feat: add percentage support for contextLimit configuration

Allow contextLimit to accept percentage strings (e.g. "40%") to specify
a percentage of the model's context window instead of fixed values.

Values are rounded and clamped between 0 and 100.

Author: tobwen

dcp.schema.json

@@ -110,7 +110,7 @@
                             "description": "Tool names that should be protected from automatic pruning"
                         },
                         "contextLimit": {
-                            "description": "When session tokens exceed this limit, a compress nudge is injected (\"model\" uses the active model's context limit)",
+                            "description": "When session tokens exceed this limit, a compress nudge is injected (\"model\" uses the active model's context limit, \"X%\" uses percentage of the model's context window)",
                             "default": 100000,
                             "oneOf": [
                                 {
@@ -119,6 +119,10 @@
                                 {
                                     "type": "string",
                                     "enum": ["model"]
+                                },
+                                {
+                                    "type": "string",
+                                    "pattern": "^\\d+(?:\\.\\d+)?%$"
                                 }
                             ]
                         }

lib/config.ts

@@ -27,7 +27,7 @@ export interface ToolSettings {
     nudgeEnabled: boolean
     nudgeFrequency: number
     protectedTools: string[]
-    contextLimit: number | "model"
+    contextLimit: number | "model" | `${number}%`
 }
 
 export interface Tools {
@@ -290,13 +290,16 @@ function validateConfigTypes(config: Record<string, any>): ValidationError[] {
                 })
             }
             if (tools.settings.contextLimit !== undefined) {
-                if (
-                    typeof tools.settings.contextLimit !== "number" &&
-                    tools.settings.contextLimit !== "model"
-                ) {
+                const isValidNumber = typeof tools.settings.contextLimit === "number"
+                const isModelString = tools.settings.contextLimit === "model"
+                const isPercentString =
+                    typeof tools.settings.contextLimit === "string" &&
+                    tools.settings.contextLimit.endsWith("%")
+
+                if (!isValidNumber && !isModelString && !isPercentString) {
                     errors.push({
                         key: "tools.settings.contextLimit",
-                        expected: 'number | "model"',
+                        expected: 'number | "model" | "${number}%"',
                         actual: JSON.stringify(tools.settings.contextLimit),
                     })
                 }

lib/messages/inject.ts

@@ -13,6 +13,20 @@ import { getFilePathsFromParameters, isProtected } from "../protected-file-patte
 import { getLastUserMessage, isMessageCompacted } from "../shared-utils"
 import { getCurrentTokenUsage } from "../strategies/utils"
 
+function parsePercentageString(value: string, total: number): number | undefined {
+    if (!value.endsWith("%")) return undefined
+    const percent = parseFloat(value.slice(0, -1))
+
+    if (isNaN(percent)) {
+        return undefined
+    }
+
+    const roundedPercent = Math.round(percent)
+    const clampedPercent = Math.max(0, Math.min(100, roundedPercent))
+
+    return Math.round((clampedPercent / 100) * total)
+}
+
 // XML wrappers
 export const wrapPrunableTools = (content: string): string => {
     return `<prunable-tools>
@@ -54,9 +68,22 @@ Context management was just performed. Do NOT use the ${toolName} again. A fresh
 
 const resolveContextLimit = (config: PluginConfig, state: SessionState): number | undefined => {
     const configLimit = config.tools.settings.contextLimit
-    if (configLimit === "model") {
-        return state.modelContextLimit
+
+    if (typeof configLimit === "string") {
+        if (configLimit.endsWith("%")) {
+            if (state.modelContextLimit === undefined) {
+                return undefined
+            }
+            return parsePercentageString(configLimit, state.modelContextLimit)
+        }
+
+        if (configLimit === "model") {
+            return state.modelContextLimit
+        }
+
+        return undefined
     }
+
     return configLimit
 }