sesf

Structured English Specification Format (SESF v4.1)

Generate well-structured specifications from user requests using formal BEHAVIOR/PROCEDURE/RULE/STEP blocks. Given a domain, complexity indicators, and user intent, produce a specification that uses explicit block syntax for visual scaffolding, WHEN/THEN conditionals for scannable rules, STEP with → $variable for data flow tracking, @route tables for multi-branch decisions, @config for centralized parameters, and consolidated error tables. Follows the correct tier format and passes structural validation.

Not in scope: executing the generated specs, providing domain expertise for spec content, maintaining the validator script, runtime interpretation of specs

Configuration

@config validator_path: ${CLAUDE_PLUGIN_ROOT}/skills/sesf/scripts/validate_sesf.py reference_path: ${CLAUDE_PLUGIN_ROOT}/skills/sesf/assets/reference.md template_path: ${CLAUDE_PLUGIN_ROOT}/skills/sesf/assets/template.md examples_path: ${CLAUDE_PLUGIN_ROOT}/skills/sesf/references/examples.md authoring_guide_path: ${CLAUDE_PLUGIN_ROOT}/skills/sesf/assets/authoring-guide.md line_budget: micro: [20, 80] standard: [80, 200] complex: [200, 400] tier_threshold: route_min_branches: 3

Inputs

• user_request: string - what the user wants specified - required
• domain: string - subject area for the specification - required
• complexity_indicators: signals that inform tier selection (concern count, shared structures, overlapping rules, stateful behavior) - optional
• target_tier: enum [micro, standard, complex] - explicit tier override - optional

Outputs

• specification: string - the complete SESF v4.1 specification document
• validation_result: pass/fail/warnings from the validator
• extracted_assets: reference material extracted to assets/ if applicable

Tier Selection

@route tier_selection [first_match_wins] single concern, ≤5 rules, no shared types or functions → micro 2-5 concerns OR shared structures OR multi-phase procedures → standard overlapping rules needing precedence OR 5+ concerns → complex

•                                                          → standard
  

When the user explicitly requests a tier via target_tier, use that tier regardless of complexity indicators. When a spec exceeds its tier's line budget, suggest promotion to the next tier.

Instructions

How to Select a Tier

Use the @route table above. Default to standard when signals are ambiguous. A micro spec that grows past 80 lines SHOULD become standard. A complex spec with only one concern after refactoring SHOULD demote to micro.

How to Structure the Document

Always required (all tiers):

• Purpose (1-3 sentences, no heading needed for micro)
• Scope (IN/OUT as bullet lists, or a single "Not in scope" line for micro)
• Configuration (if any static params exist — use @config block for 3+ values, or inline for fewer)
• BEHAVIOR and/or PROCEDURE blocks (the core content — formal blocks with RULE and STEP entries)
• Errors (as a consolidated table: Error | Severity | Action)

Required for Standard and Complex only:

• Inputs / Outputs (typed parameter lists)
• Rules (cross-cutting only — rules that span ALL behaviors/procedures)
• Examples (edge cases only, with pass/fail)

Optional (include only when they add value):

• @route tables (only for 3+ branch decision logic)
• @config blocks (only when 3+ configuration values exist)
• $variable threading with STEP → $var (only when data flows between procedures are complex enough to need explicit tracking)
• Rationale annotations (parenthetical after non-obvious rules)

Removed entirely — do NOT include:

• Meta section (version goes in YAML frontmatter if needed)
• Notation section (symbols are self-evident or explained inline on first use)
• Types section (inline field descriptions where they're used)
• Functions section (logic goes in BEHAVIOR or PROCEDURE blocks)
• Precedence section (resolve conflicts in the rules directly)
• Dependencies section (agent discovers these from context)
• Changelog section (use git history; version in frontmatter is sufficient)

How to Write BEHAVIOR Blocks

BEHAVIOR blocks group related declarative rules. Each BEHAVIOR contains one or more RULE entries that describe what should be true — not when to check it.

