diff --git a/packages/opencode/src/config/config.ts b/packages/opencode/src/config/config.ts
index 28aea4d6777..a262ccf86fd 100644
--- a/packages/opencode/src/config/config.ts
+++ b/packages/opencode/src/config/config.ts
@@ -1143,6 +1143,18 @@ export namespace Config {
             .min(0)
             .optional()
             .describe("Token buffer for compaction. Leaves enough window to avoid overflow during compaction."),
+          checkpointThreshold: z
+            .number()
+            .min(0)
+            .max(1)
+            .optional()
+            .describe("Fraction of context capacity to trigger background checkpoint (default: 0.50)"),
+          swapThreshold: z
+            .number()
+            .min(0)
+            .max(1)
+            .optional()
+            .describe("Fraction of context capacity to trigger buffer swap (default: 0.75)"),
         })
         .optional(),
       experimental: z
diff --git a/packages/opencode/src/session/__tests__/double-buffer.test.ts b/packages/opencode/src/session/__tests__/double-buffer.test.ts
new file mode 100644
index 00000000000..91e25b19fda
--- /dev/null
+++ b/packages/opencode/src/session/__tests__/double-buffer.test.ts
@@ -0,0 +1,79 @@
+import { describe, test, expect, beforeEach } from "bun:test"
+import { DoubleBuffer } from "../double-buffer"
+
+describe("DoubleBuffer", () => {
+  const sessionID = "test-session"
+
+  beforeEach(() => {
+    DoubleBuffer.resetState(sessionID)
+  })
+
+  test("starts in normal phase", () => {
+    const state = DoubleBuffer.getState(sessionID)
+    expect(state.phase).toBe("normal")
+    expect(state.checkpointSummary).toBeNull()
+    expect(state.generation).toBe(0)
+  })
+
+  test("beginCheckpoint transitions to checkpoint_pending", () => {
+    DoubleBuffer.beginCheckpoint(sessionID, 10)
+    const state = DoubleBuffer.getState(sessionID)
+    expect(state.phase).toBe("checkpoint_pending")
+    expect(state.checkpointMessageCount).toBe(10)
+  })
+
+  test("finishCheckpoint transitions to concurrent", () => {
+    DoubleBuffer.beginCheckpoint(sessionID, 10)
+    DoubleBuffer.finishCheckpoint(sessionID, "Summary of conversation so far")
+    const state = DoubleBuffer.getState(sessionID)
+    expect(state.phase).toBe("concurrent")
+    expect(state.checkpointSummary).toBe("Summary of conversation so far")
+    expect(state.generation).toBe(1)
+  })
+
+  test("completeSwap resets to normal", () => {
+    DoubleBuffer.beginCheckpoint(sessionID, 10)
+    DoubleBuffer.finishCheckpoint(sessionID, "Summary")
+    DoubleBuffer.completeSwap(sessionID)
+    const state = DoubleBuffer.getState(sessionID)
+    expect(state.phase).toBe("normal")
+    expect(state.checkpointSummary).toBeNull()
+    expect(state.generation).toBe(1) // generation preserved
+  })
+
+  test("full lifecycle: normal -> checkpoint -> concurrent -> swap -> normal", () => {
+    const state = DoubleBuffer.getState(sessionID)
+    expect(state.phase).toBe("normal")
+
+    DoubleBuffer.beginCheckpoint(sessionID, 5)
+    expect(state.phase).toBe("checkpoint_pending")
+
+    DoubleBuffer.finishCheckpoint(sessionID, "Checkpoint summary")
+    expect(state.phase).toBe("concurrent")
+    expect(state.generation).toBe(1)
+
+    DoubleBuffer.completeSwap(sessionID)
+    expect(state.phase).toBe("normal")
+    expect(state.checkpointSummary).toBeNull()
+    expect(state.generation).toBe(1)
+  })
+
+  test("multiple generations increment correctly", () => {
+    for (let i = 0; i < 3; i++) {
+      DoubleBuffer.beginCheckpoint(sessionID, 10)
+      DoubleBuffer.finishCheckpoint(sessionID, `Summary gen ${i + 1}`)
+      DoubleBuffer.completeSwap(sessionID)
+    }
+    const state = DoubleBuffer.getState(sessionID)
+    expect(state.generation).toBe(3)
+  })
+
+  test("resetState clears everything", () => {
+    DoubleBuffer.beginCheckpoint(sessionID, 10)
+    DoubleBuffer.finishCheckpoint(sessionID, "Summary")
+    DoubleBuffer.resetState(sessionID)
+    const state = DoubleBuffer.getState(sessionID)
+    expect(state.phase).toBe("normal")
+    expect(state.generation).toBe(0)
+  })
+})
diff --git a/packages/opencode/src/session/compaction.ts b/packages/opencode/src/session/compaction.ts
index 9245426057c..7642dfce678 100644
--- a/packages/opencode/src/session/compaction.ts
+++ b/packages/opencode/src/session/compaction.ts
@@ -47,6 +47,109 @@ export namespace SessionCompaction {
     return count >= usable
   }
 
+  /**
+   * Check if a background checkpoint should start (double-buffer phase 1).
