From fbd16ef93ef0cbfe7a5ec20b38faefb577476d30 Mon Sep 17 00:00:00 2001
From: Aiden Cline <aidenpcline@gmail.com>
Date: Thu, 24 Sep 2026 18:59:41 -0500
Subject: [PATCH] fix(core): fit model limits and recover compaction overflow

---
 packages/core/src/plugin/provider/openai.ts   |   8 +
 packages/core/src/session/compaction.ts       | 346 +++++++++++++-----
 packages/core/src/session/model-request.ts    |  28 +-
 packages/core/src/session/runner/llm.ts       |   5 +
 packages/core/test/config/compaction.test.ts  |   2 +-
 .../core/test/plugin/provider-openai.test.ts  |  25 ++
 packages/core/test/session-compaction.test.ts |  34 +-
 .../test/session-model-request-hooks.test.ts  |  58 +++
 .../core/test/session-model-request.test.ts   |  33 +-
 .../test/session-native-compaction.test.ts    |   6 +-
 packages/core/test/session-runner.test.ts     | 129 ++++++-
 services/www/src/docs/content/compaction.mdx  |  28 +-
 12 files changed, 560 insertions(+), 142 deletions(-)

diff --git a/packages/core/src/plugin/provider/openai.ts b/packages/core/src/plugin/provider/openai.ts
index 255cbb2cfcc..5c7bdc38aff 100644
--- a/packages/core/src/plugin/provider/openai.ts
+++ b/packages/core/src/plugin/provider/openai.ts
@@ -1,5 +1,6 @@
 import type { IntegrationOAuthMethodRegistration } from "@opencode/plugin/effect/integration"
 import { define } from "@opencode/plugin/effect/plugin"
+import type { SessionRequest } from "@opencode/plugin/effect/session"
 import { Deferred, Effect, Option, Schema, Semaphore, Stream } from "effect"
 import type { Server } from "node:http"
 import { App } from "../../app.js"
@@ -307,6 +308,13 @@ export const OpenAIPlugin = define({
         }),
       { providerID: Provider.ID.openai },
     )
+    // The ChatGPT backend rejects a requested output limit, and OpenAI counts one against rate limits.
+    const omitOutputLimit = (evt: SessionRequest) =>
+      Effect.sync(() => {
+        delete evt.options.maxTokens
+      })
+    for (const name of ["context", "compaction"] as const)
+      yield* ctx.session.hook(name, omitOutputLimit, { providerID: Provider.ID.openai })
     const refresh = () => loading.withPermit(load().pipe(Effect.andThen(ctx.provider.reload())))
     yield* bus.subscribe(Credential.Event.Switched).pipe(
       Stream.filter((event) => event.data.integrationID === Integration.ID.make("openai")),
diff --git a/packages/core/src/session/compaction.ts b/packages/core/src/session/compaction.ts
index fc9d16799a0..aed56665326 100644
--- a/packages/core/src/session/compaction.ts
+++ b/packages/core/src/session/compaction.ts
@@ -37,12 +37,17 @@ import { toLLMMessages } from "./runner/to-llm-message.js"
 import type { AgentNotFoundError } from "./error.js"
 import type { Instructions } from "../instructions/index.js"
 
-const DEFAULT_BUFFER = 20_000
+const AUTO_THRESHOLD = 0.9
 const DEFAULT_KEEP_TOKENS = 15_000
-const OUTPUT_TOKEN_MAX = 32_000
 const TOOL_OUTPUT_MAX_CHARS = 2_000
+const FALLBACK_TOOL_CHARS = 1_000
+const FALLBACK_OVERFLOW_RETRIES = 3
 const IMAGE_TOKEN_ESTIMATE = 1_500
 const PDF_TOKEN_ESTIMATE = 2_000
+const usableWindow = (limit: SessionRunnerModel.Resolved["limit"]) =>
+  Math.min(limit.context > 0 ? limit.context : (limit.input ?? 0), limit.input ?? Number.POSITIVE_INFINITY)
+const compactionCeiling = (usable: number, buffer?: number) =>
+  buffer === undefined ? Math.floor(usable * AUTO_THRESHOLD) : usable - buffer
 const SUMMARY_TEMPLATE = `You MUST use this format for your response (you may omit sections that aren't applicable). Do not include the <template> tags in your response.
 <template>
 ## Objective
@@ -89,7 +94,7 @@ const LEGACY_HEADING = "## Additional Context"
 
 export type Settings = {
   auto: boolean
-  buffer: number
+  buffer?: number
   tokens: number
 }
 
@@ -172,6 +177,12 @@ const hasInputUsage = (message: SessionMessage.Info) =>
   message.tokens.input + message.tokens.cache.read + message.tokens.cache.write > 0
 
 export const estimateTokens = (input: RequiredInput) => {
+  const prompt = estimatePrompt(input)
+  return prompt.measured + prompt.estimated
+}
+
+/** The prompt size: `measured` is what the provider reported at the latest response, `estimated` is the text since. */
+export const estimatePrompt = (input: RequiredInput) => {
   const index = input.messages.findLastIndex(hasInputUsage)
   const last = input.messages[index]
   // Keep the anchor's local tool results: they are not covered by its provider usage.
@@ -182,14 +193,15 @@ export const estimateTokens = (input: RequiredInput) => {
     .filter((message) => message.role !== "assistant" || message.id !== last?.id)
     .reduce((sum, message) => sum + message.content.reduce((sum, part) => sum + estimatePart(part), 0), 0)
   if (last?.type === "assistant" && last.tokens)
-    return (
-      added +
-      last.tokens.input +
-      last.tokens.cache.read +
-      last.tokens.cache.write +
-      last.tokens.output +
-      last.tokens.reasoning
-    )
+    return {
+      measured:
+        last.tokens.input +
+        last.tokens.cache.read +
+        last.tokens.cache.write +
+        last.tokens.output +
+        last.tokens.reasoning,
+      estimated: added,
+    }
   const transcript = SessionModelRequest.baseTranscript({
     agent: input.context.agent.info,
     model: input.resolved,
@@ -197,14 +209,16 @@ export const estimateTokens = (input: RequiredInput) => {
     initial: input.context.initial,
     messages: [],
   })
-  return (
-    added +
-    transcript.system.reduce((sum, part) => sum + Token.estimate(part.text), 0) +
-    input.context.tools.definitions.reduce(
-      (sum, tool) => sum + Token.estimate(tool.name + tool.description + JSON.stringify(tool.inputSchema)),
-      0,
-    )
-  )
+  return {
+    measured: 0,
+    estimated:
+      added +
+      transcript.system.reduce((sum, part) => sum + Token.estimate(part.text), 0) +
+      input.context.tools.definitions.reduce(
+        (sum, tool) => sum + Token.estimate(tool.name + tool.description + JSON.stringify(tool.inputSchema)),
+        0,
+      ),
+  }
 }
 
 const estimateMedia = (mime: string) => {
@@ -255,10 +269,10 @@ export const retainUsers = (
   return users.slice(start)
 }
 
