Add instructions to setup API key

.github/workflows/deploy_dev_docs.yaml

@@ -40,7 +40,7 @@ jobs:
       group: docs-deployment
       cancel-in-progress: false
     steps:
-      - uses: actions/checkout@de0fac2e4500dabe0009e67214ff5f5447ce83dd # v6
+      - uses: actions/checkout@8e8c483db84b4bee98b60c0593521ed34d9990e8 # v6
         with:
           fetch-depth: 0 # Fetch all history for .GitInfo and .Lastmod
 
@@ -56,7 +56,7 @@ jobs:
           node-version: "22"
 
       - name: Cache dependencies
-        uses: actions/cache@cdf6c1fa76f9f475f3d7449005a359c84ca0f306 # v5
+        uses: actions/cache@8b402f58fbc84540c8b491a91e594a4576fec3d7 # v5
         with:
           path: ~/.npm
           key: ${{ runner.os }}-node-${{ hashFiles('**/package-lock.json') }}

.github/workflows/deploy_previous_version_docs.yaml

@@ -30,14 +30,14 @@ jobs:
 
     steps:
       - name: Checkout main branch (for latest templates and theme)
-        uses: actions/checkout@de0fac2e4500dabe0009e67214ff5f5447ce83dd # v6
+        uses: actions/checkout@8e8c483db84b4bee98b60c0593521ed34d9990e8 # v6
         with:
           ref: 'main'
           submodules: 'recursive'
           fetch-depth: 0
 
       - name: Checkout old content from tag into a temporary directory
-        uses: actions/checkout@de0fac2e4500dabe0009e67214ff5f5447ce83dd # v6
+        uses: actions/checkout@8e8c483db84b4bee98b60c0593521ed34d9990e8 # v6
         with:
           ref: ${{ github.event.inputs.version_tag }}
           path: 'old_version_source' # Checkout into a temp subdir

.github/workflows/deploy_versioned_docs.yaml

@@ -30,7 +30,7 @@ jobs:
       cancel-in-progress: false
     steps:
       - name: Checkout Code at Tag
-        uses: actions/checkout@de0fac2e4500dabe0009e67214ff5f5447ce83dd # v6
+        uses: actions/checkout@8e8c483db84b4bee98b60c0593521ed34d9990e8 # v6
         with:
           ref: ${{ github.event.release.tag_name }}
 

.github/workflows/docs_preview_clean.yaml

@@ -34,7 +34,7 @@ jobs:
       group: "preview-${{ github.event.number }}"
       cancel-in-progress: true
     steps:
-      - uses: actions/checkout@de0fac2e4500dabe0009e67214ff5f5447ce83dd # v6
+      - uses: actions/checkout@8e8c483db84b4bee98b60c0593521ed34d9990e8 # v6
         with:
           ref: versioned-gh-pages
 

.github/workflows/docs_preview_deploy.yaml

@@ -49,7 +49,7 @@ jobs:
       group: "preview-${{ github.event.number }}"
       cancel-in-progress: true
     steps:
-      - uses: actions/checkout@de0fac2e4500dabe0009e67214ff5f5447ce83dd # v6
+      - uses: actions/checkout@8e8c483db84b4bee98b60c0593521ed34d9990e8 # v6
         with:
           # Checkout the PR's HEAD commit (supports forks).
           ref: ${{ github.event.pull_request.head.sha }}
@@ -67,7 +67,7 @@ jobs:
           node-version: "22"
 
       - name: Cache dependencies
-        uses: actions/cache@cdf6c1fa76f9f475f3d7449005a359c84ca0f306 # v5
+        uses: actions/cache@8b402f58fbc84540c8b491a91e594a4576fec3d7 # v5
         with:
           path: ~/.npm
           key: ${{ runner.os }}-node-${{ hashFiles('**/package-lock.json') }}

.github/workflows/link_checker_workflow.yaml

@@ -22,10 +22,10 @@ jobs:
     runs-on: ubuntu-latest
     steps:
       - name: Checkout Repository
-        uses: actions/checkout@de0fac2e4500dabe0009e67214ff5f5447ce83dd # v6
+        uses: actions/checkout@8e8c483db84b4bee98b60c0593521ed34d9990e8 # v6
 
       - name: Restore lychee cache
-        uses: actions/cache@cdf6c1fa76f9f475f3d7449005a359c84ca0f306 # v5
+        uses: actions/cache@8b402f58fbc84540c8b491a91e594a4576fec3d7 # v5
         with:
           path: .lycheecache
           key: cache-lychee-${{ github.sha }}

