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find_callers

Identify all direct and indirect callers of a C/C++ function, including function-pointer assignments, callbacks, and struct or array initializers, to trace usage text search misses.

Instructions

Find who calls a C/C++ function — direct calls AND indirect via function pointers, callbacks, and struct/array init lists (such as a vector table that the build generates in C). libclang-powered: detects function-pointer assignments that text-based search cannot see. A slot of an assembly vector table (ref_kind "vector") and an NVIC_SetVector install ("runtime_vector") are not callers: use find_references or get_vector_table for those.

Falls back to macro lookup when the symbol is not found as a function/method: returns the macro definition (kind="macro") and files that use it (ref_kind="macro_use").

Use when you need a quick, flat list of immediate callers. For the full transitive call tree (who calls this indirectly through other functions), use find_all_callers_recursive. For all references including reads and member accesses, use find_references. For a path between two specific symbols, use find_call_path.

Read-only. No side effects. Requires the reference index (fw-context index — refs are on by default). Only direct call sites are returned; callers more than one hop away are not included.

Indirect edges (ref_kind: "indirect") are detected when a function pointer references a function through:

  • Call arguments: callback(&Class::method, this), EventQueue::call_every(ms, obj, &handler)

  • Assignments: driver.onData = &handleData, global_cb = &handler

  • Variable initializers: static void (*fp)(int) = &handler

  • Struct/array init lists: {.on_data = &handler}, {&fn_a, &fn_b}

Args: name: Symbol name to find callers of. Uses the same three-tier resolution as find_references (exact name, exact qualified, suffix LIKE). project_root: Project root directory. Auto-detected if omitted. project: Project name or project_id — call list_projects to get them. Use it to ask about a project that is not the project of the current directory. It is an alternative to project_root, which takes a root path. Give one of the two, not both. limit: Maximum results of one page (default 50, max 200). offset: Skip this many results. Reads the next page of a symbol with many call sites; the page notice names the offset to use. variant: Variant of the build (a board, or a PlatformIO environment). On a project of more than one build, give the arguments of one build as the refusal lists them: the variant, the image, or the two. fw-context does not select a build. One query answers for ONE build. image: Image of the build: a separate program, such as the application or the bootloader. Give it when the build has an image name.

Returns: The page notice first — total, offset, shown, more — then a dict per call site with: file, line, ref_kind ("call", "indirect", "implicit_construct", or "macro_use"), caller (enclosing function name), caller_kind ("function", "method", …). Macro fallback puts a dict with kind="macro", signature (#define free: NAME or NAME(a, b)), is_function_like, value (the replacement text ALONE) and expanded_value between the notice and the rows; that answer pages too, and its total counts the uses in active code only — a use inside a comment is not one.

When *name* matches more than one symbol, such as two classes with
a method of the same name, the answer holds the call sites of all
of them.  A ``warning`` dict then comes first and names the
symbols, and each result carries ``target_qualified_name``.  Give
the full qualified name to ask about one symbol only.

A virtual method with no call site of its own answers with the
call sites of its base method and of the other overrides of that
base.  A ``warning`` dict leads such an answer, and each row
carries ``recorded_against`` (the symbol the index records the
call against) and ``reaches_this_symbol`` (true for a call on the
base method, which reaches this override at run time; false for a
call on a sibling override, which does not).  Read both before you
report a caller count.

Never empty: one dict with ``error`` (symbol not resolved) or
``info`` (no references of this kind).  Check both keys first.

Input Schema

NameRequiredDescriptionDefault
nameYesSymbol name to find callers of. Returns direct call sites and indirect calls via function pointers.
imageNoImage of the build: a separate program, such as the application or the bootloader. On a project of more than one build, give it when that build has an image name, as the refusal lists it.
limitNoMaximum results of one page (default 50, max 200).
offsetNoSkip this many results. Reads the next page of a symbol with many call sites.
projectNoProject name or project_id — call list_projects to get them. Use it to ask about a project that is not the project of the current directory. It is an alternative to project_root, which takes a root path. Give one of the two, not both.
variantNoVariant of the build (a board, or a PlatformIO environment). On a project of more than one build, give the arguments of one build as the refusal lists them: the variant, the image, or the two. fw-context does not select a build for you. One query answers for ONE build.
project_rootNoProject root. Auto-detected if omitted. This field also accepts a project name or a project_id, but project is the clear field for those.

Output Schema

NameRequiredDescriptionDefault
resultYes

Schema Changelog

Changes observed during successful MCP inspections.

