diff --git a/benches/data_loader_bench.rs b/benches/data_loader_bench.rs
index c4a743be46..b99aebc647 100644
--- a/benches/data_loader_bench.rs
+++ b/benches/data_loader_bench.rs
@@ -10,6 +10,8 @@ use criterion::Criterion;
 use hyper::body::Bytes;
 use reqwest::Request;
 use tailcall::core::config::Batch;
+use tailcall::core::error::file::FileError;
+use tailcall::core::error::http::HttpError;
 use tailcall::core::http::{DataLoaderRequest, HttpDataLoader, Response};
 use tailcall::core::ir::IoId;
 use tailcall::core::runtime::TargetRuntime;
@@ -23,7 +25,8 @@ struct MockHttpClient {
 
 #[async_trait::async_trait]
 impl HttpIO for MockHttpClient {
-    async fn execute(&self, _req: Request) -> anyhow::Result<Response<Bytes>> {
+    type Error = HttpError;
+    async fn execute(&self, _req: Request) -> Result<Response<Bytes>, Self::Error> {
         Ok(Response::empty())
     }
 }
@@ -39,11 +42,12 @@ struct File;
 
 #[async_trait::async_trait]
 impl FileIO for File {
-    async fn write<'a>(&'a self, _: &'a str, _: &'a [u8]) -> anyhow::Result<()> {
+    type Error = FileError;
+    async fn write<'a>(&'a self, _: &'a str, _: &'a [u8]) -> Result<(), Self::Error> {
         unimplemented!("Not needed for this bench")
     }
 
-    async fn read<'a>(&'a self, _: &'a str) -> anyhow::Result<String> {
+    async fn read<'a>(&'a self, _: &'a str) -> Result<String, Self::Error> {
         unimplemented!("Not needed for this bench")
     }
 }
diff --git a/benches/impl_path_string_for_evaluation_context.rs b/benches/impl_path_string_for_evaluation_context.rs
index d3e74c3184..a916b1e6d3 100644
--- a/benches/impl_path_string_for_evaluation_context.rs
+++ b/benches/impl_path_string_for_evaluation_context.rs
@@ -17,6 +17,8 @@ use reqwest::{Client, Request};
 use reqwest_middleware::{ClientBuilder, ClientWithMiddleware};
 use tailcall::core::blueprint::{Server, Upstream};
 use tailcall::core::cache::InMemoryCache;
+use tailcall::core::error::file::FileError;
+use tailcall::core::error::http::HttpError;
 use tailcall::core::http::{RequestContext, Response};
 use tailcall::core::ir::{EvaluationContext, ResolverContextLike};
 use tailcall::core::path::PathString;
@@ -70,7 +72,8 @@ impl Http {
 
 #[async_trait]
 impl HttpIO for Http {
-    async fn execute(&self, mut request: Request) -> anyhow::Result<Response<Bytes>> {
+    type Error = HttpError;
+    async fn execute(&self, mut request: Request) -> Result<Response<Bytes>, Self::Error> {
         if self.http2_only {
             *request.version_mut() = reqwest::Version::HTTP_2;
         }
@@ -90,11 +93,12 @@ impl EnvIO for Env {
 struct File;
 #[async_trait]
 impl FileIO for File {
-    async fn write<'a>(&'a self, _: &'a str, _: &'a [u8]) -> anyhow::Result<()> {
+    type Error = FileError;
+    async fn write<'a>(&'a self, _: &'a str, _: &'a [u8]) -> Result<(), Self::Error> {
         unimplemented!("Not needed for this bench")
     }
 
-    async fn read<'a>(&'a self, _: &'a str) -> anyhow::Result<String> {
+    async fn read<'a>(&'a self, _: &'a str) -> Result<String, Self::Error> {
         unimplemented!("Not needed for this bench")
     }
 }
diff --git a/src/core/errata.rs b/src/core/errata.rs
index ff55619b68..1a6d8110aa 100644
--- a/src/core/errata.rs
+++ b/src/core/errata.rs
@@ -3,8 +3,8 @@ use std::fmt::{Debug, Display};
 use colored::Colorize;
 use derive_setters::Setters;
 
-use crate::core::valid::ValidationError;
 use crate::core::error::Error as CoreError;
+use crate::core::valid::ValidationError;
 
 /// The moral equivalent of a serde_json::Value but for errors.
 /// It's a data structure like Value that can hold any error in an untyped
diff --git a/src/core/error.rs b/src/core/error.rs
index 4cc7419aee..7729479223 100644
--- a/src/core/error.rs
+++ b/src/core/error.rs
@@ -133,7 +133,7 @@ pub mod file {
 
