fix(context-pruning): prune image-containing tool results instead of skipping them (#41789)

src/agents/pi-embedded-runner/run/history-image-prune.test.ts

@@ -49,6 +49,30 @@ describe("pruneProcessedHistoryImages", () => {
     expect(first.content[1]).toMatchObject({ type: "image", data: "abc" });
   });
 
+  it("prunes image blocks from toolResult messages that already have assistant replies", () => {
+    const messages: AgentMessage[] = [
+      castAgentMessage({
+        role: "toolResult",
+        toolName: "read",
+        content: [{ type: "text", text: "screenshot bytes" }, { ...image }],
+      }),
+      castAgentMessage({
+        role: "assistant",
+        content: "ack",
+      }),
+    ];
+
+    const didMutate = pruneProcessedHistoryImages(messages);
+
+    expect(didMutate).toBe(true);
+    const firstTool = messages[0] as Extract<AgentMessage, { role: "toolResult" }> | undefined;
+    if (!firstTool || !Array.isArray(firstTool.content)) {
+      throw new Error("expected toolResult array content");
+    }
+    expect(firstTool.content).toHaveLength(2);
+    expect(firstTool.content[1]).toMatchObject({ type: "text", text: PRUNED_HISTORY_IMAGE_MARKER });
+  });
+
   it("does not change messages when no assistant turn exists", () => {
     const messages: AgentMessage[] = [
       castAgentMessage({

src/agents/pi-embedded-runner/run/history-image-prune.ts

@@ -21,7 +21,11 @@ export function pruneProcessedHistoryImages(messages: AgentMessage[]): boolean {
   let didMutate = false;
   for (let i = 0; i < lastAssistantIndex; i++) {
     const message = messages[i];
-    if (!message || message.role !== "user" || !Array.isArray(message.content)) {
+    if (
+      !message ||
+      (message.role !== "user" && message.role !== "toolResult") ||
+      !Array.isArray(message.content)
+    ) {
       continue;
     }
     for (let j = 0; j < message.content.length; j++) {

src/agents/pi-extensions/context-pruning/pruner.test.ts

@@ -45,6 +45,19 @@ function makeAssistant(content: AssistantMessage["content"]): AgentMessage {
   };
 }
 
+function makeToolResult(
+  content: Array<
+    { type: "text"; text: string } | { type: "image"; data: string; mimeType: string }
+  >,
+): AgentMessage {
+  return {
+    role: "toolResult",
+    toolName: "read",
+    content,
+    timestamp: Date.now(),
+  } as AgentMessage;
+}
+
 describe("pruneContextMessages", () => {
   it("does not crash on assistant message with malformed thinking block (missing thinking string)", () => {
     const messages: AgentMessage[] = [
@@ -109,4 +122,84 @@ describe("pruneContextMessages", () => {
     });
     expect(result).toHaveLength(2);
   });
+
+  it("soft-trims image-containing tool results by replacing image blocks with placeholders", () => {
+    const messages: AgentMessage[] = [
+      makeUser("summarize this"),
+      makeToolResult([
+        { type: "text", text: "A".repeat(120) },
+        { type: "image", data: "img", mimeType: "image/png" },
+        { type: "text", text: "B".repeat(120) },
+      ]),
+      makeAssistant([{ type: "text", text: "done" }]),
+    ];
+
+    const result = pruneContextMessages({
+      messages,
+      settings: {
+        ...DEFAULT_CONTEXT_PRUNING_SETTINGS,
+        keepLastAssistants: 1,
+        softTrimRatio: 0,
+        hardClear: {
+          ...DEFAULT_CONTEXT_PRUNING_SETTINGS.hardClear,
+          enabled: false,
+        },
+        softTrim: {
+          maxChars: 200,
+          headChars: 170,
+          tailChars: 30,
+        },
+      },
+      ctx: CONTEXT_WINDOW_1M,
+      isToolPrunable: () => true,
+      contextWindowTokensOverride: 16,
+    });
+
+    const toolResult = result[1] as Extract<AgentMessage, { role: "toolResult" }>;
+    expect(toolResult.content).toHaveLength(1);
+    expect(toolResult.content[0]).toMatchObject({ type: "text" });
+    const textBlock = toolResult.content[0] as { type: "text"; text: string };
+    expect(textBlock.text).toContain("[image removed during context pruning]");
+    expect(textBlock.text).toContain(
+      "[Tool result trimmed: kept first 170 chars and last 30 chars",
+    );
+  });
+
+  it("hard-clears image-containing tool results once ratios require clearing", () => {
+    const messages: AgentMessage[] = [
+      makeUser("summarize this"),
+      makeToolResult([
+        { type: "text", text: "small text" },
+        { type: "image", data: "img", mimeType: "image/png" },
+      ]),
+      makeAssistant([{ type: "text", text: "done" }]),
+    ];
+
+    const placeholder = "[hard cleared test placeholder]";
+    const result = pruneContextMessages({
+      messages,
+      settings: {
+        ...DEFAULT_CONTEXT_PRUNING_SETTINGS,
+        keepLastAssistants: 1,
+        softTrimRatio: 0,
+        hardClearRatio: 0,
+        minPrunableToolChars: 1,
+        softTrim: {
+          maxChars: 5_000,
+          headChars: 2_000,
+          tailChars: 2_000,
+        },
+        hardClear: {
+          enabled: true,
+          placeholder,
+        },
+      },
+      ctx: CONTEXT_WINDOW_1M,
+      isToolPrunable: () => true,
+      contextWindowTokensOverride: 8,
+    });
+
+    const toolResult = result[1] as Extract<AgentMessage, { role: "toolResult" }>;
+    expect(toolResult.content).toEqual([{ type: "text", text: placeholder }]);
+  });
 });