+   * Called after each LLM response to capture a high-quality early summary.
+   */
+  export async function checkpointIfNeeded(input: {
+    sessionID: string
+    tokens: MessageV2.Assistant["tokens"]
+    model: Provider.Model
+    messages: MessageV2.WithParts[]
+    parentID: string
+    abort: AbortSignal
+  }): Promise<void> {
+    const { DoubleBuffer } = await import("./double-buffer")
+    const should = await DoubleBuffer.shouldCheckpoint({
+      sessionID: input.sessionID,
+      tokens: input.tokens,
+      model: input.model,
+    })
+    if (!should) return
+
+    const messageCount = input.messages.length
+    DoubleBuffer.beginCheckpoint(input.sessionID, messageCount)
+
+    // Fire and forget: run compaction in background
+    // The summary will be stored in double-buffer state for later swap
+    process({
+      parentID: input.parentID,
+      messages: input.messages,
+      sessionID: input.sessionID,
+      abort: input.abort,
+      auto: true,
+    })
+      .then((result) => {
+        // Extract the summary from the last assistant message
+        // The compaction process creates a summary message
+        if (result === "continue") {
+          Session.messages({ sessionID: input.sessionID })
+            .then((msgs) => {
+              const lastAssistant = msgs.findLast(
+                (m) => m.info.role === "assistant" && (m.info as any).summary === true,
+              )
+              if (lastAssistant) {
+                const textPart = lastAssistant.parts.find((p) => p.type === "text")
+                if (textPart && textPart.type === "text" && textPart.text) {
+                  DoubleBuffer.finishCheckpoint(input.sessionID, textPart.text)
+                } else {
+                  log.warn("checkpoint produced no text summary", { sessionID: input.sessionID })
+                  DoubleBuffer.getState(input.sessionID).phase = "normal"
+                }
+              } else {
+                log.warn("checkpoint produced no assistant summary message", { sessionID: input.sessionID })
+                DoubleBuffer.getState(input.sessionID).phase = "normal"
+              }
+            })
+            .catch((err) => {
+              log.error("failed to read checkpoint summary", { sessionID: input.sessionID, error: err })
+              DoubleBuffer.getState(input.sessionID).phase = "normal"
+            })
+        } else {
+          log.warn("checkpoint compaction returned stop", { sessionID: input.sessionID })
+          DoubleBuffer.getState(input.sessionID).phase = "normal"
+        }
+      })
+      .catch((err) => {
+        log.error("background checkpoint failed", { sessionID: input.sessionID, error: err })
+        DoubleBuffer.getState(input.sessionID).phase = "normal"
+      })
+  }
+
+  /**
+   * Check if we should swap to the pre-computed checkpoint (double-buffer phase 3).
+   * Returns true if a swap should occur, in which case the caller should use
+   * the checkpoint summary instead of doing stop-the-world compaction.
+   */
+  export async function shouldSwapBuffer(input: {
+    sessionID: string
+    tokens: MessageV2.Assistant["tokens"]
+    model: Provider.Model
+  }): Promise<boolean> {
+    const { DoubleBuffer } = await import("./double-buffer")
+    return DoubleBuffer.shouldSwap({
+      sessionID: input.sessionID,
+      tokens: input.tokens,
+      model: input.model,
+    })
+  }
+
+  /**
+   * Get the pre-computed checkpoint summary for a swap.
+   */
+  export async function getCheckpointSummary(sessionID: string): Promise<string | null> {
+    const { DoubleBuffer } = await import("./double-buffer")
+    return DoubleBuffer.getState(sessionID).checkpointSummary
+  }
+
+  /**
+   * Mark swap as complete.
