feat: configurable SubQuestions via Step 1.2 system prompt page

- Split 'Step 1: Query Decomposition' into Step 1.1 (prompt template) and Step 1.2 (format config with description + max_length)

- Add create_subquestions_model() and parse_decompose_format() to decompose.py

- QueryDecomposer reads decompose_format from DB, creates dynamic Pydantic model at runtime

- PromptEditor renders Step 1.2 as textarea (description) + number input (max_length 1-5)

- Graceful fallback to static SubQuestions when decompose_format unavailable

backend/app/core/sqlite_db.py

@@ -11,12 +11,18 @@ logger = logging.getLogger(__name__)
 
 _SEED_DECOMPOSE = (
     "Given this question: '{question}'\n\n"
-    "Break it down into 2-5 simplified sub-questions that would help "
+    "Break it down into 1-3 simplified sub-questions that would help "
     "search for relevant information. Each sub-question should be short "
     "and focused on one aspect.\n\n"
     'Return a JSON array of strings: ["sub-question 1", "sub-question 2", ...]'
 )
 
+_SEED_DECOMPOSE_FORMAT = (
+    '{"description": "請將問題/任務拆解成 1-3 個簡化子問題，標籤式主題必須清楚、簡潔、具體，'
+    '一看就明白（建議 3-8 個字），若涉及地點、地區、人物、時間、金額/財政 等關鍵資訊，必須包含在標籤中 。'
+    '具體提問/要求要精準、完整 並全部轉換成以下固定格式：\\n「標籤式主題：具體提問/要求」", "max_length": 3}'
+)
+
 _SEED_FILTER = (
     "Given question '{question}' and these document chunks, rate each 0-10 for relevance. "
     "Return JSON array of scores.\n{chunks}\n"
@@ -68,6 +74,7 @@ _SEED_PROFILES = [
 
 _SEED_STEPS = [
     "decompose",
+    "decompose_format",
     "filter",
     "generate",
     "generate_per_subq",
@@ -77,6 +84,7 @@ _SEED_STEPS = [
 ]
 _SEED_TEMPLATES = {
     "decompose": _SEED_DECOMPOSE,
+    "decompose_format": _SEED_DECOMPOSE_FORMAT,
     "filter": _SEED_FILTER,
     "generate": _SEED_GENERATE,
     "generate_per_subq": _SEED_GENERATE_PER_SUBQ,

backend/app/models/decompose.py

@@ -1,11 +1,14 @@
-from pydantic import BaseModel, Field
+from typing import Any
+
+from pydantic import BaseModel, Field, create_model
 
 
 class SubQuestions(BaseModel):
-    """Structured output model for query decomposition.
+    """Structured output model for query decomposition — static fallback.
 
-    Used with LangChain's with_structured_output() to guarantee
+    When ``decompose_format`` is available from PromptService, a dynamic
-    the LLM returns a valid list of sub-questions.
+    model with DB-configured description and max_length is used instead.
+    See ``create_subquestions_model()``.
     """
 
     questions: list[str] = Field(
@@ -13,3 +16,54 @@ class SubQuestions(BaseModel):
         min_length=1,
         max_length=3,
     )
+
+
+def create_subquestions_model(description: str, max_length: int) -> type[BaseModel]:
+    """Create a dynamic SubQuestions model with configurable field constraints.
+
+    Args:
+        description: Field description text injected into the JSON format
+            instruction sent to the LLM.
+        max_length: Maximum number of sub-questions (1-5).
+
+    Returns:
+        A Pydantic BaseModel subclass with the configured constraints.
+    """
+    return create_model(
+        "SubQuestions",
+        questions=(
+            list[str],
+            Field(
+                description=description,
+                min_length=1,
+                max_length=max_length,
+            ),
+        ),
+    )
+
+
+def parse_decompose_format(raw: str) -> dict[str, Any]:
+    """Parse the decompose_format JSON string into a validated config dict.
+
+    Returns a dict with ``description`` (str) and ``max_length`` (int).
+    Raises ``ValueError`` if the JSON is invalid or fields are missing/malformed.
+    """
+    import json
+
+    try:
+        config = json.loads(raw)
+    except json.JSONDecodeError as exc:
+        raise ValueError(f"decompose_format is not valid JSON: {exc}") from exc
+
+    if not isinstance(config, dict):
+        raise ValueError("decompose_format must be a JSON object")
+
+    description = config.get("description")
+    if not isinstance(description, str) or not description.strip():
+        raise ValueError("decompose_format.description must be a non-empty string")
+
+    max_length = config.get("max_length")
+    if not isinstance(max_length, int) or max_length < 1 or max_length > 5:
+        raise ValueError("decompose_format.max_length must be an integer between 1 and 5")
+
+    return {"description": description.strip(), "max_length": max_length}

