diff --git a/apps/photocraft-cli/tests/cli.rs b/apps/photocraft-cli/tests/cli.rs
index b71c0f9a..ea330ac1 100644
--- a/apps/photocraft-cli/tests/cli.rs
+++ b/apps/photocraft-cli/tests/cli.rs
@@ -221,6 +221,24 @@ fn run_errors() {
     assert!(e.contains("layer.new.layer") && e.contains("must be a JSON object"), "{e}");
 }
 
+#[test]
+fn run_rejects_invalid_filter_parameters_without_exporting() {
+    let d = tmp("invalid-filter-params");
+    let out = d.join("result.png");
+    for (params, key) in [(r#"{"radius":"big"}"#, "radius"), (r#"{"radus":3}"#, "radus"), (r#"{"radius":5000}"#, "radius"), (r#"{"radius":-4}"#, "radius")] {
+        let result = bin()
+            .args(["run", "--new", r#"{"width":8,"height":8}"#, "--cmd", "filter.blur.gaussianBlur", "--params", params, "--out"])
+            .arg(&out)
+            .output()
+            .unwrap();
+        assert_eq!(result.status.code(), Some(1));
+        let error = String::from_utf8_lossy(&result.stderr);
+        assert!(error.contains("filter.blur.gaussianBlur") && error.contains(key), "{error}");
+        assert!(!out.exists());
+    }
+    std::fs::remove_dir_all(d).unwrap();
+}
+
 #[test]
 fn batch_applies_actions_to_directory() {
     let d = tmp("batch");
diff --git a/crates/automation/src/rpc.rs b/crates/automation/src/rpc.rs
index 520d3715..597494b2 100644
--- a/crates/automation/src/rpc.rs
+++ b/crates/automation/src/rpc.rs
@@ -306,6 +306,25 @@ mod tests {
         Mutex::new(Headless::trusted_local())
     }
 
+    #[test]
+    fn execute_reports_invalid_filter_parameters_without_changing_history() {
+        let h = session();
+        assert_eq!(respond(&h, r#"{"method":"doc.new","params":{"width":8,"height":8}}"#)["ok"], true);
+        let before = h.lock().unwrap().inspect(None).unwrap();
+        for (params, key) in
+            [(json!({"radius":"big"}), "radius"), (json!({"radus":3}), "radus"), (json!({"radius":5000}), "radius"), (json!({"radius":-4}), "radius")]
+        {
+            for wait in [true, false] {
+                let request = json!({"method":"engine.execute","params":{"command":"filter.blur.gaussianBlur","params":params,"wait":wait}});
+                let reply = respond(&h, &request.to_string());
+                assert_eq!(reply["ok"], false);
+                let error = reply["error"].as_str().unwrap();
+                assert!(error.contains("filter.blur.gaussianBlur") && error.contains(key), "{error}");
+                assert_eq!(h.lock().unwrap().inspect(None).unwrap(), before);
+            }
+        }
+    }
+
     #[test]
     fn lines_round_trip_and_errors() {
         let h = session();
diff --git a/crates/automation/src/server.rs b/crates/automation/src/server.rs
index 41e282f2..7165636a 100644
--- a/crates/automation/src/server.rs
+++ b/crates/automation/src/server.rs
@@ -894,6 +894,27 @@ mod tests {
         assert!(downscale_png(&png, 1024).is_err());
     }
 
+    #[tokio::test(flavor = "multi_thread")]
+    async fn mcp_reports_invalid_filter_parameters_without_changing_history() {
+        let mcp = PhotocraftMcp::headless();
+        mcp.headless_op(|h| h.command_run("file.new", json!({"width":8,"height":8}))).await.unwrap().unwrap();
+        let before = mcp.headless_op(|h| h.inspect(None)).await.unwrap().unwrap();
+        for (params, key) in
+            [(json!({"radius":"big"}), "radius"), (json!({"radus":3}), "radus"), (json!({"radius":5000}), "radius"), (json!({"radius":-4}), "radius")]
+        {
+            for wait in [true, false] {
+                let result = mcp
