diff --git a/README.md b/README.md
index ede0fab..a03f3e9 100644
--- a/README.md
+++ b/README.md
@@ -142,8 +142,9 @@ a string (submitted with a newline).
 If the user were to enter `fieldvalue` in response to the prompt, the `runner`
 script would then have access to an environment variable `FIELDNAME` set to
 `fieldvalue`. Field names should respect the [POSIX standard](https://pubs.opengroup.org/onlinepubs/9699919799/basedefs/V1_chap08.html)
-on environment variable naming. In particular, all field names consist solely
-of alphanumeric characters or underscores and cannot start with a digit.
+on environment variable naming. That is, field names must consist solely of
+alphanumeric characters or underscores and are not permitted to start with a
+digit.
 
 #### Types
 
@@ -151,8 +152,18 @@ The value of `type` determines how a field is prompted for. Note the value of
 `type` is case insenstive. The currently supported list of types are:
 
 * `line`
-  * The simplest prompt type. Takes in a free-form response submitted after a
-    newline (`\n`) is encountered.
+  * The simplest prompt type. Takes in a free-form response submitted after
+    encountering a newline (`\n`). The resulting environment variable has
+    leading and trailing whitespace trimmed.
+  * If `required`, whitespace-only strings are re-prompted.
+
+#### Options
+
+Additional options can be included in a field definition.
+
+* `required`
+  * A value must be specified. How this option is interpreted depends on `type`.
+  * Defaults to `true`.
 
 #### Root Directory
 
diff --git a/include/console.h b/include/console.h
new file mode 100644
index 0000000..c0d3dd3
--- /dev/null
+++ b/include/console.h
@@ -0,0 +1,31 @@
+/**
+@file
+@brief Console output.
+*/
+#ifndef _BOOTSTRAP_CONSOLE_H
+#define _BOOTSTRAP_CONSOLE_H
+
+// clang-format off
+
+#define ANSI_BLACK  "\e[0;30m"
+#define ANSI_RED    "\e[0;31m"
+#define ANSI_GREEN  "\e[0;32m"
+#define ANSI_YELLOW "\e[0;33m"
+#define ANSI_BLUE   "\e[0;34m"
+#define ANSI_PURPLE "\e[0;35m"
+#define ANSI_CYAN   "\e[0;36m"
+#define ANSI_WHITE  "\e[0;37m"
+#define ANSI_RESET  "\e[0m"
+
+#define ANSI_BLACK_F(...)  ANSI_BLACK , __VA_ARGS__, ANSI_RESET
+#define ANSI_RED_F(...)    ANSI_RED   , __VA_ARGS__, ANSI_RESET
+#define ANSI_GREEN_F(...)  ANSI_GREEN , __VA_ARGS__, ANSI_RESET
+#define ANSI_YELLOW_F(...) ANSI_YELLOW, __VA_ARGS__, ANSI_RESET
+#define ANSI_BLUE_F(...)   ANSI_BLUE  , __VA_ARGS__, ANSI_RESET
+#define ANSI_PURPLE_F(...) ANSI_PURPLE, __VA_ARGS__, ANSI_RESET
+#define ANSI_CYAN_F(...)   ANSI_CYAN  , __VA_ARGS__, ANSI_RESET
+#define ANSI_WHITE_F(...)  ANSI_WHITE , __VA_ARGS__, ANSI_RESET
+
+// clang-format on
+
+#endif /* _BOOTSTRAP_CONSOLE_H */
diff --git a/include/error.h b/include/error.h
index 19b053f..fdfc958 100644
--- a/include/error.h
+++ b/include/error.h
@@ -40,12 +40,14 @@ enum ErrorCode {
   /// A field name in `spec.json` is not alphanumeric and beginning with a
   /// non-digit.
   ERROR_VALIDATOR_FIELD_NAME_INVALID,
-  /// The `type` of a `spec.json` field is not a string.
+  /// The `type` field of a `spec.json` file is not a string.
   ERROR_VALIDATOR_FIELD_TYPE_INVALID,
-  /// The `type` of a `spec.json` field does not correspond to a known prompt
-  /// type.
+  /// The `type` field of a `spec.json` file does not correspond to a known
+  /// prompt type.
   ERROR_VALIDATOR_FIELD_TYPE_UNKNOWN,
-  /// The `prompt` of a `spec.json` field is not a string.
+  /// The `required` field of a `spec.json` file is not a boolean.
+  ERROR_VALIDATOR_FIELD_REQUIRED_INVALID,
+  /// The `prompt` field of a `spec.json` file is not a string.
   ERROR_VALIDATOR_FIELD_PROMPT_INVALID,
 
   /// The `runner` file could not be found.
@@ -75,7 +77,7 @@ static inline struct Error *priv_error_new(
   for (int i = 0; i < n; ++i) {
     string_buf_sappend(sb, messages[i]);
   }
-  e->message = string_buf_convert(sb);
+  e->message = string_buf_cast(sb);
   return e;
 }
 
@@ -126,15 +128,6 @@ It is the responsibility of the caller to free the @ref Error instance.
 
