diff --git a/Cargo.lock b/Cargo.lock
index a1f65cc..ac82fbe 100644
--- a/Cargo.lock
+++ b/Cargo.lock
@@ -2,6 +2,12 @@
 # It is not intended for manual editing.
 version = 4
 
+[[package]]
+name = "anyhow"
+version = "1.0.102"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "7f202df86484c868dbad7eaa557ef785d5c66295e41b460ef922eca0723b842c"
+
 [[package]]
 name = "bitflags"
 version = "2.10.0"
@@ -18,6 +24,7 @@ checksum = "9330f8b2ff13f34540b44e946ef35111825727b38d33286ef986142615121801"
 name = "dps-config"
 version = "0.10.1"
 dependencies = [
+ "anyhow",
  "serde_json",
  "serial_test",
 ]
diff --git a/Cargo.toml b/Cargo.toml
index ebc7c2d..d22f9df 100644
--- a/Cargo.toml
+++ b/Cargo.toml
@@ -21,6 +21,7 @@ path = "src/lib.rs"
 crate-type = ["lib"]
 
 [dependencies]
+anyhow = "1"
 serde_json = "1"
 
 [dev-dependencies]
diff --git a/README.md b/README.md
index 564e63e..0dce7f7 100644
--- a/README.md
+++ b/README.md
@@ -126,8 +126,8 @@ These properties are only available in the Rust implementation. The Bun/TypeScri
 
 | Property | Default | Description |
 |----------|---------|-------------|
-| `session_sub_to_user_id_fn` | Parses string as i64, returns 0 on failure | Function that converts a session `sub` string to an `i64` user ID |
-| `session_user_to_sub_fn` | Returns `id` property as string | Function that extracts a `sub` string from a JSON record (`serde_json::Value`) |
+| `session_sub_to_user_id_fn` | Parses string as i64, returns parse error on failure | Function that converts a session `sub` string to an `i64` user ID. Returns `anyhow::Result<i64>`. |
+| `session_user_to_sub_fn` | Returns `id` property as string, error on missing/invalid | Function that extracts a `sub` string from a JSON record (`serde_json::Value`). Returns `anyhow::Result<String>`. |
 
 Example usage:
 
@@ -136,17 +136,35 @@ let mut config = DpsConfig::new();
 
 // Use default: parses sub as i64
 let to_user_id = config.get_session_sub_to_user_id_fn();
-assert_eq!(to_user_id("42"), 42);
+assert_eq!(to_user_id("42").unwrap(), 42);
 
 // Custom converter: use length of sub as user ID
-config.set_session_sub_to_user_id_fn(|sub| sub.len() as i64);
+config.set_session_sub_to_user_id_fn(|sub| Ok(sub.len() as i64));
 
