diff --git a/CHANGELOG.md b/CHANGELOG.md
index bac0c5156..8c9daf236 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -2,6 +2,15 @@
 
 ## Unreleased
 
+### Breaking Changes
+
+* Generated HTTP clients no longer install aiohttp and urllib3 by default.
+  Install the `sync` extra for the synchronous client's default transport, the
+  `async` extra for the asynchronous client's default transport, or both, such
+  as `uv add "aws_sdk_s3[sync,async]"` or
+  `pip install "aws_sdk_s3[sync,async]"`. Clients configured with a
+  custom transport work without either extra. Imports are unchanged.
+
 ### Features
 
 * Generated AWS clients validate the checksums of response payloads for operations with the `aws.protocols#httpChecksum` trait, such as Amazon S3's `GetObject`. These clients depend on the `checksums` extra of `smithy-aws-core`.
diff --git a/codegen-output/aws-json-1-0/README.md b/codegen-output/aws-json-1-0/README.md
index 19ea5f3c1..33cbc70a4 100644
--- a/codegen-output/aws-json-1-0/README.md
+++ b/codegen-output/aws-json-1-0/README.md
@@ -3,3 +3,23 @@
 ## Sample Json 1.0 Protocol Service Client
 
 Python client for Sample Json 1.0 Protocol Service
+
+### Installation
+
+Install the `sync` extra for the synchronous client's default HTTP transport, the `async` extra for the asynchronous client's default HTTP transport, or both. In a project managed by uv, add the client with the extras you need:
+
+```sh
+uv add "awsjson10[sync]"
+uv add "awsjson10[async]"
+uv add "awsjson10[sync,async]"
+```
+
+Or install it with pip:
+
+```sh
+pip install "awsjson10[sync]"
+pip install "awsjson10[async]"
+pip install "awsjson10[sync,async]"
+```
+
+Without either extra, the models can be used, and either client works with an explicitly configured custom transport.
diff --git a/codegen-output/aws-json-1-0/pyproject.toml b/codegen-output/aws-json-1-0/pyproject.toml
index a9b4f5b94..7d9809e48 100644
--- a/codegen-output/aws-json-1-0/pyproject.toml
+++ b/codegen-output/aws-json-1-0/pyproject.toml
@@ -26,10 +26,18 @@ classifiers = [
 dependencies = [
     "smithy_aws_core[json]~=0.13.0",
     "smithy_core~=0.10.0",
-    "smithy_http[aiohttp, urllib3]~=0.7.0"
+    "smithy_http~=0.7.0"
 ]
 
 [project.optional-dependencies]
+sync = [
+    "smithy_http[urllib3]~=0.7.0"
+]
+
+async = [
+    "smithy_http[aiohttp]~=0.7.0"
+]
+
 awscrt = [
     "smithy_http[awscrt]~=0.7.0"
 ]
diff --git a/codegen-output/aws-json-1-1/README.md b/codegen-output/aws-json-1-1/README.md
index 984d1fbde..6e3791a54 100644
--- a/codegen-output/aws-json-1-1/README.md
+++ b/codegen-output/aws-json-1-1/README.md
@@ -3,3 +3,23 @@
 ## Sample Json 1.1 Protocol Service Client
 
 Python client for Sample Json 1.1 Protocol Service
+
+### Installation
+
+Install the `sync` extra for the synchronous client's default HTTP transport, the `async` extra for the asynchronous client's default HTTP transport, or both. In a project managed by uv, add the client with the extras you need:
+
+```sh
+uv add "awsjson11[sync]"
+uv add "awsjson11[async]"
+uv add "awsjson11[sync,async]"
+```
+
+Or install it with pip:
+
+```sh
+pip install "awsjson11[sync]"
+pip install "awsjson11[async]"
+pip install "awsjson11[sync,async]"
+```
+
+Without either extra, the models can be used, and either client works with an explicitly configured custom transport.
diff --git a/codegen-output/aws-json-1-1/pyproject.toml b/codegen-output/aws-json-1-1/pyproject.toml
index c6f588ad9..6366b311b 100644
--- a/codegen-output/aws-json-1-1/pyproject.toml
+++ b/codegen-output/aws-json-1-1/pyproject.toml
@@ -26,10 +26,18 @@ classifiers = [
 dependencies = [
     "smithy_aws_core[json]~=0.13.0",
     "smithy_core~=0.10.0",
-    "smithy_http[aiohttp, urllib3]~=0.7.0"
+    "smithy_http~=0.7.0"
 ]
 
 [project.optional-dependencies]
+sync = [
+    "smithy_http[urllib3]~=0.7.0"
+]
+
+async = [
+    "smithy_http[aiohttp]~=0.7.0"
+]
+
 awscrt = [
     "smithy_http[awscrt]~=0.7.0"
 ]
diff --git a/codegen-output/aws-query/README.md b/codegen-output/aws-query/README.md
index fdd59c80f..1b35dc7cf 100644
--- a/codegen-output/aws-query/README.md
+++ b/codegen-output/aws-query/README.md
@@ -4,6 +4,26 @@
 
 Python client for Sample Query Protocol Service
 
+### Installation
+
+Install the `sync` extra for the synchronous client's default HTTP transport, the `async` extra for the asynchronous client's default HTTP transport, or both. In a project managed by uv, add the client with the extras you need:
+
+```sh
+uv add "awsquery[sync]"
+uv add "awsquery[async]"
+uv add "awsquery[sync,async]"
+```
+
+Or install it with pip:
+
+```sh
+pip install "awsquery[sync]"
+pip install "awsquery[async]"
+pip install "awsquery[sync,async]"
+```
+
+Without either extra, the models can be used, and either client works with an explicitly configured custom transport.
+
 ### Documentation
 
