diff --git a/CHANGELOG.md b/CHANGELOG.md
index 00aa560a4..4a373ff5f 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -6,6 +6,7 @@ Docs: https://docs.clawd.bot
 Status: unreleased.
 
 ### Changes
+- Agents: summarize dropped messages during compaction safeguard pruning. (#2418)
 - Skills: add multi-image input support to Nano Banana Pro skill. (#1958) Thanks @tyler6204.
 - Agents: honor tools.exec.safeBins in exec allowlist checks. (#2281)
 - Matrix: switch plugin SDK to @vector-im/matrix-bot-sdk.
diff --git a/src/agents/compaction.test.ts b/src/agents/compaction.test.ts
index 1cfacda9a..32511a586 100644
--- a/src/agents/compaction.test.ts
+++ b/src/agents/compaction.test.ts
@@ -103,5 +103,47 @@ describe("pruneHistoryForContextShare", () => {
     expect(pruned.droppedChunks).toBe(0);
     expect(pruned.messages.length).toBe(messages.length);
     expect(pruned.keptTokens).toBe(estimateMessagesTokens(messages));
+    expect(pruned.droppedMessagesList).toEqual([]);
+  });
+
+  it("returns droppedMessagesList containing dropped messages", () => {
+    const messages: AgentMessage[] = [
+      makeMessage(1, 4000),
+      makeMessage(2, 4000),
+      makeMessage(3, 4000),
+      makeMessage(4, 4000),
+    ];
+    const maxContextTokens = 2000; // budget is 1000 tokens (50%)
+    const pruned = pruneHistoryForContextShare({
+      messages,
+      maxContextTokens,
+      maxHistoryShare: 0.5,
+      parts: 2,
+    });
+
+    expect(pruned.droppedChunks).toBeGreaterThan(0);
+    expect(pruned.droppedMessagesList.length).toBe(pruned.droppedMessages);
+
+    // All messages accounted for: kept + dropped = original
+    const allIds = [
+      ...pruned.droppedMessagesList.map((m) => m.timestamp),
+      ...pruned.messages.map((m) => m.timestamp),
+    ].sort((a, b) => a - b);
+    const originalIds = messages.map((m) => m.timestamp).sort((a, b) => a - b);
+    expect(allIds).toEqual(originalIds);
+  });
+
+  it("returns empty droppedMessagesList when no pruning needed", () => {
+    const messages: AgentMessage[] = [makeMessage(1, 100)];
+    const pruned = pruneHistoryForContextShare({
+      messages,
+      maxContextTokens: 100_000,
+      maxHistoryShare: 0.5,
+      parts: 2,
+    });
+
+    expect(pruned.droppedChunks).toBe(0);
+    expect(pruned.droppedMessagesList).toEqual([]);
+    expect(pruned.messages.length).toBe(1);
   });
 });
diff --git a/src/agents/compaction.ts b/src/agents/compaction.ts
index 2ab4566fd..a88447307 100644
--- a/src/agents/compaction.ts
+++ b/src/agents/compaction.ts
@@ -301,6 +301,7 @@ export function pruneHistoryForContextShare(params: {
   parts?: number;
 }): {
   messages: AgentMessage[];
+  droppedMessagesList: AgentMessage[];
   droppedChunks: number;
   droppedMessages: number;
   droppedTokens: number;
@@ -310,6 +311,7 @@ export function pruneHistoryForContextShare(params: {
   const maxHistoryShare = params.maxHistoryShare ?? 0.5;
   const budgetTokens = Math.max(1, Math.floor(params.maxContextTokens * maxHistoryShare));
   let keptMessages = params.messages;
+  const allDroppedMessages: AgentMessage[] = [];
   let droppedChunks = 0;
   let droppedMessages = 0;
   let droppedTokens = 0;
@@ -323,11 +325,13 @@ export function pruneHistoryForContextShare(params: {
     droppedChunks += 1;
     droppedMessages += dropped.length;
     droppedTokens += estimateMessagesTokens(dropped);
+    allDroppedMessages.push(...dropped);
     keptMessages = rest.flat();
   }
 
