Anthropic web_fetch tool failing type validation with PDFs

[JustinTulloss]

Description

When web_fetch is called on a PDF, I get this validation error:

Error message: [ { "code": "invalid_union", "errors": [ [ { "code": "invalid_value", "values": [ "text" ], "path": [ "content", "source", "type" ], "message": "Invalid input: expected \"text\"" } ], [ { "code": "invalid_value", "values": [ "web_fetch_tool_result_error" ], "path": [ "type" ], "message": "Invalid input: expected \"web_fetch_tool_result_error\"" }, { "expected": "string", "code": "invalid_type", "path": [ "error_code" ], "message": "Invalid input: expected string, received undefined" } ] ], "path": [ "content_block", "content" ], "message": "Invalid input" } ]

The full message appears to be valid (according to https://platform.claude.com/docs/en/agents-and-tools/tool-use/web-fetch-tool), pasted below with the PDF content elided for brevity.

Type validation failed: Value: {"type":"content_block_start","index":10,"content_block":{"type":"web_fetch_tool_result","tool_use_id":"srvtoolu_01J56AdRLf4MjaHUq6CLqKAA","content":{"type":"web_fetch_result","url":"https://assets.unilogcorp.com/187/ITEM/DOC/Simpson_Strong_Tie_101860392_Specification_Sheet.pdf","retrieved_at":"2025-12-09T22:40:10.654602","content":{"type":"document","source":{"type":"base64","media_type":"application/pdf","data":"JVBERi0xLjcNJeLjz9MNC..."},"title":null}}}}. Error message: [ { "code": "invalid_union", "errors": [ [ { "code": "invalid_value", "values": [ "text" ], "path": [ "content", "source", "type" ], "message": "Invalid input: expected \"text\"" } ], [ { "code": "invalid_value", "values": [ "web_fetch_tool_result_error" ], "path": [ "type" ], "message": "Invalid input: expected \"web_fetch_tool_result_error\"" }, { "expected": "string", "code": "invalid_type", "path": [ "error_code" ], "message": "Invalid input: expected string, received undefined" } ] ], "path": [ "content_block", "content" ], "message": "Invalid input" } ]

AI SDK Version

  "@ai-sdk/anthropic": "^3.0.0-beta.77",
  "@ai-sdk/google": "^3.0.0-beta.66",
  "@ai-sdk/openai": "^3.0.0-beta.88",
  "ai": "^6.0.0-beta.138",

Code of Conduct

• I agree to follow this project's Code of Conduct

[aayush-kapoor]

could you please provide a minimal code sample that can help reproduce the issue? will help us in ensuring the fix works. thanks!

[JustinTulloss]

@aayush-kapoor i discovered this in an tool loop agent where it had found some pdfs using web_search and then failed to fetch them using web_fetch... so not a minimal code sample. How do I run your CLI examples? I suspect that this will fail: https://github.com/vercel/ai/blob/6f7578e5dd326d5a703df99a0e19ce1d8e3b1796/examples/ai-core/src/stream-text/anthropic-web-fetch-tool-pdf.ts (and if it doesn't i'm happy to look into why)

[JustinTulloss]

Oh, I found the instructions. Stand by!

[JustinTulloss]

Ok, so actually the pdf used in your example is returned as mediaType text/plain but it's base64 encoded. That seems like an issue on anthropic's end, I dunno. Anyway, if I make it download the more complicated PDF from my example (applying the diff below), we can reproduce the error

diff --git a/examples/ai-core/src/stream-text/anthropic-web-fetch-tool-pdf.ts b/examples/ai-core/src/stream-text/anthropic-web-fetch-tool-pdf.ts
index 9a17d24fe..3358ba753 100644
--- a/examples/ai-core/src/stream-text/anthropic-web-fetch-tool-pdf.ts
+++ b/examples/ai-core/src/stream-text/anthropic-web-fetch-tool-pdf.ts
@@ -4,10 +4,10 @@ import { run } from '../lib/run';
 
 run(async () => {
   const result = streamText({
-    model: anthropic('claude-sonnet-4-0'),
+    model: anthropic('claude-haiku-4-5'),
     prompt:
-      'What does this pdf say about AI?\n' +
-      'https://raw.githubusercontent.com/vercel/ai/main/examples/ai-core/data/ai.pdf',
+      'Summarize this pdf.\n' +
+      'https://assets.unilogcorp.com/187/ITEM/DOC/Simpson_Strong_Tie_101860392_Specification_Sheet.pdf',
     tools: {
       web_fetch: anthropic.tools.webFetch_20250910(),
     },