 A query service that sends query requests and XML responses.
diff --git a/codegen-output/aws-query/pyproject.toml b/codegen-output/aws-query/pyproject.toml
index eade7c2a8..4c7311f37 100644
--- a/codegen-output/aws-query/pyproject.toml
+++ b/codegen-output/aws-query/pyproject.toml
@@ -26,10 +26,18 @@ classifiers = [
 dependencies = [
     "smithy_aws_core[xml]~=0.13.0",
     "smithy_core~=0.10.0",
-    "smithy_http[aiohttp, urllib3]~=0.7.0"
+    "smithy_http~=0.7.0"
 ]
 
 [project.optional-dependencies]
+sync = [
+    "smithy_http[urllib3]~=0.7.0"
+]
+
+async = [
+    "smithy_http[aiohttp]~=0.7.0"
+]
+
 awscrt = [
     "smithy_http[awscrt]~=0.7.0"
 ]
diff --git a/codegen-output/ec2-query/README.md b/codegen-output/ec2-query/README.md
index c24b4b691..81451933e 100644
--- a/codegen-output/ec2-query/README.md
+++ b/codegen-output/ec2-query/README.md
@@ -4,6 +4,26 @@
 
 Python client for Sample Ec2 Protocol Service
 
+### Installation
+
+Install the `sync` extra for the synchronous client's default HTTP transport, the `async` extra for the asynchronous client's default HTTP transport, or both. In a project managed by uv, add the client with the extras you need:
+
+```sh
+uv add "ec2query[sync]"
+uv add "ec2query[async]"
+uv add "ec2query[sync,async]"
+```
+
+Or install it with pip:
+
+```sh
+pip install "ec2query[sync]"
+pip install "ec2query[async]"
+pip install "ec2query[sync,async]"
+```
+
+Without either extra, the models can be used, and either client works with an explicitly configured custom transport.
+
 ### Documentation
 
 An EC2 query service that sends query requests and XML responses.
diff --git a/codegen-output/ec2-query/pyproject.toml b/codegen-output/ec2-query/pyproject.toml
index 4b6ce3a64..00b332137 100644
--- a/codegen-output/ec2-query/pyproject.toml
+++ b/codegen-output/ec2-query/pyproject.toml
@@ -26,10 +26,18 @@ classifiers = [
 dependencies = [
     "smithy_aws_core[xml]~=0.13.0",
     "smithy_core~=0.10.0",
-    "smithy_http[aiohttp, urllib3]~=0.7.0"
+    "smithy_http~=0.7.0"
 ]
 
 [project.optional-dependencies]
+sync = [
+    "smithy_http[urllib3]~=0.7.0"
+]
+
+async = [
+    "smithy_http[aiohttp]~=0.7.0"
+]
+
 awscrt = [
     "smithy_http[awscrt]~=0.7.0"
 ]
diff --git a/codegen-output/my-handwritten-json-model/README.md b/codegen-output/my-handwritten-json-model/README.md
index 006564531..c427e2bf4 100644
--- a/codegen-output/my-handwritten-json-model/README.md
+++ b/codegen-output/my-handwritten-json-model/README.md
@@ -4,6 +4,26 @@
 
 Python client for My Handwritten JSON Model
 
+### Installation
+
+Install the `sync` extra for the synchronous client's default HTTP transport, the `async` extra for the asynchronous client's default HTTP transport, or both. In a project managed by uv, add the client with the extras you need:
+
+```sh
+uv add "my_handwritten_json_model[sync]"
+uv add "my_handwritten_json_model[async]"
+uv add "my_handwritten_json_model[sync,async]"
+```
+
+Or install it with pip:
+
+```sh
+pip install "my_handwritten_json_model[sync]"
+pip install "my_handwritten_json_model[async]"
+pip install "my_handwritten_json_model[sync,async]"
+```
+
+Without either extra, the models can be used, and either client works with an explicitly configured custom transport.
+
 ### Documentation
 
 The rpcv2Json counterpart of
diff --git a/codegen-output/my-handwritten-json-model/pyproject.toml b/codegen-output/my-handwritten-json-model/pyproject.toml
index 6f1e2134b..b7753b36f 100644
--- a/codegen-output/my-handwritten-json-model/pyproject.toml
+++ b/codegen-output/my-handwritten-json-model/pyproject.toml
@@ -26,10 +26,18 @@ classifiers = [
 dependencies = [
     "smithy_aws_core[eventstream]~=0.13.0",
     "smithy_core~=0.10.0",
-    "smithy_http[json, aiohttp, urllib3]~=0.7.0"
+    "smithy_http[json]~=0.7.0"
 ]
 
 [project.optional-dependencies]
+sync = [
+    "smithy_http[urllib3]~=0.7.0"
+]
+
+async = [
+    "smithy_http[aiohttp]~=0.7.0"
+]
+
 awscrt = [
     "smithy_http[awscrt]~=0.7.0"
 ]
diff --git a/codegen-output/my-handwritten-model/README.md b/codegen-output/my-handwritten-model/README.md
index a7227557a..921e8c506 100644
--- a/codegen-output/my-handwritten-model/README.md
+++ b/codegen-output/my-handwritten-model/README.md
@@ -3,3 +3,23 @@
 ## My Handwritten Model Client
 
 Python client for My Handwritten Model
+
+### Installation
+
+Install the `sync` extra for the synchronous client's default HTTP transport, the `async` extra for the asynchronous client's default HTTP transport, or both. In a project managed by uv, add the client with the extras you need:
+
+```sh
+uv add "my_handwritten_model[sync]"
+uv add "my_handwritten_model[async]"
+uv add "my_handwritten_model[sync,async]"
+```
+
+Or install it with pip:
+
+```sh
+pip install "my_handwritten_model[sync]"
+pip install "my_handwritten_model[async]"
+pip install "my_handwritten_model[sync,async]"
+```
+
+Without either extra, the models can be used, and either client works with an explicitly configured custom transport.
diff --git a/codegen-output/my-handwritten-model/pyproject.toml b/codegen-output/my-handwritten-model/pyproject.toml
index e58cf27f1..86d58fdc1 100644
--- a/codegen-output/my-handwritten-model/pyproject.toml
+++ b/codegen-output/my-handwritten-model/pyproject.toml
@@ -26,10 +26,18 @@ classifiers = [
 dependencies = [
     "smithy_aws_core[eventstream]~=0.13.0",
     "smithy_core~=0.10.0",
-    "smithy_http[cbor, aiohttp, urllib3]~=0.7.0"
+    "smithy_http[cbor]~=0.7.0"
 ]
 
