fix: filter chat messages in place

lib/compress/search.ts

@@ -1,7 +1,7 @@
 import type { SessionState, WithParts } from "../state"
 import { formatBlockRef, parseBoundaryId } from "../message-ids"
 import { isIgnoredUserMessage } from "../messages/query"
-import { filterProcessableMessages } from "../messages/shape"
+import { filterMessages } from "../messages/shape"
 import { countAllMessageTokens } from "../token-utils"
 import type { BoundaryReference, SearchContext, SelectionResolution } from "./types"
 
@@ -10,7 +10,7 @@ export async function fetchSessionMessages(client: any, sessionId: string): Prom
         path: { id: sessionId },
     })
 
-    return filterProcessableMessages(response?.data || response)
+    return filterMessages(response?.data || response)
 }
 
 export function buildSearchContext(state: SessionState, rawMessages: WithParts[]): SearchContext {

lib/hooks.ts

@@ -22,7 +22,7 @@ import {
     consumeCompressionStart,
     resolveCompressionDuration,
 } from "./compress/timing"
-import { filterProcessableMessages } from "./messages/shape"
+import { filterMessages, filterMessagesInPlace } from "./messages/shape"
 import {
     applyPendingManualTrigger,
     handleContextCommand,
@@ -104,49 +104,50 @@ export function createChatMessageTransformHandler(
     hostPermissions: HostPermissionSnapshot,
 ) {
     return async (input: {}, output: { messages: WithParts[] }) => {
-        const messages = filterProcessableMessages(output.messages)
-        if (messages.length !== output.messages.length) {
+        const receivedMessages = Array.isArray(output.messages) ? output.messages.length : 0
+        const messages = filterMessagesInPlace(output.messages)
+        if (messages.length !== receivedMessages) {
             logger.warn("Skipping messages with unexpected shape during chat transform", {
-                received: output.messages.length,
+                received: receivedMessages,
                 usable: messages.length,
             })
         }
 
-        await checkSession(client, state, logger, messages, config.manualMode.enabled)
+        await checkSession(client, state, logger, output.messages, config.manualMode.enabled)
 
-        syncCompressPermissionState(state, config, hostPermissions, messages)
+        syncCompressPermissionState(state, config, hostPermissions, output.messages)
 
         if (state.isSubAgent && !config.experimental.allowSubAgents) {
             return
         }
 
         stripHallucinations(output.messages)
-        cacheSystemPromptTokens(state, messages)
-        assignMessageRefs(state, messages)
-        syncCompressionBlocks(state, logger, messages)
-        syncToolCache(state, config, logger, messages)
-        buildToolIdList(state, messages)
-        prune(state, logger, config, messages)
+        cacheSystemPromptTokens(state, output.messages)
+        assignMessageRefs(state, output.messages)
+        syncCompressionBlocks(state, logger, output.messages)
+        syncToolCache(state, config, logger, output.messages)
+        buildToolIdList(state, output.messages)
+        prune(state, logger, config, output.messages)
         await injectExtendedSubAgentResults(
             client,
             state,
             logger,
-            messages,
+            output.messages,
             config.experimental.allowSubAgents,
         )
-        const compressionPriorities = buildPriorityMap(config, state, messages)
+        const compressionPriorities = buildPriorityMap(config, state, output.messages)
         prompts.reload()
         injectCompressNudges(
             state,
             config,
             logger,
-            messages,
+            output.messages,
             prompts.getRuntimePrompts(),
             compressionPriorities,
         )
-        injectMessageIds(state, config, messages, compressionPriorities)
-        applyPendingManualTrigger(state, messages, logger)
-        stripStaleMetadata(messages)
+        injectMessageIds(state, config, output.messages, compressionPriorities)
+        applyPendingManualTrigger(state, output.messages, logger)
+        stripStaleMetadata(output.messages)
 
         if (state.sessionId) {
             await logger.saveContext(state.sessionId, output.messages)
@@ -174,7 +175,7 @@ export function createCommandExecuteHandler(
             const messagesResponse = await client.session.messages({
                 path: { id: input.sessionID },
             })
-            const messages = filterProcessableMessages(messagesResponse.data || messagesResponse)
+            const messages = filterMessages(messagesResponse.data || messagesResponse)
 
             await ensureSessionInitialized(
                 client,

lib/messages/inject/subagent-results.ts

@@ -1,6 +1,6 @@
 import type { Logger } from "../../logger"
 import type { SessionState, WithParts } from "../../state"
-import { filterProcessableMessages } from "../shape"
+import { filterMessages } from "../shape"
 import {
     buildSubagentResultText,
     getSubAgentId,
@@ -13,7 +13,7 @@ async function fetchSubAgentMessages(client: any, sessionId: string): Promise<Wi
         path: { id: sessionId },
     })
 
-    return filterProcessableMessages(response?.data || response)
+    return filterMessages(response?.data || response)
 }
 
 export const injectExtendedSubAgentResults = async (

lib/messages/shape.ts

@@ -24,10 +24,27 @@ export function isMessageWithInfo(message: unknown): message is WithParts {
     )
 }
 
-export function filterProcessableMessages(messages: unknown): WithParts[] {
+export function filterMessages(messages: unknown): WithParts[] {
     if (!Array.isArray(messages)) {
         return []
     }
 
     return messages.filter(isMessageWithInfo)
 }
+
+export function filterMessagesInPlace(messages: unknown): WithParts[] {
+    if (!Array.isArray(messages)) {
+        return []
+    }
+
+    let writeIndex = 0
+
+    for (const message of messages) {
+        if (isMessageWithInfo(message)) {
+            messages[writeIndex++] = message
+        }
+    }
+
+    messages.length = writeIndex
+    return messages as WithParts[]
+}

tests/hooks-permission.test.ts

@@ -113,7 +113,7 @@ test("chat message transform strips hallucinated tags even when compress is deni
     assert.equal((output.messages[0]?.parts[0] as any).text, "alpha  omega")
 })
 
-test("chat message transform ignores messages without info instead of crashing", async () => {
+test("chat message transform drops messages without info instead of crashing", async () => {
     const state = createSessionState()
     const logger = new Logger(false)
     const config = buildConfig("deny")
@@ -148,7 +148,7 @@ test("chat message transform ignores messages without info instead of crashing",
     await handler({}, output as any)
 
     assert.equal(state.sessionId, null)
-    assert.equal(output.messages.length, 1)
+    assert.equal(output.messages.length, 0)
 })
 
 test("command execute exits after effective permission resolves to deny", async () => {