-export const truncateToolOutput = (value: string) => {
-  if (value.length <= TOOL_OUTPUT_MAX_CHARS) return value
+export const truncateToolOutput = (value: string, maxChars = TOOL_OUTPUT_MAX_CHARS) => {
+  if (value.length <= maxChars) return value
   let end = 0
-  for (let count = 0; count < TOOL_OUTPUT_MAX_CHARS && end < value.length; count++) {
+  for (let count = 0; count < maxChars && end < value.length; count++) {
     const code = value.charCodeAt(end)
     end +=
       code >= 0xd800 && code <= 0xdbff && value.charCodeAt(end + 1) >= 0xdc00 && value.charCodeAt(end + 1) <= 0xdfff
@@ -319,6 +333,60 @@ const serializeRecentMessage = (message: SessionMessage.Info) => {
   return ""
 }
 
+/** Flatten provider-bound history into text so a reduced summary request carries no tool pairs or reasoning signatures. */
+const serializeFallback = (messages: readonly Message[]) =>
+  messages.flatMap((message) => {
+    const parts = message.content.flatMap((part) => {
+      if (part.type === "text" || part.type === "reasoning") return part.text ? [part.text] : []
+      if (part.type === "media")
+        return [`[Attached ${part.media.mediaType}${part.filename ? `: ${part.filename}` : ""}; content omitted]`]
+      if (part.type === "tool-call") return [`[Tool call ${part.name}(${JSON.stringify(part.input)})]`]
+      if (part.type === "tool-result") {
+        const value =
+          part.result.type === "content"
+            ? part.result.value
+                .map((item) =>
+                  item.type === "text" ? item.text : `[Attached ${item.mime}${item.name ? `: ${item.name}` : ""}]`,
+                )
+                .join("\n")
+            : typeof part.result.value === "string"
+              ? part.result.value
+              : JSON.stringify(part.result.value)
+        return [`[Tool result ${part.name}]: ${truncateToolOutput(value ?? "", FALLBACK_TOOL_CHARS)}`]
+      }
+      if (part.type === "compaction" && part.text) return [part.text]
+      return []
+    })
+    return parts.length ? [{ role: message.role, text: `[${message.role}]: ${parts.join("\n")}` }] : []
+  })
+
+/** Retain a prior checkpoint and the newest complete exchanges that fit the summary input budget. */
+const fitFallback = (entries: ReturnType<typeof serializeFallback>, budget: number) => {
+  const previous = entries[0]?.text.includes("<conversation-checkpoint>") ? entries[0] : undefined
+  const rest = previous ? entries.slice(1) : entries
+  const groups = rest.reduce<Array<string>>((groups, entry) => {
+    if (entry.role === "user" || groups.length === 0) groups.push(entry.text)
+    else groups[groups.length - 1] += `\n\n${entry.text}`
+    return groups
+  }, [])
+  const header = previous ? `${previous.text}\n\n` : ""
+  const allowance = budget - Token.estimate(header)
+  if (allowance <= 0) return
+  let start = groups.length
+  let total = 0
+  while (start > 0) {
+    const size = Token.estimate(groups[start - 1])
+    if (total + size > allowance) break
+    total += size
+    start--
+  }
+  if (groups.length && start === groups.length) return
+  return {
+    text: `${header}${start ? `[${start} older exchanges omitted from this summary input]\n\n` : ""}${groups.slice(start).join("\n\n")}`,
+    omitted: start,
+  }
+}
+
 const splitHistory = (messages: readonly SessionMessage.Info[], keepTokens: number) => {
   const tailStart = findTailStart(messages, keepTokens)
   if (tailStart === undefined) return
@@ -401,7 +469,7 @@ export const layer = Layer.effect(
 
