diff --git a/src/runner/templates/agents/node/manual/template.njk b/src/runner/templates/agents/node/manual/template.njk
index 44f8e8f..40996b8 100644
--- a/src/runner/templates/agents/node/manual/template.njk
+++ b/src/runner/templates/agents/node/manual/template.njk
@@ -45,46 +45,50 @@ function trimMessages(messages, maxSize = 10000) {
 }
 
 function buildSentryMessages(messages) {
-  const sentryMessages = [];
-  const legacyMessages = [];
   let systemInstructions = null;
 
   for (const msg of messages) {
-    const { role, content } = msg;
-
-    if (role === "system") {
-      systemInstructions = typeof content === "string" ? content : JSON.stringify(content);
+    if (msg.role === "system") {
+      systemInstructions =
+        typeof msg.content === "string" ? msg.content : JSON.stringify(msg.content);
     }
+  }
 
-    const parts = [];
-    if (typeof content === "string") {
-      parts.push({ type: "text", text: content });
-    } else if (Array.isArray(content)) {
-      for (const part of content) {
-        if (part.type === "text") {
-          parts.push({ type: "text", text: part.text });
-        } else if (part.type === "image") {
-          parts.push({ type: "text", text: "[Blob substitute]" });
-        }
-      }
-    }
+  const lastMessage = messages[messages.length - 1];
+  if (!lastMessage) {
+    return { sentryMessages: [], legacyMessages: [], systemInstructions };
+  }
 
-    sentryMessages.push({
-      role,
-      content: typeof content === "string" ? content : "[multimodal]",
-      parts,
-    });
-
-    if (Array.isArray(content)) {
-      const redacted = content.map((part) =>
-        part.type === "image" ? { type: "text", text: "[Blob substitute]" } : part
-      );
-      legacyMessages.push({ role, content: redacted });
-    } else {
-      legacyMessages.push({ role, content });
+  const { role, content } = lastMessage;
+
+  const parts = [];
+  if (typeof content === "string") {
+    parts.push({ type: "text", text: content });
+  } else if (Array.isArray(content)) {
+    for (const part of content) {
+      if (part.type === "text") {
+        parts.push({ type: "text", text: part.text });
+      } else if (part.type === "image") {
+        parts.push({ type: "text", text: "[Blob substitute]" });
+      }
     }
   }
 
+  const sentryMessages = [{
+    role,
+    content: typeof content === "string" ? content : "[multimodal]",
+    parts,
+  }];
+
+  const legacyMessages = [Array.isArray(content)
+    ? {
+        role,
+        content: content.map((part) =>
+          part.type === "image" ? { type: "text", text: "[Blob substitute]" } : part,
+        ),
+      }
+    : { role, content }];
+
   return { sentryMessages, legacyMessages, systemInstructions };
 }
 
@@ -150,6 +154,10 @@ const TOOLS = {{ agent.tools | dump }};
         agentSpan.setAttribute("gen_ai.usage.output_tokens", outputTokens{{ loop.index }});
         agentSpan.setAttribute("gen_ai.usage.output_tokens.reasoning", Math.max(0, Math.floor(outputTokens{{ loop.index }} / 4)));
         agentSpan.setAttribute("gen_ai.usage.total_tokens", totalTokens{{ loop.index }});
+        agentSpan.setAttribute("gen_ai.input.messages", otelJson{{ loop.index }});
+        if (system{{ loop.index }} !== null) {
+          agentSpan.setAttribute("gen_ai.system_instructions", system{{ loop.index }});
+        }
 
         // Chat span (child of agent)
         const chatDesc{{ loop.index }} = `chat ${model{{ loop.index }}}`;
diff --git a/src/runner/templates/agents/python/manual/template.njk b/src/runner/templates/agents/python/manual/template.njk
index 37b85aa..e863ef9 100644
--- a/src/runner/templates/agents/python/manual/template.njk
+++ b/src/runner/templates/agents/python/manual/template.njk
@@ -48,46 +48,47 @@ def trim_messages(messages, max_size=10000):
 
 
 def build_sentry_messages(messages):
-    """Build gen_ai.input.messages and gen_ai.request.messages from raw messages."""
-    sentry_messages = []
-    legacy_messages = []
+    """Build gen_ai.input.messages and gen_ai.request.messages from the last raw message."""
     system_instructions = None
 
     for msg in messages:
-        role = msg["role"]
         content = msg.get("content", "")
-
-        if role == "system":
+        if msg["role"] == "system":
             system_instructions = content if isinstance(content, str) else json.dumps(content)
 