+                    .command_run(Parameters(RunParams { id: "filter.blur.gaussianBlur".into(), params: Some(params.clone()), wait: Some(wait) }))
+                    .await
+                    .unwrap();
+                assert_eq!(result.is_error, Some(true));
+                let error = &result.content.first().and_then(|c| c.as_text()).unwrap().text;
+                assert!(error.contains("filter.blur.gaussianBlur") && error.contains(key), "{error}");
+                assert_eq!(mcp.headless_op(|h| h.inspect(None)).await.unwrap().unwrap(), before);
+            }
+        }
+    }
+
     #[tokio::test(flavor = "multi_thread")]
     async fn a_panicking_command_does_not_wedge_the_session() {
         let mcp = PhotocraftMcp::headless();
diff --git a/crates/engine/src/filters.rs b/crates/engine/src/filters.rs
index 379a6b7a..c34c9e56 100644
--- a/crates/engine/src/filters.rs
+++ b/crates/engine/src/filters.rs
@@ -13,6 +13,9 @@ use serde_json::{Value, json};
 use crate::commands::CommandSpec;
 use crate::{EngineError, Result, Session};
 
+mod validation;
+pub(crate) use validation::validate_params;
+
 fn f(p: &Value, k: &str, d: f32) -> f32 {
     p.get(k).and_then(Value::as_f64).map_or(d, |v| v as f32)
 }
@@ -27,6 +30,10 @@ fn b(p: &Value, k: &str, d: bool) -> bool {
 fn s<'a>(p: &'a Value, k: &str, d: &'a str) -> &'a str {
     p.get(k).and_then(Value::as_str).unwrap_or(d)
 }
+fn seed(p: &Value) -> u32 {
+    // New calls accept the entire documented u32 range; keep legacy record decoding otherwise.
+    p.get("seed").and_then(Value::as_u64).and_then(|n| u32::try_from(n).ok()).unwrap_or_else(|| i(p, "seed", 0) as u32)
+}
 fn undefined(p: &Value) -> UndefinedAreas {
     match s(p, "undefinedAreas", "wrap") {
         "repeat" => UndefinedAreas::Repeat,
@@ -55,7 +62,8 @@ fn preset(id: &str) -> Option<(&'static str, FilterParams)> {
 }
 
 /// Builds the algorithm parameters for a filter command id from JSON params
-/// (Photoshop dialog units).
+/// (Photoshop dialog units). Stored smart filters and previews retain their tolerant decoding;
+/// new command invocations pass through `validate_params` before any document changes.
 pub fn params_for(id: &str, p: &Value) -> Option<FilterParams> {
     if let Some((_, fp)) = preset(id) {
         return Some(fp);
@@ -94,7 +102,7 @@ pub fn params_for(id: &str, p: &Value) -> Option<FilterParams> {
             amount: f(p, "amount", 12.5).clamp(0.1, 400.0),
             distribution: if s(p, "distribution", "uniform") == "gaussian" { Distribution::Gaussian } else { Distribution::Uniform },
             monochromatic: b(p, "monochromatic", false),
-            seed: i(p, "seed", 0) as u32,
+            seed: seed(p),
         },
         "filter.noise.median" => FilterParams::Median { radius: f(p, "radius", 1.0).clamp(1.0, 500.0) },
         "filter.noise.dustAndScratches" => {
@@ -131,7 +139,7 @@ pub fn params_for(id: &str, p: &Value) -> Option<FilterParams> {
                 _ => WaveType::Sine,
             },
             undefined: undefined(p),
-            seed: i(p, "seed", 0) as u32,
+            seed: seed(p),
         },
         "filter.distort.ripple" => FilterParams::Ripple {
             amount: f(p, "amount", 100.0).clamp(-999.0, 999.0),
@@ -200,6 +208,7 @@ pub(crate) fn has_filterable_layer(s: &Session) -> std::result::Result<(), Strin
 }
 
 pub(crate) fn run_filter(s: &mut Session, id: &str, p: &Value) -> Result<Value> {
+    validate_params(id, p)?;
     // Session state some filters read (colours) becomes explicit params; external inputs resolve here.
