fix: resolve InvalidSignatureException from mismatched list header formatting during signing

Author: jonathan343

packages/aws-sdk-signers/src/aws_sdk_signers/interfaces/http.py

@@ -57,7 +57,7 @@ def remove(self, value: str) -> None:
         """Remove all matching entries from list."""
         ...
 
-    def as_string(self, delimiter: str = ", ") -> str:
+    def as_string(self, delimiter: str = ",") -> str:
         """Serialize the ``Field``'s values into a single line string."""
         ...
 

packages/smithy-http/.changes/next-release/smithy-http-bugfix-20251023131033.json

@@ -0,0 +1,4 @@
+{
+  "type": "bugfix",
+  "description": "Fix `InvalidSignatureException` caused by mismatched list header formatting during signing (e.g., `MyHeader: value1, value2` vs `MyHeader: value1,value2`)."
+}

packages/smithy-http/src/smithy_http/__init__.py

@@ -63,7 +63,7 @@ def as_string(self, delimiter: str = ",") -> str:
             return ""
         if value_count == 1:
             return self.values[0]
-        return ", ".join(quote_and_escape_field_value(val) for val in self.values)
+        return delimiter.join(quote_and_escape_field_value(val) for val in self.values)
 
     def as_tuples(self) -> list[tuple[str, str]]:
         """Get list of ``name``, ``value`` tuples where each tuple represents one