-        # New OTel format with parts
-        parts = []
-        if isinstance(content, str):
-            parts.append({"type": "text", "text": content})
-        elif isinstance(content, list):
-            for part in content:
-                if part.get("type") == "text":
-                    parts.append({"type": "text", "text": part["text"]})
-                elif part.get("type") == "image":
-                    parts.append({"type": "text", "text": "[Blob substitute]"})
-
-        sentry_messages.append({
-            "role": role,
-            "content": content if isinstance(content, str) else "[multimodal]",
-            "parts": parts,
-        })
-
-        # Legacy format with binary redaction
-        if isinstance(content, list):
-            redacted = []
-            for part in content:
-                if part.get("type") == "text":
-                    redacted.append(part)
-                elif part.get("type") == "image":
-                    redacted.append({"type": "text", "text": "[Blob substitute]"})
-            legacy_messages.append({"role": role, "content": redacted})
-        else:
-            legacy_messages.append({"role": role, "content": content})
+    if not messages:
+        return [], [], system_instructions
+
+    last_message = messages[-1]
+    role = last_message["role"]
+    content = last_message.get("content", "")
+
+    parts = []
+    if isinstance(content, str):
+        parts.append({"type": "text", "text": content})
+    elif isinstance(content, list):
+        for part in content:
+            if part.get("type") == "text":
+                parts.append({"type": "text", "text": part["text"]})
+            elif part.get("type") == "image":
+                parts.append({"type": "text", "text": "[Blob substitute]"})
+
+    sentry_messages = [{
+        "role": role,
+        "content": content if isinstance(content, str) else "[multimodal]",
+        "parts": parts,
+    }]
+
+    if isinstance(content, list):
+        redacted = []
+        for part in content:
+            if part.get("type") == "text":
+                redacted.append(part)
+            elif part.get("type") == "image":
+                redacted.append({"type": "text", "text": "[Blob substitute]"})
+        legacy_messages = [{"role": role, "content": redacted}]
+    else:
+        legacy_messages = [{"role": role, "content": content}]
 
     return sentry_messages, legacy_messages, system_instructions
 
@@ -151,6 +152,9 @@ TOOLS = {{ agent.tools | dump }}
         agent_span.set_data("gen_ai.usage.output_tokens", output_tokens)
         agent_span.set_data("gen_ai.usage.output_tokens.reasoning", max(0, output_tokens // 4))
         agent_span.set_data("gen_ai.usage.total_tokens", total_tokens)
+        agent_span.set_data("gen_ai.input.messages", otel_messages_json)
+        if system_instructions is not None:
+            agent_span.set_data("gen_ai.system_instructions", system_instructions)
 
         # Chat span (child of agent)
         chat_desc = f"chat {model}"
diff --git a/src/runner/templates/llm/node/manual/template.njk b/src/runner/templates/llm/node/manual/template.njk
index ee56df6..114d48f 100644
--- a/src/runner/templates/llm/node/manual/template.njk
+++ b/src/runner/templates/llm/node/manual/template.njk
@@ -45,48 +45,50 @@ function trimMessages(messages, maxSize = 10000) {
 }
 
 function buildSentryMessages(messages) {
-  const sentryMessages = [];
-  const legacyMessages = [];
   let systemInstructions = null;
 
   for (const msg of messages) {
-    const { role, content } = msg;
-
-    if (role === "system") {
-      systemInstructions = typeof content === "string" ? content : JSON.stringify(content);
+    if (msg.role === "system") {
+      systemInstructions =
+        typeof msg.content === "string" ? msg.content : JSON.stringify(msg.content);
     }
+  }
 