     let prepared = crate::filters_ext::prepare(s, id, p);
     let p = &prepared;
diff --git a/crates/engine/src/filters/validation.rs b/crates/engine/src/filters/validation.rs
new file mode 100644
index 00000000..a0eb171f
--- /dev/null
+++ b/crates/engine/src/filters/validation.rs
@@ -0,0 +1,335 @@
+//! Validate new core-filter invocations, without changing how old smart-filter records decode.
+
+use serde_json::Value;
+
+use crate::{EngineError, Result};
+
+#[derive(Clone, Copy)]
+enum Rule {
+    Number(f64, f64),
+    Unsigned(u64),
+    Choice(&'static str),
+    Bool,
+}
+
+impl Rule {
+    fn accepts(self, value: &Value) -> bool {
+        match self {
+            Self::Number(min, max) => value.as_f64().is_some_and(|n| n.is_finite() && (min..=max).contains(&n)),
+            Self::Unsigned(max) => value.as_u64().is_some_and(|n| n <= max),
+            Self::Choice(choices) => value.as_str().is_some_and(|s| choices.split('|').any(|choice| choice == s)),
+            Self::Bool => value.is_boolean(),
+        }
+    }
+
+    fn expected(self) -> String {
+        match self {
+            Self::Number(min, max) if min == f64::MIN && max == f64::MAX => "a finite number".into(),
+            Self::Number(min, max) => format!("a number in {min}..{max}"),
+            Self::Unsigned(max) => format!("an integer in 0..{max}"),
+            Self::Choice(choices) => format!("one of {choices}"),
+            Self::Bool => "a boolean".into(),
+        }
+    }
+}
+
+// The bounds and choices are the core filters' public dialog-unit contract. Defaults remain in
+// params_for; extended/gallery filters keep their own parsing until their validation is added.
+fn fields(id: &str) -> Option<&'static [(&'static str, Rule)]> {
+    use Rule::{Bool, Choice, Number, Unsigned};
+    Some(match id {
+        "filter.blur.gaussianBlur" | "filter.other.highPass" => &[("radius", Number(0.1, 1000.0))],
+        "filter.blur.boxBlur" => &[("radius", Number(1.0, 2000.0))],
+        "filter.blur.motionBlur" => &[("angle", Number(-360.0, 360.0)), ("distance", Number(1.0, 2000.0))],
+        "filter.blur.radialBlur" => &[
+            ("amount", Number(1.0, 100.0)),
+            ("method", Choice("spin|zoom")),
+            ("quality", Choice("draft|good|best")),
+            ("centerX", Number(0.0, 1.0)),
+            ("centerY", Number(0.0, 1.0)),
+        ],
+        "filter.blur.surfaceBlur" => &[("radius", Number(1.0, 100.0)), ("threshold", Number(2.0, 255.0))],
+        "filter.sharpen.unsharpMask" => &[("amount", Number(1.0, 500.0)), ("radius", Number(0.1, 1000.0)), ("threshold", Number(0.0, 255.0))],
+        "filter.sharpen.smartSharpen" => &[("amount", Number(1.0, 500.0)), ("radius", Number(0.1, 64.0)), ("reduceNoise", Number(0.0, 100.0))],
+        "filter.noise.addNoise" => {
+            &[("amount", Number(0.1, 400.0)), ("distribution", Choice("uniform|gaussian")), ("monochromatic", Bool), ("seed", Unsigned(u32::MAX as u64))]
+        }
+        "filter.noise.median" => &[("radius", Number(1.0, 500.0))],
+        "filter.noise.dustAndScratches" => &[("radius", Number(1.0, 500.0)), ("threshold", Number(0.0, 255.0))],
+        "filter.pixelate.mosaic" => &[("cellSize", Number(2.0, 200.0))],
+        "filter.stylize.emboss" => &[("angle", Number(-180.0, 180.0)), ("height", Number(1.0, 100.0)), ("amount", Number(1.0, 500.0))],
