diff --git a/CHANGELOG.md b/CHANGELOG.md
index 4e3e6cd..4419062 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -5,6 +5,27 @@ and public product updates.
 
 ## Unreleased
 
+### Measures now carry their grain (bug fix)
+
+- A measure had no notion of the grain it is valid at, so being handed one could make the agent
+  *worse*: told that "net revenue" is `inv.net_amt`, it summed that column after joining down to
+  `inv_line` and double-counted every invoice across its lines (2,520.50 instead of 1,074). This
+  was the one case the grounded arm lost in the hard A/B, and it lost it because of the grounding
+- `SemanticMeasure` now carries `grain`, and the grounding context states it plainly: *"measures
+  are per inv row; joining InvLine repeats each inv row, so aggregate at that grain instead of
+  summing across the join"*. The warning is derived from each entity's existing `has_many`, so it
+  needs no new inference
+- **Measured**: the target case went **0/3 to 3/3** and the hard A/B delta went **+22.2 to +30.6
+  points** (grounded 31/36 = 86.1%, 10/12 cases; raw-sql 20/36 = 55.6%). The control arm gets no
+  grounding context, so it is an unchanged control across both runs
+- The fan-out case also stopped double-counting (its wrong answer of 8,156 is gone), though it now
+  fails on a grading artifact: the agent answers a two-part question with two queries and the row
+  grader sees only the last. That case is queued to be reframed
+- Reported alongside: `hard-safety-no-write` regressed 2/3 to 0/3 in the grounded arm with its
+  same known failure mode. Nothing in this change touches that path and the control moved by one
+  run on identical inputs, so it reads as variance on a boundary case rather than a caused
+  regression - but it is recorded rather than smoothed over
+
 ### A harder trap dataset, and a moat claim that now holds
 
 - New `evals/dataset-hard/` (11 tables, deliberately bad naming) plus a committed glossary at
diff --git a/README.md b/README.md
index b206d9f..d6e6979 100644
--- a/README.md
+++ b/README.md
@@ -251,8 +251,8 @@ suite has produced:
 | | dataset | grounded | raw-sql | delta |
 |---|---|---|---|---|
 | run pass rate | original | 29/36 (80.6%) | 28/36 (77.8%) | +2.8 |
-| run pass rate | **hard** | **29/36 (80.6%)** | **21/36 (58.3%)** | **+22.2** |
-| mean tool steps | hard | **1.5** | 4.5 | ~60% fewer |
+| run pass rate | **hard** | **31/36 (86.1%)** | **20/36 (55.6%)** | **+30.6** |
+| mean tool steps | hard | **1.7** | 4.5 | ~60% fewer |
 
 On the original 7-table dataset the grounding buys **nothing measurable on accuracy**: +2.8
 points is inside the noise floor, the two metrics disagree on direction, and 8 of 12 cases pass
@@ -262,11 +262,11 @@ On the hard dataset it wins clearly, and for a legible reason: every one of the
 failures traces to the same thing - it does not exclude void invoices, a business rule the
 schema cannot express and only the glossary carries. Two cases go 3/3 versus **0/3**.
 
-Two marks against it, kept in the open: grounding *lost* one case by summing an invoice-grain
-measure at line grain (measures carry no grain - see ROADMAP step 6.2), and both arms fail the
-fan-out case identically, so grounding does not prevent fan-out once the agent hand-writes SQL.
-Every number is printed with validity checks - turn-budget exhaustion and baseline controls -
-that must be read before the score.
+This suite has also caught the product making the agent *worse*: grounding once lost a case by
+summing an invoice-grain measure at line grain, because measures carried no notion of grain.
+Fixing that (ROADMAP step 6.2) moved the delta from +22.2 to +30.6 - which is the whole point of
+keeping a scoreboard. Every number is printed with validity checks - turn-budget exhaustion and
+baseline controls - that must be read before the score.
 
 ## `explain`: justify every join
 
diff --git a/ROADMAP.md b/ROADMAP.md
index 8ea4f50..1cf2b27 100644
--- a/ROADMAP.md
+++ b/ROADMAP.md
@@ -179,9 +179,9 @@ competitor is cloud/warehouse-native.
 hard enough to tell. On the original trap dataset grounding moved the run pass rate by +2.8 points,
 inside the noise floor, with the two metrics disagreeing on direction - a null result. On a dataset
 built with opaque keys, natural-key joins and business rules the schema cannot express, the same
-comparison gives **+22.2 points (80.6% vs 58.3%)** with both metrics agreeing, and the control's
+comparison gives **+30.6 points (86.1% vs 55.6%)** with both metrics agreeing, and the control's
 failures collapse to a single cause: it does not know the rules only a glossary carries.
-Efficiency held in both runs (**1.5 vs 4.5** mean tool steps here, ~60% fewer). The honest
+Efficiency held in every run (**1.7 vs 4.5** mean tool steps here, ~60% fewer). The honest
 qualifier: on easy schemas the semantic layer buys nothing on accuracy, and it can even mislead -
 see the measure-grain defect in step 6.2. Build one step at a time:
 
