diff --git a/src/strands/ir_builders.js b/src/strands/ir_builders.js
index 465572bebb..4ae69d90f4 100644
--- a/src/strands/ir_builders.js
+++ b/src/strands/ir_builders.js
@@ -572,7 +572,11 @@ export function swizzleTrap(id, dimension, strandsContext, onRebind) {
             scalars.push(createStrandsNode(id, dimension, strandsContext));
           }
         } else {
-          FES.userError('type error', `Swizzle assignment: RHS vector does not match LHS vector (need ${chars.length}, got ${value.dimension}).`);
+          FES.dimensionMismatchError(
+            chars.length,
+            value.dimension,
+            `${target._originalIdentifier || 'value'}.${property}`
+          );
         }
       } else if (Array.isArray(value)) {
         const flat = value.flat(Infinity);
diff --git a/src/strands/strands_FES.js b/src/strands/strands_FES.js
index 3af0aca90b..245ea4a302 100644
--- a/src/strands/strands_FES.js
+++ b/src/strands/strands_FES.js
@@ -6,4 +6,11 @@ export function internalError(errorMessage) {
 export function userError(errorType, errorMessage) {
     const prefixedMessage = `[p5.strands ${errorType}]: ${errorMessage}`;
     throw new Error(prefixedMessage);
+}
+
+export function dimensionMismatchError(declaredDim,actualDim,varName){
+    userError(
+        'dimension mismatch',
+    `Cannot assign a value of dimension ${actualDim} to \`${varName}\`, which expects dimension ${declaredDim}.`
+    );
 }
\ No newline at end of file
diff --git a/src/strands/strands_api.js b/src/strands/strands_api.js
index 84fa5597a9..2b6e6eba2b 100644
--- a/src/strands/strands_api.js
+++ b/src/strands/strands_api.js
@@ -686,6 +686,17 @@ function createHookArguments(strandsContext, parameters){
             return createStrandsNode(propNode.id, propNode.dimension, strandsContext, onRebind);
           },
           set(val) {
+            const valDim = val?.isStrandsNode
+              ? val.dimension
+              : (Array.isArray(val) ? val.length : 1);
+            if( valDim !== propertyType.dataType.dimension && valDim !== 1){
+              FES.dimensionMismatchError(
+                propertyType.dataType.dimension,
+                valDim,
+                `${param.name}.${propertyType.name}`
+              );
+            }
+
             const oldDependsOn = dag.dependsOn[structNode.id];
             const newDependsOn = [...oldDependsOn];
             let newValueID;
diff --git a/src/strands/strands_node.js b/src/strands/strands_node.js
index f7638855bc..3973406659 100644
--- a/src/strands/strands_node.js
+++ b/src/strands/strands_node.js
@@ -2,6 +2,7 @@ import { swizzleTrap, primitiveConstructorNode, variableNode, arrayAccessNode, a
 import { BaseType, NodeType, OpCode } from './ir_types';
 import { getNodeDataFromID, createNodeData, getOrCreateNode } from './ir_dag';
 import { recordInBasicBlock } from './ir_cfg';
+import { dimensionMismatchError } from './strands_FES';
 export class StrandsNode {
   constructor(id, dimension, strandsContext) {
     this.id = id;
@@ -56,6 +57,16 @@ export class StrandsNode {
 
     // For varying variables, we need both assignment generation AND a way to reference by identifier
     if (this._originalIdentifier) {
+      const valueDim = value?.isStrandsNode
+        ? value.dimension
+        : (Array.isArray(value) ? value.length : 1);
+      if (valueDim !== this._originalDimension && valueDim !== 1){
+        dimensionMismatchError(
+          this._originalDimension,
+          valueDim,
+          this._originalIdentifier
+        );
+      }
       // Create a variable node for the target (the varying variable)
       const { id: targetVarID } = variableNode(
         this.strandsContext,
@@ -108,6 +119,16 @@ export class StrandsNode {
 
     // For varying variables, create swizzle assignment
     if (this._originalIdentifier) {
+      const valueDim = value?.isStrandsNode
+        ? value.dimension
+        : (Array.isArray(value) ? value.length : 1);
+      if (valueDim !== swizzlePattern.length && valueDim !== 1){
+        dimensionMismatchError(
+          swizzlePattern.length,
+          valueDim,
+          `${this._originalIdentifier}.${swizzlePattern}`
+        );
+      }
       // Create a variable node for the target with swizzle
       const { id: targetVarID } = variableNode(
         this.strandsContext,
diff --git a/test/unit/webgl/p5.Shader.js b/test/unit/webgl/p5.Shader.js
index 499dab8da8..7a1297263c 100644
--- a/test/unit/webgl/p5.Shader.js
+++ b/test/unit/webgl/p5.Shader.js
@@ -1,15 +1,24 @@
 import p5 from '../../../src/app.js';
-import { vi } from 'vitest';
+import { beforeEach, vi } from 'vitest';
 
 const mockUserError = vi.fn();
-vi.mock('../../../src/strands/strands_FES', () => ({
-  userError: (...args) => {
+vi.mock('../../../src/strands/strands_FES', () => {
+  const userError = (...args) => {
     mockUserError(...args);
     const prefixedMessage = `[p5.strands ${args[0]}]: ${args[1]}`;
     throw new Error(prefixedMessage);
-  },
-  internalError: (msg) => { throw new Error(`[p5.strands internal error]: ${msg}`); }
-}));
+  };
+  return {
+    userError,
+    internalError: (msg) => { throw new Error(`[p5.strands internal error]: ${msg}`); },
+    dimensionMismatchError: (declaredDim, actualDim, varName) => {
+      userError(
+        'dimension mismatch',
+        `Cannot assign a value of dimension ${actualDim} to \`${varName}\`, which expects dimension ${declaredDim}.`
+      );
+    },
+  };
+});
 