backend/app/routers/prompts.py

@@ -25,6 +25,7 @@ router = APIRouter(prefix="/api/v1/prompts", tags=["prompts"])
 _VALID_NAMES = {"A", "B", "C"}
 _VALID_STEPS = {
     "decompose",
+    "decompose_format",
     "filter",
     "generate",
     "generate_per_subq",

backend/app/services/prompt_service.py

@@ -14,6 +14,7 @@ logger = logging.getLogger(__name__)
 _VALID_NAMES = {"A", "B", "C"}
 _VALID_STEPS = {
     "decompose",
+    "decompose_format",
     "filter",
     "generate",
     "generate_per_subq",

backend/app/services/query_decomposer.py

@@ -24,11 +24,21 @@ logger = logging.getLogger(__name__)
 # Fallback template used when prompt_service is not provided (tests, standalone).
 _BUILTIN_DECOMPOSE_TEMPLATE = (
     "Given this question: '{question}'\n\n"
-    "Break it down into 2-5 simplified sub-questions that would help "
+    "Break it down into 1-3 simplified sub-questions that would help "
     "search for relevant information. Each sub-question should be short "
     "and focused on one aspect. Return as a JSON array of strings."
 )
 
+_DEFAULT_DECOMPOSE_FORMAT = {
+    "description": (
+        "請將問題/任務拆解成 1-3 個簡化子問題，標籤式主題必須清楚、簡潔、具體，"
+        "一看就明白（建議 3-8 個字），若涉及地點、地區、人物、時間、金額/財政 等關鍵資訊，"
+        "必須包含在標籤中 。具體提問/要求要精準、完整 並全部轉換成以下固定格式：\n"
+        "「標籤式主題：具體提問/要求」"
+    ),
+    "max_length": 3,
+}
+
 
 def _extract_json_from_markdown(response: str) -> str:
     if not isinstance(response, str):
@@ -98,12 +108,32 @@ class QueryDecomposer:
 
         prompt = template.replace("{question}", question)
 
-        from app.models.decompose import SubQuestions
+        from app.models.decompose import SubQuestions, create_subquestions_model, parse_decompose_format
+
+        pydantic_model = SubQuestions
+
+        if self._prompt_service is not None:
+            try:
+                format_raw = self._prompt_service.get_prompt_template("decompose_format")
+                format_config = parse_decompose_format(format_raw)
+                pydantic_model = create_subquestions_model(
+                    description=format_config["description"],
+                    max_length=format_config["max_length"],
+                )
+                logger.info(
+                    "Decompose format loaded from DB: max_length=%d",
+                    format_config["max_length"],
+                )
+            except Exception as exc:
+                logger.warning(
+                    "Failed to load decompose_format from DB: %s. Falling back to static SubQuestions.",
+                    exc,
+                )
 
         try:
             result = await self.llm_client.complete_structured(
                 prompt=prompt,
-                pydantic_model=SubQuestions,
+                pydantic_model=pydantic_model,
                 step_name="QueryDecomposer",
             )
             return result.questions, prompt

