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Prompt Expansion — the Creative Director Tool

creative-director is the first built-in tool. It rewrites a terse prompt into a single, vivid, self-contained prompt using Google's officially validated five-component formula, optionally injecting domain-specific (cinema / portrait / product / …) vocabulary, and deterministically strips low-signal "quality-booster" keywords. It calls the public Gemini API server-side and is never fatal — any failure degrades to your original prompt.

This is the tool. For the framework that hosts it (gflow tools, --tool, the TOML schema, MCP exposure), see TOOLS.md.

1. Quick start

# 1. Get a Gemini API key: https://aistudio.google.com/apikey
export GFLOW_CLI_GEMINI_API_KEY="..."

# 2. Preview an expansion (free of Flow credits)
gflow tools run creative-director "a cat on a couch" --style cinema

# 3. Use it on a real generation
gflow image t2i "a fox in the snow" --tool creative-director:style=cinema
gflow video t2v "a city at night"  -t creative-director:style=cinematic

Without GFLOW_CLI_GEMINI_API_KEY the tool silently leaves your prompt unchanged (see §6).

2. Domain model

Concept Where Meaning
Tool spec tools/builtin/creative-director.toml Single source of truth — name, version, the formula instruction, banned keywords, and the style libraries.
System instruction built at runtime system_template + the selected style's vocabulary + a single User prompt: marker.
Style / domain mode [[config.domains]] A named vocabulary library (camera/lens/lighting refs) injected into the instruction. 9 image + 6 video.
Expansion ExpansionResult(original, expanded, was_expanded) The result; expanded == original when nothing changed.
Provenance AppliedTool(name, version, model, config_hash, params) Recorded in history when a tool rewrote a prompt.

The tool itself lives in src/gflow_cli/tools/: spec.py (schema), runtime.py (apply_tool), expander.py (the Gemini-backed PromptExpander), banned.py (keyword stripping), invocation.py (provenance + MCP adapter).

3. The five-component formula

Based on Google's validated prompt structure for Gemini image/video models. The tool instructs the model to write natural narrative paragraphs — never comma-separated keyword lists — keeping the user's original intent and named subjects intact:

  1. Subject — who/what is the focus; specific physical characteristics, material, species, age, expression. (Never just "a person" or "a product".)
  2. Action — what the subject is doing or its primary visual state; strong present-tense verbs, or a described pose/arrangement.
  3. Location / Context — environment, time of day, atmospheric conditions.
  4. Composition — camera perspective, framing, spatial relationship (e.g. "intimate close-up from slightly below eye level, shallow depth of field").
  5. Style (lighting included) — visual register, medium, and lighting combined; references real cameras, film stock, photographers, publications, or art movements. Lighting is a sub-element of Style, not a sixth component.

The instruction also carries "Critical Rules": name real cameras/brands, include micro-details, use prestigious context anchors ("Vanity Fair editorial", "National Geographic cover"), use ALL CAPS for hard constraints, "prominently displayed" for products, and never emit banned keywords. The full text is the system_template in creative-director.toml.

4. Domain modes (styles)

Select with --style <name> (tools run) or :style=<name> (--tool). Each style injects a vocabulary library of concrete references into the instruction. Styles are category-gated — image styles apply to image commands, video styles to video commands (see TOOLS.md §4). product and abstract exist in both categories with different vocabularies.

Image styles (9)

Style Flavor of vocabulary
cinema Cine cameras (RED, ARRI Alexa 65), anamorphic lenses, Kodak Vision3 stocks, lighting setups, color grades.
product Surfaces, softbox/tent lighting, hero angles, Apple/Aesop/B&O style refs.
portrait Focal lengths (85/105/135mm), apertures, pose language, skin micro-texture.
editorial Vogue/Harper's/Kinfolk refs, styling notes, locations, editorial poses.
ui Flat vector / isometric / glassmorphism, exact hex palettes, retina sizing.
logo Geometric construction, golden ratio, 2–3 colors, monochrome-safe.
landscape Depth layers, atmospherics, blue/golden/magic hour, weather.
infographic Modular layout, data-viz types, exact-text quoting, accessible palette.
abstract Fractals/voronoi, fluid/ink textures, color harmonies, generative styles.

Video styles (6)

Style Flavor of vocabulary
cinematic Camera motion, slow-motion/speed-ramp timing, transitions, film grades.
documentary Observational/handheld, B-roll coverage, vérité aesthetic.
animation Cel/3D/stop-motion, easing curves, flat/isometric visual language.
social 9:16/1:1/16:9 formats, hook-in-2s pacing, UGC vs branded aesthetic.
product 360 orbit/macro reveal, studio lighting, infinity cove, end-card pacing.
abstract Particle/fluid sims, smoke/ink textures, gradient-shift color, VJ aesthetic.

An unknown style is ignored with a tool_unknown_style warning — never an error. Full vocabularies live in creative-director.toml; gflow tools show creative-director lists the current set.

5. Banned-keyword policy

The tool defends against low-signal "quality boosters" in two layers:

  1. Prevention — the instruction explicitly forbids them.
  2. Cleanup — strip_banned_keywords deterministically removes any that slip through, on a whole-word, case-insensitive basis, and logs tool_banned_keywords_stripped.