+        "filter.distort.twirl" => &[("angle", Number(-999.0, 999.0))],
+        "filter.distort.pinch" => &[("amount", Number(-100.0, 100.0))],
+        "filter.distort.spherize" => &[("amount", Number(-100.0, 100.0)), ("mode", Choice("normal|horizontalOnly|verticalOnly"))],
+        "filter.distort.wave" => &[
+            ("generators", Number(1.0, 999.0)),
+            ("wavelengthMin", Number(1.0, 998.0)),
+            ("wavelengthMax", Number(2.0, 999.0)),
+            ("amplitudeMin", Number(1.0, 998.0)),
+            ("amplitudeMax", Number(1.0, 999.0)),
+            ("type", Choice("sine|triangle|square")),
+            ("undefinedAreas", Choice("wrap|repeat")),
+            ("seed", Unsigned(u32::MAX as u64)),
+        ],
+        "filter.distort.ripple" => &[("amount", Number(-999.0, 999.0)), ("size", Choice("small|medium|large"))],
+        "filter.distort.polarCoordinates" => &[("mode", Choice("rectangularToPolar|polarToRectangular"))],
+        "filter.other.minimum" | "filter.other.maximum" => &[("radius", Number(0.2, 500.0)), ("preserve", Choice("squareness|roundness"))],
+        "filter.other.offset" => &[
+            // The hint specifies pixels, with no range: keep the existing rounding/saturation.
+            ("horizontal", Number(f64::MIN, f64::MAX)),
+            ("vertical", Number(f64::MIN, f64::MAX)),
+            ("undefinedAreas", Choice("wrap|repeat|transparent")),
+        ],
+        "filter.blur.blur"
+        | "filter.blur.blurMore"
+        | "filter.sharpen.sharpen"
+        | "filter.sharpen.sharpenMore"
+        | "filter.sharpen.sharpenEdges"
+        | "filter.noise.despeckle"
+        | "filter.stylize.findEdges"
+        | "filter.stylize.solarize" => &[],
+        _ => return None,
+    })
+}
+
+/// Runs both at dispatch (before committing a floating selection) and at the shared runner
+/// (direct specs/Last Filter). Filters outside this module's core family are unaffected.
+pub(crate) fn validate_params(id: &str, params: &Value) -> Result<()> {
+    let Some(fields) = fields(id) else { return Ok(()) };
+    let bad = |msg| EngineError::BadParams { cmd: id.into(), msg };
+    if params.is_null() {
+        return Ok(());
+    }
+    let params = params.as_object().ok_or_else(|| bad("params must be a JSON object".into()))?;
+    for (key, value) in params {
+        let (valid, expected) = if let Some((_, rule)) = fields.iter().find(|(name, _)| *name == key) {
+            (rule.accepts(value), rule.expected())
+        } else {
+            match key.as_str() {
+                "layer" => (value.as_u64().is_some(), "an unsigned layer id".into()),
+                "coalesce" => (value.is_string(), "a string".into()),
+                "target" => (
+                    value.as_str().is_some_and(|s| matches!(s, "pixels" | "mask" | "quickMask"))
+                        || value.as_object().is_some_and(|o| o.len() == 1 && o.get("channel").and_then(Value::as_u64).is_some()),
+                    "pixels, mask, quickMask or {\"channel\":index}".into(),
+                ),
+                _ => return Err(bad(format!("unknown parameter `{key}`"))),
+            }
+        };
+        if !valid {
+            return Err(bad(format!("`{key}` must be {expected}")));
+        }
+    }
+    Ok(())
+}
