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+# 4.1.2 Structured diagnostics with stable `E_SEMPAI_*` error codes
+
+This ExecPlan (execution plan) is a living document. The sections
+`Constraints`, `Tolerances`, `Risks`, `Progress`, `Surprises & Discoveries`,
+`Decision Log`, and `Outcomes & Retrospective` must be kept up to date as work
+proceeds.
+
+Status: DRAFT
+
+## Purpose / big picture
+
+This change stabilizes Sempai diagnostics as a contract, not just an internal
+type. After implementation, parser and validator flows will emit the same
+structured diagnostic schema with stable `E_SEMPAI_*` codes, and JSON snapshots
+will lock the schema so future refactors cannot silently break downstream
+consumers.
+
+The same implementation pass will also keep top-level CLI discoverability
+intact for bare invocation (`weaver` with no arguments): non-zero exit, short
+help with `Usage:`, the three domains (`observe`, `act`, `verify`), and
+exactly one pointer to `weaver --help`.
+
+Observable outcomes:
+
+1. Diagnostic JSON snapshots pass for parser and validator paths.
+2. Behaviour and unit tests assert the bare-invocation help contract.
+3. `make check-fmt`, `make lint`, and `make test` pass.
+4. Roadmap entry `4.1.2` is marked done. If the discoverability entry is
+   already done in the current tree (`2.2.1`), keep it as done and preserve
+   regression coverage.
+
+## Constraints
+
+- `make check-fmt`, `make lint`, and `make test` must pass after all edits.
+- No file may exceed 400 lines.
+- All new diagnostics must preserve stable string codes with `E_SEMPAI_*`
+  names as documented in `docs/sempai-query-language-design.md`.
+- Diagnostic payloads must include code, message, primary span, and notes.
+- Parser and validator diagnostics must share one canonical JSON shape.
+- Existing bare-invocation behaviour in `weaver-cli` must not regress.
+- BDD coverage must use `rstest-bdd` v0.5.0 and include happy and unhappy
+  paths.
+- Public `sempai` facade stability must be preserved: no breaking rename or
+  shape changes to existing exported types without explicit escalation.
+- Update `docs/users-guide.md` for any user-visible behaviour or payload
+  contract change.
+- Record design decisions in `docs/sempai-query-language-design.md`.
+
+## Tolerances (exception triggers)
+
+- Scope: if this work requires edits in more than 16 files, stop and escalate.
+- Dependencies: if a new external crate is required, stop and escalate.
+- Interfaces: if a public API signature in `sempai` must change incompatibly,
+  stop and escalate.
+- Iterations: if a failing gate (`check-fmt`, `lint`, or `test`) cannot be
+  fixed within 5 focused attempts, stop and escalate with failure evidence.
+- Ambiguity: if parser/validator diagnostic-path boundaries are unclear, stop
+  and resolve before implementation.
+
+## Risks
+
+- Risk: parser and validator code paths are still scaffolded, so forcing a
+  stable cross-path contract may tempt over-abstraction too early.
+  Severity: medium
+  Likelihood: medium
+  Mitigation: introduce a focused diagnostic-report schema layer and thin
+  adapters in each path; avoid speculative parser architecture.
+
+- Risk: snapshot tests may become noisy or brittle if they include unstable
+  fields.
+  Severity: medium
+  Likelihood: medium
+  Mitigation: snapshot only the stable JSON contract fields and ordering.
+
+- Risk: roadmap numbering mismatch for bare-invocation help (`5.1.1` in request
+  versus `2.2.1` in current roadmap) could cause incorrect checkbox edits.
+  Severity: low
+  Likelihood: high
+  Mitigation: treat this as the same acceptance contract, validate behaviour,
+  and only edit the checkbox that exists in the checked-in roadmap.
+
+- Risk: changing diagnostics could break current tests expecting
+  `NOT_IMPLEMENTED`.
+  Severity: medium
+  Likelihood: high
+  Mitigation: update tests in red/green order and preserve explicit coverage
+  for remaining stub paths where applicable.
+
+## Progress
+
+- [x] (2026-03-03 23:xxZ) Collected context from roadmap, Sempai design, and
+  existing ExecPlans.
+- [x] (2026-03-03 23:xxZ) Confirmed current bare-invocation behaviour and tests
+  already exist in `weaver-cli`.
+- [x] (2026-03-03 23:xxZ) Drafted this ExecPlan.
+- [ ] Implement diagnostic schema and path-specific constructors.
+- [ ] Add parser/validator snapshot coverage and BDD scenarios.
+- [ ] Re-validate bare-invocation acceptance criteria with unit and BDD tests.
+- [ ] Update design and user documentation.
+- [ ] Run full quality gates and update roadmap checkbox state.