frontend/src/components/PromptEditor.tsx

@@ -14,7 +14,8 @@ export interface PromptEditorProps {
 }
 
 const STEPS = [
-  { key: 'decompose', label: 'Step 1: Query Decomposition', placeholders: ['question'] },
+  { key: 'decompose', label: 'Step 1.1: Query Decomposition', placeholders: ['question'] },
+  { key: 'decompose_format', label: 'Step 1.2: Query Decomposition Format', placeholders: [], type: 'format_config' },
   { key: 'filter_intro', label: 'Step 2.1: Filter Intro (Preamble)', placeholders: [] },
   { key: 'filter_section', label: 'Step 2.2: Filter Section (Per Sub-Q)', placeholders: ['subq_idx', 'subq_question', 'chunks'] },
   { key: 'filter_outro', label: 'Step 2.3: Filter Outro (Format)', placeholders: [] },
@@ -23,6 +24,7 @@ const STEPS = [
 
 const VALID_PLACEHOLDERS: Record<string, string[]> = {
   decompose: ['{question}'],
+  decompose_format: [],
   filter: ['{question}', '{chunks}'],
   generate: ['{question}', '{context}'],
   generate_per_subq: ['{context_sections}'],
@@ -37,6 +39,68 @@ const findUnknownPlaceholders = (template: string, stepKey: string): string[] =>
   return found.filter((p) => !valid.includes(p))
 }
 
+const DEFAULT_FORMAT_CONFIG = { description: '', max_length: 3 }
+
+function parseFormatConfig(raw: string): { description: string; max_length: number } {
+  try {
+    const parsed = JSON.parse(raw)
+    return {
+      description: typeof parsed.description === 'string' ? parsed.description : '',
+      max_length: typeof parsed.max_length === 'number' ? parsed.max_length : 3,
+    }
+  } catch {
+    return { ...DEFAULT_FORMAT_CONFIG }
+  }
+}
+
+function serializeFormatConfig(description: string, max_length: number): string {
+  return JSON.stringify({ description, max_length })
+}
+
+interface FormatConfigFieldsProps {
+  value: string
+  disabled: boolean
+  onChange: (description: string, max_length: number) => void
+}
+
+const FormatConfigFields: React.FC<FormatConfigFieldsProps> = ({ value, disabled, onChange }) => {
+  const config = parseFormatConfig(value)
+
+  return (
+    <div className="space-y-3 pl-1">
+      <div className="space-y-1">
+        <label className="text-xs font-medium text-gray-600">Description (field description for sub-question format)</label>
+        <textarea
+          value={config.description}
+          onChange={(e) => onChange(e.target.value, config.max_length)}
+          disabled={disabled}
+          rows={5}
+          className="w-full rounded border border-gray-300 px-3 py-2 font-mono text-sm focus:outline-none focus:ring-2 focus:ring-blue-500 focus:border-transparent disabled:bg-gray-100 disabled:cursor-not-allowed resize-y"
+        />
+        <span className="text-xs text-gray-500">{config.description.length} characters</span>
+      </div>
+
+      <div className="space-y-1">
+        <label className="text-xs font-medium text-gray-600">Max Sub-Questions (1–5)</label>
+        <input
+          type="number"
+          min={1}
+          max={5}
+          value={config.max_length}
+          onChange={(e) => {
+            const v = parseInt(e.target.value, 10)
+            if (!isNaN(v) && v >= 1 && v <= 5) {
+              onChange(config.description, v)
+            }
+          }}
+          disabled={disabled}
+          className="w-24 rounded border border-gray-300 px-3 py-1.5 text-sm focus:outline-none focus:ring-2 focus:ring-blue-500 focus:border-transparent disabled:bg-gray-100 disabled:cursor-not-allowed"
+        />
+      </div>
+    </div>
+  )
+}
+
 export const PromptEditor: React.FC<PromptEditorProps> = ({
   profileName,
   prompts,
@@ -80,7 +144,9 @@ export const PromptEditor: React.FC<PromptEditorProps> = ({
 