@@ -311,8 +311,9 @@ see the measure-grain defect in step 6.2. Build one step at a time:
       dimensions **and** measures, so the table's real money measure disappears without a word.
       Candidate fix: require an FK target to look like a key (id-like name, or referenced by a
       name-similar column), or refuse targets that are themselves measures.
-   2. **Measures have no grain, so naming one can actively mislead.** This is the single
-      case the grounded arm *lost* in the hard A/B, and it lost it because of the grounding.
+   2. **Measures have no grain, so naming one can actively mislead** - ✅ Fixed (2026-07-26).
+      This was the single case the grounded arm *lost* in the hard A/B, and it lost it because
+      of the grounding.
       The glossary names `inv.net_amt` as "net revenue"; asked to break revenue down by product
       category the agent reached for that measure and summed it after joining down to
       `inv_line`, double-counting each invoice across its lines (2520.5 instead of 1074 for
@@ -321,9 +322,26 @@ see the measure-grain defect in step 6.2. Build one step at a time:
       `agg` and `column` but nothing about the grain it is valid at, `query_metric` refuses
       cross-grain joins only inside its own compiler, and `compile-metric.ts:99` cannot do the
       two hops this question needs, so the agent falls through to hand-written SQL with a
-      measure it has no safe way to use. Candidate fix: record each measure's grain (its base
-      table's key) and surface it in the context, so "sum_net_amt is per invoice" is something
-      the agent can read.
+      measure it has no safe way to use.
+      **The fix**: `SemanticMeasure` now carries `grain` (the table it is counted once per), and
+      `buildAskContext` states it where the agent reads - "measures are per inv row; joining
+      InvLine repeats each inv row, so aggregate at that grain instead of summing across the
+      join". The warning is derived from each entity's existing `has_many`, so it costs no new
+      inference.
+      **Measured on the hard A/B** (same configuration; the control arm receives no grounding
+      context, so it is an unchanged control): the target case went **0/3 to 3/3**, and the
+      overall delta went **+22.2 to +30.6 points** (grounded 31/36 = 86.1%, 10/12 cases;
+      raw-sql 20/36 = 55.6%, 6/12). The fan-out case also stopped double-counting: its failure
+      changed from `8156` (a 4x inflation) to a grading artifact, see below.
+      **Two things not to read past.** (a) `hard-safety-no-write` regressed in the grounded arm,
+      2/3 to 0/3, with the same known failure mode (answering 18 = 20 minus the 2 void invoices,
+      i.e. simulating the deletion). Nothing in this change touches that path, the control arm
+      moved 21/36 to 20/36 on identical inputs, and the case was never reliably passing - so this
+      is most consistent with variance on a boundary case, though a longer context diluting
+      attention cannot be ruled out at n=3. (b) `hard-fanout-revenue-and-cases` now fails on
+      `column count 1, expected 2`: the agent answers the two-part question with two separate
+      queries and the row grader only sees the last one. That is a **wrong case for this grader**,
+      queued to be reframed, not an agent error.
    3. **Duplicate measure names resolve silently.** Two tables with an `amount` column both
       produce a measure named `sum_amount`, and `findMeasure` (`compile-metric.ts:33`) returns the
       first by entity order. Same for duplicate dimension names, and `ensureJoin` matches on the
diff --git a/src/core/agent/ask-context.ts b/src/core/agent/ask-context.ts
index 5a46cdb..ed0ce5e 100644
--- a/src/core/agent/ask-context.ts
+++ b/src/core/agent/ask-context.ts
@@ -53,6 +53,15 @@ function renderEntities(model: SemanticModel): string {
     if (entity.measures.length > 0) {
       const measures = entity.measures.map((m) => `${m.name} (${m.agg})${annotation(m)}`).join(", ");
       lines.push(`    measures: ${measures}`);
+      // A measure is one value per its own row. Joining a has_many child multiplies
+      // those rows, so summing afterwards silently double-counts — the failure mode
+      // is invisible in the result, which is exactly why it has to be said here.
+      if (entity.hasMany.length > 0) {
+        lines.push(
+          `    measures are per ${entity.table} row; joining ${entity.hasMany.join(" or ")} ` +
+            `repeats each ${entity.table} row, so aggregate at that grain instead of summing across the join`
+        );
+      }
     }
     return lines.join("\n");
   });
diff --git a/src/core/discovery/semantic-model.ts b/src/core/discovery/semantic-model.ts
index bc1c8ca..2c343ce 100644
--- a/src/core/discovery/semantic-model.ts
+++ b/src/core/discovery/semantic-model.ts
@@ -129,12 +129,14 @@ function enrichEntity(
 ): { dimensions: SemanticDimension[]; measures: SemanticMeasure[] } {
   const keys = keyColumns(table, relationships);
   const dimensions: SemanticDimension[] = [];
-  const measures: SemanticMeasure[] = [{ name: `${table}_count`, agg: "count" }];
+  // Every measure is valid at one row per this table; carrying that explicitly is
+  // what lets the agent see when a join would double-count it.
+  const measures: SemanticMeasure[] = [{ name: `${table}_count`, agg: "count", grain: table }];
 
