diff --git a/src/Debug/debugger.cpp b/src/Debug/debugger.cpp
index 8828f2fe..c2b2ce63 100644
--- a/src/Debug/debugger.cpp
+++ b/src/Debug/debugger.cpp
@@ -3,6 +3,7 @@
 #include <algorithm>
 #include <cinttypes>
 #include <cstring>
+#include <optional>
 #ifndef ARDUINO
 #include <nlohmann/json.hpp>
 #else
@@ -332,6 +333,14 @@ bool Debugger::checkDebugMessages(Module *m, RunningState *program_state) {
             this->dumpCallbackmapping();
             free(interruptData);
             break;
+        case interruptSetOverridePinValue:
+            this->addOverride(m, interruptData + 1);
+            free(interruptData);
+            break;
+        case interruptUnsetOverridePinValue:
+            this->removeOverride(m, interruptData + 1);
+            free(interruptData);
+            break;
         default:
             // handle later
             this->channel->write("COULD not parse interrupt data!\n");
@@ -725,7 +734,7 @@ bool Debugger::handlePushedEvent(char *bytes) const {
 }
 
 void Debugger::snapshot(Module *m) const {
-    uint16_t numberBytes = 11;
+    uint16_t numberBytes = 12;
     uint8_t state[] = {pcState,
                        breakpointsState,
                        callstackState,
@@ -736,7 +745,8 @@ void Debugger::snapshot(Module *m) const {
                        stackState,
                        callbacksState,
                        eventsState,
-                       ioState};
+                       ioState,
+                       overridesState};
     inspect(m, numberBytes, state);
 }
 
@@ -885,6 +895,23 @@ void Debugger::inspect(Module *m, const uint16_t sizeStateArray,
                 addComma = true;
                 break;
             }
+            case overridesState: {
+                this->channel->write("%s", addComma ? "," : "");
+                this->channel->write(R"("overrides": [)");
+                bool comma = false;
+                for (auto key : overrides) {
+                    for (auto argResult : key.second) {
+                        this->channel->write("%s", comma ? ", " : "");
+                        this->channel->write(
+                            R"({"fidx": %d, "arg": %d, "return_value": %d})",
+                            key.first, argResult.first, argResult.second);
+                        comma = true;
+                    }
+                }
+                this->channel->write("]");
+                addComma = true;
+                break;
+            }
             default: {
                 debug("dumpExecutionState: Received unknown state request\n");
                 break;
@@ -1240,6 +1267,19 @@ bool Debugger::saveState(Module *m, uint8_t *interruptData) {
                 restore_external_state(m, external_state);
                 break;
             }
+            case overridesState: {
+                debug("receiving overridesState\n");
+                overrides.clear();
+                uint8_t overrides_count = *program_state++;
+                for (uint32_t i = 0; i < overrides_count; i++) {
+                    uint32_t fidx = read_B32(&program_state);
+                    uint32_t arg = read_B32(&program_state);
+                    uint32_t return_value = read_B32(&program_state);
+                    overrides[fidx][arg] = return_value;
+                    debug("Override %d %d %d\n", fidx, arg, return_value);
+                }
+                break;
+            }
             default: {
                 FATAL("saveState: Received unknown program state\n");
             }
@@ -1411,6 +1451,66 @@ bool Debugger::reset(Module *m) const {
     return true;
 }
 
