diff --git a/docs/CHANGELOG.md b/docs/CHANGELOG.md
index a389b71d0..705aa8663 100644
--- a/docs/CHANGELOG.md
+++ b/docs/CHANGELOG.md
@@ -10,6 +10,7 @@
 * `rugnux` writes a batch header in the unmerged MTZ for every image the observations span, not only for the images that produced one, so a scaling program reads one run per sweep.
 * `rugnux` leaves an event out of the unmerged MTZ when less of its rocking curve was captured than `--min-captured-fraction`, as the merge does, since the exported rows are declared full.
 * `rugnux` writes `FreeR_flag` with 0 for the test set and 1 for the working set, the CCP4 convention REFMAC5 defaults to; it was the other way round.
+* `rugnux` marks the free set in the merged mmCIF as `_refln.status` = `f` instead of a `_refln.status_free` column that is not in the PDBx dictionary and that `cif2mtz` refused to read.
 * `rugnux -S` refuses or re-seats a fixed space group whose symmetry axes the indexed cell does not carry, not only one whose centring differs; `--mode scale` refuses it too.
 * The rugnux results report names the groups the data cannot separate, the enantiomorph state and any refused higher point group as `SPACE_GROUP_ALTERNATIVES`, `SPACE_GROUP_ENANTIOMORPH`, `SPACE_GROUP_REFUSED_POINT_GROUP` and `SPACE_GROUP_REFUSED_REASON`; `REPORT_VERSION` is 5.
 * `rugnux` handles symmetry better: the lattice, the point group, the setting and the systematic absences.
diff --git a/docs/CPU_DATA_ANALYSIS_INTEGRATION.md b/docs/CPU_DATA_ANALYSIS_INTEGRATION.md
index ff2a64c5c..ea15fa94b 100644
--- a/docs/CPU_DATA_ANALYSIS_INTEGRATION.md
+++ b/docs/CPU_DATA_ANALYSIS_INTEGRATION.md
@@ -308,7 +308,7 @@ Each batch's $B$ is fitted on **resolution-shell means**, not on single observat
 
 ### 10.7 R-free test-set flags
 
-A fraction of the unique reflections (`rfree_fraction`, default 0.05) is flagged as a **free (test) set**, written to the output (MTZ `FreeR_flag`, mmCIF `_refln.status_free`, a text-HKL column) for model validation (§14) and for downstream refinement. The flag is a pure function of the reflection's **Friedel-merged (Laue) ASU index**, which gives three properties:
+A fraction of the unique reflections (`rfree_fraction`, default 0.05) is flagged as a **free (test) set**, written to the output (MTZ `FreeR_flag`, mmCIF `_refln.status` = `f`, a text-HKL column) for model validation (§14) and for downstream refinement. The flag is a pure function of the reflection's **Friedel-merged (Laue) ASU index**, which gives three properties:
 
 - all symmetry- and Friedel-equivalent reflections share one flag — in particular a Bijvoet pair $I(+)/I(-)$, kept as two separate merged rows in anomalous mode, is **never split** across the work and free sets (which would bias R-free);
 - the free/work decision is a deterministic hash of that key, so the same reflection always lands in the same set — reproducible run-to-run and independent of the order in which observations were merged;
diff --git a/image_analysis/WriteReflections.cpp b/image_analysis/WriteReflections.cpp
index 35ecc1678..c1de19809 100644
--- a/image_analysis/WriteReflections.cpp
+++ b/image_analysis/WriteReflections.cpp
@@ -382,7 +382,6 @@ void WriteMmcifReflections(const std::vector<MergedReflection> &reflections,
     out << "_refln.pdbx_F_plus_sigma\n";
     out << "_refln.pdbx_F_minus\n";
     out << "_refln.pdbx_F_minus_sigma\n";
-    out << "_refln.status_free\n";
     out << "_refln.status\n";
 
     // One row per unique reflection, twelve formatted floats each - tens of megabytes on a crowded
@@ -422,8 +421,10 @@ void WriteMmcifReflections(const std::vector<MergedReflection> &reflections,
                     column(Fmt(r.sigmaF_plus, 4), 14);
                     column(Fmt(r.F_minus, 4), 14);
                     column(Fmt(r.sigmaF_minus, 4), 14);
-                    s.push_back(r.rfree_flag ? '1' : '0');
-                    s.append(" o\n");   // 'o' = observed
+                    // PDBx/mmCIF _refln.status: 'f' marks the free (test) set, 'o' the working
+                    // set. This is the only place the free set is carried in the file - cif2mtz and
+                    // gemmi both read it, and neither knows any other reflection-level free-set item.
+                    s.append(r.rfree_flag ? "f\n" : "o\n");
                 }
             }
         });