     const state = State.create<Settings & { readonly native: NativeStrategy[] }, Editor>({
       name: "session-compaction",
-      initial: () => ({ auto: true, buffer: DEFAULT_BUFFER, tokens: DEFAULT_KEEP_TOKENS, native: [] }),
+      initial: () => ({ auto: true, tokens: DEFAULT_KEEP_TOKENS, native: [] }),
       editor: (editor) => ({
         configure: (settings) => {
           if (settings.auto !== undefined) editor.auto = settings.auto
@@ -470,6 +538,7 @@ export const layer = Layer.effect(
       input: ExecuteInput,
       messages: readonly SessionMessage.Info[],
       webSocket?: "session",
+      summaryPrompt?: string,
     ) => {
       const context = input.context
       const transcript = SessionModelRequest.baseTranscript({
@@ -479,6 +548,7 @@ export const layer = Layer.effect(
         initial: context.initial,
         messages,
       })
+      const prompt = estimatePrompt({ messages, resolved: context.model, context })
       return input.prepare({
         session: context.session,
         agent: context.agent.id,
@@ -490,6 +560,11 @@ export const layer = Layer.effect(
           ...(input.instructionUpdate ? [Message.system(input.instructionUpdate)] : []),
         ],
         webSocket,
+        // The instruction update and summary prompt are sent outside the history, so count them too.
+        inputTokens: {
+          measured: prompt.measured,
+          estimated: prompt.estimated + Token.estimate((input.instructionUpdate ?? "") + (summaryPrompt ?? "")),
+        },
       })
     }
     /** The durable transcript since the last local summary, re-expanding every native window. */
@@ -623,87 +698,163 @@ export const layer = Layer.effect(
       )
       // Checkpoints from the previous template ran far longer than this one asks for; its catch-all heading identifies them.
       const legacy = previous?.summary.includes(LEGACY_HEADING) ?? false
-      const prepared = yield* compactionRequest(input, history.messages)
+      const prompt = buildPrompt(previous !== undefined, legacy)
+      const prepared = yield* compactionRequest(input, history.messages, undefined, prompt)
       if (prepared.event.result) return yield* supplied(input, prepared.event.result, history.recent)
       // Hooks see the transcript alone; the summary prompt is appended after they run.
       const first = LLMRequest.update(prepared.request, {
-        messages: [...prepared.request.messages, Message.user(buildPrompt(previous !== undefined, legacy))],
+        messages: [...prepared.request.messages, Message.user(prompt)],
       })
+      const usable = usableWindow(context.model.limit)
+      const fixed =
+        prepared.request.system.reduce((sum, part) => sum + Token.estimate(part.text), 0) +
+        context.tools.definitions.reduce(
+          (sum, tool) => sum + Token.estimate(tool.name + tool.description + JSON.stringify(tool.inputSchema)),
+          0,
+        ) +
+        Token.estimate(prompt)
+      let entries: ReturnType<typeof serializeFallback> | undefined
+      let budget = usable ? compactionCeiling(usable, state.get().buffer) - fixed : undefined
+      let fallback =
+        usable > 0 &&
+        (budget ?? 0) > 0 &&
+        estimateTokens({ messages: history.messages, resolved: context.model, context }) +
+          Token.estimate(prompt) >
+          usable
+      let retries = 0
+      let last: string | undefined
+      let omitted = 0
       // Both requests share the retry allowance; rejected output never enters the reminder request.
       const transient = SessionRunnerRetry.transient(yield* SessionRunnerRetry.policy(context.session.id), {
         agent: context.agent.id,
         model: context.model.ref,
         hook: prepared.retry,
       })
-      for (const request of [
-        first,
-        LLMRequest.update(first, {
-          messages: [
-            ...first.messages,
-            Message.user(
-              "The previous response did not fill in the required summary template. Do not call tools. Return the summary as text using the exact section headings from the template.",
-            ),
-          ],
-        }),
-      ]) {
-        yield* Stream.suspend(() => {
-          chunks.length = 0
-          providerState = undefined
-          failure = undefined
-          return llm.stream(request, prepared.options)
-        }).pipe(
-          Stream.runForEach((event) => {
-            if (LLMEvent.is.providerError(event))
-              failure = {
-                type: event.classification === "context-overflow" ? "provider.invalid-request" : "provider.error",
-                message: event.message,
-              }
-            if (LLMEvent.is.textDelta(event)) {
-              chunks.push(event.text)
-              return bus.publish(SessionEvent.Compaction.Delta, {
-                sessionID: context.session.id,
-                text: event.text,
-              })
-            }
-            if (LLMEvent.is.stepFinish(event)) {
-              providerState =
-                event.providerMetadata?.[context.model.model.route.providerMetadataKey ?? context.model.model.provider]
-              const step = SessionUsage.record(event.usage, context.model.cost)
-              usage = usage ? SessionUsage.add(usage, step) : step
-            }
-            if (LLMEvent.is.finish(event)) {
-              if (event.reason.normalized === "length")
-                failure = { type: "compaction.failed", message: "Compaction summary reached the output token limit" }
-              if (event.reason.normalized === "content-filter")
-                failure = {
-                  type: "provider.content-filter",
-                  message: "Compaction summary was blocked by the provider",
-                }
-              if (event.reason.normalized === "unknown")
-                return Effect.fail(
-                  new AIError({
-                    reason: new InvalidProviderOutputError({
-                      message: "The provider response ended with an unknown finish reason.",
-                      classification: "incomplete-stream",
-                    }),
-                  }),
-                )
-              if (event.reason.normalized === "error")
-                return Effect.fail(
-                  new AIError({ reason: new UnknownProviderError({ message: "Compaction generation failed" }) }),
-                )
-            }
-            return Effect.void
+      let overflow = false
+      while (true) {
+        if (fallback) {
+          entries ??= serializeFallback(prepared.request.messages)
+          budget ??= Token.estimate(entries.map((entry) => entry.text).join("\n\n"))
+        }
+        const fitted = fallback && entries && budget !== undefined ? fitFallback(entries, budget) : undefined
+        if (fallback && (!fitted || fitted.text === last)) {
+          failure = {
+            type: "compaction.failed",
+            message: "The summary input cannot be reduced further without losing the latest exchange or checkpoint",
+          }
+          break
+        }
+        if (fitted) {
+          last = fitted.text
+          omitted = fitted.omitted
+          retries++
+        }
+        const selected = fitted
+          ? yield* input.prepare({
+              session: context.session,
+              agent: context.agent.id,
+              model: context.model,
+              tools: context.tools,
+              system: SessionModelRequest.baseTranscript({
+                agent: context.agent.info,
+                model: context.model,
+                tools: context.tools,
+                initial: context.initial,
+                messages: [],
+              }).system,
+              messages: [Message.user(fitted.text)],
+              inputTokens: { measured: 0, estimated: fixed + Token.estimate(fitted.text) },
+            })
+          : prepared
+        if (selected.event.result) {
+          yield* recordUsage
+          return yield* supplied(input, selected.event.result, history.recent)
+        }
+        const request = fallback
+          ? LLMRequest.update(selected.request, {
+              messages: [...selected.request.messages, Message.user(prompt)],
+            })
+          : first
+        for (const attempt of [
+          request,
+          LLMRequest.update(request, {
+            messages: [
+              ...request.messages,
+              Message.user(
+                "The previous response did not fill in the required summary template. Do not call tools. Return the summary as text using the exact section headings from the template.",
+              ),
+            ],
           }),
-          transient,
-          Effect.catchTag("AI.Error", (error) =>
-            Effect.sync(() => {
-              failure = toSessionError(error)
+        ]) {
+          yield* Stream.suspend(() => {
+            chunks.length = 0
+            providerState = undefined
+            failure = undefined
+            overflow = false
+            return llm.stream(attempt, selected.options)
+          }).pipe(
+            Stream.runForEach((event) => {
+              if (LLMEvent.is.providerError(event)) {
+                overflow = event.classification === "context-overflow"
+                failure = {
+                  type: overflow ? "provider.invalid-request" : "provider.error",
+                  message: event.message,
+                }
+              }
+              if (LLMEvent.is.textDelta(event)) {
+                chunks.push(event.text)
+                return bus.publish(SessionEvent.Compaction.Delta, {
+                  sessionID: context.session.id,
+                  text: event.text,
+                })
+              }
+              if (LLMEvent.is.stepFinish(event)) {
+                providerState =
+                  event.providerMetadata?.[
+                    context.model.model.route.providerMetadataKey ?? context.model.model.provider
+                  ]
+                const step = SessionUsage.record(event.usage, context.model.cost)
+                usage = usage ? SessionUsage.add(usage, step) : step
+              }
+              if (LLMEvent.is.finish(event)) {
+                if (event.reason.normalized === "length")
+                  failure = { type: "compaction.failed", message: "Compaction summary reached the output token limit" }
+                if (event.reason.normalized === "content-filter")
+                  failure = {
+                    type: "provider.content-filter",
+                    message: "Compaction summary was blocked by the provider",
+                  }
+                if (event.reason.normalized === "unknown")
+                  return Effect.fail(
+                    new AIError({
+                      reason: new InvalidProviderOutputError({
+                        message: "The provider response ended with an unknown finish reason.",
+                        classification: "incomplete-stream",
+                      }),
+                    }),
+                  )
+                if (event.reason.normalized === "error")
+                  return Effect.fail(
+                    new AIError({ reason: new UnknownProviderError({ message: "Compaction generation failed" }) }),
+                  )
+              }
+              return Effect.void
             }),
-          ),
-          Effect.onInterrupt(() => recordUsage.pipe(Effect.andThen(interrupted(input)))),
-        )
-        if (failure || hasSummarySection(chunks.join(""))) break
+            transient,
+            Effect.catchTag("AI.Error", (error) =>
+              Effect.sync(() => {
+                overflow = isContextOverflowFailure(error)
+                failure = toSessionError(error)
+              }),
+            ),
+            Effect.onInterrupt(() => recordUsage.pipe(Effect.andThen(interrupted(input)))),
+          )
+          if (failure || hasSummarySection(chunks.join(""))) break
+        }
+        if (!overflow) break
+        if (retries >= FALLBACK_OVERFLOW_RETRIES) break
+        fallback = true
+        if (retries && budget !== undefined) budget = Math.floor(budget / 2)
       }
       yield* recordUsage
       const summary = chunks.join("")
@@ -727,7 +878,9 @@ export const layer = Layer.effect(
         reason: input.reason,
         model: context.model.ref,
         providerState,
-        text: summary,
+        text: omitted
+          ? `${summary}\n\n[${omitted} older exchanges were omitted from the summary input; original session history is retained.]`
+          : summary,
         recent: history.recent,
         ...usage,
       })
@@ -754,12 +907,7 @@ export const layer = Layer.effect(
       const limit = input.resolved.limit
       const context = limit.context
       if (context <= 0) return false
-      const output = Math.min(limit.output, OUTPUT_TOKEN_MAX)
-      const promptCeiling = Math.min(
-        limit.input === undefined ? Number.POSITIVE_INFINITY : limit.input - config.buffer,
-        context - Math.max(output, config.buffer),
-      )
-      return estimateTokens(input) >= promptCeiling
+      return estimateTokens(input) >= compactionCeiling(usableWindow(limit), config.buffer)
     }
     const compactManual = Effect.fn("SessionCompaction.compactManual")(function* (input: ManualInput) {
       if (findTailStart(input.messages, state.get().tokens) === undefined)
diff --git a/packages/core/src/session/model-request.ts b/packages/core/src/session/model-request.ts
index d2277ce8ac7..c04603dc220 100644
--- a/packages/core/src/session/model-request.ts
+++ b/packages/core/src/session/model-request.ts
@@ -44,6 +44,14 @@ const IMAGE_BYTES_TARGET = 15 * 1024 * 1024 // 15 MiB
 const IMAGE_REMOVED =
   "[This image was removed to reduce the request size and is no longer visible. Do not make claims about its contents from memory. If needed, retrieve it again with an available tool or ask the user to attach it again.]"
 const GENERATION_KEYS = new Set(Object.keys(GenerationOptions.fields))
+// Default output limit caps per request kind. Titles and generate have none and keep the provider default, because
+// their reasoning is hard to budget.
+const OUTPUT_TOKEN_CAPS: Partial<Record<SessionRequestKind, number>> = { primary: 256_000, compaction: 32_000 }
+// Used when the catalog has no output limit for the model.
+const OUTPUT_TOKEN_FALLBACK = 32_000
+// Prompt text is estimated at about 4 characters per token, which can run low on dense text such as code.
+const ESTIMATE_ERROR = 0.15
+const OUTPUT_TOKEN_MIN = 1_024
 
