Bug Description

AI tool descriptions are not consistently evaluating n8n expressions before being passed to the AI Agent/model.

In a proof workflow, two AI tools are connected to the same Agent and receive the same upstream input:

{ "sessionId": "123", "user": "ABC", "thread_ts": "12345" }

Both tools use the same expression pattern in their tool description field:

={{ $json.sessionId }}{{ $json.user }}{{ $json.thread_ts }}

For one tool type, the expression resolves correctly to 123ABC12345. For another tool type, the Agent receives the raw unevaluated expression text instead.

To Reproduce

In a proof workflow, two AI tools are connected to the same Agent and receive the same upstream input.

{ "sessionId": "123", "user": "ABC", "thread_ts": "12345" } Both tools use the same expression pattern in their tool description field:

={{ $json.sessionId }}{{ $json.user }}{{ $json.thread_ts }}

Both tools show in the UI with valid evaluations and expanded content.

<img width="986" height="381" alt="Image" src="https://github.com/user-attachments/assets/3062beb9-b75a-453d-9022-f422096ca56d" /> <img width="983" height="268" alt="Image" src="https://github.com/user-attachments/assets/115774a5-279f-47bd-affa-2c1086ac9ebb" />

However when the workflow is executed for one tool type, the expression resolves correctly to 123ABC12345. For the other tool type, the Agent receives the raw unevaluated expression text instead.

<img width="1698" height="1247" alt="Image" src="https://github.com/user-attachments/assets/955a72a2-fa9c-47bb-8fe9-ac3881123d3b" /> <img width="1371" height="1018" alt="Image" src="https://github.com/user-attachments/assets/bdc79d9f-bd4e-4879-a003-30ad73ed3489" /> <img width="1159" height="1040" alt="Image" src="https://github.com/user-attachments/assets/6b2f2ffa-2ea5-4323-aa1e-24e88b181c33" />

Note, the tools that work correctly seem to use the parameter name description, the ones that don't use toolDescription.

Example Workflow { "nodes": [ { "parameters": {}, "id": "1eb91096-afad-45e6-9692-3e38b216808f", "name": "Manual Trigger", "type": "n8n-nodes-base.manualTrigger", "typeVersion": 1, "position": [ 336, 544 ] }, { "parameters": { "assignments": { "assignments": [ { "id": "test-session", "name": "sessionId", "value": "123", "type": "string" }, { "id": "test-user", "name": "user", "value": "ABC", "type": "string" }, { "id": "test-thread", "name": "thread_ts", "value": "12345", "type": "string" } ] }, "options": {} }, "id": "37a7e8cd-c678-4a85-ac1c-51f8188affaf", "name": "Set test inputs", "type": "n8n-nodes-base.set", "typeVersion": 3.4, "position": [ 528, 544 ] }, { "parameters": { "promptType": "define", "text": "I'm trying to test a bug, please confirm the description of your two tools.", "options": { "systemMessage": "You are a probe agent. The user wants you to invoke both your tools so we can inspect what description was passed to OpenAI. Call the tool with empty / placeholder args. After the call, respond with the single word 'done' and then the description of both tools." } }, "id": "b008e4d7-8e7c-4b70-9290-bc379df16634", "name": "Probe Agent", "type": "@n8n/n8n-nodes-langchain.agent", "typeVersion": 1.7, "position": [ 752, 544 ] }, { "parameters": { "model": { "__rl": true, "value": "gpt-5.5", "mode": "list", "cachedResultName": "gpt-5.5" }, "options": {} }, "id": "2313d93e-4f85-43f1-a820-eb8a54a02b3e", "name": "OpenAI GPT", "type": "@n8n/n8n-nodes-langchain.lmChatOpenAi", "typeVersion": 1.2, "position": [ 720, 784 ], "credentials": { "openAiApi": { "id": "0L1weZneNTwszqU8", "name": "n8n-proof-of-bug" } } }, { "parameters": { "description": "={{ $json.sessionId }}{{ $json.user }}{{ $json.thread_ts }}", "workflowId": { "__rl": true, "mode": "id", "value": "G37YR31SdCbWHLUg" }, "workflowInputs": { "mappingMode": "defineBelow", "value": {}, "matchingColumns": [], "schema": [ { "id": "sessionId", "displayName": "sessionId", "required": false, "defaultMatch": false, "display": true, "canBeUsedToMatch": true, "removed": false }, { "id": "user", "displayName": "user", "required": false, "defaultMatch": false, "display": true, "canBeUsedToMatch": true, "removed": false }, { "id": "thread_ts", "displayName": "thread_ts", "required": false, "defaultMatch": false, "display": true, "canBeUsedToMatch": true, "removed": false } ], "attemptToConvertTypes": false, "convertFieldsToString": false } }, "id": "8c8528b1-99df-4417-b9c0-86fb105f3618", "name": "ToolA", "type": "@n8n/n8n-nodes-langchain.toolWorkflow", "typeVersion": 2.2, "position": [ 848, 784 ] }, { "parameters": { "toolDescription": "={{ $json.sessionId }}{{ $json.user }}{{ $json.thread_ts }}", "method": "POST", "url": "https://slack.com/api/reactions.add", "authentication": "predefinedCredentialType", "nodeCredentialType": "slackApi", "options": { "response": { "response": {} } } }, "type": "n8n-nodes-base.httpRequestTool", "typeVersion": 4.2, "position": [ 960, 784 ], "id": "a479ca29-7ea3-4968-925a-dd98a3325046", "name": "ToolB", "credentials": { "slackApi": { "id": "ABC", "name": "REDACTED" } } } ], "connections": { "Manual Trigger": { "main": [ [ { "node": "Set test inputs", "type": "main", "index": 0 } ] ] }, "Set test inputs": { "main": [ [ { "node": "Probe Agent", "type": "main", "index": 0 } ] ] }, "OpenAI GPT": { "ai_languageModel": [ [ { "node": "Probe Agent", "type": "ai_languageModel", "index": 0 } ] ] }, "ToolA": { "ai_tool": [ [ { "node": "Probe Agent", "type": "ai_tool", "index": 0 } ] ] }, "ToolB": { "ai_tool": [ [ { "node": "Probe Agent", "type": "ai_tool", "index": 0 } ] ] } }, "pinData": { "Manual Trigger": [ {} ] }, "meta": { "templateCredsSetupCompleted": true, "instanceId": "ff8bc74e685205366b7c1dbd5470d650b694ebd82b586124e988b6855aef7cb2" } }