 // clang-format on
 
-#define ANSI_BLACK(...) "\e[0;30m", __VA_ARGS__, "\e[0m"
-#define ANSI_RED(...) "\e[0;31m", __VA_ARGS__, "\e[0m"
-#define ANSI_GREEN(...) "\e[0;32m", __VA_ARGS__, "\e[0m"
-#define ANSI_YELLOW(...) "\e[0;33m", __VA_ARGS__, "\e[0m"
-#define ANSI_BLUE(...) "\e[0;34m", __VA_ARGS__, "\e[0m"
-#define ANSI_PURPLE(...) "\e[0;35m", __VA_ARGS__, "\e[0m"
-#define ANSI_CYAN(...) "\e[0;36m", __VA_ARGS__, "\e[0m"
-#define ANSI_WHITE(...) "\e[0;37m", __VA_ARGS__, "\e[0m"
-
 /**
 @brief Deallocates a previously allocated @ref Error isntance.
 
diff --git a/include/string_buf.h b/include/string_buf.h
index 156e761..7c9ec53 100644
--- a/include/string_buf.h
+++ b/include/string_buf.h
@@ -75,7 +75,7 @@ of the internal array the necessary number of times to accommodate.
 void string_buf_sappend(struct StringBuf *sb, const char s[static 1]);
 
 /**
-@brief Converts a @ref StringBuf instance into a `char*`.
+@brief Casts a @ref StringBuf instance into a `char*`.
 
 This function frees the memory associated with @p sb.
 
@@ -85,7 +85,7 @@ This function frees the memory associated with @p sb.
  A null pointer if @p sb is null. Otherwise a NUL-terminated string
  corresponding to the value of @p sb. The caller takes ownership of this value.
 */
-const char *string_buf_convert(struct StringBuf *sb);
+const char *string_buf_cast(struct StringBuf *sb);
 
 /**
 @brief Deallocates a previously allocated @ref StringBuf instance.
diff --git a/include/string_utils.h b/include/string_utils.h
index e7a6d20..1095d14 100644
--- a/include/string_utils.h
+++ b/include/string_utils.h
@@ -42,4 +42,20 @@ This function operates like `strcmp` except comparison ignores case.
 */
 int strcmp_ci(const char *s1, const char *s2);
 
+/**
+Removes any leading whitespace characters from the string in-place.
+
+@param s
+ The string to trim the start of.
+*/
+void trim_leading(char *s);
+
+/**
+Removes any tailing whitespace characters from the string in-place.
+
+@param s
+ The string to trim the end of.
+*/
+void trim_trailing(char *s);
+
 #endif /* _BOOTSTRAP_STRING_UTILS_H */
diff --git a/include/validator.h b/include/validator.h
index 353ddb7..dc2fd8a 100644
--- a/include/validator.h
+++ b/include/validator.h
@@ -5,6 +5,8 @@
 #ifndef _BOOTSTRAP_VALIDATOR_H
 #define _BOOTSTRAP_VALIDATOR_H
 
+#include <stdbool.h>
+
 #include "cJSON.h"
 #include "config.h"
 #include "dyn_array.h"
@@ -42,6 +44,8 @@ struct Field {
   /// @brief The type of field. Denotes what prompt should be displayed prior to
   /// evaluation.
   enum FieldType type;
+  /// @brief Indicates the field is required.
+  bool required;
   /// @brief A reference to the name of the field. Does not take ownership of
   /// this value.
   const char *key;
diff --git a/specs/mix/runner b/specs/mix/runner
index 66936e0..135a943 100755
--- a/specs/mix/runner
+++ b/specs/mix/runner
@@ -39,8 +39,6 @@ trap cleanup EXIT
 # ENVIRONMENT
 # ============================================================
 
-# Trims away any whitespace around the module name.
-MODULE=$(echo "$MODULE" | sed -e 's/^[[:space:]]*//' -e 's/[[:space:]]*$//')
 if [ -z "$MODULE" ]; then
   MODULE_ARG=
 else
diff --git a/specs/mix/spec.json b/specs/mix/spec.json
index cca219f..d8f64ed 100644
--- a/specs/mix/spec.json
+++ b/specs/mix/spec.json
@@ -1,10 +1,11 @@
 {
   "app": {
     "type": "line",
-    "prompt": "App Name> "
+    "prompt": "App> "
   },
   "module": {
     "type": "line",
-    "prompt": "Module (optional)> "
+    "required": false,
+    "prompt": "Module> "
   }
 }
diff --git a/specs/phoenix/runner b/specs/phoenix/runner
index 4bd025a..ad0b4eb 100755
--- a/specs/phoenix/runner
+++ b/specs/phoenix/runner
@@ -39,8 +39,6 @@ trap cleanup EXIT
 # ENVIRONMENT
 # ============================================================
 
