propose: replace string hints with enriched hints_structured

propose/HINTS-STRING-REMOVAL-PROPOSE.md

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+# HINTS-STRING-REMOVAL-PROPOSE
+
+## Status
+Proposal — not yet implemented.
+
+## Problem Statement
+Two parallel hint fields (`hints: list[str]` and `hints_structured: list[StructuredHint]`) coexist on all five MCP tool outputs, carrying largely the same information in different formats.
+
+This redundancy creates concrete problems:
+
+1. **Maintenance burden** — every hint trigger requires dual emission (string template + structured object) in `mcp_hints.py`, with a parity test ensuring they stay in sync.
+2. **Information asymmetry** — non-actionable hints lose advisory context in structured form. A string like `"results look weak — narrow the query"` becomes `StructuredHint(tool="find", args={}, actionable=False)` with no explanatory text, while the string version carries the full advisory.
+3. **Wider surface than needed** — the project principles explicitly warn against widening public surface "just in case" and state that breaking changes are always allowed.
+
+The original string `hints` field predates `hints_structured` (added in #209). Now that structured hints are established and include `label` (#217), the string field's only unique contribution is the advisory reason text — which should simply be a field on `StructuredHint`.
+
+## Proposed Solution
+Consolidate to a single hint mechanism:
+
+1. **Add `reason: str` to `StructuredHint`** — carries the advisory text previously only in string hints (e.g. `"no match — try ranked fuzzy lookup"`, `"results look weak — narrow the query"`).
+2. **Remove `hints: list[str]`** from all five output models in `mcp_v2.py`.
+3. **Remove string hint generation** — `generate_hints()` returns only `list[_StructuredHint]`; all string template constants and `MCP_HINTS_FIELD_DESCRIPTION` are deleted.
+4. **Remove parity test** — `test_structured_hints_parity_with_string_hints` no longer applies.
+
+### `StructuredHint` after change
+
+```python
+class StructuredHint(BaseModel):
+    label: str = ""
+    tool: Literal["search", "find", "describe", "neighbors", "resolve"]
+    args: dict[str, Any]
+    actionable: bool = True
+    reason: str = ""
+```
+
+### `generate_hints` after change
+
+```python
+def generate_hints(
+    output_kind: Literal["search", "find", "describe", "neighbors", "resolve"],
+    payload: dict[str, Any],
+) -> list[_StructuredHint]:  # was tuple[list[str], list[_StructuredHint]]
+```
+
+### Example: actionable hint (describe finds no match)
+
+Before (two fields):
+```python
+{
+    "results": [],
+    "hints": [
+        "no match — try search(query='BankAccount') for ranked fuzzy lookup",
+        "try find(role='service') to browse by role"
+    ],
+    "hints_structured": [
+        {"tool": "search", "args": {"query": "BankAccount"}, "actionable": True, "label": ""},
+        {"tool": "find", "args": {"role": "service"}, "actionable": True, "label": ""}
+    ]
+}
+```
+
+After (single field):
+```python
+{
+    "results": [],
+    "hints_structured": [
+        {
+            "tool": "search",
+            "args": {"query": "BankAccount"},
+            "actionable": True,
+            "label": "",
+            "reason": "no match — try ranked fuzzy lookup"
+        },
+        {
+            "tool": "find",
+            "args": {"role": "service"},
+            "actionable": True,
+            "label": "",
+            "reason": "browse by role to discover related symbols"
+        }
+    ]
+}
+```
+
+### Example: non-actionable advisory hint
+
+Before:
+```python
+{
+    "hints": ["results look weak — narrow the query or try find(role=…)"],
+    "hints_structured": [{"tool": "find", "args": {}, "actionable": False}]
+}
+```
+
+After:
+```python
+{
+    "hints_structured": [
+        {
+            "tool": "find",
+            "args": {"role": "service"},
+            "actionable": False,
+            "label": "",
+            "reason": "results look weak — narrow the query or try find with a role filter"
+        }
+    ]
+}
+```
+
+## Scope
+- `mcp_hints.py` — remove string return, string templates, `MCP_HINTS_FIELD_DESCRIPTION`; add `reason` to `_StructuredHint`; update all hint templates to emit `reason`
+- `mcp_v2.py` — remove `hints: list[str]` from all five output models; add `reason: str = ""` to `StructuredHint`; update `_to_structured_hints` conversion
+- `tests/test_mcp_hints.py` — remove parity test; update assertions to check `reason` content instead of string hints
+- `README.md`, `docs/AGENT-GUIDE.md`, `AGENTS.md` — remove references to `hints` string field
+
+## Schema / Ontology / Re-index impact
+- Ontology bump: not required
+- Re-index required: no — this is an output-only change with no graph or index schema impact
+- Config/tool surface changes: `hints` field removed from all five tool outputs; `hints_structured` gains `reason` field
+
+## Tests / Validation
+- Existing hint tests rewritten to assert `reason` content on structured hints instead of string hints
+- Parity test removed (no longer applicable)
+- String-hint-specific test cases migrated to structured-hint assertions
+- Full test suite must pass — confirms no regressions in hint generation logic
+- Agent-facing behavior unchanged: structured hints already carry all tool-call information; `reason` adds advisory context
+
+## Open Questions ([TBD])
+1. Should `reason` default to `""` or be required (`str` without default)? — Recommended: default `""` for backward compatibility during transition and to avoid forcing a reason on every hint.
+2. Should non-actionable hints always carry a `reason`? — Recommended: yes, non-actionable hints without a reason are noise and should be reconsidered at authoring time.
+3. Should string hint templates be deleted entirely or archived for reference? — Recommended: delete entirely; git history preserves them.
+
+## Out of scope
+- Changing hint trigger logic or priority tiers
+- Adding new hint categories or triggers
+- Modifying the `_StructuredHint` `priority` field semantics
+- Changing the `label` field behavior (added in #217)
+
+## Sequencing / Follow-ups
+Single implementation PR covering all files listed in scope.