 [project.optional-dependencies]
+sync = [
+    "smithy_http[urllib3]~=0.7.0"
+]
+
+async = [
+    "smithy_http[aiohttp]~=0.7.0"
+]
+
 awscrt = [
     "smithy_http[awscrt]~=0.7.0"
 ]
diff --git a/codegen-output/non-query-compatible-rpcv2-cbor/README.md b/codegen-output/non-query-compatible-rpcv2-cbor/README.md
index 7b955d6a8..d70d02be3 100644
--- a/codegen-output/non-query-compatible-rpcv2-cbor/README.md
+++ b/codegen-output/non-query-compatible-rpcv2-cbor/README.md
@@ -3,3 +3,23 @@
 ## Non Query Compatible RpcV2 Protocol Service Client
 
 Python client for Non Query Compatible RpcV2 Protocol Service
+
+### Installation
+
+Install the `sync` extra for the synchronous client's default HTTP transport, the `async` extra for the asynchronous client's default HTTP transport, or both. In a project managed by uv, add the client with the extras you need:
+
+```sh
+uv add "nonquerycompatiblerpcv2cbor[sync]"
+uv add "nonquerycompatiblerpcv2cbor[async]"
+uv add "nonquerycompatiblerpcv2cbor[sync,async]"
+```
+
+Or install it with pip:
+
+```sh
+pip install "nonquerycompatiblerpcv2cbor[sync]"
+pip install "nonquerycompatiblerpcv2cbor[async]"
+pip install "nonquerycompatiblerpcv2cbor[sync,async]"
+```
+
+Without either extra, the models can be used, and either client works with an explicitly configured custom transport.
diff --git a/codegen-output/non-query-compatible-rpcv2-cbor/pyproject.toml b/codegen-output/non-query-compatible-rpcv2-cbor/pyproject.toml
index 5937a8056..5a594da75 100644
--- a/codegen-output/non-query-compatible-rpcv2-cbor/pyproject.toml
+++ b/codegen-output/non-query-compatible-rpcv2-cbor/pyproject.toml
@@ -26,10 +26,18 @@ classifiers = [
 dependencies = [
     "smithy_aws_core~=0.13.0",
     "smithy_core~=0.10.0",
-    "smithy_http[cbor, aiohttp, urllib3]~=0.7.0"
+    "smithy_http[cbor]~=0.7.0"
 ]
 
 [project.optional-dependencies]
+sync = [
+    "smithy_http[urllib3]~=0.7.0"
+]
+
+async = [
+    "smithy_http[aiohttp]~=0.7.0"
+]
+
 awscrt = [
     "smithy_http[awscrt]~=0.7.0"
 ]
diff --git a/codegen-output/query-compatible-aws-json-1-0/README.md b/codegen-output/query-compatible-aws-json-1-0/README.md
index 2f8ba8249..322f9b04e 100644
--- a/codegen-output/query-compatible-aws-json-1-0/README.md
+++ b/codegen-output/query-compatible-aws-json-1-0/README.md
@@ -3,3 +3,23 @@
 ## Query Compatible Json 1.0 Protocol Service Client
 
 Python client for Query Compatible Json 1.0 Protocol Service
+
+### Installation
+
+Install the `sync` extra for the synchronous client's default HTTP transport, the `async` extra for the asynchronous client's default HTTP transport, or both. In a project managed by uv, add the client with the extras you need:
+
+```sh
+uv add "querycompatibleawsjson10[sync]"
+uv add "querycompatibleawsjson10[async]"
+uv add "querycompatibleawsjson10[sync,async]"
+```
+
+Or install it with pip:
+
+```sh
+pip install "querycompatibleawsjson10[sync]"
+pip install "querycompatibleawsjson10[async]"
+pip install "querycompatibleawsjson10[sync,async]"
+```
+
+Without either extra, the models can be used, and either client works with an explicitly configured custom transport.
diff --git a/codegen-output/query-compatible-aws-json-1-0/pyproject.toml b/codegen-output/query-compatible-aws-json-1-0/pyproject.toml
index 0fdccd779..3441618ae 100644
--- a/codegen-output/query-compatible-aws-json-1-0/pyproject.toml
+++ b/codegen-output/query-compatible-aws-json-1-0/pyproject.toml
@@ -26,10 +26,18 @@ classifiers = [
 dependencies = [
     "smithy_aws_core[json]~=0.13.0",
     "smithy_core~=0.10.0",
-    "smithy_http[aiohttp, urllib3]~=0.7.0"
+    "smithy_http~=0.7.0"
 ]
 
 [project.optional-dependencies]
+sync = [
+    "smithy_http[urllib3]~=0.7.0"
+]
+
+async = [
+    "smithy_http[aiohttp]~=0.7.0"
+]
+
 awscrt = [
     "smithy_http[awscrt]~=0.7.0"
 ]
diff --git a/codegen-output/query-compatible-rpcv2-cbor/README.md b/codegen-output/query-compatible-rpcv2-cbor/README.md
index 5928f1851..6f5efe55b 100644
--- a/codegen-output/query-compatible-rpcv2-cbor/README.md
+++ b/codegen-output/query-compatible-rpcv2-cbor/README.md
@@ -3,3 +3,23 @@
 ## Query Compatible RpcV2 Protocol Service Client
 