-# Trims away any whitespace around the module name.
-MODULE=$(echo "$MODULE" | sed -e 's/^[[:space:]]*//' -e 's/[[:space:]]*$//')
 if [ -z "$MODULE" ]; then
   MODULE_ARG=
 else
diff --git a/specs/phoenix/spec.json b/specs/phoenix/spec.json
index cca219f..d8f64ed 100644
--- a/specs/phoenix/spec.json
+++ b/specs/phoenix/spec.json
@@ -1,10 +1,11 @@
 {
   "app": {
     "type": "line",
-    "prompt": "App Name> "
+    "prompt": "App> "
   },
   "module": {
     "type": "line",
-    "prompt": "Module (optional)> "
+    "required": false,
+    "prompt": "Module> "
   }
 }
diff --git a/src/config.c b/src/config.c
index 2b6157d..9428bec 100644
--- a/src/config.c
+++ b/src/config.c
@@ -6,6 +6,7 @@
 #include <string.h>
 #include <sys/stat.h>
 
+#include "console.h"
 #include "string_utils.h"
 
 struct Error *config_new(
@@ -19,16 +20,16 @@ struct Error *config_new(
   if (cwd == 0) {
     return ERROR_NEW(
       ERROR_CONFIG_ENV_CWD_INVALID,
-      ANSI_RED("ERROR"),
+      ANSI_RED_F("ERROR"),
       ": Could not retrieve ",
-      ANSI_CYAN("CWD"),
+      ANSI_CYAN_F("CWD"),
       "."
     );
   }
   if (root_dir == 0) {
     return ERROR_NEW(
       ERROR_CONFIG_ENV_ROOT_DIR_INVALID,
-      ANSI_RED("ERROR"),
+      ANSI_RED_F("ERROR"),
       ": Could not find root directory."
     );
   }
@@ -44,17 +45,17 @@ struct Error *config_new(
     if (errno == ENOENT) {
       error = ERROR_NEW(
         ERROR_CONFIG_TARGET_NOT_FOUND,
-        ANSI_RED("ERROR"),
+        ANSI_RED_F("ERROR"),
         ": Could not find ",
-        ANSI_BLUE(target),
+        ANSI_BLUE_F(target),
         " spec."
       );
     } else {
       error = ERROR_NEW(
         ERROR_CONFIG_TARGET_INVALID,
-        ANSI_RED("ERROR"),
+        ANSI_RED_F("ERROR"),
         ": ",
-        ANSI_BLUE(target),
+        ANSI_BLUE_F(target),
         " is invalid."
       );
     }
@@ -63,9 +64,9 @@ struct Error *config_new(
   if (!S_ISDIR(sb.st_mode)) {
     error = ERROR_NEW(
       ERROR_CONFIG_TARGET_NOT_DIR,
-      ANSI_RED("ERROR"),
+      ANSI_RED_F("ERROR"),
       ": ",
-      ANSI_CYAN(filepath),
+      ANSI_CYAN_F(filepath),
       " is not a directory."
     );
     goto cleanup;
diff --git a/src/dyn_array.c b/src/dyn_array.c
index 1fbb8ae..e338925 100644
--- a/src/dyn_array.c
+++ b/src/dyn_array.c
@@ -4,10 +4,10 @@
 
 struct DynArray *dyn_array_new(size_t capacity) {
   struct DynArray *a = malloc(sizeof(struct DynArray));
-  size_t new_capacity = capacity ? capacity : 1;
-  a->buf = calloc(new_capacity, sizeof(void *));
+  a->_capacity = (capacity == 0) ? 1 : capacity;
+  a->buf = malloc(a->_capacity * sizeof(void *));
+  a->buf[0] = 0;
   a->size = 0;
-  a->_capacity = new_capacity;
   return a;
 }
 
diff --git a/src/evaluator.c b/src/evaluator.c
index 103d674..7a7b3ab 100644
--- a/src/evaluator.c
+++ b/src/evaluator.c
@@ -7,10 +7,13 @@
 #include <string.h>
 #include <sys/stat.h>
 
+#include "console.h"
 #include "string_buf.h"
 #include "string_utils.h"
 #include "validator.h"
 
+#define BANNER_LENGTH 60
+
 static struct Error *find_run_exec(const struct Config *const config) {
   assert(config);
 
