diff --git a/CHANGELOG.md b/CHANGELOG.md
index 32ee78a..c56fa87 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -4,3 +4,7 @@
 
 - Initial port of the 2026-08-04 lecture-intro.zh-cn study into a package: config-driven scan, provenance tiers by precedence, prose-only blame, edit pairs and recurring substitutions, overwrite analysis, derived review state, freshness from state files.
 - Regression test reproducing the study's per-lecture numbers on the pinned checkout.
+- `baseline.strategy` is validated: unknown values and the planned but unimplemented `state-file` are
+  rejected instead of silently scanning with `script-jump`.
+- README and `docs/method.md` no longer claim character-level churn is reported; that work is tracked in
+  [#4](https://github.com/QuantEcon/textstrata/issues/4).
diff --git a/README.md b/README.md
index fbbd4cc..3dbeda3 100644
--- a/README.md
+++ b/README.md
@@ -44,7 +44,7 @@ One YAML file per target repository — see [docs/configuration.md](docs/configu
 ## Limits worth knowing
 
 - Squash merges hide human cleanup done inside a machine-drafted PR: `ai-initial` means *as landed*, so human effort is a **lower bound**.
-- Blame credits the last toucher: a one-character fix claims the whole line, so human shares are an **upper bound** at line granularity. Churn is also reported in changed characters.
+- Blame credits the last toucher: a one-character fix claims the whole line, so human shares are an **upper bound** at line granularity. Churn in changed characters, alongside the line counts, is planned ([#4](https://github.com/QuantEcon/textstrata/issues/4)).
 - Pairing lines inside rewritten paragraphs is heuristic; category counts are indicative, not exact.
 - Latin-script targets (e.g. French from English) have no script signal; the `source-diff` prose strategy for them is planned, not implemented.
 
diff --git a/docs/method.md b/docs/method.md
index b3aca00..9f9dbed 100644
--- a/docs/method.md
+++ b/docs/method.md
@@ -49,7 +49,7 @@ All stock metrics count **lines containing the target script**. On raw lines mos
 ## Known limits
 
 - **Squash merges** hide human work done inside a machine-drafted PR. `ai-initial` means *as landed*; human effort is a lower bound.
-- **Last-toucher blame** credits a whole line to whoever changed one character of it. Human shares are an upper bound at line granularity; churn is also available in changed characters from the pairs.
+- **Last-toucher blame** credits a whole line to whoever changed one character of it. Human shares are an upper bound at line granularity; reporting churn in changed characters is planned ([#4](https://github.com/QuantEcon/textstrata/issues/4)) — until then the before/after text in `pairs.jsonl` is the only character-level signal.
 - **Line pairing** inside rewritten paragraphs is heuristic (similarity-matched within a hunk). Category counts are indicative.
 - **Identity** is resolved by e-mail and GitHub noreply handle only; display names are ignored. Unresolved authors fall to `ai-assisted` and should be reviewed in `commits.jsonl`.
 - **Pre-engine history** has no recorded engine version. Stratify flow metrics by version downstream and label the pre-engine stratum as such; do not read its rates as the shipping engine's.
diff --git a/src/textstrata/config.py b/src/textstrata/config.py
index 7ae81ed..7e34de1 100644
--- a/src/textstrata/config.py
+++ b/src/textstrata/config.py
@@ -145,13 +145,18 @@ def load_config(path: str | Path) -> Config:
         raise ConfigError(f"prose.strategy must be script or source-diff, got {prose.strategy!r}")
     if prose.strategy == "source-diff":
         raise ConfigError("prose.strategy source-diff is planned but not implemented (Stage 2)")
+    baseline = _sub(BaselineConfig, raw.get("baseline"), "baseline")
+    if baseline.strategy not in ("script-jump", "state-file"):
+        raise ConfigError(f"baseline.strategy must be script-jump or state-file, got {baseline.strategy!r}")
+    if baseline.strategy == "state-file":
+        raise ConfigError("baseline.strategy state-file is planned but not implemented")
     cfg = Config(
         name=str(raw["name"]),
         repo=Path(raw["repo"]),
         files=str(raw.get("files", "lectures/*.md")),
         source=_sub(SourceConfig, raw.get("source"), "source"),
         prose=prose,
-        baseline=_sub(BaselineConfig, raw.get("baseline"), "baseline"),
+        baseline=baseline,
         machine=_sub(MachineConfig, raw.get("machine"), "machine"),
         disclosure=_sub(DisclosureConfig, raw.get("disclosure"), "disclosure"),
         people=_sub(PeopleConfig, raw.get("people"), "people"),
diff --git a/src/textstrata/git.py b/src/textstrata/git.py
index 08a4c4f..85a44e5 100644
--- a/src/textstrata/git.py
+++ b/src/textstrata/git.py
@@ -144,4 +144,4 @@ def diff_u0(repo: Path, sha: str, path: str) -> str:
 