 /** Tool errors, plus the user declining a permission or dismissing a question. */
 export type ExecuteError = Tool.Error | Permission.DeclinedError | QuestionTool.CancelledError
@@ -69,6 +77,16 @@ export interface Input {
   readonly toolChoice?: LLM.RequestInput["toolChoice"]
   /** Only the durable runner may use a stateful WebSocket. */
   readonly webSocket?: "session"
+  /** Prompt size, measured by the provider or estimated. The default output limit leaves room for it. */
+  readonly inputTokens?: { readonly measured: number; readonly estimated: number }
+}
+
+/** The default output limit: the catalog limit, capped, and fitted to the room the prompt leaves in the context window. */
+export const outputLimit = (limit: Model.Info["limit"], cap: number, inputTokens?: Input["inputTokens"]) => {
+  const requested = Math.min(limit.output > 0 ? limit.output : OUTPUT_TOKEN_FALLBACK, cap)
+  if (inputTokens === undefined || limit.context <= 0) return requested
+  const room = limit.context - inputTokens.measured - Math.ceil(inputTokens.estimated * (1 + ESTIMATE_ERROR))
+  return Math.min(requested, Math.max(OUTPUT_TOKEN_MIN, room))
 }
 
 export const baseTranscript = (input: {
@@ -218,8 +236,16 @@ export const layer = Layer.effect(
       const given = new Map(
         tools.definitions.map((t) => [{ description: t.description, input: { ...t.inputSchema } }, t] as const),
       )
+      // Hooks see the default output limit and may change or remove it.
+      const cap = OUTPUT_TOKEN_CAPS[kind]
       const shaped = yield* shape(
-        { sessionID: session.id, model: model.ref, system: input.system, messages: input.messages, options: {} },
+        {
+          sessionID: session.id,
+          model: model.ref,
+          system: input.system,
+          messages: input.messages,
+          options: cap === undefined ? {} : { maxTokens: outputLimit(model.limit, cap, input.inputTokens) },
+        },
         Object.fromEntries(Array.from(given, ([d, t]) => [t.name, d])),
       )
       // Match by identity first, then by key. Entries matching neither were invented by a
diff --git a/packages/core/src/session/runner/llm.ts b/packages/core/src/session/runner/llm.ts
index 82839326b46..6e68653e321 100644
--- a/packages/core/src/session/runner/llm.ts
+++ b/packages/core/src/session/runner/llm.ts
@@ -244,6 +244,11 @@ const layer = Layer.effect(
           // Keep tool definitions on the final Step to preserve the provider's cached prefix.
           toolChoice: stepLimitReached ? "none" : undefined,
           webSocket: "session",
+          inputTokens: SessionCompaction.estimatePrompt({
+            messages: loaded.messages,
+            resolved: loaded.model,
+            context: loaded,
+          }),
         })
         const outcome = yield* steps.attempt({
           isLocationClosed: lifecycle.isClosed,
diff --git a/packages/core/test/config/compaction.test.ts b/packages/core/test/config/compaction.test.ts
index e292f942ae9..45cd4a94441 100644
--- a/packages/core/test/config/compaction.test.ts
+++ b/packages/core/test/config/compaction.test.ts
@@ -172,5 +172,5 @@ const input = (tokens: number) => {
     },
   }
 }
-const bufferedInput = input(85_000)
+const bufferedInput = input(82_000)
 const nearInput = input(95_000)
diff --git a/packages/core/test/plugin/provider-openai.test.ts b/packages/core/test/plugin/provider-openai.test.ts
index 2f6725f3cf2..7ca86ee7d0e 100644
--- a/packages/core/test/plugin/provider-openai.test.ts
+++ b/packages/core/test/plugin/provider-openai.test.ts
@@ -216,6 +216,31 @@ describe("OpenAIPlugin", () => {
     }),
   )
 