+
+#[cfg(test)]
+mod tests {
+    use std::sync::Arc;
+
+    use photocraft_geom::Rect;
+    use serde_json::json;
+
+    use super::*;
+    use crate::Session;
+
+    fn session(depth: u32) -> Session {
+        let mut s = Session::new();
+        s.execute("file.new", json!({"width":16,"height":12,"depth":depth})).unwrap();
+        s.execute("layer.new.layer", json!({})).unwrap();
+        s.edit("pattern", |doc, active| {
+            let surface = doc.layer_mut(active.unwrap()).unwrap().surface_mut().unwrap();
+            surface.fill_rect(Rect::new(0, 0, 8, 12), &[0.2, 0.6, 0.8, 1.0]);
+            Ok(())
+        })
+        .unwrap();
+        s
+    }
+
+    fn rejected(s: &mut Session, id: &str, key: &str, value: Value) {
+        let doc = s.active().unwrap().doc.clone();
+        let history = s.active().unwrap().history.entries();
+        let journal = s.journal.clone();
+        let active = s.active().unwrap().active_layer;
+        let error = s.execute(id, json!({key:value})).unwrap_err();
+        assert!(matches!(error, EngineError::BadParams { .. }), "{id}: {error}");
+        let error = error.to_string();
+        assert!(error.contains(id) && error.contains(key), "{error}");
+        assert!(Arc::ptr_eq(&s.active().unwrap().doc, &doc), "{id}: {key}");
+        assert_eq!(s.active().unwrap().history.entries(), history);
+        assert_eq!(s.active().unwrap().active_layer, active);
+        assert_eq!(s.journal, journal);
+        assert!(s.jobs().is_empty());
+    }
+
+    #[test]
+    fn every_core_filter_rejects_unknown_keys_without_editing() {
+        for depth in [8, 16, 32] {
+            let mut s = session(depth);
+            for spec in crate::filters::specs().into_iter().filter(|s| s.id != "filter.lastFilter") {
+                rejected(&mut s, spec.id, "radus", json!(3));
+            }
+        }
+    }
+
+    #[test]
+    fn core_filter_numbers_reject_wrong_types_and_documented_range_violations() {
+        // Each numeric field in the core registry, independently of the validation table.
+        let cases = [
+            ("blur.gaussianBlur", "radius", 0.1, 1000.0),
+            ("blur.boxBlur", "radius", 1.0, 2000.0),
+            ("blur.motionBlur", "angle", -360.0, 360.0),
+            ("blur.motionBlur", "distance", 1.0, 2000.0),
+            ("blur.radialBlur", "amount", 1.0, 100.0),
+            ("blur.radialBlur", "centerX", 0.0, 1.0),
+            ("blur.radialBlur", "centerY", 0.0, 1.0),
+            ("blur.surfaceBlur", "radius", 1.0, 100.0),
+            ("blur.surfaceBlur", "threshold", 2.0, 255.0),
+            ("sharpen.unsharpMask", "amount", 1.0, 500.0),
+            ("sharpen.unsharpMask", "radius", 0.1, 1000.0),
+            ("sharpen.unsharpMask", "threshold", 0.0, 255.0),
+            ("sharpen.smartSharpen", "amount", 1.0, 500.0),
+            ("sharpen.smartSharpen", "radius", 0.1, 64.0),
+            ("sharpen.smartSharpen", "reduceNoise", 0.0, 100.0),
+            ("noise.addNoise", "amount", 0.1, 400.0),
+            ("noise.median", "radius", 1.0, 500.0),
+            ("noise.dustAndScratches", "radius", 1.0, 500.0),
+            ("noise.dustAndScratches", "threshold", 0.0, 255.0),
+            ("pixelate.mosaic", "cellSize", 2.0, 200.0),
+            ("stylize.emboss", "angle", -180.0, 180.0),