+
+## Surprises & discoveries
+
+- Observation: the current roadmap already marks bare invocation as done under
+  `2.2.1`, while this request references `5.1.1`.
+  Evidence: `docs/roadmap.md` section `2.2`.
+  Impact: implementation should preserve behaviour and tests instead of
+  re-implementing the feature.
+
+- Observation: diagnostics types already exist in `sempai_core`, but there is
+  not yet a dedicated, snapshot-locked parser-versus-validator contract.
+  Evidence: `crates/sempai-core/src/diagnostic.rs` and current tests.
+  Impact: this task should formalize schema stability and path-aware emission.
+
+## Decision log
+
+- Decision: treat the request's bare-invocation item as a regression contract
+  verification milestone within this plan, not a fresh feature build.
+  Rationale: the capability is already implemented and tested in the current
+  tree; acceptance now is preservation and explicit coverage.
+  Date/Author: 2026-03-03 / Codex
+
+- Decision: implement stable diagnostics through one canonical JSON schema in
+  `sempai_core`, then route parser and validator producers through it.
+  Rationale: schema centralization prevents drift and satisfies snapshot
+  stability acceptance criteria.
+  Date/Author: 2026-03-03 / Codex
+
+## Outcomes & retrospective
+
+To be completed after implementation. Must include:
+
+- which `E_SEMPAI_*` codes are emitted by each path,
+- snapshot evidence for parser and validator payload stability,
+- bare-invocation acceptance evidence,
+- quality gate results (`check-fmt`, `lint`, `test`),
+- roadmap checkbox updates.
+
+## Context and orientation
+
+Current relevant code and docs:
+
+- `crates/sempai-core/src/diagnostic.rs`: current diagnostic types and codes.
+- `crates/sempai/src/engine.rs`: current compile/execute entrypoints returning
+  stub diagnostics.
+- `crates/sempai-core/src/tests/diagnostic_tests.rs`: unit diagnostics tests.
+- `crates/sempai-core/src/tests/behaviour.rs` and
+  `crates/sempai-core/tests/features/sempai_core.feature`: current BDD for core
+  diagnostics.
+- `crates/weaver-cli/src/lib.rs`, `src/localizer.rs`,
+  `src/tests/unit/bare_invocation.rs`, and
+  `tests/features/weaver_cli.feature`: bare-invocation discoverability flow and
+  tests.
+- `docs/sempai-query-language-design.md`: required diagnostic structure and
+  code list.
+- `docs/users-guide.md`: user-facing Sempai and CLI behaviour docs.
+- `docs/roadmap.md`: status checkboxes for `4.1.2` and discoverability
+  milestone.
+
+Terminology used in this plan:
+
+- Parser path: diagnostic emitted while decoding YAML or one-liner DSL syntax.
+- Validator path: diagnostic emitted after parse, while enforcing semantic or
+  schema constraints.
+- Primary span: the principal source location for the failure; represented as
+  the diagnostic `span` in JSON.
+
+## Plan of work
+
+### Stage A: lock the diagnostic schema in `sempai_core` (red first)
+
+Add failing tests first for the stable JSON contract expected from parser and
+validator paths. Introduce snapshots that assert the exact shape and field
+names for:
+
+- parser failure report (example code: `E_SEMPAI_YAML_PARSE` or
+  `E_SEMPAI_DSL_PARSE`);
+- validator failure report (example code:
+  `E_SEMPAI_MISSING_POSITIVE_TERM_IN_AND` or `E_SEMPAI_SCHEMA_INVALID`).
+
+Use deterministic fixture data (code, message, span, notes) so snapshots are
+stable and easy to review.
+
+### Stage B: implement canonical report schema and path constructors
+
+In `sempai_core`, refine diagnostics APIs so parser and validator producers use
+the same schema builder surface. Keep output contract stable:
+
+- `code` (stable string code),
+- `message` (short human-readable message),
+- `span` (primary span; nullable only when no source location exists),
+- `notes` (ordered list of supplemental notes).
+
+If needed, add small constructor helpers for parser/validator contexts (for
+example, `from_parser_error`, `from_validation_error`) but ensure they compose
+through one underlying `Diagnostic` shape.
+
+### Stage C: wire parser/validator emission points in `sempai` scaffolding
+
+Update `Engine::compile_yaml` and `Engine::compile_dsl` so they exercise the
+new parser-path diagnostics. Add a minimal validator-path call site in the same
+scaffold layer (or shared test helper) to prove both paths emit the same
+schema.
+
+This stage does not implement full YAML/DSL parsers from 4.1.3/4.1.6; it only
+ensures stable diagnostics plumbing and contracts.