-    // New OTel format with parts
-    const parts = [];
-    if (typeof content === "string") {
-      parts.push({ type: "text", text: content });
-    } else if (Array.isArray(content)) {
-      for (const part of content) {
-        if (part.type === "text") {
-          parts.push({ type: "text", text: part.text });
-        } else if (part.type === "image") {
-          parts.push({ type: "text", text: "[Blob substitute]" });
-        }
-      }
-    }
+  const lastMessage = messages[messages.length - 1];
+  if (!lastMessage) {
+    return { sentryMessages: [], legacyMessages: [], systemInstructions };
+  }
 
-    sentryMessages.push({
-      role,
-      content: typeof content === "string" ? content : "[multimodal]",
-      parts,
-    });
-
-    // Legacy format with binary redaction
-    if (Array.isArray(content)) {
-      const redacted = content.map((part) =>
-        part.type === "image" ? { type: "text", text: "[Blob substitute]" } : part
-      );
-      legacyMessages.push({ role, content: redacted });
-    } else {
-      legacyMessages.push({ role, content });
+  const { role, content } = lastMessage;
+
+  const parts = [];
+  if (typeof content === "string") {
+    parts.push({ type: "text", text: content });
+  } else if (Array.isArray(content)) {
+    for (const part of content) {
+      if (part.type === "text") {
+        parts.push({ type: "text", text: part.text });
+      } else if (part.type === "image") {
+        parts.push({ type: "text", text: "[Blob substitute]" });
+      }
     }
   }
 
+  const sentryMessages = [{
+    role,
+    content: typeof content === "string" ? content : "[multimodal]",
+    parts,
+  }];
+
+  const legacyMessages = [Array.isArray(content)
+    ? {
+        role,
+        content: content.map((part) =>
+          part.type === "image" ? { type: "text", text: "[Blob substitute]" } : part,
+        ),
+      }
+    : { role, content }];
+
   return { sentryMessages, legacyMessages, systemInstructions };
 }
 {% endblock %}
diff --git a/src/runner/templates/llm/python/manual/template.njk b/src/runner/templates/llm/python/manual/template.njk
index b2a8925..77b2f0a 100644
--- a/src/runner/templates/llm/python/manual/template.njk
+++ b/src/runner/templates/llm/python/manual/template.njk
@@ -44,46 +44,47 @@ def trim_messages(messages, max_size=10000):
 
 
 def build_sentry_messages(messages):
-    """Build gen_ai.input.messages and gen_ai.request.messages from raw messages."""
-    sentry_messages = []
-    legacy_messages = []
+    """Build gen_ai.input.messages and gen_ai.request.messages from the last raw message."""
     system_instructions = None
 
     for msg in messages:
-        role = msg["role"]
         content = msg.get("content", "")
-
-        if role == "system":
+        if msg["role"] == "system":
             system_instructions = content if isinstance(content, str) else json.dumps(content)
 
-        # New OTel format with parts
-        parts = []
-        if isinstance(content, str):
-            parts.append({"type": "text", "text": content})
-        elif isinstance(content, list):
-            for part in content:
-                if part.get("type") == "text":
-                    parts.append({"type": "text", "text": part["text"]})
-                elif part.get("type") == "image":
-                    parts.append({"type": "text", "text": "[Blob substitute]"})
-
-        sentry_messages.append({
-            "role": role,
-            "content": content if isinstance(content, str) else "[multimodal]",
-            "parts": parts,
-        })
-
-        # Legacy format with binary redaction
-        if isinstance(content, list):
-            redacted = []
-            for part in content:
-                if part.get("type") == "text":
-                    redacted.append(part)
-                elif part.get("type") == "image":
-                    redacted.append({"type": "text", "text": "[Blob substitute]"})
-            legacy_messages.append({"role": role, "content": redacted})
-        else:
-            legacy_messages.append({"role": role, "content": content})
+    if not messages:
+        return [], [], system_instructions
+
+    last_message = messages[-1]
+    role = last_message["role"]
+    content = last_message.get("content", "")
+
+    parts = []
+    if isinstance(content, str):
+        parts.append({"type": "text", "text": content})
+    elif isinstance(content, list):
+        for part in content:
+            if part.get("type") == "text":
+                parts.append({"type": "text", "text": part["text"]})
+            elif part.get("type") == "image":
+                parts.append({"type": "text", "text": "[Blob substitute]"})
+
+    sentry_messages = [{
+        "role": role,
+        "content": content if isinstance(content, str) else "[multimodal]",
+        "parts": parts,
+    }]
+
+    if isinstance(content, list):
+        redacted = []
+        for part in content:
+            if part.get("type") == "text":
+                redacted.append(part)
+            elif part.get("type") == "image":
+                redacted.append({"type": "text", "text": "[Blob substitute]"})
+        legacy_messages = [{"role": role, "content": redacted}]
+    else:
+        legacy_messages = [{"role": role, "content": content}]
 
