fix(mcp): preserve streamable HTTP semantics

[mcowger]

Summary

Preserve streamable HTTP and MCP protocol semantics when proxying requests through the gateway. Upstream tool schemas and metadata now pass through unchanged, repeated request headers retain all values, and responses are classified as streamed strictly from their content type.

Changes

• Preserve MCP payloads

  • Keep upstream JSON-RPC responses opaque, including tool inputSchema objects and extensions such as x-mcp-header.
  • Added route and proxy regression coverage confirming these fields survive the real response path without mutation.

• Improve request header forwarding

  • Join repeated header values instead of silently keeping only the first value.
  • Join repeated Cookie values with ; , while using , for other headers such as Accept.
  • Continue forwarding MCP-specific metadata headers, including protocol version, method, tool name, and parameter headers.

• Correct response classification

  • Treat a response as SSE only when its Content-Type contains text/event-stream, case-insensitively and with optional parameters.
  • Buffer JSON responses normally, including successful JSON GET responses, rather than incorrectly exposing them as streams.

• Make SSE responses intermediary-safe

  • Preserve an upstream Cache-Control header or default it to no-cache.
  • Add X-Accel-Buffering: no to prevent buffering by compatible reverse proxies.
  • Stop emitting the hop-by-hop Connection header.

• Correct usage tracking

  • Set is_streamed from whether the proxy result contains an actual stream for POST, GET, and DELETE requests, rather than relying on method-specific defaults.

• Repository hygiene

  • Ignore .pi-subagents/ local workspace artifacts.

Testing

Added coverage for:

• Standard and mixed-case SSE content types.
• JSON GET responses being buffered.
• Preservation of x-mcp-header in tool definitions.
• Repeated Accept, Cookie, and MCP metadata headers.
• Default and upstream-provided SSE cache headers.
• Absence of the Connection response header.

[kody-ai]

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[github-actions]

PR Reviewer Guide 🔍

🏅 Score: 92

🔒 No security concerns identified

✅ No TODO sections

🔀 No multiple PR themes

⚡ Recommended focus areas for review SSE Detection Using includes classifies any content type containing text/event-stream as SSE, including values such as application/json; profile=text/event-stream or text/event-streaming. Those responses are returned as raw streams instead of being buffered and JSON-parsed. Parse and compare the media type essence case-insensitively to exactly text/event-stream. if (contentType?.toLowerCase().includes('text/event-stream')) {

[kody-ai]

Hardcoded Bearer credential sk-valid-key embeds a sensitive token directly in test source code. Replace it with an environment-provided test token or a clearly managed fixture value.

Kody rule violation: Prohibit Hardcoded Secrets

[kody-ai]

Hardcoded bearer token sk-valid-key in packages/backend/src/routes/mcp/__tests__/mcp-routes.test.ts:422 and :455 embeds credentials directly in source code. Source the test credential from an environment variable or secure test configuration instead.

Kody rule violation: Prevent Hardcoded Secrets

Prompt for LLM

File packages/backend/src/routes/mcp/__tests__/mcp-routes.test.ts:

Line 347:

Hardcoded bearer token `sk-valid-key` in `packages/backend/src/routes/mcp/__tests__/mcp-routes.test.ts:422` and `:455` embeds credentials directly in source code. Source the test credential from an environment variable or secure test configuration instead.

_{Talk to Kody by mentioning @kody}

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[github-actions]

Suggestion: Avoid comma-joining repeated singleton headers such as content-length, content-type, and authorization. Those values are not list-valued HTTP fields; joining them can produce malformed or ambiguous upstream requests, while the previous behavior safely forwarded one value. [security, importance: 6]

Suggested change

	if (value.length > 0) {
	filtered[key] = value.join(lowerKey === 'cookie' ? '; ' : ', ');
	}
	if (value.length > 0) {
	const singletonHeaders = new Set([
	'authorization',
	'content-length',
	'content-type',
	]);
	filtered[key] = singletonHeaders.has(lowerKey)
	? value[0]!
	: value.join(lowerKey === 'cookie' ? '; ' : ', ');
	}

[kody-ai]

Hardcoded bearer token sk-valid-key in packages/backend/src/routes/mcp/__tests__/mcp-routes.test.ts:422 and :455 embeds credentials directly in source code. Source the test credential from an environment variable or secure test configuration instead.

Kody rule violation: Prevent Hardcoded Secrets

Prompt for LLM

File packages/backend/src/routes/mcp/__tests__/mcp-routes.test.ts:

Line 347:

Hardcoded bearer token `sk-valid-key` in `packages/backend/src/routes/mcp/__tests__/mcp-routes.test.ts:422` and `:455` embeds credentials directly in source code. Source the test credential from an environment variable or secure test configuration instead.

_{Talk to Kody by mentioning @kody}

_{Was this suggestion helpful? React with 👍 or 👎 to help Kody learn from this interaction.}

​

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[kody-ai]

Hardcoded bearer token sk-valid-key in packages/backend/src/routes/mcp/__tests__/mcp-routes.test.ts:422 and :455 embeds credentials directly in source code. Source the test credential from an environment variable or secure test configuration instead.

Kody rule violation: Prohibit Hardcoded Secrets

Prompt for LLM

File packages/backend/src/routes/mcp/__tests__/mcp-routes.test.ts:

Line 347:

Hardcoded bearer token `sk-valid-key` in `packages/backend/src/routes/mcp/__tests__/mcp-routes.test.ts:422` and `:455` embeds credentials directly in source code. Source the test credential from an environment variable or secure test configuration instead.

_{Talk to Kody by mentioning @kody}

_{Was this suggestion helpful? React with 👍 or 👎 to help Kody learn from this interaction.}

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