+            ("stylize.emboss", "height", 1.0, 100.0),
+            ("stylize.emboss", "amount", 1.0, 500.0),
+            ("distort.twirl", "angle", -999.0, 999.0),
+            ("distort.pinch", "amount", -100.0, 100.0),
+            ("distort.spherize", "amount", -100.0, 100.0),
+            ("distort.wave", "generators", 1.0, 999.0),
+            ("distort.wave", "wavelengthMin", 1.0, 998.0),
+            ("distort.wave", "wavelengthMax", 2.0, 999.0),
+            ("distort.wave", "amplitudeMin", 1.0, 998.0),
+            ("distort.wave", "amplitudeMax", 1.0, 999.0),
+            ("distort.ripple", "amount", -999.0, 999.0),
+            ("other.highPass", "radius", 0.1, 1000.0),
+            ("other.minimum", "radius", 0.2, 500.0),
+            ("other.maximum", "radius", 0.2, 500.0),
+        ];
+        for depth in [8, 16, 32] {
+            let mut s = session(depth);
+            for (suffix, key, min, max) in cases {
+                let id = format!("filter.{suffix}");
+                for value in [json!("big"), json!(true), Value::Null, json!([]), json!({}), json!(min - 0.01), json!(max + 0.01)] {
+                    rejected(&mut s, &id, key, value);
+                }
+                // Large limits are validated without allocating a large blur kernel.
+                for value in [min, max] {
+                    validate_params(&id, &json!({key:value})).unwrap();
+                }
+            }
+            for key in ["horizontal", "vertical"] {
+                rejected(&mut s, "filter.other.offset", key, json!("4"));
+            }
+        }
+    }
+
+    #[test]
+    fn core_filter_choices_booleans_seeds_and_shared_fields_are_validated() {
+        let mut s = session(8);
+        for (suffix, key, choices) in [
+            ("blur.radialBlur", "method", "spin|zoom"),
+            ("blur.radialBlur", "quality", "draft|good|best"),
+            ("noise.addNoise", "distribution", "uniform|gaussian"),
+            ("distort.spherize", "mode", "normal|horizontalOnly|verticalOnly"),
+            ("distort.wave", "type", "sine|triangle|square"),
+            ("distort.wave", "undefinedAreas", "wrap|repeat"),
+            ("distort.ripple", "size", "small|medium|large"),
+            ("distort.polarCoordinates", "mode", "rectangularToPolar|polarToRectangular"),
+            ("other.minimum", "preserve", "squareness|roundness"),
+            ("other.maximum", "preserve", "squareness|roundness"),
+            ("other.offset", "undefinedAreas", "wrap|repeat|transparent"),
+        ] {
+            let id = format!("filter.{suffix}");
+            for value in [json!("typo"), json!(false), json!(1), Value::Null] {
+                rejected(&mut s, &id, key, value);
+            }
+            for value in choices.split('|') {
+                validate_params(&id, &json!({key:value})).unwrap();
+            }
+        }
+        for value in [json!("true"), json!(0), Value::Null] {
+            rejected(&mut s, "filter.noise.addNoise", "monochromatic", value);
+        }
+        for id in ["filter.noise.addNoise", "filter.distort.wave"] {
+            for value in [json!(-1), json!(4_294_967_296_u64), json!(0.5), json!("12")] {
+                rejected(&mut s, id, "seed", value);
+            }
+            // A valid u32 seed must not be saturated through an i32 helper.