I'll fetch and summarize that PDF for you.�[32m�[1mTool call:�[22m {
  "type": "tool-call",
  "toolCallId": "srvtoolu_01TwnFbxJnwktTRPmChafS1m",
  "toolName": "web_fetch",
  "input": {
    "url": "https://assets.unilogcorp.com/187/ITEM/DOC/Simpson_Strong_Tie_101860392_Specification_Sheet.pdf"
  },
  "providerExecuted": true
}�[0m
_TypeValidationError [AI_TypeValidationError]: Type validation failed: Value: {"type":"content_block_start","index":2,"content_block":{"type":"web_fetch_tool_result","tool_use_id":"srvtoolu_01TwnFbxJnwktTRPmChafS1m","content":{"type":"web_fetch_result","url":"https://assets.unilogcorp.com/187/ITEM/DOC/Simpson_Strong_Tie_101860392_Specification_Sheet.pdf","retrieved_at":"2025-12-10T18:49:57.475319","content":{"type":"document","source":{"type":"base64","media_type":"application/pdf","data":"..."},"title":null}}}}.
Error message: [
  {
    "code": "invalid_union",
    "errors": [
      [
        {
          "code": "invalid_value",
          "values": [
            "text"
          ],
          "path": [
            "content",
            "source",
            "type"
          ],
          "message": "Invalid input: expected \"text\""
        }
      ],
      [
        {
          "code": "invalid_value",
          "values": [
            "web_fetch_tool_result_error"
          ],
          "path": [
            "type"
          ],
          "message": "Invalid input: expected \"web_fetch_tool_result_error\""
        },
        {
          "expected": "string",
          "code": "invalid_type",
          "path": [
            "error_code"
          ],
          "message": "Invalid input: expected string, received undefined"
        }
      ]
    ],
    "path": [
      "content_block",
      "content"
    ],
    "message": "Invalid input"
  }
]
    at _TypeValidationError.wrap (/Users/justin/src/ai/packages/provider/src/errors/type-validation-error.ts:49:9)
    at safeValidateTypes (/Users/justin/src/ai/packages/provider-utils/src/validate-types.ts:71:34)
    at process.processTicksAndRejections (node:internal/process/task_queues:105:5)
    at async safeParseJSON (/Users/justin/src/ai/packages/provider-utils/src/parse-json.ts:103:12)
    at async Object.transform (/Users/justin/src/ai/packages/provider-utils/src/parse-json-event-stream.ts:29:30)
    at async transformStreamDefaultControllerPerformTransform (node:internal/webstreams/transformstream:527:12) {
  cause: ZodError: [
    {
      "code": "invalid_union",
      "errors": [
        [
          {
            "code": "invalid_value",
            "values": [
              "text"
            ],
            "path": [
              "content",
              "source",
              "type"
            ],
            "message": "Invalid input: expected \"text\""
          }
        ],
        [
          {
            "code": "invalid_value",
            "values": [
              "web_fetch_tool_result_error"
            ],
            "path": [
              "type"
            ],
            "message": "Invalid input: expected \"web_fetch_tool_result_error\""
          },
          {
            "expected": "string",
            "code": "invalid_type",
            "path": [
              "error_code"
            ],
            "message": "Invalid input: expected string, received undefined"
          }
        ]
      ],
      "path": [
        "content_block",
        "content"
      ],
      "message": "Invalid input"
    }
  ]
      at new ZodError (/Users/justin/src/ai/node_modules/.pnpm/zod@3.25.76/node_modules/zod/v4/core/core.cjs:35:39)
      at Object.<anonymous> (/Users/justin/src/ai/node_modules/.pnpm/zod@3.25.76/node_modules/zod/v4/core/parse.cjs:82:20)
      at Object.validate (/Users/justin/src/ai/packages/provider-utils/src/schema.ts:262:33)
      at safeValidateTypes (/Users/justin/src/ai/packages/provider-utils/src/validate-types.ts:63:39)
      at safeParseJSON (/Users/justin/src/ai/packages/provider-utils/src/parse-json.ts:103:18)
      at Object.transform (/Users/justin/src/ai/packages/provider-utils/src/parse-json-event-stream.ts:29:36)
      at invokePromiseCallback (node:internal/webstreams/util:172:10)
      at Object.transformAlgorithm (node:internal/webstreams/util:177:23)
      at transformStreamDefaultControllerPerformTransform (node:internal/webstreams/transformstream:527:37)
      at transformStreamDefaultSinkWriteAlgorithm (node:internal/webstreams/transformstream:573:10),
  value: {
    type: 'content_block_start',
    index: 2,
    content_block: {
      type: 'web_fetch_tool_result',