@@ -23,9 +26,9 @@ static struct Error *find_run_exec(const struct Config *const config) {
   if (stat_res == -1 && errno == ENOENT) {
     return ERROR_NEW(
       ERROR_EVALUATOR_RUNNER_NOT_FOUND,
-      ANSI_RED("NOT_FOUND"),
+      ANSI_RED_F("NOT_FOUND"),
       ": Could not find ",
-      ANSI_BLUE(config->target, "/runner"),
+      ANSI_BLUE_F(config->target, "/runner"),
       "."
     );
   }
@@ -33,9 +36,9 @@ static struct Error *find_run_exec(const struct Config *const config) {
   if (!(sb.st_mode & S_IXUSR)) {
     return ERROR_NEW(
       ERROR_EVALUATOR_RUNNER_NOT_EXEC,
-      ANSI_RED("ERROR"),
+      ANSI_RED_F("ERROR"),
       ": ",
-      ANSI_BLUE(config->target, "/runner"),
+      ANSI_BLUE_F(config->target, "/runner"),
       " is not executable."
     );
   }
@@ -43,38 +46,105 @@ static struct Error *find_run_exec(const struct Config *const config) {
   return 0;
 }
 
-static const char *prompt_field(struct Field *field) {
-  assert(field);
-  printf("%s", field->prompt);
+static void print_header(const struct Config *const config) {
+  assert(config);
 
+  struct StringBuf *banner = string_buf_new(BANNER_LENGTH * 2);
+  for (int i = 0; i < BANNER_LENGTH; ++i) {
+    string_buf_cappend(banner, '=');
+  }
+
+  struct StringBuf *header = string_buf_new(128);
+  string_buf_sappend(header, config->target);
+  string_buf_sappend(header, "/spec.json");
+  int left_padding = (BANNER_LENGTH - header->size) / 2;
+
+  printf("%s\n", banner->buf);
+  printf("%*s", left_padding, "");
+  printf("%s%s/spec.json%s\n\n", ANSI_BLUE_F(config->target));
+  printf("(%s%s%s) indicates a required field.\n", ANSI_YELLOW_F("*"));
+  printf("%s\n\n", banner->buf);
+
+  string_buf_free(header);
+  string_buf_free(banner);
+}
+
+static void print_prompt(const struct Field *const field) {
+  assert(field);
+
+  if (field->required) {
+    printf("%s%s%s%s", ANSI_YELLOW_F("*"), field->prompt);
+  } else {
+    printf("%s", field->prompt);
+  }
+}
+
+static const char *query_line(const struct Field *const field) {
+  assert(field);
+
+  // TODO: Dynamically size this value.
   char *response = calloc(1, 1024);
 
-  switch (field->type) {
-  case FT_LINE:
-    // TODO: Probably want this buffer size to be a bit more dynamic.
+  do {
+    print_prompt(field);
     if (fgets(response, 1024, stdin)) {
-      size_t len = strlen(response);
-      if (len > 0 && response[len - 1] == '\n') {
-        response[len - 1] = '\0';
+      trim_leading(response);
+      trim_trailing(response);
+      if (response[0] != 0) {
+        return response;
       }
-      return response;
-    } else {
-      free(response);
-      return 0;
+    } else {  // Likely EOF. Force-quit even if required.
+      printf("\n");
+      break;
     }
-  }
+  } while (field->required);
+
+  free(response);
+  return 0;
 }
 
 static void push_env(
   struct StringBuf *env, const char *key, const char *value
 ) {
   assert(env);
+
   for (const char *c = key; *c; ++c) {
     string_buf_cappend(env, toupper(*c));
   }
-  string_buf_sappend(env, "='");
-  string_buf_sappend(env, value);
-  string_buf_sappend(env, "' ");
+  string_buf_sappend(env, "=");
+
+  if (value) {
+    string_buf_cappend(env, '\'');
+    string_buf_sappend(env, value);
+    string_buf_sappend(env, "' ");
+  } else {
+    string_buf_cappend(env, ' ');
+  }
+}
+
+static struct Error *push_fields(
+  const struct Config *const config,
+  const struct DynArray *const fields,
+  struct StringBuf **env_buf
+) {
+  for (int i = 0; i < fields->size; ++i) {
+    struct Field *field = fields->buf[i];
+    const char *response = 0;
+    switch (field->type) {
+    case FT_LINE:
+      response = query_line(field);
+      break;
+    }
+    if (field->required && !response) {
+      return ERROR_NEW(
+        ERROR_EVALUATOR_RESPONSE_INVALID,
+        ANSI_RED_F("ERROR"),
+        ": Could not read response."
+      );
+    }
+    push_env(*env_buf, field->key, response);
+  }
+  return 0;
 }
 
 int evaluate_runner(
@@ -82,34 +152,23 @@ int evaluate_runner(
   const struct DynArray *const fields,
   struct Error **error
 ) {
-  *error = find_run_exec(config);
-  if (*error) {
+  if ((*error = find_run_exec(config))) {
     return EXIT_FAILURE;
   }
 