.github/workflows/publish-mcp.yml

@@ -29,7 +29,7 @@ jobs:
 
     steps:
       - name: Checkout code
-        uses: actions/checkout@de0fac2e4500dabe0009e67214ff5f5447ce83dd # v6
+        uses: actions/checkout@8e8c483db84b4bee98b60c0593521ed34d9990e8 # v6
 
       - name: Wait for image in Artifact Registry
         shell: bash

docs/en/getting-started/local_quickstart_go.md

@@ -46,6 +46,11 @@ from Toolbox.
     go get github.com/googleapis/mcp-toolbox-sdk-go
     ```
 
+1. Set your API key (If you don't have an API key yet, you can get one from [Google AI Studio](https://aistudio.google.com/app/apikey).)
+   ```bash
+   export GOOGLE_API_KEY="YOUR_ACTUAL_GEMINI_API_KEY"
+   ```
+
 1. Create a new file named `hotelagent.go` and copy the following code to create
    an agent:
 

internal/server/api.go

@@ -19,6 +19,7 @@ import (
 	"errors"
 	"fmt"
 	"net/http"
+	"strings"
 
 	"github.com/go-chi/chi/v5"
 	"github.com/go-chi/chi/v5/middleware"
@@ -233,11 +234,10 @@ func toolInvokeHandler(s *Server, w http.ResponseWriter, r *http.Request) {
 
 	params, err := parameters.ParseParams(tool.GetParameters(), data, claimsFromAuth)
 	if err != nil {
-		// If auth error, return 401 or 403
-		var clientServerErr *util.ClientServerError
-		if errors.As(err, &clientServerErr) && (clientServerErr.Code == http.StatusUnauthorized || clientServerErr.Code == http.StatusForbidden) {
+		// If auth error, return 401
+		if errors.Is(err, util.ErrUnauthorized) {
 			s.logger.DebugContext(ctx, fmt.Sprintf("error parsing authenticated parameters from ID token: %s", err))
-			_ = render.Render(w, r, newErrResponse(err, clientServerErr.Code))
+			_ = render.Render(w, r, newErrResponse(err, http.StatusUnauthorized))
 			return
 		}
 		err = fmt.Errorf("provided parameters were invalid: %w", err)
@@ -259,49 +259,34 @@ func toolInvokeHandler(s *Server, w http.ResponseWriter, r *http.Request) {
 
 	// Determine what error to return to the users.
 	if err != nil {
-		var tbErr util.ToolboxError
+		errStr := err.Error()
+		var statusCode int
 
-		if errors.As(err, &tbErr) {
-			switch tbErr.Category() {
-			case util.CategoryAgent:
-				// Agent Errors -> 200 OK
-				s.logger.DebugContext(ctx, fmt.Sprintf("Tool invocation agent error: %v", err))
-				_ = render.Render(w, r, newErrResponse(err, http.StatusOK))
-				return
+		// Upstream API auth error propagation
+		switch {
+		case strings.Contains(errStr, "Error 401"):
+			statusCode = http.StatusUnauthorized
+		case strings.Contains(errStr, "Error 403"):
+			statusCode = http.StatusForbidden
+		}
 
-			case util.CategoryServer:
-				// Server Errors -> Check the specific code inside
-				var clientServerErr *util.ClientServerError
-				statusCode := http.StatusInternalServerError // Default to 500
-
-				if errors.As(err, &clientServerErr) {
-					if clientServerErr.Code != 0 {
-						statusCode = clientServerErr.Code
-					}
-				}
-
-				// Process auth error
-				if statusCode == http.StatusUnauthorized || statusCode == http.StatusForbidden {
-					if clientAuth {
-						// Token error, pass through 401/403
-						s.logger.DebugContext(ctx, fmt.Sprintf("Client credentials lack authorization: %v", err))
-						_ = render.Render(w, r, newErrResponse(err, statusCode))
-						return
-					}
-					// ADC/Config error, return 500
-					statusCode = http.StatusInternalServerError
-				}
-
-				s.logger.ErrorContext(ctx, fmt.Sprintf("Tool invocation server error: %v", err))
+		if statusCode == http.StatusUnauthorized || statusCode == http.StatusForbidden {
+			if clientAuth {
+				// Propagate the original 401/403 error.
+				s.logger.DebugContext(ctx, fmt.Sprintf("error invoking tool. Client credentials lack authorization to the source: %v", err))
 				_ = render.Render(w, r, newErrResponse(err, statusCode))
 				return
 			}
-		} else {
-			// Unknown error -> 500
-			s.logger.ErrorContext(ctx, fmt.Sprintf("Tool invocation unknown error: %v", err))
-			_ = render.Render(w, r, newErrResponse(err, http.StatusInternalServerError))
+			// ADC lacking permission or credentials configuration error.
+			internalErr := fmt.Errorf("unexpected auth error occured during Tool invocation: %w", err)
+			s.logger.ErrorContext(ctx, internalErr.Error())
+			_ = render.Render(w, r, newErrResponse(internalErr, http.StatusInternalServerError))
 			return
 		}
+		err = fmt.Errorf("error while invoking tool: %w", err)
+		s.logger.DebugContext(ctx, err.Error())
+		_ = render.Render(w, r, newErrResponse(err, http.StatusBadRequest))
+		return
 	}
 