      tool_use_id: 'srvtoolu_01TwnFbxJnwktTRPmChafS1m',
      content: [Object]
    }
  },
  Symbol(vercel.ai.error): true,
  Symbol(vercel.ai.error.AI_TypeValidationError): true
}
Error: _TypeValidationError [AI_TypeValidationError]: Type validation failed: Value: {"type":"content_block_start","index":2,"content_block":{"type":"web_fetch_tool_result","tool_use_id":"srvtoolu_01TwnFbxJnwktTRPmChafS1m","content":{"type":"web_fetch_result","url":"https://assets.unilogcorp.com/187/ITEM/DOC/Simpson_Strong_Tie_101860392_Specification_Sheet.pdf","retrieved_at":"2025-12-10T18:49:57.475319","content":{"type":"document","source":{"type":"base64","media_type":"application/pdf","data":"..."},"title":null}}}}.
Error message: [
  {
    "code": "invalid_union",
    "errors": [
      [
        {
          "code": "invalid_value",
          "values": [
            "text"
          ],
          "path": [
            "content",
            "source",
            "type"
          ],
          "message": "Invalid input: expected \"text\""
        }
      ],
      [
        {
          "code": "invalid_value",
          "values": [
            "web_fetch_tool_result_error"
          ],
          "path": [
            "type"
          ],
          "message": "Invalid input: expected \"web_fetch_tool_result_error\""
        },
        {
          "expected": "string",
          "code": "invalid_type",
          "path": [
            "error_code"
          ],
          "message": "Invalid input: expected string, received undefined"
        }
      ]
    ],
    "path": [
      "content_block",
      "content"
    ],
    "message": "Invalid input"
  }
]
    at _TypeValidationError.wrap (/Users/justin/src/ai/packages/provider/src/errors/type-validation-error.ts:49:9)
    at safeValidateTypes (/Users/justin/src/ai/packages/provider-utils/src/validate-types.ts:71:34)
    at process.processTicksAndRejections (node:internal/process/task_queues:105:5)
    at async safeParseJSON (/Users/justin/src/ai/packages/provider-utils/src/parse-json.ts:103:12)
    at async Object.transform (/Users/justin/src/ai/packages/provider-utils/src/parse-json-event-stream.ts:29:30)
    at async transformStreamDefaultControllerPerformTransform (node:internal/webstreams/transformstream:527:12) {
  cause: ZodError: [
    {
      "code": "invalid_union",
      "errors": [
        [
          {
            "code": "invalid_value",
            "values": [
              "text"
            ],
            "path": [
              "content",
              "source",
              "type"
            ],
            "message": "Invalid input: expected \"text\""
          }
        ],
        [
          {
            "code": "invalid_value",
            "values": [
              "web_fetch_tool_result_error"
            ],
            "path": [
              "type"
            ],
            "message": "Invalid input: expected \"web_fetch_tool_result_error\""
          },
          {
            "expected": "string",
            "code": "invalid_type",
            "path": [
              "error_code"
            ],
            "message": "Invalid input: expected string, received undefined"
          }
        ]
      ],
      "path": [
        "content_block",
        "content"
      ],
      "message": "Invalid input"
    }
  ]
      at new ZodError (/Users/justin/src/ai/node_modules/.pnpm/zod@3.25.76/node_modules/zod/v4/core/core.cjs:35:39)
      at Object.<anonymous> (/Users/justin/src/ai/node_modules/.pnpm/zod@3.25.76/node_modules/zod/v4/core/parse.cjs:82:20)
      at Object.validate (/Users/justin/src/ai/packages/provider-utils/src/schema.ts:262:33)
      at safeValidateTypes (/Users/justin/src/ai/packages/provider-utils/src/validate-types.ts:63:39)
      at safeParseJSON (/Users/justin/src/ai/packages/provider-utils/src/parse-json.ts:103:18)
      at Object.transform (/Users/justin/src/ai/packages/provider-utils/src/parse-json-event-stream.ts:29:36)
      at invokePromiseCallback (node:internal/webstreams/util:172:10)
      at Object.transformAlgorithm (node:internal/webstreams/util:177:23)
      at transformStreamDefaultControllerPerformTransform (node:internal/webstreams/transformstream:527:37)
      at transformStreamDefaultSinkWriteAlgorithm (node:internal/webstreams/transformstream:573:10),
  value: {
    type: 'content_block_start',
    index: 2,
    content_block: {
      type: 'web_fetch_tool_result',
      tool_use_id: 'srvtoolu_01TwnFbxJnwktTRPmChafS1m',
      content: [Object]
    }
  },
  Symbol(vercel.ai.error): true,
  Symbol(vercel.ai.error.AI_TypeValidationError): true
}
 Summary