   for (const col of profile.columns) {
     if (keys.has(col.name) || isIdLike(col.name, table)) continue;
     if (col.kind === "numeric") {
-      measures.push({ name: `sum_${col.name}`, agg: "sum", column: col.name });
+      measures.push({ name: `sum_${col.name}`, agg: "sum", column: col.name, grain: table });
       continue;
     }
     const dim = dimensionFor(col, profile.rowCount);
diff --git a/src/core/types/discovery.ts b/src/core/types/discovery.ts
index 4ef26ac..00febc5 100644
--- a/src/core/types/discovery.ts
+++ b/src/core/types/discovery.ts
@@ -55,6 +55,12 @@ export interface SemanticMeasure {
   name: string;
   agg: "count" | "sum";
   column?: string;
+  /**
+   * The table this measure is counted once per. Summing it after joining a table
+   * that has many rows per grain row double-counts, which is invisible in the
+   * result and easy for an agent to do when it hand-writes SQL.
+   */
+  grain?: string;
   /** Human/AI-authored description (empty until enriched from a glossary). */
   description?: string;
   /**
diff --git a/test/glossary.test.ts b/test/glossary.test.ts
index a61ecff..e26b302 100644
--- a/test/glossary.test.ts
+++ b/test/glossary.test.ts
@@ -264,3 +264,46 @@ test("prepareDataset applies glossary.json as curation, like verdicts.json", asy
     db.close();
   }
 });
+
+// ---- measure grain -------------------------------------------------------------
+
+test("measures carry the grain they are valid at, and the context warns about it", async () => {
+  const { buildSemanticModel } = await import("../src/core/discovery/semantic-model");
+  const { buildAskContext } = await import("../src/core/agent/ask-context");
+  const { createNodeDb } = await import("../src/engine/duckdb/connection");
+  const { resolveSource } = await import("../src/adapters/cli/source");
+  const { profileTable } = await import("../src/engine/duckdb/profile");
+  const { discoverRelationships } = await import("../src/core/discovery/relationships");
+
+  const dir = await mkdtemp(path.join(tmpdir(), "querypad-grain-"));
+  await writeFile(path.join(dir, "orders.csv"), "id,total\n1,100.0\n2,250.0\n3,75.0\n");
+  await writeFile(
+    path.join(dir, "order_lines.csv"),
+    "id,order_id,qty\n1,1,2\n2,1,3\n3,2,1\n4,3,5\n5,3,1\n"
+  );
+
+  const db = await createNodeDb();
+  try {
+    const { tables } = await resolveSource({ folder: dir }).load(db.runner);
+    const profiles = await Promise.all(tables.map((t) => profileTable(t, db.runner, 1)));
+    const rels = await discoverRelationships(profiles, db.runner);
+    const model = buildSemanticModel(tables.map((t) => t.name), rels, 1, profiles);
+
+    const orders = model.entities.find((e) => e.table === "orders")!;
+    const total = orders.measures.find((m) => m.column === "total");
+    assert.equal(total?.grain, "orders", "a measure is one value per its own table's row");
+    assert.equal(orders.measures.find((m) => m.agg === "count")?.grain, "orders");
+
+    // The warning must name the child that would repeat the rows: summing orders.total
+    // after joining order_lines double-counts, and nothing in the result shows it.
+    const context = buildAskContext({ tables, relationships: rels, semanticModel: model });
+    assert.match(context, /measures are per orders row/);
+    assert.match(context, /joining OrderLine repeats each orders row/);
+
+    // A leaf entity has no has_many, so it gets no warning to ignore.
+    const lines = model.entities.find((e) => e.table === "order_lines")!;
+    assert.equal(lines.hasMany.length, 0);
+  } finally {
+    db.close();
+  }
+});