diff --git a/packages/llm/src/prompts/patterns-detection.ts b/packages/llm/src/prompts/patterns-detection.ts
new file mode 100644
index 0000000..b7b547e
--- /dev/null
+++ b/packages/llm/src/prompts/patterns-detection.ts
@@ -0,0 +1,55 @@
+import type { LLMMessage, CodeStructure } from "@codeboard/shared";
+
+export function buildPatternsPrompt(structure: CodeStructure): LLMMessage[] {
+  const sampleFunctions = structure.files
+    .flatMap((f) => f.functions.map((fn) => `${f.path}: ${fn.name}(${fn.params.join(", ")})`))
+    .slice(0, 40)
+    .join("\n");
+
+  const sampleClasses = structure.files
+    .flatMap((f) => f.classes.map((c) => `${f.path}: class ${c.name} [${c.methods.map((m) => m.name).join(", ")}]`))
+    .slice(0, 20)
+    .join("\n");
+
+  const importSources = new Set<string>();
+  for (const f of structure.files) {
+    for (const imp of f.imports) {
+      importSources.add(imp.source);
+    }
+  }
+
+  return [
+    {
+      role: "system",
+      content: `You are a code reviewer identifying patterns and conventions in a codebase.
+
+Output format:
+## Coding Conventions
+[list conventions like naming patterns, file organization, error handling approach]
+
+## Design Patterns
+[list design patterns detected: factory, singleton, observer, repository, etc.]
+
+## Architectural Decisions
+[list key architectural decisions: monorepo vs polyrepo, framework choices, state management, etc.]`,
+    },
+    {
+      role: "user",
+      content: `Analyze these code patterns:
+
+FUNCTION SIGNATURES:
+${sampleFunctions}
+
+CLASS DEFINITIONS:
+${sampleClasses}
+
+EXTERNAL DEPENDENCIES:
+${Array.from(importSources).filter((s) => !s.startsWith(".")).slice(0, 30).join(", ")}
+
+DETECTED PATTERNS FROM AST:
+${structure.patterns.map((p) => `  ${p.name}: ${p.description}`).join("\n") || "  (none pre-detected)"}
+
+Identify coding conventions, design patterns, and architectural decisions.`,
+    },
+  ];
+}