ableton-mcp

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set_crossfader_state

Destructive

Assign tracks, Groups, or Returns to crossfader sides A/B, or set the Main crossfader position in Ableton Live with guarded confirmation.

Instructions

Change either 1-64 exact track/Group/Return A/B assignments or the Main crossfader position in separate confirmations. Binds all native mixer identities and bus membership; requires recording/count-in off and known inactive Main automation. Position affects all assigned members. Immediate requests, not a timed fade or atomic audio transition. Partial work retained; no rollback/replay. Updated bridge; all profile.

Input Schema

Name	Required	Description
`dryRun`	No	False executes with token and hash.
`planHash`	No	Plan hash.
`position`	No	Native Main crossfader position; -1 left, 0 center, +1 right. No clamping.
`assignments`	No
`confirmationToken`	No	Single-use plan token.
`expectedStateVersion`	Yes	Observed stateVersion.

Output Schema

Name	Required	Description	Default
No arguments

Schema Changelog

Changes observed during successful MCP inspections.

Changed2 schema fields changedv0.3.1Oct 23, 2026

removedOutput schema / additionalProperties
```
Removed value: -true
```

addedOutput schema / anyOf

Added value: +[
+  {
+    "oneOf": [
+      {
+        "properties": {
+          "confirmation": {
+            "properties": {
+              "expiresAt": {
+                "type": "number"
+              },
+              "planHash": {
+                "pattern": "^[a-f0-9]{64}$",
+                "type": "string"
+              },
+              "token": {
+                "minLength": 1,
+                "type": "string"
+              }
+            },
+            "required": [
+              "token",
+              "planHash",
+              "expiresAt"
+            ],
+            "type": "object"
+          },
+          "dryRun": {
+            "const": true
+          },
+          "plan": {
+            "type": "object"
+          }
+        },
+        "required": [
+          "dryRun",
+          "plan",
+          "confirmation"
+        ],
+        "type": "object"
+      },
+      {
+        "properties": {
+          "dryRun": {
+            "const": false
+          },
+          "operationOutcome": {
+            "properties": {
+              "evidence": {
+                "items": {
+                  "type": "string"
+                },
+                "type": "array"
+              },
+              "reason": {
+                "type": "string"
+              },
+              "schemaVersion": {
+                "const": 1
+              },
+              "status": {
+                "enum": [
+                  "applied",
+                  "outcome_unknown",
+                  "postconditions_failed",
+                  "pending_postconditions",
+                  "pending_user_action"
+                ]
+              }
+            },
+            "required": [
+              "schemaVersion",
+              "status",
+              "reason",
+              "evidence"
+            ],
+            "type": "object"
+          }
+        },
+        "required": [
+          "dryRun",
+          "operationOutcome"
+        ],
+        "type": "object"
+      }
+    ],
+    "properties": {
+      "dryRun": {
+        "type": "boolean"
+      }
+    },
+    "required": [
+      "dryRun"
+    ],
+    "type": "object"
+  },
+  {
+    "properties": {
+      "error": {
+        "properties": {
+          "message": {
+            "type": "string"
+          },
+          "outcome": {
+            "enum": [
+              "unknown",
+              "not_executed"
+            ]
+          }
+        },
+        "required": [
+          "message",
+          "outcome"
+        ],
+        "type": "object"
+      }
+    },
+    "required": [
+      "error"
+    ],
+    "type": "object"
+  }
+]

Addedv0.3.0Oct 19, 2026

TDQS

A4/5.0

Behavior4/5

Does the description disclose side effects, auth requirements, rate limits, or destructive behavior?

Beyond the annotations (destructive, non-idempotent, write), it discloses that partial work is retained with no rollback/replay, that the position affects all assigned members, and that separate confirmations are required. These are meaningful side-effect details, though cryptic phrases like 'Updated bridge; all profile' add little clarity.

Agents need to know what a tool does to the world before calling it. Descriptions should go beyond structured annotations to explain consequences.

Conciseness3/5

Is the description appropriately sized, front-loaded, and free of redundancy?

It is reasonably front-loaded, leading with the core action, but the prose is dense and telegraphic, with fragments such as 'Binds all native mixer identities and bus membership' and 'Updated bridge; all profile' that are not self-explanatory. Every sentence is trying to carry weight, but clarity suffers for it.

Shorter descriptions cost fewer tokens and are easier for agents to parse. Every sentence should earn its place.

Completeness5/5

Given the tool's complexity, does the description cover enough for an agent to succeed on first attempt?

For a destructive mutation with an output schema and full annotations, the description covers the critical gaps: prerequisites, non-atomicity, partial-failure retention, and scope of effect. An agent has enough to call it safely without opening further docs.

Complex tools with many parameters or behaviors need more documentation. Simple tools need less. This dimension scales expectations accordingly.

Parameters4/5

Does the description clarify parameter syntax, constraints, interactions, or defaults beyond what the schema provides?

With 83% schema coverage the baseline is 3, and the description adds value by clarifying the either/or semantics of assignments versus position (the oneOf) and the 1-64 bound, plus the separate-confirmation flow implied by the token/hash params. It does not explain expectedStateVersion or the dryRun/planHash handshake in detail.

Input schemas describe structure but not intent. Descriptions should explain non-obvious parameter relationships and valid value ranges.

Purpose4/5

Does the description clearly state what the tool does and how it differs from similar tools?

The description states a specific verb (Change) and resource (track/Group/Return A/B assignments or Main crossfader position), and clearly explains the two mutually exclusive modes, which maps well to the oneOf schema. It does not name the sibling get_crossfader_context or otherwise differentiate itself explicitly from it, so it falls short of a 5.

Agents choose between tools based on descriptions. A clear purpose with a specific verb and resource helps agents select the right tool.

Usage Guidelines4/5

Does the description explain when to use this tool, when not to, or what alternatives exist?

It gives concrete preconditions for use (recording/count-in off, known inactive Main automation) and rules out misuse by noting it is an immediate request rather than a timed fade. It offers no explicit alternative tool or when-not-to-use routing, so it is clear but not exhaustive.

Agents often have multiple tools that could apply. Explicit usage guidance like "use X instead of Y when Z" prevents misuse.

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