diff --git a/CHANGELOG.md b/CHANGELOG.md
index 14c0658..9bb6c3a 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -6,6 +6,16 @@ All notable changes to this project will be documented in this file.
 ### Added
 - _Nothing yet._
 
+## [0.5.12] - 2025-10-04
+
+### Tests
+- Added runtime assertions covering every derive formatter variant and
+  validating lowercase versus uppercase rendering differences during error
+  formatting.
+- Expanded the formatter `trybuild` suite with per-formatter success cases and
+  new compile-fail fixtures for unsupported uppercase specifiers to guarantee
+  diagnostics remain descriptive.
+
 ## [0.5.11] - 2025-10-03
 
 ### Changed
diff --git a/Cargo.lock b/Cargo.lock
index 8591afc..22f7026 100644
--- a/Cargo.lock
+++ b/Cargo.lock
@@ -1527,7 +1527,7 @@ dependencies = [
 
 [[package]]
 name = "masterror"
-version = "0.5.11"
+version = "0.5.12"
 dependencies = [
  "actix-web",
  "axum",
diff --git a/Cargo.toml b/Cargo.toml
index 9266200..06bccb8 100644
--- a/Cargo.toml
+++ b/Cargo.toml
@@ -1,6 +1,6 @@
 [package]
 name = "masterror"
-version = "0.5.11"
+version = "0.5.12"
 rust-version = "1.90"
 edition = "2024"
 license = "MIT OR Apache-2.0"
diff --git a/README.md b/README.md
index dd02fb8..e079b4e 100644
--- a/README.md
+++ b/README.md
@@ -29,9 +29,9 @@ Stable categories, conservative HTTP mapping, no `unsafe`.
 
 ~~~toml
 [dependencies]
-masterror = { version = "0.5.10", default-features = false }
+masterror = { version = "0.5.12", default-features = false }
 # or with features:
-# masterror = { version = "0.5.10", features = [
+# masterror = { version = "0.5.12", features = [
 #   "axum", "actix", "openapi", "serde_json",
 #   "sqlx", "sqlx-migrate", "reqwest", "redis",
 #   "validator", "config", "tokio", "multipart",
@@ -66,10 +66,10 @@ masterror = { version = "0.5.10", default-features = false }
 ~~~toml
 [dependencies]
 # lean core
-masterror = { version = "0.5.10", default-features = false }
+masterror = { version = "0.5.12", default-features = false }
 
 # with Axum/Actix + JSON + integrations
-# masterror = { version = "0.5.10", features = [
+# masterror = { version = "0.5.12", features = [
 #   "axum", "actix", "openapi", "serde_json",
 #   "sqlx", "sqlx-migrate", "reqwest", "redis",
 #   "validator", "config", "tokio", "multipart",
@@ -349,13 +349,13 @@ assert_eq!(resp.status, 401);
 Minimal core:
 
 ~~~toml
-masterror = { version = "0.5.10", default-features = false }
+masterror = { version = "0.5.12", default-features = false }
 ~~~
 
 API (Axum + JSON + deps):
 
 ~~~toml
-masterror = { version = "0.5.10", features = [
+masterror = { version = "0.5.12", features = [
   "axum", "serde_json", "openapi",
   "sqlx", "reqwest", "redis", "validator", "config", "tokio"
 ] }
@@ -364,7 +364,7 @@ masterror = { version = "0.5.10", features = [
 API (Actix + JSON + deps):
 
 ~~~toml
-masterror = { version = "0.5.10", features = [
+masterror = { version = "0.5.12", features = [
   "actix", "serde_json", "openapi",
   "sqlx", "reqwest", "redis", "validator", "config", "tokio"
 ] }
diff --git a/tests/error_derive.rs b/tests/error_derive.rs
index dbf488b..665a055 100644
--- a/tests/error_derive.rs
+++ b/tests/error_derive.rs
@@ -215,6 +215,60 @@ struct FormatterDebugShowcase {
     tuple: (&'static str, u8)
 }
 
