diff --git a/ROADMAP.md b/ROADMAP.md
index bab0d924..5ae3b161 100644
--- a/ROADMAP.md
+++ b/ROADMAP.md
@@ -122,6 +122,7 @@ mutated and truncated movies. Field reports from the first public week are track
 
 ## Progress log
 
+- **2026-10-13:** `mix.instrument` takes the built-in instrument's parameters in `params` (validated, clamped) and refuses any other field, so a synth setting written beside `plugin` is an error instead of being accepted and ignored (#265)
 - **2026-10-12:** `soundcraft-cli convert` rejects an unparsable or zero `--rate` and an unknown `--bit-depth` (exit 1, nothing written) instead of converting with a default (#256)
 - **2026-10-12:** Timecode labels with huge fields no longer overflow: frame counts are computed in 128 bits and clamp, so `setup.current_timecode` and `session.format_time` saturate instead of failing (#248)
 - **2026-10-12:** `session.format_time` with the smallest bar number no longer fails with an internal error: the bar offset saturates (#249)
@@ -146,6 +147,7 @@ mutated and truncated movies. Field reports from the first public week are track
 
 | Date | Change | Summary |
 |---|---|---|
+| 2026-10-13 | trivial | Progress log: mix.instrument params, unknown fields refused (#265) |
 | 2026-10-12 | trivial | Progress log: convert option validation (#256) |
 | 2026-10-12 | trivial | Progress log: timecode overflow (#248) |
 | 2026-10-12 | trivial | Progress log: bar offset saturates (#249) |
diff --git a/docs/plugin-parity.md b/docs/plugin-parity.md
index f59b5981..ce995ea1 100644
--- a/docs/plugin-parity.md
+++ b/docs/plugin-parity.md
@@ -1,6 +1,6 @@
 # Plugin parity: built-in processors and plugin hosting
 
-> **Last reviewed:** 2026-10-10 · **Last updated:** 2026-10-10 · **Change:** major (first plugin checklist against Pro Tools 2026.4.1) · **Target:** Avid Pro Tools Ultimate 2026.4.1
+> **Last reviewed:** 2026-10-10 · **Last updated:** 2026-10-13 · **Change:** trivial (mix.instrument sets built-in instrument parameters in params (#265); previously major: first plugin checklist against Pro Tools 2026.4.1) · **Target:** Avid Pro Tools Ultimate 2026.4.1
 
 Pro Tools users live in plugins: the bundled set covers everyday mixing, and third-party plugins
 cover the rest. This doc tracks both. Our plugins have our own names and are written from
@@ -21,7 +21,7 @@ scratch (clean-room rule): Avid plugin names are not used here, only categories.
 | Harmonic | lo-fi, rectifier, amp simulation, saturation, tape | `lofi`, `rectifier`, `saturator` | Amp and cabinet models, tape |
 | Pitch and time | pitch shift, varispeed effects, time shift | `pitch_shifter`, `time_shift` | Formant control, varispeed effect, pitch correction |
 | Utility | trim, gain, invert, DC offset, signal generator, dither, time adjuster | `trim`, `gain`, `invert`, `dc_offset_removal`, `signal_generator`, `dither` | Time adjuster, meter/analyzer plugins, noise shaping options |
-| Instruments | several virtual instruments (multi-sampler, piano, drum machine, organ, synths, sample player; Massive X Player added in 2026.4) | `subtractive_synth`, `drum_synth` | Sampler (PR #108 open), piano, organ, wavetable, content library |
+| Instruments | several virtual instruments (multi-sampler, piano, drum machine, organ, synths, sample player; Massive X Player added in 2026.4) | `subtractive_synth`, `drum_synth` | Sampler (PR #108 open), piano, organ, wavetable, content library `mix.instrument` takes the instrument's parameters in `params` and refuses unknown fields (#265). |
 | **Total** | **≈ 80 processors and instruments** (estimate from Avid's bundle documentation) | **26 processors + 2 instruments** | |
 
 AudioSuite: every catalog AudioSuite leaf maps to one of our processes (`AUDIOSUITE` in
@@ -58,4 +58,5 @@ User report #152: a saved instrument level does not reach the bounce.
 
 | Date | Change | Summary |
 |---|---|---|
+| 2026-10-13 | trivial | mix.instrument sets built-in instrument parameters in params (#265) |
 | 2026-10-10 | major | Created: built-in processor inventory by category and third-party hosting checklist |