  1. Changed2 schema fields changedv0.32.1
    • changedInput schema / properties / image / description
      Previous value: -"Sysbuild image within the variant. Required when the variant holds several: each image is a separate program."New value: +"Image of the build: a separate program, such as the application or the bootloader. On a project of more than one build, give it when that build has an image name, as the refusal lists it."
    • changedInput schema / properties / variant / description
      Previous value: -"Build variant (multi-build project). Omit to use default_variant. One query answers for ONE build."New value: +"Variant of the build (a board, or a PlatformIO environment). On a project of more than one build, give the arguments of one build as the refusal lists them: the variant, the image, or the two. fw-context does not select a build for you. One query answers for ONE build."
  2. Changed6 schema fields changedv0.32.0
    • changedInput schema / properties / image / description
      Previous value: -"Sysbuild image name within the variant (multi-project). Omit for all images of the variant."New value: +"Sysbuild image within the variant. Required when the variant holds several: each image is a separate program."
    • changedInput schema / properties / limit / description
      Previous value: -"Maximum results."New value: +"Maximum results of one page (default 50, max 200)."
    • addedInput schema / properties / limit / minimum
      Added value: +1
    • addedInput schema / properties / name / minLength
      Added value: +1
    • addedInput schema / properties / offset
      Added value: +{
      +  "default": 0,
      +  "description": "Skip this many results. Reads the next page of a symbol with many call sites.",
      +  "minimum": 0,
      +  "title": "Offset",
      +  "type": "integer"
      +}
    • changedInput schema / properties / variant / description
      Previous value: -"Build variant name (multi-project). Omit to use default_variant or fail-closed. Use '*' for all variants."New value: +"Build variant (multi-build project). Omit to use default_variant. One query answers for ONE build."
  3. Changed3 schema fields changedv0.30.0
    • addedInput schema / additionalProperties
      Added value: +false
    • addedInput schema / properties / project
      Added value: +{
      +  "anyOf": [
      +    {
      +      "type": "string"
      +    },
      +    {
      +      "type": "null"
      +    }
      +  ],
      +  "default": null,
      +  "description": "Project name or project_id — call list_projects to get them. Use it to ask about a project that is not the project of the current directory. It is an alternative to project_root, which takes a root path. Give one of the two, not both.",
      +  "title": "Project"
      +}
    • changedInput schema / properties / project_root / description
      Previous value: -"Project root. Auto-detected if omitted."New value: +"Project root. Auto-detected if omitted. This field also accepts a project name or a project_id, but project is the clear field for those."
  4. Changed2 schema fields changedv0.25.3
    • addedInput schema / properties / image
      Added value: +{
      +  "anyOf": [
      +    {
      +      "type": "string"
      +    },
      +    {
      +      "type": "null"
      +    }
      +  ],
      +  "default": null,
      +  "description": "Sysbuild image name within the variant (multi-project). Omit for all images of the variant.",
      +  "title": "Image"
      +}
    • addedInput schema / properties / variant
      Added value: +{
      +  "anyOf": [
      +    {
      +      "type": "string"
      +    },
      +    {
      +      "type": "null"
      +    }
      +  ],
      +  "default": null,
      +  "description": "Build variant name (multi-project). Omit to use default_variant or fail-closed. Use '*' for all variants.",
      +  "title": "Variant"
      +}
  5. Changed1 schema field changedv0.2.1
    • changedInput schema / properties / name / description
      Previous value: -"Symbol name to find callers of. Returns direct call sites."New value: +"Symbol name to find callers of. Returns direct call sites and indirect calls via function pointers."
  6. First observedv0.1.0

TDQS

A4.7/5.0
Behavior5/5

Does the description disclose side effects, auth requirements, rate limits, or destructive behavior?

With no annotations provided, the description carries the full behavioral burden and does so thoroughly: read-only, no side effects, requires the reference index, only direct call sites are returned, indirect edge detection is explained with examples, macro fallback behavior is described, and output edge cases such as warnings and never-empty error/info dicts are disclosed.

Agents need to know what a tool does to the world before calling it. Descriptions should go beyond structured annotations to explain consequences.

Conciseness3/5

Is the description appropriately sized, front-loaded, and free of redundancy?

The definition is front-loaded and well organized, but it is very long, and much of the Returns section explains output shape even though an output schema exists. The indirect-edge examples and return-value edge cases are useful, but the description could be trimmed without losing essential selection or invocation information.

Shorter descriptions cost fewer tokens and are easier for agents to parse. Every sentence should earn its place.

Completeness5/5

Given the tool's complexity, does the description cover enough for an agent to succeed on first attempt?

Given the tool's complexity, the absence of annotations, and the presence of a rich output schema, the description is complete enough for correct selection and invocation. It covers indexing prerequisites, build/variant selection, pagination, macro fallback, symbol conflicts, virtual method handling, and error/info return cases.

Complex tools with many parameters or behaviors need more documentation. Simple tools need less. This dimension scales expectations accordingly.

Parameters4/5

Does the description clarify parameter syntax, constraints, interactions, or defaults beyond what the schema provides?

Schema description coverage is 100%, so the baseline would be 3, but the description adds meaningful semantics beyond the schema: `name` uses the same three-tier resolution as `find_references`, `project` and `project_root` are mutually exclusive alternatives, and `variant`/`image` must match exactly one build as refusal messages list them. These additions help an agent invoke the tool correctly.

Input schemas describe structure but not intent. Descriptions should explain non-obvious parameter relationships and valid value ranges.

Purpose5/5

Does the description clearly state what the tool does and how it differs from similar tools?

The description opens with a specific verb and resource: finding who calls a C/C++ function, and explicitly includes direct calls plus indirect calls via function pointers, callbacks, and init lists. It distinguishes itself from siblings such as `find_all_callers_recursive`, `find_references`, and `find_call_path` by name and scope.

Agents choose between tools based on descriptions. A clear purpose with a specific verb and resource helps agents select the right tool.

Usage Guidelines5/5

Does the description explain when to use this tool, when not to, or what alternatives exist?

It states exactly when to use this tool (a quick, flat list of immediate callers) and when to use alternatives: `find_all_callers_recursive` for transitive call trees, `find_references` for all references including reads, `find_call_path` for a path between two symbols. It also warns that vector-table slots and `NVIC_SetVector` installs are not callers and should be queried with `find_references` or `get_vector_table`.

Agents often have multiple tools that could apply. Explicit usage guidance like "use X instead of Y when Z" prevents misuse.