feat(knowledge): model + effort routing enter the register, dated [skip-docs]

C1: two entries the optimization lens can cite for the model/effort axis,
both read out of the primary sources in this session rather than from the
plan's 2026-07-14 summary of them.

Verifying corrected the plan's own numbers: effort is settable in SIX
places, not five (/effort, the /model slider, --effort,
CLAUDE_CODE_EFFORT_LEVEL, settings effortLevel, skill/subagent
frontmatter), the default is high on every supporting model EXCEPT Opus
4.7 (xhigh), and the level count is model-dependent (Opus 4.6 and Sonnet
4.6 have no xhigh).

- BP-MODEL-001: subagent `model` defaults to `inherit`, so a subagent
  that names no model costs what the session costs; the documented pin
  is overridable by CLAUDE_CODE_SUBAGENT_MODEL and per-invocation model
- BP-MODEL-002: effort is an axis separate from model choice, and higher
  is not universally better (`max` "may show diminishing returns and is
  prone to overthinking")
- both carry the 2026-07-07 model/effort blog as a corroborating source
  with a real `published` date, so B1's evidence-age rule has teeth:
  measured stale with reasons ['evidence-age'] at a reference date 378
  days past publication while their verified stamps are pristine. The
  docs pages themselves get NO published date — they carry none, and
  guessing one in the field whose whole job is dating evidence is the
  lie the rule exists to catch
- new blanket guard: every corroborating source must carry a parseable
  published date. Without it newestEvidenceMs() returns null and the
  entry stays green on evidence of any age — a silent hole. Seen red
  against its own defect before it was trusted
- knowledge-refresh-cli's stale branch no longer expires on every new
  entry: its reference date has to sit after every `verified` stamp, was
  bumped once for BP-SUB-001 and would have needed a third bump now, so
  it moves to a date no stamp can reach

Register 14 -> 16 entries. Frozen v5.0.0 snapshots untouched (no scanner
output changes); suite 1579/0.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01NKjojcdYYiCQP5AudUyQ5e

knowledge/best-practices.json

@@ -213,6 +213,56 @@
           "note": "Near-verbatim coverage: 'briefly describe what your repo is for, but spend most of the tokens on gotchas inside of the codebase'; 'Avoid stating the obvious things Claude should know by looking at your file system or your repo.'"
         }
       ]
+    },
+    {
+      "id": "BP-MODEL-001",
+      "claim": "A subagent's `model` frontmatter field defaults to `inherit`, so a subagent that names no model runs on the main conversation's model. Routing mechanical or read-only subagents to a cheaper alias (`haiku`, `sonnet`) while the orchestrating session keeps the stronger model is the documented way to control cost. The pin is not absolute: Claude Code resolves the model as CLAUDE_CODE_SUBAGENT_MODEL, then a per-invocation `model` parameter, then the frontmatter, then the main conversation's model.",
+      "mechanism": "model",
+      "appliesTo": "agent",
+      "recommendation": "Set `model:` explicitly on subagents whose work is mechanical or read-only (search, extraction, summarisation) and leave the orchestrator on the stronger model. Omitting the field is not a neutral default — it inherits, so every subagent costs what the session costs.",
+      "confidence": "confirmed",
+      "severity": "low",
+      "category": "model-fit",
+      "lensCheck": null,
+      "source": {
+        "url": "https://code.claude.com/docs/en/sub-agents",
+        "title": "Create custom subagents — supported frontmatter fields / choose a model",
+        "verified": "2026-08-10"
+      },
+      "sources": [
+        {
+          "url": "https://claude.com/blog/claude-model-and-effort-level-in-claude-code",
+          "title": "Choosing a Claude model and effort level in Claude Code",
+          "published": "2026-07-07",
+          "verified": "2026-08-10",
+          "note": "Verbatim: 'Pick a smaller model when the work is routine. For example, edits you can describe precisely, mechanical changes, or questions about code that's already in context.'"
+        }
+      ]
+    },
+    {
+      "id": "BP-MODEL-002",
+      "claim": "Reasoning effort is an axis separate from model choice: five levels (`low`, `medium`, `high`, `xhigh`, `max`) on current models, four on Opus 4.6 and Sonnet 4.6, which omit `xhigh`; the default is `high` on every model that supports effort except Opus 4.7, which defaults to `xhigh`. Higher is not universally better — `max` \"can improve performance on demanding tasks but may show diminishing returns and is prone to overthinking\". Effort is settable in six places: `/effort`, the slider in `/model`, the `--effort` flag, CLAUDE_CODE_EFFORT_LEVEL, `effortLevel` in settings, and `effort:` in skill or subagent frontmatter; the environment variable takes precedence over all of them.",
+      "mechanism": "effort",
+      "appliesTo": "agent",
+      "recommendation": "Treat effort as a per-task dial rather than a global maximum: pin a lower `effort:` in the frontmatter of mechanical skills and subagents, and reserve `xhigh`/`max` for work whose product is judgment. The scale is calibrated per model, so the same level name is not the same amount of thinking across models — and CLAUDE_CODE_EFFORT_LEVEL silently overrides every other source, so verify which level is actually in force.",
+      "confidence": "confirmed",
+      "severity": "low",
+      "category": "model-fit",
+      "lensCheck": null,
+      "source": {
+        "url": "https://code.claude.com/docs/en/model-config",
+        "title": "Model configuration — adjust effort level / set the effort level",
+        "verified": "2026-08-10"
+      },
+      "sources": [
+        {
+          "url": "https://claude.com/blog/claude-model-and-effort-level-in-claude-code",
+          "title": "Choosing a Claude model and effort level in Claude Code",
+          "published": "2026-07-07",
+          "verified": "2026-08-10",
+          "note": "Verbatim: 'Claude will be more predisposed to double-checking additional hypotheses or verifying correctness at higher effort levels, but it generally won't artificially inflate usage for simple tasks at higher effort levels.'; 'In fact, our team pays close attention to \"overthinking\" during model training as it degrades effectiveness.'"
+        }
+      ]
     }
   ]
 }