Syntax:

**BEHAVIOR** name: One-line description

  **RULE** rule_name:
    WHEN condition
    THEN action
    ELSE alternative
    (Rationale: why this rule exists)

  **RULE** simple_rule:
    constraint MUST be true
    (Rationale: explanation)

Guidelines:

• Every BEHAVIOR MUST have a one-line summary after the colon
• RULE entries inside a BEHAVIOR use WHEN/THEN/ELSE for conditional logic
• Simple constraints that are always true use a declarative MUST/SHOULD/MAY statement
• Rationale annotations are parenthetical and SHOULD follow non-obvious rules
• BEHAVIOR blocks are for declarative logic — use PROCEDURE for sequential logic

Example:

**BEHAVIOR** validate_payment: Ensure payment requests meet processing requirements

  **RULE** positive_amount:
    payment.amount MUST be greater than zero
    (Rationale: zero or negative payments indicate data entry errors)

  **RULE** supported_currency:
    WHEN payment.currency is not in $config.supported_currencies
    THEN reject with unsupported_currency
    (Rationale: unsupported currencies cannot be converted by our payment processor)

  **RULE** high_value_approval:
    WHEN payment.amount > $config.high_value_threshold
    THEN require VP approval before processing
    ELSE process normally

How to Write PROCEDURE Blocks

PROCEDURE blocks define ordered workflows. Each PROCEDURE contains STEP entries that execute sequentially.

Syntax:

**PROCEDURE** name: One-line description

  **STEP** step_name: Description → $output_var
    Detailed instructions for this step.

  **STEP** next_step: Description
    Uses $output_var from previous step.

Guidelines:

• Every PROCEDURE MUST have a one-line summary after the colon
• STEP entries execute in the order listed
• Use → $variable after a STEP description to declare output variables
• $variable threading tracks data flow between steps and across procedures
• Keep each STEP focused on a single logical action

Example:

**PROCEDURE** process_invoice: Validate and route incoming invoice

  **STEP** validate_fields: Check all required fields are present → $validated_invoice
    Verify vendor_name, invoice_number, invoice_date, amount, and currency
    are present and non-empty. If ANY field is missing, reject with
    missing_required_field.

  **STEP** classify: Determine invoice type → $invoice_type
    Apply the invoice_classification @route table to $validated_invoice.

  **STEP** route: Send to appropriate team
    Using $invoice_type, apply the team_assignment @route table.
    WHEN amount > $config.high_value_threshold THEN add VP approval flag.

How to Write RULE with WHEN/THEN

Inside BEHAVIOR blocks, use WHEN/THEN/ELSE for conditional rules:

• WHEN introduces the condition
• THEN introduces the action when the condition is true
• ELSE introduces the alternative (optional)
• For simple always-true constraints, use a MUST/SHOULD/MAY statement without WHEN/THEN

Use WHEN/THEN for 1-2 branch conditions. For 3+ branches, use an @route table instead.

How to Use @route and @config

These are identical to HSF v5 — they are the strongest parts of the format.

@route decision tables:

• Use ONLY for 3+ branch decision logic
• Fewer branches → use WHEN/THEN/ELSE inside a RULE
• Must specify mode: [first_match_wins] or [all_matches]
• SHOULD end with * wildcard default when a meaningful default exists

@config blocks:

• Use ONLY when 3+ configuration values exist
• Keys MUST use snake_case
• Values MUST be literals (strings, numbers, lists, nested objects)
• Reference with $config.key or $config.nested.key
• @config is for static values only — use $variable threading for runtime data

How to Write Examples

Edge cases only — boundary conditions, error paths, non-obvious behavior. If the happy path is obvious from the rules, do not exemplify it. Use the compact single-line format or multi-line worked examples.

Compact format: example_name: input_description → expected_outcome

Worked examples (for specs where derivation chain integrity matters):

[good_example]:
  Input: [concrete values]
  Expected: [concrete outcome]
  Why: [explain which rule applies and the reasoning]

How to Write Errors

Consolidate ALL errors into a single ## Errors table at the end of the spec:

## Errors

| Error | Severity | Action |
|-------|----------|--------|
| missing_field | critical | halt and report which field is missing |
| over_budget | warning | review for redundancy, suggest promotion |
| empty_result | info | generate empty report, log the date range |

Severity values: critical (halt processing), warning (continue with degradation), info (log only).

Do NOT scatter error declarations inline after individual rules. This is a key improvement over SESF v4.

How to Add Rationale Annotations

Rationale annotations explain why a rule exists. They are parenthetical and follow the rule they annotate:

**RULE** constant_time_compare:
  Signature comparison MUST use a constant-time function.
  (Rationale: prevents timing side-channel attacks that could leak signature bytes)

Add rationale to rules where the why is not obvious from the what. Skip rationale on self-evident rules like "amount MUST be positive."