 Python client for Query Compatible RpcV2 Protocol Service
+
+### Installation
+
+Install the `sync` extra for the synchronous client's default HTTP transport, the `async` extra for the asynchronous client's default HTTP transport, or both. In a project managed by uv, add the client with the extras you need:
+
+```sh
+uv add "querycompatiblerpcv2cbor[sync]"
+uv add "querycompatiblerpcv2cbor[async]"
+uv add "querycompatiblerpcv2cbor[sync,async]"
+```
+
+Or install it with pip:
+
+```sh
+pip install "querycompatiblerpcv2cbor[sync]"
+pip install "querycompatiblerpcv2cbor[async]"
+pip install "querycompatiblerpcv2cbor[sync,async]"
+```
+
+Without either extra, the models can be used, and either client works with an explicitly configured custom transport.
diff --git a/codegen-output/query-compatible-rpcv2-cbor/pyproject.toml b/codegen-output/query-compatible-rpcv2-cbor/pyproject.toml
index 8be942985..897ee5c00 100644
--- a/codegen-output/query-compatible-rpcv2-cbor/pyproject.toml
+++ b/codegen-output/query-compatible-rpcv2-cbor/pyproject.toml
@@ -26,10 +26,18 @@ classifiers = [
 dependencies = [
     "smithy_aws_core~=0.13.0",
     "smithy_core~=0.10.0",
-    "smithy_http[cbor, aiohttp, urllib3]~=0.7.0"
+    "smithy_http[cbor]~=0.7.0"
 ]
 
 [project.optional-dependencies]
+sync = [
+    "smithy_http[urllib3]~=0.7.0"
+]
+
+async = [
+    "smithy_http[aiohttp]~=0.7.0"
+]
+
 awscrt = [
     "smithy_http[awscrt]~=0.7.0"
 ]
diff --git a/codegen-output/rest-json-1/README.md b/codegen-output/rest-json-1/README.md
index 239dd0567..50ca0c053 100644
--- a/codegen-output/rest-json-1/README.md
+++ b/codegen-output/rest-json-1/README.md
@@ -3,3 +3,23 @@
 ## Sample Rest Json Protocol Service Client
 
 Python client for Sample Rest Json Protocol Service
+
+### Installation
+
+Install the `sync` extra for the synchronous client's default HTTP transport, the `async` extra for the asynchronous client's default HTTP transport, or both. In a project managed by uv, add the client with the extras you need:
+
+```sh
+uv add "restjson[sync]"
+uv add "restjson[async]"
+uv add "restjson[sync,async]"
+```
+
+Or install it with pip:
+
+```sh
+pip install "restjson[sync]"
+pip install "restjson[async]"
+pip install "restjson[sync,async]"
+```
+
+Without either extra, the models can be used, and either client works with an explicitly configured custom transport.
diff --git a/codegen-output/rest-json-1/pyproject.toml b/codegen-output/rest-json-1/pyproject.toml
index 28eae9ce9..89638e431 100644
--- a/codegen-output/rest-json-1/pyproject.toml
+++ b/codegen-output/rest-json-1/pyproject.toml
@@ -26,10 +26,18 @@ classifiers = [
 dependencies = [
     "smithy_aws_core[eventstream, json]~=0.13.0",
     "smithy_core~=0.10.0",
-    "smithy_http[aiohttp, urllib3]~=0.7.0"
+    "smithy_http~=0.7.0"
 ]
 
 [project.optional-dependencies]
+sync = [
+    "smithy_http[urllib3]~=0.7.0"
+]
+
+async = [
+    "smithy_http[aiohttp]~=0.7.0"
+]
+
 awscrt = [
     "smithy_http[awscrt]~=0.7.0"
 ]
diff --git a/codegen-output/rest-xml-with-namespace/README.md b/codegen-output/rest-xml-with-namespace/README.md
index 9c005dcfc..1ab00ba42 100644
--- a/codegen-output/rest-xml-with-namespace/README.md
+++ b/codegen-output/rest-xml-with-namespace/README.md
@@ -4,6 +4,26 @@
 
 Python client for Sample Rest Xml Protocol Service With Namespace
 
+### Installation
+
+Install the `sync` extra for the synchronous client's default HTTP transport, the `async` extra for the asynchronous client's default HTTP transport, or both. In a project managed by uv, add the client with the extras you need:
+
+```sh
+uv add "restxmlwithnamespace[sync]"
+uv add "restxmlwithnamespace[async]"
+uv add "restxmlwithnamespace[sync,async]"
+```
+
+Or install it with pip:
+
+```sh
+pip install "restxmlwithnamespace[sync]"
+pip install "restxmlwithnamespace[async]"
+pip install "restxmlwithnamespace[sync,async]"
+```
+
+Without either extra, the models can be used, and either client works with an explicitly configured custom transport.
+
 ### Documentation
 
 A REST XML service that sends XML requests and responses. This service
diff --git a/codegen-output/rest-xml-with-namespace/pyproject.toml b/codegen-output/rest-xml-with-namespace/pyproject.toml
index 2a0359aa8..cae6d76a2 100644
--- a/codegen-output/rest-xml-with-namespace/pyproject.toml
+++ b/codegen-output/rest-xml-with-namespace/pyproject.toml
@@ -26,10 +26,18 @@ classifiers = [
 dependencies = [
     "smithy_aws_core[xml]~=0.13.0",
     "smithy_core~=0.10.0",
-    "smithy_http[aiohttp, urllib3]~=0.7.0"
+    "smithy_http~=0.7.0"
 ]
 
 [project.optional-dependencies]
+sync = [
+    "smithy_http[urllib3]~=0.7.0"
+]
+
+async = [
+    "smithy_http[aiohttp]~=0.7.0"
+]
+
 awscrt = [
     "smithy_http[awscrt]~=0.7.0"
 ]
diff --git a/codegen-output/rest-xml/README.md b/codegen-output/rest-xml/README.md
index 365c8f067..4e441fc8d 100644
--- a/codegen-output/rest-xml/README.md
+++ b/codegen-output/rest-xml/README.md
@@ -4,6 +4,26 @@
 
