# Add Functional Spec > Add a single functional spec to the ***functional specs*** section of a ***plain spec file. Use whenever exactly one new functional spec is being added — whether the user explicitly asks, or another skill/workflow (e.g. forge-plain, add-feature) needs to author a new functional spec. Every new entry under ***functional specs*** must go through either this skill or `add-functional-specs` (the bulk variant for adding multiple specs in one pass); hand-authoring functional specs without invoking one of these skills is forbidden. - Skill: `codeplain-ai/add-functional-spec` (Agent Skill) - Install (CLI): `npx skillmds@latest add codeplain-ai/add-functional-spec` - Raw SKILL.md: https://api.skillmd.com/api/skills/codeplain-ai/add-functional-spec/raw - Safety review: pending - Works with: Claude Code, Claude.ai, OpenAI Codex - Category: Productivity - Author: codeplain-ai (https://skillmd.com/u/codeplain-ai) - Updated: 2026-09-17 - Page: https://skillmd.com/skills/codeplain-ai/add-functional-spec --- # Add Functional Spec Always use the skill `load-plain-reference` to retrieve the ***plain syntax rules — but only if you haven't done so yet. ## Workflow 1. **Identify the target `.plain` file.** If ambiguous, ask the user. 2. **Read the entire file** to understand existing definitions, implementation reqs, and all current functional specs (including those in `requires` modules). 3. **Draft the functional spec** following the rules below, then run the review pass in `concise-specs.md` over the draft — trim it to the shortest wording that keeps every fact before any other check. 4. **Analyze complexity** — use the `analyze-if-func-spec-too-complex` skill to verify the drafted spec implies ≤ 200 LOC. If too complex, use the `break-down-func-spec` skill to split it, then repeat from step 3 for each resulting spec. 5. **Check for conflicts** with every existing functional spec — this is critical. Run `analyze-func-specs` **once** with the new spec plus all existing specs (in the file and in any `requires` chain) as a single batch. The batched analyzer returns every conflicting pair in one call — do **not** invoke a pair-by-pair analyzer. For each conflicting pair it reports, run `resolve-spec-conflict` on that pair; re-run `analyze-func-specs` over the touched set after each resolution until the verdict is `COMPATIBLE`. 6. **Append the spec** to the end of the `***functional specs***` section (specs are chronological; new ones go last). 7. **Read the file again** to confirm correct placement and syntax. ## Rules ### Complexity Limit Each functional spec must imply a **maximum of 200 changed lines of code**. If the requirement is too large, use the `break-down-func-spec` skill to split it into multiple smaller, independent specs. Do not include LOC estimates in the spec text. ### Chronological Ordering Specs are rendered incrementally, top to bottom. The renderer has **no knowledge of future specs** — only previously rendered specs are in context. This means: - A new spec can reference behavior defined by earlier specs. - A new spec **cannot** assume anything about specs that come after it. - Place the new spec at the correct position if it must come before a future spec; otherwise append at the end. ### Language Agnosticism Write in terms of behavior, concepts, and domain logic — not implementation constructs. Avoid language-specific types, generics syntax, framework annotations, or other constructs tied to a particular language or framework. General technical terms (null values, JSON types, HTTP status codes) are fine. Language-specific guidance belongs in `***implementation reqs***`. ### No Conflicts The new spec must not contradict any existing functional spec. Conflicting specs are the most costly outcome. Before adding, review all existing specs and verify the new one is compatible. Use `analyze-func-specs` to check the new spec against every existing spec in **one batched call** — it lists every conflicting pair, so a pair-by-pair analyzer is not needed. If ambiguity exists, add explicit detail to eliminate any conflicting interpretation. For each conflicting pair the batched analyzer reports, use the `resolve-spec-conflict` skill to diagnose and fix it before proceeding. ### Disambiguation Each functional spec must be unambiguous — the renderer should have only one reasonable interpretation. If a single line is not enough to fully disambiguate the behavior, use **nested sub-bullets** to add detail. Nested lines clarify the parent spec; they do not introduce separate functionality. Even with nested detail, the spec must still imply ≤ 200 LOC. ```plain ***functional specs*** - A :User: can send a :Message: to a :Conversation:. - A :Message: must have non-empty content. - The :Message: is appended to the end of the :Conversation:. - All :Participant: members of the :Conversation: can see the new :Message:. ``` ### Deterministic Interface Specs must be detailed enough that a developer can use the built software without reading the generated code. All external interfaces must be explicit — REST endpoint paths and HTTP methods, CLI command names and arguments, file formats, message schemas, etc. Never leave interface details up to the renderer's discretion. ### Encapsulation Functionality must be self-contained in the spec text. `requires` modules only import functional specs, so do not rely on implementation reqs to convey behavior — they won't be visible in downstream modules. ## Acceptance Tests If the functional spec needs verification, use the `add-acceptance-test` skill to add acceptance tests after inserting the spec. ## Validation Checklist - [ ] Spec implies ≤ 200 lines of code changes - [ ] No conflict with any existing functional spec - [ ] Written in language-agnostic terms (no language/framework specifics) - [ ] All external interfaces are explicit (endpoint paths, methods, CLI args, formats, etc.) - [ ] All referenced `:Concepts:` are defined in `***definitions***` - [ ] Sentences are short, clear, and unambiguous - [ ] Trimmed per `concise-specs.md`: nothing obvious, no rationale, no uncheckable adjectives or hedges, plain words throughout - [ ] No redundancy with existing specs - [ ] Placed in correct chronological position (usually at the end)