     return sentry_messages, legacy_messages, system_instructions
 {% endblock %}
diff --git a/src/test-cases/checks.ts b/src/test-cases/checks.ts
index 4bd09ef..a2a4ab4 100644
--- a/src/test-cases/checks.ts
+++ b/src/test-cases/checks.ts
@@ -108,6 +108,220 @@ export function checkAISpanCount(
 // Span Type Attribute Checks
 // =============================================================================
 
+function parseInputMessages(
+  span: CapturedSpan,
+): { messages?: unknown[]; attribute?: string; error?: string } {
+  const result = getAttributeWithFallback(
+    span,
+    "gen_ai.input.messages",
+    "gen_ai.request.messages",
+  );
+
+  if (result.value === undefined) {
+    return {
+      attribute: result.usedAttribute ?? "gen_ai.input.messages",
+      error: "Missing messages attribute",
+    };
+  }
+
+  if (typeof result.value === "string") {
+    try {
+      const parsed = JSON.parse(result.value);
+      if (!Array.isArray(parsed)) {
+        return {
+          attribute: result.usedAttribute!,
+          error: `${result.usedAttribute} should be an array`,
+        };
+      }
+      return { messages: parsed, attribute: result.usedAttribute! };
+    } catch {
+      return {
+        attribute: result.usedAttribute!,
+        error: `Invalid JSON in ${result.usedAttribute}`,
+      };
+    }
+  }
+
+  if (!Array.isArray(result.value)) {
+    return {
+      attribute: result.usedAttribute!,
+      error: `${result.usedAttribute} should be an array`,
+    };
+  }
+
+  return { messages: result.value as unknown[], attribute: result.usedAttribute! };
+}
+
+function getMessageText(message: unknown): string | undefined {
+  if (typeof message !== "object" || message === null) {
+    return undefined;
+  }
+
+  const msg = message as {
+    content?: unknown;
+    parts?: Array<{ type?: unknown; text?: unknown; content?: unknown }>;
+  };
+
+  if (typeof msg.content === "string") {
+    return msg.content;
+  }
+
+  if (Array.isArray(msg.content)) {
+    const textParts = msg.content
+      .map((part) => {
+        if (typeof part === "string") {
+          return part;
+        }
+        if (typeof part === "object" && part !== null) {
+          const candidate = part as { text?: unknown; content?: unknown; type?: unknown };
+          if (candidate.type === "text" && typeof candidate.text === "string") {
+            return candidate.text;
+          }
+          if (typeof candidate.text === "string") {
+            return candidate.text;
+          }
+          if (typeof candidate.content === "string") {
+            return candidate.content;
+          }
+        }
+        return undefined;
+      })
+      .filter((part): part is string => typeof part === "string");
+
+    if (textParts.length > 0) {
+      return textParts.join("\n");
+    }
+  }
+
+  if (typeof msg.content === "object" && msg.content !== null) {
+    const candidate = msg.content as { text?: unknown; content?: unknown };
+    if (typeof candidate.text === "string") {
+      return candidate.text;
+    }
+    if (typeof candidate.content === "string") {
+      return candidate.content;
+    }
+  }
+
+  if (Array.isArray(msg.parts)) {
+    const textParts = msg.parts
+      .map((part) => {
+        if (part?.type === "text" && typeof part.text === "string") {
+          return part.text;
+        }
+        if (typeof part?.text === "string") {
+          return part.text;
+        }
+        if (typeof part?.content === "string") {
+          return part.content;
+        }