 	resMarshal, err := json.Marshal(res)

internal/server/mcp.go

@@ -23,6 +23,7 @@ import (
 	"fmt"
 	"io"
 	"net/http"
+	"strings"
 	"sync"
 	"time"
 
@@ -443,12 +444,15 @@ func httpHandler(s *Server, w http.ResponseWriter, r *http.Request) {
 		code := rpcResponse.Error.Code
 		switch code {
 		case jsonrpc.INTERNAL_ERROR:
-			// Map Internal RPC Error (-32603) to HTTP 500
 			w.WriteHeader(http.StatusInternalServerError)
 		case jsonrpc.INVALID_REQUEST:
-			var clientServerErr *util.ClientServerError
-			if errors.As(err, &clientServerErr) {
-				w.WriteHeader(clientServerErr.Code)
+			errStr := err.Error()
+			if errors.Is(err, util.ErrUnauthorized) {
+				w.WriteHeader(http.StatusUnauthorized)
+			} else if strings.Contains(errStr, "Error 401") {
+				w.WriteHeader(http.StatusUnauthorized)
+			} else if strings.Contains(errStr, "Error 403") {
+				w.WriteHeader(http.StatusForbidden)
 			}
 		}
 	}

internal/server/mcp/v20241105/method.go

@@ -21,6 +21,7 @@ import (
 	"errors"
 	"fmt"
 	"net/http"
+	"strings"
 
 	"github.com/googleapis/genai-toolbox/internal/prompts"
 	"github.com/googleapis/genai-toolbox/internal/server/mcp/jsonrpc"
@@ -123,12 +124,7 @@ func toolsCallHandler(ctx context.Context, id jsonrpc.RequestId, resourceMgr *re
 	}
 	if clientAuth {
 		if accessToken == "" {
-			errMsg := "missing access token in the 'Authorization' header"
-			return jsonrpc.NewError(id, jsonrpc.INVALID_REQUEST, errMsg, nil), util.NewClientServerError(
-				errMsg,
-				http.StatusUnauthorized,
-				nil,
-			)
+			return jsonrpc.NewError(id, jsonrpc.INVALID_REQUEST, "missing access token in the 'Authorization' header", nil), util.ErrUnauthorized
 		}
 	}
 