 suite('p5.Shader', function() {
   var myp5;
@@ -2631,6 +2640,62 @@ test('returns numbers for builtin globals outside hooks and a strandNode when ca
       assert.approximately(pixelColor[1], 0, 5);
       assert.approximately(pixelColor[2], 0, 5);
     });
+
+    test('allows scalar broadcast when assigning a scalar to a sharedVec3 (bridge)', async () => {
+  await myp5.createCanvas(5, 5, myp5.WEBGL);
+
+  expect(() => {
+    myp5.baseMaterialShader().modify(() => {
+      let worldPosX = myp5.sharedVec3();
+      myp5.getWorldInputs(inputs => {
+        worldPosX = inputs.position.x;   // scalar → vec3, valid broadcast
+        return inputs;
+      });
+    },{myp5});
+  }).not.toThrow();
+});
+
+test('reports a friendly error when assigning a vec2 to a sharedVec3 (bridge)', async () => {
+  await myp5.createCanvas(5, 5, myp5.WEBGL);
+
+  expect(() => {
+    myp5.baseMaterialShader().modify(() => {
+      let myVec = myp5.sharedVec3();
+      myp5.getWorldInputs(inputs => {
+        myVec = inputs.position.xy;   // vec2 → vec3 mismatch
+        return inputs;
+      });
+    },{myp5});
+  }).toThrow(/dimension mismatch/);
+});
+
+test('reports a friendly error on dimension mismatch via swizzle write (bridgeSwizzle)', async () => {
+  await myp5.createCanvas(5, 5, myp5.WEBGL);
+
+  expect(() => {
+    myp5.baseMaterialShader().modify(() => {
+      let myVec = myp5.sharedVec3();
+      myp5.getWorldInputs(inputs => {
+        myVec.xy = inputs.position;      // vec3 → 2-component swizzle mismatch
+        return inputs;
+      });
+    },{myp5});
+  }).toThrow(/dimension mismatch/);
+});
+
+test('does not error when shared variable assignment dimensions match', async () => {
+  await myp5.createCanvas(5, 5, myp5.WEBGL);
+
+  expect(() => {
+    myp5.baseMaterialShader().modify(() => {
+      let myVec = myp5.sharedVec3();
+      myp5.getWorldInputs(inputs => {
+        myVec = inputs.position;         // vec3 → vec3, OK
+        return inputs;
+      });
+    },{myp5});
+  }).not.toThrow();
+});
   });
 
   suite('p5.strands error messages', () => {
@@ -2648,7 +2713,7 @@ test('returns numbers for builtin globals outside hooks and a strandNode when ca
       assert.include(err.message, '// noprotect');
     };
 
-    afterEach(() => {
+    beforeEach(() => {
       mockUserError.mockClear();
     });
 
diff --git a/test/unit/webgpu/p5.Shader.js b/test/unit/webgpu/p5.Shader.js
index eb9bb79990..249fba6f87 100644
--- a/test/unit/webgpu/p5.Shader.js
+++ b/test/unit/webgpu/p5.Shader.js
@@ -1361,6 +1361,62 @@ suite('WebGPU p5.Shader', function() {
           myp5.compute(s4, 4);
         }).not.toThrow();
       });
+
+      test('allows scalar broadcast when assigning a scalar to a sharedVec3 (bridge)', async () => {
+        await myp5.createCanvas(5, 5, myp5.WEBGPU);
+
+        expect(() => {
+          myp5.baseMaterialShader().modify(() => {
+            let worldPosX = myp5.sharedVec3();
+            myp5.getWorldInputs(inputs => {
+              worldPosX = inputs.position.x;   // scalar → vec3, valid broadcast
+              return inputs;
+            });
+          }, { myp5 });
+        }).not.toThrow();
+      });
+
+      test('reports a friendly error when assigning a vec2 to a sharedVec3 (bridge)', async () => {
+        await myp5.createCanvas(5, 5, myp5.WEBGPU);
+
+        expect(() => {
+          myp5.baseMaterialShader().modify(() => {
+            let myVec = myp5.sharedVec3();
+            myp5.getWorldInputs(inputs => {
+              myVec = inputs.position.xy;   // vec2 → vec3 mismatch
+              return inputs;
+            });
+          }, { myp5 });
+        }).toThrow(/dimension mismatch/);
+      });
+
+      test('reports a friendly error on dimension mismatch via swizzle write (bridgeSwizzle)', async () => {
+        await myp5.createCanvas(5, 5, myp5.WEBGPU);
+
+        expect(() => {
+          myp5.baseMaterialShader().modify(() => {
+            let myVec = myp5.sharedVec3();
+            myp5.getWorldInputs(inputs => {
+              myVec.xy = inputs.position;      // vec3 → 2-component swizzle mismatch
+              return inputs;
+            });
+          },{myp5});
+        }).toThrow(/dimension mismatch/);
+      });
+
+      test('does not error when shared variable assignment dimensions match', async () => {
+        await myp5.createCanvas(5, 5, myp5.WEBGPU);
+
+        expect(() => {
+          myp5.baseMaterialShader().modify(() => {
+            let myVec = myp5.sharedVec3();
+            myp5.getWorldInputs(inputs => {
+              myVec = inputs.position;         // vec3 → vec3, OK
+              return inputs;
+            });
+          },{myp5});
+        }).not.toThrow();
+      });
     });
   });
 });