+        return undefined;
+      })
+      .filter((part): part is string => typeof part === "string");
+
+    if (textParts.length > 0) {
+      return textParts.join("\n");
+    }
+  }
+
+  return undefined;
+}
+
+function messagesMatchLoosely(actual: unknown, expected: unknown): boolean {
+  if (typeof actual !== "object" || actual === null) {
+    return false;
+  }
+  if (typeof expected !== "object" || expected === null) {
+    return false;
+  }
+
+  const actualMessage = actual as { role?: unknown };
+  const expectedMessage = expected as { role?: unknown };
+
+  if (actualMessage.role !== expectedMessage.role) {
+    return false;
+  }
+
+  const actualText = getMessageText(actual);
+  const expectedText = getMessageText(expected);
+
+  if (actualText !== undefined || expectedText !== undefined) {
+    return actualText === expectedText;
+  }
+
+  return JSON.stringify(actual) === JSON.stringify(expected);
+}
+
+function assertOnlyLastInputMessage(
+  spans: CapturedSpan[],
+  testDef: { inputs: Array<{ messages?: unknown[] }> },
+  spanType: "chat" | "agent",
+): void {
+  if (testDef.inputs.length <= 1) {
+    return;
+  }
+
+  const sortedSpans = [...spans].sort(
+    (a, b) => a.start_timestamp - b.start_timestamp,
+  );
+
+  if (sortedSpans.length < testDef.inputs.length) {
+    throw new CheckError(
+      `Expected at least ${testDef.inputs.length} ${spanType} span(s) for last-message validation, found ${sortedSpans.length}`,
+    );
+  }
+
+  const errors: string[] = [];
+  const locations: ErrorLocation[] = [];
+
+  for (let i = 0; i < testDef.inputs.length; i++) {
+    const span = sortedSpans[i];
+    const expectedMessages = testDef.inputs[i]?.messages;
+    const expectedLastMessage = expectedMessages?.[expectedMessages.length - 1];
+
+    if (expectedLastMessage === undefined) {
+      continue;
+    }
+
+    const parsed = parseInputMessages(span);
+    if (parsed.error || !parsed.attribute || !parsed.messages) {
+      const message = parsed.error ?? "Missing messages attribute";
+      errors.push(`${spanType} span ${i} ${message}`);
+      locations.push({
+        spanId: span.span_id,
+        attribute: parsed.attribute ?? "gen_ai.input.messages",
+        message,
+      });
+      continue;
+    }
+
+    if (parsed.messages.length !== 1) {
+      const message = `${spanType} span ${i} should keep only the last input message, found ${parsed.messages.length} message(s)`;
+      errors.push(message);
+      locations.push({
+        spanId: span.span_id,
+        attribute: parsed.attribute,
+        message,
+      });
+      continue;
+    }
+
+    if (!messagesMatchLoosely(parsed.messages[0], expectedLastMessage)) {
+      const message = `${spanType} span ${i} should keep only the last input message`;
+      errors.push(message);
+      locations.push({
+        spanId: span.span_id,
+        attribute: parsed.attribute,
+        message,
+      });
+    }
+  }
+
+  if (errors.length > 0) {
+    throw new CheckError(errors.join("\n"), locations);
+  }
+}
+
 /**
  * Check attributes on chat/completion spans (LLM API calls)
  *
@@ -177,6 +391,8 @@ export const checkChatSpanAttributes: Check = {
     if (errors.length > 0) {
       throw new CheckError(errors.join("\n"), locations);
     }
+
+    assertOnlyLastInputMessage(chatSpans, testDef, "chat");
   },
 };
 
@@ -216,6 +432,7 @@ export const checkAgentSpanAttributes: Check = {
     }
 
     assertAttributes(agentSpans, attrs);
+    assertOnlyLastInputMessage(agentSpans, testDef, "agent");
   },
 };