@@ -176,11 +172,7 @@ func toolsCallHandler(ctx context.Context, id jsonrpc.RequestId, resourceMgr *re
 	// Check if any of the specified auth services is verified
 	isAuthorized := tool.Authorized(verifiedAuthServices)
 	if !isAuthorized {
-		err = util.NewClientServerError(
-			"unauthorized Tool call: Please make sure you specify correct auth headers",
-			http.StatusUnauthorized,
-			nil,
-		)
+		err = fmt.Errorf("unauthorized Tool call: Please make sure your specify correct auth headers: %w", util.ErrUnauthorized)
 		return jsonrpc.NewError(id, jsonrpc.INVALID_REQUEST, err.Error(), nil), err
 	}
 	logger.DebugContext(ctx, "tool invocation authorized")
@@ -202,44 +194,30 @@ func toolsCallHandler(ctx context.Context, id jsonrpc.RequestId, resourceMgr *re
 	// run tool invocation and generate response.
 	results, err := tool.Invoke(ctx, resourceMgr, params, accessToken)
 	if err != nil {
-		var tbErr util.ToolboxError
-
-		if errors.As(err, &tbErr) {
-			switch tbErr.Category() {
-			case util.CategoryAgent:
-				// MCP - Tool execution error
-				// Return SUCCESS but with IsError: true
-				text := TextContent{
-					Type: "text",
-					Text: err.Error(),
-				}
-				return jsonrpc.JSONRPCResponse{
-					Jsonrpc: jsonrpc.JSONRPC_VERSION,
-					Id:      id,
-					Result:  CallToolResult{Content: []TextContent{text}, IsError: true},
-				}, nil
-
-			case util.CategoryServer:
-				// MCP Spec - Protocol error
-				// Return JSON-RPC ERROR
-				var clientServerErr *util.ClientServerError
-				rpcCode := jsonrpc.INTERNAL_ERROR // Default to Internal Error (-32603)
-
-				if errors.As(err, &clientServerErr) {
-					if clientServerErr.Code == http.StatusUnauthorized || clientServerErr.Code == http.StatusForbidden {
-						if clientAuth {
-							rpcCode = jsonrpc.INVALID_REQUEST
-						} else {
-							rpcCode = jsonrpc.INTERNAL_ERROR
-						}
-					}
-				}
-				return jsonrpc.NewError(id, rpcCode, err.Error(), nil), err
+		errStr := err.Error()
+		// Missing authService tokens.
+		if errors.Is(err, util.ErrUnauthorized) {
+			return jsonrpc.NewError(id, jsonrpc.INVALID_REQUEST, err.Error(), nil), err
+		}
+		// Upstream auth error
+		if strings.Contains(errStr, "Error 401") || strings.Contains(errStr, "Error 403") {
+			if clientAuth {
+				// Error with client credentials should pass down to the client
+				return jsonrpc.NewError(id, jsonrpc.INVALID_REQUEST, err.Error(), nil), err
 			}
-		} else {
-			// Unknown error -> 500
+			// Auth error with ADC should raise internal 500 error
 			return jsonrpc.NewError(id, jsonrpc.INTERNAL_ERROR, err.Error(), nil), err
 		}
+
+		text := TextContent{
+			Type: "text",
+			Text: err.Error(),
+		}
+		return jsonrpc.JSONRPCResponse{
+			Jsonrpc: jsonrpc.JSONRPC_VERSION,
+			Id:      id,
+			Result:  CallToolResult{Content: []TextContent{text}, IsError: true},
+		}, nil
 	}
 
 	content := make([]TextContent, 0)

internal/server/mcp/v20250326/method.go

@@ -21,6 +21,7 @@ import (
 	"errors"
 	"fmt"
 	"net/http"
+	"strings"
 
 	"github.com/googleapis/genai-toolbox/internal/prompts"
 	"github.com/googleapis/genai-toolbox/internal/server/mcp/jsonrpc"
@@ -123,12 +124,7 @@ func toolsCallHandler(ctx context.Context, id jsonrpc.RequestId, resourceMgr *re
 	}
 	if clientAuth {
 		if accessToken == "" {
-			errMsg := "missing access token in the 'Authorization' header"
-			return jsonrpc.NewError(id, jsonrpc.INVALID_REQUEST, errMsg, nil), util.NewClientServerError(
-				errMsg,
-				http.StatusUnauthorized,
-				nil,
-			)
+			return jsonrpc.NewError(id, jsonrpc.INVALID_REQUEST, "missing access token in the 'Authorization' header", nil), util.ErrUnauthorized
 		}
 	}
 