+  it.effect("omits the default output limit from OpenAI steps and compaction", () =>
+    Effect.gen(function* () {
+      yield* addPlugin()
+      const hooks = yield* PluginHooks.Service
+      const maxTokens = (providerID: Provider.ID) =>
+        Effect.gen(function* () {
+          const draft = {
+            sessionID: Session.ID.make("ses_test"),
+            model: Model.Ref.make({ providerID, id: Model.ID.make("gpt-5.5") }),
+            system: [],
+            messages: [],
+            options: { maxTokens: 128_000 },
+          }
+          const events = [
+            yield* hooks.trigger("session", "context", { ...draft, agent: Agent.ID.make("build"), tools: {} }),
+            yield* hooks.trigger("session", "compaction", { ...draft, agent: Agent.ID.make("build"), tools: {} }),
+          ]
+          return events.map((event) => event.options.maxTokens)
+        })
+
+      expect(yield* maxTokens(Provider.ID.openai)).toEqual([undefined, undefined])
+      expect(yield* maxTokens(Provider.ID.azure)).toEqual([128_000, 128_000])
+    }),
+  )
+
   it.effect("selects WebSocket only from explicit policy", () =>
     Effect.gen(function* () {
       const credentials = yield* Credential.Service
diff --git a/packages/core/test/session-compaction.test.ts b/packages/core/test/session-compaction.test.ts
index 18a020b7372..4670e9185c9 100644
--- a/packages/core/test/session-compaction.test.ts
+++ b/packages/core/test/session-compaction.test.ts
@@ -1,5 +1,5 @@
 import { expect, test } from "bun:test"
-import { LLMClient, LLMEvent, LanguageModel, ToolDefinition, type LLMRequest } from "@opencode/ai"
+import { GenerationOptions, LLMClient, LLMEvent, LanguageModel, ToolDefinition, type LLMRequest } from "@opencode/ai"
 import { OpenAIChat } from "@opencode/ai/protocols"
 import { Database } from "@opencode/core/database/database"
 import { AppNodeBuilder } from "@opencode/core/effect/app-node-builder"
@@ -153,7 +153,7 @@ test("compaction prompts prohibit task execution", () => {
     expect(SessionCompaction.buildPrompt(update)).toContain("Do not continue the task or call tools")
 })
 
-it.effect("auto compaction estimates current content against the buffered prompt ceiling", () =>
+it.effect("auto compaction uses 90% by default and a configured buffer instead", () =>
   Effect.gen(function* () {
     const compaction = yield* SessionCompaction.Service
     const session = Session.Info.make({
@@ -205,23 +205,22 @@ it.effect("auto compaction estimates current content against the buffered prompt
     }
 
     const inputLimited = { context: 400_000, input: 272_000, output: 128_000 }
-    expect(compaction.required(input(251_999, inputLimited))).toBe(false)
-    expect(compaction.required(input(252_000, inputLimited))).toBe(true)
+    expect(compaction.required(input(244_799, inputLimited))).toBe(false)
+    expect(compaction.required(input(244_800, inputLimited))).toBe(true)
     const native = (tokens: number, limit: { context: number; input?: number; output: number } = inputLimited) => {
       const selected = input(tokens, limit)
       return { ...selected, resolved: { ...selected.resolved, compaction: { type: "native" as const } } }
     }
-    expect(compaction.required(native(251_999))).toBe(false)
-    expect(compaction.required(native(252_000))).toBe(true)
+    expect(compaction.required(native(244_800))).toBe(true)
     expect(compaction.required(native(1_000_000, { context: 0, input: undefined, output: 0 }))).toBe(false)
 
     const contextLimited = { context: 100_000, output: 10_000 }
-    expect(compaction.required(input(79_999, contextLimited))).toBe(false)
-    expect(compaction.required(input(80_000, contextLimited))).toBe(true)
+    expect(compaction.required(input(89_999, contextLimited))).toBe(false)
+    expect(compaction.required(input(90_000, contextLimited))).toBe(true)
 
-    const outputLimited = { context: 100_000, output: 30_000 }
-    expect(compaction.required(input(69_999, outputLimited))).toBe(false)
-    expect(compaction.required(input(70_000, outputLimited))).toBe(true)
+    const smallWindow = { context: 32_000, output: 32_000 }
+    expect(compaction.required(input(28_799, smallWindow))).toBe(false)
+    expect(compaction.required(input(28_800, smallWindow))).toBe(true)
 
     const assistant = input(79_000, contextLimited).messages[0]
     const tool = SessionMessage.AssistantTool.make({
@@ -233,7 +232,9 @@ it.effect("auto compaction estimates current content against the buffered prompt
     })
     const grown = { ...input(79_000, contextLimited), messages: [{ ...assistant, content: [tool] }] }
     expect(SessionCompaction.estimateTokens(grown)).toBe(80_000)
-    expect(compaction.required(grown)).toBe(true)
+    expect(compaction.required(grown)).toBe(false)
+    const near = input(89_000, contextLimited)
+    expect(compaction.required({ ...near, messages: [{ ...near.messages[0], content: [tool] }] })).toBe(true)
 
     const interrupted = { ...assistant, id: SessionMessage.ID.create(), tokens: undefined }
     expect(SessionCompaction.estimateTokens({ ...grown, messages: [...grown.messages, interrupted] })).toBe(80_001)
@@ -285,6 +286,13 @@ it.effect("auto compaction estimates current content against the buffered prompt
       time: { created: 0, completed: 0 },
     })
     expect(compaction.required({ ...grown, messages: [checkpoint] })).toBe(false)
+
+    yield* compaction.transform((editor) => editor.configure({ buffer: 5_000 }))
+    expect(compaction.required(input(94_999, contextLimited))).toBe(false)
+    expect(compaction.required(input(95_000, contextLimited))).toBe(true)
+    yield* compaction.transform((editor) => editor.configure({ buffer: 0 }))
+    expect(compaction.required(input(99_999, contextLimited))).toBe(false)
+    expect(compaction.required(input(100_000, contextLimited))).toBe(true)
   }),
 )
 
