleton phase 7: the harden pass (.agents/shared/worker/references/harden-artifact.md), here the crystallize operation with the service artifact -- the scaffolded service is already on disk and the user confirmed it live, so nothing needs reconstructing and there are no worker gates.

The trigger is an explicit confirmation on the working site -- never your own sense that the code looks done. Once the usable site is in front of the user, ask a plain "this generally looks good?" and hand off only once they confirm by exercising the real behavior. (The mock confirmed the UX shape; this confirms the real behavior -- the point where deep changes actually surface, so finalizing earlier risks hardening an architecture the user is about to invalidate.)

Reading the confirmation signal:

• If the user keeps asking for changes, each one is a cheap foreground iteration that resets the clock -- you have run no gates or thorough tests yet, so pivots stay cheap. Do not hand off until their response is a confirmation rather than a change request.
• If the user starts asking for surface-level (cosmetic) tweaks, or pivots to a slightly unrelated task or follow-up, treat that as a sign the core is settled: still ask, but ground it -- "seems like we've got the core thing settled here -- good to lock it in?" -- rather than leaving it open-ended.
• Wait for an explicit confirmation rather than firing on a timeout or silence. The user is never blocked: they already hold the usable site.

On confirmation, hand the confirmed service to the crystallize-artifact skill with artifact=service. It owns the rest -- the tracking ticket, the task file (set artifact: service), launching the generic harden-worker, polling, merging on done, and refreshing the tab after merge. Give it only: the slug (the service name), and a task body naming the built lib path, the service name, the URL segment, and what the service does. The generic worker loads harden-artifact.md + op-crystallize.md + artifact-service.md and reports done once its testing contract and the review gates pass; there is no worker gate because the user already confirmed the live site.

The confirmed mock plus the confirmed working site remain the single source of truth: if finalization changes the look-and-feel, re-confirm with the user before calling the work done.

Escape hatch: wrap an existing server

For pre-existing third-party tools, do not scaffold a lib. Add a [program:<name>] block to supervisord.conf that runs forward_port.py and then your existing start command. supervisord runs commands directly (no shell), so wrap any command that chains with && in bash -c "...", and prefix the whole thing with python3 scripts/oom_tag_service.py user so this user-created service is shed before any built-in service under memory pressure (see libs/oom_priority/README.md):

[program:<name>]
command=python3 scripts/oom_tag_service.py user bash -c "python3 scripts/forward_port.py --url http://localhost:<port> --name <name> && <existing_start_command>"
directory=/mngr/code
autostart=true
autorestart=true

Two valid shapes:

• Inline (preferred when one line fits):

  [program:docs-viewer]
  command=python3 scripts/oom_tag_service.py user bash -c "python3 scripts/forward_port.py --url http://localhost:8090 --name docs-viewer && jupyter notebook --port 8090 --ip 127.0.0.1 --no-browser"
  directory=/mngr/code
  autostart=true
  autorestart=true
  

• Wrapper script (preferred for multi-step bootstrap or env exports):

  # scripts/run_<name>.sh
  #!/usr/bin/env bash
  set -euo pipefail
  python3 scripts/forward_port.py --url http://localhost:<port> --name <name>
  exec <existing_start_command>
  

  [program:<name>]
  command=python3 scripts/oom_tag_service.py user bash scripts/run_<name>.sh
  directory=/mngr/code
  autostart=true
  autorestart=true
  

After editing supervisord.conf, run supervisorctl reread && supervisorctl update to start the new program.

The forward_port.py call MUST come first in the command -- the port must be registered before the app starts listening, otherwise the app-watcher races with the backend coming up.

For the full program schema and logging knobs, see the shared .agents/shared/references/service-processes.md.

Verification and gotchas references apply identically to this path.

forward_port.py CLI reference

Used by both paths (the scaffolder generates the call; the escape hatch has you write it directly).

python3 scripts/forward_port.py --name NAME --url URL
python3 scripts/forward_port.py --name NAME --remove

Flags:

• --name: application name (must match the URL segment a user clicks: /service/<name>/).
• --url: full URL where the app is reachable from inside the container (e.g. http://localhost:8090).
• --remove: remove the named entry from runtime/applications.toml. Use this when tearing down a service.

The global (Cloudflare) URL

If the workspace has Cloudflare tunneling configured, the service is also reachable at a public URL -- with caveats about where that hostname lives and why it isn't in runtime/applications.toml. See references/public-url.md.

Cleanup

To remove a web service (drop the applications.toml entry, stop and unregister the supervisord program, and revert the scaffolded lib), see references/cleanup.md.