 Python client for Sample Rest Xml Protocol Service
 
+### Installation
+
+Install the `sync` extra for the synchronous client's default HTTP transport, the `async` extra for the asynchronous client's default HTTP transport, or both. In a project managed by uv, add the client with the extras you need:
+
+```sh
+uv add "restxml[sync]"
+uv add "restxml[async]"
+uv add "restxml[sync,async]"
+```
+
+Or install it with pip:
+
+```sh
+pip install "restxml[sync]"
+pip install "restxml[async]"
+pip install "restxml[sync,async]"
+```
+
+Without either extra, the models can be used, and either client works with an explicitly configured custom transport.
+
 ### Documentation
 
 A REST XML service that sends XML requests and responses.
diff --git a/codegen-output/rest-xml/pyproject.toml b/codegen-output/rest-xml/pyproject.toml
index 6d34622cd..28e4e01eb 100644
--- a/codegen-output/rest-xml/pyproject.toml
+++ b/codegen-output/rest-xml/pyproject.toml
@@ -26,10 +26,18 @@ classifiers = [
 dependencies = [
     "smithy_aws_core[xml]~=0.13.0",
     "smithy_core~=0.10.0",
-    "smithy_http[aiohttp, urllib3]~=0.7.0"
+    "smithy_http~=0.7.0"
 ]
 
 [project.optional-dependencies]
+sync = [
+    "smithy_http[urllib3]~=0.7.0"
+]
+
+async = [
+    "smithy_http[aiohttp]~=0.7.0"
+]
+
 awscrt = [
     "smithy_http[awscrt]~=0.7.0"
 ]
diff --git a/codegen-output/rpcv2-cbor/README.md b/codegen-output/rpcv2-cbor/README.md
index 7beeef19f..ad4cc3bfd 100644
--- a/codegen-output/rpcv2-cbor/README.md
+++ b/codegen-output/rpcv2-cbor/README.md
@@ -3,3 +3,23 @@
 ## RpcV2 Protocol Service Client
 
 Python client for RpcV2 Protocol Service
+
+### Installation
+
+Install the `sync` extra for the synchronous client's default HTTP transport, the `async` extra for the asynchronous client's default HTTP transport, or both. In a project managed by uv, add the client with the extras you need:
+
+```sh
+uv add "rpcv2cbor[sync]"
+uv add "rpcv2cbor[async]"
+uv add "rpcv2cbor[sync,async]"
+```
+
+Or install it with pip:
+
+```sh
+pip install "rpcv2cbor[sync]"
+pip install "rpcv2cbor[async]"
+pip install "rpcv2cbor[sync,async]"
+```
+
+Without either extra, the models can be used, and either client works with an explicitly configured custom transport.
diff --git a/codegen-output/rpcv2-cbor/pyproject.toml b/codegen-output/rpcv2-cbor/pyproject.toml
index 1c45fc0bd..81a0752fd 100644
--- a/codegen-output/rpcv2-cbor/pyproject.toml
+++ b/codegen-output/rpcv2-cbor/pyproject.toml
@@ -25,10 +25,18 @@ classifiers = [
 
 dependencies = [
     "smithy_core~=0.10.0",
-    "smithy_http[cbor, aiohttp, urllib3]~=0.7.0"
+    "smithy_http[cbor]~=0.7.0"
 ]
 
 [project.optional-dependencies]
+sync = [
+    "smithy_http[urllib3]~=0.7.0"
+]
+
+async = [
+    "smithy_http[aiohttp]~=0.7.0"
+]
+
 awscrt = [
     "smithy_http[awscrt]~=0.7.0"
 ]
diff --git a/codegen-output/rpcv2-json/README.md b/codegen-output/rpcv2-json/README.md
index 49b85983a..a098e3978 100644
--- a/codegen-output/rpcv2-json/README.md
+++ b/codegen-output/rpcv2-json/README.md
@@ -3,3 +3,23 @@
 ## RpcV2 JSON Protocol Service Client
 
 Python client for RpcV2 JSON Protocol Service
+
+### Installation
+
+Install the `sync` extra for the synchronous client's default HTTP transport, the `async` extra for the asynchronous client's default HTTP transport, or both. In a project managed by uv, add the client with the extras you need:
+
+```sh
+uv add "rpcv2json[sync]"
+uv add "rpcv2json[async]"
+uv add "rpcv2json[sync,async]"
+```
+
+Or install it with pip:
+
+```sh
+pip install "rpcv2json[sync]"
+pip install "rpcv2json[async]"
+pip install "rpcv2json[sync,async]"
+```
+
+Without either extra, the models can be used, and either client works with an explicitly configured custom transport.
diff --git a/codegen-output/rpcv2-json/pyproject.toml b/codegen-output/rpcv2-json/pyproject.toml
index 68835928e..005134974 100644
--- a/codegen-output/rpcv2-json/pyproject.toml
+++ b/codegen-output/rpcv2-json/pyproject.toml
@@ -25,10 +25,18 @@ classifiers = [
 
 dependencies = [
     "smithy_core~=0.10.0",
-    "smithy_http[json, aiohttp, urllib3]~=0.7.0"
+    "smithy_http[json]~=0.7.0"
 ]
 
 [project.optional-dependencies]
+sync = [
+    "smithy_http[urllib3]~=0.7.0"
+]
+
+async = [
+    "smithy_http[aiohttp]~=0.7.0"
+]
+
 awscrt = [
     "smithy_http[awscrt]~=0.7.0"
 ]
diff --git a/codegen/core/src/main/java/software/amazon/smithy/python/codegen/generators/SetupGenerator.java b/codegen/core/src/main/java/software/amazon/smithy/python/codegen/generators/SetupGenerator.java
index dda9482e2..c94f6b7d4 100644
--- a/codegen/core/src/main/java/software/amazon/smithy/python/codegen/generators/SetupGenerator.java
+++ b/codegen/core/src/main/java/software/amazon/smithy/python/codegen/generators/SetupGenerator.java
@@ -4,6 +4,7 @@
  */
 package software.amazon.smithy.python.codegen.generators;
 