+            let result = s.execute(id, json!({"seed":u32::MAX})).unwrap();
+            assert_eq!(result["filter"]["seed"], json!(u32::MAX));
+        }
+        for (key, value) in [("layer", json!("2")), ("target", json!("typo")), ("target", json!({"channel":-1})), ("coalesce", json!(false))] {
+            rejected(&mut s, "filter.blur.gaussianBlur", key, value);
+        }
+    }
+
+    #[test]
+    fn invalid_filters_leave_floating_selection_and_background_jobs_alone() {
+        let mut s = session(16);
+        s.execute("select.rect", json!({"x":0,"y":0,"width":4,"height":4})).unwrap();
+        s.execute("select.float", json!({"dx":8})).unwrap();
+        let doc = s.active().unwrap().doc.clone();
+        let history = s.active().unwrap().history.entries();
+        for params in [json!({"radius":"big"}), json!({"radus":3}), json!({"radius":5000}), json!({"radius":-4})] {
+            assert!(s.start("filter.blur.gaussianBlur", params.clone()).is_err());
+            let spec = crate::commands::find("filter.blur.gaussianBlur").unwrap();
+            assert!((spec.run)(&mut s, &params).is_err());
+            assert!(crate::float_cmds::floating(s.active().unwrap()).is_some());
+            assert!(Arc::ptr_eq(&s.active().unwrap().doc, &doc));
+            assert_eq!(s.active().unwrap().history.entries(), history);
+            assert!(s.jobs().is_empty());
+        }
+    }
+
+    #[test]
+    fn valid_filters_keep_defaults_explicit_targets_and_undo_redo() {
+        for depth in [8, 16, 32] {
+            let mut s = session(depth);
+            let layer = s.active().unwrap().active_layer.unwrap();
+            let pixels = |s: &Session| s.active().unwrap().doc.layer(layer).unwrap().surface().unwrap().read_region(Rect::new(0, 0, 16, 12));
+            let before = pixels(&s);
+            for params in [Value::Null, json!({})] {
+                let result = s.execute("filter.blur.gaussianBlur", params).unwrap();
+                assert_eq!(result["filter"]["radius"], json!(1.0));
+                assert_ne!(pixels(&s), before);
+                s.undo();
+                assert_eq!(pixels(&s), before);
+            }
+            s.execute("layer.new.layer", json!({})).unwrap();
+            let other = s.active().unwrap().active_layer.unwrap();
+            let result = s.execute("filter.blur.gaussianBlur", json!({"radius":3,"layer":layer.0,"target":"pixels","coalesce":"blur"})).unwrap();
+            assert_eq!(result["filter"]["radius"], json!(3.0));
+            let after = pixels(&s);
+            assert_ne!(after, before);
+            assert_eq!(s.active().unwrap().doc.layer(other).unwrap().surface().unwrap().tile_count(), 0);
+            s.undo();
+            assert_eq!(pixels(&s), before);
+            s.redo();
+            assert_eq!(pixels(&s), after);
+        }
+    }
+
+    #[test]
+    fn injected_channel_targets_and_extended_filter_colours_keep_working() {
+        let mut s = session(8);
+        s.execute("channel.new", json!({"fill":"white"})).unwrap();
+        let pixels = s.active().unwrap().doc.layers.clone();
+        s.execute("filter.noise.addNoise", json!({"amount":40,"seed":2})).unwrap();
+        assert_eq!(s.active().unwrap().doc.layers, pixels);
+        assert!(s.active().unwrap().doc.channels[0].surface.read_region(Rect::new(0, 0, 16, 12)).iter().any(|&v| v < 0.99));
+        s.execute("channel.target", json!({"channel":"composite"})).unwrap();
+        s.tools.foreground = [0.0, 0.0, 1.0, 1.0];
+        s.tools.background = [1.0, 0.0, 0.0, 1.0];
+        let params = crate::filters_ext::prepare(&s, "filter.render.fibers", &json!({}));
+        assert_eq!(params["foreground"], json!(s.tools.foreground));
+        assert_eq!(params["background"], json!(s.tools.background));
+        s.execute("filter.render.fibers", params).unwrap();
+    }
+}
diff --git a/crates/engine/src/jobs.rs b/crates/engine/src/jobs.rs
index 5838b4eb..b469fc77 100644
--- a/crates/engine/src/jobs.rs
+++ b/crates/engine/src/jobs.rs
@@ -618,6 +618,7 @@ impl Session {
         if id == "file.new" {
             crate::document_preset_cmds::validate_new_params(self, &params)?;
         }
+        crate::filters::validate_params(id, &params)?;
         // A floating selection drops before any other command (Undo puts it back instead).
