MCP tools being called with incorrect escaping for parameters #27079

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1. github-actions

   assigned

   nexxeln

   on May 12, 2026
2. github-actions commented on May 12, 2026

   

   github-actionsBot

   on May 12, 2026 – with GitHub Actions

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   This issue might be a duplicate of existing issues. Please check:

   • Tools with array/object parameters fail when LLM sends stringified JSON #7512: Tools with array/object parameters fail when LLM sends stringified JSON — same root cause where the LLM sends a parameter value with extra quoting/escaping (e.g. "\"markdown\"" instead of "markdown"), causing schema validation to fail
   • The write tool was called with invalid arguments: SchemaError #24604: The write tool was called with invalid arguments: SchemaError — also reported with Qwen 3.6 35B-A3B on a local provider, similar tool argument schema errors
3. lowlyocean commented on May 12, 2026

   

   lowlyocean

   on May 12, 2026

   Author

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   It isn't a duplicate of those
4. lznoob commented on May 15, 2026

   

   lznoob

   on May 15, 2026

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   packages/opencode/src/tool/webfetch.ts, line 20:
   remove .pipe(Schema.optional, Schema.withDecodingDefault(Effect.succeed("markdown" as const))),

   Should look like this:

   export const Parameters = Schema.Struct({
     url: Schema.String.annotate({ description: "The URL to fetch content from" }),
     format: Schema.Literals(["text", "markdown", "html"])
       .annotate({
         description: "The format to return the content in (text, markdown, or html). Defaults to markdown.",
         default: "markdown",
       }),
     timeout: Schema.optional(Schema.Number).annotate({ description: "Optional timeout in seconds (max 120)" }),
   })

   

   The original version sends this schema.format:

         "format": {
           "anyOf": [
             {
               "type": "string",
               "enum": [
                 "text",
                 "markdown",
                 "html"
               ],
               "description": "The format to return the content in (text, markdown, or html). Defaults to markdown.",
               "default": "markdown"
             },
             {
               "type": "null"
             }
           ]
         }

   Fixed:

         "format": {
           "type": "string",
           "enum": [
             "text",
             "markdown",
             "html"
           ],
           "description": "The format to return the content in (text, markdown, or html). Defaults to markdown.",
           "default": "markdown"
         }

   Not sure what exactly is wrong, though.
5. lowlyocean commented on May 15, 2026

   

   lowlyocean

   on May 15, 2026

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   @kitlangton Looks like your recent PR a few days ago may have introduced a regression as indicated above, can you check please?
   #26956
6. frankfletcher commented on May 31, 2026

   

   frankfletcher

   on May 31, 2026

   Last edited by frankfletcher

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   same issue.
   OpenCode v1.15.13
   Ubuntu Linux v26.04
   Qwen 3.6 27b running on beellama.cpp server.
7. frankfletcher commented on Jun 1, 2026

   

   frankfletcher

   on Jun 1, 2026

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   The workaround I found was to create a custom rules markdown with the following:

   ## webfetch Tool Usage
   
   When calling the `webfetch` tool, **never** include the `format` parameter. Omitting it defaults to `markdown`, which is the desired behavior. Only provide the `url` parameter.
   
   **Correct:**
   ```json
   {
     "url": "https://example.com"
   }
   ```
   

   I added this line in my opencode.jsonc to make it universal, but you could just add it to your AGENTS.md or whatever. "instructions": ["~/.config/opencode/custom_rules.md"],
8. nexxeln

   closed this as completedin #30215on Jun 1, 2026
9. nexxeln commented on Jun 1, 2026

   

   nexxeln

   on Jun 1, 2026

   Contributor

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   will be fixed in next release!

   👍React with 👍1Reacted by Frank Fletcher