Expose Tunable Knob

Expose Tunable Knob

An agent iterating on a perceptual parameter — a mask's size, a color threshold, an animation's timing, an audio gain — hits a hard wall the moment it can't render, watch, or listen to its own output. A human running a live webcam feed, a rendered UI, or a mixed audio track can judge instantly whether a value looks or sounds right; the agent, reasoning purely from code and possibly one static screenshot, cannot. The naive move — pick a value from domain reasoning, ship it, wait for feedback, repeat — burns a full rebuild/re-run/re-report round-trip per guess, and the agent's guess still carries no more information than the human's own eyes would supply directly.

When to Use This Skill

The pattern

1. Implement the mechanism, not the magic number. Parameterize whatever currently hardcodes the value — a mask's expansion ratio, a debounce delay, a color-mix weight — so it reads from config/state rather than a literal.
2. Pick a reasoned starting default, not an arbitrary one. Use the best available signal: the reference implementation's value, a "just a little more than currently" nudge in the diagnosed direction, or a rough calculation — record why in a doc comment, since the next reader (agent or human) needs the reasoning, not just the number.
3. Expose a live control at the layer the human already interacts with — a UI slider (egui::Slider, a web form range input), a hot-reloadable config key, a CLI flag re-read per invocation. The requirement is that the human can change it and see the result without asking the agent to redo anything.
4. Stop guessing values past this point. Once the knob exists, further "should I bump this to 0.3 or 0.4?" turns are wasted — hand the decision to the human and move on to the next piece of work.

Worked example

A Rust live face-swap app's mouth-mask feature pastes the real webcam mouth back over a swapped face, using a landmark-derived polygon expanded by a mouth_mask_size factor and shifted by a mouth_mask_offset_y bias. Neither value has an objectively correct answer — "does the opening sit between the lips, and does the boundary look smooth" is answerable only by someone watching their own live video. Rather than iterating blind (ship a guess → wait for a screen-recording or description → guess again), the fix:

• parameterized both values in ProcessingConfig (mechanism, not constant),
• set defaults reasoned from the known-wrong prior value (Python's 1.0/10% padding was empirically too tight; 4.0/40% was a deliberate, reasoned bump, not arbitrary),
• added egui::Slider controls ("Mouth Mask Size", "Mouth Mask Position") so the user could dial both in during a single just live session,

turning what would have been an open-ended sequence of "try 0.3 now" / "still not quite right, try 0.35" exchanges into one code change plus the user's own real-time tuning.

Anti-patterns

• Silent precision theater: shipping a value to three decimal places (0.347) with no note that it's a guess. A guessed value should read as a starting point, not settled science — the user needs to know it's provisional so they know to check it.
• Adding a knob nobody can reach. A config field with no UI/CLI surface and a "rebuild to test" cycle is not a knob — it's the same guess-and-wait loop with extra steps. The control has to land where the human already is (the running app), not one layer removed from it.
• Knob sprawl. Not every parameter earns a slider — reserve this for values the user has already signaled need iteration (see the "skip when" row above). A UI cluttered with tuning knobs for values nobody disputes is its own cost.

Relationship to sibling skills

• verify-before-plan — verifies facts an orchestrator assumes before dispatching work; this skill hands off judgment the agent structurally cannot make itself. Different gaps, same "don't guess — get the answer from the party who actually has it" instinct.
• mcp-management / configure-* skills often build the UI surface (sliders, config files) this pattern exposes a value through — this skill is about when to reach for that surface, not how to wire it.