diff --git a/src/validation/getFlowValidation.ts b/src/validation/getFlowValidation.ts
index 0ef915f..36e6e17 100644
--- a/src/validation/getFlowValidation.ts
+++ b/src/validation/getFlowValidation.ts
@@ -39,6 +39,25 @@ export const getFlowValidation = (
         }
     }
 
+    const functionIdentifiers = new Set(functions?.map(f => f.identifier));
+    const unreachableFunctionDiagnostics = (flow.nodes?.nodes ?? [])
+        .filter(n => n?.functionDefinition && !functionIdentifiers.has(n.functionDefinition.identifier))
+        .map(n => ({
+            nodeId: n!.id,
+            parameterIndex: null,
+            code: 0,
+            message: `The function definition "${n!.functionDefinition!.identifier}" is not reachable.`,
+            severity: "error" as const,
+        }));
+
+    if (unreachableFunctionDiagnostics.length > 0) {
+        return {
+            isValid: false,
+            returnType: "void",
+            diagnostics: unreachableFunctionDiagnostics,
+        }
+    }
+
     const sourceCode = generateFlowSourceCode(flow, functions, dataTypes);
 
     // 3. Virtual TypeScript Compilation
diff --git a/test/flowValidation.test.ts b/test/flowValidation.test.ts
index 00dd401..5ae0fda 100644
--- a/test/flowValidation.test.ts
+++ b/test/flowValidation.test.ts
@@ -934,10 +934,19 @@ describe('getFlowValidation - Integrationstest', () => {
 
         const result = getFlowValidation(flow, FUNCTION_SIGNATURES, DATA_TYPES);
 
-        expect(result.isValid).toBe(true);
-        result.diagnostics.forEach((error) => {
-            expect(error.parameterIndex).toBeDefined()
-        })
+        expect(result.isValid).toBe(false);
+        expect(result.diagnostics).toEqual(expect.arrayContaining([
+            expect.objectContaining({
+                nodeId: "gid://sagittarius/NodeFunction/1",
+                message: 'The function definition "http::response::create" is not reachable.',
+                severity: "error",
+            }),
+            expect.objectContaining({
+                nodeId: "gid://sagittarius/NodeFunction/4",
+                message: 'The function definition "http::response::create" is not reachable.',
+                severity: "error",
+            }),
+        ]));
     });
 
     it('11', () => {
@@ -1278,4 +1287,43 @@ describe('getFlowValidation - Integrationstest', () => {
         })
     });
 
+    it('fails when a node references an unreachable function definition', () => {
+
+        const flow: Flow = {
+            startingNodeId: "gid://sagittarius/NodeFunction/1",
+            nodes: {
+                nodes: [
+                    {
+                        id: "gid://sagittarius/NodeFunction/1",
+                        functionDefinition: {identifier: "std::number::add"},
+                        parameters: {
+                            nodes: [
+                                {value: {__typename: "LiteralValue", value: 1}},
+                                {value: {__typename: "LiteralValue", value: 2}}
+                            ]
+                        },
+                        nextNodeId: "gid://sagittarius/NodeFunction/2"
+                    },
+                    {
+                        id: "gid://sagittarius/NodeFunction/2",
+                        functionDefinition: {identifier: "std::imaginary::nonexistent"},
+                        parameters: {nodes: []}
+                    }
+                ]
+            }
+        };
+
+        const result = getFlowValidation(flow, FUNCTION_SIGNATURES, DATA_TYPES);
+
+        expect(result.isValid).toBe(false);
+        expect(result.diagnostics).toEqual([
+            expect.objectContaining({
+                nodeId: "gid://sagittarius/NodeFunction/2",
+                parameterIndex: null,
+                message: 'The function definition "std::imaginary::nonexistent" is not reachable.',
+                severity: "error",
+            })
+        ]);
+    });
+
 });
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