diff --git a/packages/core/src/evaluation/input-message-utils.ts b/packages/core/src/evaluation/input-message-utils.ts
index f5b9e1a9e..35d4d89b5 100644
--- a/packages/core/src/evaluation/input-message-utils.ts
+++ b/packages/core/src/evaluation/input-message-utils.ts
@@ -53,6 +53,14 @@ export function extractContentSegments(content: TestMessageContent): JsonObject[
   const segments: JsonObject[] = [];
 
   for (const segment of content) {
+    // Plain string items inside a content array are treated as text segments.
+    // This matches the validator, which accepts string items in content arrays.
+    if (typeof segment === 'string') {
+      if (segment.trim().length > 0) {
+        segments.push({ type: 'text', value: segment });
+      }
+      continue;
+    }
     if (!isJsonObject(segment)) {
       continue;
     }
diff --git a/packages/core/src/evaluation/loaders/message-processor.ts b/packages/core/src/evaluation/loaders/message-processor.ts
index 1e1a9b48d..4aa2b4ea8 100644
--- a/packages/core/src/evaluation/loaders/message-processor.ts
+++ b/packages/core/src/evaluation/loaders/message-processor.ts
@@ -79,6 +79,18 @@ export async function processMessages(options: ProcessMessagesOptions): Promise<
     const processedContent: JsonObject[] = [];
 
     for (const rawSegment of content) {
+      // Plain string items inside a content array are treated as inline text segments.
+      // This matches the validator, which accepts string items alongside structured blocks.
+      if (typeof rawSegment === 'string') {
+        if (rawSegment.length > 0) {
+          processedContent.push({ type: 'text', value: rawSegment });
+          if (textParts) {
+            textParts.push(rawSegment);
+          }
+        }
+        continue;
+      }
+
       if (!isJsonObject(rawSegment)) {
         continue;
       }
@@ -334,6 +346,14 @@ export async function processExpectedMessages(
       // Process content array, resolving file references
       const processedContent: JsonObject[] = [];
       for (const rawSegment of content) {
+        // Plain string items are treated as inline text segments (matches validator).
+        if (typeof rawSegment === 'string') {
+          if (rawSegment.length > 0) {
+            processedContent.push({ type: 'text', value: rawSegment });
+          }
+          continue;
+        }
+
         if (!isJsonObject(rawSegment)) {
           continue;
         }
diff --git a/packages/core/src/evaluation/types.ts b/packages/core/src/evaluation/types.ts
index 18144cb11..792d3bcd1 100644
--- a/packages/core/src/evaluation/types.ts
+++ b/packages/core/src/evaluation/types.ts
@@ -40,8 +40,12 @@ const TEST_MESSAGE_ROLE_SET: ReadonlySet<string> = new Set(TEST_MESSAGE_ROLE_VAL
 
 /**
  * Text or structured payload attached to a message.
+ *
+ * Content arrays may mix plain string items with structured content blocks
+ * (e.g. `{ type: 'text' | 'file' | 'image', value: ... }`). Plain string items
+ * are treated as text segments by the loader and prompt builder.
  */
-export type TestMessageContent = string | JsonObject | readonly JsonObject[];
+export type TestMessageContent = string | JsonObject | readonly (string | JsonObject)[];
 
 /**
  * System-authored instruction message.
@@ -140,7 +144,12 @@ export function isTestMessage(value: unknown): value is TestMessage {
   if (typeof candidate.content === 'string') {
     return true;
   }
-  if (Array.isArray(candidate.content) && candidate.content.every(isJsonObject)) {
+  // Content arrays may mix plain string items with structured content blocks.
+  // The loader treats string items as text segments (see message-processor.ts).
+  if (
+    Array.isArray(candidate.content) &&
+    candidate.content.every((item) => typeof item === 'string' || isJsonObject(item))
+  ) {
     return true;
   }
   // Allow messages with tool_calls but no content (for expected_output format)
diff --git a/packages/core/test/evaluation/loaders/message-processor.test.ts b/packages/core/test/evaluation/loaders/message-processor.test.ts
index 1c7f5510c..ccac4e490 100644
--- a/packages/core/test/evaluation/loaders/message-processor.test.ts
+++ b/packages/core/test/evaluation/loaders/message-processor.test.ts
@@ -162,6 +162,39 @@ describe('processMessages – image content', () => {
     }
   });
 
+  it('converts plain string items in content arrays into text segments', async () => {
+    await setupFixtures();
+    try {
+      const messages: TestMessage[] = [
+        {
+          role: 'user',
+          content: ['Use the local file.', { type: 'file', value: './test-file.txt' }],
+        },
+      ];
+
+      const textParts: string[] = [];
+      const result = await processMessages({
+        messages,
+        searchRoots: [FIXTURE_DIR],
+        repoRootPath: FIXTURE_DIR,
+        textParts,
+        messageType: 'input',
+        verbose: false,
+      });
+
+      const items = result[0].content as Record<string, unknown>[];
+      expect(items).toHaveLength(2);
+      expect(items[0].type).toBe('text');
+      expect(items[0].value).toBe('Use the local file.');
+      expect(items[1].type).toBe('file');
+      expect(items[1].text).toBe('hello world');
+      // Inline string was also surfaced through textParts for prompt building.
+      expect(textParts).toContain('Use the local file.');
+    } finally {
+      await cleanupFixtures();
+    }
+  });
+
   it('preserves existing type: text and type: file behavior', async () => {
     await setupFixtures();
     try {
diff --git a/packages/core/test/evaluation/loaders/shorthand-expansion.test.ts b/packages/core/test/evaluation/loaders/shorthand-expansion.test.ts
index 95f28200d..f0ec058a1 100644
--- a/packages/core/test/evaluation/loaders/shorthand-expansion.test.ts
+++ b/packages/core/test/evaluation/loaders/shorthand-expansion.test.ts
@@ -61,6 +61,25 @@ describe('expandInputShorthand', () => {
     const messages = [{ invalid: 'message' }, { also: 'invalid' }];
     expect(expandInputShorthand(messages)).toBeUndefined();
   });
+
+  it('accepts messages whose content array mixes plain strings and structured blocks', () => {
+    const messages = [
+      {
+        role: 'user',
+        content: ['Use the local file.', { type: 'file', value: '/README.md' }],
+      },
+    ];
+
+    const result = expandInputShorthand(messages);
+
+    expect(result).toHaveLength(1);
+    const content = result?.[0].content;
+    expect(Array.isArray(content)).toBe(true);
+    const items = content as Array<unknown>;
+    expect(items).toHaveLength(2);
+    expect(items[0]).toBe('Use the local file.');
+    expect(items[1]).toEqual({ type: 'file', value: '/README.md' });
+  });
 });
 
 describe('expandExpectedOutputShorthand', () => {