       {STEPS.map((step) => {
         const value = localPrompts[step.key] ?? ''
-        const unknownPlaceholders = findUnknownPlaceholders(value, step.key)
+        const unknownPlaceholders = step.type === 'format_config'
+          ? []
+          : findUnknownPlaceholders(value, step.key)
         const placeholderBadges = step.placeholders.map((p) => `{${p}}`).join(', ')
 
         return (
@@ -99,39 +165,49 @@ export const PromptEditor: React.FC<PromptEditorProps> = ({
               </div>
             </div>
 
-            <div className="flex items-center gap-2 flex-wrap">
+            {step.type === 'format_config' ? (
-              <span className="text-xs text-gray-500">Placeholders:</span>
+              <FormatConfigFields
-              {step.placeholders.map((p) => (
+                value={value}
-                <code
+                disabled={isSaving}
-                  key={p}
+                onChange={(desc, ml) => handleChange(step.key, serializeFormatConfig(desc, ml))}
-                  className="bg-gray-100 px-1.5 py-0.5 rounded text-xs font-mono text-gray-700"
+              />
-                >
+            ) : (
-                  {'{'}{p}{'}'}
+              <>
-                </code>
+                <div className="flex items-center gap-2 flex-wrap">
-              ))}
+                  <span className="text-xs text-gray-500">Placeholders:</span>
-            </div>
+                  {step.placeholders.map((p) => (
-
+                    <code
-            <textarea
+                      key={p}
-              value={value}
+                      className="bg-gray-100 px-1.5 py-0.5 rounded text-xs font-mono text-gray-700"
-              onChange={(e) => handleChange(step.key, e.target.value)}
+                    >
-              disabled={isSaving}
+                      {'{'}{p}{'}'}
-              rows={6}
+                    </code>
-              className="w-full min-h-[8rem] rounded border border-gray-300 px-3 py-2 font-mono text-sm focus:outline-none focus:ring-2 focus:ring-blue-500 focus:border-transparent disabled:bg-gray-100 disabled:cursor-not-allowed resize-y"
+                  ))}
-            />
-
-            <div className="flex items-center justify-between">
-              <span className="text-xs text-gray-500">{value.length} characters</span>
-
-              {unknownPlaceholders.length > 0 && (
-                <div className="flex items-center gap-1.5 bg-yellow-50 border border-yellow-200 rounded px-2 py-1">
-                  <AlertTriangle className="w-3.5 h-3.5 text-yellow-600" />
-                  <span className="text-xs text-yellow-700">
-                    Unknown placeholder{unknownPlaceholders.length > 1 ? 's' : ''}:{' '}
-                    {unknownPlaceholders.join(', ')}
-                  </span>
                 </div>
-              )}
+
-            </div>
+                <textarea
+                  value={value}
+                  onChange={(e) => handleChange(step.key, e.target.value)}
+                  disabled={isSaving}
+                  rows={6}
+                  className="w-full min-h-[8rem] rounded border border-gray-300 px-3 py-2 font-mono text-sm focus:outline-none focus:ring-2 focus:ring-blue-500 focus:border-transparent disabled:bg-gray-100 disabled:cursor-not-allowed resize-y"
+                />
+
+                <div className="flex items-center justify-between">
+                  <span className="text-xs text-gray-500">{value.length} characters</span>
+
+                  {unknownPlaceholders.length > 0 && (
+                    <div className="flex items-center gap-1.5 bg-yellow-50 border border-yellow-200 rounded px-2 py-1">
+                      <AlertTriangle className="w-3.5 h-3.5 text-yellow-600" />
+                      <span className="text-xs text-yellow-700">
+                        Unknown placeholder{unknownPlaceholders.length > 1 ? 's' : ''}:{' '}
+                        {unknownPlaceholders.join(', ')}
+                      </span>
+                    </div>
+                  )}
+                </div>
+              </>
+            )}
           </div>
         )
       })}

frontend/src/test/components/PromptEditor.test.tsx

@@ -4,6 +4,7 @@ import { PromptEditor } from '../../components/PromptEditor'
 