   return {
     messages: keptMessages,
+    droppedMessagesList: allDroppedMessages,
     droppedChunks,
     droppedMessages,
     droppedTokens,
diff --git a/src/agents/pi-embedded-runner/extensions.ts b/src/agents/pi-embedded-runner/extensions.ts
index 73deae21d..bb592e930 100644
--- a/src/agents/pi-embedded-runner/extensions.ts
+++ b/src/agents/pi-embedded-runner/extensions.ts
@@ -7,6 +7,7 @@ import type { SessionManager } from "@mariozechner/pi-coding-agent";
 import type { ClawdbotConfig } from "../../config/config.js";
 import { resolveContextWindowInfo } from "../context-window-guard.js";
 import { DEFAULT_CONTEXT_TOKENS } from "../defaults.js";
+import { setCompactionSafeguardRuntime } from "../pi-extensions/compaction-safeguard-runtime.js";
 import { setContextPruningRuntime } from "../pi-extensions/context-pruning/runtime.js";
 import { computeEffectiveSettings } from "../pi-extensions/context-pruning/settings.js";
 import { makeToolPrunablePredicate } from "../pi-extensions/context-pruning/tools.js";
@@ -75,6 +76,10 @@ export function buildEmbeddedExtensionPaths(params: {
 }): string[] {
   const paths: string[] = [];
   if (resolveCompactionMode(params.cfg) === "safeguard") {
+    const compactionCfg = params.cfg?.agents?.defaults?.compaction;
+    setCompactionSafeguardRuntime(params.sessionManager, {
+      maxHistoryShare: compactionCfg?.maxHistoryShare,
+    });
     paths.push(resolvePiExtensionPath("compaction-safeguard"));
   }
   const pruning = buildContextPruningExtension(params);
diff --git a/src/agents/pi-extensions/compaction-safeguard-runtime.ts b/src/agents/pi-extensions/compaction-safeguard-runtime.ts
new file mode 100644
index 000000000..f42cf7abe
--- /dev/null
+++ b/src/agents/pi-extensions/compaction-safeguard-runtime.ts
@@ -0,0 +1,34 @@
+export type CompactionSafeguardRuntimeValue = {
+  maxHistoryShare?: number;
+};
+
+// Session-scoped runtime registry keyed by object identity.
+// Follows the same WeakMap pattern as context-pruning/runtime.ts.
+const REGISTRY = new WeakMap<object, CompactionSafeguardRuntimeValue>();
+
+export function setCompactionSafeguardRuntime(
+  sessionManager: unknown,
+  value: CompactionSafeguardRuntimeValue | null,
+): void {
+  if (!sessionManager || typeof sessionManager !== "object") {
+    return;
+  }
+
+  const key = sessionManager as object;
+  if (value === null) {
+    REGISTRY.delete(key);
+    return;
+  }
+
+  REGISTRY.set(key, value);
+}
+
+export function getCompactionSafeguardRuntime(
+  sessionManager: unknown,
+): CompactionSafeguardRuntimeValue | null {
+  if (!sessionManager || typeof sessionManager !== "object") {
+    return null;
+  }
+
+  return REGISTRY.get(sessionManager as object) ?? null;
+}
diff --git a/src/agents/pi-extensions/compaction-safeguard.test.ts b/src/agents/pi-extensions/compaction-safeguard.test.ts
index 275e10e9f..23ab1efda 100644
--- a/src/agents/pi-extensions/compaction-safeguard.test.ts
+++ b/src/agents/pi-extensions/compaction-safeguard.test.ts
@@ -1,6 +1,10 @@
 import type { AgentMessage } from "@mariozechner/pi-agent-core";
 import { describe, expect, it } from "vitest";
 