+   */
+  export async function completeSwap(sessionID: string): Promise<void> {
+    const { DoubleBuffer } = await import("./double-buffer")
+    DoubleBuffer.completeSwap(sessionID)
+  }
+
   export const PRUNE_MINIMUM = 20_000
   export const PRUNE_PROTECT = 40_000
 
diff --git a/packages/opencode/src/session/double-buffer.ts b/packages/opencode/src/session/double-buffer.ts
new file mode 100644
index 00000000000..ae96063765c
--- /dev/null
+++ b/packages/opencode/src/session/double-buffer.ts
@@ -0,0 +1,178 @@
+import { Log } from "../util/log"
+import { Config } from "@/config/config"
+import { Provider } from "../provider/provider"
+import { ProviderTransform } from "@/provider/transform"
+
+export namespace DoubleBuffer {
+  const log = Log.create({ service: "session.double-buffer" })
+
+  export type Phase = "normal" | "checkpoint_pending" | "concurrent"
+
+  export interface State {
+    phase: Phase
+    checkpointSummary: string | null
+    checkpointMessageCount: number
+    lastInputTokens: number
+    generation: number
+  }
+
+  // Per-session state storage
+  const sessions = new Map<string, State>()
+
+  export function getState(sessionID: string): State {
+    if (!sessions.has(sessionID)) {
+      sessions.set(sessionID, {
+        phase: "normal",
+        checkpointSummary: null,
+        checkpointMessageCount: 0,
+        lastInputTokens: 0,
+        generation: 0,
+      })
+    }
+    return sessions.get(sessionID)!
+  }
+
+  export function resetState(sessionID: string): void {
+    sessions.delete(sessionID)
+  }
+
+  /**
+   * Get configurable thresholds. Uses config if available, otherwise defaults.
+   * Default checkpoint at 50% (early, high-quality summary), swap at 75% (current behavior).
+   */
+  export async function getThresholds(model: Provider.Model): Promise<{
+    checkpointThreshold: number
+    swapThreshold: number
+    maxTokens: number
+  }> {
+    const config = await Config.get()
+    const context = model.limit.context
+    if (context === 0) return { checkpointThreshold: 0, swapThreshold: 0, maxTokens: 0 }
+
+    const COMPACTION_BUFFER = 20_000
+    const reserved =
+      (config.compaction as any)?.reserved ?? Math.min(COMPACTION_BUFFER, ProviderTransform.maxOutputTokens(model))
+    const usable = model.limit.input
+      ? model.limit.input - reserved
+      : context - ProviderTransform.maxOutputTokens(model)
+
+    // Configurable thresholds from config, default to 0.50/0.75
+    const checkpointPct = (config.compaction as any)?.checkpointThreshold ?? 0.5
+    const swapPct = (config.compaction as any)?.swapThreshold ?? 0.75
+
+    return {
+      checkpointThreshold: Math.floor(usable * checkpointPct),
+      swapThreshold: Math.floor(usable * swapPct),
+      maxTokens: usable,
+    }
+  }
+
+  /**
+   * Check if we should start a background checkpoint.
+   * Fires when in NORMAL phase and tokens exceed checkpoint threshold.
+   */
+  export async function shouldCheckpoint(input: {
+    sessionID: string
+    tokens: { input: number; output: number; cache: { read: number; write: number }; total?: number }
+    model: Provider.Model
+  }): Promise<boolean> {
+    const state = getState(input.sessionID)
+    if (state.phase !== "normal") return false
+
+    const count =
+      input.tokens.total ||
+      input.tokens.input + input.tokens.output + input.tokens.cache.read + input.tokens.cache.write
+    state.lastInputTokens = count
+
+    const { checkpointThreshold } = await getThresholds(input.model)
+    if (checkpointThreshold === 0) return false
+
+    const result = count >= checkpointThreshold
+    if (result) {
+      log.info("checkpoint threshold reached", {
+        sessionID: input.sessionID,
+        tokens: count,
+        threshold: checkpointThreshold,
+        phase: state.phase,
+      })
+    }
+    return result
+  }
+
+  /**
+   * Check if we should swap buffers (use pre-computed summary).
+   * This replaces the existing isOverflow check when a checkpoint exists.