+
+### Stage D: expand tests (unit + BDD + snapshots)
+
+Add or update:
+
+- `sempai_core` unit tests for schema fields and ordering;
+- snapshot tests for parser and validator reports;
+- `rstest-bdd` scenarios that cover happy/unhappy diagnostic-path cases;
+- regression tests ensuring any remaining stub path still returns deterministic
+  diagnostics.
+
+For CLI discoverability, keep existing behaviour and strengthen assertions if
+needed so acceptance remains explicit: non-zero exit, `Usage:` line, three
+domains, exactly one `weaver --help` pointer.
+
+### Stage E: documentation and roadmap updates
+
+Update:
+
+- `docs/sempai-query-language-design.md` with decisions on diagnostic schema
+  and any naming/serialization clarifications.
+- `docs/users-guide.md` with the new stable diagnostics contract and examples
+  of parser vs validator errors.
+- `docs/roadmap.md` by marking `4.1.2` as done after verification. For
+  discoverability, keep the existing completed checkbox in its current section.
+
+### Stage F: full validation gates and evidence capture
+
+Run repository gates with logs captured via `tee` and `set -o pipefail`:
+
+```bash
+set -o pipefail
+make check-fmt 2>&1 | tee /tmp/execplan-4-1-2-check-fmt.log
+make lint 2>&1 | tee /tmp/execplan-4-1-2-lint.log
+make test 2>&1 | tee /tmp/execplan-4-1-2-test.log
+```
+
+If Markdown files changed, also run:
+
+```bash
+set -o pipefail
+make markdownlint 2>&1 | tee /tmp/execplan-4-1-2-markdownlint.log
+make nixie 2>&1 | tee /tmp/execplan-4-1-2-nixie.log
+```
+
+Do not mark completion until all applicable gates pass.
+
+## Concrete steps
+
+All commands are executed from the workspace root (`/home/user/project`).
+
+- Inspect current diagnostics and tests:
+
+```bash
+rg -n "DiagnosticCode|DiagnosticReport|E_SEMPAI|NOT_IMPLEMENTED" \
+  crates/sempai-core crates/sempai
+```
+
+- Add failing unit/snapshot tests for parser and validator JSON contracts.
+
+- Implement schema builders and path-specific constructors in
+   `crates/sempai-core/src/diagnostic.rs` (or focused submodules if needed to
+   stay under line limits).
+
+- Wire compile entrypoints in `crates/sempai/src/engine.rs` to emit parser and
+   validator diagnostics through the canonical schema.
+
+- Add/adjust BDD features and step definitions in `sempai_core` / `sempai`.
+
+- Re-run discoverability tests in `weaver-cli` and adjust only if regression
+   is detected.
+
+- Update documentation and roadmap.
+
+- Run quality gates and review logs:
+
+```bash
+set -o pipefail
+make check-fmt 2>&1 | tee /tmp/execplan-4-1-2-check-fmt.log
+make lint 2>&1 | tee /tmp/execplan-4-1-2-lint.log
+make test 2>&1 | tee /tmp/execplan-4-1-2-test.log
+```
+
+## Validation and acceptance
+
+Acceptance checks for 4.1.2:
+
+1. Diagnostics include `code`, `message`, primary `span`, and `notes`.
+2. Parser and validator diagnostic JSON snapshots are stable and pass.
+3. Unit and BDD tests cover happy and unhappy paths, including edge cases.
+
+Acceptance checks for bare invocation discoverability contract:
+
+1. `weaver` with no arguments exits non-zero.
+2. Output contains a `Usage:` line.
+3. Output lists `observe`, `act`, and `verify`.
+4. Output contains exactly one `weaver --help` pointer.
+
+Quality criteria:
+
+- `make check-fmt` exits 0.
+- `make lint` exits 0.
+- `make test` exits 0.
+- `make markdownlint` and `make nixie` exit 0 when docs are changed.
+
+## Idempotence and recovery
+
+- All edits are source-controlled and re-runnable.
+- Snapshot updates are deterministic; if a snapshot unexpectedly changes,
+  inspect semantic intent before accepting.
+- If a stage fails, revert only the incomplete stage changes and re-run from
+  the last passing gate.
+- Do not use destructive git commands (`reset --hard` / checkout overwrite).
+
+## Interfaces and dependencies
+
+Expected interfaces after implementation:
+
+- Stable diagnostic JSON contract centered in `sempai_core`.
+- Parser and validator producers emitting `DiagnosticReport` via the same
+  schema path.
+- `sempai` compile entrypoints returning deterministic `E_SEMPAI_*` diagnostics
+  where applicable.
+- No new external dependencies required.
+
+## Revision note
+
+Initial draft created for roadmap item `4.1.2` and the bare-invocation
+discoverability acceptance contract referenced in the request.