+import {
+  getCompactionSafeguardRuntime,
+  setCompactionSafeguardRuntime,
+} from "./compaction-safeguard-runtime.js";
 import { __testing } from "./compaction-safeguard.js";
 
 const {
@@ -208,3 +212,41 @@ describe("isOversizedForSummary", () => {
     expect(typeof isOversized).toBe("boolean");
   });
 });
+
+describe("compaction-safeguard runtime registry", () => {
+  it("stores and retrieves config by session manager identity", () => {
+    const sm = {};
+    setCompactionSafeguardRuntime(sm, { maxHistoryShare: 0.3 });
+    const runtime = getCompactionSafeguardRuntime(sm);
+    expect(runtime).toEqual({ maxHistoryShare: 0.3 });
+  });
+
+  it("returns null for unknown session manager", () => {
+    const sm = {};
+    expect(getCompactionSafeguardRuntime(sm)).toBeNull();
+  });
+
+  it("clears entry when value is null", () => {
+    const sm = {};
+    setCompactionSafeguardRuntime(sm, { maxHistoryShare: 0.7 });
+    expect(getCompactionSafeguardRuntime(sm)).not.toBeNull();
+    setCompactionSafeguardRuntime(sm, null);
+    expect(getCompactionSafeguardRuntime(sm)).toBeNull();
+  });
+
+  it("ignores non-object session managers", () => {
+    setCompactionSafeguardRuntime(null, { maxHistoryShare: 0.5 });
+    expect(getCompactionSafeguardRuntime(null)).toBeNull();
+    setCompactionSafeguardRuntime(undefined, { maxHistoryShare: 0.5 });
+    expect(getCompactionSafeguardRuntime(undefined)).toBeNull();
+  });
+
+  it("isolates different session managers", () => {
+    const sm1 = {};
+    const sm2 = {};
+    setCompactionSafeguardRuntime(sm1, { maxHistoryShare: 0.3 });
+    setCompactionSafeguardRuntime(sm2, { maxHistoryShare: 0.8 });
+    expect(getCompactionSafeguardRuntime(sm1)).toEqual({ maxHistoryShare: 0.3 });
+    expect(getCompactionSafeguardRuntime(sm2)).toEqual({ maxHistoryShare: 0.8 });
+  });
+});
diff --git a/src/agents/pi-extensions/compaction-safeguard.ts b/src/agents/pi-extensions/compaction-safeguard.ts
index 82ad19f2a..b2fe39884 100644
--- a/src/agents/pi-extensions/compaction-safeguard.ts
+++ b/src/agents/pi-extensions/compaction-safeguard.ts
@@ -11,6 +11,7 @@ import {
   resolveContextWindowTokens,
   summarizeInStages,
 } from "../compaction.js";
+import { getCompactionSafeguardRuntime } from "./compaction-safeguard-runtime.js";
 const FALLBACK_SUMMARY =
   "Summary unavailable due to context limits. Older messages were truncated.";
 const TURN_PREFIX_INSTRUCTIONS =
@@ -174,21 +175,28 @@ export default function compactionSafeguardExtension(api: ExtensionAPI): void {
       const turnPrefixMessages = preparation.turnPrefixMessages ?? [];
       let messagesToSummarize = preparation.messagesToSummarize;
 
+      const runtime = getCompactionSafeguardRuntime(ctx.sessionManager);
+      const maxHistoryShare = runtime?.maxHistoryShare ?? 0.5;
+
       const tokensBefore =
         typeof preparation.tokensBefore === "number" && Number.isFinite(preparation.tokensBefore)
           ? preparation.tokensBefore
           : undefined;
+
+      let droppedSummary: string | undefined;
+
       if (tokensBefore !== undefined) {
         const summarizableTokens =
           estimateMessagesTokens(messagesToSummarize) + estimateMessagesTokens(turnPrefixMessages);
         const newContentTokens = Math.max(0, Math.floor(tokensBefore - summarizableTokens));
-        const maxHistoryTokens = Math.floor(contextWindowTokens * 0.5);
+        // Apply SAFETY_MARGIN so token underestimates don't trigger unnecessary pruning
+        const maxHistoryTokens = Math.floor(contextWindowTokens * maxHistoryShare * SAFETY_MARGIN);
 