Rules

Format Compliance

• Formal blocks required: Declarative rules MUST be wrapped in **BEHAVIOR** blocks. Sequential workflows MUST be wrapped in **PROCEDURE** blocks. Do not use prose headers and bold list items — use the formal block syntax.
• RULE syntax required: Conditional logic inside BEHAVIOR blocks MUST use WHEN/THEN/ELSE syntax, not prose "if/then" sentences.
• STEP syntax required: Sequential actions inside PROCEDURE blocks MUST use **STEP** entries, not numbered lists.
• No empty sections: If a section would say "none" or be blank, omit it entirely. Do not template the absence of content.
• No duplicate constraints: Each rule MUST be stated once, in the BEHAVIOR or PROCEDURE where it applies. No separate "define" then "reference" pattern.
• No notation legend: Symbols ($, @, →) are self-evident. Explain inline on first use only if genuinely non-obvious for the target audience.
• Errors as consolidated table: All errors MUST appear in a single ## Errors table, not scattered inline after rules.
• RFC 2119 keywords preserved: MUST, SHOULD, MAY MUST be capitalized when used with their operative meanings.
• Section-level summaries: Each BEHAVIOR and PROCEDURE block MUST have a one-line description after the colon.

Notation Usage

• @route only for 3+ branches: Fewer branches → WHEN/THEN/ELSE in a RULE. The form is determined by branch count, not author preference.
• @config only for 3+ values: Fewer values → inline in the text.
• $variable threading for data flow: Use STEP → $var syntax to declare outputs. Reference $var in subsequent steps. Most micro specs do not need it.
• No @route outside BEHAVIOR/PROCEDURE blocks: @route tables belong inline in the relevant block or in a dedicated routing section — not floating unattached.

Line Budget

• Micro specs SHOULD stay within 20-80 lines
• Standard specs SHOULD stay within 80-200 lines
• Complex specs SHOULD stay within 200-400 lines

Quality Assurance

• Run the validator when the spec is complete: python3 $config.validator_path <spec.md> and fix all failures
• Concrete examples only: EXAMPLE blocks MUST use concrete values — never placeholders. Cover edge cases and boundary conditions only.
• No ambiguity: Rules MUST NOT use vague language like "handle appropriately", "use common sense", or "extract relevant fields". Specify each case explicitly.
• Reference extraction: When reference/lookup data exceeds 10 lines, extract to assets/ and add a one-line pointer.
• YAML frontmatter: When the specification is a Claude Code skill, it MUST include YAML frontmatter with name and description fields.

Quality Checklist

Check	Rule
Every behavior has a block	Rules grouped under **BEHAVIOR** name:
Every procedure has steps	Steps under **PROCEDURE** name:
WHEN/THEN for conditionals	Not prose "if/then" inside rules
@route only for 3+ branches	Fewer branches use WHEN/THEN/ELSE
@config only for 3+ values	Fewer values stated inline in the text
Errors consolidated	All errors in a single ## Errors table, not scattered inline
Rationale on non-obvious rules	Parenthetical after the rule
No empty sections	Omit unused sections entirely
RFC 2119 keywords preserved	MUST, SHOULD, MAY capitalized for precision
Line budget compliance	Micro ≤80, Standard ≤200, Complex ≤400
Edge-case examples only	No happy-path examples
Section-level summaries	Each BEHAVIOR/PROCEDURE has a one-line description

Errors

Error	Severity	Action
missing_behavior_block	critical	Wrap declarative rules in a **BEHAVIOR** name: block
missing_procedure_block	critical	Wrap sequential steps in a **PROCEDURE** name: block
prose_conditional_in_rule	warning	Convert "if/then" prose to WHEN/THEN/ELSE syntax
numbered_list_in_procedure	warning	Convert numbered list items to **STEP** name: entries
empty_section	critical	Delete the empty section entirely
duplicate_constraint	warning	Remove the duplicate; keep only the instance in the block where it applies
route_under_threshold	warning	Convert @route with fewer than 3 branches to WHEN/THEN/ELSE in a RULE
config_under_threshold	warning	Inline @config values referenced fewer than 3 times
scattered_errors	critical	Move all inline error declarations to the consolidated ## Errors table
over_budget	warning	Review for redundancy, extract reference material, consolidate rules
vague_rule	critical	Rewrite with specific conditions and actions
missing_error_table	warning	Add consolidated error table at end of spec
validation_failure	critical	Fix structural issues identified by the validator
config_key_mismatch	critical	Add key to @config or fix the reference
missing_block_summary	warning	Add a one-line description after the BEHAVIOR/PROCEDURE name
missing_rationale	info	Consider adding a rationale annotation to non-obvious rules