   struct StringBuf *env_buf = string_buf_new(512);
   push_env(env_buf, "OUT", config->cwd);
-
   if (fields) {
-    for (int i = 0; i < fields->size; ++i) {
-      struct Field *field = fields->buf[i];
-      const char *response = prompt_field(field);
-      if (!response) {
-        *error = ERROR_NEW(
-          ERROR_EVALUATOR_RESPONSE_INVALID,
-          ANSI_RED("ERROR"),
-          ": Could not read response."
-        );
-        string_buf_free(env_buf);
-        return EXIT_FAILURE;
-      }
-      push_env(env_buf, field->key, response);
+    print_header(config);
+    if ((*error = push_fields(config, fields, &env_buf))) {
+      string_buf_free(env_buf);
+      return EXIT_FAILURE;
     }
   }
 
   const char *segments[] = {config->root_dir, config->target, "runner"};
   const char *filepath = join(sizeof(segments) / sizeof(char *), segments, '/');
-  const char *env = string_buf_convert(env_buf);
+  const char *env = string_buf_cast(env_buf);
 
   struct StringBuf *command_buf = string_buf_new(1024);
   string_buf_sappend(command_buf, "cd ");
@@ -119,7 +178,7 @@ int evaluate_runner(
   string_buf_sappend(command_buf, " && ");
   string_buf_sappend(command_buf, env);
   string_buf_sappend(command_buf, filepath);
-  const char *command = string_buf_convert(command_buf);
+  const char *command = string_buf_cast(command_buf);
 
   free((void *)env);
   free((void *)filepath);
diff --git a/src/parser.c b/src/parser.c
index ef86e8f..a0249d5 100644
--- a/src/parser.c
+++ b/src/parser.c
@@ -5,6 +5,7 @@
 #include <stdbool.h>
 #include <stdio.h>
 
+#include "console.h"
 #include "string_utils.h"
 
 static struct Error *find_spec_json(
@@ -20,9 +21,9 @@ static struct Error *find_spec_json(
   if (!*handle && errno != ENOENT) {
     error = ERROR_NEW(
       ERROR_PARSER_SPEC_JSON_INVALID,
-      ANSI_RED("ERROR"),
+      ANSI_RED_F("ERROR"),
       ": ",
-      ANSI_BLUE(config->target, "/spec.json"),
+      ANSI_BLUE_F(config->target, "/spec.json"),
       " is invalid."
     );
   }
@@ -64,9 +65,9 @@ struct Error *parse_spec_json(
   if (!*parsed) {
     return ERROR_NEW(
       ERROR_PARSER_SPEC_JSON_INVALID_SYNTAX,
-      ANSI_RED("ERROR"),
+      ANSI_RED_F("ERROR"),
       ": ",
-      ANSI_BLUE(config->target, "/spec.json"),
+      ANSI_BLUE_F(config->target, "/spec.json"),
       " contains invalid JSON."
     );
   }
diff --git a/src/string_buf.c b/src/string_buf.c
index 064c8b0..9365304 100644
--- a/src/string_buf.c
+++ b/src/string_buf.c
@@ -7,9 +7,10 @@
 
 struct StringBuf *string_buf_new(size_t capacity) {
   struct StringBuf *sb = malloc(sizeof(struct StringBuf));
-  sb->buf = calloc(capacity, sizeof(char));
+  sb->_capacity = (capacity == 0) ? 1 : capacity;
+  sb->buf = malloc(sb->_capacity * sizeof(char));
+  sb->buf[0] = 0;
   sb->size = 0;
-  sb->_capacity = capacity;
   return sb;
 }
 
@@ -21,13 +22,11 @@ size_t string_buf_size(struct StringBuf *sb) {
 void string_buf_cappend(struct StringBuf *sb, char c) {
   assert(sb);
 
-  if (sb->_capacity) {
+  if (sb->size + 1 + 1 > sb->_capacity) {  // size + NUL + c
     sb->_capacity *= 2;
-  } else {
-    sb->_capacity = 2;
+    sb->buf = realloc((void *)sb->buf, sb->_capacity);
   }
 
-  sb->buf = realloc((void *)sb->buf, sb->_capacity);
   sb->buf[sb->size++] = c;
   sb->buf[sb->size] = 0;
 }
@@ -35,24 +34,20 @@ void string_buf_cappend(struct StringBuf *sb, char c) {
 void string_buf_sappend(struct StringBuf *sb, const char s[static 1]) {
   assert(sb);
 
-  double goal = sb->size + strlen(s) + 1;
-  double denom = sb->_capacity ? sb->_capacity : 1;
-  double scale = pow(2, ceil(log2(goal / denom)));
-
-  if (sb->_capacity) {
-    sb->_capacity *= scale;
-  } else {
-    sb->_capacity = scale;
+  size_t slen = strlen(s);
+  double goal = sb->size + 1 + slen;
+  if (goal > sb->_capacity) {  // size + NUL + slen
+    sb->_capacity *= pow(2, ceil(log2(goal / sb->_capacity)));
+    sb->buf = realloc((void *)sb->buf, sb->_capacity);
   }
 