src/agents/pi-extensions/context-pruning/pruner.ts

@@ -5,9 +5,8 @@ import type { EffectiveContextPruningSettings } from "./settings.js";
 import { makeToolPrunablePredicate } from "./tools.js";
 
 const CHARS_PER_TOKEN_ESTIMATE = 4;
-// We currently skip pruning tool results that contain images. Still, we count them (approx.) so
-// we start trimming prunable tool results earlier when image-heavy context is consuming the window.
 const IMAGE_CHAR_ESTIMATE = 8_000;
+const PRUNED_CONTEXT_IMAGE_MARKER = "[image removed during context pruning]";
 
 function asText(text: string): TextContent {
   return { type: "text", text };
@@ -23,6 +22,22 @@ function collectTextSegments(content: ReadonlyArray<TextContent | ImageContent>)
   return parts;
 }
 
+function collectPrunableToolResultSegments(
+  content: ReadonlyArray<TextContent | ImageContent>,
+): string[] {
+  const parts: string[] = [];
+  for (const block of content) {
+    if (block.type === "text") {
+      parts.push(block.text);
+      continue;
+    }
+    if (block.type === "image") {
+      parts.push(PRUNED_CONTEXT_IMAGE_MARKER);
+    }
+  }
+  return parts;
+}
+
 function estimateJoinedTextLength(parts: string[]): number {
   if (parts.length === 0) {
     return 0;
@@ -190,12 +205,9 @@ function softTrimToolResultMessage(params: {
   settings: EffectiveContextPruningSettings;
 }): ToolResultMessage | null {
   const { msg, settings } = params;
-  // Ignore image tool results for now: these are often directly relevant and hard to partially prune safely.
-  if (hasImageBlocks(msg.content)) {
-    return null;
-  }
-
-  const parts = collectTextSegments(msg.content);
+  const parts = hasImageBlocks(msg.content)
+    ? collectPrunableToolResultSegments(msg.content)
+    : collectTextSegments(msg.content);
   const rawLen = estimateJoinedTextLength(parts);
   if (rawLen <= settings.softTrim.maxChars) {
     return null;
@@ -274,9 +286,6 @@ export function pruneContextMessages(params: {
     if (!isToolPrunable(msg.toolName)) {
       continue;
     }
-    if (hasImageBlocks(msg.content)) {
-      continue;
-    }
     prunableToolIndexes.push(i);
 
     const updated = softTrimToolResultMessage({