+std::optional<uint32_t> resolve_imported_function(Module *m,
+                                                  std::string function_name) {
+    for (uint32_t fidx = 0; fidx < m->import_count; fidx++) {
+        if (!strcmp(m->functions[fidx].import_field, function_name.c_str())) {
+            return fidx;
+        }
+    }
+    return {};
+}
+
+std::string read_string(uint8_t **pos) {
+    std::string str = "";
+    char c = *(*pos)++;
+    while (c != '\0') {
+        str += c;
+        c = *(*pos)++;
+    }
+    return str;
+}
+
+void Debugger::addOverride(Module *m, uint8_t *interruptData) {
+    std::string primitive_name = read_string(&interruptData);
+    uint32_t arg = read_B32(&interruptData);
+    uint32_t result = read_B32(&interruptData);
+
+    std::optional<uint32_t> fidx = resolve_imported_function(m, primitive_name);
+    if (!fidx) {
+        channel->write(
+            "Cannot override the result for unknown function \"%s\".\n",
+            primitive_name.c_str());
+        return;
+    }
+
+    channel->write("Override %s(%d) = %d.\n", primitive_name.c_str(), arg,
+                   result);
+    overrides[fidx.value()][arg] = result;
+}
+
+void Debugger::removeOverride(Module *m, uint8_t *interruptData) {
+    std::string primitive_name = read_string(&interruptData);
+    uint32_t arg = read_B32(&interruptData);
+
+    std::optional<uint32_t> fidx = resolve_imported_function(m, primitive_name);
+    if (!fidx) {
+        channel->write("Cannot remove override for unknown function \"%s\".\n",
+                       primitive_name.c_str());
+        return;
+    }
+
+    if (overrides[fidx.value()].count(arg) == 0) {
+        channel->write("Override for %s(%d) not found.\n",
+                       primitive_name.c_str(), arg);
+        return;
+    }
+
+    channel->write("Removing override %s(%d) = %d.\n", primitive_name.c_str(),
+                   arg, overrides[fidx.value()][arg]);
+    overrides[fidx.value()].erase(arg);
+}
+
 Debugger::~Debugger() {
     this->disconnect_proxy();
     this->stop();
diff --git a/src/Debug/debugger.h b/src/Debug/debugger.h
index 0dbb1cca..c43a00af 100644
--- a/src/Debug/debugger.h
+++ b/src/Debug/debugger.h
@@ -49,6 +49,7 @@ enum ExecutionState {
     callbacksState = 0x09,
     eventsState = 0x0A,
     ioState = 0x0B,
+    overridesState = 0x0C,
 };
 
 enum InterruptTypes {
@@ -90,10 +91,13 @@ enum InterruptTypes {
     interruptDUMPCallbackmapping = 0x74,
     interruptRecvCallbackmapping = 0x75,
 
+    // Primitive overrides
+    interruptSetOverridePinValue = 0x80,
+    interruptUnsetOverridePinValue = 0x81,
+
     // Operations
     interruptStore = 0xa0,
     interruptStored = 0xa1,
-
 };
 
 class Debugger {
@@ -117,6 +121,9 @@ class Debugger {
 
     bool asyncSnapshots;
 
+    std::unordered_map<uint32_t, std::unordered_map<uint32_t, uint32_t>>
+        overrides;
+
     // Private methods
 
     void printValue(const StackValue *v, uint32_t idx, bool end) const;
@@ -270,4 +277,15 @@ class Debugger {
     void notifyPushedEvent() const;
 
     bool handlePushedEvent(char *bytes) const;
+
+    // Concolic Multiverse Debugging
+    inline bool isMocked(uint32_t fidx, uint32_t argument) {
+        return overrides.count(fidx) > 0 && overrides[fidx].count(argument) > 0;
+    }
+    inline uint32_t getMockedValue(uint32_t fidx, uint32_t argument) {
+        return overrides[fidx][argument];
+    }
+
+    void addOverride(Module *m, uint8_t *interruptData);
+    void removeOverride(Module *m, uint8_t *interruptData);
 };
diff --git a/src/Interpreter/instructions.cpp b/src/Interpreter/instructions.cpp
index 9046c0d0..0e4ceb3c 100644
--- a/src/Interpreter/instructions.cpp
+++ b/src/Interpreter/instructions.cpp
@@ -287,6 +287,16 @@ bool i_instr_call(Module *m) {
     }
 
     if (fidx < m->import_count) {
+        // Mocking only works on primitives, no need to check for it otherwise.
+        if (m->sp >= 0) {
+            uint32_t arg = m->stack[m->sp].value.uint32;
+            if (m->warduino->debugger->isMocked(fidx, arg)) {
+                m->stack[m->sp].value.uint32 =
+                    m->warduino->debugger->getMockedValue(fidx, arg);
+                return true;
+            }
+        }
+
         return ((Primitive)m->functions[fidx].func_ptr)(m);
     } else {
         if (m->csp >= CALLSTACK_SIZE) {