 const mockPrompts: Record<string, string> = {
   decompose: 'Decompose template with {question}',
+  decompose_format: '{"description": "test description", "max_length": 3}',
   filter_intro: 'Evaluate each chunk for relevance',
   filter_section: 'Sub-question {subq_idx}: "{subq_question}"\n{chunks}',
   filter_outro: 'Return JSON object mapping sub-question indices to scores.',
@@ -31,15 +32,16 @@ describe('PromptEditor', () => {
     defaultProps.onCancel.mockClear()
   })
 
-  it('renders 5 textareas for the active steps', () => {
+  it('renders 6 textareas for the active steps (5 regular + 1 format description)', () => {
     render(<PromptEditor {...defaultProps} />)
     const textareas = screen.getAllByRole('textbox')
-    expect(textareas).toHaveLength(5)
+    expect(textareas).toHaveLength(6)
   })
 
   it('renders labels for each step', () => {
     render(<PromptEditor {...defaultProps} />)
-    expect(screen.getByText('Step 1: Query Decomposition')).toBeInTheDocument()
+    expect(screen.getByText('Step 1.1: Query Decomposition')).toBeInTheDocument()
+    expect(screen.getByText('Step 1.2: Query Decomposition Format')).toBeInTheDocument()
     expect(screen.getByText('Step 2.1: Filter Intro (Preamble)')).toBeInTheDocument()
     expect(screen.getByText('Step 2.2: Filter Section (Per Sub-Q)')).toBeInTheDocument()
     expect(screen.getByText('Step 2.3: Filter Outro (Format)')).toBeInTheDocument()
@@ -66,18 +68,21 @@ describe('PromptEditor', () => {
 
     render(<PromptEditor {...defaultProps} />)
     const resetButtons = screen.getAllByTitle(/reset.*to default/i)
-    expect(resetButtons).toHaveLength(5)
+    expect(resetButtons).toHaveLength(6)
 
     fireEvent.click(resetButtons[0])
     expect(defaultProps.onResetStep).toHaveBeenCalledWith('decompose')
 
     fireEvent.click(resetButtons[1])
+    expect(defaultProps.onResetStep).toHaveBeenCalledWith('decompose_format')
+
+    fireEvent.click(resetButtons[2])
     expect(defaultProps.onResetStep).toHaveBeenCalledWith('filter_intro')
 
-    fireEvent.click(resetButtons[3])
+    fireEvent.click(resetButtons[4])
     expect(defaultProps.onResetStep).toHaveBeenCalledWith('filter_outro')
 
-    fireEvent.click(resetButtons[4])
+    fireEvent.click(resetButtons[5])
     expect(defaultProps.onResetStep).toHaveBeenCalledWith('generate_per_subq')
 
     confirmSpy.mockRestore()
@@ -143,10 +148,11 @@ describe('PromptEditor', () => {
     const textareas = screen.getAllByRole('textbox')
 
     expect(textareas[0]).toHaveValue('Decompose template with {question}')
-    expect(textareas[1]).toHaveValue('Evaluate each chunk for relevance')
+    expect(textareas[1]).toHaveValue('test description')
-    expect(textareas[2]).toHaveValue('Sub-question {subq_idx}: "{subq_question}"\n{chunks}')
+    expect(textareas[2]).toHaveValue('Evaluate each chunk for relevance')
-    expect(textareas[3]).toHaveValue('Return JSON object mapping sub-question indices to scores.')
+    expect(textareas[3]).toHaveValue('Sub-question {subq_idx}: "{subq_question}"\n{chunks}')
-    expect(textareas[4]).toHaveValue('Generate per-subq with {context_sections}')
+    expect(textareas[4]).toHaveValue('Return JSON object mapping sub-question indices to scores.')
+    expect(textareas[5]).toHaveValue('Generate per-subq with {context_sections}')
   })
 
   it('calls onUpdate when typing in a textarea', () => {