     #[derive(From, thiserror::Error, Debug)]
     pub enum FileError {
-        #[error("File not found")]
+        #[error("No such file or directory (os error 2)")]
         NotFound,
 
         #[error("No permission to access the file")]
@@ -145,6 +145,12 @@ pub mod file {
         #[error("Invalid file format")]
         InvalidFormat,
 
+        #[error("Invalid file path")]
+        InvalidFilePath,
+
+        #[error("Invalid OS string")]
+        InvalidOsString,
+
         #[error("Failed to read file : {0}")]
         FileReadFailed(String),
 
@@ -160,6 +166,9 @@ pub mod file {
 
         #[error("File writing not supported on Lambda.")]
         LambdaFileWriteNotSupported,
+
+        #[error("Cannot write to a file in an execution spec")]
+        ExecutionSpecFileWriteFailed,
     }
 }
 
@@ -195,6 +204,25 @@ pub mod http {
         #[error("Unable to find key {0} in query params")]
         #[from(ignore)]
         KeyNotFound(String),
+
+        #[error("Invalid Status Code")]
+        InvalidStatusCode(hyper::http::status::InvalidStatusCode),
+
+        #[error("Status Code error")]
+        StatusCode,
+
+        #[error("Invalid Header Value")]
+        InvalidHeaderValue(hyper::header::InvalidHeaderValue),
+
+        #[error("Invalid Header Name")]
+        InvalidHeaderName(hyper::header::InvalidHeaderName),
+
+        #[error("No mock found for request: {method} {url} in {spec_path}")]
+        NoMockFound {
+            method: String,
+            url: String,
+            spec_path: String,
+        },
     }
 
     #[derive(From, thiserror::Error, Debug)]
diff --git a/src/core/generator/tests/json_to_config_spec.rs b/src/core/generator/tests/json_to_config_spec.rs
index 8d208219bb..836b44da4f 100644
--- a/src/core/generator/tests/json_to_config_spec.rs
+++ b/src/core/generator/tests/json_to_config_spec.rs
@@ -3,11 +3,10 @@ use std::path::Path;
 
 use serde::{Deserialize, Serialize};
 use serde_json::Value;
+use tailcall::core::error::Error;
 use tailcall::core::generator::{from_json, ConfigGenerationRequest};
 use url::Url;
 
-use crate::core::error::Error;
-
 #[derive(Serialize, Deserialize)]
 struct JsonFixture {
     url: String,
diff --git a/src/main.rs b/src/main.rs
index 7a7c845961..16c584cf1d 100644
--- a/src/main.rs
+++ b/src/main.rs
@@ -4,10 +4,10 @@
 use std::cell::Cell;
 
 use mimalloc::MiMalloc;
+use tailcall::core::error::Error;
 use tailcall::core::tracing::default_tracing_tailcall;
 use tailcall::core::Errata;
 use tracing::subscriber::DefaultGuard;
-use tailcall::core::error::Error;
 
 #[global_allocator]
 static GLOBAL: MiMalloc = MiMalloc;
diff --git a/tailcall-cloudflare/src/runtime.rs b/tailcall-cloudflare/src/runtime.rs
index 69f423c453..9c7243b662 100644
--- a/tailcall-cloudflare/src/runtime.rs
+++ b/tailcall-cloudflare/src/runtime.rs
@@ -15,7 +15,10 @@ fn init_env(env: Rc<worker::Env>) -> Arc<dyn EnvIO> {
     Arc::new(env::CloudflareEnv::init(env))
 }
 
-fn init_file(env: Rc<worker::Env>, bucket_id: &str) -> anyhow::Result<Arc<dyn FileIO<Error = FileError>>> {
+fn init_file(
+    env: Rc<worker::Env>,
+    bucket_id: &str,
+) -> anyhow::Result<Arc<dyn FileIO<Error = FileError>>> {
     Ok(Arc::new(file::CloudflareFileIO::init(env, bucket_id)?))
 }
 
diff --git a/tests/core/file.rs b/tests/core/file.rs
index 574bd38c5b..e01e6da9cc 100644
--- a/tests/core/file.rs
+++ b/tests/core/file.rs
@@ -2,7 +2,7 @@ extern crate core;
 
 use std::path::PathBuf;
 
-use anyhow::{anyhow, Context};
+use tailcall::core::error::file::FileError;
 use tailcall::core::FileIO;
 use tokio::io::{AsyncReadExt, AsyncWriteExt};
 
@@ -19,20 +19,22 @@ impl File {
 
 #[async_trait::async_trait]
 impl FileIO for File {
-    async fn write<'a>(&'a self, _path: &'a str, _content: &'a [u8]) -> anyhow::Result<()> {
-        Err(anyhow!("Cannot write to a file in an execution spec"))
+    type Error = FileError;
+    async fn write<'a>(&'a self, _path: &'a str, _content: &'a [u8]) -> Result<(), Self::Error> {
+        Err(FileError::ExecutionSpecFileWriteFailed)
     }
 
