fix(invoke): default Accept to text/event-stream for non-MCP runtimes

[Hweinstock]

Problem

On the refactor branch the CLI invoke path sends no Accept header on the HTTP/A2A streaming path, so invoking a TypeScript runtime returns HTTP 406 unless the user manually passes --accept text/event-stream. Content-Type already defaulted to application/json; the gap was the missing SSE Accept.

Fix

In normalizeRuntimeInvokeRequest (src/handlers/runtime/invoke/request.ts) — the single normalizer shared by runtime invoke, project invoke runtime, and the invoke TUI — default Accept to text/event-stream for non-MCP runtimes. MCP keeps its own negotiation (application/json, text/event-stream), and an explicit --accept / --content-type still overrides for every protocol.

-    accept: modeled.accept ?? (mcp ? "application/json, text/event-stream" : undefined),
+    accept: modeled.accept ?? (mcp ? "application/json, text/event-stream" : "text/event-stream"),

Headers are provider-independent — the normalizer never inspects the model provider, so Bedrock and non-Bedrock TS agents get identical defaults.

Spec

agentcore invoke / project invoke runtime default Content-Type: application/json and Accept: text/event-stream on the HTTP/A2A streaming path, chosen per protocol (MCP uses its own negotiation), and overridable by explicit flags. A TS agent invokes successfully with no header flags.

Verification

Compiled binary (bun run compile:linux-x64) against live TS runtimes in the dev account (us-east-1). Payload {"prompt":"hi"} in every case.

1. Fixed binary, no --accept / --content-type flags → succeeds (spec step 1 & 4). Two distinct TS agents (container-built):

> agentcore runtime invoke --id tsTest_bob-... --payload '{"prompt":"hi"}'
data: "Hello! "
data: "👋 How"
data: " can I help you today?"
status=200 content-type=text/event-stream complete=true bytes=67

> agentcore runtime invoke --id tsTest_strands_agent-... --payload '{"prompt":"hi"}'
data: "Hello! How can I help you today"
data: "?"
status=200 content-type=text/event-stream complete=true bytes=52

A third managed (NODE_22 / S3) TS agent also returned status=200 content-type=text/event-stream (the agent then emitted its own app-level error unrelated to headers).

2. Baseline (pre-fix, built from refactor) reproduces the bug, and --accept is the workaround:

> agentcore runtime invoke --id tsTest_bob-... --payload '{"prompt":"hi"}'
Error: Received error (406) from runtime.

> agentcore runtime invoke --id tsTest_bob-... --payload '{"prompt":"hi"}' --accept text/event-stream
data: "Hello! How"
data: " can I help you today?"
status=200 content-type=text/event-stream complete=true bytes=52

3. Explicit override still reaches the wire (spec step 2) — fixed binary forced to a non-SSE Accept 406s instead of using the default:

> agentcore runtime invoke --id tsTest_bob-... --payload '{"prompt":"hi"}' --accept application/json
Error: Received error (406) from runtime.

4. MCP unaffected (spec step 3) — the MCP branch is byte-for-byte unchanged and its negotiation is covered by normalizeRuntimeInvokeRequest unit tests (no MCP runtime exists in the dev account for a live check).

Static checks: bun test 2827 pass / 0 fail · tsc --noEmit clean · oxlint clean · prettier --check clean · secretlint clean.

How to reproduce the fix

bun install
bun run compile:linux-x64                       # or your platform target
export AWS_REGION=us-east-1
dist/bin/agentcore-<platform> runtime invoke \
  --id <ts_runtime_id> --payload '{"prompt":"hi"}'
# expect: SSE data frames + status=200 content-type=text/event-stream, no --accept needed

bun test src/handlers/runtime/invoke/request.test.ts src/handlers/runtime/invoke/invoke.test.tsx

project invoke runtime --name <ts_agent> exercises the identical normalizeRuntimeInvokeRequest path (runtime.tsx:118 → invokeRuntimeTarget → normalizer).

[Hweinstock]

Superseded by upstream aws#2193 (merged into refactor). Closing this fork draft.