Interprets Arduino/ESP32/RP2040 compiler errors in plain English for beginners. Use when user shares error messages, compilation failures, upload problems, or asks "what does this error mean"...
Translates cryptic compiler errors into actionable fixes for Arduino/ESP32/RP2040 projects.
This skill includes bundled tools:
Analyze error from file:
uv run --no-project scripts/parse_errors.py --file error_log.txt
Analyze single error:
uv run --no-project scripts/parse_errors.py --message "error: 'LED' was not declared in this scope"
Interactive mode:
uv run --no-project scripts/parse_errors.py --interactive
Pipe from compiler:
arduino-cli compile 2>&1 | uv run --no-project scripts/parse_errors.py --stdin
When user pastes an error message:
error: 'xyz' was not declared in this scope
Meaning: Compiler doesn't recognize the name xyz.
Common Causes & Fixes:
| Cause | Fix |
|---|---|
| Typo in variable/function name | Check spelling, C++ is case-sensitive! |
| Variable used before declaration | Move declaration before first use |
Missing #include |
Add required library header |
| Function defined after it's called | Add forward declaration or move function up |
Example - Typo:
// WRONG
int ledpin = 13; // lowercase 'p'
digitalWrite(ledPin, HIGH); // uppercase 'P' - DIFFERENT!
// CORRECT
int ledPin = 13;
digitalWrite(ledPin, HIGH);
Example - Missing Include:
// WRONG - Servo not defined!
Servo myServo;
// CORRECT
#include <Servo.h>
Servo myServo;
error: expected ';' before 'xyz'
Meaning: Missing semicolon on the previous line.
Fix: Add ; at end of the line ABOVE the error.
// WRONG - error points to line 3
int x = 5 // ← missing ; here!
int y = 10;
// CORRECT
int x = 5;
int y = 10;
Pro Tip: The error line number is where compiler noticed the problem, not where the missing ; is!
error: expected ')' before ';'
error: expected '}' at end of input
Meaning: Mismatched parentheses or braces.
Common Patterns:
// WRONG - unmatched parenthesis
if (x > 5 { // missing )
}
// WRONG - unmatched brace
void setup() {
Serial.begin(115200);
// missing closing }
// CORRECT
if (x > 5) {
}
void setup() {
Serial.begin(115200);
}
Debugging Tip: Use IDE auto-format (Ctrl+T) to reveal mismatches.
error: invalid conversion from 'const char*' to 'char*'
Meaning: Trying to modify a string literal (which is read-only).
// WRONG
char* message = "Hello"; // string literals are const!
message[0] = 'h'; // can't modify!
// CORRECT (if you need to modify)
char message[] = "Hello"; // creates modifiable copy
message[0] = 'h'; // OK!
// CORRECT (if read-only is fine)
const char* message = "Hello";
error: no matching function for call to 'SomeClass::method(int, int, int)'
note: candidate: void SomeClass::method(int, int)
Meaning: Function called with wrong number or type of arguments.
// WRONG - too many arguments
myServo.write(90, 100); // write() takes only 1 argument!
// CORRECT
myServo.write(90);
Read the "note:" lines - they show what arguments ARE accepted.
error: multiple definition of 'xyz'
Meaning: Same variable/function defined in multiple files.
Fixes:
// In header file (.h), use 'extern':
extern int globalVar; // declaration only
// In ONE .cpp file, define it:
int globalVar = 0; // actual definition
Or for functions in header:
// WRONG - defined in header, included multiple times
int add(int a, int b) { return a + b; }
// CORRECT - use 'inline'
inline int add(int a, int b) { return a + b; }
error: 'WiFiClient' does not name a type
Meaning: Class/type not recognized.
Fixes:
| Board | Library | Include |
|---|---|---|
| ESP32 | WiFi | #include <WiFi.h> |
| ESP8266 | WiFi | #include <ESP8266WiFi.h> |
| Arduino + WiFi Shield | WiFi | #include <WiFi.h> |
// WRONG
WiFiClient client; // WiFiClient unknown!
// CORRECT for ESP32
#include <WiFi.h>
WiFiClient client;
error: redefinition of 'int x'
Meaning: Variable declared twice in same scope.
// WRONG
int count = 0;
int count = 0; // redefinition!
// WRONG in loops
for (int i = 0; i < 10; i++) {
int i = 5; // shadows loop variable!
}
// CORRECT
int count = 0; // declare once
count = 5; // assign without 'int'
avrdude: stk500_recv(): programmer is not responding
avrdude: stk500_getsync() attempt X of 10: not in sync
Meaning: Arduino IDE can't communicate with the board.
Fixes (try in order):
A fatal error occurred: Failed to connect to ESP32:
Timed out waiting for packet header
Meaning: ESP32 not entering bootloader mode.
Fix: Hold BOOT button while uploading:
Sketch too big; see https://support.arduino.cc/...
Sketch uses 34816 bytes (107%) of program storage space.
Maximum is 32256 bytes.
Meaning: Program doesn't fit in flash memory.
Fixes:
// 1. Use F() macro for strings (saves RAM + sometimes Flash)
Serial.println(F("This string in flash")); // instead of RAM
// 2. Remove unused libraries
// Each #include adds code even if not used
// 3. Use smaller data types
uint8_t x = 5; // instead of int (2 bytes saved)
// 4. Choose board with more flash (ESP32 has 4MB vs Arduino's 32KB)
fatal error: Adafruit_BME280.h: No such file or directory
Meaning: Library not installed.
Fix:
exit status 1
Error compiling for board Arduino Uno.
Meaning: Generic error - scroll UP to find the real error message.
The actual error is ABOVE this line! Look for lines containing error:.
error: cannot convert 'String' to 'const char*'
Meaning: Function expects C-string but got Arduino String.
// WRONG
String myString = "hello";
someFunction(myString); // if someFunction expects const char*
// CORRECT
String myString = "hello";
someFunction(myString.c_str()); // convert to C-string
error: invalid operands of types 'const char*' and 'const char*' to binary 'operator+'
Meaning: Can't use + with C-strings.
// WRONG
const char* a = "hello";
const char* b = " world";
const char* c = a + b; // doesn't work!
// CORRECT - use String
String a = "hello";
String b = " world";
String c = a + b; // works!
// Or use snprintf
char c[20];
snprintf(c, sizeof(c), "%s%s", a, b);
| Error Contains | Likely Problem | Quick Fix |
|---|---|---|
| "not declared" | Typo or missing include | Check spelling, add #include |
| "expected ';'" | Missing semicolon | Add ; to line ABOVE error |
| "expected ')'" | Unmatched parenthesis | Count ( and ) |
| "expected '}'" | Unmatched brace | Count { and } |
| "no matching function" | Wrong arguments | Check function signature |
| "does not name a type" | Missing library | Add #include |
| "multiple definition" | Defined in multiple files | Use extern |
| "stk500" | Upload failed | Check board/port/cable |
| "No such file" | Library not installed | Install via Library Manager |
| "too big" | Out of flash | Use F(), remove unused code |
1. Read the FIRST error (later ones often cascade)
2. Note the FILE and LINE NUMBER
3. Look at that line AND the line above
4. Check for common patterns above
5. Fix ONE error at a time, recompile
6. Repeat until clean