@@ -407,7 +415,7 @@ it.effect("manual compaction summarizes short context instead of no-op", () =>
       "x-opencode-session": sessionID,
       "x-opencode-client": "opencode",
     })
-    expect(requests[0]?.generation).toBeUndefined()
+    expect(requests[0]?.generation).toEqual(GenerationOptions.make({ maxTokens: 32_000 }))
     expect(JSON.stringify(requests[0]?.messages)).toContain("Manual compaction should include this short conversation.")
     expect(JSON.stringify(requests[0]?.messages)).toContain("Use Effect services and generators.")
     expect(JSON.stringify(requests[0]?.messages)).toContain("User shell pwd completed: /project")
diff --git a/packages/core/test/session-model-request-hooks.test.ts b/packages/core/test/session-model-request-hooks.test.ts
index 3b7b1d45f45..9e18804bd02 100644
--- a/packages/core/test/session-model-request-hooks.test.ts
+++ b/packages/core/test/session-model-request-hooks.test.ts
@@ -154,3 +154,61 @@ describe("SessionModelRequest HTTP hooks", () => {
     }),
   )
 })
+
+describe("SessionModelRequest output limit", () => {
+  const input = { session, agent: Agent.ID.make("build"), model, system: [], messages: [] }
+
+  it.effect("caps the default output limit per request kind", () =>
+    Effect.gen(function* () {
+      const requests = yield* SessionModelRequest.Service.pipe(Effect.provide(SessionModelRequest.layer))
+      const large = {
+        ...input,
+        model: SessionRunnerModel.resolved(OpenAIChat.route.model({ id: "large-output", provider: "test" }), {
+          capabilities: { tools: true, input: ["text"], output: ["text"] },
+          cost: [],
+          limit: { context: 1_000_000, output: 384_000 },
+        }),
+      }
+      const maxTokens = (prepared: SessionModelRequest.Prepared<unknown>) => prepared.request.generation?.maxTokens
+      expect(maxTokens(yield* requests.primary(large))).toBe(256_000)
+      expect(maxTokens(yield* requests.compaction(large))).toBe(32_000)
+      const inputTokens = { measured: 170_000, estimated: 8_000 }
+      expect(maxTokens(yield* requests.primary({ ...input, inputTokens }))).toBe(20_800)
+      expect(maxTokens(yield* requests.compaction({ ...input, inputTokens }))).toBe(20_800)
+    }).pipe(Effect.provideService(SessionModelTransport.Service, transport)),
+  )
+
+  // Provider plugins that remove the default limit only hook `context` and `compaction`. If titles or generate get a
+  // default, also hook `title` and `generate` in: the OpenAI plugin (`omitOutputLimit`), whose ChatGPT backend
+  // rejects any requested limit.
+  it.effect("sends no output limit for titles and generate by default", () =>
+    Effect.gen(function* () {
+      const requests = yield* SessionModelRequest.Service.pipe(Effect.provide(SessionModelRequest.layer))
+      expect((yield* requests.title(input)).request.generation?.maxTokens).toBeUndefined()
+      expect((yield* requests.generate(input)).request.generation?.maxTokens).toBeUndefined()
+    }).pipe(Effect.provideService(SessionModelTransport.Service, transport)),
+  )
+
+  it.effect("lets hooks change or remove the default output limit", () =>
+    Effect.gen(function* () {
+      const hooks = yield* PluginHooks.Service
+      const seen: Array<number | undefined> = []
+      yield* hooks.register("session", "context", (event) =>
+        Effect.sync(() => {
+          seen.push(event.options.maxTokens)
+          delete event.options.maxTokens
+        }),
+      )
+      yield* hooks.register("session", "title", (event) =>
+        Effect.sync(() => {
+          event.options.maxTokens = 100
+        }),
+      )
+      const requests = yield* SessionModelRequest.Service.pipe(Effect.provide(SessionModelRequest.layer))
+
+      expect((yield* requests.primary(input)).request.generation).toBeUndefined()
+      expect((yield* requests.title(input)).request.generation?.maxTokens).toBe(100)
+      expect(seen).toEqual([32_000])
+    }).pipe(Effect.provideService(SessionModelTransport.Service, transport)),
+  )
+})
diff --git a/packages/core/test/session-model-request.test.ts b/packages/core/test/session-model-request.test.ts
index fb3570ef5de..977a6d20656 100644
--- a/packages/core/test/session-model-request.test.ts
+++ b/packages/core/test/session-model-request.test.ts
@@ -1,9 +1,40 @@
 import { describe, expect, test } from "bun:test"
 import { Message, ToolResultPart, Media } from "@opencode/ai"
-import { boundImages, unsupportedParts } from "@opencode/core/session/model-request"
+import { boundImages, outputLimit, unsupportedParts } from "@opencode/core/session/model-request"
 
 const capabilities = (input: string[]) => ({ tools: true, input, output: ["text"] })
 
+describe("SessionModelRequest.outputLimit", () => {
+  test("requests the catalog output limit up to the cap", () => {
+    expect(outputLimit({ context: 1_000_000, output: 128_000 }, 256_000)).toBe(128_000)
+    expect(outputLimit({ context: 200_000, output: 64_000 }, 256_000)).toBe(64_000)
+    expect(outputLimit({ context: 1_048_576, output: 1_048_576 }, 256_000)).toBe(256_000)
+    expect(outputLimit({ context: 200_000, output: 64_000 }, 32_000)).toBe(32_000)
+  })
+
+  test("falls back to 32k when the catalog has no output limit", () => {
+    expect(outputLimit({ context: 200_000, output: 0 }, 256_000)).toBe(32_000)
+  })
+
+  test("fits the limit to the room the prompt leaves in the context window", () => {
+    const limit = { context: 1_000_000, output: 128_000 }
+    expect(outputLimit(limit, 256_000, { measured: 50_000, estimated: 0 })).toBe(128_000)
+    expect(outputLimit(limit, 256_000, { measured: 900_000, estimated: 0 })).toBe(100_000)
+    // Estimated text counts 15% extra, so 40k estimated takes 46k of the room.
+    expect(outputLimit(limit, 256_000, { measured: 900_000, estimated: 40_000 })).toBe(54_000)
+  })
+
+  test("keeps a minimum limit when the prompt nearly fills the context window", () => {
+    const prompt = { measured: 199_000, estimated: 0 }
+    expect(outputLimit({ context: 200_000, output: 64_000 }, 256_000, prompt)).toBe(1_024)
+    expect(outputLimit({ context: 200_000, output: 512 }, 256_000, prompt)).toBe(512)
+  })
+
+  test("ignores the prompt size when the context window is unknown", () => {
+    expect(outputLimit({ context: 0, output: 32_000 }, 256_000, { measured: 500_000, estimated: 0 })).toBe(32_000)
+  })
+})
+
 describe("SessionModelRequest.unsupportedParts", () => {
   test("replaces unsupported user media with a visible error", () => {
     const messages = unsupportedParts(
diff --git a/packages/core/test/session-native-compaction.test.ts b/packages/core/test/session-native-compaction.test.ts
index 756e93b2b61..ed2d8ce5902 100644
--- a/packages/core/test/session-native-compaction.test.ts
+++ b/packages/core/test/session-native-compaction.test.ts
@@ -393,13 +393,15 @@ it.live("only known automatic native overflow falls back locally and failed reco
     fixture.state.overflow = true
     fixture.state.localFailure = true
     expect(yield* fixture.automatic).toMatchObject({ status: "failed" })
-    expect(fixture.state.calls).toBe(5)
+    expect(fixture.state.calls).toBe(6)
     expect(yield* fixture.checkpoint).toEqual(installed)
     expect(JSON.stringify(fixture.bodies[4])).toContain("Original durable request")
     expect(JSON.stringify(fixture.bodies[4])).not.toContain("encrypted_1")
+    expect(JSON.stringify(fixture.bodies[5])).toContain("Original durable request")
+    expect(JSON.stringify(fixture.bodies[5])).not.toContain("encrypted_1")
     fixture.state.localFailure = false
     expect(yield* fixture.automatic).toEqual({ status: "completed", recoveredOverflow: true })
-    expect(fixture.state.calls).toBe(7)
+    expect(fixture.state.calls).toBe(8)
     expect((yield* fixture.load).messages).toContainEqual(
       expect.objectContaining({ type: "compaction", summary: "## Objective\n- Recovered locally" }),
     )
diff --git a/packages/core/test/session-runner.test.ts b/packages/core/test/session-runner.test.ts
index f41f61e7472..a3315c48a76 100644
--- a/packages/core/test/session-runner.test.ts
+++ b/packages/core/test/session-runner.test.ts
@@ -127,6 +127,7 @@ const fullOutputModel = testModel("full-output", { context: 262_144, output: 262
 const unknownContextModel = testModel("unknown-context", { context: 0, output: 32_000 })
 const undersizedContextModel = testModel("undersized-context", { context: 1, output: 1_000 })
 const recoveryModel = testModel("recovery", { context: 200_000, output: 1_000 })
+const fittedOutputModel = testModel("fitted-output", { context: 100_000, output: 64_000 })
 
 test("calculates step cost using the matching context tier", () => {
   expect(
@@ -2749,22 +2750,16 @@ describe("SessionRunnerLLM", () => {
     })
   }
 