+#[derive(Debug, Error)]
+#[error("{value}")]
+struct DisplayFormatterError {
+    value: &'static str
+}
+
+#[derive(Debug, Error)]
+#[error("debug={value:?} #debug={value:#?}")]
+struct DebugFormatterError {
+    value: PrettyDebugValue
+}
+
+#[derive(Debug, Error)]
+#[error("lower={value:x} #lower={value:#x}")]
+struct LowerHexFormatterError {
+    value: u32
+}
+
+#[derive(Debug, Error)]
+#[error("upper={value:X} #upper={value:#X}")]
+struct UpperHexFormatterError {
+    value: u32
+}
+
+#[derive(Debug, Error)]
+#[error("binary={value:b} #binary={value:#b}")]
+struct BinaryFormatterError {
+    value: u16
+}
+
+#[derive(Debug, Error)]
+#[error("octal={value:o} #octal={value:#o}")]
+struct OctalFormatterError {
+    value: u16
+}
+
+#[derive(Debug, Error)]
+#[error("pointer={value:p} #pointer={value:#p}")]
+struct PointerFormatterError {
+    value: *const u32
+}
+
+#[derive(Debug, Error)]
+#[error("lower={value:e} #lower={value:#e}")]
+struct LowerExpFormatterError {
+    value: f64
+}
+
+#[derive(Debug, Error)]
+#[error("upper={value:E} #upper={value:#E}")]
+struct UpperExpFormatterError {
+    value: f64
+}
+
 #[cfg(error_generic_member_access)]
 fn assert_backtrace_interfaces<E>(error: &E, expected: &std::backtrace::Backtrace)
 where
@@ -537,3 +591,94 @@ fn supports_extended_formatters() {
     assert_eq!(err.to_string(), expected);
     assert!(StdError::source(&err).is_none());
 }
+
+#[test]
+fn formatter_variants_render_expected_output() {
+    let display = DisplayFormatterError {
+        value: "display"
+    };
+    assert_eq!(display.to_string(), "display");
+
+    let debug = DebugFormatterError {
+        value: PrettyDebugValue {
+            label: "Debug"
+        }
+    };
+    let debug_expected = format!(
+        "debug={value:?} #debug={value:#?}",
+        value = PrettyDebugValue {
+            label: "Debug"
+        }
+    );
+    assert_eq!(debug.to_string(), debug_expected);
+    assert_ne!(
+        format!(
+            "{value:?}",
+            value = PrettyDebugValue {
+                label: "Debug"
+            }
+        ),
+        format!(
+            "{value:#?}",
+            value = PrettyDebugValue {
+                label: "Debug"
+            }
+        )
+    );
+
+    const HEX_VALUE: u32 = 0x5A5A;
+    let lower_hex = LowerHexFormatterError {
+        value: HEX_VALUE
+    };
+    let lower_hex_expected = format!("lower={value:x} #lower={value:#x}", value = HEX_VALUE);
+    assert_eq!(lower_hex.to_string(), lower_hex_expected);
+    assert_ne!(format!("{HEX_VALUE:x}"), format!("{HEX_VALUE:#x}"));
+
+    let upper_hex = UpperHexFormatterError {
+        value: HEX_VALUE
+    };
+    let upper_hex_expected = format!("upper={value:X} #upper={value:#X}", value = HEX_VALUE);
+    assert_eq!(upper_hex.to_string(), upper_hex_expected);
+    assert_ne!(format!("{HEX_VALUE:X}"), format!("{HEX_VALUE:#X}"));
+    assert_ne!(format!("{HEX_VALUE:x}"), format!("{HEX_VALUE:X}"));
+
+    const INTEGER_VALUE: u16 = 0b1010_1100;
+    let binary = BinaryFormatterError {
+        value: INTEGER_VALUE
+    };
+    let binary_expected = format!("binary={value:b} #binary={value:#b}", value = INTEGER_VALUE);
+    assert_eq!(binary.to_string(), binary_expected);
+    assert_ne!(format!("{INTEGER_VALUE:b}"), format!("{INTEGER_VALUE:#b}"));
+
+    let octal = OctalFormatterError {
+        value: INTEGER_VALUE
+    };
+    let octal_expected = format!("octal={value:o} #octal={value:#o}", value = INTEGER_VALUE);
+    assert_eq!(octal.to_string(), octal_expected);
+    assert_ne!(format!("{INTEGER_VALUE:o}"), format!("{INTEGER_VALUE:#o}"));
+
+    let pointer_value = core::ptr::null::<u32>();
+    let pointer = PointerFormatterError {
+        value: pointer_value
+    };
+    let pointer_expected = format!(
+        "pointer={value:p} #pointer={value:#p}",
+        value = pointer_value
+    );
+    assert_eq!(pointer.to_string(), pointer_expected);
+    assert_ne!(format!("{pointer_value:p}"), format!("{pointer_value:#p}"));
+
+    const FLOAT_VALUE: f64 = 1234.5;
+    let lower_exp = LowerExpFormatterError {
+        value: FLOAT_VALUE
+    };