 // Custom sub extractor from JSON record
 config.set_session_user_to_sub_fn(|record| {
-    record.get("sub").and_then(|v| v.as_str()).unwrap_or("").to_string()
+    record.get("sub").and_then(|v| v.as_str()).map(|s| s.to_string()).ok_or_else(|| anyhow::anyhow!("missing 'sub'"))
 });
 ```
 
+#### Overriding from a consuming crate
+
+Since the functions return `anyhow::Result<T>`, you can use `anyhow`'s convenience macros for quick error creation.
+
+```rust
+use dps_config::DpsConfig;
+
+fn configure_with_custom_errors(config: &mut DpsConfig) {
+    // Using anyhow::bail!() for convenient error creation
+    config.set_session_sub_to_user_id_fn(|sub| {
+        if sub == "invalid" {
+            anyhow::bail!("user not found: {}", sub);
+        }
+        Ok(42)
+    });
+}
+```
+
 ## Computed Getters
 
 Computed getters derive values from base properties and have no setters or environment variables.
diff --git a/docs/llm/plans/2026/05/21-add-session-sub-to-user-id-and-session-user-to-sub-fns.md b/docs/llm/plans/2026/05/21-add-session-sub-to-user-id-and-session-user-to-sub-fns.md
index 73d8147..b802e1f 100644
--- a/docs/llm/plans/2026/05/21-add-session-sub-to-user-id-and-session-user-to-sub-fns.md
+++ b/docs/llm/plans/2026/05/21-add-session-sub-to-user-id-and-session-user-to-sub-fns.md
@@ -4,8 +4,8 @@
 
 Add two new function-typed properties to the Rust `DpsConfig` struct:
 
-1. **`session_sub_to_user_id_fn`** — A function that takes a `&str` (session sub) and returns `i64` (user ID). Default: identity function that parses the string as `i64`.
-2. **`session_user_to_sub_fn`** — A function that takes a JSON-like record (`serde_json::Value`) and returns a `String` (the sub). Default: returns the `id` property of the record as a string.
+1. **`session_sub_to_user_id_fn`** — A function that takes a `&str` (session sub) and returns `Result<i64, Box<dyn std::error::Error + Send + Sync>>`. Default: parses the string as `i64`, returning a boxed parse error on failure.
+2. **`session_user_to_sub_fn`** — A function that takes a JSON-like record (`serde_json::Value`) and returns `Result<String, Box<dyn std::error::Error + Send + Sync>>` (the sub). Default: returns the `id` property of the record as a string, or an error if `id` is missing or invalid.
 
 These are Rust-only features. The Bun/TypeScript implementation remains unchanged.
 
@@ -13,94 +13,95 @@ These are Rust-only features. The Bun/TypeScript implementation remains unchange
 
 ### 1. Add fields to `DpsConfig` struct (`src/lib.rs`)
 
-Add two new fields using `Box<dyn Fn...>` to store callable functions. These fields are **non-optional** internally, ensuring they always hold a valid function.
+Add two new fields using `Box<dyn Fn...>` to store callable functions. These fields are **non-optional** internally, ensuring they always hold a valid function. The error type for `session_sub_to_user_id_fn` uses `Box<dyn std::error::Error + Send + Sync>` to allow callers to return any error type.
 
 ```rust
 pub struct DpsConfig {
   // ... existing fields ...
 
   // Session conversion functions
-  session_sub_to_user_id_fn: Box<dyn Fn(&str) -> i64 + Send + Sync>,
-  session_user_to_sub_fn: Box<dyn Fn(&serde_json::Value) -> String + Send + Sync>,
+  session_sub_to_user_id_fn: Box<dyn Fn(&str) -> Result<i64, Box<dyn std::error::Error + Send + Sync>> + Send + Sync>,
+  session_user_to_sub_fn: Box<dyn Fn(&serde_json::Value) -> Result<String, Box<dyn std::error::Error + Send + Sync>> + Send + Sync>,
 }
 ```
 
 Note: `Send + Sync` bounds ensure the functions can be used across threads safely.
 
-### 2. Initialize defaults in `DpsConfig::new()`
+### 3. Initialize defaults in `DpsConfig::new()`
 
 ```rust
 Self {
   // ... existing initializations ...
 
   session_sub_to_user_id_fn: Box::new(|sub: &str| {
-    sub.parse::<i64>().unwrap_or(0)
+    sub.parse::<i64>().map_err(|e| Box::new(e) as Box<dyn std::error::Error + Send + Sync>)
   }),
   session_user_to_sub_fn: Box::new(|record: &serde_json::Value| {
     record
       .get("id")
       .and_then(|v| {
         if let Some(n) = v.as_u64() {
-          Some(n.to_string())
+          Some(Ok(n.to_string()))
         } else {
-          v.as_str().map(|s| s.to_string())
+          v.as_str().map(|s| Ok(s.to_string()))
         }
       })
-      .unwrap_or_default()
+      .unwrap_or_else(|| Err("missing or invalid 'id' field".into()))
   }),
 }
 ```
 
-### 3. Add getters and setters
+### 4. Add getters and setters
 
 ```rust
 // session_sub_to_user_id_fn
-pub fn get_session_sub_to_user_id_fn(&self) -> &dyn Fn(&str) -> i64 {
+pub fn get_session_sub_to_user_id_fn(&self) -> &dyn Fn(&str) -> Result<i64, Box<dyn std::error::Error + Send + Sync>> {
   self.session_sub_to_user_id_fn.as_ref()
 }
 
-pub fn set_session_sub_to_user_id_fn(&mut self, f: impl Fn(&str) -> i64 + Send + Sync + 'static) {
+pub fn set_session_sub_to_user_id_fn(
+  &mut self,
+  f: impl Fn(&str) -> Result<i64, Box<dyn std::error::Error + Send + Sync>> + Send + Sync + 'static,
+) {
   self.session_sub_to_user_id_fn = Box::new(f);
 }
 