@@ -176,11 +172,7 @@ func toolsCallHandler(ctx context.Context, id jsonrpc.RequestId, resourceMgr *re
 	// Check if any of the specified auth services is verified
 	isAuthorized := tool.Authorized(verifiedAuthServices)
 	if !isAuthorized {
-		err = util.NewClientServerError(
-			"unauthorized Tool call: Please make sure you specify correct auth headers",
-			http.StatusUnauthorized,
-			nil,
-		)
+		err = fmt.Errorf("unauthorized Tool call: Please make sure your specify correct auth headers: %w", util.ErrUnauthorized)
 		return jsonrpc.NewError(id, jsonrpc.INVALID_REQUEST, err.Error(), nil), err
 	}
 	logger.DebugContext(ctx, "tool invocation authorized")
@@ -202,45 +194,31 @@ func toolsCallHandler(ctx context.Context, id jsonrpc.RequestId, resourceMgr *re
 	// run tool invocation and generate response.
 	results, err := tool.Invoke(ctx, resourceMgr, params, accessToken)
 	if err != nil {
-		var tbErr util.ToolboxError
-
-		if errors.As(err, &tbErr) {
-			switch tbErr.Category() {
-			case util.CategoryAgent:
-				// MCP - Tool execution error
-				// Return SUCCESS but with IsError: true
-				text := TextContent{
-					Type: "text",
-					Text: err.Error(),
-				}
-				return jsonrpc.JSONRPCResponse{
-					Jsonrpc: jsonrpc.JSONRPC_VERSION,
-					Id:      id,
-					Result:  CallToolResult{Content: []TextContent{text}, IsError: true},
-				}, nil
-
-			case util.CategoryServer:
-				// MCP Spec - Protocol error
-				// Return JSON-RPC ERROR
-				var clientServerErr *util.ClientServerError
-				rpcCode := jsonrpc.INTERNAL_ERROR // Default to Internal Error (-32603)
-
-				if errors.As(err, &clientServerErr) {
-					if clientServerErr.Code == http.StatusUnauthorized || clientServerErr.Code == http.StatusForbidden {
-						if clientAuth {
-							rpcCode = jsonrpc.INVALID_REQUEST
-						} else {
-							rpcCode = jsonrpc.INTERNAL_ERROR
-						}
-					}
-				}
-				return jsonrpc.NewError(id, rpcCode, err.Error(), nil), err
+		errStr := err.Error()
+		// Missing authService tokens.
+		if errors.Is(err, util.ErrUnauthorized) {
+			return jsonrpc.NewError(id, jsonrpc.INVALID_REQUEST, err.Error(), nil), err
+		}
+		// Upstream auth error
+		if strings.Contains(errStr, "Error 401") || strings.Contains(errStr, "Error 403") {
+			if clientAuth {
+				// Error with client credentials should pass down to the client
+				return jsonrpc.NewError(id, jsonrpc.INVALID_REQUEST, err.Error(), nil), err
 			}
-		} else {
-			// Unknown error -> 500
+			// Auth error with ADC should raise internal 500 error
 			return jsonrpc.NewError(id, jsonrpc.INTERNAL_ERROR, err.Error(), nil), err
 		}
+		text := TextContent{
+			Type: "text",
+			Text: err.Error(),
+		}
+		return jsonrpc.JSONRPCResponse{
+			Jsonrpc: jsonrpc.JSONRPC_VERSION,
+			Id:      id,
+			Result:  CallToolResult{Content: []TextContent{text}, IsError: true},
+		}, nil
 	}
+
 	content := make([]TextContent, 0)
 
 	sliceRes, ok := results.([]any)

internal/server/mcp/v20250618/method.go

@@ -21,6 +21,7 @@ import (
 	"errors"
 	"fmt"
 	"net/http"
+	"strings"
 
 	"github.com/googleapis/genai-toolbox/internal/prompts"
 	"github.com/googleapis/genai-toolbox/internal/server/mcp/jsonrpc"
@@ -116,12 +117,7 @@ func toolsCallHandler(ctx context.Context, id jsonrpc.RequestId, resourceMgr *re
 	}
 	if clientAuth {
 		if accessToken == "" {
-			errMsg := "missing access token in the 'Authorization' header"
-			return jsonrpc.NewError(id, jsonrpc.INVALID_REQUEST, errMsg, nil), util.NewClientServerError(
-				errMsg,
-				http.StatusUnauthorized,
-				nil,
-			)
+			return jsonrpc.NewError(id, jsonrpc.INVALID_REQUEST, "missing access token in the 'Authorization' header", nil), util.ErrUnauthorized
 		}
 	}
 