         if (newContentTokens > maxHistoryTokens) {
           const pruned = pruneHistoryForContextShare({
             messages: messagesToSummarize,
             maxContextTokens: contextWindowTokens,
-            maxHistoryShare: 0.5,
+            maxHistoryShare,
             parts: 2,
           });
           if (pruned.droppedChunks > 0) {
@@ -200,6 +208,37 @@ export default function compactionSafeguardExtension(api: ExtensionAPI): void {
                 `(${pruned.droppedMessages} messages) to fit history budget.`,
             );
             messagesToSummarize = pruned.messages;
+
+            // Summarize dropped messages so context isn't lost
+            if (pruned.droppedMessagesList.length > 0) {
+              try {
+                const droppedChunkRatio = computeAdaptiveChunkRatio(
+                  pruned.droppedMessagesList,
+                  contextWindowTokens,
+                );
+                const droppedMaxChunkTokens = Math.max(
+                  1,
+                  Math.floor(contextWindowTokens * droppedChunkRatio),
+                );
+                droppedSummary = await summarizeInStages({
+                  messages: pruned.droppedMessagesList,
+                  model,
+                  apiKey,
+                  signal,
+                  reserveTokens: Math.max(1, Math.floor(preparation.settings.reserveTokens)),
+                  maxChunkTokens: droppedMaxChunkTokens,
+                  contextWindow: contextWindowTokens,
+                  customInstructions,
+                  previousSummary: preparation.previousSummary,
+                });
+              } catch (droppedError) {
+                console.warn(
+                  `Compaction safeguard: failed to summarize dropped messages, continuing without: ${
+                    droppedError instanceof Error ? droppedError.message : String(droppedError)
+                  }`,
+                );
+              }
+            }
           }
         }
       }
@@ -210,6 +249,10 @@ export default function compactionSafeguardExtension(api: ExtensionAPI): void {
       const maxChunkTokens = Math.max(1, Math.floor(contextWindowTokens * adaptiveRatio));
       const reserveTokens = Math.max(1, Math.floor(preparation.settings.reserveTokens));
 
+      // Feed dropped-messages summary as previousSummary so the main summarization
+      // incorporates context from pruned messages instead of losing it entirely.
+      const effectivePreviousSummary = droppedSummary ?? preparation.previousSummary;
+
       const historySummary = await summarizeInStages({
         messages: messagesToSummarize,
         model,
@@ -219,7 +262,7 @@ export default function compactionSafeguardExtension(api: ExtensionAPI): void {
         maxChunkTokens,
         contextWindow: contextWindowTokens,
         customInstructions,
-        previousSummary: preparation.previousSummary,
+        previousSummary: effectivePreviousSummary,
       });
 
       let summary = historySummary;
diff --git a/src/config/types.agent-defaults.ts b/src/config/types.agent-defaults.ts
index 2a42d3623..9c6ce0211 100644
--- a/src/config/types.agent-defaults.ts
+++ b/src/config/types.agent-defaults.ts
@@ -244,6 +244,8 @@ export type AgentCompactionConfig = {
   mode?: AgentCompactionMode;
   /** Minimum reserve tokens enforced for Pi compaction (0 disables the floor). */
   reserveTokensFloor?: number;
+  /** Max share of context window for history during safeguard pruning (0.1–0.9, default 0.5). */
+  maxHistoryShare?: number;
   /** Pre-compaction memory flush (agentic turn). Default: enabled. */
   memoryFlush?: AgentCompactionMemoryFlushConfig;
 };
diff --git a/src/config/zod-schema.agent-defaults.ts b/src/config/zod-schema.agent-defaults.ts
index c4b8a8f2c..a849078ed 100644
--- a/src/config/zod-schema.agent-defaults.ts
+++ b/src/config/zod-schema.agent-defaults.ts
@@ -90,6 +90,7 @@ export const AgentDefaultsSchema = z
       .object({
         mode: z.union([z.literal("default"), z.literal("safeguard")]).optional(),
         reserveTokensFloor: z.number().int().nonnegative().optional(),
+        maxHistoryShare: z.number().min(0.1).max(0.9).optional(),
         memoryFlush: z
           .object({
             enabled: z.boolean().optional(),