-  sb->buf = realloc((void *)sb->buf, sb->_capacity);
-  for (const char *i = s; *i; ++i) {
-    sb->buf[sb->size++] = *i;
+  for (const char *c = s; *c; ++c) {
+    sb->buf[sb->size++] = *c;
   }
   sb->buf[sb->size] = 0;
 }
 
-const char *string_buf_convert(struct StringBuf *sb) {
+const char *string_buf_cast(struct StringBuf *sb) {
   assert(sb);
   const char *buf = sb->buf;
   free(sb);
diff --git a/src/string_utils.c b/src/string_utils.c
index 332d81a..57c03c9 100644
--- a/src/string_utils.c
+++ b/src/string_utils.c
@@ -45,3 +45,33 @@ int strcmp_ci(const char *s1, const char *s2) {
   }
   return s1_len < s2_len ? -1 : 1;
 }
+
+void trim_leading(char *s) {
+  int count = 0;
+  for (const char *c = s; isspace(*c); ++c) {
+    count++;
+  }
+  if (count == 0) {
+    return;
+  }
+  // Shift elements down.
+  size_t len = strlen(s);
+  for (int i = 0; i < len - count + 1; ++i) {
+    s[i] = s[i + count];
+  }
+}
+
+void trim_trailing(char *s) {
+  int last = -1;
+  size_t len = strlen(s);
+  for (int i = 0; i < len; ++i) {
+    if (!isspace(s[i])) {
+      last = i;
+    }
+  }
+  if (last == -1) {
+    s[0] = 0;
+  } else if (last < len - 1) {
+    s[last + 1] = 0;
+  }
+}
diff --git a/src/validator.c b/src/validator.c
index 8933920..4b3d7e3 100644
--- a/src/validator.c
+++ b/src/validator.c
@@ -2,6 +2,7 @@
 
 #include <ctype.h>
 
+#include "console.h"
 #include "string_utils.h"
 
 static struct Error *read_field(
@@ -12,11 +13,11 @@ static struct Error *read_field(
   if (!cJSON_IsObject(field)) {
     return ERROR_NEW(
       ERROR_VALIDATOR_FIELD_NOT_OBJECT,
-      ANSI_RED("ERROR"),
+      ANSI_RED_F("ERROR"),
       ": Field ",
-      ANSI_PURPLE(field->string),
+      ANSI_PURPLE_F(field->string),
       " in ",
-      ANSI_BLUE(config->target, "/spec.json"),
+      ANSI_BLUE_F(config->target, "/spec.json"),
       " is not a JSON object."
     );
   }
@@ -24,11 +25,11 @@ static struct Error *read_field(
   if (isdigit(field->string[0])) {
     return ERROR_NEW(
       ERROR_VALIDATOR_FIELD_NAME_INVALID,
-      ANSI_RED("ERROR"),
+      ANSI_RED_F("ERROR"),
       ": Field ",
-      ANSI_PURPLE(field->string),
+      ANSI_PURPLE_F(field->string),
       " in ",
-      ANSI_BLUE(config->target, "/spec.json"),
+      ANSI_BLUE_F(config->target, "/spec.json"),
       " may not begin with a digit."
     );
   } else {
@@ -36,11 +37,11 @@ static struct Error *read_field(
       if (*c != '_' && !isalnum(*c)) {
         return ERROR_NEW(
           ERROR_VALIDATOR_FIELD_NAME_INVALID,
-          ANSI_RED("ERROR"),
+          ANSI_RED_F("ERROR"),
           ": Field ",
-          ANSI_PURPLE(field->string),
+          ANSI_PURPLE_F(field->string),
           " in ",
-          ANSI_BLUE(config->target, "/spec.json"),
+          ANSI_BLUE_F(config->target, "/spec.json"),
           " must consist of only alphanumeric characters and underscores."
         );
       }
@@ -55,13 +56,13 @@ static struct Error *read_field(
   if (!cJSON_IsString(type)) {
     error = ERROR_NEW(
       ERROR_VALIDATOR_FIELD_TYPE_INVALID,
-      ANSI_RED("ERROR"),
+      ANSI_RED_F("ERROR"),
       ": Field ",
-      ANSI_PURPLE(field->string),
+      ANSI_PURPLE_F(field->string),
       " in ",
-      ANSI_BLUE(config->target, "/spec.json"),
+      ANSI_BLUE_F(config->target, "/spec.json"),
       " has non-string ",
-      ANSI_PURPLE("type"),
+      ANSI_PURPLE_F("type"),
       "."
     );
     goto cleanup;
@@ -72,13 +73,33 @@ static struct Error *read_field(
   } else {
     error = ERROR_NEW(
       ERROR_VALIDATOR_FIELD_TYPE_UNKNOWN,
-      ANSI_RED("ERROR"),
+      ANSI_RED_F("ERROR"),
       ": Field ",
-      ANSI_PURPLE(field->string),
+      ANSI_PURPLE_F(field->string),
       " in ",
-      ANSI_BLUE(config->target, "/spec.json"),
+      ANSI_BLUE_F(config->target, "/spec.json"),
       " has unknown ",
-      ANSI_PURPLE("type"),
+      ANSI_PURPLE_F("type"),
+      "."
+    );
+    goto cleanup;
+  }
+
+  const cJSON *required = cJSON_GetObjectItemCaseSensitive(field, "required");
+  if (!required) {
+    (*out)->required = true;
+  } else if (cJSON_IsBool(required)) {
+    (*out)->required = required->valueint;
+  } else {
+    error = ERROR_NEW(
+      ERROR_VALIDATOR_FIELD_REQUIRED_INVALID,
+      ANSI_RED_F("ERROR"),
+      ": Field ",
+      ANSI_PURPLE_F(field->string),
+      " in ",
+      ANSI_BLUE_F(config->target, "/spec.json"),
+      " has non-boolean ",
+      ANSI_PURPLE_F("required"),
       "."
     );
     goto cleanup;
@@ -90,13 +111,13 @@ static struct Error *read_field(
   } else {
     error = ERROR_NEW(
       ERROR_VALIDATOR_FIELD_PROMPT_INVALID,
-      ANSI_RED("ERROR"),
+      ANSI_RED_F("ERROR"),
       ": Field ",
-      ANSI_PURPLE(field->string),
+      ANSI_PURPLE_F(field->string),
       " in ",
-      ANSI_BLUE(config->target, "/spec.json"),
+      ANSI_BLUE_F(config->target, "/spec.json"),
       " has non-string ",
-      ANSI_PURPLE("prompt"),
+      ANSI_PURPLE_F("prompt"),
       "."
     );
     goto cleanup;
@@ -124,9 +145,9 @@ struct Error *validate_spec_json(
   if (!cJSON_IsObject(parsed)) {
     return ERROR_NEW(
       ERROR_VALIDATOR_TOP_LEVEL_NOT_OBJECT,
-      ANSI_RED("ERROR"),
+      ANSI_RED_F("ERROR"),
       ": Top-level JSON value in ",
-      ANSI_BLUE(config->target, "/spec.json"),
+      ANSI_BLUE_F(config->target, "/spec.json"),
       " is not an object."
     );
   }
diff --git a/test/suites.c b/test/suites.c
index 95dce04..f4fdcf6 100644
--- a/test/suites.c
+++ b/test/suites.c
@@ -27,6 +27,8 @@ int main(int argc, char *argv[]) {
   sput_run_test(test_join_single);
   sput_run_test(test_join_multiple);
   sput_run_test(test_strcmp_ci);
+  sput_run_test(test_trim_leading);
+  sput_run_test(test_trim_trailing);
 