+import java.util.ArrayList;
 import java.util.Collection;
 import java.util.Collections;
 import java.util.LinkedHashMap;
@@ -34,6 +35,13 @@
 @SmithyInternalApi
 public final class SetupGenerator {
 
+    // Client mode extras, each mapped to the smithy_http extra that provides that mode's default
+    // transport. These transports are opt-in rather than base requirements so that customers only
+    // install what the modes they use need.
+    private static final List<Map.Entry<String, String>> MODE_TRANSPORT_EXTRAS = List.of(
+            Map.entry("sync", "urllib3"),
+            Map.entry("async", "aiohttp"));
+
     private SetupGenerator() {}
 
     public static void generateSetup(
@@ -42,8 +50,10 @@ public static void generateSetup(
     ) {
         var dependencies = gatherDependencies(context.writerDelegator().getDependencies().stream());
         var description = resolveDescription(settings, context);
+        var extras = collectOptionalDependencies(
+                dependencies.getOrDefault(PythonDependency.Type.DEPENDENCY.getType(), Map.of()));
         writePyproject(settings, description, context.writerDelegator(), dependencies);
-        writeReadme(settings, description, context);
+        writeReadme(settings, description, context, extras);
     }
 
     /**
@@ -251,6 +261,23 @@ static Map<String, List<String>> collectOptionalDependencies(
             Map<String, SymbolDependency> dependencies
     ) {
         var extras = new LinkedHashMap<String, List<String>>();
+        var smithyHttp = dependencies.get(SmithyPythonDependency.SMITHY_HTTP.packageName());
+        if (smithyHttp == null) {
+            return extras;
+        }
+        var smithyHttpExtras = getOptionalDependencies(smithyHttp);
+
+        // Expose each requested default transport through the extra for the client mode that
+        // uses it (e.g. pip install <client>[sync]) instead of requiring it in the base package.
+        // Like awscrt below, this re-exports smithy_http's extra so the version constraint stays
+        // sourced from smithy-http. The base requirement drops these, see
+        // getRequiredOptionalDependencies.
+        for (var mode : MODE_TRANSPORT_EXTRAS) {
+            if (smithyHttpExtras.contains(mode.getValue())) {
+                extras.put(mode.getKey(),
+                        List.of("smithy_http[" + mode.getValue() + "]" + smithyHttp.getVersion()));
+            }
+        }
 
         // Let users opt into the CRT transport (AWSCRTHTTPClient) without guessing a compatible
         // awscrt version, by re-exporting smithy_http's awscrt extra. This keeps the version
@@ -258,19 +285,33 @@ static Map<String, List<String>> collectOptionalDependencies(
         // already requires awscrt as a hard dependency, where there is nothing to opt into. No
         // client defaults to CRT today, so the guard is currently always taken; it remains as a
         // safeguard should a client ever pull awscrt as a hard dependency again.
-        var smithyHttp = dependencies.get(SmithyPythonDependency.SMITHY_HTTP.packageName());
-        if (smithyHttp != null && !getOptionalDependencies(smithyHttp).contains("awscrt")) {
+        if (!smithyHttpExtras.contains("awscrt")) {
             extras.put("awscrt", List.of("smithy_http[awscrt]" + smithyHttp.getVersion()));
         }
 
         return extras;
     }
 
+    /**
+     * Get the optional dependencies to require along with a dependency, leaving out the default
+     * transports of base dependencies, which are exposed through client mode extras instead.
+     * Other dependency groups, such as test dependencies, keep their requested extras.
+     */
+    private static List<String> getRequiredOptionalDependencies(SymbolDependency dependency) {
+        var optional = getOptionalDependencies(dependency);
+        if (!dependency.getDependencyType().equals(PythonDependency.Type.DEPENDENCY.getType())
+                || !dependency.getPackageName().equals(SmithyPythonDependency.SMITHY_HTTP.packageName())) {
+            return optional;
+        }
+        var transports = MODE_TRANSPORT_EXTRAS.stream().map(Map.Entry::getValue).toList();
+        return optional.stream().filter(extra -> !transports.contains(extra)).toList();
+    }
+
     private static void writeDependencyList(PythonWriter writer, Collection<SymbolDependency> dependencies) {
         for (var iter = dependencies.iterator(); iter.hasNext();) {
             writer.pushState();
             var dependency = iter.next();
-            writer.putContext("deps", getOptionalDependencies(dependency));
+            writer.putContext("deps", getRequiredOptionalDependencies(dependency));
             writer.putContext("isLink", dependency.getProperty(SymbolProperties.IS_LINK).orElse(false));
             writer.putContext("last", !iter.hasNext());
             writer.write("""
@@ -301,7 +342,8 @@ private static List<String> getOptionalDependencies(SymbolDependency dependency)
     private static void writeReadme(
             PythonSettings settings,
             String description,
-            GenerationContext context
+            GenerationContext context,
+            Map<String, List<String>> extras
     ) {
         var service = context.model().expectShape(settings.service());
 
@@ -310,6 +352,11 @@ private static void writeReadme(
                 .map(StringTrait::getValue)
                 .orElse(StringUtils.capitalize(settings.moduleName()));
 
+        var modes = MODE_TRANSPORT_EXTRAS.stream()
+                .map(Map.Entry::getKey)
+                .filter(extras::containsKey)
+                .toList();
+
         context.writerDelegator().useFileWriter("README.md", writer -> {
             writer.pushState(new ReadmeSection());
             writer.write("""
@@ -318,6 +365,41 @@ private static void writeReadme(
                     $L
                     """, title, StringUtils.wrap(description, 80));
 