-    async fn read<'a>(&'a self, path: &'a str) -> anyhow::Result<String> {
+    async fn read<'a>(&'a self, path: &'a str) -> Result<String, Self::Error> {
         let base = PathBuf::from(path);
         let path = base
             .file_name()
-            .context("Invalid file path")?
+            .ok_or(FileError::InvalidFilePath)?
             .to_str()
-            .context("Invalid OsString")?;
+            .ok_or(FileError::InvalidOsString)?;
+
         match self.spec.files.get(path) {
             Some(x) => Ok(x.to_owned()),
-            None => Err(anyhow!("No such file or directory (os error 2)")),
+            None => Err(FileError::NotFound),
         }
     }
 }
@@ -48,20 +50,17 @@ impl TestFileIO {
 
 #[async_trait::async_trait]
 impl FileIO for TestFileIO {
-    async fn write<'a>(&'a self, path: &'a str, content: &'a [u8]) -> anyhow::Result<()> {
+    type Error = FileError;
+    async fn write<'a>(&'a self, path: &'a str, content: &'a [u8]) -> Result<(), Self::Error> {
         let mut file = tokio::fs::File::create(path).await?;
-        file.write_all(content)
-            .await
-            .map_err(|e| anyhow!("{}", e))?;
+        file.write_all(content).await?;
         Ok(())
     }
 
-    async fn read<'a>(&'a self, path: &'a str) -> anyhow::Result<String> {
+    async fn read<'a>(&'a self, path: &'a str) -> Result<String, Self::Error> {
         let mut file = tokio::fs::File::open(path).await?;
         let mut buffer = Vec::new();
-        file.read_to_end(&mut buffer)
-            .await
-            .map_err(|e| anyhow!("{}", e))?;
+        file.read_to_end(&mut buffer).await?;
         Ok(String::from_utf8(buffer)?)
     }
 }
diff --git a/tests/core/http.rs b/tests/core/http.rs
index 87192a62b8..39c0b05048 100644
--- a/tests/core/http.rs
+++ b/tests/core/http.rs
@@ -5,9 +5,9 @@ use std::str::FromStr;
 use std::sync::atomic::{AtomicUsize, Ordering};
 use std::sync::Arc;
 
-use anyhow::anyhow;
 use hyper::body::Bytes;
 use reqwest::header::{HeaderName, HeaderValue};
+use tailcall::core::error::http::HttpError;
 use tailcall::core::http::Response;
 use tailcall::core::HttpIO;
 
@@ -54,7 +54,8 @@ impl Http {
 
 #[async_trait::async_trait]
 impl HttpIO for Http {
-    async fn execute(&self, req: reqwest::Request) -> anyhow::Result<Response<Bytes>> {
+    type Error = HttpError;
+    async fn execute(&self, req: reqwest::Request) -> Result<Response<Bytes>, Self::Error> {
         // Try to find a matching mock for the incoming request.
         let execution_mock = self
             .mocks
@@ -94,12 +95,11 @@ impl HttpIO for Http {
                     });
                 method_match && url_match && headers_match && body_match
             })
-            .ok_or(anyhow!(
-                "No mock found for request: {:?} {} in {}",
-                req.method(),
-                req.url(),
-                self.spec_path
-            ))?;
+            .ok_or_else(|| HttpError::NoMockFound {
+                method: req.method().to_string(),
+                url: req.url().to_string(),
+                spec_path: self.spec_path.to_string(),
+            })?;
 
         execution_mock.actual_hits.fetch_add(1, Ordering::Relaxed);
 
@@ -110,7 +110,7 @@ impl HttpIO for Http {
         let status_code = reqwest::StatusCode::from_u16(mock_response.0.status)?;
 
         if status_code.is_client_error() || status_code.is_server_error() {
-            return Err(anyhow::format_err!("Status code error"));
+            return Err(HttpError::StatusCode);
         }
 
         let mut response = Response { status: status_code, ..Default::default() };
diff --git a/tests/core/parse.rs b/tests/core/parse.rs
index 10fef0528a..a6d43bd915 100644
--- a/tests/core/parse.rs
+++ b/tests/core/parse.rs
@@ -15,6 +15,7 @@ use tailcall::cli::javascript;
 use tailcall::core::blueprint::Blueprint;
 use tailcall::core::cache::InMemoryCache;
 use tailcall::core::config::{ConfigModule, Source};
+use tailcall::core::error::worker::WorkerError;
 use tailcall::core::http::AppContext;
 use tailcall::core::runtime::TargetRuntime;
 use tailcall::core::worker::{Command, Event};
@@ -277,19 +278,19 @@ impl ExecutionSpec {
 
         let http2_only = http.clone();
 