 // session_user_to_sub_fn
-pub fn get_session_user_to_sub_fn(&self) -> &dyn Fn(&serde_json::Value) -> String {
+pub fn get_session_user_to_sub_fn(&self) -> &dyn Fn(&serde_json::Value) -> Result<String, Box<dyn std::error::Error + Send + Sync>> {
   self.session_user_to_sub_fn.as_ref()
 }
 
 pub fn set_session_user_to_sub_fn(
   &mut self,
-  f: impl Fn(&serde_json::Value) -> String + Send + Sync + 'static,
+  f: impl Fn(&serde_json::Value) -> Result<String, Box<dyn std::error::Error + Send + Sync>> + Send + Sync + 'static,
 ) {
   self.session_user_to_sub_fn = Box::new(f);
 }
 ```
 
-### 4. Add `serde_json` dependency to `Cargo.toml`
+### 5. Add dependencies to `Cargo.toml`
 
 ```toml
 [dependencies]
 serde_json = "1"
 ```
 
-### 5. Add tests
+### 6. Add tests
 
 ```rust
 #[test]
 fn test_session_sub_to_user_id_fn_default() {
   let config = DpsConfig::new();
   let to_user_id = config.get_session_sub_to_user_id_fn();
-  assert_eq!(to_user_id("12345"), 12345);
-  assert_eq!(to_user_id("invalid"), 0);
+  assert_eq!(to_user_id("12345").unwrap(), 12345);
+  assert!(to_user_id("invalid").is_err());
 }
 
 #[test]
 fn test_session_sub_to_user_id_fn_custom() {
   let mut config = DpsConfig::new();
-  config.set_session_sub_to_user_id_fn(|sub| {
-    sub.len() as i64
-  });
-  assert_eq!(config.get_session_sub_to_user_id_fn()("hello"), 5);
+  config.set_session_sub_to_user_id_fn(|sub| Ok(sub.len() as i64));
+  assert_eq!(config.get_session_sub_to_user_id_fn()("hello").unwrap(), 5);
 }
 
 #[test]
@@ -108,21 +109,22 @@ fn test_session_user_to_sub_fn_default() {
   let config = DpsConfig::new();
   let to_sub = config.get_session_user_to_sub_fn();
   let record = serde_json::json!({ "id": 42, "name": "test" });
-  assert_eq!(to_sub(&record), "42");
+  assert_eq!(to_sub(&record).unwrap(), "42");
+  assert!(to_sub(&serde_json::json!({})).is_err());
 }
 
