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

[Hweinstock]

Problem.

On the refactor branch, the CLI invoke path doesn't send the headers the TS runtime requires, so invoking a TS agent 415/406s unless the user manually passes --accept text/event-stream (and JSON content-type).

This is due to an explicit check in the TS requiring these headers, whereas the same is not true in python.

Solution

• 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.

Verification.

Reproduce the bug with the current version on refactor:

$ agentcore runtime invoke --id testTS_agent_typescript_strands-kGTWAh9ySn --payload '{ "prompt": "hello" }'
Error: Received error (406) from runtime. Please check your CloudWatch logs for more information.
[rebuild with latest changes]
$ agentcore runtime invoke --id testTS_agent_typescript_strands-kGTWAh9ySn --payload '{ "prompt": "hello" }'
data: "Hello"

data: "! How can I help you today?\n"

status=200 content-type=text/event-stream runtime-session-id=ba742184-ae2d-44df-b51d-16dee7b6a61d mcp-session-id=- mcp-protocol-version=- trace-id=- trace-parent=- trace-state=- baggage=- complete=true bytes=54

[agentcore-devx-automation]

Claude Security Review: no high-confidence findings. (run)

[agentcore-devx-automation]

AgentCore Harness Review

Verdict: Looks good

Small, well-scoped fix. Defaulting Accept to text/event-stream for non-MCP runtimes matches what the CLI already sends from its dev-mode invocation paths (invoke.ts, invoke-a2a.ts, invoke-agui.ts, web-ui/handlers/invocations.ts), so the deployed-runtime invoke path is now consistent with local dev. classifyRuntimeResponse still handles non-streaming JSON responses correctly, so runtimes that ignore the Accept header aren't broken.

Tests cover both branches (HTTP/A2A defaults, and explicit override), no excessive mocking, and no telemetry concerns for a bugfix of this shape. LGTM to merge.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 97.15%. Comparing base (dffe6e5) to head (cf2a82b).

Additional details and impacted files

@@            Coverage Diff            @@
##           refactor    #2193   +/-   ##
=========================================
  Coverage     97.15%   97.15%           
=========================================
  Files           542      542           
  Lines         37728    37728           
=========================================
  Hits          36653    36653           
  Misses         1075     1075           

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