 def log_range(repo: Path, rng: str, path: str) -> list[tuple[str, str, str]]:
     out = run(repo, "log", f"--format=%H{SEP}%aI{SEP}%s", rng, "--", path)
-    return [tuple(l.split(SEP, 2)) for l in out.splitlines() if l.strip()]  # type: ignore[misc]
+    return [tuple(ln.split(SEP, 2)) for ln in out.splitlines() if ln.strip()]  # type: ignore[misc]
diff --git a/src/textstrata/scan.py b/src/textstrata/scan.py
index ca9da01..0515579 100644
--- a/src/textstrata/scan.py
+++ b/src/textstrata/scan.py
@@ -177,8 +177,8 @@ def tier_for(f: str, sha: str) -> str:
                 hunks = parse_hunks(diff_u0(repo, c.sha, c.path))
             except GitError:
                 continue
-            p_adds = sum(1 for h in hunks for l in h.new if prose.is_prose(l))
-            p_dels = sum(1 for h in hunks for l in h.old if prose.is_prose(l))
+            p_adds = sum(1 for h in hunks for ln in h.new if prose.is_prose(ln))
+            p_dels = sum(1 for h in hunks for ln in h.old if prose.is_prose(ln))
             churn[tier] += p_adds + p_dels
             row = rows_by_key[(f, c.sha)]
             row["prose_adds"], row["prose_dels"] = p_adds, p_dels
diff --git a/tests/test_units.py b/tests/test_units.py
index 15e3f52..96bb2a3 100644
--- a/tests/test_units.py
+++ b/tests/test_units.py
@@ -1,4 +1,6 @@
-from textstrata.config import Config, ProseConfig
+import pytest
+
+from textstrata.config import Config, ConfigError, ProseConfig, load_config
 from textstrata.git import Commit, parse_trailers
 from textstrata.pairs import categorise, line_pairs, parse_hunks
 from textstrata.prose import Prose
@@ -52,8 +54,10 @@ def make_ctx(tmp_path, overrides=None):
     ed = Person(id="ed", role="editor", emails=["ed@example.org"])
     tr = Person(id="tr", role="translator", emails=["tr@example.org"])
     for p in (ed, tr):
-        roster.people.append(p); roster.by_handle[p.id] = p
-        for e in p.emails: roster.by_email[e] = p
+        roster.people.append(p)
+        roster.by_handle[p.id] = p
+        for e in p.emails:
+            roster.by_email[e] = p
     return TierContext(cfg, roster, overrides or {})
 
 
@@ -86,5 +90,16 @@ def test_tier_precedence(tmp_path):
 
 
 def test_noreply_handle_resolution():
-    r = Roster(); p = Person(id="HumphreyYang", role="editor"); r.by_handle["humphreyyang"] = p
+    r = Roster()
+    p = Person(id="HumphreyYang", role="editor")
+    r.by_handle["humphreyyang"] = p
     assert r.resolve_email("39026988+HumphreyYang@users.noreply.github.com") is p
+
+
+def test_baseline_strategy_validated(tmp_path):
+    (tmp_path / ".git").mkdir()
+    for strategy, msg in (("state-file", "not implemented"), ("bogus", "must be script-jump")):
+        p = tmp_path / f"{strategy}.yml"
+        p.write_text(f"name: t\nrepo: {tmp_path}\nbaseline: {{strategy: {strategy}}}\n", encoding="utf-8")
+        with pytest.raises(ConfigError, match=msg):
+            load_config(p)