+            if (!modes.isEmpty()) {
+                var installs = new ArrayList<>(modes);
+                if (modes.size() > 1) {
+                    installs.add(String.join(",", modes));
+                }
+                writer.write("""
+                        ### Installation
+
+                        Install the `sync` extra for the synchronous client's default HTTP \
+                        transport, the `async` extra for the asynchronous client's default HTTP \
+                        transport, or both. In a project managed by uv, add the client with \
+                        the extras you need:
+
+                        ```sh
+                        $L
+                        ```
+
+                        Or install it with pip:
+
+                        ```sh
+                        $L
+                        ```
+
+                        Without either extra, the models can be used, and either client works \
+                        with an explicitly configured custom transport.
+                        """,
+                        installs.stream()
+                                .map(extra -> "uv add \"" + settings.moduleName() + "[" + extra + "]\"")
+                                .collect(Collectors.joining("\n")),
+                        installs.stream()
+                                .map(extra -> "pip install \"" + settings.moduleName() + "[" + extra
+                                        + "]\"")
+                                .collect(Collectors.joining("\n")));
+            }
+
             service.getTrait(DocumentationTrait.class).map(StringTrait::getValue).ifPresent(documentation -> {
                 writer.write("""
                         ### Documentation
diff --git a/codegen/core/src/test/java/software/amazon/smithy/python/codegen/generators/SetupGeneratorTest.java b/codegen/core/src/test/java/software/amazon/smithy/python/codegen/generators/SetupGeneratorTest.java
index 097191436..9c4c6800a 100644
--- a/codegen/core/src/test/java/software/amazon/smithy/python/codegen/generators/SetupGeneratorTest.java
+++ b/codegen/core/src/test/java/software/amazon/smithy/python/codegen/generators/SetupGeneratorTest.java
@@ -48,6 +48,55 @@ public void exposesAwscrtExtraForSmithyHttpDependency() {
                 extras);
     }
 
+    /**
+     * The sync (urllib3) and async (aiohttp) default transports are opt-in through {@code sync}
+     * and {@code async} mode extras rather than base requirements, while unrelated smithy_http
+     * extras (here the cbor protocol extra) stay in the base requirement. The README shows how to
+     * install them.
+     */
+    @Test
+    public void movesDefaultTransportsIntoModeExtras() {
+        var service = service("smithy.example#TestService", null);
+        var model = Model.builder().addShape(service).build();
+        var settings = PythonSettings.builder()
+                .service(service.getId())
+                .moduleName("test_client")
+                .moduleVersion("0.0.1")
+                .build();
+        var manifest = new MockManifest();
+        var delegator = new PythonDelegator(manifest, new PythonSymbolProvider(model, settings), settings);
+        var context = GenerationContext.builder()
+                .model(model)
+                .settings(settings)
+                .fileManifest(manifest)
+                .writerDelegator(delegator)
+                .build();
+        var smithyHttp = SmithyPythonDependency.SMITHY_HTTP;
+        delegator.useFileWriter("test_client/client.py", "test_client.client", writer -> {
+            writer.addDependency(smithyHttp.withOptionalDependencies("cbor"));
+            writer.addDependency(smithyHttp.withOptionalDependencies("aiohttp"));
+            writer.addDependency(smithyHttp.withOptionalDependencies("urllib3"));
+            writer.addDependency(smithyHttp.withOptionalDependencies("aiohttp", "urllib3").asTestDependency());
+        });
+
+        SetupGenerator.generateSetup(settings, context);
+        delegator.flushWriters();
+        String pyproject = manifest.expectFileString("pyproject.toml");
+        String readme = manifest.expectFileString("README.md");
+
+        var version = smithyHttp.version();
+        assertAll(
+                () -> assertTrue(readme.contains("uv add \"test_client[sync,async]\""), readme),
+                () -> assertTrue(readme.contains("pip install \"test_client[sync,async]\""), readme),
+                () -> assertTrue(pyproject.contains("\"smithy_http[cbor]" + version + "\"\n]"), pyproject),
+                () -> assertTrue(pyproject.contains("sync = [\n    \"smithy_http[urllib3]" + version + "\"\n]"),
+                        pyproject),
+                () -> assertTrue(pyproject.contains("async = [\n    \"smithy_http[aiohttp]" + version + "\"\n]"),
+                        pyproject),
+                () -> assertTrue(pyproject.contains("[dependency-groups]\ntest = [\n    \"smithy_http[aiohttp, urllib3]"
+                        + version + "\"\n]"), pyproject));
+    }
+
     @Test
     public void readmeKeepsRawTripleQuotesInCodeSpans() {
         // The README is Markdown, not a docstring, so the docstring quote escaping
diff --git a/designs/async-and-sync-duality.md b/designs/async-and-sync-duality.md
index ed825bd93..88bb0a96d 100644
--- a/designs/async-and-sync-duality.md
+++ b/designs/async-and-sync-duality.md
@@ -2,6 +2,30 @@
 
 Smithy-Python generated clients export two different classes that run in Python's synchronous and asynchronous (aio, async/await) operation modes respectively. For example, the `aws_sdk_s3` package will export both the `S3Client` (synchronous) and `AsyncS3Client` (asynchronous) classes. This allows the customer to operate one or both modes depending on application requirements.
 