@@ -169,11 +165,7 @@ func toolsCallHandler(ctx context.Context, id jsonrpc.RequestId, resourceMgr *re
 	// Check if any of the specified auth services is verified
 	isAuthorized := tool.Authorized(verifiedAuthServices)
 	if !isAuthorized {
-		err = util.NewClientServerError(
-			"unauthorized Tool call: Please make sure you specify correct auth headers",
-			http.StatusUnauthorized,
-			nil,
-		)
+		err = fmt.Errorf("unauthorized Tool call: Please make sure your specify correct auth headers: %w", util.ErrUnauthorized)
 		return jsonrpc.NewError(id, jsonrpc.INVALID_REQUEST, err.Error(), nil), err
 	}
 	logger.DebugContext(ctx, "tool invocation authorized")
@@ -195,44 +187,29 @@ func toolsCallHandler(ctx context.Context, id jsonrpc.RequestId, resourceMgr *re
 	// run tool invocation and generate response.
 	results, err := tool.Invoke(ctx, resourceMgr, params, accessToken)
 	if err != nil {
-		var tbErr util.ToolboxError
-
-		if errors.As(err, &tbErr) {
-			switch tbErr.Category() {
-			case util.CategoryAgent:
-				// MCP - Tool execution error
-				// Return SUCCESS but with IsError: true
-				text := TextContent{
-					Type: "text",
-					Text: err.Error(),
-				}
-				return jsonrpc.JSONRPCResponse{
-					Jsonrpc: jsonrpc.JSONRPC_VERSION,
-					Id:      id,
-					Result:  CallToolResult{Content: []TextContent{text}, IsError: true},
-				}, nil
-
-			case util.CategoryServer:
-				// MCP Spec - Protocol error
-				// Return JSON-RPC ERROR
-				var clientServerErr *util.ClientServerError
-				rpcCode := jsonrpc.INTERNAL_ERROR // Default to Internal Error (-32603)
-
-				if errors.As(err, &clientServerErr) {
-					if clientServerErr.Code == http.StatusUnauthorized || clientServerErr.Code == http.StatusForbidden {
-						if clientAuth {
-							rpcCode = jsonrpc.INVALID_REQUEST
-						} else {
-							rpcCode = jsonrpc.INTERNAL_ERROR
-						}
-					}
-				}
-				return jsonrpc.NewError(id, rpcCode, err.Error(), nil), err
+		errStr := err.Error()
+		// Missing authService tokens.
+		if errors.Is(err, util.ErrUnauthorized) {
+			return jsonrpc.NewError(id, jsonrpc.INVALID_REQUEST, err.Error(), nil), err
+		}
+		// Upstream auth error
+		if strings.Contains(errStr, "Error 401") || strings.Contains(errStr, "Error 403") {
+			if clientAuth {
+				// Error with client credentials should pass down to the client
+				return jsonrpc.NewError(id, jsonrpc.INVALID_REQUEST, err.Error(), nil), err
 			}
-		} else {
-			// Unknown error -> 500
+			// Auth error with ADC should raise internal 500 error
 			return jsonrpc.NewError(id, jsonrpc.INTERNAL_ERROR, err.Error(), nil), err
 		}
+		text := TextContent{
+			Type: "text",
+			Text: err.Error(),
+		}
+		return jsonrpc.JSONRPCResponse{
+			Jsonrpc: jsonrpc.JSONRPC_VERSION,
+			Id:      id,
+			Result:  CallToolResult{Content: []TextContent{text}, IsError: true},
+		}, nil
 	}
 
 	content := make([]TextContent, 0)

internal/server/mcp/v20251125/method.go

@@ -21,6 +21,7 @@ import (
 	"errors"
 	"fmt"
 	"net/http"
+	"strings"
 
 	"github.com/googleapis/genai-toolbox/internal/prompts"
 	"github.com/googleapis/genai-toolbox/internal/server/mcp/jsonrpc"
@@ -116,12 +117,7 @@ func toolsCallHandler(ctx context.Context, id jsonrpc.RequestId, resourceMgr *re
 	}
 	if clientAuth {
 		if accessToken == "" {
-			errMsg := "missing access token in the 'Authorization' header"
-			return jsonrpc.NewError(id, jsonrpc.INVALID_REQUEST, errMsg, nil), util.NewClientServerError(
-				errMsg,
-				http.StatusUnauthorized,
-				nil,
-			)
+			return jsonrpc.NewError(id, jsonrpc.INVALID_REQUEST, "missing access token in the 'Authorization' header", nil), util.ErrUnauthorized
 		}
 	}
 