 #[test]
 fn test_session_user_to_sub_fn_custom() {
   let mut config = DpsConfig::new();
   config.set_session_user_to_sub_fn(|record| {
-    record.get("sub").and_then(|v| v.as_str()).unwrap_or("").to_string()
+    record.get("sub").and_then(|v| v.as_str()).map(|s| s.to_string()).ok_or("missing 'sub'".into())
   });
   let record = serde_json::json!({ "sub": "user-123", "name": "test" });
-  assert_eq!(config.get_session_user_to_sub_fn()(&record), "user-123");
+  assert_eq!(config.get_session_user_to_sub_fn()(&record).unwrap(), "user-123");
 }
 ```
 
-### 6. Update README.md
+### 7. Update README.md
 
 Add a new section under Configuration Properties noting these are Rust-only:
 
@@ -133,8 +135,8 @@ These properties are only available in the Rust implementation. The Bun/TypeScri
 
 | Property | Default | Description |
 |----------|---------|-------------|
-| `session_sub_to_user_id_fn` | Parses string as i64, returns 0 on failure | Function that converts a session `sub` string to an `i64` user ID |
-| `session_user_to_sub_fn` | Returns `id` property as string | Function that extracts a `sub` string from a JSON record (`serde_json::Value`) |
+| `session_sub_to_user_id_fn` | Parses string as i64, returns parse error on failure | Function that converts a session `sub` string to an `i64` user ID. Returns `Result<i64, Box<dyn std::error::Error + Send + Sync>>` to allow custom error types. |
+| `session_user_to_sub_fn` | Returns `id` property as string, error on missing/invalid | Function that extracts a `sub` string from a JSON record (`serde_json::Value`). Returns `Result<String, Box<dyn std::error::Error + Send + Sync>>` for consistency. |
 
 Example usage:
 
@@ -143,23 +145,48 @@ let mut config = DpsConfig::new();
 
 // Use default: parses sub as i64
 let to_user_id = config.get_session_sub_to_user_id_fn();
-assert_eq!(to_user_id("42"), 42);
+assert_eq!(to_user_id("42").unwrap(), 42);
 
 // Custom converter: use length of sub as user ID
-config.set_session_sub_to_user_id_fn(|sub| sub.len() as i64);
+config.set_session_sub_to_user_id_fn(|sub| Ok(sub.len() as i64));
 
 // Custom sub extractor from JSON record
 config.set_session_user_to_sub_fn(|record| {
-    record.get("sub").and_then(|v| v.as_str()).unwrap_or("").to_string()
+    record.get("sub").and_then(|v| v.as_str()).map(|s| s.to_string()).ok_or("missing 'sub'".into())
 });
 ```
+
+#### Overriding from a consuming crate
+
+Since the functions return `Result<_, Box<dyn std::error::Error>>`, you can return your own error types (e.g., using `thiserror` or `anyhow`) without conversion.
+
+```rust
+use dps_config::DpsConfig;
+
+#[derive(thiserror::Error, Debug)]
+enum MyUserError {
+    #[error("user not found: {0}")]
+    NotFound(String),
+}
+
+fn configure_with_custom_errors(config: &mut DpsConfig) {
+    // Returning a custom thiserror type directly
+    config.set_session_sub_to_user_id_fn(|sub| {
+        if sub == "invalid" {
+            Err(Box::new(MyUserError::NotFound(sub.to_string())))
+        } else {
+            Ok(42)
+        }
+    });
+}
+```
 ```
 
 ## Files to Modify
 
 1. **`Cargo.toml`** — Add `serde_json = "1"` dependency
 2. **`src/lib.rs`** — Add struct fields, defaults in `new()`, getters/setters, tests
-3. **`README.md`** — Document new properties with Rust-only note
+3. **`README.md`** — Document new properties with Rust-only note and consuming crate example
 
 ## Order of Work
 
diff --git a/src/lib.rs b/src/lib.rs
index 95bfe01..c01ae3a 100644
--- a/src/lib.rs
+++ b/src/lib.rs
@@ -38,6 +38,7 @@ use std::env;
 ///
 /// Note: This struct intentionally does not perform validation — consuming
 /// crates should validate values where required.
+#[allow(clippy::type_complexity)]
 pub struct DpsConfig {
   // Global properties
   project_name: Option<String>,
@@ -60,8 +61,8 @@ pub struct DpsConfig {
   auth_api_session_ttl_seconds: Option<u32>,
 
   // Session conversion functions
-  session_sub_to_user_id_fn: Box<dyn Fn(&str) -> i64 + Send + Sync>,
-  session_user_to_sub_fn: Box<dyn Fn(&serde_json::Value) -> String + Send + Sync>,
+  session_sub_to_user_id_fn: Box<dyn Fn(&str) -> anyhow::Result<i64> + Send + Sync>,
+  session_user_to_sub_fn: Box<dyn Fn(&serde_json::Value) -> anyhow::Result<String> + Send + Sync>,
 }
 
 impl DpsConfig {
@@ -106,18 +107,20 @@ impl DpsConfig {