   sput_enter_suite("parser");
   sput_run_test(test_parser_missing);
@@ -41,8 +43,10 @@ int main(int argc, char *argv[]) {
   sput_run_test(test_validator_field_type_invalid);
   sput_run_test(test_validator_field_type_unknown);
   sput_run_test(test_validator_valid_type_ci);
+  sput_run_test(test_validator_field_required_invalid);
+  sput_run_test(test_validator_field_required_valid);
   sput_run_test(test_validator_field_prompt_invalid);
-  sput_run_test(test_validator_valid);
+  sput_run_test(test_validator_valid_no_required);
 
   sput_finish_testing();
 
diff --git a/test/test_string_buf.h b/test/test_string_buf.h
index d56d20b..db776cf 100644
--- a/test/test_string_buf.h
+++ b/test/test_string_buf.h
@@ -12,10 +12,9 @@ static void test_string_buf_sappend() {
   sput_fail_unless(
     string_buf_size(sb) == strlen("hello world!!"), "sappend size"
   );
-  const char *converted = string_buf_convert(sb);
-  sput_fail_unless(
-    strcmp(converted, "hello world!!") == 0, "sappend converted"
-  );
+  const char *cast = string_buf_cast(sb);
+  sput_fail_unless(strcmp(cast, "hello world!!") == 0, "sappend cast");
+  free((void *)cast);
 }
 
 static void test_string_buf_cappend() {
@@ -27,10 +26,9 @@ static void test_string_buf_cappend() {
   sput_fail_unless(
     string_buf_size(sb) == strlen("hello world!!"), "cappend size"
   );
-  const char *converted = string_buf_convert(sb);
-  sput_fail_unless(
-    strcmp(converted, "hello world!!") == 0, "cappend converted"
-  );
+  const char *cast = string_buf_cast(sb);
+  sput_fail_unless(strcmp(cast, "hello world!!") == 0, "cappend cast");
+  free((void *)cast);
 }
 
 static void test_string_buf_nonzero_capacity() {
diff --git a/test/test_string_utils.h b/test/test_string_utils.h
index 42f752a..9fc84cb 100644
--- a/test/test_string_utils.h
+++ b/test/test_string_utils.h
@@ -22,10 +22,51 @@ static void test_strcmp_ci() {
   const char *a1 = "aBcD";
   const char *a2 = "AbCd";
   sput_fail_unless(strcmp_ci(a1, a2) == 0, "strcmp_ci == 0");
+
   const char *b1 = "aBcDe";
   const char *b2 = "AbCd";
   sput_fail_unless(strcmp_ci(b1, b2) > 0, "strcmp_ci > 0");
   sput_fail_unless(strcmp_ci(b2, b1) < 0, "strcmp_ci < 0");
 }
 