+## Client mode extras
+
+Generated HTTP clients expose a `sync` and an `async` extra that install each mode's default HTTP transport (urllib3 for `sync`, aiohttp for `async`). In a project managed by uv:
+
+```sh
+uv add "aws_sdk_s3[sync]"
+uv add "aws_sdk_s3[async]"
+uv add "aws_sdk_s3[sync,async]"
+```
+
+Or with pip:
+
+```sh
+pip install "aws_sdk_s3[sync]"
+pip install "aws_sdk_s3[async]"
+pip install "aws_sdk_s3[sync,async]"
+```
+
+- A bare install, without either extra, does not install aiohttp or urllib3. It supports the shared models, and either client with an explicitly configured custom transport. Constructing a missing default transport fails with a `MissingDependencyError`.
+- Public import paths are unchanged.
+- Other extras keep their meaning: protocol requirements stay in the base package, and the `awscrt` extra still opts into the CRT transport.
+
+This describes the current single-distribution package. If the modes are ever split into separate distributions, how installs that use a custom transport migrate will be designed as part of that split.
+
 ## `make aio-check`
 
 `make aio-check` is a verification step that runs as part of CI's static analysis suite. It detects simple mechanical violations of sync/async duality, such as placing a class in the wrong location or with the wrong name.
diff --git a/packages/smithy-http/.changes/next-release/smithy-http-enhancement-3f9c2a7e5b1d4e08a6c4d2b9e7f1a053.json b/packages/smithy-http/.changes/next-release/smithy-http-enhancement-3f9c2a7e5b1d4e08a6c4d2b9e7f1a053.json
new file mode 100644
index 000000000..65a2307ac
--- /dev/null
+++ b/packages/smithy-http/.changes/next-release/smithy-http-enhancement-3f9c2a7e5b1d4e08a6c4d2b9e7f1a053.json
@@ -0,0 +1,4 @@
+{
+  "type": "enhancement",
+  "description": "The errors raised when aiohttp or urllib3 is missing now identify the matching smithy-http extra without prescribing a package manager."
+}
\ No newline at end of file
diff --git a/packages/smithy-http/src/smithy_http/aio/aiohttp.py b/packages/smithy-http/src/smithy_http/aio/aiohttp.py
index 078bb4d5b..f8b9b527e 100644
--- a/packages/smithy-http/src/smithy_http/aio/aiohttp.py
+++ b/packages/smithy-http/src/smithy_http/aio/aiohttp.py
@@ -213,7 +213,8 @@ def _upload_data(source: Any, body: AsyncBytesReader | None) -> Any:
 def _assert_aiohttp() -> None:
     if not HAS_AIOHTTP:
         raise MissingDependencyError(
-            "Attempted to use aiohttp component, but aiohttp is not installed."
+            "Attempted to use aiohttp component, but aiohttp is not installed. "
+            'Install the optional dependency "smithy-http[aiohttp]".'
         )
 
 
diff --git a/packages/smithy-http/src/smithy_http/urllib3.py b/packages/smithy-http/src/smithy_http/urllib3.py
index 189d28003..f78d1f0ea 100644
--- a/packages/smithy-http/src/smithy_http/urllib3.py
+++ b/packages/smithy-http/src/smithy_http/urllib3.py
@@ -262,5 +262,6 @@ def close(self) -> None:
 def _assert_urllib3() -> None:
     if not HAS_URLLIB3:
         raise MissingDependencyError(
-            "Attempted to use the urllib3 component, but urllib3 is not installed."
+            "Attempted to use the urllib3 component, but urllib3 is not installed. "
+            'Install the optional dependency "smithy-http[urllib3]".'
         )
diff --git a/packages/smithy-http/tests/unit/aio/test_aiohttp.py b/packages/smithy-http/tests/unit/aio/test_aiohttp.py
index 058407c9b..7d78d12a7 100644
--- a/packages/smithy-http/tests/unit/aio/test_aiohttp.py
+++ b/packages/smithy-http/tests/unit/aio/test_aiohttp.py
@@ -7,9 +7,11 @@
 from unittest.mock import AsyncMock, MagicMock, patch
 
 import pytest
+import smithy_http.aio.aiohttp
 from smithy_core import URI
 from smithy_core.aio.types import AsyncBytesReader
 from smithy_core.aio.utils import read_streaming_blob_async
+from smithy_core.exceptions import MissingDependencyError
 from smithy_http import Field, Fields, HTTPRequest
 from smithy_http.aio.aiohttp import AIOHTTPClient, _AIOHTTPStreamingBody
 from smithy_http.exceptions import SmithyHTTPError
@@ -36,6 +38,18 @@ def _create_request() -> HTTPRequest:
     )
 
 
+def test_missing_aiohttp_recommends_installing_the_extra(
+    monkeypatch: pytest.MonkeyPatch,
+) -> None:
+    monkeypatch.setattr(smithy_http.aio.aiohttp, "HAS_AIOHTTP", False)
+
+    with pytest.raises(MissingDependencyError) as error:
+        AIOHTTPClient()
+
+    message = str(error.value)
+    assert 'Install the optional dependency "smithy-http[aiohttp]".' in message
+
+
 async def test_close_closes_session() -> None:
     client, session = _create_client()
 
diff --git a/packages/smithy-http/tests/unit/test_urllib3.py b/packages/smithy-http/tests/unit/test_urllib3.py
index 04fb21d8d..04b61454d 100644
--- a/packages/smithy-http/tests/unit/test_urllib3.py
+++ b/packages/smithy-http/tests/unit/test_urllib3.py
@@ -9,9 +9,10 @@
 from unittest.mock import Mock
 
 import pytest
+import smithy_http.urllib3
 import urllib3
 from smithy_core import URI
-from smithy_core.exceptions import ClientTimeoutError
+from smithy_core.exceptions import ClientTimeoutError, MissingDependencyError
 from smithy_core.interfaces import is_bytes_reader
 from smithy_core.utils import read_streaming_blob
 from smithy_http import Field, Fields, HTTPRequest
@@ -75,6 +76,18 @@ def test_pool_keeps_the_configured_connections_per_host(
     client.close()
 
 
+def test_missing_urllib3_recommends_installing_the_extra(
+    monkeypatch: pytest.MonkeyPatch,
+) -> None:
+    monkeypatch.setattr(smithy_http.urllib3, "HAS_URLLIB3", False)
+
+    with pytest.raises(MissingDependencyError) as error:
+        Urllib3HTTPClient()
+
+    message = str(error.value)
+    assert 'Install the optional dependency "smithy-http[urllib3]".' in message
+
+
 def test_repeated_response_headers_are_merged_case_insensitively() -> None:
     pool = Mock()
     raw_headers = urllib3.HTTPHeaderDict()