       auth_api_sqlite_session_pool_size: load_env_u16("DPS_AUTH_API_SQLITE_SESSION_POOL_SIZE"),
       auth_api_session_secret: load_env_string("DPS_AUTH_API_SESSION_SECRET"),
       auth_api_session_ttl_seconds: load_env_u32("DPS_AUTH_API_SESSION_TTL_SECONDS"),
-      session_sub_to_user_id_fn: Box::new(|sub: &str| sub.parse::<i64>().unwrap_or(0)),
+      session_sub_to_user_id_fn: Box::new(|sub: &str| {
+        sub.parse::<i64>().map_err(anyhow::Error::new)
+      }),
       session_user_to_sub_fn: Box::new(|record: &serde_json::Value| {
         record
           .get("id")
           .and_then(|v| {
             if let Some(n) = v.as_u64() {
-              Some(n.to_string())
+              Some(Ok(n.to_string()))
             } else {
-              v.as_str().map(|s| s.to_string())
+              v.as_str().map(|s| Ok(s.to_string()))
             }
           })
-          .unwrap_or_default()
+          .unwrap_or_else(|| Err(anyhow::anyhow!("missing or invalid 'id' field")))
       }),
     }
   }
@@ -371,27 +374,33 @@ impl DpsConfig {
 
   /// Returns the function that converts a session `sub` string to an `i64` user ID.
   ///
-  /// Default: Parses the string as `i64`, returning `0` on failure.
-  pub fn get_session_sub_to_user_id_fn(&self) -> &dyn Fn(&str) -> i64 {
+  /// Default: Parses the string as `i64`, returning a parse error on failure.
+  pub fn get_session_sub_to_user_id_fn(&self) -> &dyn Fn(&str) -> anyhow::Result<i64> {
     self.session_sub_to_user_id_fn.as_ref()
   }
 
   /// Set the session sub to user ID conversion function.
-  pub fn set_session_sub_to_user_id_fn(&mut self, f: impl Fn(&str) -> i64 + Send + Sync + 'static) {
+  pub fn set_session_sub_to_user_id_fn(
+    &mut self,
+    f: impl Fn(&str) -> anyhow::Result<i64> + Send + Sync + 'static,
+  ) {
     self.session_sub_to_user_id_fn = Box::new(f);
   }
 
   /// Returns the function that extracts a `sub` string from a JSON record.
   ///
-  /// Default: Returns the `id` property of the record as a string.
-  pub fn get_session_user_to_sub_fn(&self) -> &dyn Fn(&serde_json::Value) -> String {
+  /// Default: Returns the `id` property of the record as a string, or an error
+  /// if `id` is missing or invalid.
+  pub fn get_session_user_to_sub_fn(
+    &self,
+  ) -> &dyn Fn(&serde_json::Value) -> anyhow::Result<String> {
     self.session_user_to_sub_fn.as_ref()
   }
 
   /// Set the session user to sub conversion function.
   pub fn set_session_user_to_sub_fn(
     &mut self,
-    f: impl Fn(&serde_json::Value) -> String + Send + Sync + 'static,
+    f: impl Fn(&serde_json::Value) -> anyhow::Result<String> + Send + Sync + 'static,
   ) {
     self.session_user_to_sub_fn = Box::new(f);
   }
@@ -754,15 +763,15 @@ mod tests {
   fn test_session_sub_to_user_id_fn_default() {
     let config = DpsConfig::new();
     let to_user_id = config.get_session_sub_to_user_id_fn();
-    assert_eq!(to_user_id("12345"), 12345);
-    assert_eq!(to_user_id("invalid"), 0);
+    assert_eq!(to_user_id("12345").unwrap(), 12345);
+    assert!(to_user_id("invalid").is_err());
   }
 
   #[test]
   fn test_session_sub_to_user_id_fn_custom() {
     let mut config = DpsConfig::new();
-    config.set_session_sub_to_user_id_fn(|sub| sub.len() as i64);
-    assert_eq!(config.get_session_sub_to_user_id_fn()("hello"), 5);
+    config.set_session_sub_to_user_id_fn(|sub| Ok(sub.len() as i64));
+    assert_eq!(config.get_session_sub_to_user_id_fn()("hello").unwrap(), 5);
   }
 
   #[test]
@@ -770,7 +779,8 @@ mod tests {
     let config = DpsConfig::new();
     let to_sub = config.get_session_user_to_sub_fn();
     let record = serde_json::json!({ "id": 42, "name": "test" });
-    assert_eq!(to_sub(&record), "42");
+    assert_eq!(to_sub(&record).unwrap(), "42");
+    assert!(to_sub(&serde_json::json!({})).is_err());
   }
 
   #[test]
@@ -780,10 +790,13 @@ mod tests {
       record
         .get("sub")
         .and_then(|v| v.as_str())
-        .unwrap_or("")
-        .to_string()
+        .map(|s| s.to_string())
+        .ok_or_else(|| anyhow::anyhow!("missing 'sub'"))
     });
     let record = serde_json::json!({ "sub": "user-123", "name": "test" });
-    assert_eq!(config.get_session_user_to_sub_fn()(&record), "user-123");
+    assert_eq!(
+      config.get_session_user_to_sub_fn()(&record).unwrap(),
+      "user-123"
+    );
   }
 }