Expected behavior

All AI tool description fields that support n8n expressions should evaluate those expressions before the tool schema is passed to the Agent/model.

Expected value passed to the model: 123ABC12345 Actual value passed for the HTTP Request Tool: ={{ $json.sessionId }}{{ $json.user }}{{ $json.thread_ts }}

This breaks dynamic tool descriptions in AI Agent workflows, especially where descriptions depend on runtime context such as user session, permissions, enabled tools, or metadata.

Debug Info

Debug info

core

• n8nVersion: 2.20.7
• platform: docker (self-hosted)
• nodeJsVersion: 24.14.1
• nodeEnv: production
• database: postgres
• executionMode: scaling (multi-main)
• concurrency: -1
• license: enterprise (production)
• consumerId: 1a734ee3-c22c-44bd-98f6-8fdf5af8a4de

storage

• success: all
• error: all
• progress: false
• manual: true
• binaryMode: s3

pruning

• enabled: true
• maxAge: 720 hours
• maxCount: 75000 executions

client

• userAgent: mozilla/5.0 (macintosh; intel mac os x 10_15_7) applewebkit/537.36 (khtml, like gecko) chrome/148.0.0.0 safari/537.36
• isTouchDevice: false

cluster

• instanceCount: 9
• versions: 2.20.7
• instances:
  • instanceKey: 364ce66d-ba19-4d2a-98b5-814a857efa27, hostId: worker-n8n-worker-5d4697bdc5-cmcm6, instanceType: worker, instanceRole: unset, version: 2.20.7
  • instanceKey: a9c9a202-72f7-4d54-b386-703c213326a8, hostId: worker-n8n-worker-5d4697bdc5-qb5g6, instanceType: worker, instanceRole: unset, version: 2.20.7
  • instanceKey: d3522921-fe73-42a9-95e7-c8aa284eba74, hostId: worker-n8n-worker-5d4697bdc5-zk85h, instanceType: worker, instanceRole: unset, version: 2.20.7
  • instanceKey: 59f344a3-c860-426c-8124-4caaaefcc634, hostId: main-n8n-68445b8bb4-xrvwj, instanceType: main, instanceRole: follower, version: 2.20.7
  • instanceKey: 64b17d9b-5c5b-4898-b574-18f5e98496b4, hostId: worker-n8n-worker-5d4697bdc5-wpdzp, instanceType: worker, instanceRole: unset, version: 2.20.7
  • instanceKey: f8b0b075-d6a6-488f-8ef6-d8596e7073c1, hostId: main-n8n-68445b8bb4-5fhvr, instanceType: main, instanceRole: leader, version: 2.20.7
  • instanceKey: 64fbe677-65a4-44f7-99af-f93c8bf1d55a, hostId: worker-n8n-worker-5d4697bdc5-l5vsn, instanceType: worker, instanceRole: unset, version: 2.20.7
  • instanceKey: 54781043-e832-4b50-8f80-495634f9e92b, hostId: worker-n8n-worker-5d4697bdc5-5ctmc, instanceType: worker, instanceRole: unset, version: 2.20.7
  • instanceKey: aa1f9c11-e7ea-429e-9544-9888d9f9bf75, hostId: main-n8n-68445b8bb4-qrd7b, instanceType: main, instanceRole: follower, version: 2.20.7
• checks:
  • check: hostid-clash, status: succeeded, warnings: -
  • check: lifecycle, status: succeeded, warnings: -
  • check: split-brain, status: succeeded, warnings: -
  • check: version-mismatch, status: succeeded, warnings: -

Generated at: 2026-05-21T13:42:41.895Z

Operating System

AKS

n8n Version

2.20.7

Node.js Version

24.14.1

Database

PostgreSQL

Execution mode

queue

Hosting

self hosted