x402 MCP retry omits payment metadata with Streamable HTTP transport

[mohamedkuch]

Reproduction

Using released @apify/mcpc 0.5.1 against an x402 MCP server:

1. Connect a funded Base wallet with --x402 exact.
2. Call a payment-required MCP tool.
3. The server returns a valid payment-required tool result.
4. mcpc signs the requested payment and retries.
5. The retry receives the same payment-required result. No settlement occurs.

The failure reproduces against https://api.utilia.ink/base/mcp with the solana_priority_fees tool at $0.002 USDC. The wallet balance remains unchanged, so this is a safe pre-settlement failure.

Root cause

BridgeProcess.handlePaymentRequiredRetry() caches paymentSignatureBase64 and invokes the original SDK tool call again. The fetch middleware only injects params._meta["x402/payment"] when RequestInit.body is a string. The MCP SDK transport supplies a non-string body, so the cached payment is not inserted into the MCP request metadata. The HTTP PAYMENT-SIGNATURE header alone is not sufficient for servers following the x402 MCP transport contract.

Expected behavior

Decode the signed payload and attach it to params._meta["x402/payment"] on the single retry. Preserve existing caller metadata and apply the same behavior to regular, task, and detached tool calls. The payment must be attempted at most once per tool invocation.

[jancurn]

Thanks for the report. This might be a duplicate of #247 ?

[mohamedkuch]

Thanks. #343 is separate from #247. #247 concerns session-level reuse of one payment signature across later calls. #343 occurs on the immediate retry of the same paid MCP tool call: mcpc signs the challenge, but the Streamable HTTP transport supplies a non-string request body, so the fetch middleware does not propagate the decoded payment into MCP request metadata and the retry remains unpaid. PR #344 only decodes the newly signed payload and supplies it as params._meta["x402/payment"] for that one bounded retry while preserving existing _meta. It does not add or broaden session-level payment-signature reuse.