diff --git a/harbor/cli.py b/harbor/cli.py
index 0422f26..ec4444d 100644
--- a/harbor/cli.py
+++ b/harbor/cli.py
@@ -255,7 +255,7 @@ def build_parser():
     p = sub.add_parser("who", help="who has worked on a scope (notes, reviews, claims, messages)")
     p.add_argument("scope")
     p = sub.add_parser("re", help="reverse-engineering lookups from the project's image and annotations")
-    p.add_argument("action", choices=["disasm", "xrefs", "bytes", "lookup", "ghidra", "ghidra-tools", "debugger-tools", "match", "similar", "struct-audit"])
+    p.add_argument("action", choices=["disasm", "xrefs", "bytes", "lookup", "ghidra", "ghidra-tools", "debugger-tools", "match", "similar", "struct-audit", "behaviour"])
     p.add_argument("target", nargs="?", default="", help="an address (0x4AD850) or a name; for ghidra, the tool")
     p.add_argument("arguments", nargs="?", default="{}", help="ghidra: the tool's arguments as JSON")
     p.add_argument("--decomp", help="match: the checkout or worktree to compile from")
@@ -690,8 +690,12 @@ def main(argv=None):
         out(r, f"summary #{r['id']} pinned to {r['thread']}")
     elif cmd == "re":
         from .reverse import Ghidra, Reverse
-        r = None if args.action.startswith(("ghidra", "debugger", "match")) else Reverse(harbor.project.reverse)
-        if args.action in ("similar", "struct-audit"):
+        r = None if args.action.startswith(("ghidra", "debugger", "match", "behaviour")) else Reverse(harbor.project.reverse)
+        if args.action == "behaviour":
+            from .reverse import behaviour_check
+            res = behaviour_check(harbor.project.reverse, args.target, args.decomp)
+            out(res, "\n".join(f"{r['address']} {r.get('verdict', ''):<8} {r.get('name', '')} {r.get('reason') or ''}" for r in res))
+        elif args.action in ("similar", "struct-audit"):
             from .reverse import run_toolkit
             res = (run_toolkit(harbor.project.reverse, "similar", [args.target]) if args.action == "similar" else
                    run_toolkit(harbor.project.reverse, "struct_audit", ["--struct", args.target], json_to_file=True))
diff --git a/harbor/mcp_server.py b/harbor/mcp_server.py
index c9d624d..679af1e 100644
--- a/harbor/mcp_server.py
+++ b/harbor/mcp_server.py
@@ -539,6 +539,20 @@ def re_match_program(targets: str = "", decomp: str | None = None) -> dict:
         raise ToolError(str(error)) from None
 
 
+@mcp.tool()
+def re_behaviour(targets: str, decomp: str | None = None) -> list:
+    """The emulator check: run the game's own machine code and your compiled C of each function (up to 30
+    addresses; decomp = your worktree) from the same registers, stack and memory, 12 trials each, and compare the
+    result, every memory write, the calls made and their arguments. verdict 'same' means it behaves like the game;
+    'differs' gives the first divergence in words (e.g. 'returns 1 vs 0', 'original writes 0x86BAC0=5, ours 4');
+    'weak' means no trial ran to the end in both. Use it before submitting anything that does not byte-match."""
+    try:
+        from .reverse import behaviour_check
+        return behaviour_check(harbor().project.reverse, targets, decomp)
+    except (HarborError, projects.ProjectError) as error:
+        raise ToolError(str(error)) from None
+
+
 @mcp.tool()
 def re_similar(target: str, k: int = 10, min_score: float = 0.7) -> dict:
     """Functions whose instructions look like this one (normalized: addresses blanked, registers and small
diff --git a/harbor/reverse.py b/harbor/reverse.py
index f1a3a94..72bee46 100644
--- a/harbor/reverse.py
+++ b/harbor/reverse.py
@@ -614,3 +614,31 @@ def match_program(config, targets=None, decomp=None):
             "ret_pops", "ours_ret_pops", "relocation_problems", "note")
     return {"counts": counts, "results": [{k: r[k] for k in keep if k in r} for r in rows] if wanted else
             [{k: r[k] for k in keep if k in r} for r in rows if r.get("status") != "differs"][:200]}
+
+
+def behaviour_check(config, targets, decomp=None, precision=24):
+    """Run the original machine code and our compiled C of each function in an emulator from the same inputs and
+    compare results, memory writes and calls (diffrun.py). targets: 1 to 30 addresses."""
+    import subprocess
+    import tempfile
+    config = config or {}
+    command = config.get("diffrun_command")
+    if not command:
+        raise HarborError("this project has no diffrun_command in its [reverse] table")
+    wanted = [t.strip() for t in re.split(r"[,\s]+", str(targets)) if t.strip()]
+    if not wanted or len(wanted) > 30:
+        raise HarborError("give 1 to 30 addresses")
+    command = command if isinstance(command, list) else [command]
+    with tempfile.TemporaryDirectory(dir=config.get("debugger_temp")) as tmp:
+        out = Path(tmp) / "behaviour.json"
+        argv = [*command, *wanted, "--json", str(out), "--precision", str(int(precision)), "--jobs", "4"]
+        if decomp:
+            argv += ["--decomp", str(decomp)]
+        result = subprocess.run(argv, capture_output=True, text=True, encoding="utf-8", errors="replace", timeout=1500,
+                                cwd=str(Path(command[-1]).parent),
+                                creationflags=getattr(subprocess, "CREATE_NO_WINDOW", 0))
+        if not out.exists():
+            raise HarborError("the emulator check failed: " + (result.stdout + result.stderr).strip()[-800:])
+        rows = json.loads(out.read_text(encoding="utf-8"))
+    keep = ("address", "name", "verdict", "reason", "weak", "counts", "notes")
+    return [{k: r[k] for k in keep if k in r} for r in rows]