@@ -169,11 +165,7 @@ func toolsCallHandler(ctx context.Context, id jsonrpc.RequestId, resourceMgr *re
 	// Check if any of the specified auth services is verified
 	isAuthorized := tool.Authorized(verifiedAuthServices)
 	if !isAuthorized {
-		err = util.NewClientServerError(
-			"unauthorized Tool call: Please make sure you specify correct auth headers",
-			http.StatusUnauthorized,
-			nil,
-		)
+		err = fmt.Errorf("unauthorized Tool call: Please make sure your specify correct auth headers: %w", util.ErrUnauthorized)
 		return jsonrpc.NewError(id, jsonrpc.INVALID_REQUEST, err.Error(), nil), err
 	}
 	logger.DebugContext(ctx, "tool invocation authorized")
@@ -195,44 +187,29 @@ func toolsCallHandler(ctx context.Context, id jsonrpc.RequestId, resourceMgr *re
 	// run tool invocation and generate response.
 	results, err := tool.Invoke(ctx, resourceMgr, params, accessToken)
 	if err != nil {
-		var tbErr util.ToolboxError
-
-		if errors.As(err, &tbErr) {
-			switch tbErr.Category() {
-			case util.CategoryAgent:
-				// MCP - Tool execution error
-				// Return SUCCESS but with IsError: true
-				text := TextContent{
-					Type: "text",
-					Text: err.Error(),
-				}
-				return jsonrpc.JSONRPCResponse{
-					Jsonrpc: jsonrpc.JSONRPC_VERSION,
-					Id:      id,
-					Result:  CallToolResult{Content: []TextContent{text}, IsError: true},
-				}, nil
-
-			case util.CategoryServer:
-				// MCP Spec - Protocol error
-				// Return JSON-RPC ERROR
-				var clientServerErr *util.ClientServerError
-				rpcCode := jsonrpc.INTERNAL_ERROR // Default to Internal Error (-32603)
-
-				if errors.As(err, &clientServerErr) {
-					if clientServerErr.Code == http.StatusUnauthorized || clientServerErr.Code == http.StatusForbidden {
-						if clientAuth {
-							rpcCode = jsonrpc.INVALID_REQUEST
-						} else {
-							rpcCode = jsonrpc.INTERNAL_ERROR
-						}
-					}
-				}
-				return jsonrpc.NewError(id, rpcCode, err.Error(), nil), err
+		errStr := err.Error()
+		// Missing authService tokens.
+		if errors.Is(err, util.ErrUnauthorized) {
+			return jsonrpc.NewError(id, jsonrpc.INVALID_REQUEST, err.Error(), nil), err
+		}
+		// Upstream auth error
+		if strings.Contains(errStr, "Error 401") || strings.Contains(errStr, "Error 403") {
+			if clientAuth {
+				// Error with client credentials should pass down to the client
+				return jsonrpc.NewError(id, jsonrpc.INVALID_REQUEST, err.Error(), nil), err
 			}
-		} else {
-			// Unknown error -> 500
+			// Auth error with ADC should raise internal 500 error
 			return jsonrpc.NewError(id, jsonrpc.INTERNAL_ERROR, err.Error(), nil), err
 		}
+		text := TextContent{
+			Type: "text",
+			Text: err.Error(),
+		}
+		return jsonrpc.JSONRPCResponse{
+			Jsonrpc: jsonrpc.JSONRPC_VERSION,
+			Id:      id,
+			Result:  CallToolResult{Content: []TextContent{text}, IsError: true},
+		}, nil
 	}
 
 	content := make([]TextContent, 0)

internal/server/mcp_test.go

@@ -231,7 +231,7 @@ func TestMcpEndpointWithoutInitialized(t *testing.T) {
 				"id":      "tools-call-tool4",
 				"error": map[string]any{
 					"code":    -32600.0,
-					"message": "unauthorized Tool call: Please make sure you specify correct auth headers: <nil>",
+					"message": "unauthorized Tool call: Please make sure your specify correct auth headers: unauthorized",
 				},
 			},
 		},
@@ -834,7 +834,7 @@ func TestMcpEndpoint(t *testing.T) {
 						"id":      "tools-call-tool4",
 						"error": map[string]any{
 							"code":    -32600.0,
-							"message": "unauthorized Tool call: Please make sure you specify correct auth headers: <nil>",
+							"message": "unauthorized Tool call: Please make sure your specify correct auth headers: unauthorized",
 						},
 					},
 				},

internal/tools/bigquery/bigqueryconversationalanalytics/bigqueryconversationalanalytics.go

@@ -184,7 +184,7 @@ func (t Tool) Invoke(ctx context.Context, resourceMgr tools.SourceProvider, para
 	if source.UseClientAuthorization() {
 		// Use client-side access token
 		if accessToken == "" {
-			return nil, util.NewClientServerError("tool is configured for client OAuth but no token was provided in the request header", http.StatusUnauthorized, nil)
+			return nil, fmt.Errorf("tool is configured for client OAuth but no token was provided in the request header: %w", util.ErrUnauthorized)
 		}
 		tokenStr, err = accessToken.ParseBearerToken()
 		if err != nil {

internal/tools/tools.go

@@ -17,7 +17,6 @@ package tools
 import (
 	"context"
 	"fmt"
-	"net/http"
 	"slices"
 	"strings"
 
