diff --git a/CHANGELOG.md b/CHANGELOG.md
index 444292b..dd3f6be 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -7,6 +7,21 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ## [Unreleased]
 
+## [0.9.1] - 2025-11-19
+
+### Fixed
+- **IndieAuth client_id trailing slash**: Added automatic trailing slash normalization to SITE_URL
+  - IndieLogin.com spec requires client_id URLs to have trailing slash for root domains
+  - Fixes "client_id is not registered" authentication errors
+  - Normalizes https://example.com to https://example.com/
+- **Enhanced debug logging**: Added detailed httpx request/response logging for token exchange
+  - Shows exact HTTP method, URL, headers, and body being sent to IndieLogin.com
+  - Helps troubleshoot authentication issues with full visibility
+  - All sensitive data (tokens, verifiers) automatically redacted
+
+### Changed
+- SITE_URL configuration now automatically adds trailing slash if missing
+
 ## [0.9.0] - 2025-11-19
 
 ### Added
diff --git a/starpunk/__init__.py b/starpunk/__init__.py
index 31388c3..c07e6e3 100644
--- a/starpunk/__init__.py
+++ b/starpunk/__init__.py
@@ -153,5 +153,5 @@ def create_app(config=None):
 
 # Package version (Semantic Versioning 2.0.0)
 # See docs/standards/versioning-strategy.md for details
-__version__ = "0.9.0"
-__version_info__ = (0, 9, 0)
+__version__ = "0.9.1"
+__version_info__ = (0, 9, 1)
diff --git a/starpunk/auth.py b/starpunk/auth.py
index ea02bbf..78f91c0 100644
--- a/starpunk/auth.py
+++ b/starpunk/auth.py
@@ -322,7 +322,7 @@ def initiate_login(me_url: str) -> str:
     # Store state and verifier in database (5-minute expiry)
     db = get_db(current_app)
     expires_at = datetime.utcnow() + timedelta(minutes=5)
-    redirect_uri = f"{current_app.config['SITE_URL']}/auth/callback"
+    redirect_uri = f"{current_app.config['SITE_URL']}auth/callback"
 
     db.execute(
         """
@@ -404,26 +404,52 @@ def handle_callback(code: str, state: str, iss: Optional[str] = None) -> Optiona
     token_exchange_data = {
         "code": code,
         "client_id": current_app.config["SITE_URL"],
-        "redirect_uri": f"{current_app.config['SITE_URL']}/auth/callback",
+        "redirect_uri": f"{current_app.config['SITE_URL']}auth/callback",
         "code_verifier": code_verifier,  # PKCE verification
     }
 
+    token_url = f"{current_app.config['INDIELOGIN_URL']}/token"
+
     # Log the request (code_verifier will be redacted)
     _log_http_request(
         method="POST",
-        url=f"{current_app.config['INDIELOGIN_URL']}/token",
+        url=token_url,
         data=token_exchange_data,
     )
 
+    # Log detailed httpx request info for debugging
+    current_app.logger.debug(
+        "Auth: Sending token exchange request:\n"
+        "  Method: POST\n"
+        "  URL: %s\n"
+        "  Data: code=%s, client_id=%s, redirect_uri=%s, code_verifier=%s",
+        token_url,
+        _redact_token(code),
+        token_exchange_data["client_id"],
+        token_exchange_data["redirect_uri"],
+        _redact_token(code_verifier),
+    )
+
     # Exchange code for identity (CORRECT ENDPOINT: /token)
     try:
         response = httpx.post(
-            f"{current_app.config['INDIELOGIN_URL']}/token",
+            token_url,
             data=token_exchange_data,
             timeout=10.0,
         )
 
-        # Log the response
+        # Log detailed httpx response info for debugging
+        current_app.logger.debug(
+            "Auth: Received token exchange response:\n"
+            "  Status: %d\n"
+            "  Headers: %s\n"
+            "  Body: %s",
+            response.status_code,
+            {k: v for k, v in dict(response.headers).items() if k.lower() not in ["set-cookie", "authorization"]},
+            _redact_token(response.text) if response.text else "(empty)",
+        )
+
+        # Log the response (legacy helper)
         _log_http_response(
             status_code=response.status_code,
             headers=dict(response.headers),
diff --git a/starpunk/config.py b/starpunk/config.py
index cd16a2e..a8293df 100644
--- a/starpunk/config.py
+++ b/starpunk/config.py
@@ -20,7 +20,10 @@ def load_config(app, config_override=None):
     load_dotenv()
 
     # Site configuration
-    app.config["SITE_URL"] = os.getenv("SITE_URL", "http://localhost:5000")
+    # IndieWeb/OAuth specs require trailing slash for root URLs used as client_id
+    # See: https://indielogin.com/ OAuth client requirements
+    site_url = os.getenv("SITE_URL", "http://localhost:5000")
+    app.config["SITE_URL"] = site_url if site_url.endswith('/') else site_url + '/'
     app.config["SITE_NAME"] = os.getenv("SITE_NAME", "StarPunk")
     app.config["SITE_AUTHOR"] = os.getenv("SITE_AUTHOR", "Unknown")
     app.config["SITE_DESCRIPTION"] = os.getenv(
@@ -73,6 +76,11 @@ def load_config(app, config_override=None):
     if config_override:
         app.config.update(config_override)
 
+    # Normalize SITE_URL trailing slash (in case override provided URL without slash)
+    if "SITE_URL" in app.config:
+        site_url = app.config["SITE_URL"]
+        app.config["SITE_URL"] = site_url if site_url.endswith('/') else site_url + '/'
+
     # Convert path strings to Path objects (in case overrides provided strings)
     if isinstance(app.config["DATA_PATH"], str):
         app.config["DATA_PATH"] = Path(app.config["DATA_PATH"])