         if let Some(v) = crate::float_cmds::before_command(self, id)? {
             return Ok(Started::Done(v));
diff --git a/crates/ui-egui/src/filter_dialog.rs b/crates/ui-egui/src/filter_dialog.rs
index d2d17695..420abe2e 100644
--- a/crates/ui-egui/src/filter_dialog.rs
+++ b/crates/ui-egui/src/filter_dialog.rs
@@ -527,11 +527,19 @@ pub fn preview_document_with(
     if k > 1
         && let Some(o) = p.as_object_mut()
     {
+        let spec = photocraft_engine::commands::find(command).map(|c| parse_spec(c.params)).unwrap_or_default();
         for (key, v) in o.iter_mut() {
             if is_pixel_param(key)
                 && let Some(x) = v.as_f64()
             {
-                *v = json!((x / k as f64).max(if key == "cellSize" { 1.0 } else { 0.1 }));
+                // Match the engine's minimum after scaling (e.g. Box Blur 1, Mosaic 2).
+                // Previously its tolerant decoder clamped these; new calls validate first.
+                let minimum = spec.iter().find(|p| p.key == key.as_str()).and_then(|p| match &p.kind {
+                    Kind::Range { min, .. } => Some(f64::from(*min)),
+                    _ => None,
+                });
+                let floor = minimum.unwrap_or(0.0).max(if key == "cellSize" { 1.0 } else { 0.1 });
+                *v = json!((x / k as f64).max(floor));
             }
         }
     }
@@ -732,6 +740,42 @@ mod tests {
         assert_eq!(label("wavelengthMin"), "Wavelength Min");
     }
 
+    #[test]
+    fn core_filter_previews_keep_minimum_settings_after_downsampling() {
+        use photocraft_doc::{Color, ColorMode, SampleType, Size};
+        use photocraft_geom::Rect;
+
+        for depth in [SampleType::U8, SampleType::U16, SampleType::F32] {
+            let mut doc = Document::with_background("preview", Size::new(32, 32), ColorMode::Rgb, depth, Color::WHITE);
+            let layer = doc.layers[0].id;
+            doc.layers[0].surface_mut().unwrap().fill_rect(Rect::new(0, 0, 16, 32), &[0.0, 0.0, 0.0, 1.0]);
+            for (suffix, params) in [
+                ("blur.boxBlur", json!({"radius":1})),
+                ("blur.motionBlur", json!({"distance":1})),
+                ("blur.surfaceBlur", json!({"radius":1})),
+                ("noise.median", json!({"radius":1})),
+                ("noise.dustAndScratches", json!({"radius":1})),
+                ("pixelate.mosaic", json!({"cellSize":2})),
+                ("stylize.emboss", json!({"height":1})),
+                ("other.minimum", json!({"radius":0.2})),
+                ("other.maximum", json!({"radius":0.2})),
+                ("distort.wave", json!({"wavelengthMin":1,"wavelengthMax":2,"amplitudeMin":1,"amplitudeMax":1})),
+            ] {
+                let command = format!("filter.{suffix}");
+                for k in [2, 4] {
+                    let proxy = crate::proxy::proxy_document(&doc, k);
+                    let expected = preview_document(&proxy, Some(layer), &command, &params, 1).unwrap();
+                    let out = preview_document(&doc, Some(layer), &command, &params, k).unwrap_or_else(|| panic!("{command}: {depth:?}, proxy factor {k}"));
+                    assert_eq!(
+                        out.layers[0].surface().unwrap().read_region(proxy.bounds()),
+                        expected.layers[0].surface().unwrap().read_region(proxy.bounds()),
+                        "{command}: {depth:?}, proxy factor {k}"
+                    );
+                }
+            }
+        }
+    }
+
     #[test]
     fn colour_params_parse_with_and_without_a_default() {
         let p = parse_spec(r##"{"color":color=#ffffff,"vineColor":color,"bad":color=#zz}"##);