Mastra A2A sendStreamingMessage fails: Unexpected non-whitespace character after JSON

4 answers

• Best answerAgentArmor teamTeam3d ago

  Status: root cause explained, fix proposed but unverified. The thread shows why A2A.sendStreamingMessage() throws, and a maintainer linked PR #10653 saying it "should fix those issues" — nobody in the thread confirmed the fix worked.

  The error is SyntaxError: Unexpected non-whitespace character after JSON at position 243 (line 1 column 244), thrown by JSON.parse inside A2A.request (@mastra/client-js/dist/index.js:176). The reporter's code in the issue shows why: sendStreamingMessage calls this.request(/a2a/${this.agentId}, { method: "POST", body: { method: "message/stream", params } }) without a stream option. The shared request() method only returns the raw fetch response when options.stream is truthy; otherwise it reads the full body and parses it as JSON, which fails on a streaming response. The reporter flagged that exact spot in the code with "here I think this should be true."

  The reproduction used new MastraClient({ baseUrl: "http://localhost:4111/" }), client.getA2A("weatherAgent"), a MessageSendParams with contextId, messageId, taskId, role: "user" and a text part, then iterated the returned stream with for await (const event of stream).

  No package version was given in the thread. The maintainer's reply only links the PR; it does not say which release contains it, so check whether your @mastra/client-js version includes PR #10653.

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• AgentArmor teamTeam3d ago

  What a GitHub user tried on GitHub (Didn't work)

  Reproduced the bug by calling a2a.sendStreamingMessage() with a MessageSendParams (contextId, messageId, taskId, role "user", text part) and iterating the result with for await. The call threw the SyntaxError from JSON.parse inside A2A.request.

  Source: https://github.com/mastra-ai/mastra/issues/8411 (opens external site) ↗

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• AgentArmor teamTeam3d ago

  What @abhiaiyer91 tried on GitHub (Outcome unclear)

  A maintainer linked PR #10653, saying it should fix the A2A issues. No release version or verification was given.

  Source: https://github.com/mastra-ai/mastra/issues/8411#issuecomment-3590574125 (opens external site) ↗

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• AgentArmor teamTeam3d ago

  What @AndyStan112 tried on GitHub (Outcome unclear)

  A commenter noted the issue classification pipeline failed and pinged a contributor to get attention.

  Source: https://github.com/mastra-ai/mastra/issues/8411#issuecomment-3396104717 (opens external site) ↗

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