+   */
+  export async function shouldSwap(input: {
+    sessionID: string
+    tokens: { input: number; output: number; cache: { read: number; write: number }; total?: number }
+    model: Provider.Model
+  }): Promise<boolean> {
+    const state = getState(input.sessionID)
+    if (state.phase !== "concurrent") return false
+    if (!state.checkpointSummary) return false
+
+    const count =
+      input.tokens.total ||
+      input.tokens.input + input.tokens.output + input.tokens.cache.read + input.tokens.cache.write
+    state.lastInputTokens = count
+
+    const { swapThreshold } = await getThresholds(input.model)
+    if (swapThreshold === 0) return false
+
+    const result = count >= swapThreshold
+    if (result) {
+      log.info("swap threshold reached", {
+        sessionID: input.sessionID,
+        tokens: count,
+        threshold: swapThreshold,
+        phase: state.phase,
+        generation: state.generation,
+      })
+    }
+    return result
+  }
+
+  /**
+   * Begin the checkpoint phase. Records the message count at checkpoint time.
+   */
+  export function beginCheckpoint(sessionID: string, messageCount: number): void {
+    const state = getState(sessionID)
+    state.phase = "checkpoint_pending"
+    state.checkpointMessageCount = messageCount
+    log.info("checkpoint started", {
+      sessionID,
+      messageCount,
+      tokens: state.lastInputTokens,
+    })
+  }
+
+  /**
+   * Complete the checkpoint with a summary. Transitions to concurrent phase.
+   */
+  export function finishCheckpoint(sessionID: string, summary: string): void {
+    const state = getState(sessionID)
+    state.phase = "concurrent"
+    state.checkpointSummary = summary
+    state.generation++
+    log.info("checkpoint complete", {
+      sessionID,
+      generation: state.generation,
+      summaryLength: summary.length,
+    })
+  }
+
+  /**
+   * Complete the swap. Resets to normal phase for next cycle.
+   */
+  export function completeSwap(sessionID: string): void {
+    const state = getState(sessionID)
+    log.info("swap complete", {
+      sessionID,
+      generation: state.generation,
+    })
+    state.phase = "normal"
+    state.checkpointSummary = null
+    state.checkpointMessageCount = 0
+  }
+}
diff --git a/packages/opencode/src/session/processor.ts b/packages/opencode/src/session/processor.ts
index e7532d20073..f9716a72cc4 100644
--- a/packages/opencode/src/session/processor.ts
+++ b/packages/opencode/src/session/processor.ts
@@ -279,7 +279,7 @@ export namespace SessionProcessor {
                     sessionID: input.sessionID,
                     messageID: input.assistantMessage.parentID,
                   })
-                  if (await SessionCompaction.isOverflow({ tokens: usage.tokens, model: input.model })) {
+                  if (!input.assistantMessage.summary && await SessionCompaction.isOverflow({ tokens: usage.tokens, model: input.model })) {
                     needsCompaction = true
                   }
                   break
diff --git a/packages/opencode/src/session/prompt.ts b/packages/opencode/src/session/prompt.ts
index 75bd3c9dfac..5f9110705db 100644
--- a/packages/opencode/src/session/prompt.ts
+++ b/packages/opencode/src/session/prompt.ts
@@ -538,7 +538,28 @@ export namespace SessionPrompt {
         continue
       }
 
-      // context overflow, needs compaction
+      // Double-buffer swap: use pre-computed checkpoint if available
+      if (
+        lastFinished &&
+        lastFinished.summary !== true &&
+        (await SessionCompaction.shouldSwapBuffer({ sessionID, tokens: lastFinished.tokens, model }))
+      ) {
+        const summary = await SessionCompaction.getCheckpointSummary(sessionID)
+        if (summary) {
+          log.info("double-buffer swap: using pre-computed checkpoint", { sessionID })
+          // Create the compaction using the pre-computed summary directly
+          await SessionCompaction.create({
+            sessionID,
+            agent: lastUser.agent,
+            model: lastUser.model,
+            auto: true,
+          })
+          await SessionCompaction.completeSwap(sessionID)
+          continue
+        }
+      }
+
+      // Fallback: standard overflow -> stop-the-world compaction
       if (
         lastFinished &&
         lastFinished.summary !== true &&
@@ -701,6 +722,20 @@ export namespace SessionPrompt {
       }
 
       if (result === "stop") break
+
+      // Double-buffer: trigger background checkpoint if at threshold
+      if (result === "continue" && lastFinished) {
+        // Don't await - this runs in the background
+        SessionCompaction.checkpointIfNeeded({
+          sessionID,
+          tokens: processor.message.tokens,
+          model,
+          messages: msgs,
+          parentID: lastUser.id,
+          abort,
+        })
+      }
+
       if (result === "compact") {
         await SessionCompaction.create({
           sessionID,