-        let http_worker: Option<Arc<dyn WorkerIO<Event, Command>>> =
+        let http_worker: Option<Arc<dyn WorkerIO<Event, Command, Error = WorkerError>>> =
             if let Some(script) = script.clone() {
                 Some(javascript::init_worker_io(script))
             } else {
                 None
             };
 
-        let worker: Option<Arc<dyn WorkerIO<ConstValue, ConstValue>>> = if let Some(script) = script
-        {
-            Some(javascript::init_worker_io(script))
-        } else {
-            None
-        };
+        let worker: Option<Arc<dyn WorkerIO<ConstValue, ConstValue, Error = WorkerError>>> =
+            if let Some(script) = script {
+                Some(javascript::init_worker_io(script))
+            } else {
+                None
+            };
 
         let runtime = TargetRuntime {
             http,
diff --git a/tests/core/spec.rs b/tests/core/spec.rs
index f054475ddf..b9aab9aeea 100644
--- a/tests/core/spec.rs
+++ b/tests/core/spec.rs
@@ -14,6 +14,7 @@ use tailcall::core::async_graphql_hyper::{GraphQLBatchRequest, GraphQLRequest};
 use tailcall::core::blueprint::Blueprint;
 use tailcall::core::config::reader::ConfigReader;
 use tailcall::core::config::{Config, ConfigModule, Source};
+use tailcall::core::error::Error;
 use tailcall::core::http::{handle_request, AppContext};
 use tailcall::core::merge_right::MergeRight;
 use tailcall::core::print_schema::print_schema;
@@ -301,7 +302,7 @@ pub async fn load_and_test_execution_spec(path: &Path) -> anyhow::Result<()> {
 async fn run_test(
     app_ctx: Arc<AppContext>,
     request: &APIRequest,
-) -> anyhow::Result<hyper::Response<Body>> {
+) -> Result<hyper::Response<Body>, Error> {
     let body = request
         .body
         .as_ref()
diff --git a/tests/server_spec.rs b/tests/server_spec.rs
index 297b4a9572..d2b7ac69dc 100644
--- a/tests/server_spec.rs
+++ b/tests/server_spec.rs
@@ -5,7 +5,7 @@ pub mod test {
     use std::sync::Arc;
     use std::time::Duration;
 
-    use anyhow::{anyhow, Result};
+    use anyhow::Result;
     use async_graphql::Value;
     use http_cache_reqwest::{Cache, CacheMode, HttpCache, HttpCacheOptions};
     use hyper::body::Bytes;
@@ -14,6 +14,8 @@ pub mod test {
     use tailcall::cli::javascript::init_worker_io;
     use tailcall::core::blueprint::{Script, Upstream};
     use tailcall::core::cache::InMemoryCache;
+    use tailcall::core::error::file::FileError;
+    use tailcall::core::error::http::HttpError;
     use tailcall::core::http::Response;
     use tailcall::core::runtime::TargetRuntime;
     use tailcall::core::worker::{Command, Event};
@@ -73,7 +75,8 @@ pub mod test {
 
     #[async_trait::async_trait]
     impl HttpIO for TestHttp {
-        async fn execute(&self, request: reqwest::Request) -> Result<Response<Bytes>> {
+        type Error = HttpError;
+        async fn execute(&self, request: reqwest::Request) -> Result<Response<Bytes>, HttpError> {
             let response = self.client.execute(request).await;
             Response::from_reqwest(
                 response?
@@ -95,20 +98,17 @@ pub mod test {
 
     #[async_trait::async_trait]
     impl FileIO for TestFileIO {
-        async fn write<'a>(&'a self, path: &'a str, content: &'a [u8]) -> anyhow::Result<()> {
+        type Error = FileError;
+        async fn write<'a>(&'a self, path: &'a str, content: &'a [u8]) -> Result<(), Self::Error> {
             let mut file = tokio::fs::File::create(path).await?;
-            file.write_all(content)
-                .await
-                .map_err(|e| anyhow!("{}", e))?;
+            file.write_all(content).await?;
             Ok(())
         }
 
-        async fn read<'a>(&'a self, path: &'a str) -> anyhow::Result<String> {
+        async fn read<'a>(&'a self, path: &'a str) -> Result<String, Self::Error> {
             let mut file = tokio::fs::File::open(path).await?;
             let mut buffer = Vec::new();
-            file.read_to_end(&mut buffer)
-                .await
-                .map_err(|e| anyhow!("{}", e))?;
+            file.read_to_end(&mut buffer).await?;
             Ok(String::from_utf8(buffer)?)
         }
     }