-  for (const response of ["length", "content-filter", "context overflow"] as const) {
+  for (const response of ["length", "content-filter"] as const) {
     scenario(`rejects compaction ${response} without retrying or committing its draft`, function* (s) {
       yield* s.llm.push(TestLLM.text("Earlier answer", "history"))
       yield* s.runPrompt("Earlier question")
       s.requests.length = 0
       yield* s.llm.push(
-        response === "context overflow"
-          ? Stream.fail(
-              new AIError({
-                reason: new InvalidRequestError({ message: "Too long", classification: "context-overflow" }),
-              }),
-            )
-          : TestLLM.complete(
-              { reason: { normalized: response } },
-              LLMEvent.textDelta({ id: "truncated", text: "## Objective\n- Incomplete summary" }),
-            ),
+        TestLLM.complete(
+          { reason: { normalized: response } },
+          LLMEvent.textDelta({ id: "truncated", text: "## Objective\n- Incomplete summary" }),
+        ),
       )
       const compaction = yield* s.session.compact({ sessionID })
       yield* s.resume
@@ -2778,6 +2773,98 @@ describe("SessionRunnerLLM", () => {
     })
   }
 
+  scenario("stops after three smaller compaction inputs overflow", function* (s) {
+    yield* s.llm.push(...Array.from({ length: 8 }, (_, index) => TestLLM.text(`Answer ${index}`, `answer-${index}`)))
+    yield* Effect.forEach(Array.from({ length: 8 }, (_, index) => index), (index) =>
+      s.runPrompt(`Request ${index}: ${"context ".repeat(30)}`),
+    )
+    s.currentModel = unknownContextModel
+    s.requests.length = 0
+    const overflow = () =>
+      Stream.fail(
+        new AIError({ reason: new InvalidRequestError({ message: "Too long", classification: "context-overflow" }) }),
+      )
+    yield* s.llm.push(overflow(), overflow(), overflow(), overflow())
+    const compaction = yield* s.session.compact({ sessionID })
+    yield* s.resume
+
+    expect(s.requests).toHaveLength(4)
+    expect(userTexts(s.requests[2])[0].length).toBeLessThan(userTexts(s.requests[1])[0].length)
+    expect(userTexts(s.requests[3])[0].length).toBeLessThan(userTexts(s.requests[2])[0].length)
+    expect((yield* s.messages).find((message) => message.id === compaction.id)).toMatchObject({ status: "failed" })
+    yield* s.llm.push(TestLLM.text("Continued", "continued"))
+    yield* s.runPrompt("Continue")
+    expect(userTexts(s.requests[4])).toContain("Request 0: " + "context ".repeat(30))
+  })
+
+  scenario("serializes history and omits media after a summary input overflow", function* (s) {
+    const image = "iVBORw0KGgoAAAANSUhEUgAAAAEAAAABCAQAAAC1HAwCAAAAC0lEQVR42mNk+A8AAQUBAScY42YAAAAASUVORK5CYII="
+    yield* s.session.prompt({
+      sessionID,
+      text: "Earlier question",
+      files: [{ uri: `data:image/png;base64,${image}` }],
+      resume: false,
+    })
+    yield* s.llm.push(
+      TestLLM.stop(
+        LLMEvent.toolCall({ id: "hosted", name: "web_search", input: { query: "earlier" }, providerExecuted: true }),
+        LLMEvent.toolResult({
+          id: "hosted",
+          name: "web_search",
+          result: { type: "text", value: "x".repeat(5_000) },
+          providerExecuted: true,
+        }),
+        LLMEvent.textStart({ id: "history" }),
+        LLMEvent.textDelta({ id: "history", text: "Earlier answer" }),
+        LLMEvent.textEnd({ id: "history" }),
+      ),
+    )
+    yield* s.resume
+    s.requests.length = 0
+    yield* s.llm.push(
+      [LLMEvent.providerError({ message: "Too long", classification: "context-overflow" })],
+      TestLLM.text("## Objective\n- Recovered", "summary"),
+    )
+    const compaction = yield* s.session.compact({ sessionID })
+    yield* s.resume
+
+    expect(s.requests).toHaveLength(2)
+    expect(s.requests[0]?.messages.some((message) => message.content.some((part) => part.type === "media"))).toBeTrue()
+    expect(s.requests[1]?.messages.every((message) => message.role === "user")).toBeTrue()
+    expect(userTexts(s.requests[1])[0]).toContain("[Attached image/png; content omitted]")
+    expect(userTexts(s.requests[1])[0]).toContain("[Tool result web_search]:")
+    expect(userTexts(s.requests[1])[0]).toContain("[truncated]")
+    expect(userTexts(s.requests[1])[0]).not.toContain("x".repeat(2_000))
+    expect(s.requests[1]?.system).toEqual(s.requests[0]?.system)
+    expect(s.requests[1]?.tools).toEqual(s.requests[0]?.tools)
+    expect((yield* s.messages).find((message) => message.id === compaction.id)).toMatchObject({ status: "completed" })
+  })
+
+  scenario("fits serialized compaction history by omitting oldest exchanges", function* (s) {
+    const service = yield* SessionCompaction.Service
+    yield* service.transform((editor) => editor.configure({ buffer: 3_000 }))
+    s.currentModel = testModel("large-history", { context: 1_000_000, output: 32_000 })
+    yield* s.llm.push(...Array.from({ length: 4 }, (_, index) => TestLLM.text(`Answer ${index}`, `answer-${index}`)))
+    yield* Effect.forEach(Array.from({ length: 4 }, (_, index) => index), (index) =>
+      s.runPrompt(`Request ${index}: ${"x".repeat(8_000)}`),
+    )
+    s.currentModel = testModel("smaller-history", { context: 7_000, output: 1_000 })
+    s.requests.length = 0
+    yield* s.llm.push(TestLLM.text("## Objective\n- Recovered", "summary"))
+    const compaction = yield* s.session.compact({ sessionID })
+    yield* s.resume
+
+    expect(s.requests).toHaveLength(1)
+    expect(userTexts(s.requests[0])[0]).toContain("older exchanges omitted")
+    expect(userTexts(s.requests[0])[0]).not.toContain("Request 0:")
+    expect(userTexts(s.requests[0])[0]).not.toContain("Request 1:")
+    expect(userTexts(s.requests[0])[0]).toContain("Request 2:")
+    expect((yield* s.messages).find((message) => message.id === compaction.id)).toMatchObject({
+      status: "completed",
+      summary: expect.stringContaining("older exchanges were omitted from the summary input"),
+    })
+  })
+
   scenario("records cancelled manual compaction without surfacing an internal failure", function* (s) {
     yield* s.llm.push(TestLLM.text("Earlier answer", "text-manual-interrupt-history"))
     yield* s.runPrompt("Earlier question")
@@ -2883,7 +2970,7 @@ describe("SessionRunnerLLM", () => {
         agent.steps = 2
       }),
     )
-    yield* s.llm.push(TestLLM.textWithUsage("Earlier answer", "before-native", 10_000))
+    yield* s.llm.push(TestLLM.textWithUsage("Earlier answer", "before-native", 39_000))
     yield* s.runPrompt("First real request")
     const checkpoint = (encrypted: string) =>
       CompactionCheckpointResponse.make({
@@ -2903,7 +2990,7 @@ describe("SessionRunnerLLM", () => {
     const installed = (yield* s.messages).filter((message) => message.type === "compaction")
     expect(installed).toMatchObject([{ status: "completed", reason: "auto", providerContext: { version: 1 } }])
     // New input without a post-checkpoint usage anchor must not retrigger compaction.
-    yield* s.llm.push(TestLLM.textWithUsage("Measured", "measured", 10_000))
+    yield* s.llm.push(TestLLM.textWithUsage("Measured", "measured", 39_000))
     yield* s.runPrompt("Third real request")
     expect(s.requests).toHaveLength(5)
     yield* s.llm.push(checkpoint("second"), TestLLM.textWithUsage("Continued", "continued", 10_000))
@@ -2929,7 +3016,7 @@ describe("SessionRunnerLLM", () => {
     s.currentModel = LanguageModel.make({ id: "native", provider: "openai", route: OpenAIResponses.route })
     modelLimits.set("native", { context: 42_000, output: 32_000 })
     s.compaction = { type: "native" }
-    yield* s.llm.push(TestLLM.textWithUsage("Earlier answer", "before-native", 10_000))
+    yield* s.llm.push(TestLLM.textWithUsage("Earlier answer", "before-native", 39_000))
     yield* s.runPrompt("Original durable request")
     yield* s.llm.push(
       CompactionCheckpointResponse.make({
@@ -2985,7 +3072,7 @@ describe("SessionRunnerLLM", () => {
     expect(yield* Effect.exit(s.resume)).toMatchObject({ _tag: "Failure" })
 