This specification sheet covers Simpson Strong-Tie wood construction connectors, specifically two products:

**HH - Header Hanger**
- **Purpose**: Fast, accurate installation of door/window headers and cross members; strengthens frames and eliminates need for trimmers
- **Material**: 16 gauge galvanized steel
- **Models**: HH4 (3½" × 2¹³⁄₁₆") and HH6 (5½" × 5⅛")
- **Fasteners**: Uses specified nails (diameter varies by model)
- **Load Capacity**: Varies by lumber size and configuration, ranging from 540 lbs to 2,110 lbs depending on load direction and wood type
- **Note**: When attached to 2x studs, two round holes won't be filled and load reductions apply

**RR - Ridge Rafter Connector**
- **Purpose**: Connects rafters to headers at ridge joints
- **Material**: 18 gauge galvanized steel
- **Compatible Rafter Size**: 2x6 minimum
- **Fasteners**: Uses (4) 0.148" × 1½" nails in header and rafter
- **Allowable Loads**: 330 lbs floor, 330 lbs roof, with 130 lbs uplift capacity
- **Special Feature**: Interlock provides alignment control; can accommodate rafters sloped up to 30°

Both products are code-approved and compatible with Strong-Drive SD Connector screws as an alternative installation method.

[JustinTulloss]

And I verified that the error does not occur once #11027 is applied.

I'll fetch that PDF for you.�[32m�[1mTool call:�[22m {
  "type": "tool-call",
  "toolCallId": "srvtoolu_01NABJTRocfNj9pmCHUo5AVP",
  "toolName": "web_fetch",
  "input": {
    "url": "https://assets.unilogcorp.com/187/ITEM/DOC/Simpson_Strong_Tie_101860392_Specification_Sheet.pdf"
  },
  "providerExecuted": true
}�[0m
�[32m�[1mTool result:�[22m {
  "type": "tool-result",
  "toolCallId": "srvtoolu_01NABJTRocfNj9pmCHUo5AVP",
  "toolName": "web_fetch",
  "input": {
    "url": "https://assets.unilogcorp.com/187/ITEM/DOC/Simpson_Strong_Tie_101860392_Specification_Sheet.pdf"
  },
  "output": {
    "type": "web_fetch_result",
    "url": "https://assets.unilogcorp.com/187/ITEM/DOC/Simpson_Strong_Tie_101860392_Specification_Sheet.pdf",
    "retrievedAt": "2025-12-10T19:00:29.029196",
    "content": {
      "type": "document",
      "title": null,
      "source": {
        "type": "base64",
        "mediaType": "application/pdf",
        "data": "..."
      }
    }
  },
  "providerExecuted": true
}�[0m
## Summary

This is a specification sheet from Simpson Strong-Tie® for two wood construction connectors:

**HH Header Hanger:**
- Used for fast installation of door and window headers and cross members
- Made from 16-gauge galvanized steel
- Comes in two models: HH4 and HH6
- Features an interlock for alignment control and proper nailing
- Provides load specifications for various stud and header combinations (ranging from 2x to 3x lumber)
- Allowable loads range from 540 lbs to 2,110 lbs depending on the model and lumber configuration
- Note: Attachment to 2x studs results in two unfilled round holes

**RR Ridge Rafter Connector:**
- Designed for connecting rafters at roof ridges
- Made from 18-gauge galvanized steel
- Fits 2x6 minimum rafter size and can handle rafters sloped up to 30°
- Features an interlock for proper alignment
- Allows floor loads of 330 lbs and roof loads of 330 lbs
- Uplift capacity of 130 lbs

Both products are code-approved (IBC, FL, LA) and can be installed with Simpson Strong-Drive® SD Connector screws as alternatives to traditional nails.