+static void test_trim_leading() {
+  char a1[] = {0};
+  char a2[] = {' ', ' ', ' ', 0};
+  trim_leading(a1);
+  trim_leading(a2);
+  sput_fail_unless(a1[0] == 0, "trim leading empty string");
+  sput_fail_unless(strcmp(a1, a2) == 0, "trim leading whitespace string");
+
+  char b1[] = {'a', 'b', 'c', 'd', 'e', 'f', 0};
+  char b2[] = {' ', ' ', ' ', 'a', 'b', 'c', 'd', 'e', 'f', 0};
+  trim_leading(b1);
+  trim_leading(b2);
+  sput_fail_unless(strcmp(b1, b2) == 0, "trim leading string");
+
+  char c1[] = {'a', 'b', 'c', 'd', 'e', 'f', ' ', ' ', ' ', 0};
+  char c2[] = {'a', 'b', 'c', 'd', 'e', 'f', ' ', ' ', ' ', 0};
+  trim_leading(c1);
+  sput_fail_unless(strcmp(c1, c2) == 0, "trim leading ignore trailing");
+}
+
+static void test_trim_trailing() {
+  char a1[] = {0};
+  char a2[] = {' ', ' ', ' ', 0};
+  trim_trailing(a1);
+  trim_trailing(a2);
+  sput_fail_unless(a1[0] == 0, "trim trailing empty string");
+  sput_fail_unless(strcmp(a1, a2) == 0, "trim trailing whitespace string");
+
+  char b1[] = {'a', 'b', 'c', 'd', 'e', 'f', 0};
+  char b2[] = {'a', 'b', 'c', 'd', 'e', 'f', ' ', ' ', ' ', 0};
+  trim_trailing(b1);
+  trim_trailing(b2);
+  sput_fail_unless(strcmp(b1, b2) == 0, "trim trailing string");
+
+  char c1[] = {' ', ' ', ' ', 'a', 'b', 'c', 'd', 'e', 'f', 0};
+  char c2[] = {' ', ' ', ' ', 'a', 'b', 'c', 'd', 'e', 'f', 0};
+  trim_trailing(c1);
+  sput_fail_unless(strcmp(c1, c2) == 0, "trim trailing ignore leading");
+}
+
 #endif /* _BOOTSTRAP_TEST_STRING_UTILS */
diff --git a/test/test_validator.h b/test/test_validator.h
index f46cab5..4b3248e 100644
--- a/test/test_validator.h
+++ b/test/test_validator.h
@@ -163,6 +163,46 @@ static void test_validator_valid_type_ci() {
   test_validator_teardown(fixture);
 }
 
+static void test_validator_field_required_invalid() {
+  struct TestValidatorFixture *fixture = test_validator_setup(
+    "{"
+    "  \"key\": {"
+    "    \"type\": \"line\","
+    "    \"required\": 5"
+    "  }"
+    "}"
+  );
+
+  struct Error *error =
+    validate_spec_json(&fixture->config, fixture->parsed, &fixture->prompts);
+  sput_fail_unless(
+    error->code == ERROR_VALIDATOR_FIELD_REQUIRED_INVALID,
+    "field required invalid"
+  );
+  error_free(error);
+
+  test_validator_teardown(fixture);
+}
+
+static void test_validator_field_required_valid() {
+  struct TestValidatorFixture *fixture = test_validator_setup(
+    "{"
+    "  \"key\": {"
+    "    \"type\": \"line\","
+    "    \"required\": true,"
+    "    \"prompt\": \"What value for key? \""
+    "  }"
+    "}"
+  );
+
+  struct Error *error =
+    validate_spec_json(&fixture->config, fixture->parsed, &fixture->prompts);
+  sput_fail_unless(error == 0, "required valid");
+  error_free(error);
+
+  test_validator_teardown(fixture);
+}
+
 static void test_validator_field_prompt_invalid() {
   struct TestValidatorFixture *fixture = test_validator_setup(
     "{"
@@ -183,7 +223,7 @@ static void test_validator_field_prompt_invalid() {
   test_validator_teardown(fixture);
 }
 
-static void test_validator_valid() {
+static void test_validator_valid_no_required() {
   struct TestValidatorFixture *fixture = test_validator_setup(
     "{"
     "  \"key\": {"