diff --git a/packages/core/src/types.test.ts b/packages/core/src/types.test.ts
index d7a90bef..59d4ee10 100644
--- a/packages/core/src/types.test.ts
+++ b/packages/core/src/types.test.ts
@@ -562,6 +562,65 @@ describe("createSpecStreamCompiler", () => {
     const result = compiler.getResult();
     expect(result).toEqual({ z: 1, w: 2 });
   });
+
+  describe("onError callback", () => {
+    it("calls onError for malformed JSON", () => {
+      const errors: string[] = [];
+      const compiler = createSpecStreamCompiler({
+        onError: (line) => errors.push(line),
+      });
+
+      compiler.push('{"op":"add"\n'); // Missing closing brace
+      compiler.push('not-json\n');
+      compiler.push('{"invalid":}\n'); // Invalid JSON
+
+      expect(errors).toHaveLength(2);
+      expect(errors[0]).toContain('{"op":"add"');
+      expect(errors[1]).toContain('{"invalid":}');
+    });
+
+    it("calls onError when patch application fails", () => {
+      const errors: Array<{ line: string; error?: unknown }> = [];
+      const compiler = createSpecStreamCompiler({
+        onError: (line, error) => errors.push({ line, error }),
+      });
+
+      // Valid JSON but test operation fails
+      compiler.push('{"op":"test","path":"/x","value":"wrong"}\n');
+
+      expect(errors).toHaveLength(1);
+      expect(errors[0]?.error).toBeDefined();
+    });
+
+    it("does not call onError for valid patches", () => {
+      const errors: string[] = [];
+      const compiler = createSpecStreamCompiler({
+        onError: (line) => errors.push(line),
+      });
+
+      compiler.push('{"op":"add","path":"/x","value":1}\n');
+      const result = compiler.getResult();
+
+      expect(errors).toHaveLength(0);
+      expect(result).toEqual({ x: 1 });
+    });
+
+    it("continues processing after errors", () => {
+      const errors: string[] = [];
+      const compiler = createSpecStreamCompiler({
+        onError: (line) => errors.push(line),
+      });
+
+      compiler.push('{"invalid"}\n');
+      compiler.push('{"op":"add","path":"/x","value":1}\n');
+      compiler.push('{broken\n');
+      compiler.push('{"op":"add","path":"/y","value":2}\n');
+
+      expect(errors).toHaveLength(2);
+      const result = compiler.getResult();
+      expect(result).toEqual({ x: 1, y: 2 });
+    });
+  });
 });
 
 // =============================================================================
diff --git a/packages/core/src/types.ts b/packages/core/src/types.ts
index cbc28082..a3ba5239 100644
--- a/packages/core/src/types.ts
+++ b/packages/core/src/types.ts
@@ -791,6 +791,19 @@ export interface SpecStreamCompiler<T> {
   reset(initial?: Partial<T>): void;
 }
 
+/**
+ * Options for createSpecStreamCompiler.
+ */
+export interface SpecStreamCompilerOptions<T> {
+  /** Initial state */
+  initial?: Partial<T>;
+  /**
+   * Called when a line fails to parse as a valid patch.
+   * Useful for logging malformed JSON during debugging.
+   */
+  onError?: (line: string, error?: unknown) => void;
+}
+
 /**
  * Create a streaming SpecStream compiler.
  *
@@ -798,7 +811,9 @@ export interface SpecStreamCompiler<T> {
  * line by line, and this compiler progressively builds the final spec.
  *
  * @example
- * const compiler = createSpecStreamCompiler<TimelineSpec>();
+ * const compiler = createSpecStreamCompiler<TimelineSpec>({
+ *   onError: (line) => console.warn('Failed to parse:', line),
+ * });
  *
  * // Process streaming response
  * const reader = response.body.getReader();
@@ -813,8 +828,9 @@ export interface SpecStreamCompiler<T> {
  * }
  */
 export function createSpecStreamCompiler<T = Record<string, unknown>>(
-  initial: Partial<T> = {},
+  options: SpecStreamCompilerOptions<T> = {},
 ): SpecStreamCompiler<T> {
+  const { initial = {}, onError } = options;
   let result = { ...initial } as T;
   let buffer = "";
   const appliedPatches: SpecStreamLine[] = [];
@@ -836,9 +852,19 @@ export function createSpecStreamCompiler<T = Record<string, unknown>>(
 
         const patch = parseSpecStreamLine(trimmed);
         if (patch) {
-          applySpecStreamPatch(result as Record<string, unknown>, patch);
-          appliedPatches.push(patch);
-          newPatches.push(patch);
+          try {
+            applySpecStreamPatch(result as Record<string, unknown>, patch);
+            appliedPatches.push(patch);
+            newPatches.push(patch);
+          } catch (error) {
+            // Patch failed to apply (e.g., test operation failed)
+            if (onError) {
+              onError(trimmed, error);
+            }
+          }
+        } else if (trimmed.startsWith("{") && onError) {
+          // Line looks like JSON but failed to parse
+          onError(trimmed);
         }
       }
 
@@ -853,12 +879,21 @@ export function createSpecStreamCompiler<T = Record<string, unknown>>(
     getResult(): T {
       // Process any remaining buffer
       if (buffer.trim()) {
-        const patch = parseSpecStreamLine(buffer);
-        if (patch && !processedLines.has(buffer.trim())) {
-          processedLines.add(buffer.trim());
-          applySpecStreamPatch(result as Record<string, unknown>, patch);
-          appliedPatches.push(patch);
-          result = { ...result };
+        const trimmed = buffer.trim();
+        const patch = parseSpecStreamLine(trimmed);
+        if (patch && !processedLines.has(trimmed)) {
+          processedLines.add(trimmed);
+          try {
+            applySpecStreamPatch(result as Record<string, unknown>, patch);
+            appliedPatches.push(patch);
+            result = { ...result };
+          } catch (error) {
+            if (onError) {
+              onError(trimmed, error);
+            }
+          }
+        } else if (trimmed.startsWith("{") && !patch && onError) {
+          onError(trimmed);
         }
         buffer = "";
       }