fix(fetch): auto-detect JSON body Content-Type

When a body is provided via -d and no Content-Type header is explicitly
set, auto-detect JSON bodies (starting with { or [) and set
Content-Type: application/json. Previously, string bodies defaulted to
text/plain via the Fetch spec's Request constructor, causing servers to
skip JSON parsing on payment retry.

Author: tenequm

packages/x402-proxy/CHANGELOG.md

@@ -7,6 +7,11 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ## [Unreleased]
 
+## [0.8.2] - 2026-03-24
+
+### Fixed
+- JSON request bodies sent without explicit `Content-Type` header now auto-detect as `application/json` instead of defaulting to `text/plain` - fixes servers rejecting JSON bodies on payment retry
+
 ## [0.8.1] - 2026-03-24
 
 ### Added
@@ -214,7 +219,8 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 - `appendHistory` / `readHistory` / `calcSpend` - JSONL transaction history
 - Re-exports from `@x402/fetch`, `@x402/svm`, `@x402/evm`
 
-[Unreleased]: https://github.com/cascade-protocol/x402-proxy/compare/v0.8.1...HEAD
+[Unreleased]: https://github.com/cascade-protocol/x402-proxy/compare/v0.8.2...HEAD
+[0.8.2]: https://github.com/cascade-protocol/x402-proxy/compare/v0.8.1...v0.8.2
 [0.8.1]: https://github.com/cascade-protocol/x402-proxy/compare/v0.8.0...v0.8.1
 [0.8.0]: https://github.com/cascade-protocol/x402-proxy/compare/v0.7.1...v0.8.0
 [0.7.1]: https://github.com/cascade-protocol/x402-proxy/compare/v0.7.0...v0.7.1

packages/x402-proxy/package.json

@@ -1,6 +1,6 @@
 {
   "name": "x402-proxy",
-  "version": "0.8.1",
+  "version": "0.8.2",
   "description": "curl for x402 paid APIs. Auto-pays any endpoint on Base, Solana, and Tempo. Also works as an OpenClaw plugin.",
   "type": "module",
   "sideEffects": false,

packages/x402-proxy/src/commands/fetch.ts

@@ -241,6 +241,13 @@ Examples:
     }
 
     const method = flags.method || "GET";
+    // Auto-detect JSON body and set Content-Type if not explicitly provided
+    if (flags.body && !headers.has("Content-Type")) {
+      const trimmed = flags.body.trimStart();
+      if (trimmed.startsWith("{") || trimmed.startsWith("[")) {
+        headers.set("Content-Type", "application/json");
+      }
+    }
     // Convert Headers to plain object so mppx SSE spread doesn't lose them
     const init: RequestInit = { method, headers: Object.fromEntries(headers.entries()) };
     if (flags.body) init.body = flags.body;