The 13 banned terms:

4k, 8k, ultra HD, high resolution, masterpiece, highly detailed, ultra detailed,
trending on artstation, hyperrealistic, ultra realistic, photorealistic, best quality,
award winning

Matching is longest-first (so ultra detailed is stripped before ultra), word-boundaried, and the resulting separators are tidied (no dangling , , or double spaces). Resolution intent belongs in generation parameters, not the prompt. Stripping runs only on a rewritten prompt — a prompt the tool left unchanged is never altered.

6. Never-fatal contract

The Gemini-backed PromptExpander (expander.py) is stdlib-only (urllib) and bounded. expand() returns the original prompt with was_expanded=False — never raising — in every degraded case:

  • GFLOW_CLI_GEMINI_API_KEY is missing.
  • Any HTTP error, network failure, timeout, or malformed JSON.
  • An empty, whitespace-only, or quote-only model response.

Retryable statuses (429, 500, 502, 503, 504) are retried with exponential backoff (default 3 attempts, base delay 1s, doubling); 401 / 403 fail fast (and still degrade to the original). Each attempt has a 20s socket timeout, and the whole call is bounded by an overall ~60s wall-clock budget (max_total_seconds): retries stop once the budget is spent and each attempt's timeout is clamped to the remaining budget, so sustained rate limiting can never stall a batch for the full timeout×retries schedule. (Both bounds are fixed defaults, not env-tunable.) Input is truncated to max_input_chars (4000) and output to max_output_chars (3500); a single layer of wrapping quotes is stripped from the response. The net guarantee: a tool can never abort a generation — the worst case is "your prompt went through unchanged."

7. Gemini wiring (I/O)

Aspect Value
Endpoint https://generativelanguage.googleapis.com/v1beta/models/{model}:generateContent
Auth API key in the x-goog-api-key header (never on the query string).
Default model gemini-2.5-flash (TOML config.model; GFLOW_CLI_GEMINI_MODEL overrides globally).
Request system_instruction = built instruction; user contents = the (truncated) prompt; temperature=0.7, maxOutputTokens=1024.
Response First candidate's text, cleaned (quote-stripped, truncated).

8. Provenance — what history records

When a tool rewrites a prompt, the generation is recorded with full provenance (data/recorder.py):

  • prompt column = the user's original prompt.
  • expanded_prompt column = the submitted (rewritten) prompt.
  • operations.metadata_json.tool = the applied tool descriptor.

This is redaction-aware, gated on GFLOW_CLI_HISTORY_PROMPTS (see CONFIGURATION.md and DATA_LAYER.md):

Mode expanded_prompt metadata_json.tool
store (default) stored {name, version, model, params, config_hash}
redacted withheld {name, version, params_hash, config_hash}

In redacted mode the recorder builds the minimal descriptor itself — it does not dump the full tool dict and lean on key-name redaction, because a free-text option (e.g. params.style) would otherwise leak verbatim. params_hash is a SHA-256 of the sorted params; config_hash is a SHA-256 of the resolved ToolConfig, pairing with the hand-bumped version for tamper-evidence. The metadata_json.tool write rides the post-success recorder safety wrapper, so a recording failure warns and returns exit 0 — never a non-zero exit after a paid generation.

9. Relationship to the expand_prompt MCP prompt

The MCP server also exposes an older, now-deprecated prompt template named expand_prompt (mcp/prompts.py). It predates the tools framework and overlaps functionally with creative-director, but they are different surfaces:

expand_prompt (MCP prompt) creative-director (tool)
Kind MCP prompt template — returns text for the client's model to expand. MCP tool / CLI --tool — runs server-side.
API call None (no Gemini call; the client model does the work). Calls Gemini via PromptExpander.
Input 5 structured slots (subject, action, setting, camera, lighting). A single free-text prompt + optional style.
Banned-keyword stripping No. Yes (13 terms).
Domain styles No. Yes (15).
Provenance No. Yes (expanded_prompt + metadata_json.tool).
Formula Treats lighting as a 5th separate component. Folds lighting into the Style component.

Guidance: prefer the creative-director tool (--tool / the MCP tools array) for actual generation — it is the maintained, provenance-recording, server-side path. expand_prompt is deprecated as of the unreleased line: its client-visible description carries a [DEPRECATED] marker pointing to the tool, and it is slated for removal in a future major release. It remains functional for now because some MCP clients surface prompts as user-pickable templates (a distinct UX from agent-invoked tools), but no new work should depend on it. See TOOLS.md §8.

10. Configuration

Env var Default Purpose
GFLOW_CLI_GEMINI_API_KEY — Required to actually rewrite prompts. Get a key.
GFLOW_CLI_GEMINI_MODEL gemini-2.5-flash Global override of the tool's default model.
GFLOW_CLI_HISTORY_PROMPTS store redacted withholds the expanded prompt and minimizes the recorded tool descriptor (§8).

See CONFIGURATION.md for the full precedence chain.

11. Credit

The Creative Director patterns — the five-component formula, the banned-keyword policy, and the domain-vocabulary libraries — are adapted from the banana-claude evaluation (PR #202) of Google's published Gemini prompt-engineering guidance.

12. See also