fix(typescript): stop streamEvents() sending the current query twice - #2653

Where did you find this issue?

Found by code review while comparing how run()/stream() and streamEvents() assemble agent inputs in the LangChain MCPAgent (libraries/typescript/packages/agent/src/agents/mcp_agent_langchain.ts). With memoryEnabled: true (the default) and no externalHistory, streamEvents() sends the current user query to the model twice on every call. I did not find an existing issue tracking it.

Language / Project Scope

• TypeScript
• Python
• Documentation only
• CI/CD or tooling

Changes

streamEvents() pushed the user query into conversationHistory before taking the history snapshot for the run, then appended the query again as the trailing HumanMessage of the inputs — so the current turn reached the model twice. The addToHistory() call now runs after the inputs are assembled, so inputs = [...priorHistory, new HumanMessage(query)] and the query is stored once for the next turn.

Implementation Details

1. packages/agent/src/agents/mcp_agent_langchain.ts (streamEvents()): moved the if (this.memoryEnabled) { this.addToHistory(new HumanMessage({ content: query })) } block from before const historyToUse = history ?? this.conversationHistory to after the inputs array is built. Before: turn 1 sent [human("hello"), human("hello")], turn 2 sent [hello, ai(answer), world, world].
2. run()/stream() in the same file already snapshot the history first and only record messages afterwards (accumulatedMessages = [...langchainHistory, new HumanMessage(query)], with history updated after execution); streamEvents() now matches that behavior.
3. No public API change; no dependencies added or modified.

TypeScript Checklist

Packages Modified

• tests
• Source change is in @mcp-use/agent (packages/agent/src/agents/mcp_agent_langchain.ts; the template has no agent checkbox).

Pre-commit Checklist

• Ran pnpm lint:fix — not needed: pnpm lint reports no issues in the changed files (the 41 problems it reports are pre-existing on main, all in packages/cli/src/**)
• Ran pnpm format — pnpm format:check passes
• Ran pnpm build and build succeeds without errors
• Ran pnpm changeset to create a changeset
• Added or updated tests — packages/agent/tests/stream_events_duplicate_query.test.ts
• Updated documentation in docs/ — not needed (bugfix only)

Example Usage (Before)

const agent = new MCPAgent({ llm, connectors, memoryEnabled: true }); // memoryEnabled defaults to true
for await (const event of agent.streamEvents({ prompt: "hello" })) { /* ... */ }
// executor receives messages: [human("hello"), human("hello")]

Example Usage (After)

const agent = new MCPAgent({ llm, connectors, memoryEnabled: true });
for await (const event of agent.streamEvents({ prompt: "hello" })) { /* ... */ }
// executor receives messages: [human("hello")]
// next turn: [human("hello"), ai("answer"), human("world")]

Documentation Updates

None — bugfix only, no public API change.

Testing

• Added packages/agent/tests/stream_events_duplicate_query.test.ts: drives two consecutive streamEvents() turns against a mocked agent executor and asserts the executor receives the current query exactly once per turn (turn 1: [human("hello")]; turn 2: [human("hello"), ai("answer-1"), human("world")]).
• On unpatched main the new test fails with AssertionError: expected 2 to be 1 (query counted twice in turn 1 inputs); with the fix it passes.
• pnpm --filter @mcp-use/agent test:unit — 20 test files, 149 passed, 1 skipped (skip is pre-existing).
• pnpm format:check — passes.
• pnpm lint — no new issues; the 41 reported problems exist identically on main and are all in packages/cli/src/**.
• pnpm build — succeeds.

Backwards Compatibility

Backwards compatible. The only observable change is that the current user query is no longer duplicated in the prompt sent to the model on each streamEvents() call; conversation memory contents are unchanged. Callers passing externalHistory are unaffected (external history is used instead of memory and never received the query).

Related Issues

No existing issue found in the tracker for this bug; happy to link one if a maintainer prefers an issue first.

Summary by cubic

Fixes MCPAgent.streamEvents() sending the current user query to the model twice on every call when memory is enabled (the default) and no externalHistory is provided. The query was pushed into conversationHistory before the history snapshot was taken and then appended again as the trailing HumanMessage; it is now recorded only after the inputs are assembled, matching run() and stream().

The addToHistory() call is now deferred until after the stream completes, so a stream that throws leaves no orphaned user message in conversation memory for the next turn.

Bug Fixes

• Adds regression tests covering two consecutive streamEvents() turns and a failing stream.
• Backwards compatible; conversation memory contents are unchanged.

^{Written for commit 2e0e2d5. Summary will update on new commits.}