     expect(s.requests).toHaveLength(1)
-    expect(s.requests[0]?.generation).toBeUndefined()
+    expect(s.requests[0]?.generation?.maxTokens).toBe(50)
     expect(yield* s.context).toContainEqual(
       expect.objectContaining({
         type: "compaction",
@@ -3190,6 +3277,18 @@ describe("SessionRunnerLLM", () => {
     ])
   })
 
+  scenario("fits the output limit to the prompt size", function* (s) {
+    s.currentModel = fittedOutputModel
+    yield* s.llm.push(TestLLM.textWithUsage("Earlier answer", "text-fitted-first", 50_000))
+    yield* s.runPrompt("Earlier question")
+    yield* s.llm.push(TestLLM.text("Continued", "text-fitted-final"))
+    yield* s.runPrompt("Continue")
+
+    expect(s.requests[0]?.generation?.maxTokens).toBe(64_000)
+    expect(s.requests[1]?.generation?.maxTokens).toBeLessThan(100_000 - 50_000)
+    expect(s.requests[1]?.generation?.maxTokens).toBeGreaterThan(100_000 - 50_000 - 100)
+  })
+
   scenario("publishes the original overflow when recovery summarization fails", function* (s) {
     yield* setupOverflowRecovery(s)
     yield* s.llm.push(
diff --git a/services/www/src/docs/content/compaction.mdx b/services/www/src/docs/content/compaction.mdx
index dfca69e83e8..2c5cd7dd5c3 100644
--- a/services/www/src/docs/content/compaction.mdx
+++ b/services/www/src/docs/content/compaction.mdx
@@ -78,17 +78,20 @@ Reusing that ID for another record returns a conflict.
 ## Automatic
 
 Before each model call, OpenCode estimates the final size of the system prompt,
-messages, and advertised tools. It starts compaction at this ceiling:
+messages, and advertised tools. By default it starts compaction at 90% of the
+smaller context or input limit. A configured `buffer` replaces that percentage
+with an absolute number of tokens to reserve:
 
 ```text
-estimated tokens >= min(input limit - buffer, context limit - max(output reserve, buffer))
+usable window = min(context limit, input limit if present)
+compact when estimated tokens >= (buffer configured ? usable window - buffer : floor(90% of usable window))
 ```
 
-For example, with a 128,000-token input limit and the default 20,000-token
-buffer, the input-limit side of the ceiling is 108,000 tokens.
+For example, with a 200,000-token context and a 128,000-token input limit,
+the default threshold is 115,200 tokens:
 
 ```text
-128,000 - 20,000 = 108,000
+128,000 × 90% = 115,200
 ```
 
 The estimate follows these rules:
@@ -97,11 +100,16 @@ The estimate follows these rules:
   newer content are then added.
 - Without provider usage, OpenCode estimates text, media, instructions, and
   tools locally.
-- The reserved model output is capped at 32,000 tokens.
-- A model without an input limit is constrained by its context limit instead.
+- Without a separate input limit, the context limit sets the threshold.
 - A successful checkpoint rebuilds the same pending model step. It does not
   promote the input again or spend another agent step.
 
+Compaction normally sends the existing conversation for summarization. If it
+cannot fit the selected model, OpenCode retries within the same input threshold
+with a shorter text version:
+old tool results and media are reduced first, then older exchanges are omitted
+if needed. The original messages remain in session history.
+
 OpenCode also recognizes provider errors classified as context overflow. If no
 assistant output or other retry evidence was produced, it can compact and retry
 that model step once:
@@ -132,11 +140,11 @@ Add `compaction` to any [OpenCode configuration file](/config):
 | --- | ---: | --- |
 | `auto` | `true` | Enables preflight checks and one provider-overflow recovery attempt. It does not control manual compaction. |
 | `keep.tokens` | `15000` | Approximate recent serialized context retained beside a local summary, or real user input retained for a provider checkpoint. |
-| `buffer` | `20000` | Safety margin below an explicit input limit. Without one, it is the minimum context reserve; a larger model output allowance wins. |
+| `buffer` | unset | Replaces the 90% threshold with `usable window - buffer`. Set it to reserve a fixed number of tokens. |
 
 `keep.tokens` and `buffer` accept non-negative integers. Larger
 `keep.tokens` preserves more recent detail but leaves less room for new work;
-larger `buffer` starts automatic compaction earlier.
+larger `buffer` starts automatic compaction earlier when configured.
 
 ## Providers
 
@@ -161,7 +169,7 @@ A model setting overrides the provider setting.
 
 | Topic | Provider behavior |
 | --- | --- |
-| Threshold | OpenCode uses the model's usable input ceiling configured by its context, input, and output limits plus the global compaction buffer. |
+| Threshold | OpenCode starts at 90% of the smaller context or input limit, unless `buffer` replaces it with a fixed reserve. |
 | Scheduling | Checkpoints run at normal safe step boundaries. `compaction.auto: false` disables all new automatic work. |
 | Usage | After a native checkpoint is installed, automatic checks wait for fresh model usage because encrypted checkpoint bytes cannot provide a meaningful token count. |
 | OpenAI | Responses routes use a streamed compaction trigger when supported; endpoint-only routes use the standalone compaction endpoint. Deployment and model support vary. |