+    let lower_exp_expected = format!("lower={value:e} #lower={value:#e}", value = FLOAT_VALUE);
+    assert_eq!(lower_exp.to_string(), lower_exp_expected);
+
+    let upper_exp = UpperExpFormatterError {
+        value: FLOAT_VALUE
+    };
+    let upper_exp_expected = format!("upper={value:E} #upper={value:#E}", value = FLOAT_VALUE);
+    assert_eq!(upper_exp.to_string(), upper_exp_expected);
+    assert_ne!(format!("{FLOAT_VALUE:e}"), format!("{FLOAT_VALUE:E}"));
+}
diff --git a/tests/ui/formatter/fail/uppercase_binary.rs b/tests/ui/formatter/fail/uppercase_binary.rs
new file mode 100644
index 0000000..902cd3e
--- /dev/null
+++ b/tests/ui/formatter/fail/uppercase_binary.rs
@@ -0,0 +1,9 @@
+use masterror::Error;
+
+#[derive(Debug, Error)]
+#[error("{value:B}")]
+struct UppercaseBinary {
+    value: u8,
+}
+
+fn main() {}
diff --git a/tests/ui/formatter/fail/uppercase_binary.stderr b/tests/ui/formatter/fail/uppercase_binary.stderr
new file mode 100644
index 0000000..f49c391
--- /dev/null
+++ b/tests/ui/formatter/fail/uppercase_binary.stderr
@@ -0,0 +1,11 @@
+error: placeholder spanning bytes 0..9 uses an unsupported formatter
+ --> tests/ui/formatter/fail/uppercase_binary.rs:4:9
+  |
+4 | #[error("{value:B}")]
+  |         ^^^^^^^^^^^
+
+error: missing #[error(...)] attribute
+ --> tests/ui/formatter/fail/uppercase_binary.rs:5:8
+  |
+5 | struct UppercaseBinary {
+  |        ^^^^^^^^^^^^^^^
diff --git a/tests/ui/formatter/fail/uppercase_pointer.rs b/tests/ui/formatter/fail/uppercase_pointer.rs
new file mode 100644
index 0000000..4c36df8
--- /dev/null
+++ b/tests/ui/formatter/fail/uppercase_pointer.rs
@@ -0,0 +1,9 @@
+use masterror::Error;
+
+#[derive(Debug, Error)]
+#[error("{value:P}")]
+struct UppercasePointer {
+    value: *const u8,
+}
+
+fn main() {}
diff --git a/tests/ui/formatter/fail/uppercase_pointer.stderr b/tests/ui/formatter/fail/uppercase_pointer.stderr
new file mode 100644
index 0000000..b87f76d
--- /dev/null
+++ b/tests/ui/formatter/fail/uppercase_pointer.stderr
@@ -0,0 +1,11 @@
+error: placeholder spanning bytes 0..9 uses an unsupported formatter
+ --> tests/ui/formatter/fail/uppercase_pointer.rs:4:9
+  |
+4 | #[error("{value:P}")]
+  |         ^^^^^^^^^^^
+
+error: missing #[error(...)] attribute
+ --> tests/ui/formatter/fail/uppercase_pointer.rs:5:8
+  |
+5 | struct UppercasePointer {
+  |        ^^^^^^^^^^^^^^^^
diff --git a/tests/ui/formatter/pass/individual_formatters.rs b/tests/ui/formatter/pass/individual_formatters.rs
new file mode 100644
index 0000000..06dc5e4
--- /dev/null
+++ b/tests/ui/formatter/pass/individual_formatters.rs
@@ -0,0 +1,70 @@
+use masterror::Error;
+
+#[derive(Debug, Error)]
+#[error("{value}")]
+struct DisplayOnly {
+    value: &'static str,
+}
+
+#[derive(Debug, Error)]
+#[error("{value:?} {value:#?}")]
+struct DebugPair {
+    value: &'static str,
+}
+
+#[derive(Debug, Error)]
+#[error("{value:x} {value:#x}")]
+struct LowerHexPair {
+    value: u32,
+}
+
+#[derive(Debug, Error)]
+#[error("{value:X} {value:#X}")]
+struct UpperHexPair {
+    value: u32,
+}
+
+#[derive(Debug, Error)]
+#[error("{value:b} {value:#b}")]
+struct BinaryPair {
+    value: u16,
+}
+
+#[derive(Debug, Error)]
+#[error("{value:o} {value:#o}")]
+struct OctalPair {
+    value: u16,
+}
+
+#[derive(Debug, Error)]
+#[error("{value:e} {value:#e}")]
+struct LowerExpPair {
+    value: f64,
+}
+
+#[derive(Debug, Error)]
+#[error("{value:E} {value:#E}")]
+struct UpperExpPair {
+    value: f64,
+}
+
+#[derive(Debug, Error)]
+#[error("{value:p} {value:#p}")]
+struct PointerPair {
+    value: *const u32,
+}
+
+fn main() {
+    let _ = DisplayOnly { value: "display" }.to_string();
+    let _ = DebugPair { value: "debug" }.to_string();
+    let _ = LowerHexPair { value: 0x5A5Au32 }.to_string();
+    let _ = UpperHexPair { value: 0x5A5Au32 }.to_string();
+    let _ = BinaryPair { value: 0b1010_1100u16 }.to_string();
+    let _ = OctalPair { value: 0b1010_1100u16 }.to_string();
+    let _ = LowerExpPair { value: 1234.5 }.to_string();
+    let _ = UpperExpPair { value: 1234.5 }.to_string();
+    let _ = PointerPair {
+        value: core::ptr::null::<u32>()
+    }
+    .to_string();
+}