@@ -81,7 +80,7 @@ type AccessToken string
 func (token AccessToken) ParseBearerToken() (string, error) {
 	headerParts := strings.Split(string(token), " ")
 	if len(headerParts) != 2 || strings.ToLower(headerParts[0]) != "bearer" {
-		return "", util.NewClientServerError("authorization header must be in the format 'Bearer <token>'", http.StatusUnauthorized, nil)
+		return "", fmt.Errorf("authorization header must be in the format 'Bearer <token>': %w", util.ErrUnauthorized)
 	}
 	return headerParts[1], nil
 }

internal/util/errors.go

@@ -1,77 +0,0 @@
-// Copyright 2026 Google LLC
-// Licensed under the Apache License, Version 2.0 (the "License");
-// you may not use this file except in compliance with the License.
-// You may obtain a copy of the License at
-//
-//	http://www.apache.org/licenses/LICENSE-2.0
-//
-// Unless required by applicable law or agreed to in writing, software
-// distributed under the License is distributed on an "AS IS" BASIS,
-// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
-// See the License for the specific language governing permissions and
-// limitations under the License.
-package util
-
-import "fmt"
-
-type ErrorCategory string
-
-const (
-	CategoryAgent  ErrorCategory = "AGENT_ERROR"
-	CategoryServer ErrorCategory = "SERVER_ERROR"
-)
-
-// ToolboxError is the interface all custom errors must satisfy
-type ToolboxError interface {
-	error
-	Category() ErrorCategory
-	Error() string
-	Unwrap() error
-}
-
-// Agent Errors return 200 to the sender
-type AgentError struct {
-	Msg   string
-	Cause error
-}
-
-var _ ToolboxError = &AgentError{}
-
-func (e *AgentError) Error() string {
-	if e.Cause != nil {
-		return fmt.Sprintf("%s: %v", e.Msg, e.Cause)
-	}
-	return e.Msg
-}
-
-func (e *AgentError) Category() ErrorCategory { return CategoryAgent }
-
-func (e *AgentError) Unwrap() error { return e.Cause }
-
-func NewAgentError(msg string, cause error) *AgentError {
-	return &AgentError{Msg: msg, Cause: cause}
-}
-
-// ClientServerError returns 4XX/5XX error code
-type ClientServerError struct {
-	Msg   string
-	Code  int
-	Cause error
-}
-
-var _ ToolboxError = &ClientServerError{}
-
-func (e *ClientServerError) Error() string {
-	if e.Cause != nil {
-		return fmt.Sprintf("%s: %v", e.Msg, e.Cause)
-	}
-	return e.Msg
-}
-
-func (e *ClientServerError) Category() ErrorCategory { return CategoryServer }
-
-func (e *ClientServerError) Unwrap() error { return e.Cause }
-
-func NewClientServerError(msg string, code int, cause error) *ClientServerError {
-	return &ClientServerError{Msg: msg, Code: code, Cause: cause}
-}

internal/util/parameters/parameters.go

@@ -19,7 +19,6 @@ import (
 	"context"
 	"encoding/json"
 	"fmt"
-	"net/http"
 	"reflect"
 	"regexp"
 	"slices"
@@ -119,7 +118,7 @@ func parseFromAuthService(paramAuthServices []ParamAuthService, claimsMap map[st
 		}
 		return v, nil
 	}
-	return nil, util.NewClientServerError("missing or invalid authentication header", http.StatusUnauthorized, nil)
+	return nil, fmt.Errorf("missing or invalid authentication header: %w", util.ErrUnauthorized)
 }
 
 // CheckParamRequired checks if a parameter is required based on the required and default field.

internal/util/util.go

@@ -17,6 +17,7 @@ import (
 	"bytes"
 	"context"
 	"encoding/json"
+	"errors"
 	"fmt"
 	"io"
 	"net/http"
@@ -187,3 +188,5 @@ func InstrumentationFromContext(ctx context.Context) (*telemetry.Instrumentation
 	}
 	return nil, fmt.Errorf("unable to retrieve instrumentation")
 }
+
+var ErrUnauthorized = errors.New("unauthorized")